MARINE ENGINES & PROPULSION
Ranger Hope © 2015 View as a Pdf file
This text is provided for research and study only on the understanding that users exercise due care and do not neglect any precaution which may be required by the ordinary practice of seamen or current licensing legislation with respect to its use. No copying is permitted and no liability is accepted resulting from use.
1.4 Timing- valves and injection
1.5 Air supply - turbo charging, after coolers
1.6 Engine protection – devices
7.3 Other propulsion systems - Azimuth thrusters, Voith Schneider, Jets.
Marine engines are described by their crankshafts’ revolutions per minute (rpm) – typically as low speed engines (less than 400 rpm), medium speed engines (400 -1000 rpm) or high speed engines (over 1000 rpm).
As this small commercial fleet is principally fitted with high speed diesel inboard engines, or in the smaller craft increasingly with high speed petrol outboard engines, these are described most fully.
The following technical terms are used in this text.
Bottom dead centre – the position where the piston has reached its furthest distance from the cylinder head, and will subsequently reverse its direction of stroke (bdc).
Calorific value - fuel contains heat energy which is released in the combustion process called the fuel’s calorific value, measured in joules per kilogram of fuel.
Cam lift - the distance that the peak of the cam’s lobe extends from the round which is the same as the valve opening plus the tappet clearance (valve lash).
The cam lift profile causes the valve to open rapidly then stay open momentarily before rapidly re-closing. Dwell is the angle of the cam over which the valve remains temporarily in the fully open position.
Compression ratio - is the comparison the volume of the cylinder as the piston sweeps from BDC position up to TDC position, sometimes called the ratio of the unswept volume to the swept volume of the cylinder. A compression ratio of 12:1 would mean that the air (in diesel engines) or air/fuel (in petrol engines) has been compressed to one-twelfth its original volume. Diesel engines need a high compression ratio heat the compressed air sufficiently to ignite the fuel.
Compression ratio = maximum cylinder volume ÷ minimum cylinder volume
Flash point - in order to catch on fire, a material must be heated sufficiently to cause it to partially vaporize. The lowest temperature at which the vapour arising from the fuel gives off a flammable mixture that will ignite when flame is momentarily applied under defined test conditions is called its flash point. Technically petrol and diesel are cocktails of hydrocarbons of different flash points. Commonly from - 40°C to 0°C for petrol and 60°C for diesel is regarded as the temperature that they will burn. (Diesel’s lower volatility means it is safer to use than petrol). If a fire is cooled below its flash point then the flame will not be sustained.
Force - an influence which tends to change the motion or direction of a body (such as pushing or pulling) measured in newtons (N). A force can either:
Start moving a body from rest or bring a moving body to rest.
Increase or decrease the speed of a body.
Change the direction of motion of a moving body.
Heat transfer - Heat moves (transfers) in three ways:
Convection Heat moving within a liquid or gas.
Conduction Heat moving through a solid.
Radiation Heat energy traveling out as heat rays (direct heat).
The hottest air or water experiencing convection move upwards, the coolest downwards, thus forming a gyre rotation in a sealed system. Conducted heat is movement away from its origin along metal components. Radiated heat (from exhaust manifolds) may char or set it alight material that is close, consequently removing surrounding flammable materials (boundary clearance) is essential to limit spread of fire.
Power - is the amount of work or energy expended in a given time or the capacity to do work measured as Watts (W). A watt is work at the rate of one joule per second.
Power = Force x Distance = newton/metres per second or joules per second.
Time in seconds
Force is in newtons (N), distance in metres (m) and time in seconds (s). Therefore the formula first answer is in units of Nm/s or j/s. However as1 Nm = 1 joule and 1 joule per second = 1 watt so the final answer converts to watts. Engine power is measured in kilowatts (kW) -1000 W = 1 kW.
Scavenging - describes the elimination of the burned exhaust gases from a cylinder, pushed out by the incoming stream of air induction. Valve overlap assists in the process.
Thermal efficiency - is a comparison of the work done at the flywheel to the amount of energy contained in the fuel, and is expressed as a percentage.
Top dead centre - the position when the piston has reached its least distance from the cylinder head, and will subsequently reverse its direction of stroke (tdc).
Torque - describes a force tending to cause a rotational movement about a point, also called a turning or twisting effort. Torque is the force exerted, but not moved, over a distance measured in newton/metres (Nm). For instance, the force the connecting rod exerts on the crankshaft.
Torque = Force x Distance in newton/metres (Nm).
Turbulence - a rapidly swirling motion of the air as it enters the combustion chamber. In most engines it is deliberately induced as the violent movement helps ensure even mixing of the fuel and air. It enhances flame propagation speeding up the combustion process once the fuel has ignited.
Work - the amount of energy used to overcome resistance in moving an applied force through a distance as Work = Force x Distance in joules
It could be described in newton/metres (Nm) as force is measured in newtons (N) and the distance is measured in metres (m).To prevent confusion between work and torque, the unit given to the formula for work is the joule (j), by conversion as:
One newton/metre = one joule.
Volumetric efficiency - is the ratio between the swept volume of a cylinder (between tdc & bdc) and the actual volume of air drawn in during the induction stroke. The efficiency varies depending on the design, operating conditions and engine speed. A turbo charged engine will have a higher volumetric efficiency (in excess of 100%) than that of a normally aspirated engine.
Valve overlap - is the period which both the inlet valve and exhaust valve are open simultaneously. For instance, if the inlet and exhaust valve are both open at 10° before TDC to 3°after TDC the valve overlap is over a 13° angle.
The purpose of valve overlap is to ensure that are exhaust gases are discharged from the cylinder and the cylinder receives a fresh charge of air to make it more efficient when combustion next takes place. It also has a cooling effect.
Valve rotators - rotate a valve each time it opens to ensure even wear and prevent exhaust valves from localised burn out.
Combustion is the process in which fuel heated beyond its flash point ignites and gives off energy and the waste products as exhaust gasses (carbon dioxide, carbon monoxide and water). The three elements necessary for combustion (or fire) are:
Oxygen + Heat + Fuel = Explosion or Fire
Internal combustion engines suck in air to access the oxygen and use hydrocarbon fuels including petrol, diesel and liquid propane gas.
The internal combustion process
Internal combustion engines use successive explosions of atomised air and fuel mix to force a piston down a gas tight internal cylinder. The piston is connected through a big end bearing to a cranked shaft that is weighted by a flywheel to assist its rotary momentum. Thus with three primary moving parts (piston, connecting rod and crankshaft) the piston’s reciprocating motion drives the crankshaft’s rotary motion.
A sturdy metal casting of an engine block holds the internal parts and the many ancillary components required for smooth and continuous operation. These include:
A lubrication sump (oil reserve); a cylinder head with gas outlets and inlets; and in the drawing shown below, an over head camshaft driven timing gear within a rocker box. For it to work timing is everything. The fuel (diesel, petrol or gas) and air (oxygen) charge must be inserted, the exhaust removed and a compression seal reinstated at the precise instant for the explosive ignition. The heat and noise from multiple explosions as well as wear on metal surfaces must be minimised.
To ensure a gas tight compression seal, the cylinder liners in the engine block are bored to very tight tolerances and the barrel of the piston is grooved to accept close fitting sprung metal rings - that makes for a tight sliding fit against the cylinder walls.
In order to insert the fuel/air mix and exhaust the products of the combustion, sealable inlet and outlet passageways must be provided. Typically these passageways take the form of either ports (holes in the cylinder wall that are covered and uncovered by the passing of the piston) or by valves (steel stoppers that plug and unplug holes in the cylinder head at the correct timing intervals).
The simplest engines often use ports and more the complex use valves, though hybrids like that shown below use both. The piston is connected to the big end bearing of the crankshaft by means of a connecting rod. The piston is forced down the cylinder by the expansion of combustion gases to turn the crankshaft. With the piston as low as it will go in the cylinder, it is said to be at bottom dead centre (bdc). The momentum of the flywheel weighted crankshaft rotating on its main bearings forces the piston back to the top of the cylinder, ready for the next explosion. When it is as high as it will go in the cylinder, it is at top dead centre (tdc).
The distance the piston moves from tdc to bdc is called the stroke of the engine.
The turning crankshaft drives the camshaft by means of timing gears (or by sprockets and a chain). In the drawing shown above, the side camshaft is driven by crankshaft gears. Oval cams on the camshaft open the inlet and exhaust valves at the correct time by means of push-rods actuating rockers. An inlet port (or alternately a valve) allows air or fuel/air mixture to be drawn into a cylinder ready for combustion, while an exhaust valve lets exhaust gas out of a cylinder.
The arrangement of ports or camshafts and valves differs depending on engine design. The drawing above shows a two stroke engine with its piston nearing tdc, its exhaust valve closed and inlet port covered, so compressing the fuel/air charge immediately prior to ignition. At the end of the power stroke the cam will open the exhaust valve to flush the cylinder ready for re-charge and re-ignition.
The work cycle
A sequence of four operations called the work cycle must occur for engines to work:
Induction – a charge of air (diesels) or air/fuel mix (petrol) is directed into the cylinder to provide the oxygen required for combustion.
Compression - the charge is compressed to optimise the result of combustion.
Power - the air/fuel mix (petrol) is ignited by a spark, or with diesels the fuel is injected to ignite in the superheated compressed air. The expansion of the gasses from this explosive ignition exerts a force upon the piston that turns the crankshaft.
Exhaust- The products of this explosion (the exhaust gasses) is ejected so the cycle can be repeated.
The work cycle of induction (suck), compression (squeeze), power (bang) and exhaust (push) are achieved by differing configurations or types of engines including four stroke or two stroke engines. The term stroke means the directional travel of the piston along the cylinder from tdc - bdc or from bdc - tdc.
The four-stroke engine takes two complete revolutions of the crankshaft to complete the work cycle. The two-stroke does it in one complete revolution of the crankshaft. This means that a two-stroke engine has twice as many power operations as a four-stroke engine at the same speed, so it may produce more power and acceleration but at the expense of fuel efficiency and reliability.
Petrol or diesel
Due to availability and convenience of handling, petrol (or gasoline) and diesel (or distillate) are the most common small vessel marine propulsion fuels. Larger vessels often use a cheaper and less refined tarry fuel oil requiring heat to liquefy sufficiently for the combustion process. Petrol and diesel are cocktails of hydrocarbons wth differing flash points, commonly from - 40°C to 0°C for petrol and 60°C for diesel, this being the temperature that they will vapourize to ignite. Diesel’s higher flash point makes it safer than petrol as any escaping fuel vapours can pool in a boat’s bilges with consequent risk of explosion.
The diesel engine uses the piston’s sweep of the cylinder from bdc position up to tdc position to compress the inducted air, raising its pressure and consequent heat to such an extent (1000ºC) that when fuel is injected it immediately ignites. Diesel engines need a high compression ratio compressed the air sufficient to reach the temperature for ignition. A 16:1 to 20:1 compression ratio, common to small high speed diesel engines means that the air has been compressed to one-sixteenth or one twentieth of its original volume. The wall of the cylinder needs to be heavy gauge to contain the pressure.
Considerable power is needed to overcome that amount of compression to get a diesel engine started. Larger diesel engines have decompression levers that open the cylinder to the atmosphere so relieving the force needed for the starter mechanism to get the engine turning over. When the flywheel is spinning rapidly they are closed for the first compression and power operation to start.
The petrol engine, using the more flammable fuel, can ignite the fuel/air mix by an electrically generated ignition source at its spark plug, more fully described in Chapters 1.4 and 8.2. A compression ratio of 10:1, common to small high speed petrol engines means that the air has been compressed to one-tenth of its original volume, requiring less energy to turn over in starting than the diesel.
While advances in design are ever improving, the advantages and disadvantages of the typically ponderous but reliable diesel against the responsive but explosive petrol
is demonstrated in the proliferation of diesel engines for inboard installation (required for commercial vessels) and petrol engines for outboards. In short, the strengths and weaknesses of petrol versus diesel engines can be summarised as:
Petrol driven engines use fuel with high fire risk, require electrical sparking that is not ideal in a damp environment but can be low weight for power capacity with high acceleration performance.
Diesel driven engines operate using a fuel with lower fire risk, do not rely on electrical connections but can be heavy for power capacity with slow naturally aspired acceleration balanced by reliability and longevity. Though of a heavier build than petrol engines, in order to sustain the forces resulting from higher compression, they are known for producing working power (grunt) at lower speeds.
Four stroke engines can be diesel or petrol. The four strokes of a wet sump diesel engine are shown below with exhaust outlet marked (out), air inlet (in), piston (L), con rod (M) and crankshaft (N). This crankshaft revolves in a clockwise fashion.
As the piston nears tdc the inlet valve is opened by the camshaft’s cam driven by the rotating crankshaft. As the crankshaft rotates further it drags the piston down the cylinder sucking fresh air into the cylinder through the inlet port and its inlet valve.
As the piston nears bdc the inlet valve is closed, trapping the charge of fresh air.
At near bdc both valves shut. As the crankshaft rotates it pushes the piston up the cylinder and so squeezes the trapped air (to about 5% of its normal volume). The compression of the air heats it up to about 500°C sufficient to ignite any diesel fuel.
Just before tdc the camshaft driven injector pump sprays a measure of fuel into the cylinder at high pressure through an injector. The atomised fuel sprayed mixes with the hot compressed air and explodes. As the piston passes tdc, the burning gas expands, driving the piston toward bdc, rotating the crankshaft and its attached weighted flywheel. The momentum energy of the heavy rotating flywheel keeps the engine rotating smoothly until the next power stroke.
Just before the piston reaches bdc, the exhaust valve opens and the pressure of the burnt gases is released into the exhaust outlet. As the piston passes bdc and rises up the cylinder it pushes the remaining burnt gases out of the cylinder through the exhaust valve. As the piston reaches tdc, the exhaust valve closes, and the engine is ready to repeat the work cycle.
The four stroke engine takes two complete revolutions of the crankshaft to complete the four strokes of this work cycle. A high speed engine’s crankshaft turns at more than 1200 revolutions per minute (rpm), or 20 revolutions per second, or 10 work cycles (40 strokes). Accurate valve timing is critical to efficient functioning. Typically wet sump type lubrication is used in four stroke diesel engines.
Four stroke cycle petrol engines are similar in operation to four-stroke diesel engines. Differences include that the fuel is pre-mixed with air in a carburettor. A fuel/air mixture rather than pure air is drawn into the cylinder during induction though some petrol engines use an injection system. In the power stroke, the petrol/air mix is ignited by means of an electrical ignition system and spark plug, rather than the heat of compression. The timing of the spark ignition for four-stroke petrol engines is normally synchronised by the camshaft.
Simple valveless petrol two stroke engines
The work cycle of the simplest petrol engine pump or generator is shown below with exhaust outlet port marked (out), air/fuel inlet port (in), con rod (M), crankshaft (N), carburettor (K) and spark plug (B). This crankshaft revolves in a clockwise fashion. Two stroke engines take one crankshaft revolution to complete the four work cycle operations. The simplest two strokes achieve this by using ports in the cylinder wall that are sealed or exposed during the piston’s sweep along the cylinder. Typically a proportion of oil (ratio 16 to 30:1) is added to the fuel to lubricate moving parts, this system being described as dry sump lubrication.
Power and exhaust operations-stroke 1
Just before tdc the piston is positioned above both inlet and outlet ports so sealing the cylinder with its pressurised charge of fuel/air mix. The crankshaft synchronises the timing that releases a high voltage impulse to the spark plug to explode the fuel/air mix. As the piston passes tdc, the burning gas expands, driving the piston toward bdc, rotating the crankshaft and its attached weighted flywheel.
Half way down its first stroke the piston uncovers the exhaust outlet port and directed by the piston’s contoured head, exhaust gasses are partially ejected.
Induction and compression operations-stroke 2
The inlet port shown is a transfer port as it is open to both sides of the cylinder.
In the carburettor air and fuel are premixed and atomised. To improve the induction of fuel/air mixture into the crankcase, one-way valves (check valves, reed valves, or rotary valves) may be used in the intake port to allow the fuel air charge to get into the crankcase quickly, but prevent it leaking back out.
As the piston descends, the inlet port experiences a suction that inducts the fuel/air charge around the crankcase, in the process depositing a film of oil on the moving surfaces. Once past bdc and rising on its second stroke the piston pushes the charge up through the transfer port into the cylinder. This flow of the fuel/air charge into the cylinder forces any remaining exhaust out of the cylinder, a process called scavenging. Lastly, as the piston covers the inlet port, the fuel/air charge is compressed ready for the next power operation (ignition).
Valve and port diesel two stroke engines
Power (exhaust and induction) operations-stroke 1
Just before tdc the piston is positioned above the inlet port and the exhaust valve is closed, so sealing the cylinder and its pressurised and heated air. The crankshaft driven timing synchronises the injector to release a spray of atomised fuel that explodes instantly on mixing with the super heated air.
As the piston passes tdc, the burning gas expands, driving the piston toward bdc, rotating the crankshaft and its attached weighted flywheel.
With the initial force of the expansion expended, three quarters of the way down its first stroke the exhaust valve opens and exhaust gasses begin to be ejected.
An instant later the piston exposes the inlet port and fresh air is sucked in, often under pressure from a blower (supercharger or scavenger blower). This rush of fresh air further pushes the exhaust towards the outlet valve (scavenging).
Compression (exhaust and induction) operations-stroke 2
After the piston passes bdc to begin the second stroke the fresh air compression completes the displacement of any remaining exhaust.
A quarter the way up the second stroke the piston covers the inlet port and next the exhaust valve closes. Now the air is fully compressed in the sealed cylinder ready for the next explosion.
Removal all burnt gases from the cylinder and replenishment with fresh charge (scavenging) is necessary to maintain the power and efficiency of the engine. The rapid operation of two strokes causes a flow of fresh air buffering against exhaust gasses to eject them. Improving the flow by careful design and a blower improves the scavenging efficiency. The cross flow type scavenging shown above uses a piston head profile directed towards the exhaust. Looped scavenging has inlets and outlets on the same side of the cylinder. Uniflow scavenging is arranged so the gasses all move centrally in the same direction towards the cylinder head outlet.
Engine designs with transfer inlet ports positioned evenly around the cylinder improve scavenging by allowing air to enter uniformly and push the exhaust towards the exhaust valve.
Air, blowers and after-coolers
Small engines are naturally aspirated purely by the suction created during the downward stroke of the piston. However more complex engines use additional blowers to force air into the cylinders and so increase their power and efficiency. Blowers may be the supercharger or turbocharger type.
Superchargers are driven directly by the engine and geared spin at up to twice the engine speed. A stream of air from a supercharger can also partially cool the hot running two-stroke.
Turbochargers are driven by a turbine which is powered by the engine exhaust and may spin at up to 100,000 r.p.m. Balance and lubrication is critical. At high speed turbo vanes reach high temperatures from the exhaust’s blast of flame. If the engine is instantly shut down from high speed operation the rapidly spinning turbo will run-on for a substantial time after the engine driven lubricating system has stopped. The heat from the turbo vanes will conduct into the bearings with consequent damage. To prevent these problems, hot engines should be allowed to idle for about 5 to 15 minutes before they are switched off, to allow the engine and turbo to cool down.
Turbochargers also heat the inducted air by compression. An after-cooler may be fitted to cool and increase the density the air (providing more oxygen per unit of volume) before it enters the engine, so increasing efficiency and cool running.
Timing methods vary widely in different engine designs, but all must be set exactly as even a small timing error can stop the engine or seriously damage it. Injection, and the opening and closing of the valves, is accurately timed in relation to the position of the piston, by the gear or chain drive from the crankshaft to the camshaft. The meshed teeth of the gears ensure that injection and valve operations occur at the correct point of each work cycle. At speed engines work cycles occur so rapidly that valve and injection has to be initiated prematurely to counter the miniscule delay in actuating mechanism.
Four stroke engines have twice as many teeth on the camshaft gear (or sprocket) as there are on the crankshaft gear. This means the camshaft runs at half the speed of the crankshaft. The camshaft turns (injection and valves operate) only once for every two revolutions of the crankshaft.
With two stroke engines, injection occurs and the valves will open and close on each turn of the crankshaft. In two-stroke engines, the camshaft must run at the same speed as the crankshaft.
The specific opening and closing of the inlet and exhaust valves and the period of injection of the fuel can be taken from the engine manufacturers timing diagram. Examples are shown below:
Four stroke cycle diesel engine
The diagram below represents a Caterpillar turbo charged after cooled engine. The induction stroke commences when the inlet valve opens 10° before tdc when air is drawn into the cylinder as the piston moves down. The inlet valve closes 1° before bdc. The air is now trapped in the cylinder and as the piston rises on the compression stroke, the air is compressed. As the air is compressed, it rises in temperature. When the piston reaches 19° before tdc, the injection of fuel commences and continues until 73° after tdc.
The heat in the compressed air ignites the fuel and combustion takes place. The gases expand forcing the piston down on the power stroke.
The exhaust valves opens at 26° before bdc and the exhaust gases are discharged as the piston rises on the exhaust stroke. Most of the exhaust gases have been discharged as the piston nears tdc. However, at 10° before tdc, the inlet valve opens and air enters the cylinder and helps discharge any remaining exhaust gases until the exhaust valve closes at 3° after tdc. The whole cycle is then repeated.
Both the exhaust valve and inlet valve are open from 10° before tdc to 3° after tdc, an overlap of 13°. This is referred to as “valve overlap” and ensures that all the exhaust gases are discharged from the cylinder and the cylinder receives a fresh charge of air to make it more efficient when combustion next takes place.
Therefore there is one power stroke for every cycle or two revolutions of the crankshaft.
Two stroke cycle diesel engine
The two strokes of power followed by compression are required to complete one cycle. The events of injection, combustion, expansion and compression of the gases takes place as the four stroke engine, but the exhaust of the burnt gases and the induction of air take place at the bottom of its stroke, this being a chief difference between the two stroke cycle and the four stroke cycle.
There are variations in two stroke cycle engines. The type described here is the most common to be found in marine engines. It has inlet ports and exhaust valves. The inlet holes or ports are in the lower section of the cylinder liner wall. The piston uncovers the inlet ports as it moves down the cylinder. The piston covers the inlet ports as it moves up the cylinder. This action has the same effect as a valve opening and closing. An engine driven scavenge blower is fitted and the incoming air is blown into the cylinder through the inlet ports when they are uncovered by the piston.
The above timing diagram represents a Detroit Diesel turbo charged inter cooled engine. Induction commences at 49° before bdc when the piston has uncovered the inlet ports. Air is forced into the cylinder by the scavenge blower as the piston moves down to bdc and back up again until it covers the inlet ports at 49° after bdc.
As the piston rises, the exhaust valve closes at 62° after bdc. The air is now trapped in the cylinder and as the piston rises the air is compressed and rises in temperature.
Fuel is injected before tdc and continues after tdc. Detroit Diesel do not specify the period of injection as this will vary depending upon the engine speed, the load and the size of the injectors. The camshaft contains the exhaust valve cams as well as the unit injector cams. Therefore, if the exhaust valve timing is correct, the unit injector timing will be correct providing the injector follower is adjusted to a definite height in relation to the unit injector. A special gauge is supplied to set this height.
The heat in the compressed air ignites the fuel and combustion takes place. The gases expand forcing the piston down on the power stroke.
The exhaust valve opens at 83° before bdc allowing the burned gases to escape into the exhaust manifold. However, at 49° before bdc, the inlet ports are uncovered by the piston and air enters the cylinder and helps discharge any remaining exhaust gases until the exhaust valve closes at 62° after bdc. The whole cycle is then repeated.
There is one power stroke for every revolution of the crankshaft.
Most marine engines have several cylinders for more power and smoothness. (The cylinders are identified by numbering them in order, from the front of the crankshaft (inboard end) to the back (propeller shaft end).
Above, the cylinders are numbered 1- 4. The timing assembly consist of rockers 5, valves 6, push rod 7, cams 8, camshaft 9 and timing gear 10. The crankshaft is 11 and the flywheel 12.
In engines with several cylinders, the cylinders are designed to fire one after the other, to increase the smooth delivery of power. They do not fire in consecutive order (1,2,3,4,5,6,7,8), as this would have the effect of twisting one end of the crankshaft while the other end tries to catch up. Instead, cylinders are designed to fire, first at one end of the engine, then at the other. In this way, the power thrust is more evenly balanced on each end of the crankshaft.
Typical firing orders for internal combustion engines are:
Four cylinder engines 1,3,4,2 (sometimes 1,2,4,3)
Six cylinder engines 1,5,3,6,2,4 (or 1,3,5,6,4,2)
Vee eight cylinder engines 1,5,4,8,6,3,7,2 is most common
The cylinders are usually configured in a straight line along the centre of the block, being called straight. Alternately they can be configured in the vee or v arrangement as shown below fitting greater power potential in a more compact space.
Some auto engines arrange the cylinders in a horizontal plane with pairs of pistons punching out to opposite sides of the block, hence its name the boxer engine, which provides balanced performance.
Valve timing is the critical relationship between the position of the crankshaft and the opening and closing of the inlet valves and exhaust valves. The valve train is geared or has a chain drive with sprockets on the camshaft and crankshaft. Any slight variation from the correct timing setting will result in loss of power and overheating. Any large variation and the engine will not start.
To accurately check the valve timing, it will be necessary to remove the timing cover to gain access to the timing gears. The gears or sprockets are fitted to the crankshaft and camshaft by keys so they can only be fitted in one position. However, they can be incorrectly lined up to each other.
The operators manual will indicate what the timing marks look like and in the case of chains, what the sprockets should line up with. Typical lining up marks for gears are shown below:
When timing has been found to be correct, the tappet clearances (also referred to as valve lash) should be checked. Whenever the cylinder head is overhauled, the valves are reconditioned or replaced, or the valve operating mechanism is replaced or disturbed in any way, the tappet clearance must be adjusted - also when the cylinder head has been re-tightened after the initial run in period.
As the valve and valve operating gear heats up in service, the clearance between the rocker arm and the valve stem decreases. If insufficient clearance is allowed, the valve will be prevented from seating. The correct clearance will be specified by in the operator’s manual. Some manufacturers state clearances for an engine is at its normal operating temperature, others when the engine is cold, while some give both.
Clearances will vary as much as 0.128 mm (0.005”) between a cold and the normal operating temperature of an engine. Usually, an exhaust valve will have a greater clearance than an inlet valve because of their different operating temperatures.
Too much clearance will cause excessive wear, noisy operation and altered valve timing, that is, late opening and early closing. If the clearance is insufficient and the valve does not seat properly, it will result in loss of compression through valve leakage, burning and eroding of the valve and seat and general overheating.
In the extreme, it is possible that the piston could strike the valve resulting in a bent valve stem, damaged piston or worse if the valve or piston should break.
If the valve operating mechanism is disturbed in any way and the engine is cold, but only a hot tappet clearance is given, the tappet clearance must be checked. If required, further adjust when the engine is at its normal operating temperature.
The most common form of adjustment for tappet clearance is by means of a screw and lock nut located in one end of the rocker arm. The clearance is measured by means of a feeler gauge between the valve stem and rocker arm when the valve is in
the fully closed position. This is usually done when the piston, under the valve being adjusted, is on top dead centre at the end of the compression stroke.
An easy way to identify the above is as follows:
On a six cylinder engine with a firing order of 1 5 3 6 2 4, turn the engine over in the direction of rotation. When the inlet valve and exhaust valves are rocking on number 6 cylinder (i.e. the piston finishing its exhaust stroke and starting its induction stroke) adjust the inlet and exhaust valve clearances on number 1 cylinder which will just be completing its compression stroke and commencing its power stroke.
On the crankshaft, the bottom end journals on numbers 1 and 6 are 180° to each other, 2 and 5 are 180° to each other, and 3 and 4 are 180° to each other.
What you are doing is adjusting number 1 tappets while number 6 is rocking, then adjust number 5 because it is the next one in the firing order to be on top dead centre while number 2 is rocking, adjust number 3 while number 4 is rocking, adjust number 6 while number 1 is rocking, adjust number 2 while number 5 is rocking, and adjust number 4 while number 3 is rocking.
On a Detroit Diesel, the exhaust valve/s can be adjusted on the cylinder on which the unit injector follower is fully depressed. This means that fuel injection is taking place so it is at the end of the compression stroke and the beginning of the power stroke.
Early injection - If the injection occurs too early on the compression stroke, it will result in high peak pressures. This will subject the engine to unsafe stresses caused by the tendency of the pressure to reverse the rotation of the engine and evidence by excessive detonation which is known as diesel knock.
Late injection - Retarded injection or late burning gives incomplete combustion causing too low a power output and overheating.
It will be necessary to follow the manufacturer’s instructions in the owner’s manual to time the fuel pump to the engine as different methods are employed.
Timing principle - fuel injection commences on the compression stroke just before top dead centre. With a four stroke, the piston also comes up to top dead centre on the exhaust stroke. Make sure it is on the compression stroke. As with timing inlet and exhaust valves, the fuel injection pump must be timed to inject fuel at the correct angle on the compression stroke. This means that the gear driven shaft to the pump must also be lined up in the gear wheel train, otherwise, difficulty might be experienced in lining up the holes in the drive coupling.
Timing engine to pump -The flywheel is usually marked with a tdc and with an injection mark that is before the tdc mark when turning the engine over in the direction of rotation. Turn the engine over in the direction of rotation until its number 1 cylinder is on the compression stroke and the injection mark is lined up. The fuel
injection pump must also be lined up on number 1 element or port at the commencement of injection. The owner’s manual will identify the position of the lining up marks as brands of pumps differ. When the lining up marks on the pump meet the drive couplings can be bolted together.
Alternative method of timing - Some manufacturers make provision for locking the fuel injector pump shaft at a position corresponding to tdc for number 1 cylinder. A further pin is then located in a hole in the camshaft timing gear that is tdc for number 1 cylinder. The drive couplings can then be bolted together and the pins removed.
As the pin is located in a hole in the camshaft, it can only be on the compression stroke on a four stroke engine.
Checking the timing of a fuel pump -The timing may be checked as follows:
Remove the delivery valve and spring from number 1 element in the fuel injection pump. Open the throttle to the full position. (If the throttle is left at the stop position, the slot in the plunger will be in line with the spill port and no fuel will be delivered.)
Rotate the engine in its operating direction until number 1 cylinder is on the compression stroke. Keep rotating the engine slowly and when the mark on the flywheel, indicating the start of injection is lined up with the timing indicator mark, fuel will immediately start to rise from where the delivery valve was removed. (This will mean the top of the plunger has just covered the inlet and spill ports and injection is starting). If fuel starts to rise before or after the timing marks are in line, the fuel pump timing is out and will have to be adjusted.
Detroit Diesel unit injector - On a Detroit Diesel, the cam that actuates the unit injector is on the same shaft as the cams for the exhaust valves. If the exhaust valves are correctly timed, that is they open and close at the correct angles, then the unit injector timing must be correct. It is then only a matter of adjusting the unit injector follower to get the correct height in relation to the unit injector body. A special gauge is supplied for this purpose.
Cummins PT injector - On the Cummins PT system, it is only a matter of setting the clearance between the rocker arm and the injector.
Similar timing principles and mechanisms are used in petrol engines, with the fundamental differences that a pre-mixed fuel/air charge is introduced to the cylinder during the induction stroke and that the ignition requires an electrically driven spark to initiate the power stroke.
Smaller engines may use the magneto to produce the high voltage required for an electrical spark to arc across the spark plug gap. More complex engines will use a coil, distributor and contact breakers (or points).
A magneto is an electrical generator using wire coils and magnets to produce alternating current. It produces pulses of high voltage to activate the spark plugs of small petrol driven engines such as light weight outboard motors and lawn mowers.
The ignition timing relies on a cam on the drive shaft openning the points (contact breakers) momentary interrupting the current and collapsing a coil’s electromagnetic field. This induces a voltage across the coil, which in turn supplies the energy for the spark plug firing. The size of the points’ gap opening can be adjusted to the manufacture’s recommendation. Its position in relation to the cam can be adjusted to fine tune the ignition timing.
To prevent high voltage arcing at the point’s contacts, leading to rapid decay, a capacitor (an electrical storage and smoothing device) is placed across the points to absorb the energy burst. (Capacitors were previously called condensers, are built from two films of electrical conductors separated by a film of electrical insulator).
The system is rarely used for vehicles or vessels that have electrical accessories, except for aero piston engines where simplicity and reliability are of advantage. The magneto is not used to charge batteries in marine systems but in old outboard motors may be wired to power an emergency light.
The battery, coil and distributor
More complex multi cylinder engines will use an electrical system energised by a battery bank that is kept charged by an alternator. This system induces a voltage across the coil, which in turn supplies the energy for spark plug firing.
A crank driven distributor shaft revolves in a cylindrical distributor body. The shaft drives a cam that opens the points momentary interrupting the current and collapsing the coil’s electromagnetic field. To prevent high voltage arcing at the point’s contacts, leading to rapid decay, a capacitor (an electrical storage and smoothing device) is placed across the points to absorb the energy burst. (Capacitors, previously called condensers, are built from two films of electrical conductors separated by a film of electrical insulator).
The perimeter of the distributor’s cap has contact points individually connecting to each spark plug by a high tension lead. (Don’t touch them during running or you will receive a high energy electrical shock!) As the distributor shaft rotates, the rotor arm fitted to its head brushes past each contact point in turn to distribute the electrical energy to each spark plug in the correct order and time. The size of the points’ gap opening can be adjusted to the manufacture’s recommendation. In order to fine tune the ignition timing, its position in relation to the cam can be adjusted by rotating the distributor body around the distributor shaft.
Further information on outboard engine petrol ignition is provided in Chapter 8.
A turbo charger (sometimes called a turbo blower) can be fitted to both two and four stroke engines to increase the volumetric efficiency and thus their power output. It uses the force of the expelled exhaust gasses passing through a turbine to drive a rotor blowing air into the air inlet.
Advantages - The advantage of a turbo charger is that fuel consumption is lower than that of a normally aspirated engine of the same power output. In addition, the turbo charger utilises the exhaust gases of the engine so no additional power from the engine is required to drive it.
The turbo charger inducts a larger mass of air into the cylinder to that of a same cubic capacity normally aspirated engine. This allows for a proportional increase in the amount of fuel that can be injected and burnt in the cylinder thereby providing an increase in the power output of the engine.
Components of a turbocharger
A cut away of a turbocharger’s components are shown below:
Rotor assembly - It has a rotor shaft which has exhaust gas turbine blades on one end and air compressor blades on the other end.
Casings - The exhaust gas turbine blades are housed in a casing which is attached to the exhaust manifold and to the exhaust pipe. Some casings are fresh water cooled to minimise the heat radiated out into the engine space. This allows for a cooler engine space, cooler air entering the engine air intake and therefore more power again. A nozzle ring is fitted inside the casing to direct the flow of exhaust gases to the turbine blades.
The air compressor blades are also housed in a casing which has an air cleaner on the intake side and is connected to the intake manifold on the discharge side. Where an engine is after cooled, the discharge side is connected to the after cooler which is then connected to the intake manifold. Both the above casings are attached to a centre casing which contains the bearings, seals and method of lubrication.
Bearings and lubrication - The shaft may rotate in white metal bearings which can be lubricated from the engine driven oil pump. This method of lubrication also allows the oil to remove some of the heat in the turbo charger. One bearing locates the shaft and takes the small residual thrust, the other bearing allows the shaft to move longitudinally to accommodate the differential thermal expansion of casings and shafting.
Alternatively, the smaller turbo chargers usually incorporate a ball bearing for positioning at the compressor end and a roller bearing to accommodate axial expansion at the turbine end of the rotor shaft. The bearings may have their own reservoir which forms part of the turbo charger. These reservoirs usually have round oil level sight glasses with two horizontal lines marked to indicate the high and low levels. Seals are fitted to retain the oil.
Operation of the turbo charger on a diesel engine
In a four stroke engine, exhaust gases flow from each cylinder into the exhaust manifold and then past the turbine blades of the turbo charger. With the engine running at full speed, the turbo charger can obtain speeds up to 100,000 revolutions per minute (rpm). The air compressor blades will revolve at the same speed. Air is drawn through the air cleaner and forced under pressure into the intake manifold. When the inlet valve opens on the induction stroke, with the piston descending in its cylinder, air is forced into the cylinder. It is necessary to reduce the turbo charger speed in stages or slowly for two reasons:
If the engine speed is reduced from full engine speed to stop quickly and the bearings of the turbo charger are lubricated by the main engine driven lubricating oil pump, the engine, on stopping, will cease to supply the lubricating oil to the turbo charger bearings. Because of its high speed, it will take some time for the turbo charger to come to rest and the bearings could be damaged. The exhaust gas side of the turbo charger operates at a very high temperature. It is preferable to reduce the temperature gradually rather than quickly to prevent unequal contraction of the turbo charger parts as it slows down.
Monitoring the performance
Normally, as part of the purchase of a new engine, the engine distributor or dealer will do an installation and pre-run check. The following will be recorded:
The speed of the turbo charger at a nominated engine speed.
Air flow in.
Air flow out.
Air pressure after the compressor blades.
Exhaust gas flow
The flow of air going into the turbo charger is important. The air is taken from the engine room so sufficient ventilation to the engine room is required to ensure there is enough for the engine as well as cooling the engine room. The exhaust gas flow is also important. It ensures the installation of the exhaust piping is within limits and not restricting the performance of the engine. As the above is recorded, checks can always be carried out and readings compared with the initial ones.
An after cooler is also called an inter cooler or a charge air cooler. An after cooler is fitted where an engine is turbo charged, however it is not necessary to fit one. Therefore an engine can be turbo charged or can be turbo charged and after cooled.
The reduction in air temperature will increase the density of the inlet air resulting in more air entering the cylinder. More fuel can then be injected and burnt, giving increased power.
The after cooler is fitted between the air compressor side of the turbo charger and the air intake manifold on the engine. In the after cooler, air passes over the outside of the tubes while the engine cooling water or sea water passes through the tubes usually in the opposite direction (contra flow). Fin plates are attached to the outside
of the tubes to increase the surface area for the air, thereby giving a better transfer of heat.
Sea water flowing through the tubes will tend to leave deposits in less time that if fresh water was used. The end covers can be removed and a wire brush pushed and pulled through the tubes. If the scale is not removed by the brush, the tube nest will have to be chemically cleaned.
On the air side, usually no maintenance is required if the air cleaner is doing its job and the filter is changed regularly. A leaking tube will cause the cooling water to pass into the air side. Depending on the design, the air may enter at the bottom and leave at the top to prevent water carrying over with the air. A drain cock is fitted at the bottom.
As the air passes through the after cooler, its temperature may be reduced until it is below the saturation temperature. Heavy condensation of water vapour may then follow, this water being carried into the engine. If this is a problem, a water separator can be mounted between the after cooler and the air inlet manifold.
In best practice vessels of the 25 metres class (80 tonne) will be fitted with an audible warning device to indicate a dangerous condition associated with:
engine lubricating oil pressure;
engine jacket cooling water outlet temperature; and
engine gear box lubricating oil pressure.
It should be noted that these protection devices give off an audible warning only and do not shut down the engine. Automatically shutting down an engine without any warning could have dangerous consequences and result in collision, grounding, or the loss of the vessel.
The alarm system may have a switch that must be turned on manually to put the system into operation. The danger of this system is the operator may forget to activate the system allowing the engine to run in an unprotected mode. It is preferable that there be no alarm switch. If there is an alarm switch, it is good practice to switch it on before starting the engine. It will sound until the engine is started and the minimum oil pressure registers. Similarly, it should not be switched off until the engine is stopped and the alarm sounds. This procedure also checks that the alarm is operational.
The gear box low lubricating oil pressure alarm operates in the same fashion as the engine low oil pressure alarm.
Low oil pressure alarm
The oil pressure alarm consists of a pressure switch fitted to the pressure side of the lubricating oil system, usually into an oil gallery. The oil pressure acts on a diaphragm and spring which open the contacts in a micro switch. When the spring
pressure is greater than the oil pressure, the contacts will close and sound the audible alarm.
If an alarm switch is fitted, switch it on. When the engine is started, the oil pressure switch opens when the oil pressure reaches approximately 69 kPa (10 psi) and the alarm will cease to sound.
Likewise, if the oil pressure drops below the setting of 69 kPa (10 psi), the oil pressure switch will close the circuit and sound the audible alarm.
The alarm will continue to sound until the engine is stopped or the alarm switch, if fitted, is switched off.
High temperature fresh water alarm
The high temperature fresh water alarm consists of a thermo switch. It has a bi-metal probe that activates contacts in a micro switch. It is installed in the side of the thermostat housing. When the engine is started and running at normal operating temperature, the contacts in the switch will be open. Should the engine coolant exceed 96° C (+ or - 3°) the water temperature switch will close the electrical circuit and sound the audible alarm and/or indicator light. The gear box low lubricating oil pressure alarm operates in the same fashion as the engine low oil pressure alarm.
A Detroit Diesel engine has an additional sensor fitted for the protection of their engines. In addition, advising the engineer of a slight loss of coolant, non circulation of the coolant or failure of the sea water cooling action, the alarm will also sound if there is a large loss of coolant.
A big and sudden loss in coolant may reduce the coolant level to below the probe in the thermostat housing. As the water is now not circulating over this probe, it will not detect the rise in temperature of the coolant. An additional sensor is fitted into the exhaust manifold outlet to detect the rise in temperature due to overheating.
The water temperature switch consists of a temperature-sensing valve and a micro-switch. The valve contacts a copper plug (heat probe) which extends into the exhaust manifold outlet. Engine coolant is directed over the power element of the valve and if the water temperature exceed its setting, the valve will close the contacts in the micro-switch thus closing the circuit and sounding the audible alarm. If a loss of coolant occurs, the heat of the exhaust gases will be transmitted through the copper plug to the temperature-sensing valve thus closing the circuit and sounding the audible alarm.
Emergency stop device
Numerous engines are fitted with a manually operated emergency engine shut down device, mounted in the air inlet housing, to stop the engine in the event an abnormal condition arises. If the engine continues to run after the engine throttle is placed in the no fuel position, or if combustible liquids or gases are accidentally introduced into the combustion chamber causing over speeding of the engine, the shut down device will prevent damage to the engine by cutting off the air supply stopping the engine.
The shut down device consists of an air shut off valve (flap) mounted in the air inlet housing which is retained in the open position by a latch. A cable assembly is used to remotely trip the latch. The shut off valve must be manually reset on the latch for restarting the engine after the malfunction has been rectified.
To ensure safe vessels seafaring nations developed Classification Societies to keep registers (lists of approved safe vessels). Those organisations still determine rules (specifications) and conduct surveys for construction, equipment and maintenance for each vessel class according to its trade and sea area of operations.
American Bureau of Shipping AB Det Norske Veritas NV
Lloyds Register of Shipping LR Germanischer Lloyd GL
Bureau Veritas Nippon Kaiji Kyoka
China Classification Society
For new vessels, a classification society or survey authority will approve the specifications for design plans and check the quality of materials and workmanship of all stages of the construction process at the Initial Survey. To ensure maintenance to the survey standards, regular ongoing inspections are scheduled, called Periodic Surveys. These approved specifications, the World’s best practice, are supported in Australian legislation, regulated by the Australian Maritime Safety Authority (AMSA).
The Australia domestic commercial vessel fleet (under 80 mtrs long and under 200 nm offshore) is transitioning from State survey authorities operating the Uniform Shipping Laws specifications (USL code) to a National survey authority (AMSA Domestic Vessel Division) operating the newer National Standards for Commercial Vessels (NSCV). While drawing from the USL code, the NSCV updates and provides the flexibility required by developers and operators. It retains a prescriptive approach to compliance in its deemed to satisfy standards (standards that shall be met) but also provides a flexibility with performance based equivalent solutions” (that can be proven to be as effective as those deemed to satisfy).
Vessel survey compliance will depend not only on its operations (trade and plying limits) but on its survey authority their grandfathering exemptions from past survey regimes or allowances from current equivalent solutions. In short, to confirm your vessel’s survey compliance you will need to contact its Survey Authority.
However, some features that are common in all domestic commercial vessels fuel system installations that meet best practice in structural integrity, fire control, environmental management and stability.
Due to the greater explosive risk posed by petrol, inboard installations must be diesel fuelled though petrol is allowed for vessels fitted with outboards. Some special purpose petrol inboards may be approved for petrol inboards (high powered wake and ski boats) where risk mitigation includes starter interlocked engine room purging blower and fuel management procedure.
NSCV surveys frequency is determined by risk level. Risk factors include age, attributes, operational area and nature, incident history of vessel class and performance of the operator. Greater risk category vessels include:
Class 1A, 2A, and 3A vessels (unlimited sea areas);
Class 1 > 35m in measured length;
Class 1B/1C that berth one or more pax or berth >12 persons or carry more >36 pax;
Class 1D/1E that berth one or more pax or berth >12 persons or carry >75 pax;
Class 2B vessels > 35m in measured length (workboats to 200nm offshore)
Class 2 tankers, dangerous goods carriers or tug boats.
The periodic survey inspections of a vessel are arranged in survey cycles of 5 years (inspections at 1, 2, 3, 4 and 5 years). These periodic surveys are not intended to confirm the vessel’s compliance with every requirement but to identify and verify the continued existence and functionality including trialling machinery and its fuel systems.
1 yearly inspections - for all vessels would include all pipe arrangements.
5 yearly inspections - for vessels 35 metres and over of deep tanks and double bottom tanks used exclusively for fuel oil, to be examined externally and tested to a head sufficient to give the maximum pressure that can be experienced in service.
One deep tank and one double bottom tank used exclusively for fuel oil to be surveyed internally every 5 years starting when the vessel is 10 years old.
10 yearly inspections – vessel hull thickness and examine larger fuel tanks internally
The term to examine means a process that commences with a visual inspection that identifies the evidence of damage, deterioration and/or modification (may require dismantling if deficiencies are found).
The term to test means the physical gauging of properties with the objective of ascertaining continued readiness to function, condition or conformance with standards. E.g. hammer tests, ultrasonic thickness measurements, oil analysis, starting of machinery, turning of handles
The term to trial means a specific type of rest of a system or component to ascertain functional performance and/or compliance with applicable standards. E.g. machinery trials, emergency generator trials, steering trials, fire hydrant appliance trials, anchoring trials, evacuation trials.
The term to verify means to ensure that an item exists and is as per the plan, meets or has been declared as meeting an applicable standard by a an Authority.
Survey should not be confused with maintenance. A responsible owner will have a maintenance program to open up, inspect and repair at lesser intervals than required by the survey authority. Surveyors have the discretion to survey fuel oil tanks of the vessel not due for survey if he considers such action is warranted.
Fuel storage components
Fuel tanks for storage of the fuel or for daily service tanks may form part of the hull structure, be free standing and substantially constructed of carbon steel, stainless steel, copper or marine grade aluminium. Fuel storage arrangements on board are largely dependent on the intended service of the vessel and location of refuelling stations within the area of operation of the vessel. Typical arrangements include:
Short distance -Tanks port and starboard within the machinery space. These may be free standing or form part of the hull structure plus a smaller tank for direct supply to the main engine and generator engines. Fuel from the larger tanks is transferred to the smaller tank as required.
Medium distance - A bunker or deep tank divided into port and starboard tanks forward of the machinery space the aft bulkhead of the tank being the machinery space forward bulkhead, with a small tank as for short distance, or:
Two small tanks both capable of supplying
the main engine and generator engines, each with independent lines to the main
and generator engines. The tanks would be used alternately.
Long distance - as for medium, but with the addition of a double bottom fuel tank/s.
Multiple tanks can have a cross over valve fitted to either the fuel supply or return lines enabling the engines to be run from either tank or in the event of contamination, to isolate an offending tank. Care must be taken if redirecting a fuel return line to one tank only as this effective fuel transfer can be rapid and may affect the vessels stability or even overflow the tank.
Some vessels may have two day tanks, thus the fuel return from the engines injectors should be changed over when the delivery is changed. Similarly, it is wise to close cross over fuel supply lines when refuelling from a high pressure fuel pump. The thrust of fuel entering the port tank filler pipe may depress the fuel in the tank and even force fuel up to overflow the starboard tank. The reverse will occur when the filling stops as fuel from the starboard tank can surge back to spill out from the port filler pipe.
Double bottom and void tank tops are prone to corrosion but must be more regularly inspected. A weep of water entering a double bottom tank through damage to the outer hull will suddenly become a flood if the tank’s resisting internal air pressure fails due to the tank top watertight seal corroding away.
Baffles – Perforated baffles (or not continuous baffles) are fitted inside the tank to allow limited liquid movement but minimise free surface area effects of liquids sloshing around as the vessel moves. Normally spaced not more than 1 m apart, those fitted longitudinally will reduce free surface caused by the vessel rolling and transverse baffles will reduce that caused by the vessel pitching.
Breathers- Fuel tanks, containing flammable liquids, are required to be vented to atmosphere (not into the vessel). This breather pipe will terminate in a gooseneck or swan neck (a cranked pipe), which limits rain and spray from entering. If the vent pipe is greater than 18 mm in diameter, the outlet is fitted with a wire gauze for a flame trap.
Filler pipes- Filler pipes are arranged so spillage will not enter the vessel. The inlet or delivery end of the pipe is located outside the vessel and will have a valve and fuel tight cap. The pipe between the deck and the top of the tank may be flexible, but must be reinforced and secured with twin corrosion resistant clips.
Fuel pipes - Generally unarmoured plastic or rubber flexible hose is not compliant.
Inspection port - The top or bottom of tanks, where water and condensation accumulate, are prone to corrosion and need regular inspection. The bottom of the sounding pipe can corrode or even jam the sounding device. Consequently fuel tanks of more than 800 litres capacity require opening up and inspecting at periods of not more than 12 years through a manhole or inspection port. A larger tank may also have modified vent pipes or fitted purging (by inert gas) pipes to ensure tanks are evacuated of flammable gasses before opening up. The precautions of entering a confined space must be applied.
Pumps - are used for transferring fuel between tanks. There may be a separate pump to supply fuel at pressure to the engines. Stop valves are to be provided on the suction and delivery sides of power operated pumps.
If the closed discharge pressure exceeds the maximum design pressure of the system a relief valve discharging back to the suction side of the pump shall be fitted.
Pumps located below decks shall be provided with a means to stop the pump from a safe place outside the space.
Save all – Tanks that are fitted above machinery must have drip trays (savealls) fitted to prevent leaks onto moving parts. Fillers, engines and gearboxes are similarly fitted to stop oil reaching the bilge. Save alls also need drainage arrangements.
Shut off valves - All fuel supply lines must have a means to be provided outside a propulsion machinery space, (in an accessible position not likely to be isolated by a fire in the space), to shut off the fuel to the main and auxiliary engines by means of a fire safe valve or cock. In practice this means that a valve or cock required to be fitted to each tank outlet can be operated from a safe position outside the space by means of an extended spindle, or some other method of remote operation. Any fuel
transfer or a cargo oil pump which is located below deck in a machinery space shall be provided with a means to stop the pump from both inside and outside the space.
Sludge box & drain - Sediment contaminants of water, algae and debris will gather at the tank bottom where they must be periodically removed through a self closing sludge valve. In the event of the tank rupture or for periodic inspections, all fuel tanks which are not double bottoms must be fitted with a method of draining them into another storage tank (not the bilge).
Sounding and sight gauges - Float fuel gauges are unreliable due to a vessel’s changing trim, so checking the contents of the tank can utilise poking a calibrated stick (sounding rod) down the filler pipe until it hits the bottom of the tank and reading off the height of fuel that coats the retrieved rod (sounding the tank). An alternative is to read the dry end of the retrieved rod showing the airspace above the fuel (an ullage). If the tank’s pressed up capacity is known then its remaining fuel can be calculated. Whether a filler pipe or a dedicated sounding pipe is fitted, at the tank bottom a reinforcing striker plate is welded to prevent a hole being eventually battered into the tank bottom.
An alternative measuring technique is a transparent sight glass spanning top to bottom whose fuel level reflects that of the main tank. This clear plastic/glass tube is more vulnerable to fire and impact than the main steel tank, so survey regulations specify that a self closing valve be fitted in case of rupture. Under no circumstances must these valves be left open. A recent variation is a non ferrous sight gauge containing a steel float whose height (and tank volume) can be determined by magnetic sensors. Tanks may be fitted with an overflow pipe which leads to an overflow tank or relief double bottom fuel tank. These overflows can be fitted with a sight glass and audible alarm. When re-fuelling, a safety managed procedure that utilises pollution and spill control devices must be operated to prevent spillage or fire.
Petrol and diesel fuel systems are more fully described in Chapter 3.
Unless the vessel’s fuel tanks are positioned above the level of the engine or a day tank is installed at sufficient height, fuel cannot be gravity fed to the engine’s fuel injection pump. To assist in drawing fuel from the tank/s a fuel transfer pump is fitted between the tank/s and the fuel injection pump. Fuel transfer pumps are also called transfer, lift or charge pumps.
Diaphragm type transfer pump
The diaphragm type transfer pump is mechanically driven by a special lobe on the camshaft. The lobe pushes against the lever causing the diaphragm to be pulled down against a spring pressure, creating a partial vacuum.
A first check valve opens and draws in fuel, filling the chamber between the diaphragm and check valves. As the lever moves off the lobe of the cam, the diaphragm spring pushes the diaphragm up, closing the first check valve forcing fuel through a second check valve and into the fuel pump. An external lever is provided to permit manual operation of the pump for priming purposes.
The pump will deliver more fuel than is required. The fuel not being used will build up pressure in the line between the fuel pump and the fuel transfer pump causing the second check valve to close. The downward movement of the diaphragm will allow more fuel to enter through the first check valve into the chamber. The first check valve will close and as the return spring cannot overcome the pressure in the line between the fuel pump and the second check valve, the lever will be held off the cam until more fuel is required.
This diaphragm pump could be attached to the side of the fuel pump and actuated by a cam on the camshaft for the fuel pump. Alternatively, it may be attached to the block and actuated by a cam on the main camshaft.
The plunger type fuel transfer pump is mechanically driven by a dedicated lobe on the camshaft. This pushes against the plunger in the fuel transfer pump to create the pumping action. Check valves control the direction of fuel flow, and prevent fuel bleed back during engine shut down.
As the high point on the cam lobe rotates away from the fuel transfer pump, the spring forces the piston towards the camshaft. The pressure of the fuel in the piston bore closes the first check valve and opens a second check valve forcing fuel to the low pressure supply line. As the piston moves, a third check valve opens and fuel is drawn into the spring cavity.
As the high point of the cam lobe rotates towards the fuel transfer pump, the plunger and piston are forced towards the inlet. The pressure of the fuel on the spring side of the piston causes the third check valve to close and first check valve to open, allowing the fuel in the spring cavity to flow to the other side of the piston.
A second plunger allows manual priming and bleeding of air from the system. When the plunger is depressed, the first check valve prevents back flow forcing fuel through the second check valve. When the plunger is released, the spring forces the plunger outward. This action creates a suction that causes the second check valve to close and the fuel is drawn through the open first and third check valves.
If the pump supplies more fuel than is required, the fuel will build up the pressure in the line between the plunger pump and the fuel pump. The pressure build up will hold the plunger stationary against the plunger spring an away from the arm, effectively stopping pump operation until more fuel is required.
Gear type transfer pump
This pump consists of two meshed gears in a closely fitted housing with inlet and outlet ports opposite one another. One gear is driven by the power source and in turn drives the other. As the gear teeth separate and travel past the inlet port, a partial vacuum is formed. Fuel entering the inlet port is carried to the outlet port in
pumping chambers formed between the gear teeth and the housing. As the gear teeth mesh at the outlet there is no place for the fuel to go but out.
Vane type transfer pump
In the vane type pump, a slotted rotor driven by a drive shaft rotates between closely fitted side plates, and inside of an elliptical or circle shaped ring. Polished, hardened vanes slide in and out of the rotor slots and follow the ring contour by centrifugal force. Between succeeding vanes, pumping chambers are formed which carry oil from inlet to the outlet. A partial vacuum is created at the inlet as the space between the vanes open. Fuel is squeezed out of the outlet as the pumping chamber size decreases.
Fuel arrangements must take into account its highly flammable nature, particularly in the critical operations of loading, unloading and refuelling. The only sure way to prevent accident is to ensure that personnel are trained and competent in the refuelling operation.
All fuel filling stations must be positioned outside of the machinery spaces and so arranged that any overflow cannot come into contact with any hot surface or spill overboard. The safety plan for refuelling should include but not be limited to the following:
Training - all personnel are trained to understand the safety plan and operate the safety equipment in order to comply with safety management and port regulations.
Mooring – moor the vessel securely allowing for tide and wash, secure fuel lines and pad where there are sharp edges. Pipe bends should be smooth, not leak and if necessary be earthed.
Isolating – no naked flames or smoking and have fire-fighting appliances in readiness. Plug deck scuppers, ensure tank vents are clear and have clean up spill kit gear ready. Sound tanks to ensure available capacity. Physically check all isolation valves (just visual is insufficient) and ensure incoming fuel is clean.
Connecting –approved transfer hoses and equipment correctly. The piping system should be flexible to enable pumping fuel direct from any tank to another. When filling has been completed a container should be available to catch spillage when the fuel supply line is disconnected. Both the tanker and the ship must have non drip camloc fittings on their piping connectors to minimise this risk. Metal tanks and lines must have electrical earthing. Metal delivery nozzles must be released of potential static induced sparking by contacting against an earth before placing in a filler pipe.
Draining can only be carried out if there is space in other fuel tanks to take the fuel from the tank being drained, or it is to be pumped a shore reception facility. If the tank is a double bottom tank or deep tank, it can only be drained to the height the end of the suction pipe is above the tank bottom. For tanks whose bottom is clear of any structure (eg. free standing), the valve for draining water from the bottom of the tank can be opened and the remaining fuel drained into a receptacle.
Maintaining - a constant watch to monitor flow and prevent spills, close filler caps after fuelling and clean any spills on deck. Fuel is often supplied to small vessels from a road tanker where the amount of fuel required can be measured by the fuel meter in the tanker discharge line to minimise the risk of spillage.
Fuel contamination can be caused by:
Dirt - Dirt can arise from dirty fuel taken on when bunkering. It is difficult to prevent as the receiver of the fuel has no way of knowing if the fuel is clean or dirty when being loaded. Effective filtration of fuel prior to use in the engine/s is necessary. In new vessels it may be that the tanks were not thoroughly cleaned prior to commissioning. Ensure the tanks are thoroughly cleaned.
Water/Moisture - Water can be in the fuel taken on when bunkering. Care should be taken when loading fuel in a new or remote location where fuel purity is not known.
An analysis of the fuel can indicate if the water content is excessive; a figure of around 0.1 to 0.2 % by volume is acceptable.
Moisture in the air space of the fuel tank can build up water due to condensation. Drain off of any water in the tank/s on a daily basis, and try to keep the tanks pressed up.
Microbial infection - This is a common problem in diesel and similar grades of fuel. Tests are available to identify the presence of microbes and the fuel supplier can be asked to supply a fuel which does not exceed defined standards of contamination.
It can occur due to taking infected fuel on board, and is aggravated by failing to regularly drain fuel tanks of any water that has accumulated in them. The microbes can only propagate in the presence of water and feed on the fuel at the interface between the water and oil. The waste they generate is deposited as a black slimy sludge.
On board a vessel their presence is first detected in the fuel filters which clog up more frequently. In some cases the slime gets past the filters and causes the fuel pump plungers or the injector nozzle valves to malfunction due to rusting or partial blockage.
Elimination of the bacteria is difficult. Fuel can be treated by chemicals but as noted above regular drainage of water from the fuel tanks is essential to keep it under control. In case of severe contamination it may be necessary to clean out and disinfect all affected tanks and fuel lines.
Metal tanks are considered to be confined spaces in that the atmosphere may be depleted of sufficient breathable oxygen to sustain life (due to the rusting process) and/or may contain poisonous gasses (due to build up of fuel vapours). In order to enter any confined space tank for maintenance or inspection a confined space procedure must be followed by trained personnel. This will include a specific risk assessment and control plan. Considerations will include:
Substitution –find another way rather than physically entering the tank (cameras).
Isolation – remove all services into the tank (fuel, gas, vapours, electrics)
Administrative measures – training, signage, tagging, sentry.
Engineering – provide safe access and egress, use intrinsically safe lighting.
Ppe’s – provide personnel protective devices and breathing apparatus.
Rescue plan– have rescue tripod available for personnel recovery and removal.
Gas free can be defined as when the atmosphere in an enclosed space or tank is the same as that of the outside ambient air. Enclosed spaces or tanks are required to be gas free to allow personnel to enter without danger, for the purpose of survey and inspection, maintenance or repair work.
When the tank is considered to be gas free an authorised inspector should test the space to verify the gas free condition and issue a certificate declaring it to be so.
The standard method of making a space gas free is to ventilate the space for a period of time either by forced or extractive ventilation. However additional requirements may be necessary depending on the usage of the space.
Some methods adopted to gas free spaces are as follows:
Fuel tanks carrying diesel fuel - The tank should be completely emptied of fuel.
Smoking or naked lights in the space containing the tank prohibited. Warning notices should be placed at all entrances to the space. The door or closure on any manhole or inspection opening should be removed. A portable ventilation fan (preferably one to which flexible ducting can be connected) should be situated near the tank opening, the loose end of the flexible ducting placed into the tank and ventilation commenced.
Special fans with ducting are readily available for this work. Fans can be driven by enclosed electric motor (intrinsically safe) or preferably compressed air drive. Gases will escape through the vent piping and through the manhole. Dilution of the gases and the flashpoint of diesel fuel is such that there is little danger of fire or explosion.
The time required to ventilate depends on the size of the tank and fan.
Ballast deep, double bottom and peak tanks - Tanks used for ballast may be full or empty. An empty tank should never be entered without gas freeing, especially if it has been empty for a long time (a week or more). The air becomes stagnant and any rusting that has occurred reduces the oxygen content and pockets of carbon dioxide may have formed at lower levels.
A gas free certificate must be obtained. Many gases in confined spaces have an anaesthetic action at quite low concentrations and humans exposed to the gases experience the usual sequential effects of an anaesthetic (or narcotic effect) such as drowsiness, loss of control, lack of judgement, delirium and eventually loss of consciousness. When personnel enter a tank a sentry (watch person) must be at the entrance to the tank to maintain constant communication to ensure all inside remain safe. Ventilation of the tank should be maintained whilst personnel are in the tank.
If personnel do suffer symptoms of anaesthesia as described, the sentry should call for help and rescue should only be attempted by persons with breathing apparatus.
The flash point of a fuel is the lowest temperature at which the vapour arising from the fuel gives off a flammable mixture that will just ignite (flash) when a flame is momentarily applied under defined test conditions. The flash point is determined
under laboratory conditions using various apparatus eg. the Cleveland; the Abel-Pensky; the Pensky-Martens apparatus. The former determines the open flash point of a fuel and the latter two the closed flash point. The open flash point is slightly higher than the closed flash point.
The flashpoint is an indication of the volatility of a fuel and determines the degree of fire hazard associated with the fuel. The IMDG Code mentioned earlier, classes flammable liquids as Class 3. The class is divided into sub-groups 1 (highest hazard) to 3 (lowest hazard) according to their flashpoints:
Class 3.1 - Low flashpoint liquids having a flashpoint below - 180C
Class 3.2 - Intermediate flashpoint liquids of -180C to, but not including 230C
Class 3.3 - High flashpoint liquids having a flashpoint of 230C to and including 610C.
Liquids having a flashpoint above 610C are not considered to be dangerous by virtue of their fire hazard. However, vapours from Class 3 liquids are hazardous. If inhaled, they have a varying narcotic effect. Prolonged inhalation may lead to unconsciousness and possibly death.
Flash point should not be confused with the ignition temperature. Ignition temperature is the temperature to which an explosive vapour-air mixture must be heated to cause actual explosion. There is no relationship between the flashpoint and the ignition temperature.
Petrol engines use similar tank and supply arrangements to the diesel but dependant on the engine complexity and age may use:
Petrol fuel injection.
Instead of the injector pump and injectors of the diesel engine, the fuel is atomised and mixed with air in a carburettor. The premixed fuel/air is then supplied to the cylinders through an intake manifold. The carburettor regulates the volume of air/fuel supplied to the engine at different speeds and can increase the percentage of petrol in the mix (make it richer) for easier cold starting. This control for this is called a choke.
Carburettors use the venturi principle of locally pressurising air in a narrowing tube. A fuel line (needle valve) opens into this air stream that sucks and atomises the fuel to pre-mix with the air. Carburettors are used in most outboards and small petrol engines. In the last twenty years larger petrol engines with electrical wiring circuitry have been increasingly fitted with electronics fuel injection systems because of their greater efficiency in meeting emissions control regulations. Most carburettors have the following main components:
Float bowl, float and needle valve - keeps a constant petrol level in the float bowl. The fuel pump delivers fuel to the float bowl. As the level in the bowl rises, the float rises on top of the fuel cutting off the flow of fuel into the bowl and forcing the needle valve shut. If the level drops, the float opens the valve and let in more fuel. The float bowl also helps to trap sediment, to prevent it blocking the jets.
Jets - fine brass jets regulate the amount of fuel which is delivered into the air stream drawn through the throat of the carburettor on induction. Several jets may be provided for idle, low speed, high speed, acceleration, etc.
Throttle butterfly valve - positioned in the main throat below the fuel venturi increases or decreases the flow of fuel/air to the engine (throttle).
Choke butterfly valve – positioned above the venturi restricts the flow of air, without reducing the flow of fuel. This chokes the engine for cold starts.
Further information on outboard engine petrol fuel systems is provided in Chapter 8.
Petrol fuel injection
Fuel injection is the modern alternative to the carburetion process that achieves greater control of fuel usage over a variety of throttle settings from idle to full power. This greater efficiency in fuel use meets regulatory standards for emissions controls and reduces fuel consumption.
Rather than being pre-mixed outside of the engine by induced suction in a venturi, a metered quantity of fuel is either injected into the inlet port/s or directly injected into the cylinder by a mechanical or electronic control system.
Mechanical fuel injection system (MFI)
The air is inducted through an inlet manifold to the inlet ports. A high pressure electric pump pressurises the fuel and passes it through an accumulator (reservoir) to a camshaft driven fuel distributor. This times the injectors to squirt fuel into the inlet port/s air stream during the induction stroke. The quantity of fuel injected is controlled by a throttle linked air intake flap valve that opens with throttle advance creating more piston induced suction. As the piston reaches tdc, the spark plug ignites the mix to initiate the power stroke. Carburettor type choking by restricting air flow to create a richer mix will not work with this system, so an arrangement for additional cold start fuel injection will be incorporated.
Electronic fuel injection system (EFI)
The air is inducted through an inlet manifold to the inlet ports. Fuel is injected into the inlet stream of air during the induction stroke and this mix is compressed in the compression stroke. As the piston reaches tdc, the spark plug ignites the mix to initiate the power stroke. The injection point may be positioned at the inlet manifold of all the cylinders (throttle body injection or single point injection), receiving its fuel from a pressurised common rail, or may be separately injected into each cylinders inlet port (multi point injection).
The components of an EFI single point injection system as shown below include:
Electronic control unit - monitors engine demand and controls component response.
Pump and fuel filter - supplies clean high pressure fuel to the fuel rail and injector.
Fuel rail - a common fuel pipe feeding the injectors.
Pressure regulator - controls pressure in the system.
Throttle body- controls air flow with a valve.
Injector - sprays fuel into the inducted air flow.
Air filter and flow meter –provides clean measured air flow.
Surge chamber - dampens the flow of air.
Direct fuel injection (DFI)
Similar to the injector pump and injectors of the diesel engine, the fuel and air are supplied to the cylinder separately. An injector and a spark plug are fitted to each cylinder head. The air is inducted through an inlet port principally during the induction stroke while the direct fuel injector squirts in a metered spray of petrol that mixes with the air. The mix is compressed in the compression stroke and as the piston reached tdc it is ignited to initiate the power stroke.
The advantage of the system is greater control of the injection moment and richness as may be most efficient for the engine over its operating throttle range. For instance it is most efficient for:
Light load conditions (ultra lean burn) - to inject at the compression stroke.
Moderate load conditions (stoichiometric) - to inject during the induction stroke.
High load conditions (full power) - inject during more during the induction stroke.
A simple diesel fuel supply system is shown below.
The fuel tank with filler cap and venting breather is measurable by a sight gauge. Fuel is directed to the mechanical fuel pump through the supply line, along which primary and secondary filters ensure clean fuel only reaches the mechanical pump. Additional water separators are often fitted at the primary filter position. A manual lift pump is provided to assist bleeding (priming) the fuel line of trapped air after servicing the fuel filters. The cam driven mechanical pump forces the fuel to the injectors when it is required for combustion.
Other fuel line components include:
Baffle - They are fitted to prevent free surface effect. This affects the stability of the vessel and in extreme cases can cause vessels to capsize.
Drain valve - is fitted to the lowest part of the tank. Its purpose is to drain water or sediment from the tank. A plug or cap is fitted so, if the valve vibrates open, the fuel is not lost or causes a fire risk. Water can be in the tank as a result of:
Coming with the fuel supply;
Condensation due to the level in the tank being kept low for a lengthy period;
Rain or a wave entering through the unsecured deck fittings;
Being mistaken for a water tank.
Emergency fuel shut off - This is fitted to allow the fuel to be shut off outside the engine room in the case of an emergency. It can be fitted anywhere in the metallic fuel line. It cannot be fitted after the flexible fuel line. Where fuel tanks are fitted outside the engine room and the fuel shut offs are easily accessible, emergency shuts offs are not required. An extended spindle can be fitted to the fuel shut off valve so it can be operated from outside the engine room. The fuel shut off and the emergency fuel shut off are then the one valve.
Filling pipe - is fitted to the top of the tank and it is preferable that it be piped continuously to deck level. It does not have to be piped to the deck, if in the event of an overflow; the fuel will not run onto a hot surface and ignite. The end of the pipe is to be fitted with a sealed cap or plug.
Filter/water trap - They can be a combined unit or separate units. The unit provides a secondary means of filtering the fuel from sediment and impurities while the water trap removes any moisture or water. The fuel pump and injectors have very small clearances and any impurities or water in the fuel will cause them to seize. (The fuel acts as a lubricant). In addition, moisture could cause corrosion to those finely machined components. Sometimes additional filters are fitted to the system.
Fuel contents gauge – Several methods are used to measure the amount of fuel. If a sight gauge is fitted the valves must be self closing. To take a reading, open the valves against a spring or lift a weighted handle and, on letting go, it will automatically close. If the glass breaks or the plastic tube perishes, it prevents all the fuel in the tank running into the bilges or in the case of a fire, prevents all the fuel in the tank feeding the fire. If a sounding rod is used, a striking pad must be fitted to the bottom of the tank to prevent damage to the tank through repeated soundings.
Modern arrangements now use magnetic flaps to mark the position of a steel ball floating in a sealed levelling pipe. This method resolves leakage and fire risk inherent in the sight gauge.
Fuel pick up - is fitted above the bottom of the tank. This is to allow a safety margin so as to reduce the amount of any water or sediment flowing to the fuel filter. A valve or cock must be fitted directly to the tank.
Fuel return - Excess fuel from the injectors is returned to the tank. It is good practice to operate from one tank at a time and the excess fuel returned to this tank. In this case, the fuel return valve of the tank not being used must be closed. In small vessels it is not practical to operate off one tank as the vessel would develop a list, therefore engines receive their fuel from the port and starboard fuel tanks.
Fuel lift pump - Unless there is a day tank where the fuel is fed by gravity to the engine, it will be necessary to have a fuel lift pump to get the fuel from the tanks to the fuel pump. A fuel lift pump can be a gear, diaphragm or plunger type.
Fuel injector - It is a spring loaded valve located in the cylinder head and allows the fuel, under pressure from the fuel pump, to enter the combustion space. It enters in
an atomised form to allow it to mix completely with the hot compressed air so that ignition can take place with efficient combustion. Excess fuel is returned to the tank.
Fuel injection pump - It accurately meters the fuel and delivers it under high pressure at a precise moment to the spray nozzle of the fuel injector.
Inspection opening - is fitted in a position or a number may be fitted to provide access to the whole tank. It allows the tank to be cleaned and inspected.
Vent - the purpose of the vent pipe is to allow the:
Escape of air and vapours when the tank is being filled so it is not pressurised;
Entry of air as fuel is used so a partial vacuum does not occur;
Normal expansion and contraction of the fuel due to temperature change.
It is fitted to the top of the fuel tank at the highest point when the vessel is in normal trim. This is to prevent an air lock developing. An air lock is when the tank is being filled, air or vapours become trapped in the top of the tank, are compressed, and when the pressure exceeds the filling pressure, fuel is forced out of the vent or filling pipes and a spill occurs. The smaller vent pipes terminate in a gooseneck, the end of which must be higher than the filling point.
The end of the vent pipe has an anti-flash wire gauze fitted to it. If the fuel vapours from the vent pipe ignite, the flames cannot penetrate the gauze and ignite the contents in the tank providing the size of the holes in the gauze are not too large.
Before a combustible substance will take fire, its temperature must first be raised to its point of ignition, and, if after it has ignited the temperature is reduced in some way below this point, the flame will be extinguished. A moderate flame can be extinguished by passing a current of air over it, for instance, blowing out a candle.
The reason for this is that more air than is required for combustion is supplied to the burning gas, the surplus tending to cool the flame below its point of ignition. In a similar way, gauze, which is a good conductor of heat, prevents the passage of flame, since it loses its heat very rapidly, and the flame upon coming into contact with it, is cooled below the point of ignition; consequently, no flame appears on the other side of the gauze. A good example is placing a lighted match under the gauze. The flame will not penetrate the gauze.
A metered quantity of diesel fuel is pumped by the fuel pump to the fuel injector where it is sprayed into of the cylinder to ignite in its superheated compressed air, the resulting explosion initiating the power stroke. Both the design of the fuel injector and the shape of the cylinder’s combustion chamber are critical to ensure even air/fuel mixing and efficient combustion.
Fuel injector assembly
A fuel injector is a spring controlled valve on the cylinder head that opens under pressure from the fuel pump to spray fuel into the cylinder. The pressure and fine passageways of the injector create an atomised spray to ensure good mixing. Those fine passageways required for the atomisation can be easily blocked, hence the particular precautions of filters and water separation in the diesel fuel supply arrangements.
Spray nozzle assembly
The fuel injector consists of a nozzle body and valve. The nozzle body incorporates the valve seat and has holes or orifices in it to atomise the fuel. The valve must seal effectively on the valve seat to allow for a clean cut off of fuel to the cylinder. A leaking valve causes misfiring and irregular speed, particularly on light loads. The valve and nozzle body are lapped to form a mated assembly. Therefore the valve and nozzle body cannot be exchanged individually. A nozzle cap attaches the nozzle to the body of the injector.
Types of spray nozzles
There are different types of spray nozzles. The type of spray nozzle used depends upon the design of the combustion chamber and the angle of the injector. Four types of spray nozzles are the single hole, multi-hole, pintle and pintaux.
The single and multi-hole spray nozzles eject fuel when the valve opens through one hole in the case of the single spray nozzle or through any number of holes at any angle in the case of the multi-hole spray nozzle.
The pintle and pintaux are also similar. A pintle on the valve projects past the valve seat and slightly past the end of the nozzle. There is a slight but exact clearance between the pintle and the injection hole. The pintle size and shape can be varied so as to meet any spray pattern requirement. The pintle prevents the formation of
carbon deposits in the injection hole. Pintle nozzles are used in engines with adequate air turbulence such as pre-combustion chambers or turbulence chambers.
When the fuel pressure opens the valve, the pintle causes a conical spray pattern. It also allows a relatively small proportion of the fuel to be injected as the valve starts to open, followed by the bulk of the fuel thereby slowing down the pressure rise in the cylinder bringing about smoother combustion and engine running. The pintaux differs in that it has a hole at an angle where fuel sprays out separately from the conical pattern for pilot injection.
Nozzle holder or body
A nozzle holder forms the body of the injector. It is fitted with a flange to secure it to the cylinder head. It has drilled passages for the fuel to flow to the valve and for the leak off from the valve stem.
Spring and adjustment screw
The valve is held on its seat by a spring operating on a steel spindle. The compression of the spring can be adjusted by a screw and a locknut so that the valve opens at the recommended pressure.
Cap nut - A cap nut is screwed onto the top of the nozzle holder to enclose the adjustment screw and to seal the unit.
Combustion chamber design, which includes the shape of the cylinder head, the shape of the top of the piston and the air flow through the inlet ports, is one of the most important factors in efficient operation of the diesel engine. Because of the very short space of time available in a diesel engine in which the fuel and air can mix, various methods have been devised in an attempt to give improved mixing and combustion.
Combustion chambers can be of several designs but all are concerned in creating turbulence to the air during the compression stroke. In the diesel engine, the fuel is in the form of fine particles sprayed into the cylinder after the air has been
compressed. To secure complete combustion, each particle of fuel must be surrounded by sufficient air. The mixing of the air and fuel is greatly assisted by the combustion chamber air turbulence. Some engines have helical inlet ports to provide additional swirl.
Combustion systems can be classified as direct injection or indirect injection types.
The two most common types of indirect injection are turbulence chamber and pre-combustion chamber. The larger, slow speed engines and medium speed engines do not have the same difficulty in achieving good combustion as small high speed engines.
With direct injection, the fuel is injected directly into the combustion chamber which can be formed with a cavity in the piston crown.
This cavity is carefully shaped to promote air swirl and the direction of the injector nozzle ensures that rapid mixing of the fuel and air assists complete combustion.
Advantages - direct injection gives higher thermal efficiency with lower fuel consumption. This is because no heat is lost or power wasted in pumping air through a restricted opening into the separate chamber or in discharging the gases from the chamber. This gives easier starting and generally this type of engine does not require a starting aid device, such as glow plugs.
Disadvantages - this kind of injection is prone to diesel knock (a rattling sound).
The indirect injection or separate chamber system is where a separate small chamber is connected to the main chamber by a narrow passage or orifice.
The pre-combustion chamber and the turbulence chamber (also called a compression swirl chamber) work on the same principle. The main physical difference is the location and size of the connecting passage.
With pre-combustion chambers only about 30% of the combustion air is forced into the chamber, fuel is injected and primary burning takes place in the chamber. This prevents too sudden a rise in pressure which can contribute to diesel knock. The burning mixture of fuel and air is vigorously expelled through the connecting passage into the main combustion chamber or cylinder where an excess of air permits combustion to be completed.
Advantages - lower injection pressures can be used, resulting in less wear of injector nozzles; simpler design of nozzle equipment, which are easier to maintain, and smoother idling of the engine. Engine manufacturers may in some instances use either design in their range, depending on operating requirements.
Disadvantages - not as efficient as direct injection. It can also be prone to pre-combustion burn-out.
The fuel injector pump delivers a set amount of fuel under pressure to the fuel injector. The pressure causes the valve to open against the spring and the fuel to spray into the cylinder. When the helix on the plunger of the fuel injector pump uncovers the spill port, the pressure of the fuel drops quickly and the spring in the fuel injector causes the valve to shut. The delivery valve in the fuel pump also closes and fuel is maintained in the injector pipe. The needle valve is a neat fit in the nozzle and fuel flows through the small clearance to lubricate it. This fuel is called leak off and is returned to the fuel tank.
CAUTION: Injectors deliver extremely high pressures. You should exercise due care when dealing with this equipment as there is a clear risk of serious and permanent injury.
Any injector problem at sea can be rectified by replacing the injector with a spare. However, knowing how to identify faults and how they are rectified is important.
Incorrect opening pressure
Too low an opening pressure will cause the valve to chatter on its seat. Fuel will be injected into the cylinder earlier. It is caused by insufficient compression on the spring.
Too high an opening pressure will cause the valve to hammer on its seat. Fuel will be injected into the cylinder later. It is caused by too much compression on the spring.
The spring adjusting screw has a lock nut which may have slackened off causing insufficient compression on the spring.
The spring may break. Replace the spring.
The correct opening pressure can only be obtained by placing the injector in a test rig and adjusting the tension on the spring until the correct opening pressure is obtained. Whilst on the test rig, the spray pattern of the fuel leaving the nozzle can also be checked.
Distorted spray form
Spray nozzle orifices are partially clogged. Spray nozzles should be cleaned by first soaking them in either kerosene or clean fuel to soften the dirt. The spray holes or orifices can be cleaned with a pointed piece of wood. Do not use a piece of wire.
The valve is not sealing on its seat. Grind it in with the finest grade of grinding compound. Excessive grinding causes the valve to seat too deeply in its seat causing a lagging of the fuel admission which results in late combustion and therefore loss of power. In addition, the valve stem may be bent and this will cause the valve not to seal on its seat and the valve stem will be tight in the nozzle body. The valve and nozzle body are lapped to form a mated assembly. Therefore the valve and nozzle body cannot be exchanged individually. Replace with a new valve and nozzle body. The opening pressure will then have to be adjusted.
Dirt between the valve and its seating
Spray nozzles should be cleaned by first soaking them in either kerosene or clean fuel to soften the dirt. Do not use anything metallic or abrasive to clean them. Grind it in with the finest grade of grinding compound. Excessive grinding causes the valve to seat too deeply in its seat causing a lagging of the fuel admission which results in late combustion and therefore loss of power.
Injector valve sticking in the nozzle body
The valve stem may be bent and this will cause the valve stem to be tight in the nozzle body and the valve not to seal on its seat. The valve and nozzle body are lapped to form a mated assembly. Therefore the valve and nozzle body cannot be exchanged individually. Replace with a new valve and nozzle body. The opening pressure will then have to be adjusted.
Alternately, there may be dirt between the valve stem and the nozzle body. It may be possible to clean the dirt away and reuse the assembly. If however, there has been grit passing through the fuel injector, it is most likely that there is pick up (metal from one part is transferred to its mating part) on the valve stem and body thereby scoring them. Further operation in this condition could cause the valve stem to seize in the nozzle body. Any pick up on the valve stem and nozzle body will require the assembly to be replaced.
Too much fuel escaping at the leak off pipe
Caused by excessive clearance between the valve stem and the nozzle body resulting from wear or pick up from dirty fuel or corrosion by water contaminated fuel. A fine clearance is required to maintain the fuel pressure and allow some fuel to pass by to lubricate. Replace the valve and nozzle assembly.
Jerk type fuel injection pump
A jerk type pumps can have a separate unit for each cylinder or multi-elements where several pump elements are housed together. They are comprise of a:
Rack and pinion;
Camshaft and spring
Barrel and delivery valve - Each barrel is locked into the housing in such a way that the upper section, which contains two ports placed at 180 degrees and known as intake port and spill port, is completely immersed in fuel supplied by the fuel lift pump. The barrel is closed at its upper end by a spring loaded pressure valve known as a delivery valve. An injector pipe is connected between here and the injector.
Plunger - The plunger which operates within the barrel is driven on its upward stroke by a roller tappet operating on a camshaft. Contact is kept between the plunger and the tappet by means of a spring which operates in a similar fashion to an inlet or exhaust valve spring. The plunger has a slot and a helix cut into it near the top.
Rack and Pinion - A rack is fitted to the pump to engage with a pinion on the outside of a sleeve. The sleeve fits over the plunger and has slots engaging with keys. This allows the plunger to be rotated by the fuel rack as the plunger moves up and down. The end of the fuel rack is attached to the governor.
Fuel metering principle- When the top of the plunger is below the inlet and spill ports, low pressure fuel flows through the inlet and spill ports into the barrel. It fills the space above the top of the plunger to the closed delivery valve and also down the slot of the plunger and into the space below the helix.
The cam pushes the plunger up and injection commences when the top of the plunger covers the inlet and spill ports. As the plunger moves up, the trapped fuel is delivered under high pressure through the delivery valve to the injector until the helical grove on the plunger uncovers the spill port. This allows the fuel pressure above the plunger to fall to the suction pressure through the vertical slot. The plunger will rise further to complete its stroke but no fuel will be pumped. As the lobe of the cam goes past top dead centre, the spring will cause the plunger to return to the bottom of its stroke.
To vary the amount of fuel injected into the cylinder, the plunger is rotated by the fuel rack and this causes the helical groove to uncover the spill port earlier or later depending on whether less or more fuel is required.
The fuel rack is attached to the governor. If the propeller comes out of the water the engine starts to speed up, the governor reacts by moving the fuel rack, causing the helical groove to uncover the spill port earlier or cuts off the fuel altogether. As the propeller comes back into the water, the engine starts to slow down and the opposite occurs. To cut off the fuel to stop the engine, the plunger is rotated by the rack until the vertical slot is in line with the spill port so no fuel is delivered as the plunger moves up.
Calibration and timing of a multi-element fuel injection pump
In a multi element pump, each element is calibrated and timed on a test rig. To calibrate a pump, each element is connected up to a calibrated test tube. The pump is run and then each test tube is checked to ensure that each element has delivered the same amount of fuel. Each element is timed to ensure that injection commences at the precise time in the stroke.
If the injection occurs too early on the compression stroke, it will result in high peak pressures and will subject the engine to unsafe stresses. This is caused by the tendency of the pressure to reverse the rotation of the engine and evidence by excessive detonation which is known as diesel knock.
Rotary or Distributor Pump
The distributor pump incorporates a single pumping element and automatic metering system. This makes it unnecessary to calibrate and balance a number of pumping elements which is the case of multi-element pumps. The main components of the distributor pump are:
Internal transfer pump;
Rotor and cam ring assembly (pumping element);
Timing advance mechanism;
Maximum fuel delivery adjustment.
The fuel transfer pump (or fuel lift pump) draws the fuel from the fuel tank through a pre-filter and pumps it to a filter head into a combined filter/water separator where any contaminants and water are removed.
The fuel then is pumped to the distributor pump which pressurises, controls timing, distributes and meters an amount of high pressure fuel to the injectors.
The distributor pump uses an internal transfer pump to increase the fuel pressure in relation to engine speed. Fuel then flows through a solenoid valve to the timing advance and the annular groove surrounding the rotor.
A metering valve determines the amount of fuel made available to the pumping section of the rotor. Fuel flow is either increased or decreased depending on rotation of the metering valve by the governor.
A mechanical shut down lever can also be used to move the metering valve to the closed position, stopping fuel flow to the injectors and the remainder of the engine.
The governor is connected to the engine throttle and the metering valve, controlling the fuel flow in relation to movement in the engine throttle. Fuel from the metering valve flows through a metering port into the charging port in the rotor and as the rotor revolves these two ports fall out of alignment, trapping fuel in the rotor.
As the rotor continues to revolve the fuel is placed under increasing pressure and eventually the rotor’s charging port aligns with delivery ports and the fuel escapes to the injector.
Cummins pressure time fuel injection pump
The PT (pressure/time) fuel system is used by Cummins. It uses injectors which meter and inject the fuel. Metering is based on a pressure/time principle accomplished by a fixed size opening in the injector and the pressure of the fuel supplied to the injector. The fuel is drawn from the tank through a filter by the fuel pump, delivered to the injectors with much of the fuel being returned to the fuel tank.
The main components of the fuel pump shown below are:
Throttle shaft for the standard governor;
Variable speed governor;
Shut down valve
A gear pump delivers fuel under pressure through a pulsation damper, which dampens out fuel surges created by the gear pump action. A magnetic filter is used to remove any metal debris from the fuel. The fuel flows from the magnetic filter to a standard governor which controls the engine idle speed and the fuel pressure delivered by the fuel pump.
The standard governor is located in series with the variable speed governor, which is positioned in the fuel pump housing. The variable speed governor controls the fuel flow to the injectors in relation to the engine speed.
The amount of fuel which flows to the injectors is dependent upon the fuel pressure from the fuel pump and the time the feed port is allowed to remain open. Hence the pressure - time principle of this fuel injection pump system.
Detroit diesel fuel system
Fuel is drawn from the supply tank through the strainer and enters the fuel pump on the inlet side. On leaving the pump under pressure, fuel is forced through the fuel filter into the fuel manifold and from there through fuel pipes to the inlet side of the unit injectors. Surplus fuel returns from the outlet side of the unit injectors through outlet fuel pipes into the return manifold, from where it flows back to the supply source.
The fuel pump is of the gear type with an in built relief valve. A non-return valve can be installed between the fuel strainer and the source of supply to prevent fuel draining back when the engine is not running. A restricted elbow is located at the end of the outlet manifold to maintain pressure in the fuel system between the inlet and outlet fuel passages.
The unit injector is a single unit which combines all the necessary components to provide complete and independent fuel injection to each cylinder.
The unit injector performs four functions of:
Creating the high fuel pressure needed for efficient fuel injection;
Metering and injects an accurate amount of fuel;
Atomising the fuel to assist mixture with air in the combustion chamber;
Timing the injection of fuel into the combustion chamber
Unit injectors have the advantage that there are no high pressure fuel lines and the continuous flow of fuel serves to cool injector components while also preventing vapour pockets from forming.
Detroit diesel mechanical unit fuel injector - Operation
Fuel enters the injector through a filter cap and element and flows into a supply chamber. Fuel also flows into space below the injector plunger. The plunger is moved by a special cam, push rod and rocker assembly. As the plunger moves up and down inside a bushing, fuel is fed through two ports in the bushing into the supply chamber.
Detroit diesel two stroke engine
For engine speed the plunger can be rotated within the bushing using quadrant gear which meshes with a control rack. Fuel metering is achieved by rotating the plunger which varies the relationship between two helices machined into the lower portion of the plunger and fuel inlet ports in the bushing. The positioning of the helices and ports control the plunger’s stroke and the amount of fuel injected into the cylinder.
On the pumping stroke a portion of the fuel is forced through the lower port into the supply chamber until the lower plunger helix shuts off the port. Fuel trapped below the plunger is then forced through a central hole in the plunger and so through the upper port into the supply chamber until that port is closed by the upper plunger helix. With both ports closed fuel pressure builds up during the remainder of the plunger stroke until it is sufficient to lift the injector valve from its seat, at which point injection commences. The spray tip incorporates a check-valve whose function is to prevent fuel dribble into the combustion chamber after the injection cut-off point should the injector valve fail to return to its seat.
On the injector plunger’s return stroke, the high pressure area inside the bushing is again filled with fresh fuel through the two inlet ports. This maintains a constant circulation of cool fuel which helps in reducing injector temperatures and effectively removes all traces of air. Excess fuel is fed back to the fuel return manifold and subsequently, the fuel tank, through the injector outlet opening which contains a filter element similar to the one on the fuel inlet side. When the control rack is pulled back to the cut-off position the upper port is not closed by the helix until after the lower port is uncovered. Consequently, all fuel is passed back to the supply chamber and no injection takes place.
When the control rack is in the full injection position, the upper port is shut off shortly after the lower port has been closed by the position of the helix. This rack position is set to give maximum effective plunger stroke and maximum fuel delivery.
Intermediate throttle positions are provided by the relative position of the helical contours to the inlet ports so that both the effective stroke of the plunger and the commencement timing of the injection are altered.
Increasingly, engine manufacturers are using electronic engine management systems to monitor and adjust engine performance, provide warning and improve control efficiency and data collection. The amount of fuel injected and the timing are determined by information fed into the microprocessor (Electronic Control Module) from sensors located on the engine.
The design simplifies the plunger and bushing. It also replaces the mechanical rack with an electronic solenoid. It allows precise metering and injection timing. Examples of electronic engine management systems are:
Caterpillar’s CAT Engine Monitoring System & CAT Engine Vision System
Detroit Diesel’s Electronic Controls
CAT engine monitoring systems
These systems gather data collected from an Electronic Control Module (ECM), monitoring engine and transmission parameters, providing engine diagnostics and gauge readings. The information not only aids in improving fuel economy and engine performance, but provides a record of engine faults to assist with better maintenance.
Detroit Diesel Electronic Controls
Detroit Diesel Electronic Controls (DDEC) is a computerised electronic engine governing the fuel injection system that replaces mechanical controls in Detroit Diesel engines. In addition, within its on board computer, DDEC offers engine protection and self diagnostics to identify malfunctions in its components as well as the ability to trouble shoot engine problems.
Major components of the DDEC are the Electronic Control Module (ECM) and the Electronic Unit Injector (EUI).
Electronic Control Module
It is a microprocessor that continuously monitors and analyses the DDEC system with electronic sensors during engine operation. A programmable read only memory (PROM) that provides instructions for basic engine control functions. Electronically erasable, programmable read only memory (EEP-ROM) that stores engine calibration values. A back up microprocessor to operate the engine should the main microprocessor fail. With this redundancy built into the ECM, reliability is assured.
Electronic Unit Injector
It is built on Detroit Diesel patented mechanical unit injector design that simplifies plunger and replaces mechanical rack with an electric solenoid and allows precise metering and injection timing.
The EUI is actually simpler in the area of the plunger and bushing than the mechanical unit injector. The amount of fuel injected and the timing are determined by information fed into the ECM from sensors located on the engine.
The power requirements of an engine may vary continually due to fluctuating loads, so some means must be provided to control the amount of fuel required to hold the engine speed reasonably constant during such fluctuations. To accomplish this, a governor is fitted to the engine. There are a number of different types including:
Constant speed governor - It is used to maintain the engine at the same speed. For example an auxiliary engine driving a generator may have a fixed speed of 1800 rpm. The electrical load will vary. If the load is increased, more fuel is required otherwise the speed will drop. The drop in speed will cause the governor to alter the fuel pump to supply more fuel so the 1800 rpm is maintained.
Variable speed governor - It is used to maintain a set idling speed, a maximum speed and any desired speed between these limits regardless of any load change. The desired speed is set by a speed control lever or wheel. This type of governor is used on propulsion engines. A simple mechanical and a hydraulic type are described below.
Mechanical and hydraulic governors
Mechanical governors’ sensitivity is limited due to their flyweights not only limiting the speed, but also performing the physical work of moving the fuel control mechanism. Additionally, speed droop is inherent in them, so they are incapable of maintaining constant speed with varying load without manual adjustment.
Speed droop - The steady decrease in engine speed caused by a load increase from no load to full load demand, without change in the adjustment of the governor.
Isochronism - The ability of a governor to regulate the engine speed so that there is zero speed droop. A hydraulic governor of the proper design is both isochronous and extremely sensitive as the governor flyweights are used to limit speed only, the work of moving the fuel control mechanism being performed hydraulically. A mechanical governor cannot make any adjustment to the fuel supply until the engine speed has changed (ie. they cannot anticipate, they can only correct).
Mechanical variable speed governor
The governor is engine driven causing the flyweights to rotate. When the engine is operating at normal speed, the centrifugal force acts on the rotating flyweights and is balanced by the vertical speeder spring force. The control sleeve remains stationary.
If the engine load decreases, the engine speed increases (ie. the propeller coming out of the water). The centrifugal force acting on the flyweights also increases causing the flyweights to move outwards and the control sleeve upwards. This moves the fuel rack so less fuel is delivered. The upward movement in the control sleeve increases the compression in the speeder spring and hence the speeder spring force. This increased spring force and the control sleeve remains stationary in the new position.
If the engine speed decreases (ie. the propeller going back into the water), the opposite occurs. Thus the control sleeve moves up and down as the engine speed fluctuates because of load variations. The normal operating speed of the engine can be manually adjusted by increasing or decreasing the speeder spring compression and hence the speeder spring force by the speed control lever.
Hydraulic variable speed governor
The governor is engine driven causing the flyweights to rotate. When operating at normal speed, the centrifugal force acts on the rotating flyweights and is balanced by the vertical ballhead spring force and the piston valve remains stationary.
If the engine load increases, the engine speed decreases, the centrifugal force acting on the flyweights also decreases causing the flyweights to move inwards and the ballhead spring to move the piston valve downwards. The piston valve, on moving downwards, will admit oil under the power piston. This pushes the power piston upwards, compressing the return spring and moving the fuel control towards more fuel. The movement of the compensating lever slightly decreases the force on the ballhead spring and returns the piston valve to its neutral position.
The normal operating speed of the engine can be manually adjusted on the speed adjustment wheel. To increase the engine speed, the force on the ballhead spring is increased causing the piston valve to admit more oil under the power piston which in turn increases the supply of fuel.
The compensating lever is fitted to stop the governor from hunting. Hunting is when the speed is below or above the control speed and the governor continues to adjust the fuel control. To avoid hunting, a governor mechanism must anticipate the return to normal speed and must stop changing the fuel control setting slightly before the setting required for sustaining the control speed has been reached.
The pneumatic governor operates on the principle that air passing through a pipe tends to create a vacuum in a part of smaller diameter.
The engine suction through a venturi (a tube with a narrowing throat or constriction designed to increase the velocity of the gas or fluid passing through it) provides the
necessary suction and in turn operates a diaphragm control connected directly to the control rack of the fuel injection pump.
The pneumatic governor consists of two main parts being the venturi air flow control unit (mounted between the air induction manifold of the engine and the air intake filter) and the diaphragm unit (mounted on the end of the fuel pump housing).
The venturi unit - has a butterfly valve fitted at its throat and is actuated by the throttle. The butterfly valve is limited by stops for both the idle and maximum speeds. A vacuum pipe is taken from the same centre line as the butterfly valve to the diaphragm unit.
The diaphragm unit - has a diaphragm, a light spring dampening out any oscillations and keeping the fuel pump control rack in the full open position. A manually operated lever is fitted to the control rack and is used to stop the engine by drawing the control rack into the stop or no fuel position.
When the engine is at rest, the lever is released and the spring forces the control rack into the full load position. By setting the excess fuel device rack allowed to move automatically to the extent of its travel, placing the fuel pump plungers into starting position (maximum fuel delivery). The engine is ready to start.
The excess fuel device is located at the opposite end of the fuel control rack and consists of a plunger, or latch, which when released, allows the rack to move as previously stated. As soon as the engine starts and the governor takes charge, this device resumes its normal load position automatically, preventing the control rack from again going to the starting position.
The manual control is set to running position and the engine started.
When the engine starts and is idling, the butterfly valve is almost closed. A high vacuum is immediately created in the corresponding pipe and airtight compartment in the diaphragm unit.
The air in the adjoining compartment is at atmospheric pressure, therefore the pressure on this side of the diaphragm is higher than that existing in vacuum compartment. This causes the diaphragm to move the control rack towards the stop position until the engine is running at the predetermined idling speed.
When the throttle is operated to increase the engine speed the butterfly valve is opened wider, this decreases the velocity of the air passing the mouth of the connecting tube.
The result is an increase of pressure in the vacuum compartment of the diaphragm unit with the spring forcing the diaphragm and the control rack towards the maximum speed position, so increasing the amount of fuel delivered to the engine.
Fluctuations can exist in the induction manifold. To dampen these fluctuations, an additional adjustable spring controlling the diaphragm is fitted. This auxiliary spring comes into operation at predetermined speeds and can be adjusted by a screw to suit the engine requirements.
This governor is extremely sensitive and efficient but has the bad fault that should any leakage occur that will tend to destroy the vacuum, the governing effect will be lost and the engine may race.
Electrical / hydraulic governors
The Electric Fuel Control (EFC) governor is an electrical sensing system that can be adjusted for isochronous engine speed droop. This governor will provide rapid fuel rate changes to improve the transient response to the load change. It consists of:
Magnetic pick up - This is an electromagnetic device that is mounted in the flywheel housing. As the flywheel gear teeth pass the pick-up, an alternating current (AC) voltage is induced, one cycle for each gear tooth. This electrical signal is directly proportional to the engine speed and is fed to the governor control.
Governor control - The governor control is an all electric solid state module which compares the pulses (electric signal) from the magnetic pick-up with a speed control reference point. A current output is supplied to the actuator which rotates the actuator shaft to control the fuel flow to the engine.
Actuator - The actuator is an electromagnetic rotary solenoid valve, the turning action of the shaft regulates the fuel pressure and therefore determines the engine speed and power. (In other governors, there are variations in that they still have an electromagnetic solenoid valve but it is not a rotary type, and it still controls the fuel pressure).
Drive - Governors are driven by some part of the engine that rotates. It may be off the camshaft, or be mounted on the scavenge blower and driven by the upper blower rotor or be attached to the end of the fuel pump, or enclosed in the fuel pump housing, or by some other method.
Lubrication - Mechanical governors are lubricated by oil splash. Oil entering the governor is directed by the revolving flyweights to the various moving parts requiring lubricating
Faults - Governor faults are usually indicated by speed variations of the engine. However, speed fluctuations are not necessarily caused by the governor so if improper speed variations occur, the unit should be checked for excessive load, misfiring or bind in the governor operating linkage. Dirty oil is a common cause of hydraulic governor troubles.
Remote control of governors - Some hydraulic governors are equipped with a reversible synchronising motor which is mounted on the governor cover. This motor makes a close adjustment of the engine speed possible by remote control and is especially valuable for synchronising two generators from a central control panel or bridge control.
Testing and setting a mechanical variable speed governor
All governors are properly adjusted before leaving the factory. However, if the governor has been recondition or replaced, minor adjustment might be required.
As the procedure for adjustment vary between makes and models of governors, the Owner’s manual must be followed.
Caution: To prevent maladjustment, it is the practice of some manufacturers to seal the governor mechanism after it has been adjusted on the test bed, and, if the seal is broken, to decline responsibility for failure in performance. Interference with the tension of the governor springs may cause the speed of the engine to rise beyond the safety limit. Interference with the maximum fuel stop may result in the injection of too much fuel, thus causing excessive exhaust smoke and overheating.
The usual adjustments are for the maximum no-load speed and the idling speed, although there maybe a number of steps to affect a setting.
Adjustments should only be made after the engine has reached normal operating temperature. An accurate tachometer should be used for the engine speed.
Maximum no-load speed
A stop is used to limit the compression of the governor spring which determines the maximum speed of the engine.
This adjustment will only affect the maximum speed and have no effect on intermediate speed control positions. Set the throttle at full speed and when it is running at this speed, turn the adjusting screw so that the maximum speed, as recommended by the manufacturer, is obtained. Tighten the lock nut on the adjustment screw.
A stop is used to limit the travel of the fuel pump rack so that the slot in the plunger does not line up with the spill port and stop the engine.
With the throttle in the idle position, loosen the lock nut and turn the adjusting screw until the engine is running at the manufacturer’s recommended idling speed. Tighten the lock nut.
Exhaust is directly emitted from the cylinder and therefore will indicate the operation and condition of the engine and its combustion process.
For efficient combustion, the ratio of fuel to air must be maintained otherwise incomplete combustion will take place resulting in black smoke. This could be the result of:
blocked or partially blocked air cleaner;
turbo charger not attaining sufficient speed;
incorrect fuel pump timing, Faulty fuel pump;
incorrect valve timing;
faulty injectors - dirty nozzle, opening pressure, leak off; valve not seating;
Indicates that lubricating oil is being burnt, caused by:
worn, broken or sticking piston rings and/or worn cylinder liner bores;
worn valve guides;
valve stem seals leaking;
turbo charger seals leaking;
oil bath type air cleaner overfull
white exhaust vapour indicates water or moisture.
water in the fuel;
moisture in the air;
cold cylinder liner bores and combustion space when first starting engine;
leaking cylinder head gasket between cylinder and cooling water passage.
Changing Fuel Filters
Fuel filters are a critical part of the fuel system that remove the dirt, water and other foreign matter from the fuel supply, which could otherwise cause damage and expense.
As the quality of fuel can never be guaranteed, it is necessary to change the fuel filters as part of the vessel’s maintenance program to the schedule in the manufactures service manual, usually being from 200-400 running hours intervals. This will ensure that the fuel supply remains uncontaminated. Follow this common procedure to change a single in-line filter and bleed air from the system:
Close - the nearest valves either side of the filter.
Open - vent and drain valves (if provided) and drain filter into a container.
Remove - filter cover or body as appropriate.
Clean - filter and re-assemble filter or fit new filter (if the renewable type).
Shut - drain.
Open - valves either side of filter.
Close - vent when fuel appears.
It will be necessary to bleed the system after changing the fuel filter. To assist the process fill the filter and casing with fuel oil prior to bolting or screwing the filter cover down. Given the many different types of filters (both single and duplex), such a method is not always possible. Also, it will probably be necessary to prime other equipment downstream of the filter (the fuel pump and possibly injectors).
Bleeding a diesel fuel system
Diesel fuel systems rely on fuel’s incompressibility. An injector pump forces fuel through the injector under great pressure. If any air gets into the system, it will be compressed so preventing proper injection. Air must be completely removed from the fuel system from tank through to injector, a process called bleeding or priming. Bleeding is necessary if you run out of fuel, change the fuel filter or air has been introduced, possibly from a leaking connection. There are specific methods to bleed depending on the engine type as described in its manufacture’s manual along with the bleed access points or bleed screws.
A Generic procedure will ensure that the tank has clean fuel available, that the shut off valve is open, and that personnel protective equipment, rags and clean up materials are at hand.
Low pressure supply line – the fed water separator bowl is drained from its underside cock. Next its top bleed screw is opened and re-closed once the gravity driven solid stream of fuel without air bubbles flows freely. The same action is taken with the next downstream component of the primary filter. The manual lift pump is required to lift against gravity to push the fuel up to the secondary filters. Here its top bleed screw is opened and re-closed once a solid stream of fuel without air bubbles flows freely.
Continue pumping to get fuel up to the bottom of the mechanical pump and similarly its bleed screw is opened and re-closed once a solid stream of fuel without air bubbles flows freely.
High pressure pump and distribution lines – low pressure lift pump cannot push fuel through the high pressure mechanical pump. It will be necessary to turn over the pump with the starter motor. The Detroit two strokes that use unit injectors (called self priming) fed by a common rail should start at this point. Other jerk type fuel pumps may need to be bled at the pumps top bleed screw/s or distributor lines by opening and re-closing once a solid stream of fuel without air bubbles flows freely. If the engine still this does not start or runs unevenly then the injectors must be cracked (loosened and re-tightened once a solid stream of fuel without air bubbles flows freely).
Caution - An injector pump may apply thousands of kPa pressure to force the fuel through an injector into the cylinder. In opening high pressure side PPE’s must be used and great care taken against the flesh penetrating capability and fire risk of squirting fuel oil.
Bearing in mind the caution above, if the engine runs unevenly the faulty injector can be identified by loosening one by one. If a loosened injector makes no difference to even running, but all the others make running worse, then you have identified the unserviceable injector that makes no difference.
The use of oil in an engine, called lubrication, assists in its efficient running by:
Lubrication - providing a slippery film between moving parts that reduces wear.
Cooling - dissipating and removing heat resulting from friction.
Cleaning - removing contaminants, debris and residues of combustion.
It assists the piston rings to seal on the cylinder liner walls, especially on the compression and power strokes where higher pressures are involved. It acts like a detergent by removing metal dust and carbon and keeps them in suspension so the filter can remove them. It prevents the metal parts from rusting due to the presence of corrosive gas and/or moisture.
Three systems are commonly used being the dry sump lubrication for small two stroke engines, the wet sump for two and four stroke engines and increasingly for larger outboards, oil injection.
Dry sump/oil premix
The dry sump method uses pre-mixed oil in the petrol fuel supply in ratios of from 40:1 to 20:1 as per the manufacturer’s recommendation. The fuel/oil mix is inducted through the crankcase so depositing a film of oil on the moving surfaces on its travel to the cylinder. Dry sump engines have the advantage of lightness and simplicity. The cooling fins on cylinder heads castings used by air cooled motors are not as ideal for slow marine applications as for fast motorbikes, so water cooling is often required leading to greater complexity in design. Two strokes premix engines are also prone to oiling of the spark plug gap if start up is not immediate. Accurate mixing the oil and fuel may not be convenient in a seaway.
The engine runs from a fuel tank but has a separate tank for the oil. Under pressure from a crankshaft driven oil pump, the oil is injected into the inlet port/s by way of the crankcase, so lubricating the engine. In simpler engines the pump is operated by a cable co-joined to the throttle that may be adjusted for volume with a fine tuning screw. While low oil level alarms are fitted to sophisticated motors, their simper cousins’ oil tanks must be visually monitored constantly to avoid the serious damage that would result from running dry. In more complex engines electronic management systems are independent and adjust the oil volume to the engines optimum requirements throughout its throttle range, so providing smoke free operation with ideal lubrication.
The disadvantage of oil pumps driven by the crankcase is that they do not fully function until the engine is operating, thus allowing momentarily dry spots. While this is of minimal consequence for the small dry sump two stroke that supply oil/fuel for
the starting power stroke, in larger diesel engines electrically driven oil pumps may be fitted to pre-lube the system for some minutes prior to start up. Larger outboards use direct oil injection that is more fully described in Chapter 8.
The drawing below shows a basic wet sump lubrication system. Oil is stored in the wet sump from where it is sucked through a sediment strainer, through an oil cooler arrangement to the oil pump.
The oil pump passes it on to the oil filter to separate contaminants and debris. An oil pressure gauge is fitted to indicate the viscosity of the oil and the effectiveness of the system, along with a low pressure warning light and/or alarm. In the case that the filter or other lines become blocked and an over pressure bypass relief valve will open to return flow back to the bilge rather than damaging the pump or breaching the oil lines connections.
While some oiling occurs in the crankcase by the eccentric lobes of the crankshaft splashing the surface of the wet sump (splash lubrication), the body of pumped oil is
directed by internal drillings (galleries) to the crankshaft bearing journals of the bottom engine. Similarly the engine head gear including camshaft, rockers and valves is serviced through galleries. In the drawing above the oil is also directed the mechanical fuel pump. The oil supplied to the rockers can return down through drillings to the sump.
The following components are described in more detail:
Relief and regulating valves
Full flow and bypass filters, magnetic filters
Heat exchangers and purifiers
Lubricating oil pumps are positive displacement of the gear, rotor or vane type. Being engine driven in most engines, they have the disadvantage that it takes several seconds for the oil to be pumped through the engine on start up. Wear takes place, especially on cold engines due to metal to metal contact until oil is received. Larger engines have electrically driven lubricating oil pumps so oil can be circulated for prior to the engine being started. The oil can also be heated to bring up the temperature of the engine and minimise differential expansion of the different metals.
Gear type pump
Gear pumps consist of two meshed gears within a closely fitted housing that has inlet and outlet ports opposite one another. One gear is driven by the engine and in turning, drives the other. As the gear teeth separate and travel past the inlet, a partial vacuum is formed. Oil entering the inlet is carried to the outlet in the pumping chambers formed between the teeth and the housing. They withstand heat and will pump relatively viscous liquids at medium to high pressure.
Rotor type pump
In a rotor type pump, there is an inner rotor driven by the engine. The inner rotor has a number of cam like lobes which mesh with mating parts in a rotor ring. The inner rotor causes the rotor ring to revolve within its housing with its inlet and outlet port positioned at 90° to each other. Oil entering the inlet is carried to the outlet by pumping chambers formed between the cam lobes.
Vane type pump
A vane pump’s slotted driveshaft rotates between closely fitted side plates, and inside of an elliptical shaped ring. Polished, hardened vanes slide in and out of the rotor slots and follow the ring contour by centrifugal force. Between succeeding vanes, pumping chambers are formed which carry oil from the inlet to the outlet. A partial vacuum is created at the inlet as the space between the vanes open. Oil is squeezed out at the outlet as the pumping chamber size decreases.
The fixed rotor vane pump shown above is a modification suitable for hydraulic pumps for steering or winches. It operates by the solid vanes housed in a slotted rotor being flung by centrifugal force into the eccentric (nylon) housing on rotation. The drive direction (by belt, chain, air or hydraulic) determines the flow direction. It is best suited for clean fluids only. Lube vane pumps are unidirectional and of all metal construction to cope with metal debris in the oil, however the principles of operation are similar for a lube dedicated pump.
Gauges and alarms
Standard commercial vessel practice requires control station positioned low pressure lights/buzzers connected to a sender unit on the engine block. When the starter panel is energised (the key is turned on) the sender will earth and activate the alarms. Once the engine fires and the alternator is charging, the electrical flow is reversed so that the circuit does not earth (the light will go out). High pressure alarms and oil filter pressure alarms may be fitted in more complex control systems.
As the oil pumps are of the positive displacement type, a relief valve must be fitted to protect the pumps and the lubricating oil system from excess pressure. The relief valve is usually incorporated in the pump body but can be fitted externally to the pump. Upon opening, the relief valve will cause oil to discharge back to the suction side of the pump or back to the sump. At start up a cold engine has high oil pressure, (causing the relief valve to open) dropping as the oil thins out as the engine reaches normal operating temperature (causing the relief valve to close).
A pressure regulating valve is fitted to the system to maintain a pre-determined system oil pressure. The spring pressure can be adjusted to set the valve at the pre-determined pressure. The valve would normally be opened when the engine is at its normal operating temperature with the excess pressure being discharged back to the sump. Any drop in oil pressure caused by wear in the engine or in the pump, is automatically compensated by the pressure regulating valve until the spring causes the valve to shut completely.
An engine may have one or more oil filters of the following types:
Full flow element type filter
The full flow filter requires all the oil to pass through it before entering the oiling channels. As all oil flowing to the engine must pass through this filter, a by-pass valve is incorporated in the design to prevent oil starvation in the event of the filter element becoming blocked. When the outlet pressure of the filter is below that of the inlet pressure by a predetermined amount, the by-pass valve will open allowing oil to pass. A typical opening pressures are 1.24 -1.45 bar (124-145 kPa or 18 to 21 psi). Whilst the oil now flowing will not be filtered, this if preferable to insufficient oil. To prevent this undesirable situation, filter element must be changed at the periods recommended by the engine manufacturer or less if required.
Bypass element type filter
The bypass filter continually filters a small portion of the lubricating oil that is bled off and returned to the sump. The main portion of the oil goes to the engine. (Other types return the oil back into the flow to the cooler). Eventually all of the oil passes through the filter.
Centrifugal type filter
This filter is a unit which does not employ an element. It can be used by itself or in conjunction with replacement element filters. These are centrifugal type filters and may be driven by the oil pressure or direct from the engine. Any solids in the oil are flung by a revolving drum to the sides of the rotor, where they will remain until the unit is dismantled for cleaning. Washing in a suitable cleaner is all that is required to put the unit back into service.
A magnetic oil filter is used to remove small particles of metal usually from the result of wear. A new or overhauled engine will shed minute particles of metal until it beds in. A magnetic filter is therefore beneficial in this instance. Regular inspections of the filter may also draw early attention to a problem. The magnetic filter cannot be used on its own as it will not filter out the non metallic foreign particles.
The filter is a full flow type without a by-pass valve. It is so designed that metal particles fill horizontal gaps between the iron rings from the top and working downwards but still leave vertical spaces for the oil to flow. The element consists of a permanent magnet enclosed by a non-magnetic cylinder. A number of iron rings fit over and are attached to the cylinder. There are small gaps between the iron rings to attract the metallic particles. The element is situated in a non-magnetic casing.
The oil flow into the top of the filter and out at the bottom. Metallic particles are attracted and fill the horizontal gaps between the iron rings.
To clean the filter, the cover is taken off and the element is removed. One half of the iron rings are removed at a time to avoid demagnetisation and cleaned.
Some engines are fitted with a full flow filter that removes the larger foreign particles without restricting the normal flow of oil plus a bypass filter to remove the minute particles of foreign particles that may be present.
Some engines may be fitted with three filters. The first removes the larger foreign particles, the third removes the minute foreign particles while the second removes the intermediate foreign particles.
When changing the elements on both of the above types of filter, the engine manufacturer’s recommendation as to the element number must be followed. This is to ensure that the filter has the correct degree of filtration. In addition, some elements have a built in relief valve while others don’t.
Some engines are set up with dual filters but only one is used at a time. Change over to the clean, from the dirty filter, can be accomplished whilst the engine is running.
Where a large quantity of lubricating oil is used in an engine, it is costly to carry out oil changes. A purifier is therefore piped into the system to remove impurities and water so it is possible to use the oil practically indefinitely. Oils used in a purifying system are usually non-detergent oils as the purifier removes the detergent as well as the impurities.
Where a large quantity of lubricating oil is used in an engine, it is not cost effective to carry out oil changes. A purifier is therefore piped into the system to remove impurities and water so it is possible to use the oil practically indefinitely.
Oils used in a purifying system are usually non-detergent oils as the purifier removes the detergent as well as the impurities.
It is preferable to install the purifier in a continuous by-pass system. The oil is drawn from the lowest part in the system, heated to approximately 80° C, passed through the purifier and returned to the system. Heating the oil helps to separate the water from the oil.
In the purifier, provision is made to water wash the oil. Hot water together with the oil is fed into the purifier. When it passes through the water seal of the bowl, the two are separated and in doing so, a washing action occurs. Water washing rids the oil of acids which are then discharged with the water.
The purifier works on centrifugal action. It has a high speed revolving bowl filled with cone shaped metal discs with holes in them. The oil is fed into the bowl at the top, and flows down the centre of the discs. Due to centrifugal force, the heavier solids are thrown out to the side of the bowl. The water and lighter solids move between the discs to their outer edge and are discharged. The clean oil, being lighter than the water, passes through the discs holes to the clean oil discharge. The heavier solids build up on the side of the bowl and regular cleaning is required. Most purifiers these days are of the self-cleaning type.
Heat exchangers or coolers for a lubricating oil system are commonly of types:
Shell and tube type or;
Shell and tube type:
Shell -The shell has an oil inlet and outlet and contains the tube nest. Baffles may be fitted to increase the oil’s contact period with the entire tubes’ surface. On some coolers, the oil on the inlet side is prevented from direct contact with incoming oil on the tubes.
Tube nest - The tube nest consists of a tube plate at either end with tubes fitted in between. Allowance for expansion is required due to the different rates of expansion of the metals used in the construction and prevent any undue stress on the tubes. One tube plate is secured at one end of the shell and expansion allowance is by using twin O rings at the other end for of the tube nest. The gap between them will indicate a leak from either the oil or the cooling medium.
End covers - An end cover is fitted to each end of the shell. Where there is a flange on each end cover, the sea water will flow in at one end and out the other end. Where there is two flanges on one end cover, the sea water will enter and leave via this cover. A division plate will be fitted in this cover and seals against the tube nest across its diameter to separate the inlet and outlet flows. The other end cover will have no flanges.
Electrolytic action - To protect the tube nest from electrolytic action, a zinc anode is fitted in the sea water inlet.
Flow of liquids - In the typical shell and tube type cooler shown below, oil enters and leaves by the flanges on the shell and circulates around the outside of the tubes. Sea water is circulated through the tubes and enters and leaves by the end cover/s.
The flow of sea water is opposite to the flow of the oil. The best efficiency is obtained by liquids moving in opposite directions to each other (by contra-flow).
Cleaning - The sea water will leave behind more deposits than the oil as scale in the tubes so it will be necessary to periodically clean them. The end covers can be removed and a wire brush pushed and drawn through each tube, this being the reason why the sea water flows through the tubes.
Plate type cooler
Construction - The plates are corrugated metal pressings of horizontal or chevron pattern to make them stiffer but of thinner material. This also increases turbulent flow so contributing to efficient heat transfer. Turbulence, as opposed to smooth flow, causes more of the liquid contact with the plates. It also breaks up the boundary layer of liquid which adheres to the metal and acts as a heat barrier in smooth flow. However, turbulence can cause plate damage due to erosion.
Joining the plates - Each plate is separated from the adjacent plate with a joint material (nitrile rubber), which is bonded to the plate with adhesive. The joint material is positioned to stop the oil and sea water from leaking out of the cooler and to direct the flow of both liquids along their correct paths. This allows the oil to be pumped along the top of the plates and flow down every second pair of plates, returning out of the bottom outlet. The sea water flows in at the bottom of the plates and passes up the adjacent pair of plates, returning out of the top outlet. The best efficiency is obtained by liquids moving in opposite directions to each other (by contra-flow).
Cooler assembly - The inlets and outlets are attached to the fixed end plate. The movable end sits in the horizontal carrying bars and the plates are also located and supported by these. The flow ports at the corner of the plates are arranged so that the cooling liquid and the liquid being cooled pass between alternate pairs of plates.
Cleaning - If cleaning is needed to remove deposits, use should be made of special soft brushes. Chemically cleaning may be recommended where hard deposits have accumulated. Before cleaning, coolers are isolated from the system by valves and blanks or by removing pipes and blanking the cooler flanges. Flushing is necessary after the cleaning agent has been drained from the cooler.
Advantages - Plate coolers are smaller and lighter than a tube cooler giving the same performance. No extra space is needed for dismantling (a tube cooler needs clearance at one end to remove the tube nest). Plates can be added in pairs, to increase capacity and similarly damaged plates are easily removed, if necessary without replacement. Cleaning is simple as is maintenance. Turbulent flow helps to reduce deposits which would interfere with heat transfer.
Disadvantages - In comparison with tube coolers in which leaking tubes are easily located and plugged, leaks in plates are sometimes difficult to find because they cannot be pressurised and inspected with the same ease. Deteriorating joints may be difficult to remove and new joints difficult to re-bond. Tube coolers are preferred for lubricating oil because of their pressure differential. There are a large number of expensive joints and plates on plate coolers.
Types and grades of oil
Oils are slippery viscous liquids that do not mix easily with water but mix easily with other oils. They consist of long chain organic molecules principally of carbon, hydrogen and oxygen. They are derived from organic (plants or animals) or from mineral (fossilised plants or animals), the latter source being termed petrochemical. Oils used in engines have principally been derived from the distilled and purified petrochemical sources of crude oil or oil bearing shale deposits. Such natural distillation results in a mix of molecule lengths.
Increasingly organic and mineral oils are being mixed with polymers to create designer oils to specifications including homogenous molecular chain lengths that improve consistency in viscosity and slipperiness. These oils are called synthetic or semi-synthetic (the later when containing less than 30% synthetic oil).
American Petroleum Institute (API) tests and classifies oils by their quality into Groups I, II, III, IV, and V with another 3 sub categories. The Society of Automotive Engineers (SAE) grades motor oil viscosity as 5, 10, 15, 20, 25, 30, 40, 50 or 60. The numbers 5, 10, 15 and 25 are attributed with the letter W (winter) meaning they have a cold start viscosity appropriate to a lower temperature. Grade 20 indicates a hot viscosity grade and 20W indicates a cold viscosity grade.
Oils viscosity SAE may be described as single grade or multigrade (blends). This is to allow for the engine’s range of operational temperatures and the consequent viscosity of the oil. For instance, at start up on a very cold morning thin viscosity sufficient to freely oil all moving parts may require SAE 15W whereas on the hot afternoon SAE 30 will be the limit of thinness required, hence a blend of SAE 15W – 30 would meet the operational range.
The engine manufacturer specifies the oil that is suitable for a particular engine taking into account the operating conditions and the contamination problems that could arise. This is stated by its API group and the SAE viscosity. It is therefore essential that the correct type and grade of oil be used to prolong the engine’s life.
Premix two stroke petrol engines require an oil designed to mix thoroughly but still deposit onto the engine’s moving parts before the mix is combusted.
Generally petrol engines require a lower viscosity than diesel engines as diesel engines, relying on oiling to seal the cylinder for high compression, require heavy-duty lubricating oils.
Oils used for hydraulic situations must primarily be incompressible.
The basic requirements of all oils include:
High heat resistance;
Control of contaminants;
Lubricating quality - The reduction of friction and wear by maintaining an oil film between moving parts is the primary requisite of a lubricant. Film thickness and its ability to prevent metal-to-metal contact of moving parts is related to oil viscosity. For instance, the optimums for GM Detroit Diesel 471/671 are SAE 40 or SAE 30.
High heat resistance - Temperature is the most important factor in determining the rate at which deterioration or oxidation of the lubricating oil will occur. The oil should have adequate thermal stability at elevated temperatures, thereby precluding formation of harmful carbonaceous and/or ash deposits.
Control of contaminants - The piston and compression rings must ride on a film of oil to minimise wear and prevent cylinder seizure. At normal rates of consumption, oil reaches a temperature zone at the upper part of the piston where rapid oxidation and carbonisation can occur. In addition, as oil circulates through the engine, it is continuously contaminated by soot, acids, and water originating from combustion.
Until they are exhausted, detergent and dispersant additives aid in keeping sludge and varnish from depositing on engine parts. But such additives in excessive quantities can result in detrimental ash deposits. If abnormal amounts of insoluble deposits form, particularly on the piston in the compression ring area, early engine failure may occur.
Oil that is carried up the cylinder liner wall is normally consumed during engine operation. The oil and additives leave carbonaceous and/or ash deposits when subjected to the elevated temperatures of the combustion chamber. The amount of deposits is influenced by the oil composition, additive content, engine temperature, and oil consumption rate.
Diesel engines require special lubricating oil, because of the diesel fuel and the higher pressures and therefore temperatures in the cylinder, compared to a petrol engine. Additives are therefore used to assist the oil in performing its duties and also in overcoming contamination problems and include:
Anti-oxidants or oxidation inhibitors - Prevent varnish and sludge accumulations on engine parts. They also prevent corrosion of alloy bearings.They decrease the amount of oxygen taken up by the oil thereby reducing formation of acidic bodies. Additive generally oxidises in preference to the oil.
Anti-corrosives, corrosion preventative or catalyst poisons - Prevent failure of alloy bearings and other metal surfaces by corrosive attack. Such additives inhibit oxidation preventing acidic body formation and providing protective films on bearings and metal surfaces. Chemical film formation on metal surfaces decreases catalytic oxidation of the oil.
Detergents - Keep engine surfaces clean and prevent deposits of all types of sludge
by chemical reactions. Oil soluble oxidation products are prevented from becoming insoluble and deposited on various engine parts.
Dispersants - Keep potential sludge forming insolubles in suspension and prevent their depositing on engine parts. Agglomeration of fuel soot and insoluble oil decomposition products is prevented by breakdown into finely divided state. In colloidal form, contaminating particles remain suspended in oil.
Extreme pressure agents - Prevent unnecessary wear of moving parts as well as scuffing or scoring. By chemical reactions, film is formed on metal surfaces which prevents welding or seizure when the lubricating oil film is ruptured.
Rust preventative - Prevents rust in new and overhauled engines during storage or shipment by preferential wetting of metal surfaces through added adhesiveness.
Pour point depressants - Lower pour points by coating the oil’s wax crystals to prevent growth and oil absorption at reduced temperatures.
Viscosity index improvers - Lower the rate of change of viscosity with temperature.
Improvers are less affected by temperature change of oil. They raise viscosity at 93° C (200° F) more in proportion than at 37° C (100° F).
Foam inhibitors - Prevent formation of stable foam and enable foam to break up quickly.
Dust and metallic particles from wear
Dust can enter through faulty or dirty air cleaner or leaks in the air intake system and enter the cylinder. The dust deposits itself in the oil on the cylinder wall and causes wear to the liner and piston rings. It can make its way to the sump if there is wear in the liner or rings to deposit and/or form sludge.
Metallic particles result from normal metal to metal wear, especially when the engine is starting from cold and lacks initial lubrication. On an overhauled engine, the high metal contact spots wear off creating debris. There will also be rust and scale from storage tanks or pipes. Some engines have a magnet in the sump or a magnetic filter to attract any metal particles and stop them from entering the system. Particles may deposit and form a sludge that accelerates oxidation.
A leaking injector pipe situated under the rocker cover will allow fuel to drain with the oil that is returning by gravity back into the sump. A leaking diaphragm on the fuel lift pump or on some engines the seal on the fuel pump will allow fuel into the sump.
Fuel contamination will thin out the oil and it will run easily off the dip stick. There will be a rise of the level in the sump. The dip stick will also have a fuel smell. Fuel dilution of lubricating oil will cause a reduction in viscosity and flashpoint. Lowering the viscosity impairs the oils lubricating properties. Lowering the flashpoint increases the risk of a crankcase explosion.
A fuel dilution exceeding 2.5 % by volume indicates an immediate need for an oil change and corrective maintenance action.
Lack of compression in a cylinder will result in insufficient air with lower temperature causing combustion to be incomplete. An engine with worn piston rings or cylinder liner will allow the products of incomplete combustion into the sump, called blow by. A misfiring engine will also create this type of contamination from incomplete combustion. Blow by gases in the oil will cause oxidation which could lead to corrosion and subsequent wear. A harmful varnish could adhere to engine parts. Carbon and soot will contaminate the oil. The oil will go darker in colour and a sludge could form.
(Oxidisation is when the oil is subjected to a high temperature and intimate contact with air. The products of oxidisation are acidic.)
Unburnt fuel will wash the lubricating oil off the cylinder liner bore causing liner and piston ring wear. It will dilute the oil in the sump.
Fresh or salt water leakage from cooling systems
Water created by a cold engine contaminates the oil. A normal by-product of combustion is water and when the cylinder liner wall temperature is too low, the water will condense in the cylinder and pass the piston rings or is scraped off the cylinder liner walls, by the oil scraper ring, into the sump.
Fresh water can also enter the sump from leaking water jackets, cracked cylinder heads or liners, faulty cylinder liner seals (O rings), loose cylinder head or a blown head gasket from a cylinder to a cooling water passage and the water could leak into the sump when the engine is stopped. Fresh water contamination of the lubricating oil will result in an increased level on the dipstick and a drop in the fresh water level in the header tank.
Cooling water could enter the sump from a leak in the tube nest of the oil cooler but not while the engine is running. While running, the oil pressure is greater than the cooling water pressure so any leak will cause the oil to flow into the cooling water. However, when the engine is stopped, all the oil drains into the sump. If it is sea water cooling and the sea water line is above the cooler, it forms a head (pressure) on the sea water. It will flow through the leaking tube into the sump.
If it is fresh water cooling, the fresh water in the header tank forms a head (pressure) on the fresh water. It will flow through the leaking tube into the sump. Indications of a leak in the tube of the cooler cooled with sea water can be seen in oil being discharged overboard with the sea water cooling whilst the engine is running.
Indications of a leak in the tube of the cooler cooled with fresh water can be seen in oil floating on the top of the fresh water in the header tank.
Water mixing with oil will result in emulsified oil which is grey/white or sometimes described as milky in colour. Depending upon the degree of emulsification, it can clog small openings and oil passages and prevents proper circulation and heat transfer. It will form as sludge and corrosion will occur. The fresh water could contain glycol which, when mixed with the oil, is damaging to the engine.
Lubricating oil analysis
Regular laboratory analysis of the oil should be carried out, especially when a purifier is fitted and periodic oil changes do not take place. The oil sample tendered for analysis should be taken from the circulation system, preferably while the engine is running, so that it is a true representation of the oil. If this is not possible, it should be taken as soon as the engine is stopped. An analysis will give valuable information as to the engine’s condition, any change to the oil and the nature of contaminants.
A laboratory will advise on the level of contaminants, what may be causing them and what steps can be taken to remove them. The problem causing the contamination must be rectified immediately. Contaminants may include:
Foreign mineral matter;
Carbonaceous material; and;
The Detroit Diesel Operators Manual provides used lube oil analysis warning values
The presence of ethylene glycol in the oil is damaging to the engine. Its presence and need for an oil change and for corrective maintenance action may be confirmed by glycol detector kits which are commercially available.
Fuel dilution of the oil may result from loose fuel connections or from prolonged engine idling. A fuel dilution exceeding 2.5% by volume indicates an immediate need for an oil change and corrective maintenance action. Fuel dilution may be confirmed by ASTM D-322 test procedure performed by oil suppliers or independent laboratories.
Additionally, if any of the following occur, the oil should be changed.
The viscosity at 100° F of a used oil sample is 40 percent greater than the viscosity of the unused oil measured at the same temperature (ASTM D-445 and D-2161).
Iron content is greater than 150 parts per million.
Pentane insoluble’s (total contamination) exceed 1% by weight (ASTM D-893).
The total base number (TBN) is less than 1.0 (ASTM D-664).
Note - The sulphur content of the diesel fuel used will influence the alkalinity of the lube oil. With high sulphur fuels, the oil drain interval will have to be shortened to avoid excessive acidity in the lube oil.
Faults, maintenance and servicing
Time between changes
An engine manufacturer typically recommend the oil and oil filter change intervals in each operator’s manual at between 200 and 500 hours, or less if the engine lays idle for long periods. Kubota recommend for the 38 kW an initial run in period of 50 hours and continuing at 200 hours intervals. Detroit Diesel recommend 150 hours for their Series 53, 71 and 92 naturally aspirated and turbo charged engines and 500 hours for their Series 149 naturally aspirated engines and 300 hours for the Series 149 turbo charged engines. Detroit state that the oil change intervals may be extended if supported by used oil analysis.
Factors affecting oil change periods
The amount of wear, oil contamination and efficiency of filtration are factors determining oil change scheduling. Frequent long voyages at high speed with the resultant high engine operating temperatures, may oxidise the oil and may result in the formation of sludge and varnish. Short runs and in cold weather do not permit thorough warming up of the engine, and water may accumulate in the crankcase from condensation of moisture produced by the burning of the fuel and cold engine parts. The use of effective filters, maintaining the engine in good condition and preventing overheating of the oil, will improve the efficiency of the oil.
Additives are used to increase the performance of the oil and some of them break down, in preference to the oil, as they perform their duty. Some can only do so much work before they must be removed from the engine along with the undesirable side effects of combustion. Oil change should occur before the additives wear out. This is not possible unless an analysis is carried out.
The best policy is to follow the engine manufacturer’s recommendations on oil and filter changes. These recommendations apply to normal engine operation. If the operational conditions of the engine are not normal, change the oil and filters more regularly. An oil change may be required at sea to enable the vessel to reach port so sufficient spare oil should be carried on the vessel.
Changing the oil
Containment precautions must be in place before the actual oil change including tagging off any remote starting controls. Every effort to avoid and to deal with unexpected spills into the bilge must be in place, and a plan to temporarily store then dispose of waste oil ashore must have been negotiated with a shoreside disposal facility.
The oil will flow better if the engine warms it up for a few minutes before draining. Many vessels will have dedicated oil change manual pump that most conveniently draws off the waste oil. Even so, it may be necessary to remove the lowest sump plug to drain off any remnant sludge not ejected by the manual pump.
Once the sump is dry the oil filter/s can be renewed. Disposable cartridge type filters are removed using a suitably sized oil filter strap clamp tool and a container to catch splash from any remnant oil. The used cartridge is disposed of. The new cartridge is filled with clean oil, its rubber sealing washer lightly smeared with oil and the cartridge is tightened back into the threaded housing. Do not over tighten.
The process is the same for replaceable element filters except that the waste element is disposed of, the filter bowl cleaned and a new filter element installed. A formal record must be kept in the oil record book detailing quantity, disposal date and engine hours. Engineers will often write on the filter the date and engine hours as a future scheduling reminder, as shown below.
Faults in a lubricating oil system
Contamination – As has been described above.
Low oil pressure - Oil pressures will differ between types and makes of engines. As a guide, on a Detroit Diesel, normal oil pressure at 2100 rpm is 276-414 kPa (2.7 – 4.1 bar or 40- 60 psi) with a minimum oil pressure of 207 kPa (30 psi). At 1200 rpm, it is 207-414 kPa (30 to 60 psi) with a minimum oil pressure of 124 kPa (1.2 bar or 18 psi). A lower minimum oil pressure is acceptable at a lower speed compared to the minimum at a higher speed. This is due to the lower loadings on the bottom end bearings whilst on the power stroke.
The reduction in the normal operating pressure of lubricating oil can be a gradual process or happen instantaneously. The total or significant loss of oil pressure will cause the parts under most load to fail first, often the bottom end bearings due to the load placed on them during the power stroke. In an emergency situation where engine power is still required, reducing the engine speed will reduce the load on the bearings. If there is still some oil pressure a reduction in load maybe sufficient to save them. Should the oil pressure drop instantaneously, the engine must be stopped immediately.
Should there be no oil pressure within 10 to 15 seconds after starting the engine, immediately stop the engine. Most modern vessels are fitted with low pressure alarms but no equipment is 100% reliable, especially in a marine environment. The engineer must rely on his/her senses to monitor the engine performance.
Insufficient level of oil in the sump - May cause a fluctuation of the oil pressure as the vessel rolls, the pump loses suction and air enters it.
Lubricating oil pump strainer clogged - The additives in oil keep contaminants in suspension for the filter to remove and minimise this problem.
Faulty lubricating oil pump - If the drive to the pump has sheared, there would be no oil pressure at all requiring immediate engine shut down to avoid severe damage. Should the pump gears/rotors be worn or have too much clearance between them and the backing plate a gradual drop in oil pressure would be experienced.
Faulty pressure relief valve - It may stick open or its spring may have broken. A cold engine on start up will have a high oil pressure causing the relief valve to open. As the engine reaches its normal operating temperature the oil pressure drops as the oil thins out resulting in the relief valve closing. Should the relief valve stick in the open position or the spring break, the oil pressure will drop below normal.
Filter partially clogged - With the filter being partially clogged, the flow of oil will gradually be restricted. Lower oil pressure will occur and be indicated on the pressure gauge until the filter by pass valve opens.
Oil temperature too high - A high oil temperature will thin the oil out causing it to flow more easily with a resulting drop in oil pressure. This could be caused by a worn engine which would have fresh water overheating as well. Alternately, it could be caused by a dirty oil cooler on the sea water side.
Faulty oil pressure gauge - A faulty oil pressure gauge could indicate a low oil pressure where in fact the actual pressure is correct. If the oil pressure gauge is suspected, try another one.
Fractured lubricating oil pipes – This may result in a sudden pressure drop.
Excessive clearance in a bearing or bearings - In a main/bottom end bearing the clearance is very small and places a restriction on the flow of oil increasing oil pressure. If the bearing clearance is excessive, the oil is less restricted and its pressure will drop. Usually, the bottom end bearing is the problem.
Water in the oil - Water mixing with oil results in emulsified oil described as grey/white or milky in colour. Emulsified oil loses its lubricating propertiesand the oil pressure will drop below normal.
Fuel in the oil - Fuel contamination will thin the oil and it will run easily off the dip stick. It will smell of fuel and there will be a rise in the level in the sump. The oil will lose its lubricating properties and the oil pressure will drop.
The purpose of a cooling system is to maintain a constant temperature throughout the engine that minimises hot spot expansion leading to seizing of the moving parts. Small lightweight auxiliary engines use ambient air cooling with carefully designed cooling fins around the hottest component, the cylinder head. More complex auto engines use a heat exchanger system of a wind cooled fresh water radiator circulating fresh water coolant around the water jackets within the engine block.
Water cooling systems are universally used in marine main engines where speed driven cooling is insufficiently constant for reliability. These systems include:
Direct water cooling (fresh or salt);
Heat exchanger cooling (tube or keel cooling).
Direct water cooling
All vessels have an unlimited supply of cooling water over the side. Simpler engines working in fresh water have no difficulty in pumping water up into the internal water jacket that surrounds the engine’s cylinder and dumping the used water back overboard. The water pick up pipe will exclude large waterborne debris with a clam shell external grate. A fine strainer and sacrificial zinc anode may be included. Crude systems such as these rely on flow designed around an average water temperature, and may cool the engine more than the optimum running temperature.
Outboards typically use direct (salt) sea water cooling with its added problem of salt induced corrosion and salt deposit build up. The cooling water pick up is positioned in the outboard leg so designed that water is both directed towards the gear box driven impellor pump and strained of debris. Anti corrosive constructional metals, strategically placed sacrificial anodes and flow design are critical in the piping and water-jacket layout. Moreover, a strict regime of flushing out the motor with fresh water after every trip is essential. Outboard motors have a leak off pipe from the cooling water system called a tell tale placed in a visible position to indicate that water is circulating freely, a requirement being that the leg and water pick up are constantly immersed. The outboard is more fully described in Chapter 8.
Heat exchanger cooling
The heat exchanger cooling system maintains a constant temperature throughout the engine by removing heat from the hottest part of the engine in the vicinity of the combustion space and transfers it to the cooler parts. Used salt cooling water is ejected directly overboard or via the exhaust. Marine systems are designed to operate at temperatures of typically 85°- 90° C and may be pressurised to raise coolant boil temperature and provide more efficient circulation. An engine can operate intermittently up to 96° with a header tank cap pressure of 103 kPa (1.03 bar or 15 psi). The cooling water high temperature alarms are set at around 96° C.
In the schematic system shown below, engine driven pump sucks sea water from the sea chest through the cooler (header tank and heat exchanger) and another circulates fresh water through the engine water jackets and the cooler. The close contact of the engine’s hot fresh water is cooled (heat exchanged) by the piped cold salt water on its way to be ejected (and cool) the exhaust.
The salt water or raw water components include:
Rose or grid - The sea water intake is through a corrosion resistant (bronze or stainless steel 316 grade) rose or grid on the vessel’s hull in order to prevent large pieces of foreign matter entering or blocking the flow.
Sea cock and valve - is attached directly to the inside of the hull so that the sea water can be shut off during maintenance or repair whilst the vessel is afloat.
Multi sea cocks are used on catamaran hulls allowing a cross over facility in case one strainer (screen) gets blocked.
On some vessels there is a high and low sea water intake valve. Only one is used at a time. The high one is used for smooth water operations where there is shallow water so that sand and mud are not sucked up into the system. The low one is used at sea so when the vessel rolls, the intake does not come out of the water. It can also be used on smooth water operations where there is deep water.
Strainer or screen- is fitted into the pipe work before the pump, to avoid small foreign matter being drawn into the system. The strainer may be fitted with a sight bowl. The strainer must be easily accessible for frequent cleaning and inspection. A sacrificial zinc anode plug is often fitted to the top of the sea chest.
Sea water pump - is an engine or belt driven positive displacement pump, usually of the flexible impellor type (often called after the common brand, Jabsco)
Fresh water cooler - The cooler is on the discharge side of the sea water pump and is usually protected against the corrosive effect of the water by a sacrificial zinc anode plug. To further protect against salt water corrosion the constructional materials used are non corrosive and consistent in their electrolytic reactivity.
Exhaust box or swan neck – In a wet exhaust system the raw water is ejected into the exhaust flow to cool and quieten the exhaust. It is injected into the exhaust pipe after a gravity lock (a bend or bucket) in such a way that water will not drain back through the exhaust manifold and into the cylinders. This is to prevent the potential for serious damage of turning over uncompressible water filled cylinders (hydraulicing) and/or introducing a corrosion source. This remains a possibility if an engine is repeated turned over without starting, as the build up of salt water may over flow the water lock. More complex engines may have a salt water leak off pipe with control valve whereby exhaust back pressure generated by the salt water injection into the exhaust can be optimally adjusted to manufacturer’s specification.
Some vessels use an insulated dry exhaust and eject the used raw water directly overboard.
The fresh water or coolant circulation components include:
Fresh water or coolant circulation – The circulating liquid may be fresh water of a mix of water and additives. Coolant glycol based additive (often called antifreeze) assists to reduce corrosion, minimise freezing, raise boiling point and promote efficient coolant flow. Such corrosion inhibitors have a maximum service life and should be replaced as recommended by the manufacturer. In most high speed engines the fresh water system is pressurised both to raise the boiling point and to promote efficient pumping. Care must be taken when opening the cap of a warm header tank that the release of pressure does cause a gushing release of boiling water and steam.
The top of the cylinder liner and the inlet and exhaust valve area are where combustion takes place. Fresh water with additives called coolant is circulated under pressure through the block and cylinder head to cool this hottest part of the engine.
There are spaces around the cylinder liner and the block that are called jackets. The fresh water circulates through them. The fresh water then flows through the holes in
the top of the block and corresponding holes in the cylinder head gasket and cylinder head. In the cylinder head the water circulates around the inlet and exhaust gas passages to the thermostat.
The fresh water in some engines may be fed through the exhaust manifold and turbo charger. Cooling them minimises the radiant heat given off thereby keeping the engine space cooler and providing better engine power. Fresh water cooling of these parts does not leave the deposits that sea water cooling does.
Fresh water or coolant pump - is engine driven and as it is constantly immersed (needing no priming) is usually of the low maintenance centrifugal type.
Header tank or expansion tank - This allows for expansion of the fresh water as it heats. Modern engines typically also have a vent and additional overflow reservoir. The tank provides an adequate method of venting air and combustion gases from the fresh water system during engine operation. Since combustion gases may enter the fresh water system, it will be necessary to remove these gases before they cause deterioration of the engines cooling water system’s performance. Gases in the system reduce coolant flows and may result in engine failure if adequate venting capacity is not provided.
In many engines the header tank is situated above the fresh water cooler to form one unit.
Venting - Initial venting of the system is critical in order to ensure that the system is completely filled with coolant. Fill the engine slowly to allow the coolant to fill from the bottom up. Quickly filling the expansion tank can fill the vent pipes with coolant and result in slow or incomplete fill of the engine. If vent cocks are fitted, they are to be used to help any air to escape. After initial filling of the system, run the engine, keep checking and if necessary top up the water level until the coolant operating temperature is reached. Any trapped air should make its way up to the header tank in the form of bubbles.
Thermostat - A thermostat is a heat sensitive valve that enables the engine to reach its normal operating temperature quicker to minimise warming up wear. When the engine is cold, the closed thermostat closes the coolant off from the heat exchanger. When the engine reaches its normal operating temperature the thermostat opens and directs the water to the heat exchanger.
Heat exchanger or fresh water cooler – The heat exchanging process takes place below the header tank. When the thermostat is open, fresh water will flow through the bath of the cooler (shell) and be cooled by the nest of tubes through which the raw water flows, hence the name of a tube nest and shell heat exchanger.
Shell and tube nest heat exchanger
Shell -The shell has a fresh water inlet and outlet and contains the tube nest. Baffles may be fitted to increase the fresh water contact period with the tubes’ surfaces.
Tube nest - The tube nest consists of a tube plate at either end with tubes fitted in between. Allowance for expansion is required due to the different rates of expansion of the metals used in the construction and prevent any undue stress on the tubes. One tube plate is secured at one end of the shell and expansion allowance is by using twin O rings at the other end for of the tube nest. Any gap between them will indicate a leak from either the raw water or coolant.
End covers - An end cover is fitted to each end of the shell. Where there is a flange on each end cover, the sea water will flow in at one end and out the other end. Where there is two flanges on one end cover, the sea water will enter and leave via this cover. A division plate will be fitted in this cover and seals against the tube nest across its diameter to separate the inlet and outlet flows. The other end cover will have no flanges.
Cleaning - The sea water will leave behind more deposits than the fresh water as scale in the tubes so it will be necessary to periodically clean them. The end covers can be removed and a wire brush pushed and drawn through each tube, this being the reason why the sea water flows through the tubes. Dismantling and cleaning heat exchangers is a scheduled service task.
In this system coolant is circulated through pipes or tanks mounted externally on the hull below the waterline. Seawater on the outside of the keel cooling pipes cools the fresh water on the inside. These coolers may be a channel or half a round pipe welded to the hull or may be a copper tube grid. A sea water pump can eliminated though attention to convectional flow in design is paramount.
Keel cooling has the great advantage that there is no external water inlet that may become clogged and restrict engine cooling, particularly if the vessel is operating in silty shallow areas. The disadvantages include that marine growth on the pipe will impair heat transfer requiring slipping to clean the keel cooling pipes. Unless shielded the external the pipes are subject to impact damage from flotsam or grounding. A large fresh water tank and fresh water reserves are required.
Due to the disadvantages of a fully keel cooled system hybrids are commonly found, particularly when auto engines are modified for marine applications.
The hybrid shown above uses a closed system salt water circulation cooling the fresh water by way of a standard heat exchanger. The advantage of this design is that replenishment of the reservoir can be by fresh or if necessary while at sea with readily available salt water. Also damage or leaks in the external pipes will not immediately affect the systems cooling efficiency.
Additional cooling features
Other coolers - Sea water may be piped through an oil cooler before it goes into the fresh water cooler, as described in Chapter 4. A turbo charged engine fitted with an after cooler may also be cooled as described in Chapter 1. If the engine employs a wet exhaust system, a proportion or all of the sea water will then be pumped directly into the exhaust pipe. In a dry exhaust system, all the sea water would be pumped overboard as it leaves the fresh water cooler or after cooler.
Other pumps - On large engines, the fresh and sea water pumps are driven by electric motors rather than the engine. No thermostat is fitted, the fresh water circulates through the engine as well as the cooler. The advantage is that the fresh water can be pre-circulated through the engine and its temperature gradually raised until at normal operating temperature so when the engine fires and the sudden rise in temperature is not so great. To control the temperature of the fresh water, a control valve is fitted for the sea water to by-pass the cooler until it is required.
Two types of pumps are employed in cooling water systems:
Flexible impeller type; (raw water)
Centrifugal type pump (fresh water).
Flexible impeller type pump
This rotary positive displacement pump is so widely used for cooling and bilge systems that the type is often called by its trade name, a Jabsco pump. The bronze casing in which the neoprene or rubber impeller revolves is not uniformly circular, having a constriction (or cam) over a third of its diameter between the inlet and outlet. As the impeller blades pass the cam a partial vacuum is created because the space between the impeller blades expands around the inlet (drawing water in) and contracts around the outlet (pushing water out).
The partial vacuum created is so good that it is a self priming action which makes this type of pump popular as a bilge pump.
The engine driven shaft is sealed by packing or a mechanical seal. All suction side connections must also be air-tight as leaks will stop or slow flow through the pump.
A cover plate over a gasket gives easy access to the casing and impeller. The impeller is a drive fit onto a splined shaft or one with a keyway. Although it is a self-priming pump the flexible impellor relies on the pumped fluid for lubrication so it will be damaged if the pump runs dry.
The impeller is clearly a delicate component that will easily be stripped of its vanes if a nail or metal fragment is sucked up from the inlet. Other failures of flexible impellers result from chemical attack (from polluted bilge water), water flow cavitations (from narrow piping or over speed pumps) or more traumatically solid materials that evade the inlet gratings and screens and are drawn into the pump. A sudden increase in wet exhaust engine noise is a sign that salt water cooling has dramatically failed, and the impeller is a prime suspect. Pumps that are not used for extended periods can develop misshapen and brittle impellers that need to be replaced and can adhere to the pump cover.
Spare impellers sets should always be onboard so timely replacement can be carried out by removing pump cover and gasket beneath and sliding the impeller off the splined drive shaft to inspect for damage. They can be reluctant to let go and may have to be carefully prised off with levers. Check for broken blades, impeller end clearance, worn casing wear plate and leaking seals. The end plate and impeller must be a good fit to pump and self prime. Old end plates may have become grooved so will have to be honed flat again. Repairs may include attention to the gasket or replacing a worn bearing. To separate the bearings from the shaft use a wood block to support the unit while tapping out the shaft.
A new impeller can be just as reluctant to squeeze back onto the shaft and into the casing. A smear of soap and the assistance of a rubber mallet may be required.
Before starting the pump ensure that drive belts (if fitted) do not slip. It may be necessary to initially prime the system especially if the pump is fitted high in the vessel. Smaller portable electric pumps are unlikely to pump up to more than one to two metres so outlets may have to be initially positioned by trial and error.
Loss of pressure is usually due to too much clearance between the impeller and the back cover allowing water to escape from the pumping chambers.
If the pump is allowed to stand idle for a considerable period, the rubber impeller blades will stick to the housing and tear when the pump is started.
The water flowing through the pump helps to lubricate and keep them cool. Running a pump dry will result in a damaged impeller. In a cooling water system, this would only happen if there is air in the system.
Metal debris will strip an impeller in seconds.
Centrifugal type pump
A centrifugal type pump must be full with the medium it is pumping, whether gas or liquid. It is not a positive displacement pump and will not self prime. As a water pump the water enters at the centre of the impeller and is thrown radially out of the impeller by centrifugal force to the periphery from where it is discharged. The centrifugal pump can therefore be recognised by the suction or inlet entering the centre of the pump housing and the outlet or discharge on its circumference.
The centrifugal pump consists of a shaft attached to an impeller. The impellor will have internal curved vanes if it is hollow or curved vanes on one or both sides if it is not hollow.
Centrifugal type pumps are generally used for large quantities of water at low pressures. The pressure can be increased by having a number of impellers in series, although this is not required in a cooling water system. It is preferable that a centrifugal pump has a positive suction (the level of liquid is above the inlet of the pump) or is used when the lift is very small. They are not self priming pumps. They are suitable for closed fresh water cooling systems.
To be effective the impeller must have the minimum of clearance between the suction and discharge openings.
Erosion is a problem in centrifugal pumps caused by the flow of the fluid, especially if used for sea water in shallow sandy conditions.
Cavitation can also be a problem.
Corrosion and electrolysis in cooling water systems
Galvanic corrosion (often incorrectly referred to as electrolysis) is the corrosion of the more active (or less noble) member of a pair of metals in physical contact in a corrosive environment. The more active metal is the anode, whereas the less active metal (more noble) is the cathode and does not corrode. Although the current flowing is often very small, it is a continuous process and the attack is made worse if the anode is small and the cathode large. Where the more corrodible metal is very much larger in area (e.g. stainless steel bolts in an aluminium hull) galvanic corrosion may not happen.
Where corrosion on dissimilar metals occur, the particular metal that will corrode can be determined from the Galvanic Series, a table in which the common construction metals are listed with the active (or anodic) metals at the top and the noble (or cathodic) metals at the bottom.
Active Metals (anodic)
Magnesium and magnesium alloys
Zinc and galvanised steel
Iron, cast iron, mild steel
Stainless steels (activated)
Lead, tin, tin-lead solders
Naval brass, high tensile brass
Nickel and High nickel alloys (activated)
Copper, Admiralty brass
Phosphor bronze, silicon bronze, gun metal
Monel, Inconel, high nickel alloys (passivated)
Stainless steels (passivated)
Platinum, gold graphite
Noble Metals (Cathodic)
If two metals are coupled, the metal nearer the top of the series will be the anode and suffer increased corrosion, whereas the metal nearer the bottom will be the inert cathode. Generally, the further apart the two metal are in the Galvanic Series, the worse will be the galvanic corrosion of the more active metal. Where two metals are listed close together in the series, there is little likelihood that damaging galvanic corrosion will occur.
As can be seen from the above, when two dissimilar metals are placed in sea water, galvanic action loosely referred to as electrolysis will take place. In an engine cooling system, a number of different metals are used which are close together in the Galvanic Series. However to prevent the slight galvanic action (electrolysis) that will take place, a sacrificial anode is placed in the fresh water, if an inhibitor is not used, and sea water inlets of the cooler (heat exchanger). Regular inspections should take place of these anodes. They should cleaned with a wire brush so they are more effective. If they are excessively eaten away, they are to be replaced. If in doubt about the condition of the anode, strike it sharply against a hard surface. A weakened anode will break.
Sea water system faults causing rise in temperature
A gradual rise is where the temperature rises over a period of time. It could be caused by a gradual build up of scale on the cooling water surfaces or a strainer gradually becoming clogged.
A sudden rise in temperature could be caused by the thermostat stuck in the closed position, a pump impeller revolving on its shaft or the engine overloaded.
Sea water temperature too high - Not normally a problem in southern Australian waters, however must be considered when a vessel is operating in warmer northern waters. The engine speed should be reduced to bring the temperature back to its normal operating temperature.
Sea water intake rose or grid - Could become clogged over a period of time so there would be a gradual increase in the fresh water cooling temperature.
Reduce the engine speed until the normal operating temperature is obtained.
Alternately, a plastic bag may get sucked onto the grid and a sudden rise in temperature would occur. Gradually slow down the engine to reduce the heat slowly and stop the engine. With no suction holding the plastic bag on the grid and with the vessel moving through the water, the plastic bag will come away from the intake grid. Start the engine and let it idle until temperatures stabilise.
Clogged sea water strainer - Could become clogged over a period of time so there would be a gradual increase in the fresh water cooling temperature. Reduce the engine speed gradually and stop the engine. Clean out the strainer. Start the engine and let it idle until temperature stabilises. Alternately the vessel may have voyaged through matter which quickly clogged the strainer. Effect the same remedy as before.
Faulty impeller in sea water pump -The rubber flexible impeller in the sea water pump could be damaged. Damage usually occurs when the pump is run dry. Indications would be the pump discharge pipe would be warm and not at the same
temperature as the sea water. In addition there would be no or a reduced sea water discharge overboard.
Reduce the engine speed gradually and stop the engine. Replace the impeller. Should you be at sea and have no replacement impeller, it may be possible to reach port at reduced speed if the impeller is only partially damaged and still can pump some water. Alternately, a sea water hose from the fire pump or the wash deck hose could be connected up to the system at the discharge side of the sea water pump to get the vessel back to port.
With a centrifugal type pump, the pin holding the impeller onto the shaft may have sheared. The indications would be as above. It should be possible to make up and fit a new pin.
Faulty seal in sea water pump - Sea water will leak out and will cause no problems provided it does not spray over anything, especially electrical equipment. The seal can be replaced when the vessel gets back to port.
Air in sea water cooling system - On a lot of vessels, air is trapped in the sea water cooling system when the vessel re-enters the water after slipping.
With the engine stopped, the air can be bled off by slackening off the backing plate on a jabsco pump or loosening a join in the seawater cooling pipe on the suction side of the pump that is below the water line. If it is a jabsco pump and has run dry until the engine overheated, the rubber impeller will be severely damaged.
Insufficient speed of sea water pump - On some vessels the sea water pump is belt driven from the engine. The adjustment of the belt may cause it to slip.
Reduce the engine speed gradually and stop the engine. Adjust the belt tension. Start the engine and let it idle until temperature stabilises. It may be that the pump does not attain sufficient speed as the driver or driven pulleys may be the wrong size. Reduce the engine speed gradually until normal operating temperature is attained. When back in port, change the pulleys.
Dirty or fouled tubes in the cooler - The sea water discharged overboard would be restricted. It is unusual for the cooler to be completely blocked. Reduce engine speed until normal operating temperature is attained. Stop engine and clean the cooler or return to port under reduced speed.
Electrolysis in the cooler - It would appear that the zinc anode/s have wasted and require replacing.
Leaking sea water hoses or pipes - Sea water will leak out and will cause no problems provided it does not spray over anything, especially electrical equipment.
The hose can be replaced, or a piece of rubber fastened with a hose clip, can be fitted to the leaking pipe.
Keel cooling pipes not effective due to marine growth - This causes a gradual increase in the fresh water temperature. Reduce the engine speed gradually until the normal operating temperature is obtained. The vessel will have to be slipped to clean the keel cooling pipes.
Keel cooling pipes leaking - Corrosion or electrolysis on the keel cooling pipes could cause a leak to develop. The fresh water would flow into the sea water as the fresh water pressure is greater than the head of sea water, even when the engine is stopped, that is until the fresh water level drops to the same level as the water line of the vessel. If the leak is not too bad, the engine can be run provided the fresh water tank is kept topped up. The vessel will have to be slipped to repair the leak.
Fresh water system faults causing rise in temperature
Fresh water cooling level is too low - A leak has developed in the fresh water system causing a loss of water in the header tank. It could be a leak in the piping, seal in the pump or a blown cylinder head gasket. Reduce the engine speed gradually and if the fresh water system is the unpressurised type, very slowly top up the header tank to its correct level. If the fresh water system is of the pressurised type, reduce the engine speed gradually and stop the engine. Let the engine cool down further before placing a rag over the header tank cap. Turn the cap anti-clockwise until it reaches the position where the pressure is released. When the pressure is released, remove the cap. Start the engine and very slowly top up the header tank to its correct level.
If there is very little water in the header tank, it is advisable to let the engine cool right down before adding fresh water. When the engine is hot and the fresh water level in the header tank is low, cold water should be introduced very slowly whilst the engine is running. The cold water will then be heated sufficiently before it circulates around the combustion space. Cold water suddenly coming into contact with the hot cylinder liner and cylinder head may crack them.
If possible, the leak in the piping should be stopped or the pump seal replaced.
In an emergency or in distress the engine could be run with a blown head gasket between the cylinder and a cooling water passage to get the vessel back to safety providing the leak is not too severe and the engine is not stopped. If the engine is stopped, water could make its way into the cylinder and hydraulic the engine resulting in severe damage.
Thermostat not opening fully - The thermostat is in the closed position when the engine is cold and first started. In the closed position, the water is circulated through the engine only. As the engine reaches its operating temperature, the thermostat opens and now allows the water circulating through the engine to pass through the fresh water cooler or the keel cooling pipes. Should the thermostat stay in its closed position or not open fully, the engine will overheat. Feeling the pipe from the thermostat housing to the fresh water cooler will indicate whether or not water is flowing through it. Reduce the engine speed gradually and stop the engine. When the engine has cooled down replace the thermostat. Start the engine. Should you be at sea and have no replacement thermostat, the engine can be run without one to get the vessel back to port.
Faulty impeller in fresh water cooling pump - The rubber flexible impeller in the fresh water pump could be damaged. Reduce the engine speed gradually and stop the engine. Replace the impellor. Should you be at sea and have no replacement impellor, it may be possible to reach port at reduced speed if the impellor is only partially damaged and can still pump some water.
Alternately, the impeller from the sea water pump could be used if it is the same size and a sea water hose from the fire pump or the wash deck hose could be connected up to the system at the discharge side of the sea water pump to get the vessel back to port.
Faulty seal in fresh water pump - Fresh water will leak out and will cause no problems provided it does not spray over anything, especially electrical equipment, until the level in the header tank is low and overheating will start to occur. See Fresh water cooling level is too low above.
Leaking fresh water hoses or pipes - Fresh water will leak out and will cause no problems provided it does not spray over anything, especially electrical equipment, until the level in the header tank is low and overheating will start to occur. The hose can be replaced, or a piece of rubber fastened with a hose clip, can be fitted to the leaking pipe.
Build up of scale on cylinder water jackets - Fresh water contains impurities which will come out of solution at high temperatures and adhere to hot surfaces. The hottest part of the engine is in the combustion space at the top of the cylinder. Scale will deposit on the cylinder liner walls in this area, on the passages to the cylinder head and around the exhaust valve. The scale will stop the transfer of heat from the combustion process to the fresh water cooling and in the case of passages, will restrict the flow. This will be a gradual process.
Reduce the engine speed until normal operating temperature is attained. Back in port, the cooling water system will have to be chemically cleaned.
Air in fresh water cooling system – This is not normally a problem when the engine is running. Air can get into the system when repairs are carried out and the cooling system is refilled. On starting the engine, bubbles will be sighted in the header tank as the air makes its way out. As the water replaces the air, the water level in the header tank will drop. As it drops, it can be topped up slowly.
The cooling system is like any other part of machinery. It requires routine maintenance and preventative maintenance to minimise costly engine overhauling due to overheating. The cooling water system should always be maintained at its correct level in the header tank. Inhibitors are used in the cooling water to provide corrosion protection, pH control, water softening, prevent freezing and increasing the boiling point.
The manufacturer’s instructions should be followed as to what type of inhibitor should be used. It is recommended that one brand of inhibitor be used as mixing different brands may have serious consequences if they are not compatible. Care should be taken to insure the inhibitor is only used at the correct strength, is pre-mixed prior to filling the engine and is used when topping up the system. The inhibitor has a maximum service life and should be replaced as recommended.
Hoses on the cooling water system tend to deteriorate due to age and heat. In marinised auto engines they may not be of a suitable durable marine quality and may be fastened with ferrous (rust prone) hose clamps. A hose found to be soft, is bulging or its outside surface is covered with minute cracking should be replaced immediately. It is recommended that all hoses be replaced regularly in order to avoid possible engine overhaul, water damaging electrics and inconvenience.
Repair leaks as they occur as they will only get worse.
Keep a regular check on the cooling water temperature. A rise in temperature could be attributed to a dirty cooler. The cooler should therefore be cleaned regularly, especially if the vessel is to proceed to an area of higher sea water temperatures.
Any anodes are to be inspected regularly, cleaned, checked for weakness and replaced if circumstances dictate.
Flexible impeller replacement is a service scheduled item.
The cooling water temperature alarm consists of a thermo switch. It has a bi-metal probe that activates contacts in a micro switch. It is usually situated in the thermostat housing. The manufacturer will state at what temperature the engine operates at and what temperature the alarm will sound.
The probe can be inserted into a container of water with a thermometer making sure they do not touch the container.
Heat and agitate the water to ensure uniform water and probe temperature. Place a meter over the contacts of the micro switch. The contacts should close when the thermometer reaches the temperature within the required tolerances. If the water is left to cool, the contacts will open.
Should the contacts not close within the required tolerances, the micro switch will have to be adjusted. However, most of them are sealed units and cannot be adjusted. A replacement would therefore have to be fitted.
Manufacturers supply a shut down device that would energise a solenoid to shut off the fuel when the cooling water temperature is at the alarm setting.
Ships with low speed engines typically use direct drive from the crankshaft to the tailshaft with its propeller. Smaller domestic commercial vessels typically operate with high speed engines where direct drive shafts would revolve too fast to drag water across their propellers without efficiency losses due to cavitation (bubbling in the water flow). Consequently a gearbox is used to provide reduced speed of the shaft, reverse propulsion and through a clutch mechanism, disengaged propulsion.
Principles of marine gear boxes
Gearing is used in drive trains (sets of intermeshed gear wheels) to alter direction, position or speed/mechanical advantage of propulsion and auxiliary equipment such as winches, pumps and steering. The gear and shaft driven directly by the motor can be called the input and the final gear and shaft it drives can be called the output. Drive train A shown below illustrates reduction and reversal of the output, drive train B illustrates reduction only with further displacement (to the right) of the output, drive train C illustrates reduction and splitting into two outputs.
The efficient crankshaft speed of a small high speed motor is greater than the efficient speed of a propeller that revolves in relatively dense salt water. The solution to this is to use of a drive train with an input shaft and drive gear wheel enmeshed with a set of follower gear wheels connected to an output shaft (a gear box). The output shaft follower (to the propeller) has more gear teeth than the input shaft driver (from the crankshaft), and therefore it rotates more slowly. The number of teeth in each gear wheel has a direct relationship to not only the speed of the follower but also the transfer of torque. The reduction ratios shown in the drawings above are calculated by the:
number of gear teeth (cogs) in the large
wheel = 16 = 2 or a Reduction Ratio of 2:1
number of gear teeth in the small wheel 8
Therefore if the engine speed is 1000 rpm and the propeller speed is 500 rpm then:
rpm of the engine = 1000 = 2 or a Reduction Ratio of 2:1
the rpm of the propeller 500
While the speed has been halved, the mechanical advantage has been doubled (not allowing for efficiency losses due to friction and heat). Marine gear boxes are designed to reduce the speed of the propeller shaft but some applications require a greater speed from the output shaft. This is simply achieved by using a smaller follower to give higher output speed with less mechanical advantage. It must also be noted that by adding a following gear wheel it will turn in the opposite direction to its driving wheel (in reverse).
Gear selection and engagement
Gear box arrangements to select reverse gear include mechanically (using a gear lever directly or through a cable or linkage), electro-mechanically (using electrical solenoids which change the gears directly or hydraulically through clutches) or by hydraulic operation alone. The gearbox control and engine throttle (speed) may use separate levers for thrust and speed, or be combined in a single control lever as drawn below.
Marine transmission systems are heavy and generate considerable momentum so changing gear is smoothed by clutches. To avoid clashing of gears and overload on clutches the sequence of changing requires a delay for the propeller and shaft to slow down. Modern systems operate sequentially, or have override safety systems, however good practice remains to follow the manufacturer’s instructions always.
In most operations it is convenient to be able to positively engage and disengage an input shaft from an output shaft. The mechanism that smooths this operation is called a clutch.
Basic clutch mechanisms
Dog clutch - The simplest arrangement as found on windlasses and winches is the dog clutch. Shown below, a collar is splined to the driving motors shaft that can be pushed by the clutch lever to engage hard against a lug in the winch drum’s side. This arrangement only allows engagement when winch and motor are stopped or both moving slowly at the same speed (a neat trick if at all possible for the arrangement shown below).
Dog clutches are not used in propulsion engines.
Cone clutch - The cone clutch does allow for engagement while the motor is running and is suitable for slow running operations such as capstans, windlasses and winches.
The warping drum (rope carrier) or gypsy (chain carrier) is machined with a tapered central hole. The drum is fitted over a matching tapered axle and can revolve smoothly when in the disengaged mode. The tapered axle can also be turned by a directly fitted motor. When the drum is tightened down onto the axle it grips (engages) and will turn with the motor driven axle. The system can snatch as it engages and although usually of heavy duty build, as with all friction type clutches there will always be momentary slip before engagement resulting in heat and wear.
More sophisticated systems are held in engagement with a heavy coil spring that can be manually compressed to disengage the drum from the axle. For a better grip the driving axle may be machined to mate inside a wide tapered drum and the mating cone’s surface be coated with a friction lining material. Cone clutches are rarely used in propulsion engines.
Centrifugal clutch - A driving shaft and output coupling drum can revolve independently at their common bearing. The driving shaft has hinged weighted friction pads that at rest are restrained clear of the drum’s surface by springs. When the driving shaft is revolved the friction pads are thrown outwards by centrifugal force sufficient to overcome the springs and bind onto the drums surface (engaging the coupling). Light duty pumps, power tools and the smallest outboards often use a centrifugal clutch for its light weight and economy.
Double de-clutching - Before the modern synchromesh gearbox the earliest motor cars used a crash gearbox. To change gear, a lever is first applied to rip the currently engaged gearwheel away from the output gear train and into neutral (disengaging the engine from the road wheels). While temporarily in neutral the accelerator is revved to boost the engine speed to match the speed of the lower gear wheel (determined by the road wheel speed). This is called double de-clutching. When the driver judges that the speeds are matched the gear lever is applied to enmesh the lower gear by crashing it into the engine gear wheel, with hopefully not too much crunching of the gear teeth. Such foot and hand control is not available in marine gearboxes so some means of smoothing the change of gears is required.
Synchronising clutch mechanisms
Single plate clutch - a clutch plate disc is held firmly squeezed by heavy duty springs between friction pads on the flywheel and on the pressure plate disc.
A sliding muff coupling can be withdrawn using a lever mechanism to release the spring tension and enable disengagement. Not shown in the drawing for simplicity are the photos of the flexible links between the clutch plate and the output shaft that allow for shock loads and natural oscillation in engine output.
Multiple plate clutch - The single plate clutch mechanism operates externally of the gear box and is typical for auto applications. For weight and space saving, marine applications usually have clutches that operate within the gear box. The large single clutch plate is replaced by several smaller ones enabling equivalent surface area contact in a more compact unit.
Hydraulic oil is pumped in through the input shaft which forces the internal piston disc to overcome the hold off resistance of the return spring and move towards the output assembly. This in turn squeezes together the plates that are alternately splined (embedded) inside the rim of the input shaft assembly and on the outer rim of the output shaft assembly. Adjusting the hydraulic pressure enables accommodation for heavy loading operations as may be required for low gear or reverse gear.
Clutch slip - Clutch slip describes when there is incomplete engagement between the driving and driven side of the clutch. The power developed by the engine is therefore not fully transmitted to the propeller. Wear of friction plates or linings is unavoidable due to momentary slippage when clutch is being engaged. Each engagement results in a slight loss of the friction material. In the course of time, this accumulated wear allows slippage. With regular maintenance, adjustments can be made to the clutch to reduce this problem but eventually wear will be such that the friction material will need to be replaced.
Fluid (hydraulic) coupling – while a hydraulic linkage may be the activating mechanism to enable positive engagement in the previous clutch types, the fluid coupling transfers thrust by smooth fluid action. Consider a household electric fan pointed towards another. The second fan will slowly rotate in the breeze. This induced motion in the driven turbine (second fan) enables a cushioned engagement/disengagement between a driving motor shaft and a driven shaft albeit with some slip (poor thrust transfer) at low speed.
A fluid coupling transmits rotation from one shaft to another by accelerating hydraulic fluid inside its housing containing closely fitted rotors; the input driving shaft pump (impeller) and the output driven shaft turbine (runner). The flywheel, impeller and housing (shell) are fixed and the cavity filled with hydraulic fluid. The impeller spins the fluid from the centre where the velocity is low, to the periphery where the velocity becomes high. A net force of multiplied torque on the turbine causes it to rotate with the direction of the impeller.
Fluid couplings have a stall speed where the impeller is turning but not fast enough to coax the turbine into rotation (i.e. when the car driver stopped at traffic lights selects the gear while his brakes are still on). In this condition for excessive periods or if the runner becomes jammed, the engine's power could transfer its energy as overheated fluid, possibly leading to damage. A fluid coupling only achieves about 94% transmission efficiency due to fluid friction and turbulence.
Torque converter - Unlike the two rotor fluid coupling, the torque converter has at least three rotors- the impeller (motor driven), the turbine, (load driving) and positioned between them the stator (that modifies oil flow returning from the turbine to the impeller).
The fluid flow returning from a fluid coupling’s turbine can oppose the direction of impeller rotation during slip, causing lost efficiency. The torque converter uses its stator to redirect the returning fluid to assist the rotation of the impeller so improving efficiency and output torque.
Though simple stators can be fixed often they are mounted on a one way clutch allowing forward motion only. Since the returning fluid is initially travelling in a direction opposite to impeller rotation, the stator will likewise attempt to counter-rotate as it forces the fluid to change direction, an effect prevented by the one-way stator clutch. The typical torque converter in automatic transmissions has three stages of operation:
Stall- the motor turns the impeller but the turbine cannot rotate. (When the car driver stopped at traffic lights selects the gear while the brakes are still on).
Acceleration-the motor accelerates with the impeller spinning at a greater rate than the turbine.
Coupling-The turbine has reached approximately 90% of the speed of the impeller. Torque multiplication has ceased and the torque converter is behaving like a fluid coupling.
Lock-up clutches such as the Voith TurboSyn coupling overcome the problem of slip and improve fuel efficiency by coating the periphery of a multi-section impeller with friction plates. Centrifugal force throws the impeller onto the turbine at higher revolutions, creating a positive clutch lock at speed.
The gear system below has an input shaft and forward shaft (often combined), a reverse shaft and an output shaft turning within an oil bath called the sump (not shown). The forward and reversing shafts have pinions (driving gear wheels) engaged by clutches. When disengaged as below the reverse pinion gear wheel rotates freely on its shaft.
Ahead mode operation - The forward clutch pushes the forward pinion to engage on the forward/input shaft. The forward pinion then drives the driven shaft as shown above. The reverse clutch is not active so the reverse pinion rotates freely on the reverse shaft.
Astern mode operation - The reverse clutch pushes the reverse pinion to engage on the reverse shaft. The reverse pinion then drives the driven shaft as shown above. The forward clutch is not active and the forward pinion rotates freely on the forward/input shaft. An input shaft driven oil pump provides oil flow for lubrication and operation of the clutches. Oil pressure is regulated by a pressure relief valve before distribution to the gears, bearings, clutch and control via passages in the gearbox. A control box is mounted on the top of the gearbox and connected by linkage (electronic, hydraulic, pneumatic) to the throttle and forward/reverse control at the wheelhouse and gearbox (for manual emergency operation). An oil cooler and oil filters may be mounted on the gearbox.
Epicyclic or planetary gearing
Epicyclic or planetary gearing systems provide the advantages weigh saving, balance and compactness for a gear box suitable for marine applications. Variations of three basic geared components of a sun gear, planet gear and annulus are utilized.
One or more outer gears (planet gears) revolve about a central gear wheel (sun gear). In more complex systems groups of planet gears can be mounted on a movable arm (carrier) which as a body can rotate around the sun gear. An outer ring (annulus) with inward-facing teeth meshes with the planet gear. In operation one of the three components (star, planet or annulus) is held stationary, another is used as a power input and the last component is used as the output. The ratio of input to output is dependent upon the number of teeth in each gear, and upon which component is held stationary.
In the reversing gearbox shown below the output shaft can revolve in ahead mode (anticlockwise) with the annulus free to rotate within the shaft. If however the annulus is locked on the output shaft by the outer tightening collar (brake band) the annulus and shaft are rotated in reverse (clockwise) by the planet gears.
Epicyclical gear arrangements can be simple or compound. Compound arrangements enable large reduction ratios, weight saving and flexibility. They may be meshed-planet (at least two or more planets in mesh within each planet train), stepped-planet (a shaft connects two planets within each planet train), or multi-stage structures (two or more planet sets).
The main thrust bearing is located on the output shaft near the aft end of the gearbox. Its function is to carry the reaction to the thrust of the propeller and transmit it through the gearbox casing/frame to the framework of the vessel. This ensures the gears and engine is not affected and remains in the correct longitudinal alignment.
Gear box maintenance
Assuming that normal maintenance has been carried out on the gearbox, for daily pre-departure checks it should be sufficient to:
Check the oil level in the gearbox sump.
Run the engine with the gearbox in neutral position and check for leaks.
Operate the gearbox in the head and astern mode to ensure satisfactory operation after ensuring that the lines holding the vessel are secure during testing.
Gearboxes are fairly reliable. Faults tend to be due to lack of normal maintenance such as:
Adjustment of clutches and operating mechanism
Maintenance of filters
Cleaning of oil coolers
The traditional transmission system of a vessel is a system of gears, bearings, and couplings that drive a shaft (solid metal rod) onto which a propeller is fitted, called a shaft driven transmission.
Direct driven propellers - The thrust block/bearing is a short length of shafting with a single collar with tilting white metal faced thrust pads either side of the collar. The forward side pads resist the thrust of the propeller when operating in the ahead direction and the aft pads when going astern. The shaft is supported on both sides of the collar by bearings and all are carried in a housing bolted to the vessel frames. The pads are fitted in carriers that transmit the loads through the housing to the frames. In some vessels self aligning roller thrust bearings are used. The rollers are often angled to combine both thrust and bearing loads. The load is transmitted through the roller carriers through housing to the engine frames.
Reduction gear driven propellers - The reduction gear is interposed between the engine and the propeller to reduce the speed of the propeller relative to the engine speed. This is to allow the engine to run at its optimum or design speed. Propellers running at lower speeds are generally more efficient. The thrust bearing is in many cases built into the aft end of the reduction gear.
The purpose of the thrust bearing is to prevent the thrust of the propeller from being transmitted to the engine. A thrust load on the crankshaft would result in longitudinal misalignment of main bearings with the crankshaft and, crankpins with pistons and connecting rods resulting in overheating of the parts and premature wear.
The intermediate shaft is a length (or lengths) of shaft between the thrust block and tail shaft. They are required when the engine is located well forward of the stern. The shaft is supported by a bearing (plummer block).
The propeller shaft (screwshaft or tubeshaft) is the final link between the engine and propeller. It must pass through the hull of the vessel and the arrangements must be such that water cannot enter the vessel. This is done by enclosing the shaft in a tube that passes through the aft bulkhead of the engine room through the peak tank and the stern plating or frame, the outer diameter of the tube being made watertight at the aft engine room bulkhead and where it passes through the stern.
The shaft is supported in the stern tube by bearings at each end of the tube. The forward end is flanged and connects to the intermediate shaft or thrust block. At the forward end of the tube where it passes through the aft engine room bulkhead, a packing gland or mechanical seal is fitted to prevent water entering via the annulus between the tube and shaft.
The purpose of the propeller shaft is to carry the propeller. To achieve this the shaft protrudes through the stern sufficient to fasten the propeller to it. The most common fastening arrangement is to cut a taper with keyway and key in the shaft end with a threaded extension; a matching taper and keyway is cut in the bore of the propeller. The propeller is pushed on to the shaft taper and the nut is made tight; the nut is then prevented from turning by fitting a suitable locking device.
Couplings and intermediate shaft
A muff coupling is a sleeve joining two in-line shafts. Unlike flange type couplings which butt solidly together, the muff coupling allows for a gap between the joining shafts so that expansion of one shaft (due to heating) can be accommodated. It is therefore used where one of the shafts must expand longitudinally due to heating (such as steam turbine shafts). The coupling shown below is heated and shrunk on. To remove, oil is pumped under pressure into grooves assisting release of one half of the coupling.
Alternative arrangements include keyed half couplings that are a press fit on shafts or half couplings that are forged integral (are part of) with the shaft
A flange is fitted to the end of a shaft and bolted to a similar flange on the end of the adjoining shaft. The flange may be an integral part of the shaft or separate from it. If separate it has to be keyed and secured to the shaft. The mating flanges are butted and bolted together using fitted bolts with accurate alignment to avoid wear on the bolts.
Faulty couplings are rare, the problem usually being misalignment or loose bolts, the later often created by excessive vibration from misalignment. Care should be taken in alignment adjusted on the slipway with older wooden boats as the hull may alter shape when fully supported when back in the water.
Some flange type couplings (known as flexible couplings) have bolts that are secured in one half of the coupling and loose in the other. The loose half bolt has a rubber or synthetic sleeve which allows for flexibility in alignment. This type of coupling is often used as a less than ideal last resort where satisfactory alignment has not been successfully achieved in a marine shafting.
Water lubricated - The water lubricated stern tubes (as below) allows the stern tube to fill with sea water, and the shaft runs in water lubricated bushes. Some systems can also use pumped engine cooling (raw) water from the jabsco supply to inject water around the stern gland housing. A gland sealed by compressible rings of rope like gland packing material the inboard end stops too much sea-water leaking into the vessel, but some dripping is acceptable as this helps to lubricate the gland.
Some systems provide a grease nipple on the inner stuffing box. Though the grease will initially stop leaking it should be used with discretion as grit and sand can create a paste that actually wears the shaft and ultimately increases leaking. The gland can be periodically checked and tightened slightly to reduce dripping if necessary. Feel the bearing after tightening for excessive heat due to friction, and loosen if required.
The stern gland can be re-packed as the gland packing material wears away. Dig out the old gland packing carefully to avoid scoring the bearing. New gland packing coils must be cut to the correct length from the continuous lengths supplied by your marine dealer.
Consult your manufacturer’s instructions for the correct size (usually at least three coils of gland packing sized appropriately to the shaft diameter). When cutting the coils make a diagonal cut (scarf) to increase the overlap at the ends. A minimal wipe of grease on the shaft may be acceptable to ease fitting, but don’t overdo it. Tighten the gland and monitor for heat and dripping for a few days until it settles in. Re-adjust as required.
The propeller fits tightly onto a tapered shaft and is positioned with a keyway. This keyway needs to support the propeller over 1.6 x diameter of its hub length along the shaft (i.e. a shaft diameter 50 mm x 1.6 = 80 mm keyway).
Routine management - Check the stern gland for leakage. Though it is common practice to allow a slight leakage from the gland to keep it cool, many modern stern gland packings (due to their low coefficient of friction) will operate satisfactorily without leakage. Listen to the noise generated by the rotation of the propeller. Increase in noise generally means wear in the after stern tube bearing.
Oil-filled stern tubes - give better lubrication and reduce corrosion. Sea-water is kept out of the stern tube by a complex outboard seal with springs and o rings that retains the oil bath. An inboard mechanical or standard packing gland seals oil inside the stern tube to lubricate the white metal bearings. The stern tube oil header tank should be regularly checked and topped up. A pump to circulate the oil around the system may be provided and valves for venting and draining are also provided.
Routine management - Check header tank to ensure there is no loss of oil. Check oil for water dilution. Most oils are designed to emulsify if water enters the stern tube. If the forward gland is the packing type there should be no leakage. Adjust to stop leakage if necessary. Mechanical seals at the forward end are usually trouble free but should be regularly checked. A save all should be provided to catch any oil leakage thus preventing it from entering the bilges.
Mechanical shaft seals or PPS –The shaft is contained in an oil bath within a rubber boot and sealed with o rings running in grooves. These are often called drip-less shaft seals with the obvious advantage of maintaining a dry bilge. They are very common in smaller craft with small diameter shafts.
Outboard engines have a sealed, oil filled foot containing gearbox, bearings, shafts and seals. The oil level in the foot should be regularly checked and periodically changed as shown in the engine maintenance schedule. The differences between oil lubricated and water lubricated stern tubes include:
Water lubricated stern tubes -
A tried and tested sturdy system.
Simple to repair wearing parts or replace, but usually requires dry dock for repair.
No complex mechanical seals.
High wear rates at the aft bearing and shaft liner or shaft as it carries the weight of the propeller.
Carbon steel shafts have to be protected from corrosion by a sleeve or liner. On smaller ships the shaft is often made from corrosion resistant steel or monel and a liner is not required.
Oil lubricated stern tubes -
The shaft runs in oil.
Friction is reduced and more power is available at the propeller due to white metal bearings.
Less wear and longer life from reduced friction.
For carbon steel shafts, liners are not required for protection of the shaft from corrosion as the shaft is not exposed to sea water.
Initial cost is greater as additional piping.
An oil header tank is required.
After end of the stern tube requires a mechanical seal which, to ensure reliability, is of a fairly complex design. The forward end can be fitted with either a mechanical seal or conventional packing gland.
Running checks should include:
Gear box - oil level; inspect for leaks around casing; ensure pins and bolts in mechanical linkages are secure.
Shaft bearings - check oil level in bearing sump; leakage through end seals; temperature of bearing.
Stern gland - a leaking stern gland is a normal routine and is adjusted as a matter of course. See stern tubes in section above.
Propeller shaft - check noise level to determine whether it is increasing.
Regular maintenance schedules will include:
Gearboxes - Adjust clutches and operating mechanism, clean filters and clean oil coolers.
Drive train – Engine and shaft bearing moutings, loose shaft coupling bolts (perhaps due to mis-alignment) may need to be tightened. Mis-alignment of intermediate bearings will show up by overheating of the bearing.
Survey servicing and repair schedules will include:
Weardown survey- Allowance is determined by the manufacturer, but rules of thumb range from 3% of diameter as due for replacement to 6% being condemned (3% of a 50mm diameter shaft is 1.5mm permissible movement within the bearing).
Water lubricated tailshaft wear - The aft bearing is accessible via the small gap between the aft end of the stern tube and the front end of the propeller boss. If a rope guard is fitted over this gap, it must be removed. The shaft rests on the bottom half of the bearing. The gap between shaft and stern tube bearing can be measured by using long feeler gauges inserted at the top of the shaft. Many bearings have longitudinal grooves to allow water to circulate. Ensure measurement is taken at the bearing surface not the groove. If the gap above is too small to allow access of the feeler gauges another method of measuring the weardown is to clamp a dial indicator gauge to the hull with the pointer resting on the top of the shaft between the stern tube and propeller. Note the dial reading, then jack up the prop until resistance to jacking increases. Note the new reading. The difference is the weardown (the sum of the wear in the bearing and shaft wear).
Oil lubricated tailshafts wear - These shafts can have a mechanical seals at each end. The standard method of checking wear is by depth gauge. A collared plug in the gland housing or the stern tube just forward of the gland is removed. The depth from the face of the plug boss to the top of the shaft is measured. This should be compared to the original measurement when the shaft was installed. The difference is the weardown.
Checking intermediate shaft alignment