(Ranger Hope © 2008, contains edits of material courtesy of A.N.T.A.  Publications.)

            1 Marine Engines                             

            2 Gearboxes and Stern-tubes        

            3 Engine Cooling Systems             

            4 Engine Lubricating Systems       

            5 Engine Fuel Systems                   

            6 Routine Checks                            

            7 Engine Maintenance                    

            8 Troubleshooting    

1.1 Diesel and Petrol Engines

Diesel and Petrol engines are internal combustion engines. They burn a fuel/air mixture in ‘combustion chambers’ or ‘cylinders’ inside the engine, to produce power from the rotating engine.

Heat is taken away from the engine by the oil, cooling water and exhaust. Poisonous fumes, produced by the combustion process, are collected and removed by the exhaust system.

‘Diesel’ engines use diesel oil as a fuel. Diesel oil is also known as ‘diesel’ or ‘distillate’. Diesel oil burns strongly, but it is safer to handle than petrol.

Diesel engines are generally heavier than petrol engines, but:

·        they produce a lot of ‘lugging’ or ‘working’ power at lower speeds

·        they are generally more efficient, with better fuel economy

·        they are generally more reliable over a longer operating life

All these characteristics make diesel engines more suitable for commercial vessels.

‘Petrol’ engines use petrol, (or petrol containing a special lubricating oil in a ‘petroil’ mix), as fuel. Petrol engines may use more fuel, and wear out more quickly, but they are often light-weight, high-speed ‘zippy’ engines–so they are very popular for high speed recreational boats, specific purpose commercial vessels (abalone boats, etc) and tenders.


Petrol is a highly flammable fuel which will form fumes at air temperature. Petrol fumes may explode if ignited.

Great care must be taken when handling petrol fuels.

In all internal combustion engines, four essential operations occur over and over.  These are:

1.         Induction:        Fresh air or fuel/air mixture is drawn into the cylinder
                                    ready for combustion (burning or firing)

2.         Compression:            The air or fuel/air mix is squeezed or compressed into a
                                    small space at the top of the cylinder. This makes it
                                    more explosive.

3.         Combustion:  The fuel/air is ignited, and combustion (burning)
                                    produces hot expanding gas to drive the piston down in
                                    the cylinder. This turns the crankshaft to drive the load.

4.         Exhaust:                     Burnt gases are driven from the cylinder through the
                                    exhaust system.

1.2 Typical Engine Components

Typical engine components are shown in Figure 1 below:

Figure 1: Typical internal components of an engine

In Figure 1, the crankshaft turns in main bearings.  The piston, which is connected to the crankshaft by means of a connecting rod, moves up and down in the cylinder as the crankshaft turns.  With the piston as low as it will go in the cylinder, it is said to be at bottom dead centre (bdc).  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).  Oval ‘cams’ on the camshaft open the inlet and exhaust valves at the correct time by means of push-rods and rockers.  (An inlet 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 camshafts and valves can be very different for different engine designs.)

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).

1.3 Two-stroke and four-stroke engines

Diesel and petrol engines can be either   ‘two-stroke’  or   ‘four-stroke’ cycle designs:

In four-stroke engines,  the four essential operations (induction compression, combustion and exhaust) occur one after the other,  in four separate ‘strokes’ of the piston  (down,  up,  down,  up). The four-stroke cycle takes two complete turns (720° rotation) of the crankshaft.

In two-stroke engines,  the four essential operations occur in two ‘strokes’ of the piston (down,  up). The two-stroke cycle occurs in one complete turn (360° rotation) of the crankshaft. This means that a two-stroke engine has twice as many power strokes as a four-stroke engine at the same speed, so it may produce more power. However, there may be losses in fuel efficiency and reliability.

1.3.1 Firing Order

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

1.4 Principles of Marine Diesel Engines

1.4.1 Operation of Four-Stroke Marine Diesel Engines

There are many different designs for diesel four-stroke cycle engines. Each has it’s own special features and characteristics. However, the general operation of a diesel four-stroke cycle engine is outlined on the following pages. 

Induction Stroke

When the piston is at about top dead centre (tdc), the inlet valve is opened by the cam on the camshaft as the crankshaft rotates, and the induction stroke begins ......

As the crankshaft rotates, the piston goes down in the cylinder (as shown in the diagram).

Fresh air rushes into the cylinder through the inlet port and inlet valve,  to fill the space left by the piston as it goes down. When the piston reaches about bottom dead centre (bdc),  the inlet valve is closed,  trapping the charge of fresh air. The induction stroke is then complete.

Figure 3: Compression Stroke


Compression Stroke

At bdc, with both valves shut, the compression stroke begins .......

As the crankshaft rotates, the piston moves up the cylinder toward tdc,  and the air trapped in the cylinder is compressed (to about 5% of its normal volume) by the rising piston.

The compression of the charge of air heats it up to about 500°C,  ready for the power stroke.

       Most of us have felt this effect!

Have you ever felt a pump get hot as you pumped up a tyre?

 How hot is 500°C ?

(Pretty Hot!   Steel heated to about 600°C would be just starting to glow!)


Figure 4:

At about tdc, the compression stroke is complete.

Combustion or ‘Power’ Stroke

Just before tdc, with both valves shut and hot compressed air trapped in the cylinder, the combustion or ‘power’ stroke begins......

An ‘injector pump’ which is driven by the camshaft, forces a small measure of diesel fuel into the cylinder at high pressure, through an ‘injector’.  The injector breaks the fuel up into a fine mist which mixes with the hot compressed air,  causing it to burst into flames (ignite).

As the piston passes tdc,  the burning gas expands, driving the piston toward bdc. This rotates the crankshaft and drives the flywheel (and the load).

The mass (weight) of the flywheel is very important!   The energy contained in the heavy rotating flywheel will keep the engine rotating smoothly until the next power  stroke.

At about bdc, the power stroke ends.



Figure 5:  Exhaust Stroke

Exhaust Stroke

Just before the piston reaches bdc, the exhaust stroke begins......

The exhaust valve is opened by a cam on the camshaft, and the pressure of the burnt gases is released into the exhaust.

As the piston reaches bdc and begins to rise,  the piston pushes all the remaining burnt gases out of the cylinder through the exhaust valve.

As the piston reaches tdc,  the exhaust valve is closed, and the exhaust stroke is complete.

At this point, the inlet valve is opened by it’s cam,  ready for the induction stroke of  the next four-stroke ‘cycle’.

1.4.2 Operation of Two-Stroke Diesel Engines

There are many different designs for diesel two-stroke cycle engines, and each has it’s own special features and characteristics. However, the general operation of a diesel two-stroke cycle engine is outlined on the following pages.

Figure 6: Exhaust & Induction
                  of Fresh Air

Stroke 1 - Upward

Exhaust and Induction of Fresh Air.

With the piston approaching bdc, the exhaust valve is opened by its cam. (Some engines have multiple exhaust valves to let burned gases out more quickly.) The pressure of combustion is released into the exhaust.

The inlet ‘ports’ which are machined through the bottom of the cylinder wall, are then uncovered by the piston.

Pressure from a ‘blower’ forces fresh air into the cylinder through the inlet ports, and drives the burnt gases out of the cylinder through the exhaust valves.

As the piston begins to rise, first the exhaust valves are closed, then the inlet ports are covered by the piston, locking in the charge of fresh air.

Exhaust and Induction are now complete.

Stroke 1 - Upward

Figure 7:  Compression almost complete.

Compression, and start of power stroke.

The piston continues to rise in the cylinder. This compresses the trapped air, and makes it hot.

Injection and Start of Power Stroke.

Just before tdc as compression is completed,  a measured quantity of fuel is injected into the hot compressed air,  and it ignites.

Figure 8: Injection and Power Stroke

Stroke 2 - Downward

Power Stroke.

After injection, the expanding burning gas drives the piston down the cylinder on the power stroke, turning the crankshaft and flywheel.

Start of Exhaust and Induction.

Before the piston reaches bdc, the exhaust valves are opened by gears and cams to release the burnt gases into the exhaust.

The inlet ports are then uncovered by the descending piston as it approaches bdc.  Fresh air is forced into the cylinder to replace the burnt gases.  As the piston passes bdc, the next two-stroke cycle begins (see previous page).

Figure 9:  ‘Scavenging’


Scavenging is the name given to removing all the burnt gases from the cylinder.

Burnt gases left in the cylinder will reduce the power and efficiency of the engine.

Two-stroke engines do not use the piston to force burned gas from the cylinder, but rely on the incoming air to push it out.

Improving the airflow through the engine with a blower, and careful airflow design, improves scavenging.


‘Blowers’ and ‘Intercoolers’

Blowers force air into the engine on the induction stroke for better ‘breathing’. They increase the power and efficiency of the engine, and also help to cool two-stroke engines, which tend to run hotter than four-strokes.

Blowers may be ‘superchargers’ or  ‘turbo-chargers’.

Superchargers are driven directly by the engine. They are geared up to spin at 1½ to 2 times the engine speed (perhaps up to about 8000 r.p.m.)

Turbochargers are driven by a turbine which is powered by the engine exhaust.  Some turbochargers may spin at up to 100,000 r.p.m.  Balance and lubrication is critical under these conditions.

At high throttle settings, turbo vanes reach high temperatures in the blast of flame from the exhaust. If the engine is instantly shut down from high speed operation, this heat from the turbo vanes will ‘soak’ into the bearings and lubricating oil. The bearings may be damaged, and the oil burned by this heat. Due to its speed, the turbo might also ‘run-on’ for a substantial time after the engine lubricating system has stopped. This may also cause damage. To prevent these problems, hot engines should be allowed to idle for about 10 to 15 minutes before they are switched off, to allow the engine and turbo to cool down.

Blowers tend to heat the air by compression. ‘Intercoolers’ may be used to cool the air from the blower before it enters the engine. This increases the efficiency, and allows the engine to run cooler.

1.4.3 Timing of Valves and Injection

Valve and injection timing will vary widely for different engine designs, but both must be set exactly for any engine–even a small timing error can stop the engine, perhaps even 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 ‘cycle’.

In a four-stroke engine, there are 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.

In 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.

1.5 Petrol Engines

Petrol engines (mainly ‘outboards’), are often used in recreational boats, but are also used in specific purpose commercial vessels, tenders etc.

Petrol ‘inboard’ engines are rarely used, except for recreational vessels which may use ‘marinised’ car engines.

1.5.1 Operation of Four-stroke Cycle Petrol Engines

Four-stroke cycle petrol engines are similar in operation to four-stroke diesel engines, with some differences.

Review the operation of the four-stroke diesel engine now, to make sure you clearly remember how it operates.

Four-stroke petrol engines have the following major differences to diesels:

Induction Stroke:  In most petrol engines, fuel is mixed with air in a ‘carburettor’. The fuel/air mixture is drawn into the cylinder during induction, instead of pure air. Some petrol engines may use injection, however.

Power Stroke: At the start of the power stroke, the petrol/air mix is ignited by means of an electrical ‘ignition’ system and a ‘spark plug’, instead of the heat of compression.  (The compression ratio of petrol engines is lower than diesel engines.)

The timing of the spark ignition for four-stroke petrol engines is normally taken off the camshaft.

The operating strokes of a four-stroke petrol engine can be outlined as:

Induction Stroke     Piston going down in cylinder, inlet valve open. Fuel/air mixture is drawn into the cylinder.

Compression Stroke            Piston rising toward tdc, both valves shut. Fuel/air mixture is compressed in the cylinder.

Power Stroke           Electrical ignition system causes an electrical spark to jump the gap between the spark plug electrodes. The spark ignites the compressed fuel/air mixture. The piston is driven down by the expanding gases.

Exhaust Stroke       Piston Rising, exhaust valve open. The burnt gases are driven into the exhaust by the rising piston.

Operation of Two-stroke Cycle Petrol Engines

Two-stroke engines are often used for outboard engines, and may be used for small auxiliary engines (pumps, generators etc).

Instead of conventional valves, two-stroke petrol engines most often use ‘ports’ cut into the cylinder wall.  These are opened and closed by the piston as it moves up and down inside the cylinder.

The crankcase is normally used as part of the fuel path. A two-stroke petrol engine is usually lubricated by adding two-stroke oil to the fuel (petroil mix). Alternatively, an oil injection system may inject oil into the engine at pressure. (This system does not re-use the oil–an oil supply tank is regularly topped up.)

Figure 10:

Upward Stroke

Compression stroke & Induction of fuel/air into the crankcase

The piston rising from bdc, covers the transfer and exhaust ports in the cylinder wall, and then uncovers the intake port into the crankcase. (Figure 10)

The trapped fuel/air mixture is compressed in the top of the cylinder (Compression)

At the same time,  the rising piston creates suction in the crankcase. This suction draws fuel/air mixture from the carburettor,  through the intake port,  to fill the crankcase. (Induction into crankcase).

Figure 11:

Downward Stroke

Power stroke & compression of fuel/air in the crankcase

As the piston reaches tdc, the spark ignites the fuel/air mixture forcing the piston down on the power stroke. (Combustion)

The descending piston closes the intake port, and then compresses the fuel/air mixture trapped in the crankcase. (Figure 11)

Figure 12:

Figure 13:

Downward Stroke

Exhaust stroke and transfer of fuel/air mix into the cylinder

When the piston approaches bdc,  it uncovers the exhaust port in the cylinder wall,  allowing the burned gases to escape through the exhaust port. (Figure 12)

Shortly after, the transfer port in the cylinder wall is uncovered, and the compressed fuel/air in the crankcase rushes through the transfer port into the cylinder (Induction stroke and scavenging). (Figure 13)

The shape of the engine components (combustion chamber, ports, and piston crown) are designed so the incoming fuel/air charge drives out any remaining burned gases without too much of the fresh charge escaping through the exhaust port. (Figure 13)

Many different port, combustion chamber, and piston designs have been used by different manufacturers to try to improve scavenging, engine efficiency, and power.

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. These allow the fuel air charge to get into the crankcase quickly, but prevent it leaking back out.

Petrol two-stroke engines are simple, and powerful for their light weight, but they are normally less efficient and less reliable than either four-strokes or diesel two-strokes.

2 Gearboxes and Stern Tubes

2.1 Reversing Gearboxes

Smaller commercial vessels are normally equipped with a ‘Thrust Reversing Gearbox’ which has three gear positions–‘Ahead’,  ‘Neutral’, and ‘Astern’.

This allows the vessel to navigate and maneuver safely.

An example of a typical thrust reversing gearbox is shown in Figure 14

Figure 14: ‘Marine Gear’, thrust reversing Gearbox

Reduction gear-sets are often included in a gearbox to match the engine and propeller speed ranges.

With different gearboxes, the gears may be changed:

·        mechanically, using the movement of the gear lever to change the gears, either directly, or through a cable or linkage.

·        electro-mechanically, using a switch to operate electrical solenoids which change the gears

·        hydraulically, using mechanical clutches operated hydraulically, or by using gearboxes with hydraulic operation (more like car automatic gearboxes).

Oil levels in gearboxes must be regularly checked according to the maintenance schedule.

The gearbox control and engine throttle (speed) may use separate levers for thrust and speed, or be combined in a single control lever (see Figure 15).

Figure 15: Throttle and Gear Control Arrangements

2.2 Stern Tubes

The stern tube contains the propeller shaft, and shaft bearings and seals. The stern tube may be water lubricated, or oil-filled.

With water lubricated stern tubes (Figure 16), the stern tube fills with sea water, and the shaft runs in water lubricated bushes.

A gland at the inboard end stops too much sea-water leaking into the vessel, but some weepage is acceptable as this helps to lubricate the gland. The gland can be periodically checked and tightened slightly to reduce weepage if

necessary, and re-packed as the packing material wears away.

Figure 16 Water lubricated stern tube

Oil-filled stern tubes (Figure 17) give better lubrication and reduce corrosion. Sea-water is kept out of the stern tube by an outboard seal. An inboard seal holds oil inside the stern tube to lubricate the bearings. The stern tube oil header tank should be regularly checked and topped up.


Figure 17:  Oil-filled stern tube

The propeller shaft must be correctly lined up with the gearbox shaft. Misalignment will cause noise and vibration while the shaft is running. Gearbox and shaft bearings will overheat and collapse more quickly.

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.

3 Engine Cooling Systems

The ‘engine cooling system’ is really a ‘temperature control system’. It keeps the engine temperature within the best operating range,  at all speeds and under all load conditions.  It should not keep the engine cold.

Engine cooling systems may use air or liquid coolant to take heat from the engine.

3.1 Air Cooling


Figure 18: Air Cooling

Air cooling (Figure 18) is seldom used for marine engines, but may be used on auxiliary engines.

A fan on the engine shaft, blows air past ‘cooling fins’ on the engine castings.  Metal cowling concentrates the air flow where it is most effective for removing heat.

Maintenance is reduced to keeping air cowls and cooling fins free of dust and rubbish.

Air cooled engines should be operated in open, well ventilated   areas.

3.2 Liquid Cooling (Water Cooling)


Figure 19: Liquid Cooling

Liquid cooling (Figure 19) is usually called water cooling, but due to the rust and corrosion caused by water, other liquids are often used.

Liquid coolants pass through special water ‘galleries’ in the engine castings, to cool the engine. 

Liquid cooling systems often consist of:

·        Pump/s to push the coolant through the system

·        Engine coolant galleries

·        Thermostat to allow a cold engine to quickly reach operating temperatures

·        A cooling device may be used to cool down the hot coolant

Several different types of liquid cooling systems are used (these are outlined on the following pages).

3.2.1 Direct or Raw Water Cooling


Figure 20: Raw Water Cooling

This is a simple method of liquid cooling. Raw (sea) water is pumped from outside the vessel, though the engine cooling system, and back out again (see Figure 20).

This method is normally used for outboard engines, but is seldom used for larger engines because of the corrosive nature of sea water.  In outboards, the water pump is mounted in the drive foot of the engine.

Intake strainers must be regularly cleaned, and pump impellers will periodically need to be replaced.

3.2.2 Indirect or Closed Circuit Cooling


Figure 21: Closed Circuit Cooling

This method (Figure 21) is similar to automotive systems.

The coolant in the engine is pumped around and around through a cooling circuit,  consisting of:

·        a circulating pump

·        the engine cooling galleries

·        temperature regulating thermostat

·        a device to take heat away from the coolant

The coolant takes the heat from the engine and carries it to a cooling device which removes heat from the coolant. The coolant then goes back to the engine to remove more heat.

In this system, only clean anti-corrosive coolant passes through the engine.

There are several ways to remove heat from the coolant:

·        Keel cooling. Piping carries the coolant through the hull and along underneath the vessel, then back to the engine. As the hot coolant passes through the pipe, it is cooled by the sea-water outside.

·        Skin Tank cooling for metal vessels. A metal tank is formed against the bottom skin of the vessel. The coolant passes through the tank, and is cooled by the hull in the sea water.

·        Keel and skin tank systems are both fully closed.  There are no sea water strainers to block, but marine growth on the outside of the keel pipes and hull reduces the effectiveness of both systems.

·        Heat Exchanger cooling (see Figure 22).  Hot coolant flows in a closed circuit from the engine, through the cooling tubes inside a heat exchanger, and back to the engine. Raw sea water is pumped from outside the vessel via a separate system, and through the heat exchanger to cool the coolant. The raw water then returns to the sea.
This system needs two separate pumps–one for the coolant, and one for raw sea-water. This system is not affected by marine growth, but the raw water system and strainer must be kept clean.

The raw water flows through the cooling tubes in the top section.

Figure 22: Heat Exchanger Cooling

The heat exchanger in Figure 23 is designed to remove heat from the coolant and from the engine lubricating oil.

Figure 23: Typical Heat Exchanger

Hot coolant flows into the top section, and around the cooling tubes where heat is removed by the raw water. The (cooled) coolant then flows down through the tubes in the bottom section and back out to the engine.

Engine lubricating oil flows around the tubes in the bottom section, and is cooled by the coolant on its way through the heat exchanger.

The oil retains some warmth as cold raw water is not used to cool it.

Like automotive systems, indirect or closed marine cooling systems may operate under pressure.

If you remove the pressure cap while the engine is hot, the drop in pressure in the cooling system can instantly boil the coolant and spray boiling liquid over you.

Do not remove the pressure cap while the engine is hot!


3.3.3 ‘Thermostat’ for Temperature Regulation

In direct cooling systems, the thermostat (Figure 24) will reduce–but not stop–the flow of water while the engine is cold. This allows the engine to warm up quicker.

In indirect or closed systems, the thermostat bypasses the heat exchanger, so the coolant only circulates through the engine. When the engine is warm, the thermostat closes the bypass, and sends the coolant to the heat exchanger.

All systems must keep some coolant flowing through the engine, so it does not boil and cause local hot spots in the engine.

Figure 24: Bellows Type Thermostat

Several different types of thermostats are made, using:

·        Wax Elements

·        Bellows

·        Bi-metal springs

All of them operate in a similar way, but it is essential that exactly the right type be fitted to the engine, to prevent temperature problems.

Occasionally a thermostat will stick.

A sticking thermostat may cause the engine to take too long to warm up, or it may cause overheating.

Where the engine is overheating while under way, a stuck thermostat can be temporarily removed (after the engine cools down), to get you back home.

Either way, the sticking thermostat must be changed for the correct replacement as soon as possible.

3.3.4 Water Circulating Pumps

Water pumps may be positive displacement pumps, or non-positive pumps.


Figure 25: Positive Flexible Impeller Pump

Positive pumps work on the principle of spaces between the vanes expanding at the input (so they suck in liquid), and shrinking at the output (so they force the liquid out). Flexible rubber impellers are one way of doing this (Figure 25).

Non-positive pumps use a simple vaned impeller spinning at high speed.

Water is drawn in at the centre and flung around to the outlet by centrifugal force. Non-positive pumps must be almost full of water to allow them to pump.

Pumps may use mechanical seals or packing to minimise leakage.

The sea water inlet to cooling systems includes a sea-cock to close the inlet and a strainer and weed-trap. These must be regularly checked and cleaned.

Cooling system pumps should not be run dry as this may damage impellers and/or seals.

Some flexible impeller pumps may also be damaged by turning them backwards, as this tends to turn the vanes ‘inside out’, cracking the rubber.

3.3.5 Temperature Indicators

Temperature gauges or warning lights may be used for engine coolant temperature, engine oil temperature, transmission oil temperature, even hydraulic oil temperature.

Temperature gauges and warning lights may be operated:

·        mechanically using a ‘Bourdon’ tube. (A coiled ‘Bourdon’ tube unrolls and turns a gauge pointer as it heats up.)

·        electrically using electrical ‘sender’ units. Senders may:

-    vary the current through an electrical gauge

-    have a switch which operates at a critical temperature to light

     a warning light

4 Engine Lubrication Systems

Lubricating oil has three main functions:

·        it keeps working surfaces apart to minimise friction and heat under conditions of temperature,  pressure,  and contamination

·        it helps to carry heat, grit and contaminants away from machine surfaces

·        it increases the seal between moving components  (for example, piston to cylinder gas seal,  which is needed to maintain compression).

4.1 Force Feed Systems

Except for petrol two-strokes, and small four-stroke auxiliary engines, most marine engines use a ‘force feed’ lubrication system (Figure 23). Typical components of a force feed system are:

·        oil supply tank or sump

·        oil pump

·        oil filter

·        oil galleries feeding critical engine components

·        oil cooler

·        pressure and temperature gauges, if required.

In the force feed lubrication system, oil is pumped from a sump or tank, filtered, and supplied under pressure through oil galleries to critical components (bearings, etc.) of the engine. Less critical parts of the engine (gears, chains, etc.) are lubricated by oil splashed from bearings and other oil fed parts.

The used oil drains into the sump of the engine, where it is returned to the oil pump or supply tank, to be used over and over again.

Most force feed systems have the oil reserve stored in the sump of the engine (like car engines). These are called ‘wet-sump’ lubrication systems. Others have the oil reserve stored in an external supply tank. These are called ‘dry-sump’ systems

The system in Figure 26 uses a ‘wet sump’. The oil supply is stored in the sump below the engine. A pump in the sump circulates the oil to the engine. After lubricating the engine, used oil drains back to the sump to be constantly re-circulated through the engine.

Figure 27 shows a ‘dry sump’ system— oil is returned to the external oil tank by a scavenge pump. The scavenge pump is larger than the delivery pump to stop oil ‘pooling’ in the sump. Dry sump systems have larger oil tank capacity, and are more reliable in heavy seas. The oil also remains cooler, and is less likely to leak past engine seals.

Figure 26: ‘Wet-Sump’ Force Feed Lubrication System

Figure 27: ‘Dry-Sump’ Force Feed Lubrication System

The oil delivery pump can develop extremely high pressures. A relief valve on the oil pump will open to limit the oil pressure, to prevent damage to the engine.
If the oil filter blocked up, no oil would get to the engine. The bypass valve will let unfiltered oil past a blocked filter to supply the engine. (Slightly dirty oil will do less damage than no oil at all.)

Acids and contaminants build up in the oil. Always replace the oil and filters when indicated by the service schedule, or earlier if they need it.

Use the correct type and grade of lubricant, and never mix them. Oils and greases with different chemical bases may interact and cause expensive damage.

4.2 Lubrication of Two-Stroke Engines

Diesel two-stroke engines normally have a conventional ‘wet-sump’ force feed system as just described.

Most two-stroke petrol engines are lubricated by ‘petroil’ or ‘outboard’ mix as it passes through the crankcase during induction. To ensure proper lubrication, and to prevent starting and running problems, you must make sure that:

·        only the correct type of two-stroke oil is used

·        the correct quantity of oil is added to the petrol

·        the petrol and oil are thoroughly mixed together before being used

A few high performance two-strokes, force feed a small amount of undiluted oil directly into the bearings and critical components from an oil supply tank. This is called ‘oil injection’. The used lubricating oil is eventually burned with the fuel, and the oil tank must be occasionally topped up to replace the lost oil.

Straight petrol is used with oil-injected two-strokes.

5 Engine Fuel Systems

5.1 Diesel Fuel Systems

Figure 28: An overview of typical fuel system components.

Typical components of a marine diesel fuel system are shown in Figure 28.


Main Tank/s. Mounted low in the vessel– high tanks may affect vessel stability. Emergency shut-off valve to allow the fuel supply to be cut off outside the engine room, in the event of emergency. Vent pipe (high) with a flash trap allows the tank to ‘breathe’. Drain tap allows water and sludge to be drained from the tank. Filters may be fitted in the filler neck and fuel pipe. Standard filler coupling, filling line and valve to allow tank to be filled safely. Fuel gauge may be fitted, but sounding rods are more reliable and accurate.

Service tanks with sludge drain valve and low fuel level alarm may be fitted .

Filters. The primary filter (sludge trap) settles out water and sediment. It may be periodically drained (into a container). The secondary fuel filter is a fine element type filter which must be changed according to service schedule.

Pumps. The fuel lift pump may be an engine driven mechanical pump or an electric pump to lift fuel at low pressure from the tank to the injector pump. The hand priming pump allows the injector pump to be primed, and ‘bleeding’ of the fuel system if necessary. The injector pump is timed to inject a squirt of fuel into each cylinder at the correct moment for combustion. Injector pump timing is critical, and must be set by an expert.

Good Injector - Full Spray Pattern

Faulty Injectors - Deflected Spray Pattern, and Non-atomised Stream

Injectors. Deliver the fuel as a flammable mist into the cylinders at the moment of injection. Spray patterns for typical good and faulty injectors,  are shown in Figure 29.

Figure 29:

Spray patterns from good and faulty injectors

Faulty injectors will often cause starting problems, poor performance, black exhaust smoke, and unburned oil as droplets in the exhaust.