ENGINE CYCLES

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The Four Stroke Cycle

The four-stroke cycle is so called because there are four distinct parts to each

complete cycle. The cycle keeps the piston moving upwards and downwards and the

crankshaft revolving, and the sequence is as follows:

The Induction Stroke – The piston is moving down the cylinder and sucking in clean

air through the air inlet filter, down through the inlet manifold and into the cylinder,

via the inlet valve. The inlet valve has opened for this specific operation and closes

when the piston is at the bottom and the cylinder is full of clean air.

The Compression Stroke – The piston is now moving back up the cylinder, both the

inlet valve and the exhaust valve are shut and no air can escape. The air is now being

compressed (compression) and as this occurs it heats up. When the piston is at the top

of the stroke and the air is at its most compressed state, diesel fuel is injected into the

cylinder as a high-pressure spray.

The Power stroke – The fuel and air mixture compressed within the cylinder, burn

rapidly and expand, driving and forcing the piston down, producing the power.

Marine Engineering – Third Class – Study Guide and Training Manual

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The Exhaust stroke – The piston travels back up the cylinder pushing all spent gases

out of the opened exhaust valve, which closes when the piston reaches the top of its

travel. The inlet valve then opens as the cycle starts again.

The power produced is, in part, directly related to the volume of the cylinders,

although other factors enhance performance. It is the amount of fuel/air burnt that

produces the power.

Combustion (the fire triangle) – All internal combustion engines require Heat, Fuel

and Air for combustion. Combustion takes place when energy is released by the

atomising of the fuel under pressure and bringing it into contact with the high

temperature of the compressed air. This mix of fuel, air and heat causes an increase in

temperature and pressure in the top of the cylinder. The ignition and expansion that

occur will push the piston downward.

The Two Stroke Cycle

The exhaust valve opens much earlier in the power stroke than in the case of the 4-

stroke cycle. With the piston still descending inlet ports cut into the base of the

cylinder walls are uncovered. Air enters the cylinder until the piston again covers the

inlet ports as it ascends. The exhaust valve has by this time closed so that a charge of

air is trapped in the cylinder. The piston continues to rise, compressing air until just

before the top of the stroke when the fuel is injected. The fuel is ignited by the heat of

compression and the burning gases drive the piston down in the power stroke.

NOTE: THE CRANKSHAFT WILL ROTATE TWO REVOLUTIONS TO

COMPLETE ONE FULL CYCLE IN THE 4-STROKE CYCLE AND

ONE REVOLUTION IN THE 2-STROKE CYCLE.