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Monday, October 22, 2012

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
- 36 -
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.

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