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15 - Engine
00 - General
00 - General
15 - 14 9806/6900-3 15 - 14
Operation
The Four Stroke Cycle
This section describes the cycle sequence, for the 6
cylinders of the diesel engine.
The stages in the four stroke cycle for each cylinder
are as follows:
Table 6. The Four Stroke Cycle
Stage number Piston operation Valve operation
1 The piston is at the top of its Com-
pression stroke and is about to
start its Power stroke.
Inlet and exhaust valves closed.
2 The piston is at the bottom of its
Power stroke and is about to start
its Exhaust stroke.
Inlet valves closed, exhaust valves
about to open.
3 The piston is at the bottom of its In-
duction stroke and is about to start
its Compression stroke.
Exhaust valves closed, inlet valves
about to close.
4 The piston is at the top of its Ex-
haust stroke and is about to start
its Induction stroke.
Valve operation exhaust valves
about to close, inlet valves about to
open.
Firing order
A cylinder is said to be firing, when the fuel/air mixture
ignites and the piston is about to start its power
stroke.
Four Stroke Cycle
Induction
As the piston travels down the cylinder, it draws
filtered air at atmospheric pressure and ambient
temperature through an air filter and inlet valves into
the cylinder.
Compression
When the piston reaches the bottom of its stroke the
inlet valves close. The piston then starts to rise up the
cylinder compressing the air trapped in the cylinder.
This causes the temperature and pressure of the air
to rise. Fuel is injected into the cylinder when the
piston is near to top dead centre.
Power
The piston continues to rise after the start of fuel
injection causing a further increase in pressure and
temperature.
The temperature rises to a point at which the fuel/air
mixture ignites. A cylinder is said to be firing, when
the fuel/air mixture ignites.
This combustion causes a very rapid rise in
both temperature and pressure. The high pressure
generated propels the piston downward turning the
crankshaft and producing energy.
Exhaust
Once the piston has reached the bottom of its travel,
the exhaust valves open and momentum stored
in the flywheel forces the piston up the cylinder
expelling the exhaust gases.
In a running engine these four phases are
continuously repeated. Each stroke is half a
revolution of the crankshaft, thus, in one cycle of a
four stroke engine, the crankshaft revolves twice.