Four-Stroke
Cycle
The
sequence of four operating strokes (inlet, compression, power,
exhaust) of the piston in its cylinder by which most automotive
engines are powered.
Inlet stroke: With the
inlet valve open and the crankshaft rotating, the piston is moving down. As it descends, it draws in a mixture of fuel and air through the intake manifold
and past the open inlet valve.
Compression stroke: With
the inlet and exhaust valves shut, the rotating crankshaft
pushes the piston upwards, compressing the air/fuel mixture
into the combustion chamber.
Power stroke: With the valves
still shut, the spark plug generates a spark which ignites the compressed
air/fuel mixture. This burns rapidly and expands, forcing
the piston down the cylinder and rotating the crankshaft.
Exhaust stroke: The rotating
crankshaft pushes the piston back up the cylinder, forcing
the burnt gases out past the open exhaust valve and into the
exhaust manifold. As the piston reaches the top
of the cylinder, the exhaust valve closes, the inlet valve
opens and the cycle begins again. The crankshaft rotates twice
during the four strokes, and the camshaft once every four
strokes, as it only needs to open the valves once during the
cycle.
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