TM 5-4240-501-14&P
THEORIES OF OPERATION
Compression
Figure 1.
First is the intake stroke. With the exhaust valve closed
and the intake valve open, the piston moves downward
and the air-fuel mixture is drawn into the cylinder. (A -
Fig. 1)
Then, the intake valve closes, and the piston moves
upward on the compression stroke. The air-fuel mixture
becomes greatly compressed in the small space
between the top of the piston and the cylinder head. (B -
Fig. 1)
Figure 2.
The spark occurs, igniting the mixture, and the force of
the expanding gases push the piston down. This is the
power stroke. (C - Fig. 2)
The exhaust valve opens, and the upward movement of
the piston on the exhaust stroke forces
the burnt gases out of the cylinder. (D - Fig. 2) Then the
exhaust valve closes, the intake valve opens, and the
engine is ready to repeat the cycle just described. Thus
four strokes complete the cycle.
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