d. Exhaust stroke. After the airfuel mixture has turned, it must be
cleared from the cylinder. This is done by opening the exhaust valve just as
the power stroke is finished and the piston starts back up on the exhaust
stroke. The piston forces the burned gases out of the cylinder, past the
opened exhaust valve. The four strokes (intake, compression, power, and
exhaust) must be automatically repeated over and over in the same sequence in
each cylinder if the engine is to run (Figure 110).
Figure 110. Exhaust Stroke.
4. FourStroke and Twostroke Engines. The engines described in paragraphs 2
and 3 and illustrated in Figures 16 through 110 are a fourstroke cycle
engine. Four strokes of the piston, with two revolutions of the crankshaft and
one revolution of the camshaft are required for the complete cycle of events.
This type of engine is also called a fourstroke Ottocycle engine because it
was Dr. N.A. Otto who, in 1976, first applied the principles of this engine.
In the twostroke cycle engine, the entire cycle of events (intake,
compressions, power, and exhaust) takes place in two piston strokes.
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