a. TwoStroke cycle. A twostroke cycle gasoline engine is shown in
Figure 111. Every other stroke on this engine is a power stroke. Each time
the piston moves down, it is on the power stroke. Intake, compression, power,
and exhaust still take place, but they are completed in just two strokes.
Intake and exhaust ports are cut into the cylinder wall instead of being placed
at the top of the combustion chamber as in the fourstroke cycle engine. As
the piston moves down on its power stroke, it first uncovers the exhaust port
to let burned gases escape and then it uncovers the intake port to allow a new
airfuel mixture to enter the combustion chamber. Then on the upward stroke,
the piston covers both ports and at the same time, compresses the new mixture
in preparation for ignition and another power stroke. In the engine shown in
Figure 111, the piston is shaped so that the incoming airfuel mixture is
directed upward, sweeping out ahead of it the burned exhaust gases. Also,
there is an inlet into the crankcase through which the airfuel mixture passes
before it enters the cylinder. This inlet is opened as the piston moves
upward, but it is sealed off as the piston moves downward on the power stroke.
The downward moving piston slightly compresses the mixture in the crankcase,
giving the mixture sufficient pressure to pass rapidly through the intake port
as the piston clears this port. This improves the "sweepingout", or
scavenging effect of the mixture as it enters and clears the burned gases from
the cylinder through the exhaust port.
Figure 111. Events in A TwoStroke Cycle,
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