Lesson 1/Learning Event 1
FIGURE 16. THE TWO-STROKE CYCLE.
When the piston reaches the top of its stroke, it has compressed the fuel-air mixture in the cylinder and filled
the crankcase with a fresh mixture. Note that this has taken place in half a turn of the crankshaft.
The ignition system produces a spark at the spark plug gap igniting the fuel-air mixture in the cylinder, and the
piston moves downward on the power stroke. As the piston moves downward, it decreases the space in the
crankcase which tries to push the mixture out of the crankcase through the intake valve. The valve closes,
sealing the crankcase. The descending piston now compresses the mixture in the crankcase.
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