Toro 51643 Service Manual page 50

Engine Operation (cont'd)
EXHAUST
Figure 87
4.
The upward movement of the piston builds
pressure (compression) in the combustion
chamber. See Fig. 88.
HIGH PRESSURE
VACUUM
Figure 88
5.
The lower skirt of the piston rises above the intake
port and allows a fresh air/fuel mixture to be drawn
into the lower crankcase. See Figure 89.
AIR/FUEL
MIXTURE
LOWER CRANKCASE
1
Figure 89
6. The spark plug fires before the piston reaches the
top of its stroke. This sets off combustion of the
air/fuel mixture sealed in the combustion chamber.
Note that the piston continues its upward
movement even after the plug has fired. This "post
combustion compression" continues until the
piston reaches top dead center and results in a
more complete burn. See Figure 90.
Figure 90
7. The piston then begins its downward stroke. As
this occurs, pressure begins to build in the lower
crankcase. See Figure 91.
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