Toro V Service Manual page 60

OPERATION (cont'd)
The crankshaft continues its rotation and the
piston begins its travel toward the spark plug. As
it moves, the piston seals off the transfer port
first, then the exhaust port. When both ports are
sealed, the remaining travel compresses the
air/fuel mixture to prepare it for ignition. The
travel of the piston towards the spark plug is
called the compression stroke.
The movement of the piston towards the spark
plug also has an important effect in the
crankcase. Once the transfer port is sealed, a
vacuum is created in the crankcase. This action
pulls the reed valve open and draws in a fresh
charge of the fuel/air mixture.
Just before the piston reaches top dead center
(TDC), the spark plug fires. When the engine is
functioning properly, the fuel/air mixture ignites,
causing a burn that occurs evenly through the
power stroke. The piston changes direction and
begins its travel towards the crankcase.
NOTE: I f the compression ratio is too high, the
fuel/air mixture actually explodes before it is
ignited by the sparkplug. This can be heard and
is identified as detonation (also called "knock").
When the piston opens the exhaust port,
high-pressure exhaust gases exit the exhaust
port. Further movement of the piston towards
the crankcase uncovers the transfer port which
allows a fresh charge of air/fuel mixture to enter
the combustion chamber. As the exhaust gases
continue to exit the exhaust port the engine is
readied for another cycle.
REED
VALVE
VALVE
REED
CLOSED
\
751.2.5130.817
SECOND STROKE
REED
VALVE
CLOSED,
EXHAUST GASES
_-
I - -
751.2.5130.818
9-4
Service Section