Engine Operation - Toro 51643 Service Manual

SECTION 6 ENGINE
Engine Operation
The engine used in the TC3000, TC4000, and TC5000 is
based upon a two-stroke, port to port design. The term
"two-stroke" refers to the number of operations the
piston goes through in order to complete a single
combustion cycle. Those two operations are called an
intake stroke (where an air/fuel mix is drawn into the
combustion chamber) and an exhaust stroke (where the
exhaust gases are purged from the combustion
chamber).
The term port to port refers to the way in which the intake
and exhaust gases enter and exit the engine. These
engines use no valves, but rather, rely on the piston
passing by the intake and exhaust ports to control the
flow of gases. Hence the term: port to port.
There are four major components that are involved in the
combustion cycle:
Piston
Intake Port
Exhaust Port
Scavenger Ports
Their relative locations can be seen in Figure
8 4 .
EXHAUST PO SCAVENGER
INTAKE PORT
Figure 84
Proper operation of these engines relies on nine
separate phases. Beginning with the piston at a midway
point of its upward stroke, those nine phases are
described below.
1.
The piston closes the scavenger ports. This
prevents prevents equalization of pressures
between the lower crankcase and the combustion
chamber. See Fig. 85.
38
SCAVENGER
Figure 85
2.
As the piston continues its upward stroke, vacuum
begins to build within the lower crankcase. That
vacuum will later be used to draw the intake
mixture. See Fig.
8 6 .
VACUUM
Figure 86
3.
The piston covers the exhaust port which
completely seals the combustion chamber. See
Fig.
87.