Generac Power Systems 941-2 Service Manual page 37

Section 2.1
GENERAL INFORMATION
INTRODUCTION
The engine used on Impact 36 plus Series recreational
vehicle AC generators is a Generac Series GV-220,
vertical shaft, single cylinder, overhead valve type.
These engines are not equipped with a mechanical
engine governor. Instead. variable engine speeds are
controlled by a computer circuit board. The circuit
board signals a stepper motor to move the carburetor
throttle linkage.
4-CYCLE ENGINE THEORY
Figure 1. Intake Stroke
GENERAL:
Series GV-220 engines require four (4) strokes or
cycles to complete one power cycle. This is often
called the "4-stroke, 5-event" cycle. The 4 strokes and
5 events that occur are (1) Intake, (2) compression,
(3) Ignition, (4) power and (5) exhaust
INTAKE STROKE (FIGURE 1):
The intake valve is open. The exhaust valve is closed.
The piston travels downward, creating a suction
which draws the air-fuel mixture from the carburetor
into the cylinder and just above the piston.
COMPRESSION STROKE (FIGURE 2):
Figure 2. Compression Stroke
As the piston reaches bottom dead center (BDC), the
intake valve closes. The exhaust valve remains
closed, as well. The piston starts to move outward in
the cylinder. Since both valves are closed, the air-fuel
mixture in the cylinder is compressed.
POWER STROKE (FIGURE 3):
Both valves remain closed. At some point before the
piston reached top dead center (TDC), the spark plug
fires to Ignite the fuel-air mixture. The piston moves to
its top dead center position and the burning, expand-
ing gases of combustion force the piston downward.
EXHAUST STROKE (FIGURE 4):
The expanding gases of combustion force the piston
downward to its bottom dead center (BDC) position.
Figure 3. Power Stroke
The exhaust valve then opens, as the piston starts its
movement toward top dead center (TDC). Piston
movement then forces the exhaust gases out through
the open exhaust valve. The 4-stroke cycle of events
then starts over again.
Figure 4. Exhaust Stroke
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