Generac Power Systems NP-40G - Manual (Page 34)

Section 2.1- GENERAL INFORMATION
Introduction
The engine used on Series NP-30G and NP-40G recre­
ational vehicle AC generators Is a Generac Series GN190
or GN220, 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
GENERAL;
Series GN190 and GN220 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):
As the piston reaches bottom dead center (BOC), 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, expanding
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 (BOC) position. The
exhaust valve then opens, as the piston starts its move­
ment 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.
TIMING;
Valve timing and Ignition timing must be precisely
controlled If the engine Is to operate properly and effi­
ciently. Intake and exhaust valves must open and close
In a precise timed sequence If the four strokes are to
occur. Ignition must occur at exactly the correct piston
position, just prior to the start of the power stroke.
Timing of valve opening and closing, as well as of spark
occurence. Is given in relation to the piston position and
the degrees of crankshaft rotation.
Ignition Is timed to occur several degrees before top
dead center (TDC) of the piston, to allow time for the
air-fuel mixture to ignite and start to bum before the
piston reaches top dead center
There must be no leakage past the valves in their
closed position or compression will not develop. Like­
wise, there must be no leakage past the piston-
Figure 3. Power Stroke