4-Cycle Engine Theory - Generac Power Systems GN190H Service And Repair Manual

4-CYCLE ENGINE THEORY

If the engine is to run properly, four (4) events must occur
in the proper sequence and at the correct time. These
events are (a) intake, (b) compression, (c) ignition and
power, and (d) exhaust.
A
The piston is travelling from top dead center (TDC) to
bottom dead center (BDC). The cam has opened the intake
valve. The piston's downward movement in the cylinder
creates a partial vacuum in the cylinder. Air at atmos-
pheric pressure is drawn into the cylinder through the
carburetor and is mixed with fuel in the carburetor. The
fuel-air mixture flows through the open intake valve into
the cylinder. When the piston reaches BDC, the intake
stroke is over.
B
COMPRESSION
As the piston reaches bottom dead center (BDC), both the
intake and exhaust valves are closed. The piston moves
upward toward TDC and the fuel-air mixture is com-
pressed. Just before the piston reaches TDC, ignition
occurs.
INTAKE
C
IGNITION AND
By the time the piston reaches TDC , combustion is already
in progress. The intake and exhaust valves remain closed
as the expanding gases of combustion force the piston
downward.
The exhaust stroke begins when the piston has reached
BDC and has started its upward movement. The intake
valve is closed. The exhaust valve is open to let gases
escape.
8
POWER
D
EXHAUST