Generac Power Systems GTV99O Service Manual page 10

+CYCLE ENGINE THEORY
ftheengine isto run properly, four (4) events rnustoccur in
the proper sequence and a!the corect time. These events are
(a) intake, (b) compression, (c) ignition and power, and (d)
exhaust.
A INTAKE
The piston is travellinq from top dead center (IDC) to bottom
dead center(BDC).
The cam has opened the intake valve. The
piston s downward movementin the cylindercreatesa partial
vacuurn in the cylinder Air at atmosphefic pressure is drawn
into the cylinderthrough the carburetor and is mixed with fuel
in the ca lburetor. The fuel air mixturc flows thrcugh the open
intake valve into the cylinder When the piston reaches BDC,
the intake stoke is over.
C rcNrT|oN AND powER
By the time the piston reaches TDC , combustion is akeady
in progress. The intake and exhaustvalves
remain closed as the
exprnding gases of combustion force the piston downwad.
D EXHAUsr
The exhauststroke beginswhen the piston has rcached BDC
and has started its upward movement. The intake valve is
closed. The exhaunva ve is open to let gases escape.
B coMPREsstoN
As the piston reaches bottom de6d center (BDC), both the
intake and exhaustvalves
are closed. The piston moves upward
toward TDC and thefuel-air mixture is compressed.lust
before
the piston rcaches TDC, ignition occLirs.