8
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 INTAKE
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 atmospheric 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 compressed. Just before
the piston reaches TDC, ignition occurs.
C
IGNITION AND
POWER
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.
D EXHAUST
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.
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