4-Cycle Engine Theory - Generac Power Systems GTH760 Service 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 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. As the piston moves upward toward TDC,
the fuel-air mixture becomes compressed. Just before the piston
reaches TDC, ignition occurs.
INTAKE
(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)
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.
4
EXHAUST