The engine is the heart of the car as it
supplies the power that drives the wheels.
Controlling the combustion of
the air/fuel mixture is a very important and complex process. It requires that
four essential steps take place at the correct time and in the correct sequence.
This has to happen in every cylinder of the engine. This process is known as the
four-stroke cycle.
The steps include:
The name "four-stroke" is derived from the number of
piston strokes required to complete the combustion cycle. A stroke is the
movement of the piston from its highest position in the cylinder
(top-dead-center) to its lowest (bottom-dead-center); or from the lowest to
highest position. The four strokes are called intake, compression, power, and
exhaust. Each cylinder of the engine completes the combustion cycle at a
different time.
The intake stroke is the first of the four strokes in the
combustion cycle. As the piston moves away from the top of the cylinder, the
intake valve opens. The downward movement of the piston creates a vacuum
(negative pressure) in the cylinder. The greater pressure outside the cylinder
(ambient pressure) pushes a mixture of air and fuel into the cylinder. Just
after the piston reaches the bottom of the cylinder, the intake valve closes.
The second stroke in the four-stroke cycle is the
compression stroke. The compression stroke begins as the piston starts to move
upward in the cylinder. The intake valve closes, trapping the air-fuel mixture
in the cylinder. Upward movement of the piston compresses the air-fuel mixture
into a very small area.
Compressing or squeezing the mixture is very
important for developing maximum power. The higher the compression, the greater
pressure exerted on the piston when the air/fuel mixture is ignited. Compression
also "pre-heats" the mixture which helps it to burn more efficiently.
The third stroke in the four-stroke cycle is the power
stroke. The power stroke begins as the compressed air/fuel mixture is ignited in
the combustion chamber. A spark plug, located in the cylinder head, creates an
electrical spark in the combustion chamber which ignites the air/fuel mixture.
The burning fuel rapidly expands, creating a very high pressure against the top
of the piston.
This pressure drives the piston downward. The downward
motion provides the power that turns the crankshaft and drives the wheels to
propel the vehicle. Up and down movement of the piston on all four strokes is
converted to rotary motion by the crankshaft. 
The final stroke of the cycle is the exhaust stroke. As
the piston approaches the end of the power stroke, the exhaust valve opens.
Pressure in the cylinder causes the exhaust gases to rush past the valve and
into the exhaust system. The piston moves up the cylinder, pushing most of the
remaining exhaust gases from the cylinder. As the piston nears the top of this
stroke, the exhaust valve begins to close as the intake valve begins to open.
The exhaust stroke completes the combustion process. The opening of the
intake valve signals the beginning of a new cycle. This cycle occurs in every
cylinder and will be repeated as long as the engine is running. 