How Does The Four-Stroke Cycle Works

Four-Stroke Cycle

Automobile engines normally use a four-stroke cycle. Four separate piston strokes (up or down movements) are needed to produce one cycle (complete series of events). The piston must slide down, up, down, and up again to complete one cycle. As the four strokes are described below, study the image below.

The four stroke cycle of an automotive engine.
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1. The intake stroke draws the air-fuel mixture into the engine’s combustion chamber. The piston slides down while the intake valve is open and the exhaust valve is closed. This produces

a vacuum (low-pressure area) in the
cylinder. Atmospheric pressure (outside air
pressure) can then force air and fuel into the
combustion chamber.

2. The compression stroke prepares the air-fuel
mixture for combustion. With both valves
closed, the piston slides upward and compresses
(squeezes) the trapped air-fuel mixture.

3. The power stroke produces the energy to operate
the engine. With both valves still closed, the
spark plug arcs (sparks) and ignites the compressed
air-fuel mixture. The burning fuel
expands and develops pressure in the combustion
chamber and on the top of the piston. This pushes
the piston down with enough force to keep the
crankshaft spinning until the next power stroke.

4. The exhaust stroke removes the burned gases
from the combustion chamber. During this
stroke, the piston slides up while the exhaust
valve is open and the intake valve is closed. The
burned fuel mixture is pushed out of the engine
and into the exhaust system.
During engine operation, these four strokes are
repeated over and over. With the help of the heavy flywheel,
this action produces smooth, rotating power
output at the engine crankshaft.
Obviously, other devices are needed to lubricate the
engine parts, operate the spark plug, cool the engine, and
provide the correct fuel mixture.