THE BEECHMONT CREST AUTOMOTIVE LEARNING ZONE
Four-Stroke Cycle
Most automobile engines have a four-stroke cycle, consisting of four piston strokes. These four piston strokes create one power stroke of energy that the vehicle can actually use.
The intake stroke brings gasoline and air into the engine. When this occurs, the intake valve is open and the exhaust valve is closed. The piston slides down in the cylinder, creating a vacuum that draws both air and fuel into the cylinder.
The compression stroke squeezes the air-fuel mixture. This is a preparation for combustion, as the compressed mixture is more flammable. During the compression stroke, the piston slides up in the cylinder. Both the intake valve and the exhaust valve are closed.
The power stroke occurs when the spark plug (of a gasoline engine) fires. Both valves are still closed as a result of the compression stroke, so pressure builds at the top of the piston. This pressure propels the piston downward with an intense force.
Of all the strokes, only the power stroke does not consume energy. The power stroke creates energy.
In the exhaust stroke, the piston moves upward in the cylinder. The exhaust valve is opened (the intake valve is still closed). The burned gases are pushed out of the engine and into the exhaust system.