A gasoline engine typically operates on a four-stroke cycle. During the intake stroke, the piston slides down while the intake valve is open and the exhaust valve is closed. This creates a vacuum in the cylinder. The vacuum draws the air-fuel mixture into the engine's combustion chamber. The compression stroke prepares the air-fuel mixture for combustion. With both valves closed, the piston slides upward and compresses the trapped air-fuel mixture. The power stroke produces the energy to operate the engine. With both valves still closed, the spark plug fires, igniting the compressed air-fuel mixture. The burning mixture expands and develops pressure in the combustion chamber to push the piston downward. The exhaust stroke removes the burned gases from the combustion chamber. During this stroke, the exhaust valve is open and the intake valve is closed. As the piston slides up, it pushes the burned mixture out of the engine and into the exhaust system.
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