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What happens during the compression stroke of a gasoline engine?

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Labeled cross-sectional diagram of a V-type gasoline engine for an automobile, showing internal components such as pistons, crankshaft, camshaft, intake and exhaust valves, connecting rods, spark plugs, and intake manifold. — gasoline engine Cross section of a V-type engine.

During the compression stroke of a gasoline engine, the piston ascends within the cylinder while both the intake and exhaust valves remain closed. This action compresses the previously drawn-in mixture of gasoline vapor and air into a significantly smaller volume.1

The degree to which this mixture is compressed is determined by the compression ratio, which is the ratio of the cylinder's volume at bottom dead center to its volume at top dead center. By reducing the volume of the air-fuel charge, the engine increases its theoretical efficiency. As the piston nears the end of this stroke, the compressed charge is ignited by an electric spark from the spark plug, initiating the power stroke.2314