Why are both valves closed during the compression stroke of a four-stroke 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.

In a four-stroke cycle engine, both the intake and exhaust valves must remain closed during the compression stroke to create a sealed environment within the cylinder. This closure is essential to allow the piston to compress the ignitable mixture of gasoline vapor and air into a smaller volume before it is ignited by a spark.1

If the valves were not closed, the pressure required for efficient combustion could not be maintained, and the fuel mixture would escape or fail to reach the necessary compression ratio, which is critical for engine performance and efficiency. Keeping the valves sealed ensures that the energy released during the subsequent power stroke is directed toward moving the piston rather than being lost through open ports.21