In a four-stroke engine, what occurs during the power stroke?

During the power stroke in a four-stroke engine, the piston moves down the cylinder due to the force generated by the explosive combustion of the air-fuel mixture. This occurs after the mixture has been compressed in the preceding compression stroke. Once the piston reaches the top of its stroke, the spark plug ignites the compressed mixture, resulting in a rapid expansion of gases that push the piston downward. This downward motion is what ultimately drives the crankshaft, creating the mechanical power necessary for the engine to function.

The energy released during combustion not only causes the piston to move but also contributes to the overall efficiency and performance of the engine. In this context, the specific mention of the explosion highlights the critical role of combustion in generating power, emphasizing why this option accurately describes the power stroke's function in the cycle.