On a 4 stroke engine, what happens during the compression stage?

During the compression stage of a 4-stroke engine, both the intake and exhaust valves close, and the piston moves upward in the cylinder. This upward movement of the piston compresses the air-fuel mixture that was drawn in during the intake stroke. The compression is crucial because it increases the efficiency of combustion when the mixture is ignited in the following power stroke. A higher compression ratio typically leads to more power output and improved engine efficiency.

The choice indicating that both valves close while the piston moves up is correct as it reflects the essential mechanics of this stage, where the engine is preparing for the combustion that will occur immediately afterward. Properly sealing the combustion chamber by closing both valves ensures that the maximum pressure can be achieved, leading to an effective combustion event.

The other options do not accurately describe the functioning of the engine during this critical stage. For instance, having only one valve open or keeping both valves open would disrupt the necessary containment and compression needed for the upcoming power stroke.