During which strokes are both valves on a four-stroke cycle reciprocating aircraft engine open?

In a four-stroke cycle reciprocating aircraft engine, both valves are open during the exhaust and intake strokes. This occurs because, at the end of the exhaust stroke, the exhaust valve remains open briefly to allow any residual gases to escape completely. Simultaneously, during this phase, the intake valve begins to open to allow the next charge of air-fuel mixture to enter the cylinder for the subsequent intake stroke.

The design of the four-stroke cycle ensures that these overlaps are effectively timed to enhance engine performance and efficiency. This phenomenon is called valve overlap and is crucial for improving the intake of the air-fuel mixture and reducing emissions, as it allows a smoother transition between the cycles while the engine operates.

During the power and exhaust strokes, or during the intake and compression strokes, one of the valves will be closed, which is not the case during the exhaust and intake combination. Thus, the correct answer reflects the key aspect of valve timing that optimizes the efficiency of the four-stroke operation.