On which stroke of a four-stroke cycle reciprocating engine are both valves open?

In a four-stroke cycle reciprocating engine, both the intake valve and the exhaust valve are open during the intake and exhaust stroke, also known as the overlap phase. This occurs at the end of the exhaust stroke and the beginning of the intake stroke, allowing for a more efficient transition between these two processes.

During this phase, the exhaust gases are expelled while fresh air-fuel mixture is being drawn into the cylinder. This overlap between the two strokes helps improve engine efficiency by enhancing the scavenge of exhaust gases, reducing the amount of unburned fuel remaining in the combustion chamber and allowing for a better air-fuel mixture to enter.

Other strokes, such as the compression stroke and the power stroke, do not involve both valves being open simultaneously. The compression stroke requires the intake valve to be closed to build pressure in the cylinder, while the power stroke occurs after the combustion of the air-fuel mixture is completed, with both valves also closed to maximize pressure from combustion. The exhaust stroke primarily focuses on expelling gases, with the intake valve closed. Thus, the only time both valves are open is during the intake and exhaust stroke, making it the correct response.