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

In a four-stroke cycle reciprocating aircraft engine, both valves are open during the exhaust stroke and the intake stroke. This design allows for an efficient transition between exhaust gases being expelled from the cylinder and the next fuel-air mixture being drawn in for combustion.

During the exhaust stroke, the exhaust valve opens to allow the spent gases from the previous combustion cycle to exit the cylinder. At the end of this stroke, as the piston moves down, the intake valve also opens shortly afterward, creating a period where both valves are open. This overlap is critical because it helps use the momentum of the outgoing exhaust gases to assist in drawing in the next charge of air-fuel mixture, thus enhancing engine efficiency and performance.

The other strokes—compression and power—are characterized by both valves being closed to ensure that the air-fuel mixture is compressed properly and that the combustion can occur without any leakage or interference. Therefore, the period of overlap where both valves are open is specifically during the transition between the exhaust stroke and the intake stroke.