When does valve overlap occur in the operation of an aircraft reciprocating engine?

Valve overlap is a crucial phase in the operation of an aircraft reciprocating engine where both the intake and exhaust valves are open simultaneously. This overlap occurs specifically at the end of the exhaust stroke and just before the intake stroke begins.

During this period, the exhaust gases are still being expelled from the combustion chamber when the intake valve begins to open, allowing for the fresh air-fuel mixture to enter. This design effectively enhances the engine's efficiency by increasing the flow of gases in and out of the cylinder. It promotes better scavenging of exhaust gases, which can improve the power output and overall performance of the engine.

The other scenarios listed do not represent valve overlap, as they describe points where either one valve is fully closed or where the operations of the strokes do not coincide in a manner that permits both valves to be simultaneously engaged.