How is valve overlap defined?

Valve overlap is defined as the period during which both the intake and exhaust valves of a cylinder are open simultaneously. This condition is strategically important in the operation of an engine, as it helps optimize the efficiency of the combustion process. During overlap, fresh air-fuel mixture enters the combustion chamber while exhaust gases are still being expelled, aiding in the scavenging process and improving performance.

The other options describe different states of the valves but do not accurately capture the concept of valve overlap. For instance, the scenario where the intake valve closes while the exhaust valve opens does not represent overlap but rather signifies the transition between the exhaust and intake strokes. In contrast, having both valves on their seats refers to a state where neither is allowing flow into or out of the cylinder, and when both are completely closed, the engine is in a non-active phase of operation.