What do you understand by 'valve overlap'?

Valve overlap refers to the condition in an internal combustion engine where both the intake and exhaust valves are open at the same time during a portion of the engine's cycle. This simultaneous opening occurs typically at the end of the exhaust stroke and the beginning of the intake stroke. It is a critical aspect of engine performance as it allows for improved scavenging of exhaust gases and enhances the intake of fresh air-fuel mixture, contributing to more efficient operation and better power output.

In performance-tuned or high-revving engines, valve overlap can play a significant role in optimizing the engine's power band, especially at higher RPMs, by ensuring that the transition between exhaust and intake is smooth and rapid.

The other options presented do not accurately describe valve overlap. For instance, the idle state of an engine refers to its operation at low throttle and does not involve the simultaneous actuation of valves. The time taken for a piston to reach the top dead center (TDC) is related to the mechanical movement of the piston rather than the interaction between valves. Lastly, the duration of valve closure during a cycle pertains to the period when valves are not open, which is the opposite of valve overlap. Therefore, the correct understanding is that valve overlap is fundamentally about the