What is it called when both valves are open during an engine cycle?

The situation where both the intake and exhaust valves are open simultaneously during an engine cycle is known as valve overlap. This overlap occurs at the end of the exhaust stroke and the beginning of the intake stroke, allowing for better exhaust scavenging and improved intake flow. During this phase, the engine can expel the remaining exhaust gases while simultaneously drawing in fresh air-fuel mixture, which enhances the overall efficiency and performance of the engine.

The other terms relate to different aspects of valve operation. Valve clearance refers to the space between the valve stem and the rocker arm, ensuring that the valve properly seats when closed. Valve timing pertains to the precise moment the valves open and close in relation to the position of the piston, which is crucial for optimizing engine performance throughout the RPM range. Finally, a lift event refers to the physical movement of the valve off its seat, but it does not imply that both valves are open at the same time. Thus, valve overlap is the correct term to describe the phenomenon where both valves are open during an engine cycle.