How is valve overlap defined in relation to crankshaft travel?

Valve overlap is defined as the period during which both the intake and exhaust valves are open at the same time. This phenomenon occurs at specific points in the engine cycle and is crucial for efficient engine performance.

When both valves are off their seats, it indicates that they are both open, allowing for a smoother transition between the exhaust phase of the cycle and the intake phase. This overlap facilitates the expulsion of exhaust gases and the subsequent admission of the fresh fuel-air mixture, enhancing scavenging and improving thermodynamic efficiency.

In contrast, the other options describe different phases of valve operation, but they do not capture the essence of overlap. Options referring to either valves being closed, only one valve being open, or focusing specifically on the closing of the exhaust valve do not accurately describe the simultaneous opening of both valves, which is the key characteristic of valve overlap.