Valve overlap occurs when a piston in a cylinder is located at ____________.

Valve overlap refers to the period in the four-stroke engine cycle when both the intake and exhaust valves are open simultaneously, allowing for the expulsion of exhaust gases while fresh air-fuel mixture enters the cylinder. This occurs specifically when the piston reaches the top dead center (TDC) position.

At TDC, the piston is at its highest point in the cylinder, which is critical during this phase of the cycle. The exhaust valves begin to close just before the piston reaches TDC, and the intake valves begin to open just as the piston reaches TDC. This strategic timing promotes better scavenging of exhaust gases, enhances engine efficiency, and helps achieve smoother transitions between the exhaust and intake strokes.

The other positions mentioned—after top dead center (ATDC), before top dead center (BTDC), and below bottom dead center (BBDC)—do not align with the conditions needed for valve overlap. At ATDC, the exhaust valve is no longer open, while BTDC and BBDC reference positions where only one valve (either intake or exhaust) is active, thereby missing the critical simultaneous operation that defines valve overlap.