When does valve overlap occur in a four-stroke engine?

• A. at TDC of the combustion stroke
• B. at BDC of exhaust stroke
• C. at TDC of intake stroke
• D. at BDC of the compression stroke

Answer: (A)

Valve overlap is the brief window when both intake and exhaust valves are open at the same time. This happens around the top of the compression stroke as the engine transitions into the power stroke—the crank is essentially near the top dead center when combustion begins. At this moment the exhaust valve is still opening/closing while the inlet valve has already opened, so both valves are open together long enough for the incoming fresh charge to start filling the cylinder as remaining exhaust gas is scavenged out. That’s why the correct timing is around TDC when the cylinder is ready to fire (the moment the combustion stroke begins). The other times listed don’t put both valves open simultaneously: the exhaust stroke bottom dead center is mostly exhaust-only; the intake stroke top dead center is mainly intake-only; and the compression-bottom dead center point is when both valves are typically closed.