Which statement best describes the period when intake and exhaust valves are open?

• A. It is the time frame when both intake and exhaust valves are open
• B. It is when only the intake valve is open
• C. It is when both valves are closed
• D. It is when the piston is at BDC

Answer: (A)

Valve overlap is the moment when both the intake and exhaust valves are open at the same time. This occurs briefly around the top of the compression stroke, as the piston is near top dead center. During this overlap, exhaust gases are still leaving while fresh air-fuel mixture starts entering, helping push out spent gases and improve cylinder charging, especially at higher RPMs. The statement describes exactly that simultaneous opening of both valves, which is why it’s the best description. If only the intake valve were open, you’d be in the intake portion of the cycle; if both valves were closed, you’d be in the compression or power portion; and the piston being at bottom dead center doesn’t define the overlap period.