What is Valve Overlap?

         Valve overlap is the period during engine operation when both intake and exhaust valves are open at the same time.

        Valve overlap occurs when the piston nears TDC between the exhaust event and the intake event. The duration of valve overlap is between 10°-20° of crankshaft rotation, depending on the engine design. The intake valve is opened during the exhaust event just before TDC, initiating the flow of a new charge into the combustion chamber.

Valve Overlap

        As the exhaust gases are evacuated from the combustion chamber, a small but distinct low-pressure area is created on the surface of the piston head. By opening the intake valve earlier than TDC, the charge begins to fill this low-pressure area while exhaust gases exit. The low-pressure area on the head of the piston assists the fresh charge in filling the combustion chamber to its maximum capacity.

Valve Overlap Period

        Valve overlap is designed into the engine and is most useful at higher speeds. At higher speeds, the head start in filling the combustion chamber provides a substantial increase in available power. The amount of time that both valves are open is directly related to engine rpm. The higher the engine rpm, the shorter the amount of time that both valves are open. The degrees of crankshaft rotation when both valves are open do not change. Only the amount of time both valves are open varies with the engine rpm. For example, at idle, the amount of time both valves are open is relatively long compared to top no-load speed.