WHAT IS VALVE OVERLAP

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In order to promote effective scavenging and cooling of combustion components in four stroke engines, large valve overlaps between the inlet and exhaust valve are provided. During this time the scavenge pressure is significantly above the exhaust manifold, hence a flow of cooling air will result.

However when the engine load falls, the scavenge air pressure also falls. As the exhaust back pressure is dictated by the frictional flow through a fixed orifice, this pressure will fall more slowly than the air input. Thus reverse flow will occur from the exhaust to the scavenge for the initial part of the overlap period until the exhaust pressure falls. This will lead to fouling of the inlet ports, and hence produce throttling of the air inlet flow. A reduced valve overlap would be more beneficial in this situation, and this can be effected by delaying the opening of the air inlet valve. Remember that it takes 30-40^o to open/close an inlet/exhaust valve, so that a valve beginning to open at say 60^o BTDC will not be fully open until 20^o BTDC. Until the valve is fully open, flow through the valve will be restricted.