四冲程发动机的工作循环是以活塞在上止点、进气门打开为开始的。
The cycle of operation of the four-stoke engine begins with the piston at TDC and the intake valve open.
在上止点前打开进气门并在下止点增大气缸内的油气混合物之后关闭。
Opening the intake valve before TDC and closing it after BDC increase the fill of air-fuel mixture in the cylinder.
但是像进气门一样，排气门需要时间到达充分打开和关闭的位置。
But like the inlet valve, the exhaust valve needs time to reach the full-open position, it also needs time to reach the full-closed position.
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