The piston travels down on the intake stroke, up on the compression stroke, down on the power stroke and up on the exhaust stroke.
进气行程，活塞下行；压缩行程，活塞上行；做功行程，活塞下行；排气行程，活塞上行。
By the time the crankshaft completes two revolutions, each cylinder in the engine will have gone through one power stroke.
曲轴转两周，发动机里的气缸完成一个作功行程。
On the power stroke, the intake valve and exhaust valve close. The spark plug ignites the air-fuel mixture, the piston moves down.
在作功行程，进气门和排气门关闭，火花塞点燃混合气，活塞向下移动。
Power stroke. As the piston reaches TDC on the compression stroke, an electric spark is produced at the spark plug.
做功行程：当活塞到达压缩行程的上止点时，火花塞产生电火花。
Power stroke: As the fuel is compressed and the piston nears the top of the cylinder the spark plug fires and ignites the fuel and air.
作功行程：活塞处于汽缸中的上止点，火花塞点燃充分压缩后的混合气体。
Power stroke: As the fuel is compressed and the piston nears the top of the cylinder the spark plug fires and ignites the fuel and air.
作功行程：活塞处于汽缸中的上止点，火花塞点燃充分压缩后的混合气体。
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