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。
The piston travels down on the intake stroke, up on the compression stroke, down on the power stroke and up on the exhaust stroke.
这并不意味着活塞将以两倍于正常速度上下运动，因为浪费的火花出现在排气行程中，和没有任何压缩。
This does not mean that the plugs will wear at twice the normal rate, as the wasted spark is on the exhaust stroke, and is therefore under no compression.
在压缩行程，活塞向上运动，进气门和排气门关闭，活塞压缩混合气。
On the compression stroke, the piston moves up. The intake valve and exhaust valve close. The piston compresses the air-fuel mixture.
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