当活塞上行压缩气缸中的混合气体，火花塞点燃气体燃烧，通过气缸上另一个孔排气。

As the piston moves up it is compressed, the spark plug ignites combustion, and exhaust exits through another hole in the cylinder.

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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.

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