When the engine runs in compression stroke and power stroke, the valves must close tightly on their seats to produce a gas-tight seal and thus prevent the gases escaping from the combustion chamber.

• 当发动机处于压缩行程和做功行程时，阀门必须紧紧地关闭以产生一个不透气的气封，以防止气体逃离燃烧室。