Desmodromic valve

A desmodromic valve is a reciprocating engine poppet valve that is positively closed by a cam and leverage system, rather than by a more conventional spring.

In general mechanical terms, the word desmodromic is used to refer to mechanisms that have different controls for their actuation in different directions.

The valves in a typical four-stroke engine allow the air/fuel mixture into the cylinder at the beginning of the cycle and exhaust spent gases at the end of the cycle. In a conventional four-stroke engine, valves are opened by a cam and closed by return spring. A desmodromic valve has two cams and two actuators, for positive opening and closing without a return spring.