In conventional four-stroke engines, intake and exhaust valves are opened by a cam and closed by a return spring. Desmodromic valves use two cams, one for positively opening the valve and the other for closing it without the use of a return spring.
For racing engines on cars and motorcycles, their high revving nature makes conventional spring-loaded valves inefficient and prone to metal fatigue with sustained use at high rpms, thereby seriously limiting high performance. As maximum rpm rises, the springs in conventional valvetrains would encounter higher forces leading to increased cam drag and wear leading to eventual failure. Desmodromic valves address this problem because it does not have to overcome the static energy of the spring.
Mercedes Benz used desmodromic valves in their 1954 Formula One car as well as on the Mercedes Benz 300 SLR of 1955. Ducati developed a desmodromic valve system for their Ducati 125 Grand Prix motorcycle in 1956 to force valves to comply with the timing diagram as consistently as possible. Ducati went on to deliver a number of patents related to desmodromic valvetrains. If in a racing application, a normal valve spring had an upper limit of 10,000 rpm, that same engine would be capable of an upper limit of 15,000 rpm when equipped with a desmodromic valve, and therefore much higher power and better efficiency.
Modern Formula One engines use a pneumatic system to close valves that allow very high rpm without valve float, catastrophic failure due to valve coming in contact with the piston head, and for proper valve seating before the next combustion cycle begins.
Desmodromic valves are still used on current Ducati motorcycle engines from standard production bikes to superbikes and MotoGP bikes.
Aerospace Dynamics 