Desmodromic valve
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- In general mechanical terms, the word desmodromic is used to refer to mechanisms that have different controls for their actuation in different directions.
Desmodromic valves are those which are positively closed by a cam and leverage system, rather than relying on the more conventional valve springs to close them. The term is derived from two Greek roots, desmos (controlled, linked) and dromos (course, track).
The valves in question are those in an internal combustion engine that allow the air/fuel mixture into the cylinder and (usually different ones) that allow exhaust gases out. In a conventional engine valve springs close the valves, and the camshaft (directly or indirectly) opens them. This system is satisfactory for traditional mass-produced engines that do not rev highly.[1] Steels used for valve springs were a major engine performance limitation until vacuum melt processes to extract impurities from the steel were developed in the 1950s. Even so, in the 1950s, valve springs often would fatigue and break after extended operation above 8000 rpm. The desmodromic system was devised to enable sustained high rpm operation.[2]
A desmodromic system uses extra cam lobes with rocker arms that close the valves, instead of valve springs. There is total control of the opening and closing action of the valves.
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[edit] Historical Examples
Famous examples include the successful Mercedes-Benz W196 and Mercedes-Benz 300 SLR race cars, and modern Ducati motorcycles.
Fully controlled valve movement was thought of in the earliest days of engine development, but devising a system that worked reliably and was not overly complex took a long time. Desmodromic valve systems are first mentioned in patents in 1896 by Gustav Mees, and in 1907 the Aries is described as having a V4 engine with "desmodromique" valve actuation, but details are scarce. The 1914 Grand Prix Delage used a desmodromic valve system (quite unlike the present day Ducati system). [3]
Azzariti, a short lived Italian manufacturer from 1933 to 1934, produced 173 cc and 348 cc twin cylinder engines, some of which had desmodromic valve gear, with the valve being closed by a separate camshaft.[4]
In 1956 Fabio Taglioni, a Ducati Engineer, developed a desmodromic valve system for the Ducati 125 Grand Prix, creating the Ducati 125 Desmo. The engineers that came after him continued that development, and Ducati holds a number of patents relating to desmodromics. Desmodromic valve actuation has been applied to top-of-the-range production Ducati motorcycles since 1968, with the introduction of the "widecase" Mark 3 single cylinders. Ducati motorcycles with desmodromic valves have won numerous races and championships, including World Superbike Championships from 1990-92, 1994-96, 1998-99, 2001, 2003-04 and 2006. Ducati's return to Grand Prix motorcycle racing was powered by a desmodromic-valved V4 990 cc engine, which went on to claim a one-two victory at the final 990 cc MotoGP race at Valencia, Spain in 2006.
[edit] See also
[edit] Sources
- ^ Rivola, A., et al: "Modelling the Elastodynamic Behaviour of a Desmodromic Valve Train", Proceedings of SMA2002 International Conference on Noise & Vibration Engineering,16-18 September, 2002 - Leuven, Belgium
- ^ MRS.org The Art and Materials Science of 190 mph Superbikes. Retrieved on November 2, 2006.
- ^ [1] Jansen Desmodromology (Retrieved 31 October 2006)
- ^ Title: The Illustrated Encyclopedia of Motorcycles, Editor: Erwin Tragatsch, Publisher: New Burlington Books, Copyright: 1979 Quarto Publishing, Edition: 1988 Revised, Page 81, ISBN 0-906286-07-7
