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 engines that do not rev highly.
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.
The primary benefit of desmodromic (abbreviated to "desmo") systems is to improve valve timing at higher engine revolutions. On very high-revving valve spring engines, the spring does not always have enough force to keep the valve in contact with the camshaft lobe. This is called "valve float". To a point, this can be compensated for by stiffer valve springs, but at the cost of increased wear and power consumption. (In some engines valve float from over revving can result in valves damaging pistons.)
A desmodromic valve system camshaft can have steeper opening and closing ramps on its lobes, as the inertia of a quickly opening valve is kept in check by the closing camshaft lobe; and likewise a quickly closing valve can not bounce off the valve seat since it is retained by the opening camshaft lobe. The Desmo system makes the valve movement conform precisely to the camshaft profile, with no opportunity to stray. The benefits of this system are only found at high engine rpms, and would normally only be considered necessary for racing and high performance applications.
The more precise valve control allows higher valve acceleration and deceleration (without risk of collision between valves and piston), the elimination of "valve float" at high rpm , and lower friction (partly due to the lack of valve spings).
The disadvantages of the desmodromic valve system are the higher service cost and shorter service intervals, due to the high mechnical precision required to set up the more complex system, and there are possible cold starting issues. There are more moving parts involved, and more adjustments to be made, than in a comparable valve spring engine. However, once the parts have all "run in", there is very little wear in the system, and clearance adjustment intervals increase. Neglecting the valve adjustment intervals on a Desmo is an invitation to disaster.
Desmodromic valve actuation has been applied to top-of-the-range Ducati motorcycles since 1968, with the introduction of the "widecase" Mark 3 single cylinders. Ducati motorcycles with desmo valves have won numerous races and championships, including the 2006 World Superbike Championship and a one-two finish at the final 2006 Moto Grand Prix race at Valencia. Other famous examples include the successful Mercedes-Benz W196 and Mercedes-Benz 300 SLR race cars.
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). [1]
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.[2]
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.
[edit] See also
[edit] Sources
- ^ [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 0906286077
