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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. The term desmodromic is derived from two Greek roots, desmos (controlled, linked) and dromos (course, track).

The valves in question are those in a typical otto cycle internal combustion engine that allow the air/fuel mixture into the cylinder at the beginning of the cycle and allow exhaust gases to be expelled at the end of the cycle. In the conventional otto cycle engine, a spring is used to apply pressure to valve and return it to the valve seat or closed position. The valve is either directly or indirectly opened by the camshaft.

Ideal

The common valve spring system is satisfactory for traditional mass-produced engines that do not rev highly and which design requires low maintenance.[1] Materials used for valve springs, at the period of initial desmo development, were a major engine performance limitation due to springs breaking from fatigue. Vacuum melt processes developed in the 1950’s helped remove impurities in the steel used for the springs, though valve springs often would still fatigue and break after extended operation above 8000 RPM. Thus the desmodromic system was devised to enable sustained high RPM operation.[2]

Design and History

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]

The Mercedes-Benz W196 Formula One racing car of 1954-55, and the Mercedes-Benz 300SLR sports racing car of 1955 all had desmodromic valve actuation.

In 1956 Fabio Taglioni, a Ducati engineer, developed a desmodromic valve system for the Ducati 125 Grand Prix, creating the Ducati 125 Desmo.

He was quoted to say…

The specific purpose of the desmodromic system is to force the valves to comply with the timing diagram as consistently as possible. In this way, any lost energy is negligible, the performance curves are more uniform and dependability is better.

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.

How it Works

A desmodromic valve system uses an extra set of cam lobes within each valve conglomerate to not only open but also close the valve during the engine cycle. Each valve contains two shims, one for the opening and one for the closing of each valve. This allows for adjustments of the valve as tolerance varies with engine wear. These clearances are usually kept set within a few thousandths of an inch. A clip is placed on the shaft of the opening rocker that may be removed allowing the rocker arm to slide laterally and expose access to the opening shim. If the valve is then held in place and the engine is rotated, the closing rocker arm will expose access to two collets holding the closing shim in place within a notch on the valve, thus allowing removal. The closing shim will slide up and off the valve. The shims can then be replaced for either a too tight or too loose valve. There is one small spring used in the desmodromic system, but it is merely to remove any rotational lash out of the system and keep the system taut.

File:Bluming desmo 8x6.jpg

Benefits

In modern engines, valve spring failure at high RPMs has been mostly remedied. The main benefit of the desmodromic system is the prevention of valve float. In traditional spring valve actuation, as engine speed increases, the spring loses its ability to close the valve completely before the piston reaches TDC (Top Dead Center). This can lead to several problems. First, and most catastrophic, the piston will collide with the valve and thus damage it permanently. Second, the valve will not completely return to its seat before combustion begins. This allows cylinder gases and pressure to escape prematurely, which causes a major decrease in engine performance. In valve spring engines the traditional remedy for valve float is to make the springs stronger. This increases the seat pressure of the valve, which is the static pressure that holds the valve closed. This is of benefit at higher engine speeds. The drawback is that to open the valve the engine has to work harder. This causes greater pressure (hence temperature and wear) in the valve actuation mechanism. The desmodromic system avoids this problem, because although it has to work against the directional energy of the valve opening and closing, it does not have to overcome the static energy of the spring.

Examples

Famous examples include the successful Mercedes-Benz W196 and Mercedes-Benz 300 SLR race cars, and most commonly, modern Ducati motorcycles.

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 V4 990 cc engine, which went on to claim several victories, including a one-two finish at the final 990 cc MotoGP race at Valencia, Spain in 2006. With the onset of the 800cc era in 2007, they are generally still considered to be the most powerful engines in the sport, and have powered Casey Stoner to the 2007 MotoGP Championship and Ducati to the constructors championship.



See also

Sources

  1. ^ 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
  2. ^ "MRS.org The Art and Materials Science of 190 mph Superbikes" (PDF). Retrieved 2006-11-02.
  3. ^ [1] Jansen Desmodromology (Retrieved 31 October 2006)
  4. ^ 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
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