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Image File history File links Rockhopper. ...
Image File history File links Rockhopper. ...
How it works
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). A poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. ...
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. The internal combustion engine is an engine in which the burning of a fuel occurs in a confined space called a combustion chamber. ...
A piston and cylinder from a steam engine A cylinder in an internal combustion engine is the space within which a piston travels. ...
Computer animation of a camshaft The camshaft is an apparatus often used in piston engines to operate poppet valves. ...
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.
Advantages 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.) Valve float is a condition which occurs when the valves on an internal combustion engine do not return to the fully closed position under high rpms due to valve springs incapable of overcoming the momentum of the valvetrain. ...
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 principle of inertia is one of the fundamental laws of classical physics which are used to describe the motion of matter and how it is affected by applied forces. ...
This article or section does not cite its references or sources. ...
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).
Desmodromic valve drive seemed to be the solution for high speed engines in the days before computer analysis was available. Lift, velocity, acceleration, and jerk curves can easily be analyzed to reveal where limitations in valve control lie. As these analyses became common, valve adjustment, hydraulic tappets, and valve float became a thing of the past, and one would expect that desmodromic valve drive as an expensive relic of mistaken engineering would also fade into history.
High performance cams are symmetrical today because limiting contact pressure of lifting the valve off the base circle is identical to that of catching it again when it descends. Therefore, rapidity of lift and catch is the same and limited by the moving mass that results in cam contact pressure. In this respect, the mass of desmodromic mechanism outweighs its supposed advantages because valves cannot be raised or lowered any faster than with conventional cam and spring.
Valve float was analyzed and found to be caused by resonance in valve springs that generated oscillating compression waves among coils, much like a Slinky. High speed photography showed that at specific resonant speeds, valve springs were lifting off at both ends leaving the valve floating before crashing into the cam on closure.
Current valve springs are wound with varying pitch coils so that the number of active coils varies throughout the stroke, thereby giving the spring no resonant frequency, its active mass and force varying over the stroke.
Damage from valve float (lift off from the cam contour) is mainly at the apex of lift where a benign contact area can be furnished in the piston crown. Valve overlap, when both are partially open is where valves could be damaged but this is not where lift-off might occur, that being at maximum lift. This has also been found to not be an issue now that acceleration curves are analytically known.
In other words, Desmodromic valve motion is an anachronism that with diligence can be made to work at significant cost and design effort.
Disadvantages 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 distribution on a Ducati engine Image File history File linksMetadata Download high-resolution version (1600x1200, 369 KB) Picture of a desmodromic distribution on a Ducati engine Author Gérard Delafond File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Desmodromic valve Metadata This file...
Image File history File linksMetadata Download high-resolution version (1600x1200, 369 KB) Picture of a desmodromic distribution on a Ducati engine Author Gérard Delafond File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Desmodromic valve Metadata This file...
Historical Examples Famous examples include the successful Mercedes-Benz W196 and Mercedes-Benz 300 SLR race cars, and modern Ducati motorcycles. (see below) The Mercedes-Benz W196 was the Formula 1 entry of Mercedes-Benz in the 1954 and 1955 season, winning 10 of 14 races at the hands of Juan Manuel Fangio and Stirling Moss. ...
The Mercedes-Benz 300 SLR was a sports racing car in 1955. ...
Ducati Motor Holding (NYSE: DMH) is an Italian motorcycle manufacturer located in Bologna. ...
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. 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. Fabio Taglioni (born September 10, 1920, Lugo di Romagna, Italy; died July 18, 2001, Bologna, Italy) was chief designer and technical director of Ducati Motor Holding from 1954 until 1989. ...
Ducati Motor Holding is an Italian motorcycle manufacturer. ...
Superbike racing is a category of motorcycle road racing that employs modified production motorcycles, in the same manner that Touring car racing employs production cars. ...
Ducati Motor Holding is an Italian motorcycle manufacturer. ...
Grand Prix motorcycle racing refers to the premier category of motorcycle road racing, currently divided into three distinct classes: 125 cc, 250 cc and MotoGP (as of 2007, up to 800 cc). ...
Grand Prix motorcycle racing refers to the premier categories of motorcycle road racing. ...
See also Ducati Motor Holding is an Italian motorcycle manufacturer. ...
Engine tuning or engine building is the adjustment, modification or design of internal combustion engines to yield more performance, either in terms of power output or economy. ...
The sleeve valve is a type of valve for piston engines that has a number of advantages over the more common poppet valve, used in most engines, as well as disadvantages that have precluded its widespread adoption to date. ...
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 0-906286-07-7
External links - "Desmodromology": for further investigation and visualisation.
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