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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.

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Desmodromic poppet valve

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).

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.

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.)

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).

Disadvantages

At a time when combustion advantages of overhead valves were recognized, desmodromic valve drive seemed to be the solution for problems that arose in operating engines at higher speeds, long before computer analyses were available. Today, lift, velocity, acceleration, and jerk curves for cams have been modeled with computation to identify what limits valve motion.^{[citation needed]} As analytic methods came into use, valve adjustment, hydraulic tappets, push rods, rocker arms and valve float became largely a thing of the past,^{[citation needed]} making desmodromic valve drive a cumbersome engineering relic.

Today most engines use an overhead camshaft with symmetric cam profiles because maximum acceleration for opening valves is the same as that of catching them when closing.^{[citation needed]} Therefore, speed of lift and catch is limited only by cam contact pressure of reciprocating valve mass. This conclusion escaped discovery until computer modeling showed that limiting stresses were mirror image for beginning and end of the valve stroke. Consevation of momentum should have made this obvious without computers, but sometimes investment in an idea causes mental blindeness.^{[citation needed]}

Additional mass of the desmodromic mechanism outweighs its supposed advantage because valves cannot be raised or lowered any faster than cam-to-tappet stress allows without galling. Neither lift nor catch is affected by spring load in conventional cams because both occur at the base circle [2] where spring force is at its minimum. Additionally, desmodromic cams use curved (lever) tappets that cause higher contact pressures than a flat tappet does for the same lift profile, thereby limiting valve acceleration.

For flat spring loaded tappets the highest cam contact stress occurs at full lift when turning at zero speed (engine cranking) and diminishes with increasing speed, while the desmodromic cam has essentially no load at zero speed because, in the absence of springs, its load is entirely inertial,^{[citation needed]} increasing with speed, its greatest acceleration occurring at its smaller contact radii. Acceleration forces for both types of cams increase with the square of velocity because they result from kinetic energy [3]. ^{[citation needed]}

Desmodromic actuation was often justified by claims that springs could not close valves reliably at high speed and that the forces caused by suitably strong springs exceeded what cams could withstand. Since then, valve float was analyzed and found to be caused largely by resonance in valve springs that generated oscillating compression waves among coils, much like a Slinky.^{[citation needed]} High speed photography showed that at specific resonant speeds, valve springs were no longer making contact at one or both ends, leaving the valve floating before crashing into the cam on closure. For this reason as many as three concentric valve springs, press fit into each other, were often used, not for more force (the inner ones having no significant spring constant), but to act as snubbers to prevent spring oscillations.^{[citation needed]}

An early response to spring problems led to the use of hairpin (mouse trap) [4] springs that avoided resonance but were ungainly to locate in cylinder heads. Today's formula-one racing engines use gas springs that have no resonant parts, the bellows having a spring constant insignificant to the force of the pressurized gas. These springs are expensive and short lived, therefore, offering no benefit for most motors.

High performance valve springs are wound with varying pitch (progressive) or varying diameter, called beehive springs from their shape. [5] ^{[citation needed]} whose number of active coils varies during the stroke, the more closely wound coils being on the static end, becoming inactive as the spring compresses. In the beehive spring the small diameter coils att he top compress first. Both mechanisms prevent resonance because spring force and mass varies with stroke.^{[citation needed]} This advance in spring design removed valve float, the main justification for desmodromic valve drive.

Damage from valve float formerly occurred at the apex of lift, where a benign contact with the piston crown can be seen on most racing engines during overhaul. Overlap, when both valves are partially open, is where damaged could occur, but this is not where lift-off occurs, that being at maximum lift when only one valve is raised and is no longer an issue now that acceleration profiles are understood.^{[citation needed]}

Today desmodromic valve drive is an anachronism that, with diligence, can be made to work but at significant cost and design effort Ducati.^{[citation needed]} That overhead cams using flat tappets and spring valve closure offer advantages over desmodromic is seen in current automobile engines, none of which use desmodromic drive.^{[citation needed]} Why other motor companies are not using desmodromic valve drive is mentioned in Ducati "Motorcycle Designs".

Historical Examples

Famous examples include the successful Mercedes-Benz W196 and Mercedes-Benz 300 SLR race cars, and modern Ducati motorcycles. (see below)

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.

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.

[1] [1] Jansen Desmodromology (Retrieved 31 October 2006)

[2] 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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@@ Line 21: / Line 21: @@
 At a time when combustion advantages of overhead valves were recognized, desmodromic valve drive seemed to be the solution for problems that arose in operating engines at higher speeds, long before computer analyses were available.  Today, lift, velocity, acceleration, and jerk curves for cams have been modeled with computation to identify what limits valve motion.{{fact}}  As analytic methods came into use, valve adjustment, hydraulic tappets, push rods, rocker arms and [[valve float]] became largely a thing of the past,{{fact}} making desmodromic valve drive a cumbersome engineering relic.
-Today most engines use an [[overhead camshaft]] with symmetric cam profiles because maximum acceleration for opening valves is the same as that of catching them when closing.{{fact}}  Therefore, speed of lift and catch is limited only by cam contact pressure of reciprocating valve mass.  This conclusion escaped discovery until computer modeling showed that limiting stresses were [[mirror image]] for beginning and end of the valve stroke.{{fact}}
+Today most engines use an [[overhead camshaft]] with symmetric cam profiles because maximum acceleration for opening valves is the same as that of catching them when closing.{{fact}}  Therefore, speed of lift and catch is limited only by cam contact pressure of reciprocating valve mass.  This conclusion escaped discovery until computer modeling showed that limiting stresses were [[mirror image]] for beginning and end of the valve stroke.  Consevation of [[momentum]] should have made this obvious without computers, but sometimes investment in an idea causes mental blindeness.{{fact}}
 Additional mass of the desmodromic mechanism outweighs its supposed advantage because valves cannot be raised or lowered any faster than cam-to-tappet stress allows without [[galling]].  Neither lift nor catch is affected by spring load in conventional cams because both occur at the base circle [http://www.webcamshafts.com/pages/cam_glossary.html] where spring force is at its minimum.  Additionally, desmodromic cams use curved (lever) tappets that cause higher contact pressures than a flat tappet does for the same lift profile, thereby limiting valve acceleration.