| Revision as of 04:53, 24 January 2007 editJobstbrandt (talk | contribs)333 edits →Disadvantages← Previous edit | Revision as of 05:20, 24 January 2007 edit undoJobstbrandt (talk | contribs)333 edits →DisadvantagesNext edit → | ||
| Line 23: | Line 23: | ||
| Today high performance cams are overhead with symmetrical profiles because maximum acceleration for lifting valves from the base circle is identical to that of catching them when returning to the closed position.{{fact}} Therefore, speed of lift and catch is limited by cam contact pressure of reciprocating mass. It should be obvious that this condition is the same for both ends of the stroke.{{fact}} | Today high performance cams are overhead with symmetrical profiles because maximum acceleration for lifting valves from the base circle is identical to that of catching them when returning to the closed position.{{fact}} Therefore, speed of lift and catch is limited by cam contact pressure of reciprocating mass. It should be obvious that this condition is the same for both ends of the stroke.{{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 permits without galling. Neither |
Additional mass of the desmodromic mechanism outweighs its supposed advantage because valves cannot be raised or lowered any faster than cam to tappet stress permits without ]. Neither lift or catch is affected by spring load because they occur adjacent to the base circle of the cam where spring force is at its minimum. Additionally desmodromic cams use curved (lever) tappets that cause higher contact pressures than a flat tappet for the same lift profile, thereby limiting valve acceleration. | ||
| The highest contact stress on flat tappets occurs at full lift when turning at zero speed (cranking) and diminishes with increasing speed, while the desmodromic cam has essentially no load at zero speed{{fact}} and increases with speed, its greatest acceleration occurring at its smaller contact radii. Acceleration forces for both types of cams increase with the square of velocity as in the relationship (F = ½ mv²).{{fact}} | The highest contact stress on flat tappets occurs at full lift when turning at zero speed (cranking) and diminishes with increasing speed, while the desmodromic cam has essentially no load at zero speed{{fact}} and increases with speed, its greatest acceleration occurring at its smaller contact radii. Acceleration forces for both types of cams increase with the square of velocity as in the relationship (F = ½ mv²).{{fact}} | ||
| Line 36: | Line 36: | ||
| 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.{{fact}} | 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.{{fact}} | ||
| Today desmodromic valve drive is an anachronism that, with diligence, can be made to work but at significant cost and design effort.{{fact}} 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.{{fact}} |
Today desmodromic valve drive is an anachronism that, with diligence, can be made to work but at significant cost and design effort ].{{fact}} 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.{{fact}} Why other motor companies are not using desmodromic valve drive is mentioned in ] "Motorcycle Designs". | ||
| ] |
] 05:20, 24 January 2007 (UTC) | ||
| ==Historical Examples== | ==Historical Examples== | ||
Revision as of 05:20, 24 January 2007
- In general mechanical terms, the word desmodromic is used to refer to mechanisms that have different controls for their actuation in different directions.
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
 | This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Desmodromic valve" – news · newspapers · books · scholar · JSTOR (January 2007) (Learn how and when to remove this message) |
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
| This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Desmodromic valve" – news · newspapers · books · scholar · JSTOR (January 2007) (Learn how and when to remove this message) |
Desmodromic valve drive seemed to be the solution for high speed engines at a time when the combustion advantages of overhead valves were introduced, before computer analyses were available. Since then, lift, velocity, acceleration, and jerk curves for cams have been analyzed to isolate where limitations in valve control lie. As analytic methods became common, valve adjustment, hydraulic tappets, push rods, rocker arms and valve_float became largely a thing of the past, making desmodromic valve drive a cumbersome engineering relic.
Today high performance cams are overhead with symmetrical profiles because maximum acceleration for lifting valves from the base circle is identical to that of catching them when returning to the closed position. Therefore, speed of lift and catch is limited by cam contact pressure of reciprocating mass. It should be obvious that this condition is the same for both ends of the stroke.
Additional mass of the desmodromic mechanism outweighs its supposed advantage because valves cannot be raised or lowered any faster than cam to tappet stress permits without galling. Neither lift or catch is affected by spring load because they occur adjacent to the base circle of the cam where spring force is at its minimum. Additionally desmodromic cams use curved (lever) tappets that cause higher contact pressures than a flat tappet for the same lift profile, thereby limiting valve acceleration.
The highest contact stress on flat tappets occurs at full lift when turning at zero speed (cranking) and diminishes with increasing speed, while the desmodromic cam has essentially no load at zero speed and increases with speed, its greatest acceleration occurring at its smaller contact radii. Acceleration forces for both types of cams increase with the square of velocity as in the relationship (F = ½ mv²).
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 showedthat at specific resonant speeds, valve springs were lifting off at 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.
Current high performance valve springs are wound with varying pitch (progressive) 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. This prevents resonance because spring force and mass varies with stroke.
An early response to spring problems led to the use of hairpin (mouse trap) 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.
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.
Today desmodromic valve drive is an anachronism that, with diligence, can be made to work but at significant cost and design effort Ducati. 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. Why other motor companies are not using desmodromic valve drive is mentioned in Ducati "Motorcycle Designs".
Jobst 05:20, 24 January 2007 (UTC)
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).
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.
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.
See also
Sources
- 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