Definitions

Desmodromic_valve

Desmodromic valve

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 four-stroke 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 four-stroke engine, a spring is used to apply pressure to the 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 are of a design that requires low maintenance. At the period of initial desmo development, valve springs were a major limitation on engine performance because they would break from metal fatigue. Vacuum melt processes developed in the 1950s helped remove impurities in the steel used to make valve springs, although after sustained operation above 8000 RPM often springs would still fail. The desmodromic system was devised to remedy this problem. Furthermore, as maximum RPM increases, higher spring pressure is required to return the valve, leading to increased cam drag and higher wear on the parts at all speeds, problems addressed by the desmodromic mechanism.

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).

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.

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.

In 1959, the Maserati brothers introduced one of their final designs: a desmodromic four cylinder, 2000cc engine for their last O.S.C.A. Barchetta.

Benefits

In modern engines, valve spring failure at high RPM 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 inertia of the valvetrain overcomes the spring's ability to close the valve completely before the piston reaches TDC (Top Dead Centre). 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 and can cause the valve to overheat, possibly causing warping or catastrophic valve failure or both. In valve spring engines the traditional remedy for valve float is to make the springs stronger (stiffer). 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 by reducing the aformentioned valve float. The drawback is that to open the valve the engine has to work harder. The higher spring pressure 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.

Disadvantages

Before the days when valve drive dynamics could be analyzed by computer, desmodromic drive seemed to offer solutions for problems that were worsening with increasing engine speed. Famous examples of successful desmodromic engines were Mercedes-Benz W196 and Mercedes-Benz 300 SLR racing cars. Since those days, lift, velocity, acceleration, and jerk curves for cams have been modeled by computer to reveal that cam dynamics are not what they seemed. With proper analysis, valve adjustment, hydraulic tappets, push rods, rocker arms, and above all, valve float, became things of the past...without desmodromic drive.

Today most automotive engines use overhead cams, driving a flat tappet to achieve the shortest, lightest weight, and most inelastic path from cam to valve, thereby avoiding elastic elements such as pushrod and rocker arm. Computers enabled accurately designing acceleration profiles, the most important dynamic of valve motion, because it defines forces from F=Ma (Force equals Mass times Acceleration).

Before numerical computing methods were readily available, acceleration was only attainable by differentiating cam lift profiles twice, once for velocity and again for acceleration. This generates so much hash (noise) that the second derivative (acceleration) was uselessly inaccurate. Computers permitted integration from the jerk curve, the third derivative of lift, that is conveniently a series of contiguous straight lines whose vertices can be adjusted to give any desired lift profile.

Integration of the jerk curve produces a smooth acceleration curve while the third integral gives an essentially ideal lift curve (cam profile). With such cams, that mostly do not look like the ones "artists" formerly designed, valve noise (lift-off) went away and valve train elasticity came under scrutiny.

Today's cams have mirror image (symmetric) profiles with identical positive and negative acceleration while opening and closing valves. An asymmetric cam either opens or closes valves more slowly than it could, speed being limited by Hertzian contact stress between curved cam and flat tappet from accelerating the mass of valve, tappet and spring.

In contrast, desmodromic drive uses two cams per valve, each with separate rocker arm (lever tappets). Maximum valve acceleration being limited by cam-to-tappet galling stress, is governed by moving mass and cam contact area. Rigidity and contact stress are best achieved with conventional flat tappets and springs whose lift and closure stress is unaffected by spring force, both occurring at the base circle where spring load is minimum and contact radius is largest. Curved (lever) tappets of desmodromic cams cause higher contact stress than flat tappets for the same lift profile, thereby limiting rate of lift and closure.

With conventional cams, stress is highest at full lift, when turning at zero speed (engine cranking), and diminishes with increasing speed as inertial force of the valve counter spring pressure, while a desmodromic cam has essentially no load at zero speed (in the absence of springs), its load being entirely inertial, and therefore increasing with speed. However, its greatest inertial stress bears on its smallest radius. Acceleration forces for either method increase with the square of velocity resulting from kinetic energy.

Desmodromic valve drive 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. 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, today as many as three concentric valve springs are sometimes nested inside one other; not for more force (the inner ones having no significant spring constant), but to act as snubbers to reduce oscillations in the outer spring.

An early solution to oscillating spring mass was the mousetrap or hairpin spring used on Norton Manx engines. These avoided resonance but were ungainly to locate inside cylinder heads. Today, formula-one racing engines use gas springs that have no resonant parts, their working parts having an insignificant mass compared to the force of their compressed gas. These springs are expensive and short lived, therefore, offering no benefit for most engines.

Valve springs that do not resonate are progressive, wound with varying pitch or varying diameter called beehive springs from their shape. The number of active coils in these springs varies during the stroke, the more closely wound coils being on the static end, becoming inactive as the spring compresses or as in the beehive spring, where the small diameter coils at the top are stiffer. Both mechanisms reduce resonance because spring force and its moving mass vary with stroke. This advance in spring design removed valve float, the initial impetus for desmodromic valve drive.

Overhead cams using flat tappets and springs offer advantages over desmodromic drive in current automotive engines. Currently the only major manufacturer using desmodromic drive is Ducati, in motorcycle engines. Reasons for not using a desmodromic approach are increased maintenance and valve noise which can be uncomfortably loud in engines with four or more cylinders.

Controversy

While the desmodromic system is not the most ideal in a practical world of mechanics, it still survives and performs without problem to this day. While it can be more expensive to maintain than traditional spring actuated valve systems, there are many aftermarket precision machined components which can extend the maintenance interval to that of the almost magical spring actuated systems (in comparable motorcycles).

While newer, high performance, pneumatic systems may follow more specific design and engineering specifications (computer aided) they are typically limited to race only applications (Formula 1, Moto GP, etc). Currently, there is no method of determining longevity or extended maintenance intervals of such systems in practical, everyday, systems such as the automobile.

While the design can be noisy, it is typically overridden by road noise from tires and other engine components such as intake and exhaust noise. Though stated above the noise is "uncomfortably loud in engines with four or more cylinders", if true, this is limited (in terms of Ducati) to the MotoGP and MotoGP Race Replica bikes which are the only current production desmodromic motors that feature four cylinders; they are intended for race use. (It should be noted that exhaust noise levels can exceed 110dB on full race systems.)

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 Superbike World 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 in the GP3 (Desmosedici) bike, 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 with the GP7 (Desmosedici) bike.

See also

Sources

External links

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