Desmodromic valve - AbsoluteAstronomy.com

A desmodromic valve
Poppet valve
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 shaft guides the plug portion by sliding through a valve guide...

is a reciprocating engine valve that is positively closed by a cam and leverage system, rather than by a more conventional spring. The term desmodromic derives 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

Cylinder (engine)

A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...

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

Camshaft

A camshaft is a shaft to which a cam is fastened or of which a cam forms an integral part.-History:An early cam was built into Hellenistic water-driven automata from the 3rd century BC. The camshaft was later described in Iraq by Al-Jazari in 1206. He employed it as part of his automata,...

Etymology

The word itself comes from the Greek

Greek language

Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

words desmos (δεσμός, translated as "bond" or "knot") and dromos (δρόμος, "road" or "way"). Denoting this way the major characteristic of the valves being continuously bound to the camshaft, a tied way.

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

Vacuum Induction Melting

Vacuum induction melting is a process for melting metal under vacuum conditions using electromagnetic induction. It works by creating electrical eddy currents in the metal which heats the "charge" to melt it...

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 conceived during 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 Ariès

Ariès

The Ariès was a French automobile manufactured by a company in Asnières-sur-Seine from 1903 to 1938. The first cars were two- and four-cylinder vehicles built 20 chassis at a time in a large factory. These shaft-drive cars had a rather unusual double rear axle, while the engines were built by Aster...

is described as having a V4 engine with "desmodromique" valve actuation, but details are scarce. The 1914 Grand Prix Delage

Delage

Delage was a French luxury automobile and racecar company founded in 1905 by Louis Delage in Levallois-Perret near Paris; it was acquired by Delahaye in 1935 and ceased operation in 1953.-History:...

used a desmodromic valve system (quite unlike the present day Ducati

Ducati Motor Holding

Ducati Motor Holding S.p.A. is a motorcycle manufacturer in Bologna, Italy. It produces motorcycles for both road use and motorcycle racing.- History :...

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

Mercedes-Benz W196

The Mercedes-Benz W196 was the Mercedes-Benz Formula One entry in the and Formula One seasons, winning 9 of 12 races entered in the hands of Juan Manuel Fangio and Stirling Moss....

Formula One

Formula One, also known as Formula 1 or F1 and referred to officially as the FIA Formula One World Championship, is the highest class of single seater auto racing sanctioned by the Fédération Internationale de l'Automobile . The "formula" designation in the name refers to a set of rules with which...

racing car of 1954-55, and the Mercedes-Benz 300SLR sports racing car of 1955 both had desmodromic valve actuation.

In 1956, Fabio Taglioni

Fabio Taglioni

Fabio Taglioni was an Italian engineer.Born in Lugo di Romagna, he was chief designer and technical director of Ducati from 1954 until 1989. His desmodromic L-twin design is still used in all current Ducati motorcycle engines...

, 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

Maserati Brothers

The Maserati Brothers were involved with automobiles from the beginning of the 20th century. All were born to Rodolfo Maserati and his wife Carolina in Voghera, Lombardy, Italy. Rodolfo was a railway worker from Piacenza, driving a heavy Krupp locomotive, and married Carolina Losi...

introduced one of their final designs: a desmodromic four cylinder, 2000cc engine for their last O.S.C.A.

O.S.C.A.

Officine Specializzate Costruzioni Automobili - Fratelli Maserati SpA was an Italian brand of sports car automobiles, usually abbreviated to O.S.C.A., OSCA or Osca.- History :...

Barchetta.

Comparison with conventional valvetrains

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

Valve float

Valve float is an adverse condition which occurs when the poppet valves on an internal combustion engine valvetrain do not remain in contact with the camshaft lobe during the valve closure phase of the cam lobe profile...

at high rpm. It has the primary disadvantages of complexity, since there are more components, and lack of understanding, which prevents people from straying from the well-known conventional valvetrain with its valve springs.

In traditional sprung-valve actuation, as engine speed increases, the momentum of the valve will eventually overcome the spring's ability to close it completely before the piston reaches TDC (Top Dead Centre). This can lead to several problems. First, and most damaging, the piston

Piston

A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...

collides with the valve and both are destroyed. Second, the valve does not completely return to its seat before combustion begins. This allows combustion gases to escape prematurely, leading to a reduction in cylinder pressure which causes a major decrease in engine performance. This can also overheat the valve, possibly warping it and leading to catastrophic failure. In sprung-valve engines the traditional remedy for valve float is to stiffen the springs. This increases the seat pressure of the valve (the static pressure that holds the valve closed). This is beneficial at higher engine speeds because of a reduction in the aforementioned valve float. The drawback is that the engine has to work harder to open the valve at all engine speeds. The higher spring pressure causes greater friction (hence temperature and wear) in the valvetrain.

The desmodromic system avoids this problem, because it does not have to overcome the static energy of the spring. It still needs to work against the momentum of the valve opening and closing, and that force still depends on the effective mass of the moving parts. The effective mass of a traditional valve with spring includes one-half of the valve spring mass and all of the valve spring retainer mass. However, a desmodromic system must deal with the moment-of-inertia of the two rocker arms per valve, so this advantage depends greatly on the skill of the designer. Another disadvantage is the contact point between the cams and rocker arms. It is relatively easy to use roller tappets in conventional valvetrains, although it does add considerable moving mass. In a desmodromic system the roller would be needed at one end of the rocker arm, which would greatly increase its moment-of-inertia and negate its "effective mass" advantage. So desmo systems have generally needed to deal with sliding friction between the cam and rocker arm and therefore may have greater wear. The contact points on most Ducati rocker arms are hard-chromed to lessen this wear issue. Another possible disadvantage is that it would be very difficult to incorporate hydraulic valve lash adjusters in a desmodromic system, so the valves must be periodically adjusted.

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

Mercedes-Benz W196

The Mercedes-Benz W196 was the Mercedes-Benz Formula One entry in the and Formula One seasons, winning 9 of 12 races entered in the hands of Juan Manuel Fangio and Stirling Moss....

and Mercedes-Benz 300 SLR

Mercedes-Benz 300 SLR

The Mercedes-Benz 300 SLR was a sportscar racing car for the 1955 World Sportscar Championship season, which it won.-Technical highlights:...

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 tappet

Tappet

A tappet is the noise made by a worn cam follower, however the term is widely used to represent the cam follower itself. In mechanical engineering it is a projection which imparts a linear motion to some other component within an assembly...

s, push rods, rocker arms, and above all, valve float

Valve float

, became things of the past without desmodromic drive.

Today most automotive engines use overhead cams

Overhead camshaft

Overhead cam valvetrain configurations place the engine camshaft within the cylinder heads, above the combustion chambers, and drive the valves or lifters in a more direct manner compared to overhead valves and pushrods...

, 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

Rocker arm

Generally referred to within the internal combustion engine of automotive, marine, motorcycle and reciprocating aviation engines, the rocker arm is a reciprocating lever that conveys radial movement from the cam lobe into linear movement at the poppet valve to open it...

. Computers have allowed for fairly accurate acceleration modelling of valvetrain systems.

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

Mirror image

A mirror image is a reflected duplication of an object that appears identical but reversed. As an optical effect it results from reflection off of substances such as a mirror or water. It is also a concept in geometry and can be used as a conceptualization process for 3-D structures...

(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

Galling

Galling usually refers to the adhesive wear and transfer of material between metallic surfaces in relative converging contact during sheet metal forming and other industrial operations....

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 counters 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. Its greatest inertial stress bears on its smallest radius. Acceleration forces for either method increase with the square of velocity resulting from kinetic energy

Kinetic energy

The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...

.

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

Slinky

Slinky or "Lazy Spring" is a toy consisting of a helical spring that stretches and can bounce up and down. It can perform a number of tricks, including traveling down a flight of steps end-over-end as it stretches and re-forms itself with the aid of gravity and its own momentum.-History:The toy was...

. 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

Norton Manx

The Norton Manx or Manx Norton is a British racing motorcycle that was made from 1947 to 1962 by Norton Motors Ltd. A Norton had contested every Isle of Man TT race from the inaugural 1907 event through into the 1970s, a feat unrivalled by any other manufacturer, and the development and honing of...

engines. These avoided resonance but were ungainly to locate inside cylinder heads.

Valve springs that do not resonate are progressive

Poppet valve

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 shaft guides the plug portion by sliding through a valve guide...

, 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

Valve float

, the initial impetus for desmodromic valve drive.

Controversy

While the desmodromic system is not ideal in a practical world of mechanics, it still survives and performs without problem. While it can be more expensive to maintain than traditional spring actuated valve systems, many aftermarket precision machined components can extend the maintenance interval to that of 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 and are intended for racing. (Note that exhaust noise levels can exceed 110 dB on full race systems.)

Examples

Famous examples include the successful Mercedes-Benz W196

Mercedes-Benz W196

The Mercedes-Benz W196 was the Mercedes-Benz Formula One entry in the and Formula One seasons, winning 9 of 12 races entered in the hands of Juan Manuel Fangio and Stirling Moss....

and Mercedes-Benz 300 SLR

Mercedes-Benz 300 SLR

The Mercedes-Benz 300 SLR was a sportscar racing car for the 1955 World Sportscar Championship season, which it won.-Technical highlights:...

race cars, and most commonly, modern Ducati

Ducati Motor Holding

Ducati Motor Holding S.p.A. is a motorcycle manufacturer in Bologna, Italy. It produces motorcycles for both road use and motorcycle racing.- History :...

motorcycles.

Ducati motorcycles with desmodromic valves have won numerous races and championships, including Superbike World Championships from 1990 to 1992, 1994–96, 1998–99, 2001, 2003–04, 2006 and 2008. Ducati's return to Grand Prix motorcycle racing

Grand Prix motorcycle racing

Road Racing World Championship Grand Prix is the premier championship of motorcycle road racing currently divided into three distinct classes: 125cc, Moto2 and MotoGP. The 125cc class uses a two-stroke engine while Moto2 and MotoGP use four-stroke engines. In 2010 the 250cc two-stroke was replaced...

was powered by a desmodromic V4

V4 engine

A V4 engine is a V form engine with four cylinders and three main bearings.-Automobile use:Lancia produced several narrow-angle V4 engines from the 1920s through 1960s for cars like the Lambda, Augusta, Artena, Aprilia, Ardea, Appia, and Fulvia....

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 800 cc era in 2007, they are generally still considered to be the most powerful engines in the sport, and have powered Casey Stoner

Casey Stoner

Casey Stoner is an Australian professional motorcycle racer. Born in Kurri Kurri, New South Wales, Australia and raised in Southport, Queensland, Stoner raced from a young age and moved to the United Kingdom to pursue a racing career...

to the 2007 MotoGP Championship and Ducati to the constructors championship with the GP7 (Desmosedici) bike.

External links

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