In general mechanical terms, the word desmodromic is used to refer to mechanisms that have different controls for their actuation in different directions.
Desmodromic poppet valve in Ducati engine
A desmodromic valve is a reciprocating engine poppet valve that is positively closed by a cam and leverage system, rather than by a more conventional spring.
The valves in a typical four-stroke engine allow the air/fuel mixture into the cylinder at the beginning of the cycle and exhaust spent gases at the end of the cycle. In a conventional four-stroke engine, valves are opened by a cam and closed by return spring. An engine using desmodromic valves has two cams and two actuators, each for positive opening and closing without a return spring.
Etymology
The word comes from the Greek words desmos (δεσμός, translated as "bond" or "knot") and dromos (δρόμος, "track" or "way"). This denotes the major characteristic of the valves being continuously "bound" to the camshaft.
Idea
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.[1] At the period of initial desmodromic development, valve springs were a major limitation on engine performance because they would break from metal fatigue. In the 1950s new vacuum melt processes helped to remove impurities from the metal in valve springs, increasing their life and efficiency greatly. However, many springs would still fail at sustained operation above 8000 RPM.[2] The desmodromic system was devised to remedy this problem by completely removing the need for a spring. Furthermore, as maximum RPM increases, higher spring force is required to prevent valve float, leading to larger springs (with increased spring mass, and thus greater inertia), cam drag (as the valve springs require energy to compress, robbing the engine of power), and higher wear on the parts at all speeds, problems addressed by the desmodromic mechanism.
Design and history
Desmodromic poppet valve example
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.[citation needed] Austin's marine engine of 1910 produced 300bhp and was installed in a speedboat called "Irene I"; its all-aluminium, twin-overhead-valve engine had twin magnetos, twin carburettors and desmodromic valves.[3] The 1914 Grand Prix Delage and Nagant (see Pomeroy "Grand Prix Car") used a desmodromic valve system (quite unlike the present day Ducati system).[4]
Azzariti, a short-lived Italian manufacturer from 1933 to 1934, produced 173cc and 348cc twin-cylinder engines, some of which had desmodromic valve gear, with the valve being closed by a separate camshaft.[5]
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:
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 held 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.
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 at high rpm.
In traditional spring-valve actuation, as engine speed increases, the inertia of the valve will eventually overcome the spring's ability to close it completely before the piston reaches top dead centre (TDC). This can lead to several problems. First,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. Second, and most damaging, the piston collides with the valve and both are destroyed. In spring-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 force of the spring. It must still overcome the inertia of the valve opening and closing, and that depends on the mass distribution of the moving parts. The effective mass of a traditional valve with spring includes one-half of the valve spring mass for symmetric springs and all of the valve spring retainer mass. However, a desmodromic system must deal with the 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. Thus, 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 reduce this wear. 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, but this is true of typical performance oriented motorcycles as valve lash is typically set using a shim under a cam follower.
Before the days when valve drive dynamics could be analyzed by computer,[when?] desmodromic drive seemed to offer solutions for problems that were worsening with increasing engine speed. Since those days, lift, velocity, acceleration, and jerk curves for cams have been modelled by computer[6] to reveal that cam dynamics are not what they seemed. With proper analysis, problems relating to valve adjustment, hydraulic tappets, push rods, rocker arms, and above all, valve float, became things of the past without desmodromic drive.
Today[when?] 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 have allowed for fairly accurate acceleration modelling of valve-train 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, which mostly do not look like the ones "artists" formerly designed, valve noise (lift-off) went away and valve train elasticity came under scrutiny.
Today,[when?] most cams have mirror image (symmetric) profiles with identical positive and negative acceleration while opening and closing valves. However, some high speed (in terms of engine RPM) motors now employ asymmetrical cam profiles in order to quickly open valves and set them back in their seats more gently to reduce wear. As well, production vehicles have employed asymmetrical cam lobe profiles since the late 1940s, as seen in the 1948 Ford V8.[7] In this motor both the intake and exhaust profiles had an asymmetric design. More modern applications of asymmetrical camshafts include Cosworth's 2.3 liter crate motors, which use aggressive profiles to reach upwards of 280 brake horsepower.[8] An asymmetric cam either opens or closes the valves more slowly than it could, with the speed being limited by Hertzian contact stress between curved cam and flat tappet, thereby ensuring a more controlled acceleration of the combined mass of the reciprocating componentry (specifically the valve, tappet and spring).
In contrast, desmodromic drive uses two cams per valve, each with separate rocker arm (lever tappets). Maximum valve acceleration is limited by the cam-to-tappet galling stress, and therefore is governed by both the moving mass and the cam contact area. Maximum rigidity and minimum contact stress are best achieved with conventional flat tappets and springs whose lift and closure stress is unaffected by spring force; both occur at the base circle,[9] where spring load is minimum and contact radius is largest. Curved (lever) tappets[10] 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 (initiation of 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.[11]
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[12] before crashing into the cam on closure.
For this reason, today[when?] 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.[citation needed]
An early solution[when?] to oscillating spring mass was the mousetrap or hairpin spring[13] used on Norton Manx[14] engines. These avoided resonance but were ungainly to locate inside cylinder heads.
Valve springs that do not resonate are progressive, wound with varying pitch or varying diameter called beehive springs[15] 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.
Examples
Ducati motocycle desmodromic valve timing engine parts: camshaft, valves, opening rocker arm and closing rocker arm
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, 2008 and 2011. Ducati's return to Grand Prix motorcycle racing was powered by a desmodromic V4 990cc engine in the GP3 (Desmosedici) bike, which went on to claim several victories, including a one-two finish at the final 990cc 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.
↑ Rivola, A., et al.: "Modelling the Elastodynamic Behaviour of a Desmodromic Valve Train", Proceedings of SMA2002 International Conference on Noise & Vibration Engineering,16–18 September 2002 - Leuven, Belgium
↑ Falco, Charles M. (July 2003). "The Art and Materials Science of 190mph Superbikes"(PDF). MRS Bulletin. p.514. Archived from the original(PDF) on 2007-03-07. Retrieved 2006-11-02. Thus, neglecting all other factors, the faster an engine can be made to turn, the more power can be generated. Unfortunately, through at least the 1950s, valve springs often would fatigue and break when engines were operated for significant periods of time much above 8000 rpm.
↑ Baker, John. "Austin Marine Engines". Austin Memories. Archived from the original on August 21, 2015. In 1910 Herbert Austin decided to build a Marine engine that at the time was very advanced. It produced 300bhp and was installed in a speedboat called "Irene I" which was named after his eldest daughter who had married Colonel Waite. The all aluminium twin ohv engine had twin magneto, twin carburettor and desmodronic valves.
Ducati is an Italian motorcycle-manufacturing company that is part of the Ducati group, and is headquartered in Bologna, Italy. The company is directly owned by Italian automotive manufacturer Lamborghini, whose German parent company is Audi, itself owned by the Volkswagen Group.
A camshaft is a shaft that contains a row of pointed cams, in order to convert rotational motion to reciprocating motion. Camshafts are used in piston engines, mechanically controlled ignition systems and early electric motor speed controllers.
A four-strokeengine is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either direction. The four separate strokes are termed:
Intake: Also known as induction or suction. This stroke of the piston begins at top dead center (T.D.C.) and ends at bottom dead center (B.D.C.). In this stroke the intake valve must be in the open position while the piston pulls an air-fuel mixture into the cylinder by producing a partial vacuum in the cylinder through its downward motion.
Compression: This stroke begins at B.D.C, or just at the end of the suction stroke, and ends at T.D.C. In this stroke the piston compresses the air-fuel mixture in preparation for ignition during the power stroke (below). Both the intake and exhaust valves are closed during this stage.
Combustion: Also known as power or ignition. This is the start of the second revolution of the four stroke cycle. At this point the crankshaft has completed a full 360 degree revolution. While the piston is at T.D.C. the compressed air-fuel mixture is ignited by a spark plug or by heat generated by high compression, forcefully returning the piston to B.D.C. This stroke produces mechanical work from the engine to turn the crankshaft.
Exhaust: Also known as outlet. During the exhaust stroke, the piston, once again, returns from B.D.C. to T.D.C. while the exhaust valve is open. This action expels the spent air-fuel mixture through the exhaust port.
VTEC is a system developed by Honda to improve the volumetric efficiency of a four-stroke internal combustion engine, resulting in higher performance at high RPM, and lower fuel consumption at low RPM. The VTEC system uses two camshaft profiles and hydraulically selects between profiles. It was invented by Honda engineer Ikuo Kajitani. It is distinctly different from standard VVT systems which change only the valve timings and do not change the camshaft profile or valve lift in any way.
In internal combustion engines, variable valve timing (VVT) is the process of altering the timing of a valve lift event, and is often used to improve performance, fuel economy or emissions. It is increasingly being used in combination with variable valve lift systems. There are many ways in which this can be achieved, ranging from mechanical devices to electro-hydraulic and camless systems. Increasingly strict emissions regulations are causing many automotive manufacturers to use VVT systems.
An overhead camshaft (OHC) engine is a type of piston engine in which the camshaft is located in the cylinder head above the combustion chamber. This contrasts with earlier overhead valve engines (OHV), where the camshaft is located below the combustion chamber in the engine block.
Redline refers to the maximum engine speed at which an internal combustion engine or traction motor and its components are designed to operate without causing damage to the components themselves or other parts of the engine. The redline of an engine depends on various factors such as stroke, mass of the components, displacement, composition of components, and balance of components.
A tappet is most commonly a component in an internal combustion engine which converts the rotating motion of the camshaft into linear motion of the valves, either directly or indirectly.
Valve float is an adverse condition which can occur at high engine speeds when the poppet valves in an internal combustion engine valvetrain do not properly follow the closure phase of the cam lobe profile. This reduces engine efficiency and performance. There is also a significant risk of severe engine damage that can include valve spring failure, pistons contacting the valves, or catastrophic lifter and cam lobe failure, especially with roller lifters.
In the context of an internal combustion engine, a rocker arm is a valvetrain component that typically transfers the motion of a pushrod to the corresponding intake/exhaust valve.
A valvetrain or valve train is a mechanical system that controls the operation of the intake and exhaust valves in an internal combustion engine. The intake valves control the flow of air/fuel mixture into the combustion chamber, while the exhaust valves control the flow of spent exhaust gasses out of the combustion chamber once combustion is completed.
The Ducati 888 was a motorcycle manufactured by Ducati as an upgrade to the Ducati 851. The earlier 851 had introduced liquid cooling, computerized fuel injection and four-valve heads to the company's two cylinder motors. In 1991 Ducati increased the capacity of the 851 to 888 cc to create the 888. Both engines featured the Desmoquattro valvetrain concept in which a four valve per cylinder motor was given desmodromic valve actuation, with cams both opening and closing the valves. Ducati's desmodromic system reduces the frictional penalty from conventional valve springs.
A hydraulic tappet, also known as a hydraulic valve lifter or hydraulic lash adjuster, is a device for maintaining zero valve clearance in an internal combustion engine. Conventional solid valve lifters require regular adjusting to maintain a small clearance between the valve and its rocker or cam follower. This space prevents the parts from binding as they expand with the engine's heat, but can also lead to noisy operation and increased wear as the parts rattle against one another until they reach operating temperature. The hydraulic lifter was designed to compensate for this small tolerance, allowing the valve train to operate with zero clearance—leading to quieter operation, longer engine life, and eliminating the need for periodic adjustment of valve clearance.
Pneumatic valve springs are metal bellows filled with compressed air used as an alternative to the metal wire springs used to close valves in high-speed internal combustion engines. This system was introduced in Formula One in 1986 with the Renault EF-Type.
The Ducati singles were single cylinder motorcycles, made by Ducati from 1950 to 1974. Chief Engineer Fabio Taglioni developed a desmodromic valve system in these years, a system that opens and closes the valves using the camshaft, without the need for valve springs. This valve system has become a trademark feature of Ducati motorcycles.
The Ducati 350 Mark 3 is a 340 cc (21 cu in) single cylinder bevel drive SOHC motorcycle produced by the Italian manufacturer Ducati from 1968 to 1974. It was one of the first 'wide case' Ducati singles produced. A higher performance version, the Ducati 350 Mark 3D, which used desmodromic valves was also available.
The Ducati 239 Mark 3 is a 340 cc (21 cu in) single cylinder bevel drive SOHC motorcycle produced by the Italian manufacturer Ducati in limited quantities for the French market in 1974. The French Government has announced that they were to increase VAT on motorcycles of 240 cc and above in 1975. Ducati responded by producing the 239 to take advantage of the lower 20% VAT rate on sub-240 machines. To compensate for the reduced capacity, the engine was tuned to produce more power with a different camshaft, slipper piston, 30 mm carburettor and a different exhaust using a Lafranconi silencer.
The Ducati 450 Mark 3 is a 436 cc (26.6 cu in) single cylinder bevel drive SOHC motorcycle produced by the Italian manufacturer Ducati from 1969 to 1974. The 450 was largest displacement version of the OHC single series produced by Ducati and used the 'wide case' engine. A higher performance version, the Ducati 450 Mark 3D, which used desmodromic valves was also available. The 450 was criticised for vibration and lack of performance compared to other models such as the Mach 1.
The Ducati 250 Mark 3 is a 249 cc (15.2 cu in) single cylinder bevel drive SOHC motorcycle produced by the Italian manufacturer Ducati from 1967 to 1974. Initially produced using the 'narrow case' engine, the newly introduced 'wide case' engine was used from 1968. A higher performance version, the Ducati 250 Mark 3D, which used desmodromic valves was also available.
The Ducati 450 R/T (road/trail) is a 436 cc (26.6 cu in) single cylinder bevel drive desmodromic SOHC motorcycle produced by the Italian manufacturer Ducati from 1971 to 1974. Initially produced at the request of the American importers Berliner Motor Corporation as a pure motocross machine exclusively for the American Market, only a few hundred machines were made of this type. It is the only motocross bike to use desmodromic valves. An optional street equipment kit was available. From 1972 it was produced for the European Market as a street legal on/off road machine, which was sometimes known as the 450 T/S.
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