Desmodromic Mechanism

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                                DESMODROMIC MECHANISM

Hey guys it been a very long time since I  have posted ,in this post I would be discussing about the desmodromic mechanism.

A desmodromic valve is a reciprocating engine 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 gases to be expelled 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.
                                                            
These types of valves are used because the normal puppet valves fail at higher RPM thus making it difficult for the engine to operate at higher speeds as result these types of mechanism came into play and the were first mentioned in patents in 1896 by Gustav Mees. Austin's marine engine of 1910 produced 300 bhp and was installed in a speedboat called "Irene I"; its all aluminium twin overhead-valve engine had twin magneto, twin carburettor 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]  

Prominent uses of desmodromic valves.

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.ne had twin magneto, twin carburettor and desmodromic valves. The 1914 Grand Prix Delage and Nagant (see Pomeroy "Grand Prix Car") used a desmodromic valve.

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a ducati bike demodromic valve
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Desmodromic poppet valve example

Comparison with old valvetrains.
The main benefit of the desmodromic system is the prevention of valve float at high rpm.
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 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 must 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 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.
If you have any doubts just mention it in the comment section.Thanks for viewing. 

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