Desmodromic valve
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- 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 springs to close the valves. It is derived from two Greek roots, desmos (controlled, linked) and dromos (course, track).
The benefit of desmodromic valves lies in the ability to allow higher valve acceleration/deceleration without the risks of causing a collision between valves and piston. Also the friction is lower. Famous examples include the successful Mercedes-Benz W196 and Mercedes-Benz 300 SLR race cars as well as Ducati motorcycles.
This is in the context of internal combustion engines. The valves in question are the ones that allow air into the cylinder and (usually different ones) that allow exhaust gases out. This refers, for example, to the valve control system used in Ducati engines: both valve movements (opening and closing) are "operated". It is usual to say that action on the valve is positive in both cases, in other words, both strokes are controlled.
Desmodromic valve actuation has been applied to top-of-the-range Ducati motorcycles since the early 1970s. Two primary mechanical methods have been used to transfer timing information, from the crankshaft, to the camshaft and ultimately the rocker arms and valves. Initially bevel-driven camshafts were used. This involved transferring the timing information via several bevel (part conical gears where the rotating axis' of the two lie on an angle - 90° for example) gears and a shaft running on the outside of the engine block. Then at around 1977, Chief Design Engineer Fabio Taglioni completed and tested an actuation system that used rubberized metal belts with timing teeth. These teeth would mesh with timing pulleys, also external to the main engine block, and transmit the timing information to the valves.
The primary reason for Desmodromic (Colloquial - "Desmo") systems is to improve valve timing at higher engine revolutions. On very high revving valve spring engines, the spring does not always have force to keep the valve in contact with the camshaft lobe. This is called "valve float". A Desmodromic valve system camshaft can also 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, likewise a quickly closing valve will not bounce off the valve seat since it is retained by the opening camshaft lobe.
The disadvantage of Desmodromic valve system is its high service cost and more frequent service durations, due to the low-tolerant precision required to run the system, and possible cold starting issues. However, once the parts have all "broken in", there is very little wear in the system, and clearance adjustment intervals increase at roughly a geometric rate.
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