Desmodromic valve
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.
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.
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 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 allows combustion gases to escape prematurely, leading to a reduction in cylinder pressure which causes a major decrease in engine performance.
The desmodromic system avoids some of the shortcomings of spring-loaded valves because it is not subject to the high loads associated with compressing stiff springs.
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.
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.
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.
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).
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.
