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What are Desmodromic Valves?

Paul Scott
Paul Scott

Desmodromic valves are internal combustion engine valves which are closed by a mechanical actuator rather than the more conventional spring. This actuation method utilizes separate opening and closing rocker arms. These rocker arms are either driven by separate lobes on a common camshaft or by two individual camshafts. Desmodromic valves were developed to counter the problems of valve spring breakage and valve float in high revving, performance engines. Recent engineering and metallurgical advancements have, however, addressed these problems, and desmodromic valves are now only used by a small group of engine manufacturers.

Conventional internal combustion engine inlet and exhaust valve arrangements utilize a camshaft driven rocker arm to depress and open the valve against spring pressure. When the rocker arm lifts, the spring returns the valve to its closed position. These valve springs are exposed to considerable stress; before the principles of metal fatigue were fully understood, valve spring breakage was commonplace, particularly in high speed performance engines. Desmodromic valves were developed in an attempt to address this and other common valve-related performance issues. The system relied on two rocker arms per valve, one for opening the valve and one that closed it again, thereby completely eliminating the need for a valve spring.

Man with hands on his hips
Man with hands on his hips

The rocker arms were driven either by a single camshaft with individual open and close lobes for each valve or by two separate camshafts. This valve actuation method also did away with another common valve-related problem: valve float. This phenomenon occurs where the valve's inertia at high speeds defeats the spring's ability to close it before the piston reaches its top dead center (TDC) position. Apart from a serious loss of cylinder pressure and its resultant negative performance effects, valve float may cause the piston to affect the open valve, thereby bending the valve stem and possibly damaging the piston. The desmodromic valve system does not have to contend with static spring energy and therefore eliminates valve float altogether.

Although generally effective, desmodromic valves are typically more expensive to install and maintain and are noisy, particularly in engines with four cylinders or more. This, along with issues of valve float and spring failure, has almost entirely been eliminated by modern engineering. Moreover, metallurgy technologies have made the system largely obsolete. There are, however, some manufacturers such as Ducati which still produce “desmo” valve motors for their racing bikes. In fact, Ducati wins at the Superbike World Championships as late as 2008 were achieved on bikes with desmodromic valve engines.

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