Desmodromic valve
reciprocating engine valve actuation mechanism
In general mechanical terms, the word desmodromic is used to refer to mechanisms that have different controls for their actuation in different directions.
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. A desmodromic valve has two cams and two actuators, 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. 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.
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This entry incorporates text from “Desmodromic valve” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.