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Desmodromic Valvetrains; A short history. This is a condensed version of a presentation on desmodromic valvetrains I did for an automotive restoration class I was taking. For more complete information and a few illustrations of what these systems look like, go to www.ducati.com or http://members1.chello.nl/~wgj.jansen/ . Those sites contributed most of the information here, with a few tidbits coming from the book "Ducati Desmo" by Mick Walker.
The term "desmodromic" is derived from two Greek words: "desmos" (controlled or linked), and "dromos" (stroke, course, track). It is used to describe a system that provides "positive valve actuation" as in the cam controls both the opening and closing of the valve.
Why? "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". -Fabio Taglioni
In the early days, competition engines had unreliable valve springs. Coil springs reliability dropped as speeds increased over 6000 rpm and lift increased above about 3/8". Springs would fail due to surging/vibration of coils. Early motorcycles often had external hairsprings for valves; in part because they could be changed easily without removing the head and their natural frequency was outside the normal operating range of the engine. Desmodromic valvetrains were one method tried to avoid such problems.
History 1910 Arnott (England) - Widely accepted to be first desmodromic timing system. Patent applied for, but process was never completed? 1914 Delage (France) - Grand Prix car uses "desmodromique" system, 4 valves per cylinder, pent roof combustion chambers, twin overhead cams. One cam opened the valve, the other closed it. 1917 Miller (USA) - "Golden Submarine", built for Barney Oldfield at a cost of $15,000. Uses a 4 cylinder aluminum alloy engine, 289 cid, Single overhead cam with desmodromic valves, dual intake ports for each cylinder, dual sparkplugs and magnetos. 1920 Bignan (France) - Sports car, won Spa 24 hours race and on the Brooklands circuit 1923 Fiat (Italy) - Patents variation of Bignan's system. 1924 Norton (England) - Patents desmo design, but never produces it? 1954 Arnott (England) - F. H. Arnott renews interest in engines. Converts 500cc JAP ohv single cylinder engines to desmodromics. Noting the use of springs in most earlier designs (due to manufacturing tolerances, etc) he states "the introduction of springs into a device intended to eliminate them had been an unnecessary complication, which in retrospect hampered the development of the principle" 1954 Mercedes (Germany) - Employs desmodromics on it's W196 GP car; 2.5L, 260hp straight eight engine. During development all springs are removed from system. Complexity goes down, performance unchanged. W196 proceeds to win two world championships. Also used on Mercedes' 300 SLR Sport race car. (Enzo Ferrari is said to have considered using desmodromics following the success of Mercedes. He reportedly talks to Fabio Taglioni about it. Taglioni goes on to develop system for Ducati… Ferrari experiments with Desmo systems in the'90s) 1956 Ducati (Italy) - Taglioni develops desmo system for 125 GP bike; wins it's first race. 1958 Norton (England) - Develops four-cam system for 500cc motorcycle. Considered too bulky, never reaches production. (Aftermarket conversions are now available for various motorcycles however…) 1968 Ducati - Introduces the first mass produced system on Mark 3 Desmo 350 motorcycle. Similar system is still being used on current two-valve Ducatis. 1980 Ducati - Entire line of production motorcycles are now "Desmos" 1987 Ducati - Prototype four-valve system is used on 750cc twin cylinder motorcycle. Promptly wins Daytona "Battle of the Twins" race. (Designed in cooperation with Cosworth?) 1988 Ducati - Four-valve "Desmoquattro" system is introduced. Many World Superbike Championships follow. 1988 Ford (USA) - Receives patent for it's version of desmodromic valve system 2001 Delphi Technologies (USA) - Receives patent for Desmodromic Cam Driven Variable Valve Timing Mechanism Note this is by no means a complete timeline! Many companies have patented desmo systems over the years. This is only meant to give an overview. For more information, go to the sites listed at the top of the page! Advantages of desmodromic actuation Aggressive cam profile - steeper ramps, higher accelerations than with springs Controls position of valve better - No float No springs to bind/flutter/break - only spring in system is hairspring to aid in starting and to improve idle. (Typical race prep includes removing this spring. ) Uses less power - no spring forces to overcome. (Yes, some may argue this one. They'll say the spring pushes on the cam as the valve closes to return some of that energy.)
Disadvantages High maintenance - clearances very critical, need to be checked every 6000 miles. Complex cams/mechanisms - twice as many lobes. Rockers always used, two for each valve, more critical tolerance zones during machining. Cost - more parts, more close tolerance machining.
Alternatives Rotary valves Disk valves Sleeve valves - Piston aircraft engines, typically radials, circa 1930-40 Pneumatic actuation Electromechanical actuation Torsion springs - Honda 450
For a list of companies and individuals that have used desmodromic engines/conversions, along with links to pages with more information on specific systems, go to http://members1.chello.nl/~wgj.jansen/
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