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Technical Curiosities:
The Desmodromic Valve

February 8, 2013 by Matt

Desmodromic Valves Valvetrain Stem Cam

Much like our earlier installment of the Technical Curiosities series discussing the Laycock de Normanville overdrive, I couldn’t not talk about desmodromic valves for the awesomeness of the name alone.

In essence, what we have here is a valve not only opened using positive mechanical pressure from a camshaft lobe (as in nearly every conventional reciprocating piston engine), but also closed by a camshaft lobe and follower, doing away with the valve spring. With precise timing, one lobe clicks the valve open; the other slams it closed.

By eliminating the valve spring, all possibility of valve float is removed. Valve float occurs in a conventionally-sprung valvetrain arrangement at high rpm when a valve spring is unable to pull the valve shut quickly enough, disrupting timing and in extreme cases allowing the valve to contact the piston, leading to their mutual destruction. Engines with desmodromic actuation can thus rev higher without the threat of internal damage, and the multiple camshaft arrangement allows for slightly more sophisticated valve opening and closing profiles.

Desmodromic Valves Valvetrain Stem Cam Diagram Schematic Drawing Illustration

The downsides of the a desmodromic valvetrain include added complexity, twice as much mechanical noise (clicking and tapping and such) as a conventional setup, and more required maintenance in the way of valve adjustments. Also, if maintenance is neglected and the clearance between the closing cam lobe and follower is allowed to become too large, the valves might not held fully closed, leading to a whole host of running problems.

In spite of its advantages, the configuration has only been adopted by a few manufacturers, most prominently Italian motorcycle firm Ducati, who use desmodromic valves in their bikes to this day. On the automotive side, only Mercedes, back in the 1950s, designed a handful of racing engines (fitted to their W196 and 300 SLR racers) around the unique valvetrain. Eventually, as with the sleeve valve, more sophisticated engineering and metallurgy all but eliminated the disadvantages of a conventional engine arrangement, and the alternative solution fell into obscurity (with the noted exception).

Image credits: oocities.org

Editor’s note: This post is part of an ongoing series spotlighting obscure automotive engineering solutions. Read the other installments here:

Filed under: Technical, Technical Curiosities

6 Comments

  1. areopagitica says:

    While this idea was expected to take over in the fifties, somehow the valve dynamics of conventional springs managed to work to about 12,000 rpm in Formula One, after which something called pneumatic springs took over. Something a little different and patented is now being used by FIAT which combines hydraulics in an active way, not passively like hydraulic lash adjustment. It is said to have the ability to vary not only initiation and duration of the valve “event” but also the rate of acceleration of both opening and closing, all independently. In combination with valve lift and perhaps total deactivation of specific cylinders these are is all useful variables to alter in modern computerized valve actuation systems.

  2. Mort says:

    In the first picture (photo) there is a torsion spring involved in the “Desmo” valve operation. I was under the impression that the Desmo valve theory eliminated the spring due to the poor quality of spring material at the time. Why does this version have a spring?

    • Matt says:

      Good question! I assume it’s there not to actually effect valve operation but to eliminate any slop in the system, to reduce (or eliminate) the need for valvetrain adjustments, in other words.

      • Boo says:

        As there always has to be some clearance in the valvetrain, the spring is there to make sure the valve is held against the seat when not actively being lifted.. The closing cam will make sure it heads towards the closed state at any RPM, but the spring is needed to make sure the valves actually seat rather than staying slightly open at the other end of the few thousands of an inch clearance. Without the clearance the closing cam would actually be hammering the valves into the valve seats!
        The spring is particularly essential on the exhaust valve, as the cylinder vacuum during the intake stroke would certainly unseat the valve without it.

  3. Sam D says:

    I remember reading an article by my namesake, on an OSCA of the 1950’s using desmodronic valves. It allowed a relatively small engine make excellent power as it could rev so high.

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