19

Ducati's use a unique method to open and close their valves. It's called the desmodromic valve train.

The desmodromic valve system does not use springs to close the valve, it uses the cam and has a valve closing lobe.

Desmo Valve Train

What are the benefits to removing valve springs from the valve train?

Why did Ducati use this type of valve mechanism as opposed to a more tradition under the bucket shim design?

  • @racefever I kinda want to see what people come up with. There's a bunch of controversy about it's use in wider applications so I took the link out. Thanks though! Experimenting. Cheers! – DucatiKiller Dec 29 '15 at 20:48
  • Yes, of course. Just inserted it to make it simple for people like me to look up what desmodromic valvetrain is. (: – race fever Dec 29 '15 at 20:56
  • No problem! I know your just trying to contribute! TY – DucatiKiller Dec 29 '15 at 20:58
13

Ducati use desmodromic valve systems because it provides for;

  1. A more faithful adherence to both;

    • (1A) Not just high speed Valvetrain timing.

    • (1B) But also high acceleration Valvetrain rates; regardless of what weight/material the valve is made from.

The latter (1B) - which can provide an advantage over the pneumatic Valvetrain design approach - allows for a more optimal and square-wave like Valvetrain function; a function that most typical valve-spring Valvetrains (even at moderate speeds) simply can’t accommodate.

  1. An alternative to pneumatic Valvetrain design approaches used by other manufacturers in MotoGP; that - if implemented correctly - can also provide both weight savings and also cylinder head port geometry benefits.

Footnote

Ducati's desmodromic valve system was (contrary to popular belief) not invented by them, it does actually employ springs (see links below), and (some of) the significant considerations and/or drawbacks of the desmodromic valve approach are as follows;

  1. A complicated design involving phase-conjugate camshaft design and machining that performs marginally better than most modern day high performance valve-spring alternatives.

Please note how:

  1. :

    • a) Most comparable Japanese (and some European) motorcycles using traditional valve-spring alternatives not only reliably rev higher than Ducati's desmodromic valve system, but also do so without loss of power when compared to Ducati.

    • b) Several other Japanese and European motorcycles using both (i) the same 90 degree “L” and/or “V” twin engine configuration and capacity as Ducati, and (ii) traditional valve-spring alternatives; not only produce similar and/or more power than Ducati - but also do so quite reliably.

    • c) Ducati’s desmodromic valve system employed in their earlier 900 SS models (that shared an almost identical engine 860cc design to their 860/900 GTS range) provided no real meaningful power/torque benefit over the their 860/900 GTS range - at least not one that could not easily be attained by doing the same (non desmodromic valve system) “delta” modifications to the their 860/900 GTS range.

  2. Ducati’s desmodromic valve system's camshaft (timing and conjugate) designs are considerably more complex and therefore susceptible to change engine behaviour with wear; more than, say, other traditional valve-spring alternatives/designs exhibiting the same degradation.

  3. Maintenance and servicing intervals. Take a look at Ducati's (real-world and much shorter) homolgated product line's servicing intervals to most Japanese (and some European) motorcycles using traditional valve-spring alternatives/design; and note also how much greater the crankshaft rotational speeds (friction/wear?) are of the Japanese counterparts that usually have greater servicing intervals.

  4. Mercedes Benz whom (between them, Maserati, Ducati, and Austin, are often cited for inventing the desmodromic system) have a considerable racing/engineering success and history, and they were one of - if not the - first to use the desmodromic system in F1. Mercedes Benz didn't drop the idea of the desmodromic valve actuation because it performed better in ways that made the additional complexities worthwhile.

  5. The desmodromic valve actuation system that Ducati have some patents for is considerably different than the first desmodromic valve actuation systems that were invented and aforementioned.

  6. All currently working desmodromic valve actuation systems that offer timing diagram and other performance benefits over traditional racing and/or high performance valve spring Valvetrains must employ extremely aggressive (even by traditional racing and/or high performance standards) Valvetrain acceleration rates in order to make their advantages realizable over other options; and even then the cost effectiveness and the performance benefits of currently working desmodromic valve actuation systems are still debatable - hence their lack of popularity with the racing fraternity.

  7. Rotary valves - when correctly implemented - are a far better idea than currently working desmodromic valve actuation and also other popular Valvetrain systems; that lost popularity and funding for to F1 and other decisions. Theoretically this (rotary valve) approach resolves many limitations of the poppet valve whilst also offering many other benefits.

  8. It still requires springs to be implemented properly and reliably; defeating one of its most commonly considered benefits.

  9. The whole idea of any desmodromic system is to force the valves to remain utterly complaint with the Valvetrain/camshaft timing diagram as faithfully as possible. As such the way in which Ducati implement their desmodromic valve actuation in MotoGP means that (unlike pneumatic and traditional valve-sprung Valvetrain systems) there is no chance that the intake/exhaust valve - as it transcends through its line of theoretically allowed linear movement - can move in any other way than the timing diagram intends.

To be clear on point 9; conversely, pneumatic and traditional valve-sprung Valvetrain systems can often (and sometimes are actually designed {within 1/4 mile and/or engine power shoot-out contests} to) accelerate their poppet valves at rates that are faster and/or not entirely adherent to the camshaft and/or overall Valvetrain timing diagram.

Finally, I believe it is mostly for reasons of tradition, heritage, and marketing, that Ducati persist with their desmodromic system on their non MotoGP machines; as it provides very little real-world benefits in those guises.

enter image description here

enter image description here

  • Beautiful answer. Thanks. Happy New Year and cheers! – DucatiKiller Dec 30 '15 at 6:26
  • You should have enough rep to comment now :D – DucatiKiller Dec 30 '15 at 6:28
  • If you register a user account in the system it may be easier for you to consistently sign in with the same user account so you gain reputation on a single account. It will help your reputation score that, as it increases, allows access to various features of the site. :D Cheers and Happy New Year. – DucatiKiller Dec 30 '15 at 18:45
  • I agree that part of why they continue to use it is nostalgia and that the Ducatisti drive the loyalty to the mechanism, much like HD owners and their special V-Twins. – DucatiKiller Jan 5 '16 at 2:31
6

The goal was to prevent valve float at higher rpms.

Given the metallurgy of the day it required a lot of spring pressure to ensure that the spring was push the valve closed. The desmo unit mechanically forces the valve closed as opposed to using spring pressure.

This would allow an engine to run at a higher rpm then a unit with a conventional spring poppet valve.

This was more important on racing motors but Ducati used it on street bikes as well.

  • 1
    It also frees up frictional losses you see with typical valve spring design. – Pᴀᴜʟsᴛᴇʀ2 Dec 29 '15 at 23:29
  • I'd like to add that other motorcycle makers who race on Moto GP would use a conventional valvetrain and most later switched to a pneumatic one (en.wikipedia.org/wiki/Pneumatic_valve_springs) a lá Formula 1. Although Ducati has dominated the SuperBike World Championship since the 90s. – race fever Dec 30 '15 at 2:06

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.