Ducati use desmodromic valve systems because it provides for;
- A more faithful adherence to both;
1A) Not just high speed Valvetrain timing.
1B) But also high acceleration Valvetrain rates.
The latter - which can provides an advantage to pneumatic Valvetrain design approaches - allows for a more optimal and square-wave like Valvetrain function; that most typical valve-spring Valvetrains ( even at moderate speeds) can not accommodate.
- 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 considerations and/or drawbacks of the desmodromic valve approach are;
A complicated design involving phase-conjugate camshaft design and machining that performs marginally better than most modern day high performance valve-spring alternatives.
The 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.
Maintenance and servicing intervals. Look at Ducati's (real-world) homolgated product line's servicing intervals to most Japanese (and some European) motorcycles using traditional valve-spring alternatives/design.
Mercedes Benz whom (between them, Maserati, Ducati, and Austin, are often cited for inventing the desmodromic system) have a considerable racing/engineering history, and first used the desmodromic system in F1; didn't drop the idea of the desmodromic valve actuation because it performed better in ways that made the additional complexities worthwhile.
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.
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 valvesprung Valvetrain systems) there is no chance that the intake/exhaust valve - as it transcends through its line of allowed linear movement - can move in any other way than the timing diagram intends.
To be clear on point 6; conversely, pneumatic and valvesprung Valvetrain systems can often (and sometimes are actually designed {within 1/4 mile and/or engine power shoot-out contests} to) accelerate their valves at rates that are faster and/or not entirely adherent to the camshaft and/or overall Valvetrain timing diagram.
Jim.
