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Pnumatic Valvs... Why?

Discussion in 'Racing, Motorsports, and Track Days' at netrider.net.au started by FALCON-LORD, Sep 11, 2007.

  1. I have three pieces of information that don't add up (So obviosly at least one of them has to be wrong or things just don't make sence)
    1) From what I have gathered pnumatic valves are capable of faster return rates allowing an engine to rev higher.
    2) GP Bikes are rev limited so that they don't suck through there tank before they get to the finish line.
    3) Yamaha have started looking into Pnumatic valves to try to overcome there power disadvantage relatvice to Ducati.

    So something doesn't add up, Any one able to help?

  2. 1. Correct.
    2. Self imposed rev limiter to keep a lid on fuel consumption. As they understand the engines better, fuel consumption is better known. Fuel management strategies can still be used to control fuel consumption
    3. Correct. They are (I think) the only spring return engine left.
  3. In order to make more power, the engine generally has to rev higher. In order to rev higher, stiffer springs must be used to make the valves shut faster. The stiffer the springs, the greater the valve-train losses (and hence greater fuel consumption).

    By going pneumatic, valve-train losses are reduced, and the engine can make more power at the same revs while consuming less fuel.
  4. That too :p
  5. O.K. so pnumatics don't make the return faster, they mean less power loss in the timing cams because they don't have to fight against teh return spring.
    Is that correct?
  6. why not just use desmo then?
  7. speed and adjustability spring to mind

    instead of changing cams you can change the power of the engine and how it produces power via the ecu by when and how long the valves are held open
  8. Well, pneumatic do make the valves return faster, if they want them to.

    Another thing pneumatics allows for is variable valve timing (think like Honda's VTEC stuff). Effectively grants you ultimate flexibility in the valve timing advance/retardation for different portions of the rev range. Can map it for torquey operation at lower revs, and "high lift cam" like action at higher revs.

    Can make the valves do whatever is needed, and not even desmo gives you that.

    Desmo, I think (I cound be wrong), has less losses than pneumatics, because you don't have to drive the pneumatics system either, so that's where Desmo gets its advantages. Desmo involves the greatest amount of spinning and reciprocating valve-train mass though, and it can't allow you to vary the valve timings like pneumatics can.

    I recall the F1 engineers looking at Desmo, because even though pneumatics allows for greater flexibility, Desmo provides an almost as effective solution, for less cost, when all the pros and cons are considered. It was Ducati's success with the 999's that more or less made the F1 engineer's more aware of Desmo's capabilities.
  9. http://en.wikipedia.org/wiki/Desmodromic_valve

    valid question..
    Looks like a stable technolodgy

    Thanks PP There is some interesting stuff there.
    Thanks flux That explains it
  10. The biggest problem with Desmo, is the size of the valve-train heads and its weight (comparatively). Desmo is heavier than any other solution, and it adds around an extra 5cm or so to the head height to fit it all in, and on a small and compact motorbike, that can make things hard to fit.

    This is probably why Ducati are doing so well now though. With the 990cc engines, they probably had to make some tradeoffs to make it all fit, and that's why the bikes lagged behind the others. Once they went to 800cc, everything that was a slightly too-hard squeeze all fitted nicely, and the Desmo engine came to the fore.

    It really is a delicate balance. Desmo, space wise, is a harder fit. If you're old enough, you'll remember the troubles that Ducati had with the 916's with ensuring that the front wheel didn't come into contact with the front cylinder head, and this is actually why the bike couldn't be made any smaller, even though Ducati had wanted to.
  11. Desmo's have an advantage over normal cams in that by changing either the rocker arm lengths and pivot position (in the rocker arm as well as its pickup), the timing of the valve (effective cam profile for direct acting cams) can be altered significantly without altering the cam. Which gives a superior amount of control over any other mechanical system.

    The power lost in engines is not only due to the amount of mass but its inertia, a bigger cam will take more energy (fuel) to acelerate through the rev range even if they weigh the same amount.

    Hence the system with the least mass, inertia & the highest efficiency will use the least fuel. Mechanical driven gears and bearings are in the high 90's, in an engine the biggest loss it the piston rings (related to speed squared).

    As for the inertia of a desmo system compared to a pneumatic or conventional system i have no idea as to which has the least, and no idea of the loss' in a pneumatic system.

    The other interesting idea i've heard of is solenoid operated valves, apparently a bit heavy though.
  12. Flux nailed it in one. The desmodromic actuation is the perfect way of valve timing with high reving, dr T refined it perfectly. Id argue its exactly why the ducati is the most powerful straightline weapon this season. That said, its trade off is size and weight. That said, with the amount of lightweight super strong components floating about now, it wont be long until the desmodromic system itself becomes even more refined, lighter, smaller etc.

    In terms of the opening and closing of valves, the desmo system is damn near perfect from everything ive read, but its trade off is the space required.

    That said, im not a mechanic, but my dad is, and thats what he said.
  13. Too bad rotary valves did not get the development they should have. They're very promising, but have been overlooked. With teh ability to adjust cam timing on modern engines, they could well be the go.
    I recall Siemens and Detroit Diesel playing around with electric solenoids to operate valves, dunno what came of that.

    Regards, Andrew.
  14. I had researched a bit on rotary valves, and their main issue, even today, is the tolerances required to achieve a good enough mechanical seal during the power stoke phase.

    Don't know much about the solenoid approach. Should be lighter weight and require less power than pneumatics. Might be a longevity, heat and strength issue though getting the magnetic windings strong enough to close the valves fast enough, and not overheat with the amount of current flowing through them. Might be why they were working on them for diesel, with the lower revs and so lower closing forces required?
  15. IIRC Lotus has also been developing something similar, using a computer-controlled electrohydraulic system for valve actuation, doing away with cams. Last I heard of it was they made a single cylinder prototype and were moving on to a full 4cyl engine, but that was probably 2yrs ago now I think.