Pneumatic vs. Desmodromic

[wiley]

Allrigh powerplant junkies!!!!!

What advantage does pneumatic valve return have over desmodromic valve return? (or vice versa)

I know that mercedes introduced desmo to racing, and ducati uses the sytem on all(?) of it's bikes, even current moto gp racers.

Why is desmo not in F1

[G-Rock]

The Desmo system is simpler but has more friction than a pneumatic system. The pneumatic system runs on air under pressure and the beauty of it is that it's a closed system. No air has to be pumped into it. Once the system is charged it will last the whole race.
The Desmo system also can't eliminate valve to valve seat bounce at high rpms (16000 and up) because it still relies on a small spring to make the seal complete under different temperatures due to different expansion rates of the different metals.
One more thing that's very important in F1 is compactness. The Desmo system requires a few extra parts in the cylinder head which can make a big difference in the size of an engine. The pneumatic system can have shorter valve stems making the cylinder heads very compact.

[joseff]

The upside of desmo is that the camshaft does not have to fight the constant spring tension, reducing frictional losses at lower rpms and hence improving low end torque. I'm not sure how this relates to pneumatic springs. Is the air pressure constant? If so, then desmo still has a low rpm advantage, at least according to Ducati.

The supposed holy grail of valvetrain design is the electropneumatic system. I read about it in '94 or '95. Honda had a prototype engine with electrically actuated valves that totally eliminated the need for a throttle body (a la valvetronic). So it's been around a LONG time. Quite surprising that it never saw production, considering BMW's fuel efficiency/power/torque improvement claims with their valvetronic.

[Saribro]

How about reliability? Those constructions of beams and hinges seem more wear-prone than a stick sliding in a tube.

[DaveKillens]

A desmodromic valve train has traditional cams opening the valves, and cmas and fingers closing the valves. This form of technology is very meture, and if properly assembled and the tolerances are proper, very reliable and give great power. The traditional valve spring is eliminated, and replaced by a much lighter spring used just to seat the valve in the last few millimeters (or less). As mentioned by joseff, there is a lot less spring to fight, and frictional losses are much lower, especially at lower RPM. Usually, because a finger is required to close the valve, we rarely see the camshaft riding directly over the valve, but instead beside it, with rockers or fingers on pivots.
http://en.wikipedia.org/wiki/Desmodromic_valve
Pneumatic valves eliminatwe the traditional valve spring, allowing much less mass in the valvetrain, thus higher RPM's. They can be very compact. But they do occasionally develop leaks, and during a race that requires recharging the pressurized bottle in pit stops. And before you start an engine, it is critical you have pressure, or all the valves will be instantly destroyed when the pistons rise. That is why they have a severe handicap in road car engines.
http://www.everything2.com/index.pl?node_id=1222194
They are very different systems, each with strengths and handicaps.

[G-Rock]

One correction for Joseph's comment. He claimed the Desmo system would increase low end torque.
This is actually not accurate but not an unusual incorrect description of an engines power characteristic.
What he means is low end horsepower. Torque is a measurement of a constant pulling power while horsepower is a measurement of an engines acceleration under load. Torque is good for explaining how a lorry pulls a big load while horsepower is for discussing an engines ability to accerate an F1 car for instance.
In a race car you would want good a good horsepower curve from low to high rpms, not an easy feat, especially this year with the banning of most variable cams, trumpets etc. [/quote]

[DaveKillens]

With the desmo system, there are no valve springs for the cam to fight with. That reduces the friction within the engine by a heck of a lot. At higher RPM inertial loads start to make themselves known, but at lower RPM, it's like free horsepower.