Ducati Desmo Dreaming | Drawing the Line

As I'm writing this, a Ducati, with Casey Stoner aboard, has won three of the first four MotoGP races of 2007. There is lots of talk about the Ducati's power, and even some speculation in the press of cheating. Claudio Domenicali, Ducati Corse managing director, credits electronics, desmodromics and aerodynamics with saving fuel and thus allowing the engines to make more power. He notes the desmo system's gains at low to mid engine speeds, but says nothing about high revs.

Why is saving fuel so important in the power equation? For 2007, the fuel capacity of the MotoGP bikes was reduced from 22 liters to 21. Bikes that run out of fuel aren't going to win races, so fuel consumption is carefully measured by the engine-management electronics. If necessary, rpm and power are restricted to make sure there is enough fuel to finish. Indications are that the bikes could make more power, but only if there were more fuel.

I think maybe Ducati's desmo system is the real hero here. The positive-valve-actuation system uses cam lobes to both open and close the valves, unlike the competition which uses cam lobes to open the valves and either metallic or pneumatic springs to close them. These springs must be quite stiff to control valve motion at the near-18,000-rpm peak of today's 800cc MotoGP racers, and the engine has to do considerable work to open them. This work is not all lost, however, as the compressed springs store energy and then release it, pushing against the tappets and against the face of the cam, turning the cam.

So far, the desmo engine may have a bit of an advantage. Now let's look at very high revs, and to do that take a lesson, ironically, from the low-tech, low-revving world of NASCAR. To get big power from their comparatively primitive engines, NASCAR teams have studied what happens to their valve trains in the (for them) very high 9000- to 10,000-rpm range. At these speeds they have valve toss, in which the valve train above the cam is tossed into motion with such violence that the spring can no longer keep the parts in touch with the cam. Just beyond valve toss is valve float, in which the parts start bouncing in truly chaotic fashion, and the valves hit the pistons. Computer simulations can chart this action accurately enough that engines can be made to live with this seeming chaos going on inside.

With this in mind, let's look at Ducati's competition at maximum revs, twice as high as their NASCAR counterparts. Much, much lighter and more sophisticated than NASCAR's pushrod valve trains, these valves, springs, and tappets are still operating very close to their limits. At or near the point at which valve toss occurs, the stored energy of the springs cannot act to rotate the cam, because the parts are unweighted. The parts come together with the tappet farther down the face of the cam, where the spring force is less effective at rotating the cam. The system must still use power to open the valve, but it gets less power back from the spring. Meanwhile, the desmo engine needn't work to compress the spring in the first place, though it does use some power to close the valve.

So, what are the relative gains and losses? We may never know. What would be necessary here would be to build two engines, absolutely identical except for valve gear, develop each to a high level and then test them side by side. Computer simulations might tell us, but the smallest error can derail that approach. My hunch is that there may be a few percentage points' gain for the desmo system at high revs, but that might be more significant when we remember the fuel situation.

Greater efficiency means less fuel is consumed at comparable power settings. We've seen Casey Stoner set his fastest laps very late in races. What this may mean is the desmo system has saved enough fuel in the course of the race to allow an increase in fuel usage-and thus power production-right at the end. The reduced fuel capacity in MotoGP has introduced a fascinating new level of competition in which rpm, power, mechanical efficiency, fuel use and the strategies combining them all provide a complex background to the already compelling battles of riders, brands and teams.

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