CW Tech: Why Desmo? A short history of valve control and why Ducati sticks with desmodromics.



Why Desmo diagram #1 After World War II, when future Ducati engineer Fabio Taglioni wrote his original engineering-school paper on desmodromic valve drive, there was good reason to seek such a system: Valve springs broke at random as an era of rising rpm opened. In the 1950s, NSU pushed to 12,000 rpm, and by 1957, Mondial’s 125 was revving to 13,000. Something better than metal springs was needed, and Ducati, by 1958, was ready to win the 125cc rpm race. The Bologna-based company was second, third, and 4th in the 125cc TT, then smoked the MVs in Belgium and Sweden. This success started Ducati down a path it has never left. In 2003, when I asked Claudio Domenicali, then in charge of racing and now CEO, why Ducati continued with desmo in MotoGP, he said, “Because it is the system we know best.” The company had, naturally, looked into pneumatic springs but understood: 1) it would cost a lot of money, and 2) there would then be a totally unfamiliar learning curve.

Something better than metal springs was needed, and Ducati, by 1958, was ready to win the 125cc rpm race.

Domenicali then said to me, “I would hope you can regard desmo as the developed equal of any other system of valve control.” As in a valve-spring engine, a cam lobe operates a pivoted finger follower that presses against the end of the valve stem to open the valve. A second, complementary cam lobe on the same camshaft operates an L-shaped closing lever whose clevis-shaped other end pulls the valve closed, acting against the underside of a collar fixed to the valve stem. Thus desmo eliminates the usual problems with springs (read on), while the absence of conventional spring load saves some frictional loss at low to mid-rpm. Ultimately, Ducati uses desmo because it knows desmo and also because it is known for desmo.

DIAGRAMS: [1] OPENING CAM: Shaped conventionally [2] CLOSING CAM: Flat section allows opening [3] PNEUMATIC VALVE: Air chamber [4] AIR PISTON: Moves with valve, black cross-sections are seals [5] COIL SPRING: Double coils help reduce spring fatigue

METAL VALVE SPRINGS Metal springs overcame the problems of the 1950s by combining super-clean vacuum-remelted wire with shot-peening to put the wire’s surface in compression. These advances came initially from the US producer S&W (Art Sparks and Tim Witham) but were soon adopted worldwide for high-reliability springs. No sooner do improved materials appear than they are exploited. More fatigue-resistant spring wire allowed cam designers to use faster-opening ramps, leading to higher-load accel­eration. Back and forth went the cycle—materials improvement, followed by ever-more-vigorous valve acceleration. By 2006, Suzuki was changing springs in its MotoGP engines every night. The problem is that metal wire has inertia. When a high-acceleration cam begins to lift a valve, wire coils pile up against the spring retainer then rebound from it as the valve reaches one-quarter lift. This can cause rapid end-to-end bouncing of the spring coils, greatly increasing the number of fatigue cycles the wire must endure. There are tricks to suppress this “spring surge,” but even when they work, springs become very hot in vigorous operation, which accelerates metal fatigue. Formula 1 had reached the point of “one-day spring life” in the 1980s, triggering an intense search for better solutions. Several F1 engine constructors prototyped desmo systems of their own (Ferrari and Cosworth, for example) but did not pursue them.
Why Desmo diagram #2 PNEUMATIC SPRINGS Jean-Pierre Boudy, at Renault in 1984, devised an essentially massless spring that was immune to both metal fatigue and to the inertia-driven resonant coil vibration that drove it. The spring material he chose was a pressurized gas. Although pneumatic springs are regarded as exotic, they are in fact little more sophisticated than the gas struts that support the hood of your car when open. As with a metal spring design, the cam operates a finger follower that presses against the end of the valve stem to open the valve. But instead of the normal spring retainer and spring, there is a small piston, sealed to the inside of a bore in the head. The space under this piston contains nitrogen gas at a moderate pressure, such as 150 psi. To prevent leakage of gas between valve stem and guide, a highly effective seal is located there. Because there will be some leakage of gas, pressure is maintained by a small port that is open only when the valve is closed, connected to an on-board pressure bottle and regulator or to an engine-driven pump. A special advantage of pneumatic springs is that they are highly progressive. That is, as the valve lifts, the pressure under the gas piston rises steeply, allowing valve return force to be very much greater at full lift than when the valve is on its seat. Titanium valve supplier Del West is also a provider of pneumatic-spring technology. The company would like to see economy cars save fuel with a variable-pressure pneumatic system that provided just enough “spring” for the rpm being used. With metal springs, a car engine on the freeway running at 2,600 rpm must bear the friction of springs chosen for that engine’s 6,500-rpm redline.

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