DESMODROMIC VALVE PDF

Desmodromic valve drive was often justified by claims that springs could not close valves reliably. There was once a time when Desmodromic valve technology was cutting edge and a necessary competitive advantage for Ducati in racing. Ducatisti around the world know that the red bikes have desmo valves. Not all of them can tell you what that means, exactly. The Greek words.

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Understanding the Operation of a Desmodromic Valve

After World War II, when future Desmodrmoic 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. Something better than metal springs was needed, and Ducati, bywas ready to win the cc rpm race. This success started Ducati down a path it has never left.

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.

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Thus desmo eliminates the usual problems with springs read onwhile 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. No sooner do improved materials appear than they are exploited. Back and forth went the cycle—materials improvement, followed by falve valve acceleration. BySuzuki was changing springs in its MotoGP engines every night. The problem is that metal wire has inertia.

Ducati: Moto, MotoGP & Superbike

When a high-acceleration cam begins to lift a valve, wire coils pile up against the spring desodromic 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. Several F1 engine constructors prototyped desmo systems of their own Ferrari and Cosworth, for example but did not pursue them.

Jean-Pierre Boudy, at Renault indevised 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 valvs 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.

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The space under this piston contains nitrogen gas at a moderate pressure, such as 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 Deskodromic is desmodormic a provider of pneumatic-spring technology.

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