Desmodromic valve mechanism

What are desmodromic valves?

Desmodromic valves are those which are positively opened and closed by a cam and leverage system, rather than relying on the more conventional valve springs. The term desmodromic is derived from two Greek roots, desmos (controlled, linked) and dromos (course, track).The Desomodromic valve system was invented by Dr. Fabio Taglioni.

Why desmo?

What pushed “ingegner” Fabio Taglioni, and everybody else at Ducati, to persevere with the desmo system?

In an interview of 1989, Taglioni himself explained that, in mechanics (which means, in real life) there is not ONE best solution in absolute terms. The secret of success is to develop one’s intuition with devotion, logic and ingeniousness.

“…it was just by doing calculations that I had to conclude, at the end of a series of logically connected, very clear engineering considerations, that it was necessary to eliminate the spring preloading!”

Taglioni knew that it would be a road bristling with obstacles connected with designing and manufacturing, but he wanted to leave behind the two traditional drawbacks of a spring system:

• heavy spring loading, which meant more engine work (and the resulting power loss)
• “valve bounce” at high RPM.

These problems are all solved by a desmodromic system: smoothness (and consequently, decreased losses) at low RPM and reliability at high RPM (without valve bounce) are obtained.

The valves in question are those in a typical four-stroke engine that allow the air/fuel mixture into the cylinder at the beginning of the cycle and allow exhaust gases to be expelled at the end of the cycle. In the conventional four-stroke engine, a spring is used to apply pressure to the valve and return it to the valve seat or closed position. The valve is either directly or indirectly opened by the camshaft. Whereas in a desmodromic engine, the operation of the valves is are controlled by the cam. The diagram given below is self explanatory.

Who are the groundbreakers of this technology?

Ducati and Mercedes are two major companies who have used the desmodromic system in racing engines. Ducati has the most experience of any manufacturer in the world at successfully applying desmodromic valve control to production machines

What are its advantages? 

• There are no heavy power robbing springs used to close the valves in conventional engines.

• Better protection of the engine if it is over revved. This is because when the motor is over revved, the  valves are still controlled, whereas when they are returned by springs the valves can “float” and hit the piston.

• Better performance and efficiency.

• Finally, the manufacturer can use wilder cam grinds for better performance.

Any disadvantage with the system?

 

The only disadvantage of this system is the complexity of design and cost.

 

What is a desmosedici engine?

 

A desmodromic engine is one with two cyliders in V arrangement. A desmosedici is a 16 valve V4 engine. Sedici in Italian means 16. Hence the name

 

 

 

                                              A desmodromic engine

 

 

 

A desmosedici engine

 

If in a racing engine, a normal valve spring engine has an upper rpm limit of about 10,000 rpm, that same engine design when equipped with a Desmodromic valve system would be capable of revving till 15,000 rpm and the result is more power.

 

Ducati has consistently used its desmodromic system ever since 1956. It is the only manufacturer in the world to have applied it to anything from standard production bikes to Superbike glory: the achieved standard of excellence mirrors Ducati corporate technology.

 

 

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