Desmodromic valve

 

GET TO KNOW WITH DESMODROMIC VALVE

 

Figure 1: Desmodromic valve system schematic

    A Desmodromic valve is a reciprocating engine poppet valve that is positively closed by a cam and a leverage system. This valve was the secret sauce of every Ducati's high-performance motorcycle engine. 

WHY A DESMODROMIC VALVE?

    Desmodromic valve was often justified by claims that springs could not close valves reliably at high-speed revving and that the forces caused by suitably strong springs exceeded what cams could withstand. Since then, valve float was analyzed and found to be caused largely by resonance in valve springs that generated oscillating compression waves among spring coils.

    High-speed photography showed that at specific speeds, valve springs were no longer making contact at one or both ends, leaving the valve floating before making a contact with the cam on closure.  For this reason as many as three concentric valve springs, press fit into each other, were often used, not for more force, but to act as snubbers to reduce oscillations in the outer spring. 

    In the world of racing, a traditional valve spring engine had an upper rpm limit of about 10,000 rpm. Meanwhile, an engine equipped with a Desmodromic valve system would be capable of revving up to blistering 15,000 rpm and able to produce more power. These are the reason why Ducati only choose one valvetrain option on every of their motorcycle engine.

DESMODROMIC VALVETRAIN DIAGRAM

Diagram 2: Desmodromic valvetrain main components

DESMODROMIC VALVETRAIN WORKING ANIMATION

ADVANTAGES 

1. Desmodromic valve opens and closes the valves mechanically. No spring required.
2. Prevention of valve float at high engine speeds.
3. Avoids higher spring pressure that causes greater friction in the valvetrain system.

DISADVANTAGES 

1.  Engine become noisy. 
2.  Valvetrain system has a complex mechanical and expensive when in come to maintenance cost.
3.  Valve clearance is larger when the engine is cold.