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Ducati Desmodromic

Ducati Desmodromic (Desmo) Valve control system - A brief insight.


The Desmodromic valve control system is totally unique to Ducati engines. Why is it used?...

Ducati engines are short stroke high revving engines (as far as twins go that is). The more conventional valve control systems used in Japanese motorcycles use a valve spring to return the valve to its closed position when the lobe on the camshaft releases its downward force. The problem with this system can be that as engine revs build up, a spring can suffer lag which in turn means that the valve may still be slightly open when the piston reaches top dead centre. If you are lucky, this will give loss of compression, for those less fortunate it can cause total destruction.
Expensive..............

The Desmo system uses a second rocker arm to positively lift the valve back to its closed position ensuring that there is always piston to valve clearance.

Below is an animation of a working model that will explain the actions and reactions of the Desmo system better.

Animation and images courteously supplied by Bluming Inc.

Please click on thumbs for large images....

 

Ducati Testastretta Engines - What's New?


Everything has a life . As time moves on and we petrol heads demand more and more power for our Superbikes, Ducati found that the introduction of larger and larger valves was required. Unfortunately the space within the cylinder head for these larger valves particularly retaining the valve angles that were being used just was not available any more.

As a for instance, the 916 engine used 33mm valves. The 996 increased to 36mm (both of these engines being pre - testastretta). In fact in order to house these valves in the 996 whilst still retaining valve overlap clearance, the inlet timing had to be retarded. This could be termed as a compromise which is generally a word not known in the vocabulary of the great Ducati.

As can be seen from the pictures, the valves on the 998 are more vertical and further apart at the base. The rocker pivot point is now on the outside of the cylinder head. A 10 mm plug aperture replaces previous 12mm housing.
Ducati 996 Cylinder Head
Ducati 998 Cylinder Head

It was time for a makeover - Enlisting the services of an engineer who had just retired from the Ferrari Formula 1 team and applying the futuristic development techniques that he had experienced. An all new larger diameter cylinder head that not only allows the use of 41mm valves, donates additional space to permit more upright, 12 degrees on intake and 13 degrees on exhaust - total 25 degrees as opposed to the former 40 degree valve angles whilst also achieving a larger and more efficient combustion path between inlet and exhaust valves. Inlet and exhaust tracts also had their entry and exit angles reduced in order to reduce the amount of "bend" of the gas flow path these have also been enlarged in order to cope with the additional flow. Less aggressive camshafts that lift higher than previously with a more robust oil fed cam bearing system and drive belts beefed up by some 20+% all form an important part of this dramatic re-design.
One thing that does remain unchanged is the use of the Desmo valve gear explained above.

In order for a larger cylinder head to be mounted, a larger cylinder bore diameter is provided which in turn requires a shorter stroke to ensure that displacement capacity remains pretty much unchanged. Shorter pistons = more rock within the bore. To counter this, the top piston ring is now steel as opposed to cast iron - 1mm as opposed to 1.5mm and chamfered in such a way that when the piston angle does change on the up stroke the ring sits flat against the bore wall.

Fuel injection was altered to a single injector with a much narrower spray pattern (30 degrees reduced to 15) ensuring that the fuel remains atomsied prior to combustion rather than being sprayed on to the bore wall. This now being controlled by a faster 20mhz processor that has doubled the ignition and fuel mapping area capabilities. More control = more power.

In real terms what this means is that the Testastretta engine revs higher, passes gasses faster and burns fuel far more efficiently whilst every component reaction has been catered for with regards to longevity of life and reliability. A win win win situation.

Welcome to the new life ........................... The Ducati Testastretta Life................... and with it the benefits of future development possibilities.

Watch this space....

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