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THEORY AND HISTORY OF THE DUCATI DESMODROMIC ENGINE

Second Part

[...] This valve system, known as “Desmodromic” (which means controlled movement), is based on a very simple concept: as early as in the beginning of 1900s, the first examples of its application were tested with sometimes disastrous, sometimes excellent results.This, in theory, extremely simple concept encounters, when applied in practice, mechanical and construction difficulties not easy to overcome, depending on the construction solution used. It is useful at this point to briefly dwell upon the characteristics of some of the most representative valve motion law diagrams.

Knowing beforehand the distance covered, i.e. the lift h, and the time required to cover it, we are able to establish the motion law trend by assuming that certain fundamental geometric parameters are verified. These include the absence of mechanical straining in the parts and of derived oscillatory motion, to be considered at a later stage.

The most intuitive diagram is the symmetrical rectangular acceleration diagram, in which the time is represented on the abscissa and the acceleration, velocity and space are all represented on the ordinate axis. The resulting motion is uniformly accelerated, therefore:

If n¹ is the number of engine revolutions per minute, and

is the number of camshaft revolutions, the angle covered in one minute is 2 π n radians.
The angular distance covered in a second will be

Consequently, the ascent angle expressed in degrees is equal to

or 360° = 2 π n radians

If θ^°₁ is the ascent angle in degrees, in radians it will be equal to

and the time required will be

But because half of the lift is covered accelerating and the other half decelerating, after the time

Therefore

Speed being a linear function, it is represented by straight lines.
The distance covered (i.e. the lift h + ∆h) being a quadratic function, it is represented by two equal parabola arcs.

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