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Hieronder treft je een artikel aan die gaat over de  redenen waarom Ducati voor de MotoGP afgeweken is van hun befaamde 2-cilinder techniek, de befaamde L-twin. Dit artikel stond op de site van Ducati, maar door het nieuwe ontwerp van de website is dit artikel ( en nog vele andere mooie pagina's over de historie en techniek van Ducati niet meer voor handen).

Ik heb deze pagina min of meer kunnen redden en de uitleg over de keuze voor de 4 cilinders wil ik graag zichtbaar houden voor iedereen.


For the

Based on an idea by Lorenzo Pavan
Student of Mechanical Engineering at Università degli Studi of Padua

Ducati's choice to tackle the MotoGp adventure with the new Demosedici engine left certain supporters somewhat perplexed.
The amazing sequence of wins in the Superbike World Championship only confirmed how extremely competitive the Testastretta twin-cylinder is, and the lap times obtained by the factory 998F Ducati suggested that this bike, minus 30 kg according to the MotoGp regulations, would be immediately competitive in the new class too.

The same considerations were obviously made at Ducati: in fact, the feeling (and the evidence) is that a more conservative choice would probably enable Ducatis to become immediately competitive. But what the Ducati Corse engineers have done is tolook forward, laying the foundations of a successful project with enormous margins of development.
The reasons behind this choice are summed up by a sentence uttered by Massimo Bordi, the engineer who, with engineer Gianluigi Mengoli, "fathered" the latest 4-valve twin-cylinder Ducati engines. 
Among other things he said, "regulations make the engine".
This statement, confirmed by Filippo Preziosi, from whose "pencil" the new Desmodici was born, stems from the experience of great designers, but it only takes a few simple calculations to make everybody see the reasons behind Ducati's choice to develop a new engine architecture, whilst knowing all the while that a few Ducatisti wouldn't agree.
A fairly good conclusion can be drawn from the analysis of a simple formula, taken from a bibliography in this field, and very useful for an approximate calculation of the power: although it is subject to heavy approximation, it contains two very important engine behaviour indications, i.e. the Mean Effective Pressure (MEP) and average piston speed (Um).
Notwithstanding the cylinder number and arrangement, we can write:
P = K · MEP · Stot · Um da cui
Stot =
  K · PME · Um  
P = Maximum power 
as Hp
PME = Mean Effective Pressure 
as bar
Stot = Total piston surface 
as mm2
Um = Average piston speed 
as m/s
K = Coefficiente dimensionale pari a 100/3

By assuming a maximum power of 230 cv, a MEP of 13 bar, and an average piston speed Um of 25 m/s (all values not far from the truth in racing engines), we will obtain a total piston surface of:
Stot = 21230,77 mm2
This basic information enables us to carry out a more detailed analysis of 2- and 4-cylinder engines.

 •In the case of a twin-cylinder engine, Stot must clearly be divided by 2, therefore, each piston surface is
Stwin =
= 10615,39 mm2
with a resulting bore of
Øtwin = 116,25 mm
Knowing that, according to the regulations, the total displacement must not exceed 990 cm3, the resulting stroke will be
Ctwin = 46,63 mm

Given the Um to n ratio

n =
  60 · Um
  2 · C
con: n = rev as RPM
c = bore as m
we can see that the maximum power value initially set can be achieved at a rev of about 16.000 RPM.
We can immediately see that the Øtwin value is rather high. As a term of comparison, consider that, in the Testastretta engine fitted to the Ducati 998R 2002 version, the bore is 104 mm.
Unfortunately, such a high Øtwin bore currently causes combustion problems with dramatically decreased efficiency.
This stems fundamentally from the need to augment the injection advance and from the worsening of the "shape factor" of the combustion chamber which, with the reduction of the bore/stroke ratio, becomes ever broader and flatter. The "shape factor" is a critical synthetic value to check a combustion chamber's good operation, and a good indicator of its compactness and "thermal efficiency".
It should be borne in mind that aspirated racing engines require rather extreme valve lift and overlap angles, therefore, cavities are made in the piston crowns to prevent contact with the half-open valves. The combustion chamber is therefore practically contained in the piston cavities, such cavities becoming bigger as the stroke/bore ratio decreases, which makes it hard to obtain the high compression ratios required by high specific power engines.

On top of all this, a very high bore leads to an increase in the weight of the masses in reciprocating motion(con-rods and pistons), exposing parts such as the crankshaft and main bearings to more and more severe stress as the RPM increases, and cancelling all the advantage given by a reduced stroke/bore ratio.
On this subject, the MotoGp regulations ban the use of lightweight materials such as beryl, used in the past in F1 too, and now forbidden because it is noxious; aluminium alloys, on the other hand, cannot be used below a certain weight threshold or reliability would be affected.
To sum up, a twin-cylinder engine with the required specifications appears right from the start to be a highly stressed engine.

 • Let's now consider the four-cylinder engine.
By applying (1) again and keeping the original figures, i.e. 230 HP maximum power, 13 bar MEP and Um = 25 m/s, we will obtain once again:
Stot = 21230,77 mm2
Bearing in mind that we have 4 cylinders this time, we will obtain a piston surface of
Sfour =
= 5307,69 mm2
and therefore a bore of
Øfour = 82.21 mm
while the stroke remains
cfour = 46.63 mm
Since Um has not changed, the maximum power value will be achieved again at 16.000 RPM.
Our resulting stroke/bore ratio is therefore 0,567, very close to 0,565 as in the latest 998R SBK, which is undoubtedly a good starting point for further development.
If we wish to achieve even more power at higher RPM, there are sufficient margins to increase the bore and decrease the stroke some more, thus making the engine "readier" to rev.
In this sense, the choice of an "L" configuration (with banks of cylinders at 90°) has the advantage of bringing into effect an intrinsically balanced engine, minimizing the vibrations and increasing mechanical efficiency and reliability to the highest value.
As regards an in-line 4 cylinder engine, a L4 has also a shorter crankshaft and, so, is more rigid in relation to weight and less cumbersome in the traversal direction, as illustrated in the sketch on the right. This allows for a decreased width in the frontal section of the bike and, as a consequence, improved aerodynamics.
Ducati have been studying a possible "twin-pulse" configuration, where combustion in cylinders belonging to the same block occurs simultaneously, which is expected to make the new Desmosedici's behaviour more similar to a traditional twin-cylinder's, imitating its excellent performance in terms of drive and low-RPM torque.

On this subject, it should be borne in mind that in motorcycle engines, delivery is critically important to the point that it sometimes outweighs maximum power. Not so in Formula 1, where "grounding" problems are not so critical and where engines have specific power values much higher than the value assumed in the calculations here above.
Additionally, a "twin-pulse" at 17000 RPM sounds like a twin-cylinder, which would gratify at least the hearing of the unyielding supporters of twin-cylinders.

 • The MotoGp World Championship regulations also accept oval pistons, but given the current state of technological progress in this field, this type of architecture seems an "exercise in style" rather than a truly advantageous option.

The MotoGp class regulations also take into account the advantage, in terms of performance, of "over 4 cylinder" engines, prescribing different weights according to the chosen architecture.
The regulations section concerning the minimum permitted weights indicates:
  • for twin-cylinder and three-cylinder engines: 135 Kg
  • for four- and five-cylinder engines: 145 Kg
  • In the event that oval pistons are used, 10 kg must be added to the above-listed weights.
A 10 kg advantage granted to less-than-4-cylinder bikes can certainly prove beneficial in terms of handling and braking behaviour, but these are not essential factors. Evidence comes from years of 500cc class competitions, where twin-cylinder and 3-cylinder 2-stroke bikes, notwithstanding they were lighter, never managed to be really competitive. Also in the Superbike World championship, the success of twin-cylinders had everything to do with engine specifications and very little to do with the weight advantage, which by the way, was progressively reduced over the years without any apparent effect on races results.

You can download from here a simple file that allows you to make the calculations we have shown so far, with the additional possibility of being able to vary the principal parameters and directly observe how the characteristic dimensions of the engine change. I would like to underline that these calculations lay no claim to even coming close to the accuracy of the powerful calculation codes used for in-depth studies on this matter.
I think, however, that this paper can give a rather clear general idea of the reasons behind Ducati Corse's technical choices.

Niko Baldini
laatste wijziging:  29 augustus 2010