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MotoGP-4-cilinder-ducati
MotoGP-4-cilinder-ducati
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.
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MotoGP: WHY A FOUR CYLINDER?
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For
the
technically
minded |
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Based
on an idea by Lorenzo
Pavan
Student of Mechanical Engineering at Università degli Studi
of Padua |
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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. |
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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". |
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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 |
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where:
| 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. |
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•In
the case of a twin-cylinder engine,
Stot must
clearly be divided by 2, therefore, each piston surface is
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 |
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n = rev as RPM
c = bore
as m |
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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. |
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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. |
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• 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
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. |
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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. |
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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. |
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