[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 1) (Locked)

@Frits -> Perhaps skip the variable intake length in the first prototype to simplify the design

A simple idea how to measure the torque need to move the rotary disc. Don't know if it's good or bad.

Take a tandem twin, remove ignition on one cylinder and make a dyno run. Then remove the disc on the cylinder not firing and make a new run. Then the difference in the runs would be an indicator of the torque needed.

Mic a écrit:

A simple idea how to measure the torque need to move the rotary disc.... Take a tandem twin, remove ignition on one cylinder and make a dyno run. Then remove the disc on the cylinder not firing and make a new run. Then the difference in the runs would be an indicator of the torque needed.

The gas dynamics in the non-firing cylinder are quite different from those in the firing cylinder, as will be the crankcase pressure variation. So this is not a good way to measure the disc friction.
But measuring the friction is pointless anyway, because the inertia torque is much larger.
Imagine a rider shifting down for a corner: dataloggers on works Aprilias have registered spikes of 17.500 rpm. Just think of what this means for the disc drive torque...

Back to basics: the shape of the power curve is determined by the exhaust resonance and varying the total opening angle of a rotary inlet disc is rather useless. It is however possible to keep the total angle constant and rotate the disc position on the crankshaft a little: open and close it early in the lower part of the power band and shift to late at higher revs. This has already been practiced on some kart engines, with a simple coarse-pitch drive between the crankshaft splines and the disc hub.

Is it needed to run the disc for overreving under braking?

The motor isn't pulling anyway so why not just leave the inlet constant open until the engine speed drops down to 0.8*peak power rpm.

For RSA this would mean that the disc is only needed until 10.400 rpm. Above that constant open.

Mic a écrit:

....For RSA this would mean that the disc is only needed until 10.400 rpm. Above that constant open.

You'll get there eventually, Mic . A good 125 cc rider can always keep the revs above 10.500 rpm, so you do not need any power below that rpm. And if you don't need power, why would you bother with a rotary disc? Use a simple reed valve and swing it out of the way as soon as the engine runs in the power band: that is the philosophy behind my 24/7-system.

In my previous post I wrote something about the inertia forces on the disk during gearshifting. What you propose, takes this to the next level: synchronizing a stationary disk with a crankshaft running at 10.000 rpm within one revolution may well require 50 HP....

The reason for I considered the rotary valve solution for you 24/7 system was because of the perfect flow.

You could have any hole shape and with a perfect smooth inlet.

Mic a écrit:

The reason for I considered the rotary valve solution for you 24/7 system was because of the perfect flow. You could have any hole shape and with a perfect smooth inlet.

I appreciate the thought, Mic. But a lateral rotary valve messes up the inflow to the transfer ducts at one side, and a rear disc requires a complicated drive system (setting up the double conical drive of the RSA125 is a nightmare). A smooth rectangular hole (with the starting-reed swung out of the way) flows fine. And remember the abbreviation KISS : Keep It Simple, Sport .

I reckon that my idea is more complicated than a simple reed based solution.

Just tried to come with a different solution

The "swing" motor for the reedcase would also have to react very fast in order not to disturb the flow with a half open reedcase during change from reed to 24/7. If it takes to long the flow rate would drop along with the power during the change over.

Mic a écrit:

The "swing" motor for the reedcase would also have to react very fast in order not to disturb the flow with a half open reedcase during change from reed to 24/7. If it takes to long the flow rate would drop along with the power during the change over.

No sweat, Mic. The reed can take its time swinging out of the way; a half-open reed has less flow resistance than a 'normal' reed.

Citation :

@Hakkelaar: your proposal of shifting the phase of the disk by shifting the belt tensioners was used by Porsche over 20 years ago; it was their first version of a variable inlet camshaft drive.

Damn those Germans... Hahahahaha!

The idea of shifting the fase like that can probably be found in lots of books about solutions for mechanical drives, etc. Now I'm just curious, has anyone ever tried to construct a system with variable rotary disc timing? I guess that it would only be an advantage when accelerating out of a corner while the revs are below the powerband. A little slip of the clutch would probably achieve the same ;-)

The idea of the 24/7 intake really intrigues me. I was wondering... Would it be of an advantage to use a relatively long and small diameter intake tract between the reed valve and the crank case? The small diameter and long tract would raise the speed and therefore the inertia of the intake charge, making it harder for the flow to reverse.

Maybe I'm just acting like a parrot and repeating things that are already mentioned. It's hard to remember all the 37 pages of this thread...

Greetz,

Jacob

Hakkelaar a écrit:

... has anyone ever tried to construct a system with variable rotary disc timing? I guess that it would only be an advantage when accelerating out of a corner while the revs are below the powerband. A little slip of the clutch would probably achieve the same

Variable disk valve phasing has been tried in the past in some kart engines, as I mentioned above. These were direct drive, so slipping the clutch was no option.

Citation :

The idea of the 24/7 intake really intrigues me. I was wondering... Would it be of an advantage to use a relatively long and small diameter intake tract between the reed valve and the crank case? The small diameter and long tract would raise the speed and therefore the inertia of the intake charge, making it harder for the flow to reverse.

You are on the right trail, Jacob. The combination of crankcase volume, inlet tract length and diameter should be chosen so that the direction of inlet flow never reverses when the engine is running in the power band.

Hakkelaar a écrit:

.... Now I'm just curious, has anyone ever tried to construct a system with variable rotary disc timing? I guess that it would only be an advantage when accelerating out of a corner while the revs are below the powerband. A little slip of the clutch would probably achieve the same ;-)...


Greetz,

Jacob

I was also curious,seems that not many efforts have been utilized after all or the results
have not been known.
It all comes down to what we need from an engine.
Ill toss the reed valve out of my KTM 125 exc and try to keep the revs above 10500,
on hard rock or muddy terrain...

And when I retarded inlet closure 2 or 3 degrees my engine lost a lot of power at 10.500 revs!
This might work on an aircraft engine with constant rpm.
Or with a flow diode!
A small diameter and long trackt would destroy power.

Jan Thiel a écrit:

And when I retarded inlet closure 2 or 3 degrees my engine lost a lot of power at 10.500 revs!
This might work on an aircraft engine with constant rpm.
Or with a flow diode!
A small diameter and long trackt would destroy power.

Where did you gain with retarded inlet closure 2 or 3 degrees?
But we love motorcycles and we have to ride them out of corners and open the throtle asap
with the little tyre contact patch we have available,we want power AND progressively ...

I gained above 12500 rpm

Jan Thiel a écrit:

I gained above 12500 rpm

So if you could keep the timing at +3 degrees @ 10500
and retard at 12500 you could have the best of both worlds?

Sure, but with a variable timing I would like to have more than 3° variation!

Jan Thiel a écrit:

A small diameter and long tract would destroy power.

But you never tried it in combination with 360° inlet timing, did you?

No!

Anyway, small is relative. I might use a 38mm carburetter on a 125 cc engine in combination with a very large crankcase volume.

Jan Thiel a écrit:

Sure, but with a variable timing I would like to have more than 3° variation!

So something based on the beauty of the rotary valve and the luxury of a simple form of variable timing is something you would have tried?

Frits Overmars a écrit:

Anyway, small is relative. I might use a 38mm carburetter on a 125 cc engine in combination with a very large crankcase volume.

Funny, makes me think of the other Thiel(*)... who gave his name to the Thiel-Small parameters for parameters for loud-speaker design... including oscillation of bass-reflex ports.

Wouldn't a long tract and large crankcase volume both tend towards lowering the resonant frequency to the point where you would end up with a sharply-peaked interaction with engine rpm?

(*)... just checked, spelling was different: Neville Thiele...

GrahamB a écrit:

Wouldn't a long tract and large crankcase volume both tend towards lowering the resonant frequency to the point where you would end up with a sharply-peaked interaction with engine rpm?

They would both lower the resonant frequency, ideally to the point where the inlet flow would never come to a full stop. But being a Helmholtz resonance, there would not be any sharp peaks, just a lazy fundamental sine.

Frits Overmars a écrit:

But being a Helmholtz resonance, there would not be any sharp peaks, just a lazy fundamental sine.

A sinewave is a very sharp peak viewed in frequency domain... the width of the peak is determined by the amount of damping (1/Q), which I'd expect to be less for a long than a short tract: mostly it comes from where the ends of the oscillating column of gas flow into the outside atmosphere... which would stay roughly constant with the length and hence smaller relative to the oscillating mass in a longer tube.

bonjour tous le monde

Mr frits. pour votre système de boite a clapet 24/7
Comment pourrait t' on calculer une résonance avec un carter moteur qui voit son volume modifier constamment par le déplacement du piston?
Ou prendrait on une moyenne du volume carter, ou encore un moment précis dans le temps moteur qui donnerait un volume carter
Il faudrait aussi calculer avec la section de passage du carburateur au moment donner (encore une autre variable).
Enfin est ce que lorsque les conditions le permettent le "phénomène" se déclenche instantanément ou il y a t' il un délai de déclenchement?

Si vous pouviez éclairer ma lanterne.
edit: et aussi la pression dudit carter qui change non seulement avec le déplacement du piston mais aussi avec les phase d'admission et échappement.
Merci

If the inlet tract is open 360 degrees, smooth and free of obstructions, the diameter shouldn't have to be too large I guess.

Almost 10 years ago, I was in the Formula Student competition and we were restricted to a max. displacement of 610 cc and an intake restrictor of 20mm The white thing pointing up in the picture. Using CFD, we and other teams tried to maximize the flow. Some (turbocharged) teams claimed up to 90 hp at the crankshaft (Yamaha R6 engine). The turbo had to be placed down stream of the restrictor. Of course we could use an intake plenum and we had to use fuel injection. We also mounted a supercharger as you can see in the pciture below.

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To get back on topic.... In theory the intake diameter doesn't need to be very large when a high velocity can be reached.

I was day dreaming about the effect of supercharging combined with the 24/7 system to reduce possible back flow and to increase the total intertia of the intake system, so to speak... Or would this just increase the crankcase pressure in the same ratio as the intake pressure is raised and the effect will be cancelled...?