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

Hakkelaar a écrit:

...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...?

Supercharging would offer a lot of additional possibilities, but has one major drawback: it is forbidden in the types of motorsport that I am interested in. If it were allowed, I would not need any form of inlet flow control, but then I would not use the crankcase either; I would blow directly into the transfer ports.
And yes, if you do blow into the crankcase, you just raise the pressures and the backflow behaviour will remain the same.

lesolexeur a écrit:

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.

Bonjour Mr. Lesolexeur,
You are right; it is not easy to calculate inlet resonance when so many factors are constantly changing. In fact is is too complicated to explain it here in the forum. It made me write a simple story, called:
Helmholtz blues
"A Helmholtz resonator consists of a volume connected to a duct". That is what Wikipedia tries to tell you.
But that is a Helmholtz resonator in its simplest form; one that you won't find anywhere in an engine.

What you do find in an engine is an intake tract with a variable cross-sectional area. This tract is from time to time connected to a variable volume (the crankcase) through a very variable window (the intake port / reed valve / rotary inlet).
The crankcase is connected to a number of transfer ducts with variable cross-sectional areas, which are from time to time connected to a very variable volume (the cylinder) through a number of very variable windows (the transfer ports).
The cylinder is from time to time, through a very variable window (the exhaust port), connected to an exhaust pipe with a very variable cross-sectional area who at the same time doubles as a volume. This pipe volume is constantly connected to a big volume (the outside world) through a tailpipe with a constant cross-sectional area and constant entry and exit windows (thank God, finally someting that's not variable).

We call this a compound Helmholtz resonator .

The various papers also tell us how to calculate the resonator's frequency:
" frequency = speed of sound / (2*pi)* Sqr ( cross-sectional area of the neck / ( volume of the resonator * effective neck length ) ) ".

O yes, the speed of sound... It is dependent on temperature, which is not really constant in the intake tract and the crankcase, rather variable in the transfer ducts and very variable in the cylinder and the exhaust pipe.

Now the above frequency formula is not exact; it is an approximation that is usable as long as the volume of 'the' tract is very small compared to the resonator's volume.
So when engines are concerned, that formula goes very far out the window.

Who said gas dynamics is simple dull?

PS:
It's a similar story with acoustics. That is a sub-branch of gas dynamics, simplified with a lot of assumptions that are acceptable as long as the sound pressure does not exceed a certain limit. The wave pressures in a two-stoke exhaust exceed that limit by a factor of thousand. Bye bye, acoustics...

Some times, some car manufacturers and motorbikes (yamaha) put a derivative duct attached to the main flow pipe, to smooth the resonance; I think it can be usefull to explore this combination with 24/7 system rather than to play with the other dimensions(lenght-diam size).
The exhaust is a resonator , why don't put another in the entrance and trying to optimise with avoiding putting some complicated moving parts....

cristogrr a écrit:

Some times, some car manufacturers and motorbikes (yamaha) put a derivative duct attached to the main flow pipe, to smooth the resonance; I think it can be usefull to explore this combination with 24/7 system rather than to play with the other dimensions(lenght-diam size).
The exhaust is a resonator , why don't put another in the entrance and trying to optimise with avoiding putting some complicated moving parts....

The 24/7-system has no moving parts, except for a very simple reed that is only there to start the engine;
once the engine is running in the power band, the reed is swung aside and does not move at all any more.
And like the exhaust, the inlet system is also a resonator; no need to complicate it with additional ducts or volumes.

Jan Thiel a écrit:

............
At high rpm there is some reverse flow, the transfer ducts becoming black, and even the
crankcase and carburetter bellmouth because of the entering burned gasses.
This happens because with raising rpm. the pre-exhaust flow becomes insufficient.
When rpm raises still more there is no more transfer flow and the engine stops completely.
So transfer flow depends completely from what is 'allowed' by the exhaust flow!

.

Also yesterday stated the need of a flow diode with in a sense is a reed valve but meaning without the restrictions of one i assume...

So GrahamB a full 360 inlet port will have serious practical drawbacks
Or else to build two stroke engine based on the theoretics of a pulse jet engine will have us with
a motor running at example .13000 rpm....but we will need a starter motor to reach that speed...
and never close the throttle again...(or with the 24/7 principle)
Anyway thats my(theoretical)opinion... if i had the resources maybe I could test them too,and prove myself wrong...
And yes it says transfer flow but can we separate them that "easily"(transfer/exhaust -inlet tract/crankcase)? have i got somethin totaly wrong?
edit.about the pulse jet:I havent got deep on how it works and i dont intend to either.
im just using it as an example to state the differences of application and needs.

Frits Overmars a écrit:

(The Cagiva was cured by inserting 1 mm strips between the reed casing and the reeds, so the reeds did not really close anymore. It cost more than 10 hp, but the laptimes improved...)

I was reading this famous thread again and came across this. Frits, do you understand why it would have lost so much power? I'd have thought it was a little way towards a 24/7 system, so it might work slightly better near peak torque?

Or does having the reeds almost closed mean they start resonating?

l'occitan a écrit:

Voila une des rares photos de son moteur
En 50 on comprimé à 11/1, ça marché bien quand on le prenais pas dans la gueule !!!
@+ L’occitan

[Vous devez être inscrit et connecté pour voir cette image]

motore bbft

GrahamB a écrit:

Frits Overmars a écrit:

(The Cagiva was cured by inserting 1 mm strips between the reed casing and the reeds, so the reeds did not really close anymore. It cost more than 10 hp, but the laptimes improved...)

I was reading this famous thread again and came across this. Frits, do you understand why it would have lost so much power? I'd have thought it was a little way towards a 24/7 system, so it might work slightly better near peak torque? Or does having the reeds almost closed mean they start resonating?

The 24/7-system has the 'mechanical' advantage that there is no reed hindering the flow, and the gasdynamical advantage that the flow does not need to be accelerated from a complete standstill with every revolution, provided that the inlet system's resonance matches the rpm. The Cagiva had neither of these advantages.
Moreover, soft reeds, and to an even greater extent reeds that do not close sufficiently, mess up the carburation: the suction signal to the needle jet will be weak. So it is not surprising that Cagiva became one of the pioneers of injecting a two-stroke.

Reed resonance can become a problem if the reed's root frequency falls somewhere in the engine's power band instead of well above it. That can happen with too much free length of the reeds and / or with reeds that are too soft. To my knowledge Caviva did not alter the reed's dimensions so I don't think reed resonance was an issue.

Good news: BMW predicts the return of the two-stroke.
[Vous devez être inscrit et connecté pour voir ce lien]

Rasputin, you were in the middle of it. Would you care to comment? Or are you still sworn to secrecy?

and he argument are full of sense, contrary to policies liar

I would like to show the scooter engine we are building in the Italian machining factory FTM - (we produce the CVT FR4 variator if anybody knows...)

It is much inspired from RSA, but reed valved to speed up development and cut costs, power will be enough even with reeds i hope.

It is made by welding a new aluminium block in place of the original 50cc crankcase, and then machining it.
engine is thought to be "not very expensive" so I had to choose some compromises.

[Vous devez être inscrit et connecté pour voir cette image]

cylinder is a small RSA replica aswell (50mm bore 48mm stroke, due to available pistons and capacity limitations). bridged exhaust because we are keeping it small (95% bore, no piston plug caps)
[Vous devez être inscrit et connecté pour voir cette image]

crankcase volume is high and I have a 2-stroke fuel injection kit running around, so 24/7 is OK to be tested here. (but i have much work to do before that...)

I went directly for high inertia cranks, because rubber belt CVT suffers much the crankshaft speed fluctuation, I feel high inertia improves power transmission. Also the engine speed will be almost steady (thanks to CVT) so a quick revving engine is not that important.

here the (unfinished) crank webs (next to original 50cc Piaggio ones)
[Vous devez être inscrit et connecté pour voir cette image]
We are going with 100mm conrod, 18mm crank pin and 14mm piston pin.

Exhaust will be tuned for max power around 14.6k rpm, or approx. 23 m/s with 48mm stroke.

Much informations comes from this topic, so thanks everybody, and also I feel the need to personally thank Frits and Martijn (Emot)
the ignition side of the case will be machined from full block in these days...

40hp are possible with 13 bar bmep @ 23m/s, I think is possible to reach this target, what do you think?

If i remember Jan Thiel's words, Aprilia did 14.8 bar. but we don't have that technology...

C est un post de folie, domage qu il soit en anglais; C est comme si on essayait de converser avec une très jolie fille qui ne parle pas la même langue; Frustration!!! Moi y en a vouloir comprendre quoi Jan Thiel veut dire.

@nasone32

Something to think about while the rest of the family occupy themselves with boiling, hiding, finding and eating easter eggs: a couple of recent videos from the Dutch 50 cc scene.
[Vous devez être inscrit et connecté pour voir ce lien]

very nice, great work there.

the exhaust variaton looks very long to me,
from my (small) experience on kart engines there are 2-3 centimeters on the muffler that make quite a difference, but after that you don't gain that much.

maybe they can trade off lenght variation for precision and faster exhaust response?

the "conrod actuator" looks promising and reliable! a stepper motor can provide high torque and position accuracy. (do they use a stepper?)

The length variation is from 70 mm longer to 20 mm shorter than standard. I would say these are excellent values. The servo motor comes from an Aprilia RS250 street bike where it controls the powervalves and the oil pump.

A 90mm variation looks quite excessive to me!
Even a 2mm variation has a big influence on carburation!

I like it, its kind of sexual

what sort of sealing ring have you used on the pipe to keep it airtight ?

cheers, david

rgdavid a écrit:

.....what sort of sealing ring have you used on the pipe to keep it airtight ?

A viton O-ring, for sealing and low-friction guidance. But the piece of pipe that is attached to the cylinder, gets very hot because it is shielded from the surrounding air by the pipe that moves over it. Too hot for the Viton, apparently. Any bright ideas anyone?

A silicone O-ring with a silicone grease(Dow Corning DC4) should work well.Suzuki used silicone O rings on all their 2T street bike exhaust head pipe to muffler connections in the 70's..

nice concept

nasone32 a écrit:

I would like to show the scooter engine we are building in the Italian machining factory FTM - (we produce the CVT FR4 variator if anybody knows...)

It is much inspired from RSA, but reed valved to speed up development and cut costs, power will be enough even with reeds i hope.

It is made by welding a new aluminium block in place of the original 50cc crankcase, and then machining it.
engine is thought to be "not very expensive" so I had to choose some compromises.


cylinder is a small RSA replica aswell (50mm bore 48mm stroke, due to available pistons and capacity limitations). bridged exhaust because we are keeping it small (95% bore, no piston plug caps)


crankcase volume is high and I have a 2-stroke fuel injection kit running around, so 24/7 is OK to be tested here. (but i have much work to do before that...)

I went directly for high inertia cranks, because rubber belt CVT suffers much the crankshaft speed fluctuation, I feel high inertia improves power transmission. Also the engine speed will be almost steady (thanks to CVT) so a quick revving engine is not that important.

here the (unfinished) crank webs (next to original 50cc Piaggio ones)

We are going with 100mm conrod, 18mm crank pin and 14mm piston pin.

Exhaust will be tuned for max power around 14.6k rpm, or approx. 23 m/s with 48mm stroke.

Much informations comes from this topic, so thanks everybody, and also I feel the need to personally thank Frits and Martijn (Emot)
the ignition side of the case will be machined from full block in these days...

40hp are possible with 13 bar bmep @ 23m/s, I think is possible to reach this target, what do you think?

If i remember Jan Thiel's words, Aprilia did 14.8 bar. but we don't have that technology...

Great work, 40Bhp? that will make 50cc scotters faster than most road 125's 2t. They will be scary

I would have loved the oportunity to work on a project like that

Frits Overmars a écrit:

rgdavid a écrit:

.....what sort of sealing ring have you used on the pipe to keep it airtight ?

A viton O-ring, for sealing and low-friction guidance. But the piece of pipe that is attached to the cylinder, gets very hot because it is shielded from the surrounding air by the pipe that moves over it. Too hot for the Viton, apparently. Any bright ideas anyone?

Maybe you could try a few turns of Kapton film or similar Polymide compounds

I had on my old car a special joint in the exhaust , it was made with metallic foam with carbon-graphite coating because the two pieces weren't the same diameter , the shape was like a trumpet connected in a tube , if you want this kind of product for your researches...(opel make)
I'm working in a polymers company , these temperatures are too high for them (400-800°c); the more resistant is the teflon with glass fibers charged but it burns above 440°c...
The ceramics rings could be another possibility , there the limit is higher than the metallic pipes
Cristo