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

Institute of TwoStrokes a écrit:

Frits anymore info you wish to share on you flow bench as I wish to build one. As I'm unsure if 'blowing' from under the transfers or 'sucking' out the exhaust duct is the correct method. Also I note on the Aprilia cylinder the unusual shape to the exhaust duct outlet(pipe step) , is that to minimise exhaust gas and fresh gas mixing or to keep flow velocity up (or both).

like I mentioned before, I will not reveal too much about pulsating flow benches. Not because you misspelled my name (just joking) but because many projects are realised in collaboration with others and I cannot reveal such collective achievements; not if I wish to continue working with them in the future....
Regarding the question of sucking or blowing: it depends which ports you want to flow. If you fix a cylinder up with a piston and a cylinder head, and you suck at the exhaust, you are really flowing the transfers ducts and the exhaust duct in series, and there is no direct way of telling which of them is limiting the flow. The logical way would be to remove the head and blow through the transfers if you want to flow those, and to suck at the exhaust duct if you want to flow the exhaust.

The stepped cylinder/pipe-junction is not all that unusual. I first saw it on the Van Veen Kreidler works cylinders in 1970. They had unvoluntary steps because of the flattened shape of the exhaust duct casting. It seemed logical to remove these steps, but that cost a lot of bottom power.
Nowadays I am still not 100% clear about the exact reasons, but I think there are two factors. First, returning flow from the exhaust pipe to the cylinder gets kicked up by the threshold and so concentrates on the upper part of the duct, thereby somewhat avoiding a collision with the piston and entering the cylinder with fewer losses.
Secondly, the volume of the exhaust duct should be as small as possible, obviously without impairing the flow. Why? Imagine that you have to pump up a bicycle tyre with a hand pump. If the hose between pump and tyre has a large volume, you will never succeed in pumping up the tyre, you will just fluctuate the pressure in the hose. It's the same for an exhaust pipe trying to shove escaped mixture back into a cylinder...

By the way, I was pleasantly surprised by the contributions of our new forum member Fügner. I did not get much sleep last night. Here's why:
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Hello @ all,

I would like to introduce myself quickly: I am 24 years old (or young:-)),motorcycle mechanic and I love 2stroke engines…..

In general I got say this Forum seems to be the most interesting place online for people who are interested in motorcycle technique , specially this topic is incredible!!!!

@ Frits,

Citation:
Regarding the FOS cylinder what will be the best method of induction ?
The best method of induction, when you consider the conventional solutions, is a rotary valve. But there are also unconventional posrisibilities; I am working on a system that I call the 24/7-inlet. I will explain later.

Could you tell me perhaps more about your idea about this 24/7 inlet? Sounds like the opening hours from a gas station:-) is it permanent? But how?

By the way it’s a pleasure to got the possibility talking to you

Willkommen Michael, und danke für die Blumen.
You guessed right: the 24/7-inlet is named after its 'opening hours'. Originally I wanted to keep the idea a secret until it was fully developed. But I suffer from an ever-increasing lack of time and I would love to see my ideas realised, so I have decided to reveal some of them, like I did with the symmetrical scavenging.

The 24/7-inlet idea arose in 1974 when the rotary inlet disk of my racer stuck open and it just kept running, as long as I kept the engine in the power band. It meant that I rould ride back to the pits instead of pushing through the grass alongside the track side (which I had already done often enough...).
This experience proved what I already knew in theory: that the pumping effect of the crankcase only serves to start the engine; as soon as it runs in the power band, the exhaust pipe takes care of all the gas movement.
So how do you build a rotary disk that stops in the open position when the engine revs reach the power band?
I had no idea...
But then why would you need a rotary disk anyway? Because it flows better than a reed valve? That doesn't matter when you only need it to start the engine. And a reed valve is much simpler to open...

Below you see an old prototype with a typical design error: it is too complicated. You do not need two gear-coupled reeds; one reed that swings out of the way would be enough. And it can be operated by the same servomotor that operates the exhaust power valve .

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I like this kind of parts: when you see them, it appears obvious!



And then, you tell us it is a wrong system...
(fortunately, you also tell us why... and more!)

hello guys,(bien vu Stéphane!)

And what about a "camembert" like air chamber in front of the carburetor to pulse or compress a little the air...?
I had seen that device in lateral square fours motors 500cc..

regards..

"guys" would have been better

avec ce système de boite à clapets les pertes par pompage dans les hauts régimes sont théoriquement moindres! Quelle belle idée!

Thanks for the reply and sharing so genius ideas with us Frits.
It’s very funny, because before two years I had a bit similar experience with an turbocharged scooter engine. Its got an central rotaryvalve taken from an italsytem (SV31 not100% shure) kart engine,and at the dyno by full throttle the belt popt off,the rotary valve also stopt at the right moment and the engine was taken even more RPM.
But I was thinking all the time it was the turbocharger who did it possible.The funny thing about it is that on page 6 in this topic is a picture from this engine and they think it was never working .:-)

Concerning your idea with the reed valve with one side that moves is as great than simple. I even think it would take no really long time to realize it, but to find the right exhaust chamber design will take some time,or not?. I will try to build such a reed valve and than i will tell you how it works. Thanks for this great idea.

Michael Burgard a écrit:

to find the right exhaust chamber design will take some time,or not?

Not . You may need to spend some time finding the optimum combination of crankcase volume, inlet diameter and inlet length, and with the carburettor settings, but you don't have to change anything about the exhaust system. It should suck and blow at the right moments, just as it should with a conventional inlet system. So if you have an exhaust that works well with a reed valve or a rotary disk, it should work just as well with my 24/7-inlet.

Thats great:-)
i will construct some kind of reed valve and show it next week in this topic.

really nice design Frits and thanks for sharing...
yesterday I was looking at intake mach numbers (flow velocity) on my freshly designed 100cc and noted how the reed valve phase lasts 300° degrees of crank rotation.
in the remaining 60° degrees there is a negative wave travelling through the inlet that shuts the reeds closed.
probably with further intake lenght tuning the intake phase would last slightly less than 360°... then why have a reed valve, that is an obstacle to the flow?

So Frits, nice idea.

Hello ,

very impressive and interesting way of seing things of two stroke. Photos and your knowledge Frits you are my (our with Boogie = Burgard) hero .

@ Boogie pour le système de boite à clapets au niveau emcombrement ça serait idéal sur du piaggio avec les "dentures" vers le cylindre et à l'extérieur pour voir comment agissent les lamelles .étancher le système et voir ce que ça donne en terme de perf.

Juste une question concernant le ressort , ne va t'il pas s'affoler à MI régimes (avant que ceux ci ne restent ouverts à hauts régimes) ? Sont ils supposés être en constante tension ?

Si le deux temps n'a pas vraiment évolué ces dernières années , il semblerait que certaines idées ingénieuses de ce type puissent changer la donne .

En tous cas merci pour les rêves.

Hello @ all,

Here my construction idea :

What do you think about, Frits ?


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Hallo Michael,
I see only one hinged reed in your drawing but I get the impression that you are planning to use two reeds, like I did in my original design.
Keep it simple and use only one reed, and shape the duct so that the flow is optimal when the reed is opened.

With todays cheap programmable ignitions this idea could be quite easily driven off a servo or solenoid and be throttle position/rpm dependent. I would look to have a central splitter (to direct flow at the reed tip) incorporated in the design.

Bon marché programmable allumages cette idée pourrait être chassés un servo ou solénoïde et être accélérateur emplacement/rpm contrôlée,très facilement. J'aimerais examiner d'avoir un répartiteur central (à directement à la clappets astuce) sont incorporées dans la conception.

Institute of TwoStrokes a écrit:

I would look to have a central splitter (to direct flow at the reed tip) incorporated in the design.

Forget the reed tip. Forget the whole reed! It is only there to start the engine. As soon as the engine is running in the power band, the reed has to hinge out of the way so that it will ly against a wall of the inlet duct. Then the duct should be just a rectangular channel with nothing disturbing the flow within it.

Hello Frits,
thanks again for the information,now i think i can way better imagine the construction you think about.I will show as soon as possible some new drawings.

Frits Overmars a écrit:

Institute of TwoStrokes a écrit:

I would look to have a central splitter (to direct flow at the reed tip) incorporated in the design.

Forget the reed tip. Forget the whole reed! It is only there to start the engine. As soon as the engine is running in the power band, the reed has to hinge out of the way so that it will ly against a wall of the inlet duct. Then the duct should be just a rectangular channel with nothing disturbing the flow within it.

I was thinking more about the part between starting and on pipe /in phase. Frits from your research is directly under the rising piston the lowest pressure during the intake phase? Did you ever experiement with enclosed cranks to keep intake away from the spinning crank?

Je pensais plus au sujet de la partie entre démarrage et le tuyau /en phase. Frits de votre recherche est directement sous l'augmentation la plus basse pression piston pendant l'admission phase? Avez-vous jamais, avec joint manivelles à garder apport loin de la filature manivelle?

The part between starting and coming into resonance is utterly unimportant compared to the flow when the engine is running in the power band.
You cannot say that the lowest pressure is under the rising piston; any pressure differences in the crankcase move with the speed of sound (well, not quite but almost, if you consider the laws of gas dynamics). The piston is only moving at about 10% of that speed so any difference that it might cause, is immediately evened out. And anyway, not the piston but the exhaust pipe is doing the major part of the suction.

In theory enclosed cranks are good. Jan Thiel did some experiments at Aprilia with a kart engine that had its reed valve at the front: the incoming mixture had to move against the direction of crank rotation. And although the crankcase stretched over the crank webs, reversing the direction of rotation brought another HP. So the crankshaft does have an influence.
But in practice, if you reduce the distances between crankshaft and crankcase walls to less than 1 mm, the viscous friction of the mixture between the surfaces really costs power at high rpm. And if you make the clearances so tight that lubricating oil can no longer reach the big-end and crankshaft bearings, it will also cost engines
Another negative aspect: any volume with a narrow 'entrance' between the crankshaft and crankcase surfaces acts as an hydraulic damper on the Helmholtz-resonance in the crankcase.
Aprilia has avoided this by making the space between the crank webs as wide as the big-end bearing. As a result the crankcase volume of the 125 cc RSA engine at TDC is about 650 cc, so the exhaust pipe really has some volume to breathe from.
So much for the fairy tale of high crankcase compression

By the way, Jan Thiel also measured the influence of the exhaust pipe on engine power. A good engine produced 52 hp then. Then the pipe was removed, and carburation and ignition were meticulously
re-optimized for the pipe-less engine. Its best power was 17,5 hp...
So you can say that a good exhaust pipe adds 200% extra power to a racing two-stroke :lol:

This is not mathematic!
17,5 + P = 52
Means the pipe without engine produce 34.5 hp...

OK

Nice logic, Marc! OK, the engine produces 52 hp. But if you remove the spark plug, it produces zero hp. So the spark plug alone produces 52 hp? Let's build twin spark engines then! Or triple spark. Or quadruple spark. Or ..... what comes after quadruple ?

you have the right to whip Marc
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he likes this

Frits

Frits Overmars a écrit:

So much for the fairy tale of high crankcase compression

I was glad to read your opinion on that.

Hello Fritz, thank you for your participation in this forum. A quick question, have you ever used ceramic bearings on your crankshaft, and what do you think of this application.