[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)

Thanks Frits !

Good morning Frits, good morning everyone, trying to understand the two-stroke exhaust I assimilated this to a helmholtz resonator, but then analyzing the pulsations that occur inside I seemed to understand that in the end the resonance does not occur but is exploited "only" "the reverberation of the exhaust impulse. Is this a correct interpretation in your opinion? Thanks

Quand vous dites 140% de remplissage c'est en rapport a la cylindré, 124.8cc, ou en rapport a la cylindré plus la chambre de combustion, 124.8cc+8.6cc=133.4cc ?

Comment faites-vous pour calculer ce taux de remplissage, c'est une opération que vous pouvez nous décrire ici ou cela est trop complexe?

Dans votre formule mathématique pour le calcul de la chambre de résonance(pot d'échappement) pourquoi visez-vous le régime de puissance maximal, je pensai qu'il serai logique de viser le régime de couple maximal?

Est-ce que quelqu'un a le lien de la discussion sur kiwibiker qui traite du conduit d'échappement sur les moteurs deux temps? je l’avais vu(dans ce sujet) sans l'enregistrer en pensant pouvoir le retrouver facilement, maintenant je ne le trouve plus!!

Merci d'avance.

Bonjour jan, cette question est pour vous si vous venez occasionnellement ici, plus tôt dans la discussion vous avez dit que si vous faisiez un nouveau moteur il aurait une valve d'échappement en acier inoxydable d'une épaisseur de 1.5mm, est-ce que j'ai bien compris? Pourquoi en acier inoxydable et pourquoi si fine, 1.5mm me parait fragile, cela serait peut être pour une valve diabolo évidé?

Bonne journée a tous! NONO

My apologies if this topic has already been discussed elsewhere, I didn't have any luck finding it. I'm interested in pipe centre section diameters. I was under the assumption that bigger the better as long as you can make it fit.
Using engmod2T, I ran a series of simulations comparing pipes derived using the program's Blair pipe generator. I kept all parameters the same except exhaust port to centre section coefficient. 6 pipes ranging from 2.5 to 3.75x. For the most part, the simulation results show the larger the pipe, the better. Although Max peak hp was from the 3.25 pipe, over rev seemed to be better on the larger pipes.
I'm wondering if these results would match reality, or if there is an ideal pipe to cylinder volume ratio that would supersede these findings?
TIA


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there is limited energy in the exhaust gases. the bigger the center section, the more energy is used for sucking and the less is left for pushing back. so eventualy there will be a tipping point when bigger becomes to big.

Hello,

for my current project, a 250cc tandem prototype I need an exhaust system made in titanium.
The silencers are homemade, in titanium, too.
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Flanges, restrictors, ect. are milled yet. All sheet material ect. is existent. Unfortunately I don´t have the machines to get this done.

SAMBA-racing is not responding. Anybody here knows an adress to get this done? Sould be up to 1000km around Frankfurt (Main).
Here a picture, but without cooling and some other stuff.
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Thanks in advance for your help.
Best regards,
Bernd

Il me semblait avoir compris que tout matériau exotique comme le titane ou l'inox est à proscrire pour les pots de détente. Une question de résonnance.
Je me trompe ?

De mémoire les pots des dernières 500 de gp étaient en titane, a remplacer tous les 3 gp pour cause de modification des caractéristiques de résonance.

crigar a écrit:

Il me semblait avoir compris que tout matériau exotique comme le titane ou l'inox est à proscrire pour les pots de détente. Une question de résonnance.
Je me trompe ?

C'est largement employé sur les 4 temps, pourquoi pas sur les 2 strokes si c'est bien fait avec les bons matériaux et épaisseurs idoines?
Dan

Théoriquement le pot de détente joue le rôle des soupapes et assure un meilleur remplissage pendant la phase admission. D'où la forme du pot et les résonnance d'ondes de retour moins importantes sur un 4T.
Ce n'est pas une affirmation de ma part car je n'ai ni compétence ni expériences valables.

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Voilà, juste à temps pour les vacances.

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Avec ce lien, vous pouvez télécharger les versions les plus récentes de

Logiciel FOS

et

Conseils et concepts FOS

J'ai toujours écrit mon propre logiciel. J'ai écrit ces programmes au cours de plus de cinq décennies pour mon usage personnel et je n'ai jamais pris la peine de les rendre conviviaux, vous devrez donc peut-être faire quelques expériences par vous-même, car je n'aurai pas le temps de répondre aux questions.

Le logiciel FOS fonctionne sur les systèmes d'exploitation depuis MS-DOS 6.2 jusqu'à Windows 98 (je recommande Win98SE), donc déterrez un vieil ordinateur ou utilisez un émulateur. Sur mon ordinateur portable Win10, j'utilise VMware Player.
Je déconseille DOSBox 0.74 car il s'agit essentiellement d'une version allégée de MS-DOS 5 ; il exécutera certains des programmes, mais pas tous.

Tout ce que je mets à disposition ici ou via des forums ouverts tels que Pit-Lane et KiwiBiker, peut être librement distribué.

Joyeux noël et bonne année !
**********************************

Merci pour ce travail, passe de bonne fêtes

Hi Frits,

thank you for offering your FOS-data.
Regarding the file " bellmouth end correction": What´s the matter with "curtain area" and "end correction"? Can you please give an explanation?

Do you have a guess what axial distance should be kept between the end of the carburetor bellmouth to other parts, like airbox walling or fairing, ect.?
Carburetors are 42mm in diameter. Is there a rule of thumb?

Best regards,
Bernd

Döllinger a écrit:

Regarding the file " bellmouth end correction": What´s the matter with "curtain area" and "end correction"? Can you please give an explanation?

The 'Bellmouth end correction' in my 'FOS Tips & Concepts' should be self-explaining. I wouldn't know what else to say about it, Bernd.

Döllinger a écrit:

Do you have a guess what axial distance should be kept between the end of the carburetor bellmouth to other parts, like airbox walling or fairing, ect.? Carburetors are 42mm in diameter. Is there a rule of thumb?

The 'End correction is the minimum axial distance. A bit more will not hurt, but a bit less will.
Again, the drawing should provide all the math you need, independent of carburetror diameter.

Hi Frits,

thank you for your reply.
You are right, actually it´s self explaining. I thought so, but was was not completely sure about your intention.
The math is clear. It´s just... 4,96mm axial clearance is far less than i had expected with my amateur thoughts.
Fortunately most answers are just one question away. Even if i made a fool of myself this time...
Thank you for sharing your knowledge, Frits.
Best regards,
Bernd

Döllinger a écrit:

Do you have a guess what axial distance should be kept between the end of the carburetor bellmouth to other parts, like airbox walling or fairing, ect.?

Bernd, I feel that your question deserves a more detailed answer.
The primary purpose of this end correction approach is to give a guide line when performing wave calculations. But for flow calculations other factors play a role as well.
On a static flow bench the flow will remain constant as long as the axial distance from the bellmouth to the fairing or other shielding parts is at least Dcarb² /(4*Dbellmouth). This will ensure that the flow velocity through the curtain area will not need to be higher than the flow velocity through the carb throat area.
I am assuming that the bellmouth has a nicely rounded outer shape, like a donut, which will help guide the flow from radial to axial. If it has not, the flow coefficient will suffer and a flow bench will tell you so.

However, there is no static flow anywhere in an engine; it is always accelerating or decelerating, in other words: pulsating. And as the distance from the bellmouth to the fairing nears the above-mentioned end correction value, the mixture passing through the curtain area needs to be accelerated to a higher flow velocity, creating more flow resistance. So it does make sense to make the distance from bellmouth to fairing larger than the calculated end correction.  If you can find the space, use a distance of 3 to 4 times the end correction.
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Hi Frits,

thank you very much for your profound answer.
The bellmouth shape is calculated via NT-Project´s "intake bellmouth design" and milled of PA66.
Not completely pushed on here. The inner shapes fit edgeless, when pushed to end.
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My "problem" is, that the airbox must not be very wide around the very bottom of the bellmouth.
Due to the engine layout i have to go lanes apart normal in some points.
Here a picture at 55°(+ some extra) lean angle, forks fully compressed.
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Your posts in this forum and answers to my questions had really great impact to many aspects of this project.
Best regards,
Bernd

Hi,

another little question to Frits:

We are about to built an exhaust according to your "tubo102". The "tubo102" drawing is assuming an exhaust duct lenght of 63mm and a flange lenght of 40mm.
The total lenght of the system is 803mm from piston skirt.
On the cylinders i use the exhaust duct lenght is 55mm and the flange lenght is 30mm.  That´s a difference of 18mm.
Is it a good idea to just elongate the the first cone by these 18mm, to get the total lenght to 803mm? The incline of the cone would get a bit flatter.

On the other hand i could use a straight part (18mm long) with 41,5mm diameter and then start the fist cone as used by the "tubo102".

With "first cone" i mean the cone from 41mm to 49,9mm diameter (156mm lenght) in the tubo102 drawing.
What´s your advise?

Kind regards,
Bernd

P.S. Here one restrictor we milled to your drawings. Was much work to get the two 23,3mm radii exactely done on the milling machine.
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Bernd, unless you are building an exhaust system for an engine that is identical to an Aprilia RSA in all respects, including the 202° exhaust timing and the BMEP of 15,76, my advice would be to forget Tubo 102 and stick to the FOS exhaust concept, with a length from piston to the end of the header equal to 34% of the total length. And this total length will probably not need to be equal to the 803 mm of the Tubo 102 either.

Hallo Frits,

the engine is not identical but related.(manufactures words) The engine manufacturer uses your "tubo21". He gave me a drawing and the only difference is a longer section after the flange. Instead of  155mm in "tubo21" he uses 161mm while having a shorter cylinder duct and shorter flange.
So this should work and give a good starting point. Tubo21 and tubo102 are quite similar so i thik this should work aswell.
As the engine is actually a kart engine ( i made some modifications) i`m a bit unsure if a shorter exhaust is suitable for a motorbike.
I consider to use the "tubo102" instead, because a few riders i know use this setup in their bikes.
The thing i´m doubtfull about is: should i add the missing lenght to reach approx 803mm. And if yes, should i use a cylindrical section or
stretch the fist section/cone).
Would these differences in the frontmost section have a strong effect on the engine characteristic or would only rise the powerband with a shorter system?
Can you give an advise? This would be very kind.
Best regards,
Bernd

Hi Frits,

i just made a measurement on the engine: Exhaust timing is exactely 202°.
FOS exhaust concept is telling me that 34% of LMax is approx 273mm.
Depending on the speed of sound. I assumed 550-570°C.
LMax is variying around 800mm, also depending on the SOS. Rising 20° is virtually shortening the exhaust by 30mm.
In my case: 273mm-85mm= 188mm for the first cone section.
It´s really a bit confusing... 30mm seams quite much to me for choosing without experieance.
Best regards,
Bernd

I think the best solution would be to use a longer flange that would fit the exhaust duct at the cylinder without any steps, so you might need to make the flange conical, and  with an end diameter of 41.5 mm Ø, so it would fit the Tubo 102-header, again withoud any steps. Mind you, never eliminate any steps by removing metal from the cylinder duct.

Whether the Tubo 102 is suitable for karting, depends on the type of kart and the type of circuit. A 125cc kart on a kart track goes through the gears so quickly that the pipe never reaches the temperature that it would on the Aprilia GP machine, so you might need a slightly shorter pipe. On the other hand, a 250cc twin-cylinder superkart should perform fairly well with the Tubo 102 pipe.

Hi Frits,

thank you for your profund answer.

Please see the picture:
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The black flange is part of the step.(PVP-made) I´ve got no ambition to touch the cylinder with a grinder.
The black flange is conical and got an end diameter of 41,0mm. The flange i milled (right part in the picture in previous post) got the right size to create 41,0mm inside diameter together with a welded in 0,8mm sheet metal, without any steps.
Creating a new, lengthened black flange is not that easy, as i don´t have any CAD-data of this part.
You think a lengthened first sheet metal section would do a similar job, than a new lenthened flange? Conical or better cylindric?
Sorry for my questions. English is not my mother tongue and i don´t want to make a mistake.
Best regards,
Bernd