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

op76 a écrit:: Frits, as far as I have understood, you are using chordal width and open height of the port for angle area, getting area of xx mm² for each degree separately, and sum all degrees together, getting angle area unit of °mm², and then again STA units of °mm²/cm³/1000rpm. Is there some good reason for using this instead of what Jennings did, because at least in manual measuring, this seems much bigger job?

I am using effective cross flow area. Open height only works when the axial angle of a port is zero (which it should never be) and chordal width may or may not be applicable, depending on the port shape just before the window. I use the "effective passage width", as shown in the top left part of the picture below.

As for the "Jennings method", I think Vannik already answered your question.
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Nombre de messages : 32 Localisation : Suomi Date d'inscription : 26/08/2018

Ok, so for me it seems, that you all (except Jennings) are using same method for calculating angle-area, but you have bit different approach to STA.

STA (Frits) - Angle-Area °mm²/(RPM of max torque/1000)/Swept Volume cm³

STA (Vannik/Blair) - Angle-Area °mm²/(RPS of max power*360*SV) giving you sec-mm²/cm³

And Frits is using Swept Volume with head, but Blair used only Cylinder capacity. Is that right?

Yes. One of the issues with a lot of definitions used in 2T engines is the use of SV vs SV+ChamberVolume.

But the difference when used for semi-empirical equations should not matter, as long as you are consistent.

Nombre de messages : 12 Localisation : USA Date d'inscription : 30/09/2019

Could one of you guys explain to me why the toroidal head design works better than a hemi head design?? And what is the main advantage is??

Nombre de messages : 32 Localisation : Suomi Date d'inscription : 26/08/2018

Frits, when I'm comparing my own designed cylinder to RSA, I would need somehow correct the flow rate of transfers. Cylinder will be machined from billet, and cylinder tube is pressed in, so those transfers will be pretty much L shaped. Of course I will try to make some shape to that curve, but it will be far from optimal. What would be your estimated correction factor for such design, and should I add that correction while performing angle-area calculation (chordal and vertical flow)?

op76 a écrit:: Frits, when I'm comparing my own designed cylinder to RSA, I would need somehow correct the flow rate of transfers. Cylinder will be machined from billet, and cylinder tube is pressed in, so those transfers will be pretty much L shaped. Of course I will try to make some shape to that curve, but it will be far from optimal. What would be your estimated correction factor for such design, and should I add that correction while performing angle-area calculation (chordal and vertical flow)?

You do know that I hate pressed-in sleeves, don't you? They will never let you achieve the rate of cooling that you need for a reliable high-power two-stroke.
Apart from that, I cannot give you a straight answer. You would have to measure the mass flow through the various duct shapes on a flow bench in order to collect the necessary data for a realistic correction factor.
And then you still could not be sure that the transfer streams will exit your L-shaped ducts in their desired directions (or rather, you could be pretty sure that they wouldn't).

Incidentally, measuring the mass flow could lead to a better alternative for the angle.area approach. Instead of adding the steps of open time x open area, we should add the steps of open time x flow. But even then we would not be able to fully simulate a running engine, as we cannot duplicate the pressure fluctuations inside a running engine on a test bench.

Nombre de messages : 105 Localisation : Biblis Date d'inscription : 14/01/2017

Hallo,

the parts for the "injector" are ready to be welded to the carburetor housing.
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@ Frits: You told me to keep the volume between the front of the difusor and the jet as small as possible.
My thought is to weld on a "jet-seat" straight to the big aluminium part you see on the pictures.
So the jet would be as close to the difusor as possible. On the other hand the jet is above the fuel level in the float chamber.
You think that can cause problems?

The second possibility is to create a bypass outside the float chamber with the jet within the fuel level.
The solution with a tube inside the float chamber will result in a too long bore to lead the fuel to the valve unit as the carburetor is not constructed for a PJ.
Can you please tell me your thoughts?

Best regards,
Bernd

Döllinger a écrit:: @ Frits: You told me to keep the volume between the front of the difusor and the jet as small as possible. My thought is to weld on a "jet-seat" straight to the big aluminium part you see on the pictures. So the jet would be as close to the difusor as possible.
On the other hand the jet is above the fuel level in the float chamber. You think that can cause problems?

On the contrary. Fuel flow through all jets in the system is dependent on suction anyway.
Provided that the entry of the PJ-circuit is always below the float fuel level, you should be fine.

Nombre de messages : 2 Localisation : Slovenia Date d'inscription : 25/09/2013

Hi, My name is Matej and Im a 2 stroke enthusiast from Slovenia. Im making a new cylinder and I cant find anything on cylinder wall thickness [Vous devez être inscrit et connecté pour voir cette image]
Dear Frits, have you ever tried what is the minimum or best cylinder thickness wall for a 50 or 125cc motor?
Does anyone have a picture of RSA cylinder head cap?
Kind regards

Dobrodošel Matej [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5) - Page 35 980796

.
I cannot give you a simple answer because much will depend on the shape that you are going to choose for the coolant ducting, the placing of the cylinder studs, and so on.
You can find all available Aprilia pictures, and a lot more from many other motorcycle brands, here:
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Nombre de messages : 105 Localisation : Biblis Date d'inscription : 14/01/2017

Hallo Frits,

thank you for your answer. That´s the best and most simple solution.
Then i have another question for you:
Is a distance of 5-6mm between the slide and the center of the difusor OK?
Or is it better to give it an angle towards the venturi to come closer? Then the difusor would not be 90° to the inlet but 80°-75°.
I don´t know if giving an angle to the difusor can cause a problem with the fuel-suction./If the venturi effect is troubled if it´s not 90°.
Can you please give me an advise?
Best regards,
Bernd

Nombre de messages : 2 Localisation : Slovenia Date d'inscription : 25/09/2013

Frits Overmars a écrit:: Dobrodošel Matej .
I cannot give you a simple answer because much will depend on the shape that you are going to choose for the coolant ducting, the placing of the cylinder studs, and so on.
You can find all available Aprilia pictures, and a lot more from many other motorcycle brands, here:
[Vous devez être inscrit et connecté pour voir ce lien]

Thank you for the pictures, you have a beautifull collection of nice pictures, but unfortunatley not the one I hoped to find.
I measured a 125cc Aprilia rsw rotax cylinder and its 8mm thick with no supports on the side. [Vous devez être inscrit et connecté pour voir ce lien] The same thickness is on a RSA cylinder.
I would make a same style as a new ktm sx 50 cylinder with 6 nuts head assembly. But I know that a SX50 cylinder is a bit to thick for a 50cc. Can I scale a 125cc design to a 50cc and make a wall thickness 5-6mm?

Döllinger a écrit:: Is a distance of 5-6mm between the slide and the center of the difusor OK? Or is it better to give it an angle towards the venturi to come closer? Then the difusor would not be 90° to the inlet but 80°-75°. I don´t know if giving an angle to the difusor can cause a problem with the fuel-suction./If the venturi effect is troubled if it´s not 90°.

I suspect that with 'diffusor' you mean the bellmouth at the carburetter entrance, but I can't be sure. A sketch might be helpful Bernd.

Rookie j. a écrit:: I measured a 125cc Aprilia rsw rotax cylinder and its 8mm thick....
I would make a same style as a new ktm sx 50 cylinder with 6 nuts head assembly. But I know that a SX50 cylinder is a bit to thick for a 50cc. Can I scale a 125cc design to a 50cc and make a wall thickness 5-6mm?

If you are talking about cylinders with the same bore/stroke ratios, you can simply scale their dimensions up and down using a scale factor of { (cylinder capacity A) / (cylinder capacity B) }^(1/3).
But the rigidity of a structure is proportional to its dimensions ^2 , so if you scale down, you should use relatively larger material dimensions in order to retain sufficient rigidity. 6 mm wall thickness for a 50 cc cylinder seems to me rather little.

Nombre de messages : 105 Localisation : Biblis Date d'inscription : 14/01/2017

Hallo Frits,

here two little sketches.

It shows the bellmouth and the slide. In the first sketch the angle between the "difusor"/"jet-tube" and the intake direction/air flow is 90°.
So the middle of the 1,2mm bore inside the difusor will have a distance of 5-6mm to the surface of the slide.
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In the second sketch the complete powerjet unit is angled towards the slide. So it is possible to get the difusor tip extremely close to the slide surface.
But so the bore inside the difusor is not 90° angled towards the intake direction/air flow.
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My questions are: Is a 5-6mm distance between difusor and slide surface acceptable or is it better to get the tip as close as possible?
Does an angled difusor trouble the effect of sucking fuel out of the powerjet system?

Thanks for your help!
Best regards,

Bernd

In your second sketch the air flow over the powerjet tip is highly dependent on the throttle slide position; at small slide openings there will be no fuel flow out of the powerjet. I cannot say if this is desirable. Maybe you can compensate for this by increasing the fuel flow through the stationary circuit.

The angled position of the powerjet in your second sketch will have more or less the same effect as a screen around the needle jet exit that is open towards the engine side: air or mixture flowing backward will create a ram air pressure that pushes the fuel down in the jet, counteracting the overrich mixture that would otherwise arise from mixture passing several times over the jet.

Assuming that the throttle slide position is high enough, the angled powerjet will experience at least the same amount of sucking as the 90°-version; maybe even a bit more. If you wish to delve deeper into this phenomenon, you might study 'Pitot-Tube' and 'Prandl-Rohr'.

Bonjour,
Sur le carburateur Dellorto de la RSA, le diffuseur du powerjet est positionné à 20mm de la guillotine.
Et perpendiculaire au flux.

Nombre de messages : 105 Localisation : Biblis Date d'inscription : 14/01/2017

Hallo Frits,
thank you for your answer.
I will read about Prandl-Rohr" and Pilot tube.

Great you described the effects of the angled tube, that´s what i wanted to know.

@ Francis:

Thank you for your Information. On photos it always appeared a bit closer to me.
If a 20mm gap to the guillotine is not too much on the RSA things are fine.
With 20mm distance there is no issue to get inside the tunnel of the Guillotine.

What distance you use in your carburetor between the difusor and the guillotine?

Best regards,
Bernd

Nombre de messages : 6492 Localisation : Limoges Date d'inscription : 19/02/2016

[quote="Döllinger"]Hallo Frits,
thank you for your answer.
I will read about Prandl-Rohr" and Pilot tube.

I think it’s pitot tube,like on the planes .

Nombre de messages : 105 Localisation : Biblis Date d'inscription : 14/01/2017

Yes, my fault. I noticed yet.
Best regards,
Bernd

Nombre de messages : 86 Localisation : Australia Date d'inscription : 05/05/2012

Frits disc valves, the old engines has a steel disc, probably a spring steel of sorts, carbon valves wear the non surface protected engine cases away, what’s your thoughts on a titanium disc ?
I wonder if there is a suitable grade re-fatigue, I’m also query it’s gauling characteristics,

RAW a écrit:: Frits disc valves, the old engines has a steel disc, probably a spring steel of sorts, carbon valves wear the non surface protected engine cases away, what’s your thoughts on a titanium disc ?
I wonder if there is a suitable grade re-fatigue, I’m also query it’s gauling characteristics,

Yes, the old discs were spring steel; they worked well in combination with aluminium, cast-iron, bronze, teflon and pertinax covers.
Carbon discs are much more abbrasive; they need to run against hard-anodised or ceramic-coated surfaces.
Titanium discs have been tried but I have no first-hand knowledge of their suitability. Galling is always a problem with titanium and I suspect that you cannot run them against any type of uncoated metal. I think i'd try pertinax or full-ceramic covers, as offered by emot.nl.
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Nombre de messages : 86 Localisation : Australia Date d'inscription : 05/05/2012

Thank you Frits. I’m working with a 92 model Rotax, I was more thinking of a solution to not wearing out the splined shaft from the mass unbalance of the spring steel disc in these older engines.
If there was to be a suitable grade of titanium perhaps it could be DLC coated therefore leaving the cases STD
Interesting the emotion ceramic valve cover

Good afternoon all,

What do you mean by "gauling", do you mean "wear" or "galling"?

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