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

ORC a écrit:

Finally I have read the whole thread!! Is like the final part of a peregrination, then I win the rigth to make an answer to the monks of knowledge
Well, my question is about the interpretation of the drawings that Jan and Frits share about the RSA cylinder. I found on a "Diagramma angolare delle luci" that the floor and roof of the A ports are parallel and the normal direction of the slope (marked as red arrow) points sligthly back of the middle of the cylinder.
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On the other hand, there is a section of the cylinder in wich the normal directions of roof and floor of the A ports are not parallel and pointing slightly more close to the exhaust direction.
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My question... what is the correct one? Or is just a problem of my interpretation of the projections on drawings? Maybe the normal direction of the slope change among the heigth? Help!
Thanks to everybody for the knowledge on this forum and specially to Frits and Jan!

ORC, the monks wish to thank you for your kind words  .
Now the first thing we need to do is to establish our terminology. The roof of a port is the top plane, the first plane to be opened when the piston moves downward. The port floor is its bottom plane; in the case of the transfer ports it is the plane that matches the piston crown in Bottom Dead Center.
Your drawings do not show the port roofs and the port floors; they only show the leading flanks and the trailing flanks of the ports. And your first drawing does not show the axial direction angles of the ports; it only shows their position angles, i.e. the points where the ducts intersect the cylinder bore.
Your second drawing does show the axial direction angles of the leading and trailing port flanks. But both your drawings were superseded by newer drawings that I could not yet publish at the time.
The picture below shows all relevant angles of all transfer ports.
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Thanks for the clarification about the new scanvenge design.

Sorry for my wrong explanations about roof and floor of port, the lack of experience together with the english skills...   , I was really refering to the planes that continues the floor and roof. Let's go 3D... in the next figure of a supersimplified A-port is highlighted in red the plane I refered in the previous post, and shown in yellow the axis that is normal to that plane and I drew in red in the prev post.
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My problem was more concern about the orientation of that plane which shape that angle marked in blue in the next figure.
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But analizing your words and the scavenge design that you shared I understood that this angles are not so importatn, the key for direct properly the jet of the port is the axial angles of roof and floor and the angles and positions of leading and trailing port flanks, please correct me if I'm wrong.

Thnks again!!!  

ORC a écrit:

analizing your words and the scavenge design that you shared I understood that this angles are not so importatn, the key for direct properly the jet of the port is the axial angles of roof and floor and the angles and positions of leading and trailing port flanks, please correct me if I'm wrong.

You are quite correct .

Ok, then, last but not least... I know Frits don't like to figth with problems already resolved, but I'm designing a cylinder for a classic bike category and I have as limiter our famous friends f***ing studs.

I have two options for try to respect the scavenge design that Frits showed... make A-port equal to B-port (in chordal length terms), or make the B port a little bit bigger. I show the tow options above.
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¿Any other idea for avoid studs or improve that scavenge angle design?

Many thanks in advance!

ORC a écrit:

I know Frits don't like to figth with problems already resolved, but I'm designing a cylinder for a classic bike category and I have as limiter our famous friends f***ing studs. I have two options for try to respect the scavenge design that Frits showed... make A-port equal to B-port (in chordal length terms), or make the B port a little bit bigger. Any other idea for avoid studs or improve that scavenge angle design?

If I had to choose between option #1 and option #2, I would have a slight preference for option #1.
But how about option # 3?[Vous devez être inscrit et connecté pour voir cette image]

I expected it, Frits.

Nice...

I was thinking about put three ports at the back part instead of two, but would be better just two bigger. And about the axial angles? Should point as the C port or a less than 50°?

Two big C-ports would leave a center bridge for the piston ring gap. And for the same cross flow area two big ports would have less wall area than three smaller ports: less flow friction and less heat transfer to the mixture.
With the limited available information I can only guess the axial angles: maybe 45°.

very reasonable
what other info do you need? bore x stroke 72x60. Enduro engine with peak power near 7.5-8k. Reed valve intake.

thanks!!

ORC a écrit:

what other info do you need? bore x stroke 72x60. Enduro engine with peak power near 7.5-8k. Reed valve intake.

I would need quite a lot of other info, but for now I will keep guessing. For example, the transfer timing: in most veteran engines it is much too high, around 140°, but for starters let's assume a modern value of 130°.
The transfer layout of option #3, in combination with 130° transfer timing, could give good cylinder filling up to 7500 rpm.
But creating sufficient exhaust blowdown angle.area in a short-stroke engine is problematic anyway, and in your case the cylinder studs will make it impossible to add auxiliary exhausts. A single exhaust port with the optimum width of 70% of bore and an exhaust timing of 190° (a nice value for an enduro bike) would only allow sufficient blowdown up to 3600 rpm.

Sufficient blowdown at 7500 rpm would require 208° exhaust timing! That is out of the question, so our 130° transfer timing would be useless. 120° would be better; that would be good for maximum power at 6000 rpm and then you would need an exhaust timing of 'only' 196°. The result could be an enduro engine with a bulldozer-character.
As you remarked quite correctly: "I know Frits don't like to fight with problems already resolved."
I think you will share my point of view when you try to solve these problems .

Ok, I untderstand you quite well. Thank a lot for your help!!!

I will continue reading your posts religiously!

just question for Mr Overmars, what do you think of the injection on the V4 sutter, that they are the advantages and disadvantages of this engine, your answer would be very useful to me because I think we could advance in conclusions on this type of motorization
Thanks for you reply
Alain Freret

I have not been involved in this Suter project and everything I 'know' about it, stems from hear-say and press releases,
so I feel not qualified to offer a judgement, Mr Freret.

Too bad because to advance projects like this one is an extraordinary advance, I participated in the development project on the V4 MUZ in 500cc I made the reeds gage, I continue to develop my process you can see the brand Twister boite à clapet, I also noticed that you are not fooled by the advances of KTM on the motorization of 2T engines, which are already developed in the Pacific on the YZ250 engine, I still manufacture but for me, I think today to be able to do walk a two-stroke injection engine with some specific parts a 0.2% oil lubrication that gives emissions readings well below the current 4T, that's your feeling
Thank you
Alain

Hallo,
due to the information which could be interesting for others i ask my question here.
I set up one air cooled racing engine with a Maico MD 250 rotary engine as base.
As the original transfer port times are far to long i want to change them to a better opening angle.
A few detailes: Bore 76mm, stroke 54mm rod lenght 137mm target rpm-band 6500-9500
outlet diameter on the exhaust flange is 44,5mm.
On the picture you can see the ports. Interesting would be also the radius measure of the exhaust port right and left of the bridge which is usefull for more power.

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What opening angles for exhaust port and transfer ports would you recommend? Frits wrote in the other thread that he would increase the widh of the exhaust port(s). That´s a good point.
I´m happy for every information.

Best regards,

Bernd

Döllinger a écrit:

I set up one air cooled racing engine with a Maico MD 250 rotary engine as base. As the original transfer port times are far to long i want to change them to a better opening angle.
A few details: Bore 76mm, stroke 54mm, rod lenght 137mm, target rpm-band 6500-9500, outlet diameter on the exhaust flange is 44,5mm. Interesting would be also the radius measure of the exhaust port right and left of the bridge which is usefull for more power.
What opening angles for exhaust port and transfer ports would you recommend? Frits wrote in the other thread that he would increase the width of the exhaust port(s). That´s a good point.

The 'other thread' that Bernd mentions above, was 'Cylinder head design'
( http://www.pit-lane.biz/t7360-cylinder-head-design#361045 )
There I wrote:

Citation :

The past few days we have been dealing here with questions that have little to do with the title of this Cylinder head thread.
I will gladly answer all questions as well as I can, but it would be a pity if some discussions cannot be found back because people are mislead by the title of a thread.

May I suggest that we concentrate questions about two-strokes in
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This will make it easier for everyone to access the available information.

I don't like to wrestle with problems that have already been solved. But since this Maico is rather unique, I'll give it a try anyway.
Why is this engine so unique? Because originally it was a 125 cc engine with a bore and stroke of 54 x 54 mm. Then Maico made a 250cc version the easy way: they just fitted a cylinder with a bigger bore. A much bigger bore: 76 mm.
I've explained a couple of times why big-bore, short-stroke two-strokes are not a good idea. And this Maico MD250 with a bore/stroke-ratio of 1,4 is the worst example I've ever seen. But calculating extreme cases can be quite instructive,
so I decided to have a go at this worst-case scenario.

I based my calcuations on Bernd's data plus the above transfer layout option #3.
All transfer timings: 130°
Transfer port widths:     A=30mm, B=19mm, C=15mm
Axial scavenging angles: A=18°,    B=8°,     C=45°
These values will give sufficient transfer angle.area for good cylinder filling up to 7500 rpm.
The cylinder studs do not permit the application of auxiliary exhaust ducts, but the central exhaust bridge allows us to make two exhaust windows of 32mm width each. With 200° exhaust timing that gives sufficient blowdown angle.area for 7500 rpm; a good match with the transfer rpm value. That ought to give a power band from about 6500 rpm to 8500 rpm;
you'd better forget that 9500 rpm limit, Bernd.
Regarding the corner radii of the exhaust windows: 4 mm for the top corners and 10 mm for the bottom corners should be good values.
Bernd reported a 36 mm carburetter. That would be too small even for a 125 cc rotary engine, so this 250 cc engine would definitely have asthma problems. Maybe just as well, because I don't think its cooling could cope with real power.
That is the main problem when trying to extract today's power from yesterday's engines.

This little story has occupied three hours of my time. I did it because calculating extreme cases can be instructive, for myself as well as for the forum members, but please do not expect me to perform similar jobs on a regular basis.

Hi Frits,

thank you very much for your efforts! 3 hours... What can i say?
My father loves these engines, though they are anachronistic. I stick to my PVP/Rotax.
The Maico is just a charming bike.

I will change the porting and exhaust angles to your values. 130° and 200°.

The port width for B and C are OK or 2mm less, so just a little work. The A ports are 22mm, so much filing work. But Ok, i filed my TZ 350 Roll-Cylinder for more than 20 working hours.
The exhaust windows are rectangular and have a widht of 30m. The 30mm are radial, not a chord measure.
My question: What is the top lenght of the window you recommend? With 32mm on each side a could not gain much more window area.

For the carburettor, yes i know the 36mm is not sufficient for a 250, but the diameter of the ratory valve is only 190mm. The inlet is just a kind of a bottle-neck. I have to measure if i can increase the diameter of the inlet for an 38mm equivalent. The intention is not to gain 15 or 20 hp, just to have an engine with about 40 hp and a good natured characteristic.

For the word "Axial scavenging angles". I am not sure if i understood this right. Is that the angle the transfer duct is getting narrow at the upper end?
Is that correct? An enlightment would be very kind.

Thanks again for that big effort you made!

The areas of the exhaust windows above the transfers should be as wide as you can make them, and then you might get away with less than 200° timing. But the necessary radial outflow (pictures below) will be restricted by the cylinder studs.
I would need to have the cylinder in my hands before I can say any more about the exhaust windows.
A 38 mm carb is better than a 36mm but still too small. Keep the complete inlet tract as short as possible. And you would have to experiment with the closing angle of the inlet disc. You might start with 70° after TDC and then try step by step how much later you can close it before the engine character becomes too capricious.

The bottom drawing shows the axial transfer angles.
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Hi Frits,

ok, now i got it. I have to take a look if these angles are possible to produce, but i think yes. I will see what i can do with the width, the studs are not far away.
If this engine is so interesting to you i can offer you to send one cylinder of mine to you to have a look. I would NOT tie ANY expectations to that! Just as a gratitude for your efforts.

Thank you for the tip with the rotary disc. I will cut out one with 70° closing angle, so i can cut of piece by piece.

Best regards,

Bernd

Döllinger a écrit:

If this engine is so interesting for you i can offer you to send one cylinder of mine to you to have a look. I would NOT tie ANY expectations to that! Just as a gratitude for your efforts.

No thanks, Bernd. But I have been wondering: your name makes me suspect that you are living in Germany, and at the moment so am I. My Postleitzahl is 01326; if that happens to be in your neighbourhood, we could meet for Kaffee und Kuchen

Hi Frits,
you´re right i live in Germany.
My post code is 68647, Dresden is more than 500km. I think it´s too far, though it would be very interesting for me.

Best regards,

Bernd

Bonjour A Tous !
Mon nom est Patrice , je demeure au Canada , je modifie des moteur 2 temps de motoneige depuis 30 ans environ pour le plaisirs.
je suis tombé par hazard sur ce forum cette été et j'ai lu attentivement  les 5 parties que j'ai vraiment adoré !
c'est mieux que tout les livres que j'ai lue dans le passé et j'aimerais félicité et remercié Frits et Jan de bien vouloir partagé leurs savoir faire !
merci

Frits Overmars a écrit:

The areas of the exhaust windows above the transfers should be as wide as you can make them, and then you might get away with less than 200° timing. But the necessary radial outflow (pictures below) will be restricted by the cylinder studs.
I would need to have the cylinder in my hands before I can say any more about the exhaust windows.
A 38 mm carb is better than a 36mm but still too small. Keep the complete inlet tract as short as possible. And you would have to experiment with the closing angle of the inlet disc. You might start with 70° after TDC and then try step by step how much later you can close it before the engine character becomes too capricious.

The bottom drawing shows the axial transfer angles.
[Vous devez être inscrit et connecté pour voir cette image]

Frits!
Dans votre formule pour connaitre le degré axial que veut dire ATN ?
merci

porttiming124 a écrit:

Frits! Dans votre formule pour connaitre le degré axial que veut dire ATN ?merci

ATN = Arctangent
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Merci beaucoup Frits!