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

Hallo,

theses days i got an ignition from PVL for a Maico 250.
It´s fully adjustable. I found for the start a time/avance sheet of an excisting aftermarket CDI (a update CDI-box which uses the stock Kröber rotor/stator) ) with values that seem weird to me.
Bore 76mm, stroke 54mm, conrod lenght 137mm.
1500-16°
2000-16,5°
2500-17°
3000-15,5°
3500-18,5°
4000-19°
5000-20°
6000-20°
7000-21°
8000-21,5°
9000-21,5°
10.000-21°
11.000-20°
The advance is getting less at 10k rpm, a number the Maico will hardly reach.The original Kröber ignition got a curve with even more advance.
My engine is ported to Frit´s transfer times, got a 38mm Dellorto, and a resonance exhaust, ect, ect.
You think that´s a curve to start with and get on the dyno, or will it end in tears. (piston melting ect.)
I read Frit´s "Zündkurven" and think that the suggestions in this essay don't match with the ignition timing i postet above.(and the other ignition curves i use for other engines)
It´s hard to find an engine with comparable technical data.

Best regards,

Bernd

PS. Bob van der Zijden told me that at these low revs the strenght of the spark is far more important than an advance curve. What is your opinion?

Bernd, with my 'Zündkurven'-essay at your disposal there is an easy way to generate your own ingnition curve.
Al you need to do is multiply the rpm-values in my curve with the rpm-value at which your Maico produces its maximum power, and divide each outcome by the 13000 rpm at which the RSA produces its maximum power.

At low revs the compression pressure and the turbulence in the combustion chamber are not very strong, so I don't think you will need a strong spark in those circumstances. But because the density and purity of the fresh mixture will also be mediocre, chances are that there is not always an igniteable mixture present at the right moment between the spark plug electrodes, so you will need a spark with a long duration, in order to improve the hit-or-miss chances.

Hi Frits,
thank you for your fast reply. The RSA-curve is close to the one for my Rotax 256.
I calculated the RPM's and supposed to have the max. power at 7500 rpm. I assume the finally max power at 8500, the 2nd curve to switch is calculated for that, but for the first start i want to keep the engine undamaged.
Here´s the comparison between the updated Kröber and the converted RSA curve.
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I assume that the flat characteristic of the Kröber is accounted by it´s wiring with an electronic filter.

Good explanation with the spark duration and an interesting issue. I will have a look at the flashover on the oscilloscope.
Another question: Is it only possible to operate with such a high advance in the power band (Kröber and others) on an engine with low torque at high rpm. A torque that is so poor, that the actually to high advance has no damaging effect?

Best regards,

Bernd

Döllinger a écrit:

Is it only possible to operate with such a high advance in the power band (Kröber and others) on an engine with low torque at high rpm. A torque that is so poor, that the actually to high advance has no damaging effect?

Well, the torque is only so poor outside the power band. And then 30° ignition advance is not too high.
The early ignition results in a high expansion rate after the end of combustion, so the exhaust gases will contain very little energy and the wrongly-timed exhaust pipe pulsations will not upset scavenging too much. It's all explained in the Zündkurven-essay.

A word of advice: do not retard the ignition timing too much after maximum power. An advance of only 5° will yield quite hot exhaust gases and your engine may not be thermally sound enough to endure this. I wouldn't go later than 10° advance.

Hi Frits,

thank you for your advice. I will change the ignition timing after the power band.

I`m not sure if the aspect i meant came out clear enough. When you see the Kröber ignition timing, it´s got much more advance after, let´s say 7000rpm. (in the power band)
If i would have so much advance in my Rotax, the engine would not survive that. My question is: How can an engine survive an advance as high like on the green curve?
For the first passage you wrote. I understand this. I read your essay many times.
Thanks for your efforts,

Bernd

PS. My CNC operator is coursing the day he agreed to mill the cylinder head. It looks good so far, but he is not ready, yet.

Döllinger a écrit:

When you see the Kröber ignition timing, it´s  got much more advance after, let´s say 7000rpm. (in the power band).
If i would have so much advance in my Rotax, the engine would not survive that.
My question is: How can an engine survive an advance as high like on the green curve?

I'm not sure that it can. It depends on the specific power it's making. In the period when the Maico was built, many racing engines had a fixed 20° ignition timing. You would hardly get away with that on a modern competition engine.

Hi Frits,

thank you for your efforts. Here are two pics of the Maico cylinder head, nearly ready, only some cosmetics are missing. Combustion chamber exactey like in your drawing.
It´s very interesting to watch and listen how he´s milling that out of a solid block. My CNC wizard needed ca. 25h of work until now, without drawing. (all fins are conical)
Probably i have to weld some thin stripes on the top of each fin to keep them from cracking under vibrations.
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While on the spark advance curve, do some of you have more information about the coil charging timing curve ?
I'm designing my own ignition module but I'm struggling to find more information on the dwell especially on high revs engines.
I suppose this aspect of the ignition affects the power through the spark intensity.

Hi,

the cylinder heads are ready. The sphere top is a little kept a few 1/10mm higher. So i can define the volume of the combustion chamber exactely to mill out the missing volume. The surface is approx 3 times the original head.
Thank you Frits for your advice in case of the combustion chamber design! Only little bridges between the single fins are missing aside a few hours of finishing with file and sandpaper. About 55 hours of CNC work.
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working on an excell to calculate Time.area's etc. a question about the actual width of ports to use :

for A-port, it's simple I thinck : perpendicular to the leading port angle (closest to exhaust)
but B-port is troubling me a bit , do you use :
A perpendicular to leading angle
B perpendicular to trailing angle
C chordal width

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and does anyone have a suggestion of how to calculate needed Time.Area's based on input (like rpm, BMEP,...)

JanBros a écrit:

working on an excell to calculate Time.area's etc. a question about the actual width of ports to use: B-port is troubling me a bit, do you use :
A perpendicular to leading angle
B perpendicular to trailing angle
C chordal width

There is no simple A-, B- or C-answer, Jan.
As a global description I would say: a line from the end of the leading edge, perpendicular to the trailing edge, marked as line D in the drawing below. But as you can see, the shape of the duct, just before it reaches the cylinder window, can also play a role.

I think the passage of any port is best defined as the diameter of a circle that will pass right through it, as shown by the yellow circle in the bottom half of the drawing. In fact, this was the method used by Jan Thiel. The photo below shows a collection of his 'coin-on-a-stick' port gauges.
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I was afraid it would be dificult to determine the correct width. finding a formula to determine the diameter of your circle will be very dificult.
looking at the drawing, B seems a bit smaller than the circle, D more bigger than B is smaller. thinking about taking the average of 2xB and 1xC. Will make some different set-ups to see if I can figure out the best way to get the average closest to the circle.

JanBros a écrit:

I was afraid it would be dificult to determine the correct width. finding a formula to determine the diameter of your circle will be very dificult. looking at the drawing, B seems a bit smaller than the circle, D more bigger than B is smaller. thinking about taking the average of 2xB and 1xC. Will make some different set-ups to see if I can figure out the best way to get the average closest to the circle.

In our example the diameter of the circle is limited by the hook in the trailing edge of the B-port. If the hook were less obstructive, dimension D would be fine.

Happy Birthday Jan Thiel (in Thailand it's already saturday the third)[Vous devez être inscrit et connecté pour voir cette image]

Gelukkige verjaardag Jan  

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Attention  Frits, les bougies NGK n'aiment pas un mélange trop riche… elles vont s'encrasser avec autant de crème

Happy Birthday Jan

Now that's quite a cake! Happy Birthday!!

Good morning All!

I'm really impressed by the quality of the info displayed on this forum and especially on this topic, special thanks to Jan and Frits.
As I'm only a modest agricultural racer (as will say Frits) and long-time 2 stroke addict, I barely dare ask a question, as most of mine were already answered, and I did not finished yet the reading of all the 200+ pages.
I would like to ask a simple translation question to the knowing French guys here, as I would like to be sure my understanding is correct.
What is the correct French word for "scavenging"?

scavenging = balayage

OK merci beaucoup, c'est ce que je supposais mais je n'en étais pas certain.

Here is the first test-file of my Porting Excel : [Vous devez être inscrit et connecté pour voir ce lien]

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I've already tested it a lot, but might be better if other people test it to . any comment is welcome. Especialy if someone with Engmod would like to compare the results

It is based arround the Bimotion-Idea (only the idea, it's all mine ) because it gives total freedom in port-design. Ports are not limited to any geometrical shapes at all. I know it is not the "handiest" way to design ports quickly, but that does not outweigh the advantage of free design in my opinion.

It shows actual portmap, portlay-out from top, calculates : A.A, T.A, width between all bridges, exhaust port width % of bore, the direction of the scavenging column, the ideal safe-exhaust-port-roof-depending on the width of the port, and lot's more basic stuff.

still need to make a "how to use it" file, but most is obvious and there are lot's of remarks on it with explanation.
feel free to do anything you want with it, as long as you leave me as the creator and Frits for his help (not personaly, but through the forum) mentionned somewhere, even if it is only small, stashed away in a corner out of sight.

made an error while re-organising tables. same link as above with new file.

Is this thread still alive? And if so, can I ask some theoretical questions to Frits Overmars?

Yes, I do think you can ask.

OK. I am a stupid amateur. My only experience with 2T engines is as a rider. I have owned Kreidler RMCS (50 cc), and I currently own an Aprilia RS250 and a Suzuki RG400. I restored the RG400 from "basket case" to "as good as new" over the last three years.

I have read the book "Design and Simulation of Two Stroke Engines" by Gordon P. Blair several times; I have studied the book "Internal combustion engines" by Pulkrabek, and I have also studied the text book by Cossalter about motorcycle dynamics.

I have a question to Frits Overmars about the timing of the exhaust port, specifically about his assertion that the timing of the exhaust port should be 180 degrees to obtain optimal resonance behavior. However, before I spend (a lot of?) time typing up my question, I would like to ask whether or not Frits would be willing to entertain a question from an interested amateur.