[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked)

Forgi a écrit:: ....So you are telling me that an engine at 6000 rmp have to has an exhaust port with an opening angle of 190 degrees, and it is the ideal. I think that in this case the exhaust port has to be very small to avoid to get a too huge angle area, am I rigth? Well it is obviously a marginal example but according to the theory it is right, isn't it?

Yes, that is what I meant. But when I wrote 'any two-stroke', I meant any racing two-stroke, where the influence of the exhaust pipe must be used to the maximum to be competitive. If power is not important and all you want is an easy to ride bike without temperament, you can use lower timings.

Citation :: By the way what is your suggestion for a given rpm which is the best angle area?

This picture of the Aprilia values may give you an impression:
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Citation :: So if we take an average exhaust port, after its opening at 10 degrees will be the maximum of the amplitud, that's why you said 190 degrees, didn't you? The simulators count with 15 degrees as a maximum of the amplitud after the opening of the port. But I think it is not the same in case of a port with or without auxiliary port. In your opinion the difference is it significant?

A single port cannot open the same amount of area per crankshaft degree as a double or triple port because its shape is limited by piston ring life. So yes, the difference can be significant.

Citation :: Regarding to the transfer port why 130 degrees are the best? Have you any explanation?

That is rather empirical; lower transfers limit the rpm at which the cylinder can still be adequately scavenged; higher transfers limit the exhaust blowdown angle.area. In my FOS scavenging concept (picture below) there is so much blowdown angle.area available that the transfers could have been higher than 130°, but I stuck with these 130° because higher transfers will narrow the powerband; exhaust pulses returning too early would have more opportunity to push the fresh charge back from the cylinder into the crankcase.
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Nombre de messages : 55 Localisation : Germany Date d'inscription : 20/02/2012

@romeuh80:

I meant Frits' concept.

p12palof a écrit:: This is the most interesting thread that I have read in my life about 2 strokes technology. Thanks to all the people that share your knowledge in this thread, especially Jan and Frits. Your generosity are extraordinary

We want a book written by Jan and Frits!!!! That book would be priceless

A book would be great [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked) - Page 23 771973

A book would be great..... but you cannot ask questions to a book!

In fact, yes, you can.
But the book will not answer you...

[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked) - Page 23 809262

[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked) - Page 23 809262

Frits or Jan , .... 200 Deg. ex duration and 130 deg. transfer duration means exhaust open at 80 deg. atdc and transfer open at 115 deg atdc which gives a blowdown interval of 35 deg. This is at least 3 deg. more blowdown interval than we have been using... Our Rotax snowmobile race engines come stock with more generous timings by a couple of degrees ... The auxilliary exhaust on these cylinders in stock form is only approx 85% chordal .. Most of our race prepped cylinders with auxilliaries widened have eo of 79 deg. with 32deg. blowdown ... If we widen the auxilliaries to closer to 100 % chordal , which will greatly increase blowdown and widen the transfers so that the septums are like the RSA should we be reducing the timing angle of all the ports to less than stock timings and also increase the blowdown interval to 35 deg..? [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked) - Page 23 980796

[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked) - Page 23 980796

Hard to say, Mike. So your transfers have a timing of 138°? By today's standards that is huge; it will not do your powerband any good, but then you have a CVT to take care of that problem, haven't you?
The right way to go about this is to establish the angle.areas for blowdown and transfer, divide them by cubic capacity and by rpm of max.torque, which will give you the specific time.area values, and compare those to the Aprilia values.

Nombre de messages : 30 Localisation : Greece Date d'inscription : 09/07/2011

I think that blowdown should not be primarily referred to as a degree quantity, but as an area quantity (which can't be irrelevant to engine speed, so it becomes time-area).
If -in an imaginary engine and left aside all other exhaust/transfer timing effects- we could have all the required blowdown t-a, within 1 degree of revolution, for every rpm, cylinder would be evaquated, pressure would fall, pwer would be produced etc etc.
Obviously there can't be such an extreme, since neither blowdown is irrelevant to both exhaust and transfer timings nor can be infinite rising area, but hypothetically speaking, it wouldn't matter if blowdown is 1 or 40 degrees, as long as you have the required area to do its job in time.

Dernière édition par Frits Overmars le Lun 19 Nov 2012, 16:48, édité 1 fois

Quite right, Vagelis.
In my FOS scavenging system I use 192°/130° timing, giving 31° blowdown angle. Yet the angle.areas will allow the 125 cc version to produce its maximum power at 16,500 rpm.
Now where do I find a big end bearing that will play along? scratch

Yes ; the CVT is great for high strung small displacement race powerbands ... I agree that 138 deg. is lots of transfer timing and we will reduce it some seeing as how we are now widening them more to prevent seizure . That timing is for the C ports , our a and b ports are a bit less.. I can see that it will still be an empirical task to improve on what we have..... Your advice and experience has sped this process up and for that we are grateful... thankyou ... [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked) - Page 23 980796

Dernière édition par Frits Overmars le Lun 19 Nov 2012, 17:06, édité 1 fois

You're welcome Mike. It will indeed still be an empirical task, as the angle.areas are only part of the story.
You can have two ports with identical timing and angle.areas, and one may flow 20% better than the other....

Ideally we should not use the angle.area concept; instead we should use angle.flowrate. Computational Fluid Dynamics should be able to do that if you can persuade CFD to get all the pressure fluctuations right.
At the moment CFD for two-stroke development mainly means: garbage in > garbage out silent

.

Frits Overmars a écrit:: A single port cannot open the same amount of area per crankshaft degree as a double or triple port because its shape is limited by piston ring life. So yes, the difference can be significant. That is rather empirical; lower transfers limit the rpm at which the cylinder can still be adequately scavenged; higher transfers limit the exhaust blowdown angle.area. In my FOS scavenging concept (picture below) there is so much blowdown angle.area available that the transfers could have been higher than 130°, but I stuck with these 130° because higher transfers will narrow the powerband; exhaust pulses returning too early would have more opportunity to push the fresh charge back from the cylinder into the crankcase.

Thank you Frits for your answer. I understand everything.

Changing the topic a little bit...

1.) In case of a piston controlled inlet port is it right, that the suggested opening angle is 160 degrees which value is independent from the rmp? Is this true even if the time area or the angle area are too small?

2.) If I'd like to get the time or angle area I should count it from the size of the carburator's area, shouldn't I? If the area of the inlet port is bigger than the area of the carburator will I get appropriate values for the time and angle area, or not?

Forgi a écrit:: In case of a piston controlled inlet port is it right, that the suggested opening angle is 160 degrees which value is independent from the rmp? Is this true even if the time area or the angle area are too small?

No, the inlet event is completely different. Here you should try an inlet tract as short as possible, a large carburator and the shortest timing that will produce sufficient power.
Any inlet tract is much too short for pulse resonance; between Inlet Open and Inlet Close a pulse would cover as much as five times the tract length and it would have hardly any energy left by the time the inlet closes. For the inlet system the Helmholtz resonance is all-important.

Citation :: If I'd like to get the time or angle area I should count it from the size of the carburator's area, shouldn't I? If the area of the inlet port is bigger than the area of the carburator will I get appropriate values for the time and angle area, or not?

If you calculate the angle.area of the carburator you will get huge values because it is always open. Calculating angle.areas only makes sense if you do it at the port.

Frits Overmars a écrit:: You're welcome Mike. It will indeed still be an empirical task, as the angle.areas are only part of the story.
You can have two ports with identical timing and angle.areas, and one may flow 20% better than the other....

Ideally we should not use the angle.area concept; instead we should use angle.flowrate. Computational Fluid Dynamics should be able to do that if you can persuade CFD to get all the pressure fluctuations right.
At the moment CFD for two-stroke development mainly means: garbage in > garbage out .

Thats why we love our dyno .....It sorts things out .... [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked) - Page 23 980796

Frits Overmars a écrit:

Forgi a écrit:: In case of a piston controlled inlet port is it right, that the suggested opening angle is 160 degrees which value is independent from the rmp? Is this true even if the time area or the angle area are too small?

No, the inlet event is completely different. Here you should try an inlet tract as short as possible, a large carburator and the shortest timing that will produce sufficient power.
Any inlet tract is much too short for pulse resonance; between Inlet Open and Inlet Close a pulse would cover as much as five times the tract length and it would have hardly any energy left by the time the inlet closes. For the inlet system the Helmholtz resonance is all-important.

Citation :: If I'd like to get the time or angle area I should count it from the size of the carburator's area, shouldn't I? If the area of the inlet port is bigger than the area of the carburator will I get appropriate values for the time and angle area, or not?

If you calculate the angle.area of the carburator you will get huge values because it is always open. Calculating angle.areas only makes sense if you do it at the port.

Well, I'm affraid I wasn't clear enough. I mean what if the size of a carburetor is limited for example because of a rule of competition...? In this case we only have a given small sized carburetor so if we'd like to get a convenient time area, we have to make a significant increase of the inlet port's angle.

Concerning the second question I tried to say that we calculate the time area applying the value of the inlet port's angle and the value of the carburetor's area. So the inlet port which is bigger than the size of the carburetor is unnecessary and if we take into account such area it will lead us to a wrong result, or we can easily mislead ourselves.

Drawins are also attached.

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I have a question, maybe a bit offtopic.

A nearby company makes coatings, titanium coatings, one worker from there said to me that the surface of the cylinder I showed him would cost around 130€ to coat with 0.03 mm thick titanium.

Why not titanium coating instead of nikasil?
It would be possible to make the honing with soo thin thikness?

I don't know exactly what temperature they need to heat the surface wall of the cylinder...
I don't know if the ring material would have to be changed...

Or the titanium would eat the piston??? lol!

Regards

Nombre de messages : 41 Localisation : Italia Date d'inscription : 20/11/2012

Jan Thiel a écrit:

ridley a écrit:: "At Aprilia we had a very good epoxy, called 'stucco verde' in Italian.
Never worked loose and even withstood replating!
Sorry I do not know were it was made!"

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Yes, that's it!

Hi Jan, which one, exactly?
Thanks a lot to share with us your huge knowledge!

The one above Filandro!

Thanks for that Jan.

This place can expect to be swamped with orders [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked) - Page 23 809262

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Nombre de messages : 41 Localisation : Italia Date d'inscription : 20/11/2012

Thanks a lot, ordered just now.

Nombre de messages : 28 Localisation : Slovenia Date d'inscription : 19/03/2012

I would like to ask a question; what kind of gaskets is best to use between the cylinder and carter, metal or "paper" type?
Could a "paper"-type gasket help transfer less heat to the carters and thus have less heat transferred to the fresh intake charge?
I noticed competition kart engines use paper-type gaskets.

Thanks for your answer.

Paper gaskets are fairly good heat insulators, compared to metal gaskets. Their drawback is that they set during use. That is why Honda used no gaskets at all on their two-stroke works racers; instead they fitted O-rings between cylinders and crankcase.
Of course, when cylinders and crankcase are both well cooled, there is hardly a temperature difference and in that case the insulating property of a gasket becomes unimportant.

Sujet: Auxiliary exhaust ports Jeu 29 Nov 2012, 02:14

Hi Jan,
Was there much HP gain from extending the auxiliary exhaust ports below the front of transfer port A? – (the side closest to the main exhaust)

Thanks for your answer in advance.
Regards
Allan.

GtG001 a écrit:: Hi Jan,
Was there much HP gain from extending the auxiliary exhaust ports below the front of transfer port A? – (the side closest to the main exhaust)

Thanks for your answer in advance.
Regards
Allan.

I think that was already explained in the first pages.

What I did not find in my search was the C port Open angle?

What changes are made by the angle of C port?

Do we really lost charge at high rpm by directing it towards the exhaust?

Thanks

Nombre de messages : 30 Localisation : Greece Date d'inscription : 09/07/2011

I 'm not sure of the aux-ex full-open point, probably just below 115°, judging from this [Vous devez être inscrit et connecté pour voir ce lien]
But if you look at the earlier ap- cylinders, auxiliaries were closer to the front TFR than in the latest cylinders. And of course, Jan said that, in general, they should be as far as possible, to prevent direct fresh charge loss at tfr opening.

C port opens at 114°, same as B ports. Axial angle of the older APC (I think) cylinder was 52.3°. I guess this hasn't changed much, given the role of the port.

And... I couldn't help you about the 2nd and 3rd question [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked) - Page 23 858879

[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked) - Page 23 858879

Nombre de messages : 28 Localisation : Slovenia Date d'inscription : 19/03/2012

Frits Overmars a écrit:: Paper gaskets are fairly good heat insulators, compared to metal gaskets. Their drawback is that they set during use. That is why Honda used no gaskets at all on their two-stroke works racers; instead they fitted O-rings between cylinders and crankcase.
Of course, when cylinders and crankcase are both well cooled, there is hardly a temperature difference and in that case the insulating property of a gasket becomes unimportant.

Thanks for your answer mr. Overmars!
I think I'm going to try it out. Perhaps also I'll try an insulation coat for the crankcase insides, our CVT produce quite a lot of heat that also dissipates on the engine block. And the cooling system is hard to improve.

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