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

Jan Thiel a écrit:

It would be most interesting to see if a 50cc can reach the same level as a 125!

Panas a écrit:

50cc power level of a 125?!!!!

Panas, Jan did not write power level; Jan was talking about the development level, the Brake Mean Effective Pressure.
When you scale-down an engine, it is very difficult the maintain the BMEP of the original engine. For example, if you halve the bore and the stroke, all surface areas that come into contact with combustion, become 1/4 the size of the original areas,
but the cylinder volume becomes 1/8 of the original volume. So the area / volume ratio becomes twice as big, and the specific heat losses are doubled.

I didn't really mean 54 hp...from a 50cc
,enthusiastically got carried away...
also others have made weird claims "up there"
I also believed you(dutch )have gotten everything possible out of a 50cc...
(no true if we consider  jans/your work the pinnacle of 2t development?)
so how much power  do you get from 50cc and how much do you think one would  start making
from downsizing a rsa
ps the god like figure of Jan makes our minds go...num!

Panas a écrit:

so how much power  do you get from 50cc and how much do you think one would  start from downsizing a rsw

Hard to say.
Some people published calculations based on cylinder capacity and rpm values, but they forgot these heat losses

i wish you guys were here, id bring the beers and chops and all you had to do is tell the stories...!
il erase as irrelevant...

Toop a écrit:

This one is not a 125cc  

[Vous devez être inscrit et connecté pour voir cette image]

Jan,care to elaborate on this project?

Yes, this is the Garelli 250 engine.
It should never have been made, as Garelli did not have the resources to develop it.
Which I should have realized, but didn't!
I decided on a V-engine because of all the stories I had heard about badly vibrating twins.
That was a good decision, it did not vibrate at all, even with the 72° V-angle.
The crankshaft was 100% reliable.
But I made a mistake with the cylinder.....
Too much confidence in flow bench tests led me to believe that I could use
the same short port-timings I was used to in 50cc and 125/2 engines. Less than 190°
This made very small tailpipe diameters a necessity.
And this, in turn, led to piston overheating.
Later, working at Aprilia, I learned that short exhaust timing is useless on a 125 single!
The engine was fast when cold, but lost power due to piston overheating.....
So it never really was successful.
But when cold, it easily passed the Aprilias on the straights!

Jan Thiel a écrit:

Yes, this is the Garelli 250 engine.
It should never have been made, as Garelli did not have the resources to develop it.
Which I should have realized, but didn't!
I decided on a V-engine because of all the stories I had heard about badly vibrating twins.
That was a good decision, it did not vibrate at all, even with the 72° V-angle.
The crankshaft was 100% reliable.

The crankshaft throws were 0°/360?
ie the 2nd cylinder fired 72 after the other?

Jan Thiel a écrit:

But I made a mistake with the cylinder.....
Too much confidence in flow bench tests led me to believe that I could use
the same short port-timings I was used to in 50cc and 125/2 engines. Less than 190°
This made very small tailpipe diameters a necessity.
And this, in turn, led to piston overheating.
Later, working at Aprilia, I learned that short exhaust timing is useless on a 125 single!
The engine was fast when cold, but lost power due to piston overheating.....
So it never really was successful.
But when cold, it easily passed the Aprilias on the straights!

Why such short port timings on the 50's?Why was it "useful" on them?
was it just the development level?
which means,as you have implied by previous
answers that more exh.degrees were on their way?
Can you please explain the "troubleshooting" process after the fact that
190 were not enough.
If i'm not mistaken (but probably I am...)
and by tailpipe you mean expansion chamber then....
short port timing necessitated a short exhaust to make up for power,
this made the power curve very steep,but a not very high revving engine,
the piston overheated at part throttle just like you have mentioned for the aprilia.

I read somewhere that you left Garelli distraught.
there was also a 125

First of all, many (late) wishes to Jan, happy birthday!      

Hi Frits,
some time ago I asked you for some suggestions to make a head for my karting engine, to help me reduce the detonation, and also on the configuration of the scavenging (thanks again for your generous explanations):

initially I modified an original head by reducing the squish to 0.65 mm instead of the original 0.90 and I made it parallel to the piston instead of the previous 2 degrees (the piston is a 4-degree conical type).
You advised me to go down to 0.50 but honestly I didn't feel well like reaching such an extreme value, because in karts gear changes are very violent and we often reach 16.500 rpm.

Also, you have to know that the original head has a particular design, in practice after the squish band, instead of a 90 degree edge, it has a half-ring about 0.6 mm wide and about 2.5 mm high (see attached image).

In order to have further feedback on performance differences, I also made a head on the style of your design, always with squish 0.65 mm parallel to the piston, but with 5.25 mm wide squish band, slightly wider than the original 5 mm (see image attached), also to get feedback on the relationship between temperature and width of the squish.

Obviously both heads have identical volume (11 cc) to remain compliant with the regulation.

[Vous devez être inscrit et connecté pour voir cette image]

I would therefore like to share with you, and all the readers of the forum, the feedback I had: please note that these are just my impressions without the help of a test bench, so they are qualitative and not quantitative (except for the exhaust temperatures):

Original modified head (the one with the inner half-ring):
- it produced the desired effect, it eliminated detonation
- the minimum and maximum exhaust temperatures have slightly lowered, respectively from 485 to 465 the minimum and from 610 to 595 ° C the maximum (the carburation tends to be rich);
- the driving feeling is that it has slightly improved the torque delivery to the extreme low of the power band (9.500 - 10.500 rpm) and made the torque start slightly less "pointed", very little ...

Head FOS-Like design:
- this also eliminated the detonation;
- require a slightly leaner carburetion in the idle circuit (63 instead of 66) to make the engine drivable as before;
- made the torque significantly smoother between 9,500 and 11,500 rpm, which is good when there is little grip, less good when there is a lot of grip or on fast corners;
- has increased - against all my expectations - the minimum and maximum exhaust temperatures, which respectively reached 505 and 630°C; since the squish band was slightly enlarged, as you explained to me, I expected a slight decrease;
- another unexpected result was the piston crown aspect: raising temperatures – generally – increase carbon deposits due to the mixture oil carbonizations, but this did not happen, those remain the same (meteo condition was unchanged too!!!).

After the head modifications, I changes a little bit the cylinder (for the moment I made minor adjustments):
- raised the T1 from 130.8 to 131.8 degrees
- raised the T2 from 127 to 128 degrees
- raised the T5 from 124.5 to 128 degrees
- little increased the radial angle of the boosters
- the result was very satisfactory, with both heads, which led to a greater linearity of the torque, maintaining the good rpm extension already existing, and above all improving the shot in the lower power-band.

At this point I would like to ask you, why did the temperatures rise with the head with the widest squish band?
Could it be because the semi-ring realizes a "virtual" extension of the squish band?
If yes, why?

Thanks again for your fantastic explanations.

Thank you for your kind words and your feedback, Scalvo.
As for the unexpected temperature rise, I'd have to guess because I agree that it was unexpected.
I'd say that the most probable cause might be a slight change in the air/ruel ratio. This can occur when changes are made to an engine, even though the carburettor setting is unaltered.

Jan Thiel a écrit:

Yes, this is the Garelli 250 engine.... I decided on a V-engine because of all the stories I had heard about badly vibrating twins. That was a good decision, it did not vibrate at all, even with the 72° V-angle.

Panas a écrit:

The crankshaft throws were 0°/360? ie the 2nd cylinder fired 72 after the other?

Both big end pins were in line, so with 0° offset, and indeed the firing order was 72°, as was the included cylinder angle.

Jan Thiel a écrit:

But I made a mistake with the cylinder. Too much confidence in flow bench tests led me to believe that I could use the same short port-timings I was used to in 50cc and 125/2 engines. Less than 190°. This made very small tailpipe diameters a necessity. And this led to piston overheating.

Panas a écrit:

Why such short port timings on the 50s?
Why was it "useful" on them?
Was it just the development level?
Which means, as you have implied by previous answers that more exh.degrees were on their way?
Can you please explain the "troubleshooting" process after the fact that 190° were not enough?

Some time ago the administrator of this fine forum proposed a 'one question per post' line of conduct.
I think he meant a maximum rather than a minimum, Panas   .

Citation :

by tailpipe you mean expansion chamber?

Working with Jan Thiel taught me that he always means what he writes. So when he writes tailpipe, he means tailpipe.
Perhaps the Americanism stinger is more familiar to you? Or the Ialian expression terminale? Or the German Endrohr?
Kidding apart, you'll know it as σωλήνα εξαγωγής   .

Citation :

I read somewhere that you left Garelli distraught. There was also a 125

Yes, I too heard about that Garelli-125. I also 'heard' that it won six straight world championships in a row (I was there, still have the T-shirt. And the mechanic overalls).
And I don't for one moment think that Jan left Garelli distraught. It's just that racing there came to an end because of the economic situation.

Frits Overmars a écrit:

Thank you for your kind words and your feedback, Scalvo.
As for the unexpected temperature rise, I'd have to guess because I agree that it was unexpected.
I'd say that the most probable cause might be a slight change in the air/ruel ratio. This can occur when changes are made to an engine, even though the carburettor setting is unaltered.

Many thanks to you Frits, also for fast reply...

I can assure you that the test was carried out without any change in the carburetor settings and with the same weather conditions: I did a couple of stints with the original modified head and immediately after with the FOS-like one.

As I said, I removed 3 points from the minimum jet to improve driveability, but the temperatures remained essentially the same.

The first test was carried out with the original cylinder, while a couple of weeks later I did the same experiment with the modified cylinder, but first with the FOS-like head and then with the original modified head and the results was confirmed.

The explanation I gave myself (of pure fantasy ...) is that the semi-ring after the squish band creates a sort of virtual extension: practically when the mixture ignites, the flame front flows on the wall of the head, meets the ring and is pushed towards the inside of the combustion chamber, creating the virtual extension.

In your opinion, could this be a plausible explanation?

Scalvo a écrit:

The explanation I gave myself (of pure fantasy ...) is that the semi-ring after the squish band creates a sort of virtual extension: practically when the mixture ignites, the flame front flows on the wall of the head, meets the ring and is pushed towards the inside of the combustion chamber, creating the virtual extension.
In your opinion, could this be a plausible explanation?

Could be.
The question that arises is: how to create a situation where you can measure what is happening? Or at least exclude other possible explanations? Four-strokes are so much easier in this respect (and two-strokes are so much more fun ).

Σωλήνας εξαγωγής is very general.(exiting  pipe or pipe of exit)
I prefer tailpipe,No dutch?
I m sure it would be  more familiar sounding
to my ears...

So why a small diameter tail pipes a necessity of short exit duration?
oh ...2 questions here please forgive me!
So if you have the time and patience we will like to know on the development of the 125.

Panas a écrit:

Σωλήνας εξαγωγής is very general.(exiting  pipe or pipe of exit). I prefer tailpipe,No dutch? I'm sure it would be  more familiar sounding to my ears...
So why a small diameter tail pipes a necessity of short exit duration? oh ...2 questions here please forgive me!
So if you have the time and patience we will like to know on the development of the 125.

Let's stick with tailpipe then, Panas. Or, if you prefer the Dutch word: tailpijp (we borrow a lot from the English language )
It is not my experience that a short exhaust timing always necessitates a small diameter tailpipe. It may be the case in combination with a relatively long expansion chamber; that would be a question for Jan to answer.

Re 'the development of the 125': I'm not sure whether you mean the Garelli 125/2 or the Aprilia RSA 125.
I like to think that I have a fair amount of patience, but I have definitely not enough time to start digging into a cubic metre of documentation. Besides, the Garelli 125/2-era was over 30 years ago.
If you are interested in two-stroke development, I would recommend reading about the much more up-to-date Aprilia RSA.
If you have time, you could start reading here: [Vous devez être inscrit et connecté pour voir ce lien]
That wil keep you out of the pub for the next couple of weeks .

Frits Overmars a écrit:

Scalvo a écrit:

The explanation I gave myself (of pure fantasy ...) is that the semi-ring after the squish band creates a sort of virtual extension: practically when the mixture ignites, the flame front flows on the wall of the head, meets the ring and is pushed towards the inside of the combustion chamber, creating the virtual extension.
In your opinion, could this be a plausible explanation?

Could be.
The question that arises is: how to create a situation where you can measure what is happening? Or at least exclude other possible explanations? Four-strokes are so much easier in this respect (and two-strokes are so much more fun ).

My small-sized brain could be has not a reasonable answer, but I try:
the basic of any good measure is to have every external condition fixed: in my tests there were (same air temperature, UR and Pressure, same circuit, same carburation, same driver, etc.), but if we are looking to "perfection" these are not comparable to a test bench.

If we are speaking about what happen into the combustion chamber, I think we cannot have a direct misurament, so we must look to an indirect measurement. For example realizing other 2 heads, one with "semi-ring" halved than actual, and another one with "semi-ring" doubled, then measure the exhaust temperature.
This could be confirm - or not - if it really works like i found.

Another method could be use a flow bench with polarised laser to direct visualize how flux move into the chamber, but it can not have the same condition that happen during the combustion.

Sincerly, I don't know how to exclude anything

So, what do you think about that?

Scalvo a écrit:

the basic of any good measure is to have every external condition fixed: in my tests there were (same air temperature, UR and Pressure, same circuit, same carburation, same driver, etc.)

We need track time to optimize the carburation, the gearing and the suspension, but I would not perform engine research on a circuit; there are far too many uncontrollable circumstances.
The track may be dirty, the wind may change, the rider might have had a wild night, the engine might perform better with a gearing that is different from the previous test, and, as I wrote above, changes in the air/fuel ratio can occur when changes were made to an engine, even though the carburettor setting is unaltered.

Citation :

If we are speaking about what happen into the combustion chamber, I think we cannot have a direct measurement.

We can; google Comsol Multiphysics. They have everything you could possibly want. In return they want your weight in gold.

lol no pubs Frits never really been a fan...
id prefer the fireplace usually with a grill on top of it!
no much free time either...
the "j" is like in the english "yes" i assume(or silent?)
Sometimes i understand the topic of conversation if i hear dutch
( contributing is the constant use of English in almost every sentence)
sometimes I know its dutch but the language sounds very different,like a dialect.
anyway for the moment  my head is in this  http://www.classic50racingclub.co.uk
then ill have to do..2 years of catching up where you suggested
this is what i meant
edit.
theres supposedly attached a picture of a garreli 125 engine...

Panas a écrit:

the "j" is like in the english "yes" i assume(or silent?)
edit: there's supposedly attached a picture of a garreli 125 engine...

Do you mean that you tried to attach a picture of a Garelli 125 engine? I can't see anything and that's understandable because attaching pictures in this forum is not the simplest of things. It took me some time to get the drift.
I have so many Garelli pictures that I wouldn't know which ones to post first. Below are a few examples.

The pronunciation of the character j in the word pijp is not silent. The i and the j together stand for a sound, for which I could not find an English or French example. If you have German, you could pronounce it as äi. But you can find the pronunciation of almost every word in almost every language here: [Vous devez être inscrit et connecté pour voir ce lien]
[Vous devez être inscrit et connecté pour voir cette image]

And some more:
[Vous devez être inscrit et connecté pour voir cette image]

Frits Overmars a écrit:

We need track time to optimize the carburation, the gearing and the suspension, but I would not perform engine research on a circuit; there are far too many uncontrollable circumstances.
The track may be dirty, the wind may change, the rider might have had a wild night, the engine might perform better with a gearing that is different from the previous test, and, as I wrote above, changes in the air/fuel ratio can occur when changes were made to an engine, even though the carburettor setting is unaltered.

Thanks again Frits,
I agree, on track we can not perform correctly any research, and I’m absolutely aware that changes in any engine component changes carburetion, (I’m quite well skilled in carburetion).

What really surprise me is the temperature delta I discovered…. This is not the first time I change something on the head (and also other components), but I assure this is the first time I see a delta of 35+ degrees!!! Usually there are small differences from 5 to 10°C.
Fortunately I decided to start with a rich carburetion, otherwise seizure would have arrived immediately.

Frits Overmars a écrit:

We can; google Comsol Multiphysics. They have everything you could possibly want. In return they want your weight in gold.

I know this simulation software suite, I work for a company that have software solutions for product development (CAD, PLM, IoT, etc), and many of our customer are in automotive and in motorsports sectors (I don’t want to say here the firm and customers, but you can imagine, there are only 3 respectable firms worldwide in software sector, and not many automotive customer).

Unfortunately I do not work for them and also my money pocket doesn't contain so much gold...

Next step will be a new cylinder, with scavenging 132/130/130, booster exit angle about 15/20 degrees more than the perpendicular angle and raise 2-3 mm the exhaust port bottom.

We will see here

thanks again

Hi frits, i saw this old photo that you have posted time ago. Could you explain what's happen if the direction angle is greater than position angle? Thanks in advance.
Eric
[Vous devez être inscrit et connecté pour voir ce lien]

Eric_91 a écrit:

Hi frits, i saw this old photo that you have posted time ago.  Could you explain what's happen if the direction angle is greater than position angle? Thanks in advance. Eric [Vous devez être inscrit et connecté pour voir ce lien]

If the direction angle is greater than the position angle, you will increase the exhaust duct volume without improving the flow.

PS: I saw your hydraulic dyno [Vous devez être inscrit et connecté pour voir ce lien] Nice job
I wonder: is the stepper motor reaction always quick enough?

Thanks a lot!
The stepper motor reactions is not quick enough to brake engine with pid control system, but works well if you put engine at full throttle and then stepper start to gradually close the flow valve until engine reach endpoint of test.
Of course you can set the velocity of the sweep and the starting position of flow valve. At the moment is the only brake control system that works well with hydrauil dyno. (orher guys that have built simikar dyno agree with me)

Hi frits, I have a stupid question.....if I increase the lenght of boost ports (see photo) without increase the section of ducts, I think the flow won't benefit, but the angle area increase or not?
[Vous devez être inscrit et connecté pour voir cette image]

Eric_91 a écrit:

Hi frits,  I have a stupid question.....if I increase the lenght of boost ports (see photo) without increase the section of ducts, I think the flow won't benefit, but the angle area increase or not?

I'm not sure that I understand what you mean Eric. In your above picture I see a higher port, but not a longer one. Heightening the port will certainly increase its angle.area. It will not improve the maximum flow velocity through the ducts, but because of the longer port opening time it will increase the mass flow through the ducts.