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

I prefer not to talk about a stinger because that would imply a tube of some length. And length is not the issue here; the issue is flow resistance and diameters are far more decisive than lengths in that respect. Therefore I always apply a venturi as the flow-controlling element at the end of the reflector cone. It can be made exchangeable which allows experiments, and because the flow is controlled by the venturi, it allows for a tailpipe with a much larger diameter that is less susceptible to dents and deposit build-up. It also allows for a longer tailpipe in case you need to move a silencer out of the way.

You may be wondering about waves running up and down the tailpipe. Don't. Those waves will never be able to influence the events in the exhaust pipe because the flow through a correctly dimensioned venturi is sonic. Those waves are like birds flying against a storm: they will lose all their energy but they will never make any progress.

Your experience is correct: more flow restriction causes a higher average pressure in the exhaust pipe, hence less expansion and a higher average temperature of the gas in the pipe, and hence a higher speed of sound and a higher resonance rpm. And if you create too much flow restriction, the exhaust gases in the cylinder will not completely have left the building by the time the transfer ports open; they will enter the transfer ducts and that flow of hot gases will severely heat up the cylinder, the piston and the fresh mixture in the transfer ducts.

The required flow restriction depends solely on the amount of generated exhaust gas, and that is directly proportional to the generated horsepower. I am almost certain that somewhere on this forum I already posted a simple exhaust concept that included the venturi calculation. But I can't find it through the search option (forum search options and I never seem to get along) so here it is once more, together with a sketch of the exchangeable venturi (red in the sketch).

Brian, you are right about the flow restriction influencing the mixture strength of the MB40 model aero engine. For those of you who are less familiar with it: these model airplanes keep their fuel in a bladder. That bladder is contained in a tank, and the exhaust pipe pressure is fed to that tank. The pipe pressure tries to squeeze the bladder and the fuel is pushed to the inlet port via a regulator needle. But since the needle setting is optimized for each flight anyway, it doesn't matter if the fuel pressure is not always the same.

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As far as I know the flow restrictor was first used by Helmut Fath.

splendid frits as always!! is there any thumbrule on D from your graph?

on another note, yesterday i found the vid of the injection CR500 - today i was browsing along and found this article:

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it is mainly dealing with injection and their possibilites for the future, quite interesting but a bit "light" stuff. especially when they are comparing a 450 2-stroke with a 450 4-stroke, oh wonder it has the same power? this should rather be embarrassing for orbital...

but what attracted me was the passage about an injection equipped rs250

Citation :

“I’ve ridden the future”
Jeremy Bowdler is the editor of Australia’s Two Wheels magazine and the only man outside Aprilia to ride the RS250ADI – the Orbital-equipped next-generation RS.

anyone knows if its real? the article seems somehow a bit light as i said...

koenich a écrit:

is there any thumbrule on D from your graph?

Yes. It is called Dtailrestrictor...

koenich a écrit:

what attracted me was the passage about an injection equipped rs250

Citation :

“I’ve ridden the future”
Jeremy Bowdler is the editor of Australia’s Two Wheels magazine and the only man outside Aprilia to ride the RS250ADI – the Orbital-equipped next-generation RS.

anyone knows if its real? the article seems somehow a bit light as i said...

It was real. That bike must be about ten years old now. It never got beyond the prototype stage. Aprilia ceased developing street two-strokes a long time ago .

Frits Overmars a écrit:

It was real. That bike must be about ten years old now. It never got beyond the prototype stage. Aprilia ceased developing street two-strokes a long time ago .

sad, that would have been a nice bike!

hopefully bmw won't give up (at least from the last articles it does not seem so) and ktm pushes sth in the MX market. I know a hell lot of people from MX (including myself) that would love to go back to twostrokes. Once you have blown a fourstroke, which will happen if u ride it above hobbyspeed, your wallet will be damn empty and the garbage bin full of camchain, valves, a cylinder, maybe a cylinder head, a piston and probably a crank.

Jan or Frits, during development of the engine, did you use a fuel balance and exhaust gas analysis, so that you could calculate delivery ratio, trapping efficiency, total engine efficiency and so on to thermodynamically analyse your test bench results?

For example: primary compression is lowered
result: more power when in the powerband and even more in the overrev

what happened (example): total delivery ratio decreased slightly, but charging efficiency increased more, thus trapping efficiency increases which is where the extra power comes from (and the lower specific fuel consumption). Looking at the total engine efficiency across the speed range, you notice that it now drops later. It does so, because of the reduced pumping losses at BDC which rise quadratically with engine speed.

But why was trapping efficiency increased? For this you'd need an indication system and I remember somewhere in this thread you mentioned that you did not use indication. With the pressure transducer located near the transfer port exit you could see that with the lower primary compression the transfers actually start flowing a couple of degrees later which (in this example) reduces scavenging losses.

Haufen a écrit:

pumping losses at BDC which rise exponentially with engine speed.

A small soap-box issue for the ex-mathematician that I am: They do NOT rise exponentially. They rise quadratically,
or maybe cubically, but certainly not exponentially.

Now carry on

Thanks, you are right, quadratically is the right term. Or can you say squarish, also?

yes you can! sounds strange though

We just measured fuel consumption and exhaust temperature.
There was very little instrumentation!
So we never calculated trapping ratio und such things!
Our only interest was more HP!

Jan Thiel a écrit:

The aux. exhaust ports were 2mm lower than the central one.
The exhaust valve was fully open at 12.000 rpm.

Thanks for answering my question Jan.
What rpm did you start to open the power valve at?

You mention previously that your used 0.8mm piston rings; what was the HP gain from using these compared to the 1.00mm ring and were there any losses in the lower rpm range?

Thank you,
regards
Allan.

We started to open the power valve after 10.000, it was fully opened at 12.000.

The 0,8mm piston ring gave a couple of tenths more HP.
At no point in the power range was there any loss.

Thanks Jan.

Do you think that the RSA crankcase volume would work as well on a Reed Valve equilvent motor?

Sincerely I do not know.
But reed valve engines may need smaller crankcase volumes.

Thanks Jan,
My experience with crankcase volume with GP reed valve motors is that there is a right volume /resonances for a particular RPM range.
I am wondering if this limits the rod length a reed valve motor can use as it also varies crankcase volume away from the optimum.

crankcase volume seems to be somehow strange - at least if u can't test it on a dyno. I am looking for a small engine dyno (30hp is enough for me) at the moment and already did some calculations how I could build one. Would be cool just to have a engine stand with access all around...

BTW @Frits: How is you FOS cylinder? Did you make any progress?

"Guys from the world", to answer, most of the time you just have to use the orange "Répondre" button, not the blue "Citer" one...
Thank you!

Hello Jan ,
I have to say what a great pleasure it is to be able to talk to you in this way , i have followed your exploits since the
early days at Jamati . Just a small question about temperatures , what readings were you getting at the exhaust duct/
pipe junction , where the thermocouple was located , at the pipe mid point , and perhaps at the spark plug ?
I look forward to hearing from you ,

Kind regards, Mel .

Mel,

Temperatures at the exhaust flange were usually between 500 and 600°C if I remember well after 4,5 years!
Mid-exhaust temperatures were not taken,but I think they are higher.
The temperature sensor in the exhaust flange might have been cooled a little by escaping fresh charge!
We sometimes measured under-spark plug temperatures, they proved too high so we modified the head insert,
bringing the water nearer to the plug. This was very succesful, afterwards we never bothered with the sparkplug temperature anymore!

GtG001 a écrit:

Thanks Jan,
My experience with crankcase volume with GP reed valve motors is that there is a right volume /resonances for a particular RPM range.
I am wondering if this limits the rod length a reed valve motor can use as it also varies crankcase volume away from the optimum.

I think it would be no problem to fit a longer conrod and keep the crankcase volume the same!
You could put the pistonpin higher in the piston for example, at Aprilia we did this.

Hi Jan,
Can you tell us what the volume of the airbox was on the RSA and can you tell us how much HP the airbox was worth please?

Thank you

Jan ,
Many thanks for that information , i must say i was anticipating a higher value for the exhaust flange temp .
Using that number would indicate a lower average temp for my wave speed calculations . Oh well , back to the
drawing board , calculator and pencil !
I bet you get a hell of a kick out of all of this , chatting to enthusiasts around the world and helping others
to enjoy twostroke technology .

All the best , Mel

GtG001 a écrit:

Hi Jan,
Can you tell us what the volume of the airbox was on the RSA and can you tell us how much HP the airbox was worth please?

Thank you

I am sorry not to know the airbox volume.
On the dyno the engine gave less HP with the airbox fitted.
On track it was better of course, due to more air pressure at speed.
How much better I do not know!
Much of the improvement comes from the engine not being fed with hot air!

Melvyn,

Frits may have better information about exhaust temperature, as I gave all my notes to him when I stopped working!

GtG001 a écrit:

Hi Jan,
Can you tell us what the volume of the airbox was on the RSA and can you tell us how much HP the airbox was worth please?

Thank you

i think a good thumb rule for airbox volume is at least 10 x capacity, even better is 20 x capacity