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

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

You should be able to calculate the effect. Worst case, imagine that no air was able to enter the box during the period that the motor is drawing from it. Then the motor sucks a certain volume and the result is that the density
in the box must drop in proportion. So with a box of 10x capacity, the density would drop 10%... huge!

-In fact the motor will draw a bit more than it's capacity because of the crankcase/resonance/pipe issues;
-of course air continues to fill the box, but particularly if there are longish intake tubes there will be a frequency-dependent impedance;
-20 x capacity for a 125 is still tiny!

The 10x rule of thumb comes from multi-cylinder engines with even firing. A conventinal inline 4 is breathing almost continually, but the situation is very different for a single or v-twin.

There was a nice description on the Sigma website about running a Ducati supermono at Daytona. A fourstroke, but still a single, hence non-uniform breathing from the air-box. They got best performance by cutting a big hole in the top! Similarly Durbahn obtained good gains on an RC51 by increasing volume past 20 litres... again, a V-twin breathes irregularly.

I'm pleased to see Jan making the temperature comment : the whole ram-air thing is over-stated, it will give you at best 2% more density at 280km/h. The same benefit as having air 6° cooler. You don't want to breathe through the radiator!

The air box works like a resonator cavity in the four strokes, the pressure waves blown the plastic boxes (there are pressure sensor inside on some makes of twins like aprilia 1000rsv- formula laverda ); the box helps to maintain a constant pressure( not real density) and for the altitude corrections also .

The box helps at first the middle range torque (avoiding losses of irregular feeding caused by the waves exiting the pipes (long or short ones)).It plays like a damper but the noise recorded is louder because of the volume of this cavity acts like a woofer.

I don't think the volume for 2 strokes must be big as 4 strokers because the waves are twice frequencies and surely more intense(no valve in the duct)> the wall must be softer also to damp effectivelly..



What you think?

Ah the cat is out of the bag! Frits that is! You have all Jan's notes. A book is in the works hopefully. What will it take to get you (Frits) to publish the notes with your interpertations added?

Howard Gifford a écrit:

A book is in the works hopefully. What will it take to get you (Frits) to publish the notes with your interpertations added?

A second life i guess.

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GrahamB,

Some time ago engines mostly breathed air warmed by the radiator.
And do'nt forget the exhaust pipes!
Inside the fairing they heated the air quite a lot.
Putting them outside the fairing you lost revs.
The best solution was outside the fairing, but shielded from the airstream.
A friend of mine once tested his MBA 125 on a dyno. With and without fairing.
With fairing he lost 7HP!
So when I once had the opportunity of wind tunnel testing we looked at this.
There proved to be a quite big depression at the carburetors, at a wind speed of 120km/h.
This depression would of course have been even bigger had the engine been running!
The airbox resolved all these problems.
The bigger the better I heard at Aprilia!
Mostly there is not enough room at the bike however!