[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 3) (Locked)

Frits Overmars a écrit:

Ciao Stefano, come stai?
Everything is different between the Aprilia RSA125 engine and the 100cc kart engine. I think you have been able to find sufficient data regarding the RSA125 in all the forums you are visiting , so I will confine myself to the kart engine.
It has only three transfer ports, with a timing of 134°. The exhaust timing is 187° and there are no power valve, no powerjet and no variable ignition timing.
Both the primary and secondary compression ratios are high and the exhaust pipe is deliberately made low-efficient so that the exhaust pulses do not generate much power, but they do not cause a deep torque dip either.
The axial scavenging angles are rather steep so that at low revs a lot of fresh mixture is washed-through, lowering the exhaust gas temperature and the pipe resonance rpm.
What you would need for your ideal power curve, are a programmable ignition, a power jet and a power valve or, best of all, a trombone exhaust pipe.

Hi Mr Frits. i'm ok thanks
i can't use variable ignition , and not even power-jet... the regulation does not allow.
I can modify my tube, and all to engine parameters... remaining within the limits of space accepted.

I sent you one message on facebook...

pierre95 a écrit:

A question for you Frits. Concerning exhaust systems , all calculation we do are doing on linear exhausts so my question is, how do you convert a linear shape and design to a no linear one for keeping characteristics from calculations?
What would be the most important? Example on a diffusor. Keeping the volume after shaping? Keeping the same length on the neutral fiber.. or something else? Of course i know in any case it will be after, testing and testing....

My starting point is to maintain the length along the neutral line.

bengui a écrit:

Frits Overmars a écrit:

It has only three transfer ports, with a timing of 134°. The exhaust timing is 187° and there are no power valve, no powerjet and no variable ignition timing.

Frits, are you sure about the exhaust timing ? In the old ICA the maximum allowed was 177°, I have already seen 182° but not more. You confirm 123° for A port and 133° for C port ?

No, I am not sure, Bengui. It's too long ago and I never occupied myself seriously with direct-drive kart engines.
Maybe my memory is playing tricks on me and it was 178° instead of 187°.
In the old rules the timing was measured with a gauge between piston and port roof; this may explain the 1° deviation.

SB07 a écrit:

I sent you one message on facebook.

Please don't. The forums are already taking up too much of my time. Facebook would only make it worse.

Frits Overmars a écrit:

SB07 a écrit:

I sent you one message on facebook.

Please don't. The forums are already taking up too much of my time. Facebook would only make it worse.

This is graph of my engine to compared another engine.
My engine is the red curve...
My engine is very explosive out of the curves... i gain a lot of road ... but the engine near to 14500 rpm stop screaming...

Mr Jan and Mr Frits, is possible for you get off more slowly the final part?

decrease thikness squish? current value is 5.5mm
modify angle of the transfer? (not the diagrams) only vertical angle...
decrease section of exhaust port? (not the diagrams) only internal section with one insert or resin...

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SB07 a écrit:

Mr Jan and Mr Frits, is possible for you get off more slowly the final part?

You power curve already looks very good, Stefano. But your question is impossible to answer because I know nothing about your engine: no bore, no stroke, no conrod length, no timings, no port areas, no port directions, no volumes, no pipe dimensions, no carburation setting, no ignition timing: nothing.

SB07 a écrit:

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

I return an old graphics by Mr. Fos...
which is the principal characteristic that mark the difference of power curves?
Angles timing?
Volume crankcase?
Pipe study?
I need of increase rpm of my 50cc cylinder... but any change that i make, i increase only max power and increases the difficulty of driving...
i can give up a little power for increase rpm...
How can i do?
This is example of my situation.

Regards Stefano

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hi Stefano,

without detailed knowlwdge of your engine that is hard to tell, as already stated by Frits. But, depending on your dyno accessibility (can you get on the dyno for several days without paying a whole lot of money?) and familiarity with exhaust pipes (can you make them yourself? do they perform well?), it should be possible to make your engine softer on the beginning and on the end of your power curve, like the green line in your diagram.

You can achieve your desired shape of the power curve by converting your exhaust pipe into a kind of "semi kart pipe" by drilling holes into your end cone and connecting a volume to it. This weakens the effect of the end cone, and spreads its working range further. As you would have to add a couple of holes, then test with different volumes, then add another few holes and retest with varying volumes, eventually rejet and adjust ignition timing (yes, every time if you are unlucky), this is not a few hours work, could well take days.

If you look at the homologation files of some of the 100cc kart engines, you can see a section view of such a pipe. As you only want your engine a little softer, you would probably need less holes and a smaller volume (in relation to engine size).

Hi, thank you at all

The dyno bench is mine... i haven't problems of time.
I can use one analogic ignition and with fixed advance, this is imposed by regulation.
I can move the ignition point at my liking.
Yesterday, i tried to change thikness (with more hard)of the petals of reed valve...but the curve is not changed in that point...
I tried move the ignition point so as to delay... but has moved the curve a little in front... without modify the inclination.
i would like to try restrict the cylinder exhaust transfer... not the exhaust port...i think that in this system the extraction of gas will be slower...the problem is find a good putty for high temperatures ... if this test will be succesful, i modify my core box...

SB07 a écrit:

Hi, thank you at all

The dyno bench is mine... i haven't problems of time.
I can use one analogic ignition and with fixed advance, this is imposed by regulation.
I can move the ignition point at my liking.
Yesterday, i tried to change thikness (with more hard)of the petals of reed valve...but the curve is not changed in that point...
I tried move the ignition point so as to delay... but has moved the curve a little in front... without modify the inclination.
i would like to try restrict the cylinder exhaust transfer... not the exhaust port...i think that in this system the extraction of gas will be slower...the problem is find a good putty for high temperatures ... if this test will be succesful, i modify my core box...

I think you should play with shallow baffle angles, instead of going with 14º (as an example) I would use 12º for a start, also increasing the volume of the exhaust It seems that makes a bit the effect you are searching.

So the first thing I would try with the exhaust would be a slight divergent mid section (1,5º) and also a bit longer . then a 12º baffle? And maybe a more enclosed restrictor to build some egt after some seconds of WOT??

Lower compression might helping you revv better too.

Haufen a écrit:

{...} If you look at the homologation files of some kart engines, you can see a section view of such a pipe. {...}

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

From a Vortex RKF engine

and (different category)

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

from a Maxter MXO engine.

----

By the way, how would you calculate the cone length from these dimensions?
I average the side lengths and assume an equal length sided cone, from which I calculate the length@middle through trigonometry. Not sure if it's correct..
Furthermore, those diameters are always external, eh? How about the stated volume?

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This is the YZ250 EFI I talked about earlier. This is just a small clip of running it round the yard, early days of tuning still but it goes quite well. Injection at the transfer ports.

SB07 a écrit:

Hi, thank you at all

The dyno bench is mine... i haven't problems of time.
I can use one analogic ignition and with fixed advance, this is imposed by regulation.
I can move the ignition point at my liking.
Yesterday, i tried to change thikness (with more hard)of the petals of reed valve...but the curve is not changed in that point...
I tried move the ignition point so as to delay... but has moved the curve a little in front... without modify the inclination.
i would like to try restrict the cylinder exhaust transfer... not the exhaust port...i think that in this system the extraction of gas will be slower...the problem is find a good putty for high temperatures [Vous devez être inscrit et connecté pour voir cette image] ... if this test will be succesful, i modify my core box...

SBO7.... In addition to the suggestions for extending the meaty part of your power band you have already received.. Also consider the
effects of the combustion chambers shape on ignition timing requirements. It is well understood that a parallel squish such as Fritz and Jan used on the
Aprilla is more efficient for max power. But when dealing with ignition timing and gearing that is fixed. Efficiency viewed as an average over a wider range takes precedent. For a short introduction..... much the same effect as an ignition that retards with rising rpm's will be found when the squish angle cut in the head is greater than the pistons crown angle. Larger radiuses at the junction between the combustion dome and the squish angle have much less but somewhat similar effects. Double or even exponential squish angles in the head, will make the effect more pronounced, as well as reducing some of the trapped volume losses that are inevitable with a single angle squish. A slower burning fuel with more initial timing is also a variable to add to your experiments with squish angle shapes. 2005bully

Why do we speak of cylinders. Someone has already made new cylinders for the engine Suzuki vj22?
This engine is very widespread.
We have designed and built new crankshafts. This is the second version.
[Vous devez être inscrit et connecté pour voir cette image]
But for us the realization of the cylinders is more difficult

I might be able to help you there, based on The RSV design coming soon..........ie next couple of months.
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Frits, Jan, or anyone with experience designing power valve systems could you please chime in with your experiences as to what worked well and what didn't? I'm considering designing a simple blade type valve for my cylinder and don't have a lot of information on the subject. It is my understanding the valve should come as close to the piston as possible when down. Are there large negative effects when the valve is up and there is a large gap in the port roof? Is this an area where a compromise must be made? Below are a couple pics exaggerating what I'm talking about and also one of the new Arctic Cat C-Tec engine which has a valve which doesn't seem to adhere to any of the things I've said.

On a premixed engine what is the best way to keep the valve lubricated and/or from sticking? Could a greased brass bush or similar be used for the sliding mechanism? As always any info is greatly appreciated.

Regards,

Paul

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Paul,

Yes the power valve needs to be as close as possible to the piston. The sliding gate design is usuallly a two piece affair to maintain that as the slide rises. I'm a big fan of the Honda RC pivoting flapper design, as used on the NSR, as its simple and can maintain the close gap all thru its range by a dint of geometric maths.

Looking at your second piccy, that ding in the roof of the exhaust duct is going to produce a very strong reversion wave upsetting your initial wave in a bad way. Any exhaust valve must conform to the profile of the roof when it's in the fully open postion.

In sliding valve arrangement the engine oil will provide all the lube necessary for the valve to operate. Don't use brass, use steel for the entire valve and its mechanism.

Forget about the Arctic Cat, not a good example of how to apply an exhaust valve to a racing engine

dutch fisher a écrit:

Paul,

Yes the power valve needs to be as close as possible to the piston. The sliding gate design is usuallly a two piece affair to maintain that as the slide rises. I'm a big fan of the Honda RC pivoting flapper design, as used on the NSR, as its simple and can maintain the close gap all thru its range by a dint of geometric maths.

Looking at your second piccy, that ding in the roof of the exhaust duct is going to produce a very strong reversion wave upsetting your initial wave in a bad way. Any exhaust valve must conform to the profile of the roof when it's in the fully open postion.

In sliding valve arrangement the engine oil will provide all the lube necessary for the valve to operate. Don't use brass, use steel for the entire valve and its mechanism.

Forget about the Arctic Cat, not a good example of how to apply an exhaust valve to a racing engine

At Aprilia we tried the 'flapper system' as I thought it would be better.
But it proved to be worse....
I think because of the relatively big volume behind the valve.
Of course the valve has to be flush with the exhaust duct when open!
If I had to make one now, I would make it from 1,5mm stainless steel.
And make the 'opening' for it by spark erosion.
An exhaust valve needs no lubrication at all.
It is always full of oil when you take it out!

Jan, you mention that even a 1mm change in the exhaust pipe dimensions can be seen on dyno – what are your thoughts on the area where each cone join the other cone or section in the exhaust – some people beat these area into a smooth radius, thereby taking any shape transition out – is this good or bad?

Thank you in advance,
Regards
Allan.

GtG001 a écrit:

Jan, you mention that even a 1mm change in the exhaust pipe dimensions can be seen on dyno – what are your thoughts on the area where each cone join the other cone or section in the exhaust – some people beat these area into a smooth radius, thereby taking any shape transition out – is this good or bad?

Thank you in advance,
Regards
Allan.

Very good in my opinion!

husaberger a écrit:

I might be able to help you there, based on The RSV design coming soon..........ie next couple of months.
[Vous devez être inscrit et connecté pour voir cette image]

Thank you, keep me informed.

Hi Jan, how much was the clearance piston-barrel on Aprilia engines?

Filandro a écrit:

Hi Jan, how much was the clearance piston-barrel on Aprilia engines?

0,04mm
I tested with 0,05, the engine gave more power.
But nobody wanted to know!

Jan Thiel a écrit:

Filandro a écrit:

Hi Jan, how much was the clearance piston-barrel on Aprilia engines?

0,04mm
I tested with 0,05, the engine gave more power.
But nobody wanted to know!

Thanks Jan!
Aprilia pistons were cast or forged?
what is the function of the "niches" above wristpin?
I remember you said ring gap must be at least 0.4, am I right?
Last question, could you tell me the clearance on Garelli 50 GP?

Filandro a écrit:

Jan Thiel a écrit:

Filandro a écrit:

Hi Jan, how much was the clearance piston-barrel on Aprilia engines?

0,04mm
I tested with 0,05, the engine gave more power.
But nobody wanted to know!

Thanks Jan!
Aprilia pistons were cast or forged?
what is the function of the "niches" above wristpin?
I remember you said ring gap must be at least 0.4, am I right?
Last question, could you tell me the clearance on Garelli 50 GP?

There were cast and forged pistons.
The cast pistons were the best.
The right ring gap was 0,35
The 'niches' are there for lightening, I do not like them at all.
But with a forged piston it is the only way.
The Garelli 50 had a clearance of 0,45