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

Thank you for your answer!

I really like your theory of 180 degrees exhaust resonance and I would like to add a thought to it.

The returning pulse from the end cone also gets a boost from the rarefaction wave reflected back into the system from the cylinder walls/piston top.
Since it has to propagate through the divergent section again (after reflected from the cylinder) it gets inverted and reflected back in the direction of the cylinder but now as a compression wave. And this will enhance the effect of the oncoming compression wave reflected from the end cone to stuff any fresh charge back into the cylinder. .

Laci a écrit:

I really like your theory of 180 degrees exhaust resonance and I would like to add a thought to it.
The returning pulse from the end cone also gets a boost from the rarefaction wave reflected back into the system from the cylinder walls/piston top. Since it has to propagate through the divergent section again (after reflected from the cylinder) it gets inverted and reflected back in the direction of the cylinder but now as a compression wave. And this will enhance the effect of the oncoming compression wave reflected from the end cone to stuff any fresh charge back into the cylinder. .

That is correct Laszlo. When we talk about waves, we are dealing with local density changes in the gas that move with the local speed of sound, which can vary from 200 m/s for a rarefaction wave to 600 m/s for an overpressure wave.
But apart from the wave there is also the oscillating mass flow of the gas, the Helmholtz resonance. This is often neglected but just as important as the wave action, and its efficiency largely depends on a correct choice of the header diameters and the volume of an expansion pipe.
On a molecular level however, wave action and Helmholtz resonance are part of one and the same phenomenon, as decribed by Gordon Blair and incorporated in the EngMod simulation software.

Hallo,

as the cylinder head for the Maico is for CNC-machining, there's time for other work. I prepared the lower end of the transfer ports in the case and the cylinder, as far as it is possible in such an old engine, to a modern design.
Now i have a suitable exhaust system and think about to invest a little time to make a socket by my own.
The inner diameter of the first cone of the exhaust is 49,7mm, but the diameter inside the cylinder and the original socket is only 44,5mm
The exhaust ports will be extended as Frits showed me a few pages back in this thread.
Now i have two thoughts without knowing the right one.
The first is to increase the flange diameter in the cylinder and the socket to 49,7mm and to adapt the inner shape of the cylinder right up to the exhaust window.
To get a good blowdown without a big resistance.
The second thought ist to keep the 44,5mm in the cylinder and the socket to get an edge in the exhaust, related to the egde i know of my Aprilia RSW style PVP cylinders.

What do you think?

Best regards,

Bernd

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Little Off-Topic: the footrests + adapter in 7020 tube i machined for the Maico. The chassis is completely done, only the engine is not ready yet.
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Döllinger a écrit:

... i have a suitable exhaust system... The inner diameter of the first cone of the exhaust is 49,7mm, but the diameter inside the cylinder and the original socket is only 44,5mm... Now i have two thoughts without knowing the right one.
The first is to increase the flange diameter in the cylinder and the socket to 49,7mm and to adapt the inner shape of the cylinder right up to the exhaust window to get a good blowdown without a big resistance.
The second thought is to keep the 44,5mm in the cylinder and the socket to get an edge in the exhaust

Bernd, I assume you call your exhaust system suitable because it fits the frame, but I don't think it will suit the engine.
The flange diameter should be about 42 mm Ø. If you are going to use a conical exhaust header, its initial diameter should also be 42 mm Ø. And if you are going to use a parallel header, its diameters should be about 47 mm Ø.
Whatever you do, do not increase the duct diameter and the flange diameter.
I think the best thing to do is forget about that 'suitable' pipe altogether and make a new one.

Hi Frits,
you think even the original flange is already too big? It will be difficult to change the diameter in the cylinder
Is it better to have a hard step to a smaller diameter on the level cylinder-header or is it better to make a conical section from 44,5mm to 42mm and behind a conical from 42 to 49,7mm?
I´m astonished that you think the exhaust is that bad. You mean it´s too long? I´m always open for critique!
On my TZ is was difficult too, to make an exhasut which character fit´s the engine.
Thanks for your honest reply.
Best regards,

Bernd

Döllinger a écrit:

I´m astonished that you think the exhaust is that bad. You mean it´s too long?

That exhaust may be excellent, but its initial diameter of 49,7 mm is too big for your engine and trying to adapt it will deliver a makeshift result.
I don't know its length so obviously I cannot say it's too long. But anyway I think it is advisable to start with a clean sheet.
Maybe you can enter an insert in order to reduce the exhaust duct diameter in the cylinder. And try to avoid any steps.

Hi Frits,

obviously you don´t have the length. But on many exhaust you see with one glimpse that i won´t work with a specific engine. My thought was in that direction.
It´s possible to get an insert in the cylinder, but not very deep. 2-3mm, and i have to adapt it to the shape of the exhaust duct.
Your intention is to increase the exhaust gas velocity with a smaller diameter? The reason for your diameter would be very interesting for me.
You mean a header like this? That would cause no problems in the construction.
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Best regards,

Bernd

Döllinger a écrit:

It´s possible to get an insert in the cylinder, but not very deep. 2-3mm, and i have to adapt it to the shape of the exhaust duct. Your intention is to increase the exhaust gas velocity with a smaller diameter?

The purpose of the insert is not to increase the gas velocity but to reduce the duct diameter and the header diameters to values that will give the desired Helmholtz frequency.  As an additional benefit the insert will reduce the duct volume.
The insert should go into the duct as far as possible which of course means it should be adapted to the shape of the original duct.
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Maybe you can bore out the original duct and press an insert in, as shown in the pictures below. On the left is an original KTM 50SX cylinder in which the exhaust duct diameter is too big. The cylinder on the right has been bored out and provided with a pressed-in insert.
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Dear Frits,

I am aware you already mentioned a couple of times that the cylinder-exhaust pipe-crank case-transfers-etc make up a coupled Helmholtz system. But for the Helmholtz theory you need to consider the whole system as an acoustic device. But in the case of exhaust systems the waves are a good way beyond acoustic.

Plus the exhaust system is considered to behave as a closed pipe. But closed pipe's acoustic mass is only 1/3 of an open pipe. But from the view point of the Helmholtz theory (and in reality) the exhaust pipe is open to the atmosphere.

How do you incorporate these (someway contradicting) things into your Helmholtz theory? If it treated as a closed pipe is it just a division by three? Or how do you merge the two theories together in the case of exh.systems (wave reflecting device+Helmholtz)?

Laci a écrit:

Dear Frits, I am aware you already mentioned a couple of times that the cylinder-exhaust pipe-crank case-transfers-etc make up a coupled Helmholtz system. But for the Helmholtz theory you need to consider the whole system as an acoustic device. But in the case of exhaust systems the waves are a good way beyond acoustic.
Plus the exhaust system is considered to behave as a closed pipe. But closed pipe's acoustic mass is only 1/3 of an open pipe. But from the view point of the Helmholtz theory (and in reality) the exhaust pipe is open to the atmosphere.
How do you incorporate these (someway contradicting) things into your Helmholtz theory? If it treated as a closed pipe is it just a division by three? Or how do you merge the two theories together in the case of exh.systems (wave reflecting device+Helmholtz)?

Laszlo, it is true that the waves in an exhaust system are far beyond acoustic. But the Helmholtz approach is non-acoustic. And a two-stroke exhaust system is not just a closed or an open system; it is very much a leaking system.
Here is something I wrote on the subject a couple of years ago:
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Both the wave theory and the Helmholtz theory can be useful when we try to understand what goes on in an exhaust system. The wave theory helps us to establish system lengths; the Helmholtz theory helps us to determine the desirable diameters and volumes in addition to these lengths. But as I said, on a molecular level there are no two theories but just one gasdynamics phenomenon.

Explaining the merger of wave theory and Helmholtz theory is clearly beyond the scope of this forum.
I can recommend "Design and Simulation of Two-Stroke Engines" by Gordon P.Blair, ISBN 1-56091-685-0.
If the book is out of print, you can try to download it here: [Vous devez être inscrit et connecté pour voir ce lien]

Hi Frits,

thank you for your profund answer. I will get to work.

Best regards,

Bernd

Thank you!

I have his books and many of his papers. I will read them again.

Derby readvalve engine 72cc bore 47.6 stroke 40mm
Result 32.75 hp this means 97% of Aprilia level
150m dragrace 6.62sec with total weight 130kg

If there exist a breeding ground for horsepower in motorsport, than it is the NSSC dragrace competition. Several experienced tuners prepare there engines every year to the limit close for the new season at 150 meter distance competition. The accompanying example shows that the Aprilia RSA level was almost achieved in 2017. Moreover, it was a reedvalve engine of which up to now it was assumed that approaching the power up to 3% would be impossible. Maybe we should adjust our opinion on this now! Or is it the advance of 4x cranckshaft bearing, which were not on the Aprilia RSA.

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LucF a écrit:

More information and pictures: [Vous devez être inscrit et connecté pour voir ce lien]

Thank you very much LucF.
There is just one small problem: I cannot see that Facebook-page because LucF has blocked me. I am not sure why,
because I never wrote anything about him on Facebook. Maybe he does not want me to see what he writes about me...

I asked Jan Thiel to take a look at that Facebook-page, so he might tell me what it was all about.
Jan's answer: "I cannot see it either; LucF has blocked me too."

I seem to remember a Ryger-argument in which LucF publicly accused Jan Thiel of not knowing how a two-stroke works.
Thank you again LucF, and welcome on this thread, called:
"All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars".

(We do not block anybody)

I prefer not to comment on this.
My results through many years speak for themselves.
But I must admit I don't understand how the Ryger engine works.
And I am not sure anybody else does.......
No results at all so far!!!!

Thanks Jan, That is an honest reply, which I can understand. But I´m not allowed to say anthing about this subject at this moment.

When Frits will also become honest about "his FOS" than everything will change.
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Hallo,
for my part i can say that i highly appreciate the expertise of Jan and Frits. And that they even share their knowledge. Thumbs up!
The Maico cylinder is now welded and bored out, ready to get Frit´s porting times.
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I made a exhaust header as close to the advise (diameters ect.) of Frits as possible on this old engine.
The new cylinder heads are half ready, i will post a photo of them as they are ready.
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Best regards,

Bernd

LucF a écrit:

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you have to be registered, I have done and the administrator does not validate at this time.

What is it used for if you can not read ?

Toop a écrit:

LucF a écrit:

More information and pictures: [Vous devez être inscrit et connecté pour voir ce lien]

you have to be registered, I have done and the administrator does not validate at this time.

What is it used for if you can not read ?

You have to wait. It works. You'll receive a mail. I'm registered for more one year.

Toop a écrit:

LucF a écrit:

More information and pictures: [Vous devez être inscrit et connecté pour voir ce lien]

you have to be registered, I have done and the administrator does not validate at this time.

What is it used for if you can not read ?

See pictures Toop !

Onboard video of Ryger in action

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LucF a écrit:

Onboard video of Ryger in action

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this video says nothing. it's not even proof that it actualy IS a Ryger engine staying alive for a couple of laps

JanBros a écrit:

it's not even proof that it actualy IS a Ryger engine

This tells me enough ! Thanks Jan !!

LucF a écrit:

JanBros a écrit:

it's not even proof that it actualy IS a Ryger engine

This tells me enough ! Thanks !!

oh no, are you gonna ban me now ? luckily I'm not even on f$ckbook so 1-0 for me