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

Happy birthday Jan !

carlovitch1 a écrit:

I have a question for Frits and Jan about the transfer ducts orientation.
If my understanding of the "tower of Pisa" transfer flow concept is correct, the A and B ducts are angled upwards at different angles (A somewhat 18-20°, B somewhat 8-10°) so the flow doesn't lose too much energy by a collision at right angle and is directed upwards.
What if we put a slightly different upward angle to the A ducts (left 16° and right 20° for example) and the same for the B ones, so the collision is even less energy consuming but the general flow direction is kept ?
Was this tested once (at least maybe because of cylinders having a foundry not that symmetrical)?

I wonder why your axial angles are somewhat different from the values quoted  in 'The leaning tower of Pisa'
(A=25° , B=10° , C=50°).
Disturbing the right-left-symmetry of the transfer flow will cause a lot of turbulence during the transfer phase and the spent exhaust gases will be mixed with the fresh charge instead of being expelled. This will pollute and heat the fresh mixture, causing power loss and the detonation risk.

Thanks for your reply, Frits, about the angles values I was just giving as examples, sorry for that.

OK, this is not a good idea, I was forgetting about the turbulence issue.

While I am at weird things, I'm working these days on a 1974 Suzuki 125 GT engine to rebuild it.
When I take the cylinders off, I see this:
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There's a kind of closing of the crankcase so only a small part of it can let the fresh gases (this is a piston-port inlet of course) go down to the down end.

Of course, there's separated oil injection, but I'm not sure at all this kind of separation is good for getting the maximum fresh gas volume in the engine as it substantially reduces the crankcase volume.
What do you think about it?

Il me semble qu’il est toujours intéressant de réduire le volume « mort »du carter sur un 2 tps,pour ne pas stocker des gaz frais. Ce cloisonnement vise peut-être à diriger les gaz sur le maneton pour favoriser le graissage,au détriment du transfert ?
Bien entendu je laisse la réponse à Frits,c’était pour participer...

En fait les gaz frais arrivent directement sur les cloisons, seule une partie passe dans le carter.
Après réflexion, je pense que ça sert à maintenir l'huile (qui bénéficie d'un système de graissage sous pression bien spécial) vers le maneton et les roulements et à diriger la majeure partie des gaz frais (qui viennent du cylindre, admission jupe du piston, sans clapets) vers les transferts.
L'inconvénient que j'y vois est que ça doit créer de sacrées turbulences.

En ce qui concerne le volume dit "mort", il me semble que Frits avait parlé de "fairy tale" à ce sujet. Il en faut un minimum sinon tu ne peux pas "remplir" ton haut moteur.

c'est pour diminuer le volume global quand le piston descend, la "compression" vers les transfers
est plus importante, Jacques Roca faisait ça sur les Derbi dans le temps jadis !

Oui, bien sûr, mais ma question est de savoir si c'est réellement efficace ou si les turbulences générées ne font pas perdre tout le bénéfice. Le 125 GT n'est pas réputé pour sa souplesse.

And while we are at it, maybe Frits can answer the following question: when you have only one pair of lateral transfer ports mated with a C transfer port, which axial angle would you aim for? The 10° of the B (maybe to try to keep the piston crown cooling) or the 25° of the A or something else?

carlovitch1 a écrit:

And while we are at it, maybe Frits can answer the following question: when you have only one pair of lateral transfer ports mated with a C transfer port, which axial angle would you aim for? The 10° of the B (maybe to try to keep the piston crown cooling) or the 25° of the A or something else?    

Carlo, while we are at it, if there is only one pair of lateral transfer ports, there cannot be a C-transfer.
Sure, there can be a transfer port opposite the exhaust port, but in this case it should be called a B-transfer.
The naming is as follows: the transfer ports nearest to the exhaust are called the A-transfers. If there is another set of transfers, then those will be called the B-transfers, and so on.
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Now for the serious part of your question. If you have only one set of lateral transfer ports, I would give them an axial angle of about 15°.
But the radial angles are important too, and I would try to give them the same radial angles that I advised for the A-transfers in the "Leaning Tower of Pisa" treatise. You should be able to download it here: [Vous devez être inscrit et connecté pour voir ce lien]

Thanks for your reply, Frits.
Sorry for my confusion with the transfer letters, I think you understood anyway, indeed there are 2 rear transfers angled upwards on the 125 GT cylinder but what is quite special is that the entry is located like if they were exiting laterally.

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Hi Frits, I do hope you can help me, and I guess a lot of other people as well. I am struggling to visualize the cylinder conditions that exist in a race engine at peak torque rpm and at the point of exhaust port opening with a single port at 70% width, curved top edge on a 54mm bore. Is the gas still in a post combustion, turbulent state with differing pressure and temperature areas or in a basically quiescent, expanded state influenced only by downward piston movement and increasing cylinder volume? Can it be judged at what point the accelerating gas out flow into the duct is at its maximum efficiency, set against increasing cylinder volume and decreasing cylinder pressure? I will be most grateful for any insight you can offer. Many thanks, Mel

melvyn trevor a écrit:

Hi Frits, I do hope you can help me, and I guess a lot of other people as well. I am struggling to visualize the cylinder conditions that exist in a race engine at peak torque rpm and at the point of exhaust port opening with a single port at 70% width, curved top edge on a 54mm bore. Is the gas still in a post combustion, turbulent state with differing pressure and temperature areas or in a basically quiescent, expanded state influenced only by downward piston movement and increasing cylinder volume? Can it be judged at what point the accelerating gas out flow into the duct is at its maximum efficiency, set against increasing cylinder volume and decreasing cylinder pressure? I will be most grateful for any insight you can offer. Many thanks, Mel

Hi Melvin, answering your question about the turbulent state of gases is easy: all gases in a running engine are turbulent, be they inhaled mixture, compressed mixture in the crankcase, transferred mixture in the transfer ducts and in the cylinder, compressed gases in the cylinder and in the combustion chamber, combustion products in the cylinder and blowdown flow in the exhaust duct and the pipe. Or, to to sum it up succinctly, the only time those gases are not turbulent, is when the engine is not running.

If a pressure difference occurs between various areas in the cylinder, moleculair movement will try to even things out as quickly as possible, in some places with the speed of sound, in other place a bit slower, but in any case with a velocity that is a multiple of the piston speed. So I think it is fair to say that the pressure is the same everywhere in the cylinder at all times during compression, combustion and blowdown.
Only during the scavenging phase will there be zones where the kinetic enery of colliding transfer streams is converted into local pressure with the result that the transferred mixture is flowing in the direction that we want.

I do not quite know how to answer your question "at what point the accelerating gas out flow into the duct is at its maximum efficiency". How would you define the efficiency of a gas flow?

Frits, Many thanks for the prompt reply you have explained a lot for me, re-reading the post I can now see my confusion, would velocity be more appropriate, factoring in the driving pressure and exhaust diffuser action?
Grateful thanks, Mel

melvyn trevor a écrit:

Frits, Many thanks for the prompt reply you have explained a lot for me, re-reading the post I can now see my confusion, would velocity be more appropriate, factoring in the driving pressure and exhaust diffuser action?
Grateful thanks, Mel

Yes Melvyn, asking about the maximum velocity of gas flow makes more sense to me than asking about the maximum efficiency of the flow. The picture below shows a velocity profle of blowdown outflow. Note the effect of a radius at the top egde of the port. The maximum velocity at this point can be Mach 1. This will not be influenced by exhaust diffuser action, as the pressure differential during the onset of the blowdown phase is supercritical; lowering the downstream pressure will not raise the flow velocity any higher.
s[url=https://servimg.com/view/19520277/404][Vous devez être inscrit et connecté pour voir cette image][/url

Wow that is a great image, if ever a picture spoke a thousand words, It clearly shows the benefit of raising the exhaust duct floor and of the port top edge radius. I see also that mach1 velocity cannot be improved on by any diffuser action, chocked flow is the result?
Again, very many thanks for your time and insight, it greatly helps people such as myself.
Regards Mel

Indeed this image is really interesting. About the radius on the top, I understand it modifies the flow main direction, so may I conclude it should be avoided or made as small as possible?

carlovitch1 a écrit:

Indeed this image is really interesting. About the radius on the top, I understand it modifies the flow main direction, so may I conclude it should be avoided or made as small as possible?

On the contrary, Carlo. The radius helps changing the flow direction. It is called the Coanda effect: [Vous devez être inscrit et connecté pour voir ce lien] .
Without the radius the flow would have to suddenly change direction from downward to leftward, and it cannot do that. There would be flow separation, flow contraction, turbulence, increased pressure loss and decreased flow coefficient directly after any sharp edge.

OK, thanks for clarification, Frits.

Hello Frits:
Are we still using a 5% of the stroke radius as a rule-0f-thumb for the top edge of the exhaust port? Thank you. Jeff

jfn2 a écrit:

Hello Frits:
Are we still using a 5% of the stroke radius as a rule-0f-thumb for the top edge of the exhaust port? Thank you. Jeff

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

Hi frits, in working on 100cc direct drive kart engine. His working range Is from 9000 to 20000rpm.
It has 3 ports exhaust. It has a very large exhaust duct volume (and area), measured at 28mm from piston skirt the area of duct Is about 1196mm2. For comparison newer kz engine TM kz10b at same point has 933mm2.
What do you think to reduce the duct volume by rising the floor? In planning to achieve 900mm2 area.The cylinder duct Is very short, about 28mm, does the volume reduction must continue in exhaust manifold?
Thanks a lot, and greets from italy[Vous devez être inscrit et connecté pour voir cette image]

Eric_91 a écrit:

Hi frits, in working on 100cc direct drive kart engine. His working range Is from 9000 to 20000rpm.
It has 3 ports exhaust. It has a very large exhaust duct volume (and area), measured at 28mm from piston skirt the area of duct Is about 1196mm2. For comparison newer kz engine TM kz10b at same point has 933mm2.
What do you think to reduce the duct volume by rising the floor? In planning to achieve 900mm2 area.The cylinder duct Is very short, about 28mm, does the volume reduction must continue in exhaust manifold?
Thanks a lot, and greets from italy.

Eric, direct-drive engines are a very special case. When I first encountered such an engine, I thought it was horrible and I was convinced I could do better. The first thing I did was make a new exhaust pipe for it. With this pipe, with a much narrower, tapered header, 'my' kart could overtake two or three competitors every time it went onto the main straight.
Small problem: it would be overtaken by two or three competitors at every corner exit. And all circuits have more corners than straights...

I learned that maximum power is completely useless if the engine's power range is too narrow for its intended purpose.
This was the reason for devising my 'power range' concept, which I presented here on Pit-Lane (April 2013 if I remember correctly), and which is now also included in the EngMod2T software of professor Neels van Niekerk (Vannik).

The character of a two-stroke engine is largely determined by its exhaust pipe. The more effective the pipe, the higher the maximum torque and power values will be.
But at 2/3 of the rpm of maximum torque, everything goes wrong. Then the charge pulse from the exhaust pipe returns to the cylinder when the transfer ports are not yet closed again, and the combustible mixture present in the cylinder is forced back into the crankcase so that barely enough of it remains in the cylinder for combustion.

The more effective the pipe is, the stronger that pulse is, and the deeper the dent in the torque curve will be.
For a direct-drive engine which has to go through that critical rev range every time, it is therefore much more important to weaken that dent as much as possible instead of producing a high maximum power after the dent.
Time lost while accelerating out of the corners can never be compensated for with a higher top speed at the straight.

Below is the power graph of a world champion direct-drive kart engine. You can see that it is willing to rev to the moon, which allows the rider to fit a short gearing that will help with accelerating out of the corners. The maximum power is nothing special, but the power range is.

Conclusion: I hesitate to give any advice regarding your exhaust. You may think, as did I, that the exhaust systems of these direct-drive engines are not very effective, but bear in mind that they are the result of many fine technicians trying everything to improve them over many years.
[Vous devez être inscrit et connecté pour voir cette image]

PS: if the rulebook allows it, you can gain a lot with a programmable ignition.....

Thanks frits for the reply. I Just saw other 2 cylinders  100cc direct drive like rotax and TM and at that point of duct both has less area (so less volume in duct) than cylinder that i talked about.
With Narrower exhaust duct im expect less gas expansion so higher exhaust temp so more Max Power but less bottom end, correct?
Thanks in advance