[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 think to be courteous to the people answering questions that it would be best if any newcomers take the time to read this thread from start to finish. It is some of the best reading on the planet and is like an encyclopedia of information about real world 2 stroke performance. I think Jan and Frits are becoming somewhat tired of re-answering questions. That said I hope that Mr Thiel and Mr Overmars don't mind that I have printed this thread in its entirity and have a copy in my library, next to the porcelain throne for reference.


Sorry to disturb you, gentlemen!

As you probably don't know, there is an automatic process to split a topic after 50 pages...

The first part of this gorgeous topic is there : [Vous devez être inscrit et connecté pour voir ce lien]

         

Howard Gifford a écrit:

I think to be courteous to the people answering questions that it would be best if any newcomers take the time to read this thread from start to finish. It is some of the best reading on the planet and is like an encyclopedia of information about real world 2 stroke performance. I think Jan and Frits are becoming somewhat tired of re-answering questions.

Most considerate of you, Howard .
As you may have noticed I stopped answering questions four days ago and I am glad Jan took over .

Citation :

That said I hope that Mr Thiel and Mr Overmars don't mind that I have printed this thread in its entirity and have a copy in my library, next to the porcelain throne for reference.

Everybody needs some form of paper next to that throne and I feel honoured that you choose our life's work

Howard Gifford a écrit:

Im still waiting for Frits to explain why hydroforming is not a good as welded cones

and nobody could give me an answer on the crankcase volume

koenich a écrit:

and nobody could give me an answer on the crankcase volume

Did you read through the thread? Crankcase volume and the non-importance of primary compression were discussed at length, in part around the idea of the 24/7 reed valve. It was spelt out several times that it's the pipe that pulls gas through the trannsfers, not the crankcase compression which pushes.

On the other hand, I'm surprised at the anti-hydroforming suggestion : I've always viewed multi-stage diffusers as an approximation to an exponential horn, which gives an equal proportion of energy returned per unit time.

Has it been discussed somewhere in the thread previously, or is it a new claim?

GrahamB a écrit:

koenich a écrit:

and nobody could give me an answer on the crankcase volume

Did you read through the thread? Crankcase volume and the non-importance of primary compression were discussed at length, in part around the idea of the 24/7 reed valve. It was spelt out several times that it's the pipe that pulls gas through the trannsfers, not the crankcase compression which pushes.

On the other hand, I'm surprised at the anti-hydroforming suggestion : I've always viewed multi-stage diffusers as an approximation to an exponential horn, which gives an equal proportion of energy returned per unit time.

Has it been discussed somewhere in the thread previously, or is it a new claim?

i read it several times graham and love it. I've read pretty much all books that you can get, but this 51 pages feature more knowledge then all of those together. especially when it is going to practical stuff. i really appreciate every single person taking part in it!

Thats sth I learned from here that the pipe sucks, but somehow i think there is and from tested it seems like there is a limit. thats why I am asking...

hiya,
if anyone is interested in pipes, ive made an expansion chamber forum that a freind (coralex & stephane) helped to put on line,
its all new,never been used,

please please Jan, Frits, Graham, Howard, etc come & talk about your ideas,techniques & feelings about chambers,
its in english so as to be international,
[Vous devez être inscrit et connecté pour voir ce lien]

thanks very much, david

please come & put your arguments up on my forum,

thanks, david

You can also open a new topic here.... if rgdavid agrees, of course!



[Vous devez être inscrit et connecté pour voir ce lien]

roost a écrit:

Talking about cranks, I would like to ask a question mr. Thiel or mr. Overmars.
Is it possible that too big clearance between the crankshaft webs and crankcase could reduce performance? I lately built an engine that has about 2mm of space between the crankshaft webs and crankcase, and the engine compared to other similar engines has substantially lower performance. It's a small 70cc engine, the crankshaft webs diameter is 71mm and crankcase is 75.
Thanks.

Hi Marko, it is interesting. The same thing is with Malossi crankshaft for 180 Piaggio crankcase which has 2 mm less in webs diameter than original one(to fit also older 125-150 crankase) so that means 4mm space between webs and crankcase like in your case and the power is not lower, but the webs on Malossi are fuller and crankshaft is heavier.

Jan Thiel a écrit:

Making the aux exhaust ports wider still, going past the cylinder center by more than 1mm gave less power.

Hi Jan,
As you had capped the piston pin in the RSA to reduce the connection between the Aux exhaust ports and the Transfer “A” ports, did you try extending the end of the Aux exhaust port closest to the centre of the cylinder down to the top of the transfer “A”, and if so what was the result.

Thanks for your answer.

GtG001 a écrit:

Jan Thiel a écrit:

Making the aux exhaust ports wider still, going past the cylinder center by more than 1mm gave less power.

Hi Jan,
As you had capped the piston pin in the RSA to reduce the connection between the Aux exhaust ports and the Transfer “A” ports, did you try extending the end of the Aux exhaust port closest to the centre of the cylinder down to the top of the transfer “A”, and if so what was the result.

Thanks for your answer.

Doing so the engine gave less power, I think part of the charge made a 180° bend and went from the A-port into the aux. exhaust ports.
Just removing 0,5mm gave less power!

roost a écrit:

Talking about cranks, I would like to ask a question mr. Thiel or mr. Overmars.
Is it possible that too big clearance between the crankshaft webs and crankcase could reduce performance? I lately built an engine that has about 2mm of space between the crankshaft webs and crankcase, and the engine compared to other similar engines has substantially lower performance. It's a small 70cc engine, the crankshaft webs diameter is 71mm and crankcase is 75.
Thanks.

I do not know the answer to this question, because I never tried this!
Normally high crankcase cv. likes short timings and low crankcase cv. likes longer timings.
I think there is also a limit of how low you can go, but I do not know where this limit is!
The flow from the inlet into the transfer ducts is very important but little known I think.
It is impossible to simulate on a flow bench!
And difficult to understand, with the moving conrod, piston and crankshaft.
One thing I am sure about is: always give priority to flow over crankcase volume!

Sabijator a écrit:

roost a écrit:

Is it possible that too big clearance between the crankshaft webs and crankcase could reduce performance? I lately built an engine that has about 2mm of space between the crankshaft webs and crankcase, and the engine compared to other similar engines has substantially lower performance.

The same thing is with Malossi crankshaft for 180 Piaggio crankcase which has 2 mm less in webs diameter than original one .

A smaller crank web diameter means less material around the big end hole, a weaker press fit and a flabbier crankshaft. That may well nullify the benefits of a larger crankcase volume.
As a rule of thumb, the amount of material around the big end hole should be at least 50% of the big end pin diameter. So if you have a crankshaft with a 40 mm stroke and a 16 mm big end pin, the web diameter should be at least 72 mm.

Our engines have typically a 2 mm clearance (give or take for casting flaws) between the crank webs and the case. We tried closing the clearance between the crank and the case to .5mm by machining an aluminum plug that was exactly 1 mm bigger than the crankshaft and injecting epoxy to fill the void. It was a lot of work and did reduce the volume of the cases substantially but resulted in less power. We then went the otherway and machined a crankshaft down adding an extra 2 mm clearance on another case which gave now 4 mm clearance. It resulted in a small gain in power. My dilema is that we do not know if the reduction in power was a result of added viscous drag or as a result of tighter volume and was the increasein power as a result of the fact that inlet flow was less impeded by the crank web or was the added volume the cause. My recommendation for crankcase volume is bigger is better than smaller (in moderation) anything that adds extra airflow always adds power. A good airflow turbine is a great tool.

While on the subject of crankshafts I have noticed that the RSA uses a full circle crankshaft and there is no indication of what we call "turbo" machining on the webs. Basically it is machined grooves in the crank webs that act as a sort of fan blades creating forced flow out to the transfers. We have tested torbo crankshafts and they seem to give a small increase in power and an increase in airflow. Jan did you ever try a turbo crankshaft in the RSA and what were the results?

Can you show us a picture of a "turbo-crank", Howard?

Howard Gifford a écrit:

Our engines have typically a 2 mm clearance (give or take for casting flaws) between the crank webs and the case. We tried closing the clearance between the crank and the case to .5mm by machining an aluminum plug that was exactly 1 mm bigger than the crankshaft and injecting epoxy to fill the void. It was a lot of work and did reduce the volume of the cases substantially but resulted in less power. We then went the otherway and machined a crankshaft down adding an extra 2 mm clearance on another case which gave now 4 mm clearance. It resulted in a small gain in power. My dilema is that we do not know if the reduction in power was a result of added viscous drag or as a result of tighter volume and was the increasein power as a result of the fact that inlet flow was less impeded by the crank web or was the added volume the cause. My recommendation for crankcase volume is bigger is better than smaller (in moderation) anything that adds extra airflow always adds power. A good airflow turbine is a great tool.

from what I learned, you should not go lower than 1mm clearance to avoid viscous drag. so maybe 1,5 would have worked well. to much testing with too low budget...

and howard: pretty much all bike cranks i know are full circles nowadays. standard derbi, tuning derbi, mx bikes and so on. this has also a major influence on the crankcase volume and inertia...

just found this vid:



seems like someone was able to fit fuel injection to 500...pretty cool! and especially i have not heard of a fuel injected 2 stroke mx bike so far. might flat out the worse power bump of 500.

Jan Thiel a écrit:

The flow from the inlet into the transfer ducts is very important but little known I think.
It is impossible to simulate on a flow bench!
And difficult to understand, with the moving conrod, piston and crankshaft.
One thing I am sure about is: always give priority to flow over crankcase volume!

When I read this, a question arises to me. I once heard, that the surface of the last part of the inlet tract should not be bigger than the carburetor surface (or the effectice surface of the reed valve in a reed valve engine)
But if flow from the inlet into the transfers is really as important as you say, then maybe a big radius would make sense!? I have made a little sketch to show what I mean.
[Vous devez être inscrit et connecté pour voir cette image]

Regards

edit: oh sorry, I just saw that it has already been discussed on page 44, okay, so it should be worth a try

Frits, Jan, Howard, or anyone, could you say a few words about how to size the stinger diameter? My experience is that as you go smaller everything gets better especially top end power and then at some point the piston gets too hot or else you get into too much detonation. I always considered the stinger last, after designing all the rest of the pipe: is this a bad idea? Frits, I imagine this was difficult with the .40 R/C engine because changes to the stinger also change the fueling curve!

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.

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

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:

[Vous devez être inscrit et connecté pour voir ce lien]

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.