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

Does anyone have a better version of this picture

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jfn2 a écrit:

Does anyone have a better version of this picture

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Page 3 and 26 of part one of the topic i think contains the information you seek:good:

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I would like to take this oportunity to offer my thanks to both Jan and Frits for sharing some wonderful insight of the intricities of high performance 2 stroke design and function and for their patience with the less knowledgeable. which is almost everyone.........

Questions for Jan

A couple on cooling...

Jan did you ever try different temperatures of the fuel ie. above and below ambient to see what effect that may have on fuel atomistation, crankcase cooling and oil dropout?

The Aprilia APC and APE cylinders have an open deck design for coolant flow control, did that design have any drawbacks (or advantages)?
The later APF cylinder has a closed desk design, what was the thinking in that change?



One on in-cylinder gas flow....

Tumble and swirl are vital in 4 stroke power production, what attempts where made to advance this in the your 2 stroke designs in the open cycle after transfer closing?

jfn2 a écrit:

Does anyone have a better version of this picture

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P34

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Dear Frits
Having only recently stumbled across this website, I have spent too much of the last few days reading and re-reading the information provided. In line with many of the others may I add my thanks for the information Jan and yourself have provided; it is fascinating.

Earlier you explained the thinking behind the shape of the auxiliary port on the RSA/RSW and why they do not extend beyond +/- 90° of the front of the barrel. Can you provide an insight in to the timing of the opening of the auxiliary ports relative to the main port?

I ask as the Aprilia barrel in common with others (Yamaha TZ) has the auxiliary port opening after the main port. On the Kawasaki parallel 250 (KR-1S) the auxiliary opens first (on this engine the KIPS exhaust valve system controls only the auxiliary port which is either open or closed). As an interested uninformed observer there is, in my mind, some sense in what Kawasaki have done as the length of the path from the auxiliary to the front of the barrel (expansion chamber) is greater than the distance from the main port. On the Kawasaki the difference in path length is about 2cm.

Thanks
Mark Jordan

Hello Mj , after a few years ....

You'll find some stuff for you bikes , definitly , most of all .... Get rid of the 50.6 crap stroke
Main weekness is form the port layout that can't allow to port correctly the transfers around the Kips booster ( not enough metal )

Mark, I don't have all of my documentation at hand here, but I think you'll find your answer in FPE's posts. I'm sorry I can't give you the link; I never seem to find anything with the forum's search function but it must be somewhere around here: [Vous devez être inscrit et connecté pour voir ce lien] .
The Kawa KR-1S is a great little bike, grossly underestimated in my opinion.
The drawback of having auxiliary ports that are higher than the main exhaust, is that the bulk of the heat will flow through the aux. ducts, whose cross-section area / wall surface ratio is worse than the main duct's, so more heat is fed into the cylinder.
In theory Seb is right about the short stroke, but it has one advantage: it allows you to fit 56 mm Yamaha single ring pistons that are stronger, lighter and 1 mm lower than the Kawa's and give perfect port timings for both the transfers and the aux. exhausts. Then raise the main exhaust port to the same height as the aux's, shorten the inlet rubbers as much as possible or better still, fit larger carbs, get some decent pipes and you have a missile.

Frits thank you for the reply I will try and hunt down the relevant information in the FPE thread.

I have had a little bit of success with tuning the engines but have hit a barrier with the physical strength of the barrels. The mounting flanges are too thin and under full load the barrels flex and crack across the back. My attempts at strengthening them resulted in the problem moving from the back of the barrel to the front - the last set failed at the front

I have a few things I need to try. In the short term I am going to try a crank I had assembled with 3mm longer rods (109mm instead of 106mm) and use a 3mm plate to strengthen the barrel (this rod length will be closer to the 2.18 to 2.2 times the bore that Jan mentioned earlier in the thread). Long term my aim is to cast my own barrel so understanding what is good and what can be improved is important - this thread has been very informative but I need to translate to my 50.6 stroke or maybe I need to make a 54.4 mm crank and just copy your barrels

I also want to try the piston pin plug as the shorting problem on the KR is more significant than on the Aprilia - for about 25% of the stroke (~12mm) the pin hole bridges the gap between the auxiliary and the front transfer port. I had been puzzling how to make such a plug (16mm) and retain it reliably.

dutch fisher a écrit:

Questions for Jan

A couple on cooling...

Jan did you ever try different temperatures of the fuel ie. above and below ambient to see what effect that may have on fuel atomistation, crankcase cooling and oil dropout?

The Aprilia APC and APE cylinders have an open deck design for coolant flow control, did that design have any drawbacks (or advantages)?
The later APF cylinder has a closed desk design, what was the thinking in that change?



One on in-cylinder gas flow....

Tumble and swirl are vital in 4 stroke power production, what attempts where made to advance this in the your 2 stroke designs in the open cycle after transfer closing?

Our fuel was kept in the dyno room and so had always the same temperature.

There seemed to be no drawbacks or advantages in the open deck design.
I remember once, when working for Garelli I transformed a closed deck cylinder to open deck.
Of course it was tested before and afterwards.
There was no difference at all!

I suppose turbulence goes on after transfer closing.
And later there is the squish.
What more could you do?

Jan were you doing the Laser Doppler turbulence tests yourself or supervising the results ?

In regards to the piston pin plug.... I have had found pure teflon as a good material to use. It can be easily machined with a protrousion that helps to secure it by a press fit into the wrist pins ID. Plus if any part of the plug ever rubs the cylinder wall or becomes un lodged, damage to motor parts is highly unlikely. 2005bully

2005bully a écrit:

In regards to the piston pin plug.... I have had found pure teflon as a good material to use. It can be easily machined with a protrousion that helps to secure it by a press fit into the wrist pins ID. Plus if any part of the plug ever rubs the cylinder wall or becomes un lodged, damage to motor parts is highly unlikely. 2005bully

It will probably melt.
All non-metallic plugs we tried did!
The only reliable solution was to close them by welding.
This was done by Pankl in Austria.

Institute of TwoStrokes a écrit:

Jan were you doing the Laser Doppler turbulence tests yourself or supervising the results ?

We never did such tests.
And we never had sophisticated instruments.
'Try and see' was always used.
And no simulations!

Was the flow bench much help in finding ways of reducing turbulence out of the ducts?

Institute of TwoStrokes a écrit:

Was the flow bench much help in finding ways of reducing turbulence out of the ducts?

No, but it was very useful for improving the exhaust duct.
We flowed the transfers, but I think it was fairly useless!

Thanks for that Jan

Hi again. Usually, some 54mm pistons used in reed valve engines have in the back side one rectangular port.

Some say it's to cool the top end bearing and piston, others say it helps filling the area underside the piston...

But, Honda RS and Aprilia does not use that.... Why? Is there some inconvinience?

I leave a photo

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Maybe because they use Rotary valve :)

This piston window has two meanings in engines with a reed valve inlet entering to the cylinder (not to the crankcase):
1. the inlet can be open even at bdc
2. the C-port (if existing) gets its filling from there

I've got another question for everyone:

What is the maximum diffusor cone angle one should use? I've heard if the angle gets too big, the exhaust flow could detach from the exhaust wall, what will mess up the whole flow.

I once heard everything above 10° was critical, but I already used an exhaust with 10,5° at its steepest "sucking part" and it worked fine, too.

Sidenote: The steepest part of the Aprilia exhaust has just 8,6°, but I suspect that for reed valve engines steeper angles might work better.

Regards

If you want the flow to keep attached to the diffuser wall, a total angle of about 8° is the maximum.
But as you can calculate from the Aprilia drawing below, the total angle of the steepest diffuser is 17,2°.
So the flow will detach.
This large angle is not designed for optimum flow, but for optimum pulse reflection.
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If you play a bit with the simple exhaust concept below, you will discover the coherence between engine rpm, exhaust timing, speed of sound and cone angles.
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I have a Gas Gas EC125 it has crankcase intake via reed valves and the piston is just like that a big milled window in the back of the piston, I though that it was to improve cooling/lurbication of the top-end bearing and the piston crown has there will be(in my head at least) flow through the C port and that milled window in the piston.

But, I may as well be wrong.

Okay, it was clear for me, that f.e. higher revs require steeper angles in general, but in case of your exhaust concept the angle won't be "really" big anyway, because it's a 2-stage diffusor.
Exhaust pipes with a 3-stage diffusor most time have a relatively big angle at the third stage and I am wondering, if in this case there is a specific critical angle so that, if exceeded, the flow will get REALLY bad, what could result in a clearly noticeable powerloss.

Daniel A. a écrit:

This piston window has two meanings in engines with a reed valve inlet entering to the cylinder (not to the crankcase):
1. the inlet can be open even at bdc
2. the C-port (if existing) gets its filling from there

Regards

I mean reed valve to the crankCase of course or Rotary Valve

Daniel A. a écrit:

2. the C-port (if existing) gets its filling from there

Booth Aprilia and Honda Gp Machines do not use this...So is not a need for filling, I think.

Some 125 MX bikes use this, others not.

[quote="Frits Overmars"]

Jeram a écrit:

...Another concern would be the width of the exhaust port (74%) which is at the theoretical maxiumum. As the bore size increases will the negative effects (ring life) of such a large exhaust port become more apparent or will it remain the same when scaled up?

The optimum exhaust port width is 70%. If you make it wider, you will need ever larger radii in the corners to keep the pistong ring happy. Those radii take away port area where it is most needed, at the very beginning of port opening (the yellow areas in the picture below) which will reduce blowdown angle.area.
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Thank you Jan and Frits for the interesting topic.
In regards to the opening of the exhaust port top edge, if I understand your explanation; the exhaust port top edge would open completely at once if you did not have to worry about the ring being eased in and out of it – no generous radiuses in the corner – is this correct or is the gradual opening of the exhaust main port intentional to widen the power band at the expense of maximum power
Along this same line, did you have plans to add power valves to the RSA auxiliary exhaust ports so you could increase their port timing to the same as the main port?
Best Regards
Allan.

Jan Thiel a écrit:

2005bully a écrit:

In regards to the piston pin plug.... I have had found pure teflon as a good material to use. It can be easily machined with a protrousion that helps to secure it by a press fit into the wrist pins ID. Plus if any part of the plug ever rubs the cylinder wall or becomes un lodged, damage to motor parts is highly unlikely. 2005bully

It will probably melt.
All non-metallic plugs we tried did!
The only reliable solution was to close them by welding.
This was done by Pankl in Austria.

I guess PEEK was among the tested materials?
If not, that would be very high on my list of possible candidates with a Tg of ~145 degrees C, and a melting point above 340 degrees C.