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

Muciek a écrit:

I've read whole thread and all this stuff almost blow my mind , so much valuable information about 2T engines and so many good people who want to share their knowledge. I'm curious about one thing I have seen your new FOS pipe formula [Vous devez être inscrit et connecté pour voir ce lien] and what about the old one that you posted in magazine long time ago this is screen shot of 1st page [Vous devez être inscrit et connecté pour voir ce lien] . I want to ask is there a English version somewhere of it? Dutch is a lil bit hard to translate especially if you are using google , and do you recommend using this new one formula or this older one for moderate tuned engine (not racing or track one i'm lil bit concerned because using this new formula I'm getting strange dimensions, and could somebody tell me what is that tailrestrictor?)?? Thank you anyway. Greetings from Poland.

Witaj Muciek,
The first link you posted, [Vous devez être inscrit et connecté pour voir ce lien] , shows a program that calculates exhaust pipes based on my 'simple exhaust concept'. I developed that concept all right, but I did not write any of these programs that can be found in the internet, so I am not going to answer any questions about them.
The screenshot [Vous devez être inscrit et connecté pour voir ce lien] that you posted, is from a series of technical articles that I wrote in 1978 / 1979. As far as I know, there is no English version, but I would not be surprised if someone had translated it and put it on the internet.
A tail restrictor is the narrowest part of the tail pipe.
Pozdrowienia z Holandii.

jfn2 a écrit:

Hello Mr Overmars: In your post about 'Transfer Theory' could you elaborate more on the explanation of abbreviations. I understand your explanation of the radial scavengingg direction resultant, but do not understand what the axial scavenging direction resultant is.
In your example MB cylinder, since I don't know this engine I can only guess, to improve it would you lower the axial angle of the A port and adjust the leading direction angle of the A port for a little more angle. On the C port maybe lower the axial angle a little and adjust the flanks straighter to decrease the pressure over a wider area of the gas column. Am I on the right track? Also a little more explanation about the height H for the ports.
One other question is about the part of the port right before the port opens into the bore. On a 4-stroke this is called the port throat area. As I understand it, the gas needs some length to get itself started into the angle otherwise the gas would continue to loop from the port outside radius. Now I know the gas wouldn't loop but if you had a 25 deg axial angle how far back into the port should this angle go? On all the drawings I've seen the port radius is shown to go pretty close to the port opening.
Thank you very much for all the information your sharing with us.
Jeff

Hi Jeff,
In part 4 of my Transfer Theory I tried to explain the axial scavenging direction resultant as best I could.
Trying to explain it once more would only lead to repeating myself. And the same goes for the height H of the ports.
I can only go so far in detailed explanations here; after all this is an open forum, not a university lecture.

To improve the scavenging of the MB cylinder that I used as an example in the Transfer Theory, the axial angles of the B-ports were reduced from 25° to about 10°. In the original scavenging set-up I had concentrated on cooling the glow plug that had given some reliability problems in the past. I succeeded all right; the plug could hardly be kept on temperature! So we reduced the axial angles of the B-ports in order to lower the scavenging velocity while maintaining the transfer angle.area.

The inner radii of the transfer ducts are paramount. You should fit the largest inner radii that you can, and that means going all the way to the port openings.

For all piston motors= the longer the conrod, the lesser the friction on the cylinders( it's easy to understand) some manufacturers move the pin axle to the exhaust to compensate this, but you obtain these effect of friction during the compression stroke then.[Vous devez être inscrit et connecté pour voir cette image]

Hello Frits, Jan, Everybody!

Well, I'm back again, as I have new question, this time about the crankcase compression ratio. First of all please leave out of consideration the Helmholcz resonation and others.

So the question is: According to the old doctrine in case of the road race engine the best ratio is between 1.4 an 1.5, while in case of a low rmp the best ratio is 1.2-1.3. But in case of the modern engines (for example RSA cc 1.24) the abovementioned ratio is between 1.2 an 1.3. Unfortunatelly the technical texts do not answer the question that what is the best choice in case of piston port, reed valve or rotary valve engines? I know that it depends on several things, as the angle of the transfer ports and other things, but does it make any sense to raise a general rule?

Well, thanks for the answers and I wish you a Very Happy New Year!

Balazs

Forgi a écrit:

...I have new question, this time about the crankcase compression ratio.
First of all please leave out of consideration the Helmholcz resonation and others.

You are like the guy who wrote: "please tell me how much is 1 + 1 ? PS: the answer '2' does not suit me."
Nevertheless, a Very Happy New Year to you too, Forgi.

Hello Mr Overmars:
Thank you very much for your reply. After rereading (about 10-15 times) your transfer theory post I believe I've got it. My only stumbling block is I can't figure out where you come up with the number 40.510 deg. I don't know what the stroke is of your MB engine. And is the H measurement taken from TDC or BDC? I'm still working on it. Again thank you very much.

Thank you Jan, Frits and all the other two stroke gurus on here for their contributions to this threrad. We have been steadily making gains with our race motor using what we have learned and we now have an engine package that we feel will be a winner. Of course there is the other 99% of the equation that is the driver and the chassis combination but at least with the new engine we should be the first to turn one. After that it is up to the driver...
Happy new Year and keep it on the pipe!!!
Howard..

Thought some of you would like this....

'Twas the night before christmas ,
and all through the shop .
Not a machine was running,
the phone even stopped .

My tools were scattered ,
all about on the floor .
Between the cardboard boxes ,
I had a path to the door .

Down the hallway I walked ,
and with a flick of key .
I locked the place down ,
for tonight I was free .

As I walked to my home ,
a sound I did here .
It was up in the sky ,
and it was not a reindeer .

It was a worn out old two-stroke ,
jetted lean on the main .
The guy who was running it ,
must have been half insane .

The engine was pinging ,
all the RPM's he did squeeze .
" Easy there dude ,
That things going to seize !!"

Just as I thought this ,
the motor did blow .
Downward circles he spun ,
til he crashed in the snow .

I ran out to greet him ,
it was a heck of a crash .
He got up rather easy ,
although the front bumper was trash .

Politely I said ,
"can I help you I'm Rick "
He replied in in an instant ,
"Yes please , I'm St. Nick "

"The reindeer are home tonight ,
the folks at PETA said no .
They said it's far to cruel ,
to put a deer in the snow. "

"So I picked up a '95 scrambler,
for five hundred bucks .
The owner was late on his payments
on his new pick-up truck . "

"So quad I did hook,
to the front of my sleigh.
POS Polaris ,
it didn't make it half way !"

Upset at it he was,
he was still a jolly old elf .
His round little belly ,
kind of reminded me of myself.

To what was left of his bumper,
I hooked on a chain.
I towed him back to my shop ,
Then the radiator I drained .

Off with the head,
and off with the jug.
A thick coat of aluminum ,
was melted on to his plug .

Quickly did I bore,
and then hone the jug .
I installed a 280 mainjet ,
and the 17 millimeter plug .

A brand new wiseco piston ,
and new gaskets too .
Back together went the motor ,
as he watched and drank my brew .

I pulled the rope starter ,
cause the button didn't work .
The motor fired up and idled ,
at a nice little perk .

We rolled it out side ,
hooked back onto his sleigh .
In the blink of an eye,
he was back on his way .

And I yelled to him ,
as he rode out of sight .
'BREAK IT IN EASY !!
IT'S BORED KIND OF TIGHT !!!!

Great little poem, Howard. I love it!

jfn2 a écrit:

After rereading (about 10-15 times) your transfer theory post I believe I've got it. My only stumbling block is I can't figure out where you come up with the number 40.510 deg. I don't know what the stroke is of your MB engine. And is the H measurement taken from TDC or BDC? I'm still working on it.

Jeff, I tried to make that whole transfer theory dimensionless, so the stroke shouldn't matter (but if you wish to know: the MB engine in the picture had a 21 mm bore and a 19 mm stroke). The H measurement was supposed to have been taken from BDC.
The 40.510° angle is the resultant of all fore-aft and axial vectors, plus, and this may make it hard to comprehend, the effect of the axial flow emanating from the thrust at the base of the central scavenging column, where all left-right and fore-aft vectors collide. This 40.510° is not something I would expect anybody to figure out.
Like I wrote the other day, this is not the place to dive that deep into gas dynamics.

Thanks to all who have made this such an interesting tour.
The Transfer Theory article was the best explanation of the subject that I have read.

Frits, could you explain the function of the drastic "hook" seen in the back wall of the B transfers on some of the dirt bike cylinders? They seem to be aimed directly at the exhaust port.

Also, what determines the inclination of the exhaust port duct? Does the blowdown pulse have a downward vector? Or is it important to direct the returning charge from the pipe against the top of the "Leaning Tower"?

Hello Mr Overmars:
Again thank you for your reply. The reason I thought about the stroke of your MB engine was I thought it would help me figure out the 40.510°. Thank you for the explanation.

Hi Frits and Jan,
I have been watching over this thread since the beginning but have not had any specific questions until now which may require your specific input.

The item I wish to discuss is the further applications of the Aprilia cylinder technology, and more specifically replicating the Aprilia 125 cylinder for larger displacement engines.

I am currently redrawing the Aprilia 125 cylinder designs into CAD so that I can then scale them up to 80x80mm bore and stroke.
I intend to use such a cylinder in a KTM 380 bottom end which is housed in an aluminium road racing chassis.
This bike would certainly be an odity with no 'specific' class to race in, but it is more of an engineering exhibition than anything else.

Currently, I am simply scaling up the cylinder from 125cc to 400cc, But I thought it would be best to approach both of you to see if you have any tips you could give me regarding anything I should watch out for when performing such an alteration of scale/size, and is there anything in general that should be optimised now that the engine size is larger.

This will be my first attempt at designing a cylinder casting, but I have the assistance of an experienced cylinder maker to reveiw my designs and proposed charges prior to having him cast the first cylinder.

I do hope that an essentially scaled-up RSA-400 cylinder will be a formidable beast!

Larry Wiechman a écrit:

Thanks to all who have made this such an interesting tour. The Transfer Theory article was the best explanation of the subject that I have read.
Frits, could you explain the function of the drastic "hook" seen in the back wall of the B transfers on some of the dirt bike cylinders? They seem to be aimed directly at the exhaust port.

Thank you for your kind words, Larry.
In my opinion the 'hook' came into being when existing cylinders were modified in order to enlarge the cross-sectional width of the B-windows, approaching their chord width as much as possible. Completely radially-aimed B-ducts would have been nice, but cylinder studs were in the way, original castings should still be usable, and so on.

Citation :

Also, what determines the inclination of the exhaust port duct? Does the blowdown pulse have a downward vector? Or is it important to direct the returning charge from the pipe against the top of the "Leaning Tower"?

Yes, the blowdown pulse does have a downward angle, of about 25°. Aiming the returning charge at the top of the 'leaning tower' is also a consideration, but since by that time the piston is heavily in the way of the flow, it only helps a little. One advantage of my FOS cylinder layout is that the exhaust duct floors lie above the transfer ports, so both the exiting and the returning flow are guided considerably better.

Jeram a écrit:

I am currently redrawing the Aprilia 125 cylinder designs into CAD so that I can then scale them up to 80x80mm bore and stroke. Currently, I am simply scaling up the cylinder from 125cc to 400cc, But I thought it would be best to approach both of you to see if you have any tips you could give me regarding anything I should watch out for when performing such an alteration of scale/size, and is there anything in general that should be optimised now that the engine size is larger.
I do hope that an essentially scaled-up RSA-400 cylinder will be a formidable beast!

It will certainly be a beast, in more ways than one. In my younger years (about 100 years ago) I raced a 370 cc two-stroke single. Racing was allright, but cruising was nearly impossible. To keep the engine from jerking, I had to keep it pulling all the time. But then it would go much too fast for public road conditions.
You made the right choice in selecting 80x80 mm bore and stroke dimensions; that should compare almost 1 on 1 with the 54x54.5 mm sizes of the RSA125.
The second important factor in scaling up is the mean piston speed. Considering the stroke ratios of your engine and the RSA, maximum power should be at 54.5/80 * 13000 = 8856 rpm and maximum revs at 9878 rpm.
Keep us imformed .

That is an interesting calculation Frits.
Divide the RSA stroke into the scaled up stroke and multiply by 13000. It seems towork very well for our engine with a 65.8 mm stroke. 54.5/65.8X13000+10767. Our engine makes peak power at 103000. We are using basically the same port timings as the RSA so should we be targeting the maximum to 10760?

Howard Gifford a écrit:

Divide the RSA stroke into the scaled up stroke and multiply by 13000. It seems to work very well for our engine with a 65.8 mm stroke. 54.5/65.8X13000=10767. Our engine makes peak power at 103000. We are using basically the same port timings as the RSA so should we be targeting the maximum to 10760?

Multiplying the RSA's 13000 rpm of maximum power with the stroke ratios only works if the bore/stroke ratios of the respective engines and their cylinder layouts are identical. With all due respect I do not expect your engine to have the same specific blowdown and specific scavenging angle.areas as the RSA, so I would not aim at those 10760 rpm.

Thanks Frits. We also have a CVT that looses efficiency above 10,000 rpm so even if we had identical gasflow and made more at 10760 we would probably not gain any speed by upping the rpm. We have tried to duplicate as much as possible the shapes and angles of the RSA but our work is still done by hand so the porting is always a product of skilled hands and interpertation with the grinder. My brother Mike is quite adept at porting but he is only human! We also use those damn VM34 mm carbs that restricts the airflow through the engine so yes that was a dumb question on my part. I think we have done quite well with what we have learned and hopefully it will result in a championship win this winter. Iwill keep you posted.
Thanks again,
Howard...

Frits Overmars a écrit:

Forgi a écrit:

...I have new question, this time about the crankcase compression ratio.
First of all please leave out of consideration the Helmholcz resonation and others.

You are like the guy who wrote: "please tell me how much is 1 + 1 ? PS: the answer '2' does not suit me."
Nevertheless, a Very Happy New Year to you too, Forgi.

Dear Frits,

As I can see this post of mine is not gonna be your favourite!

Then in other words please consider that the ratio mentioned before is 1.2 in the first case, and 1.5 in the other, and in both cases all the other parameters are right so the resonance Helmholtz is ideal.
I suggest to take the resonance Helmholtz as a constant, because it is hard to calculate due to the numerous variables, and because of the fact that this system is changing all the time due to the motion of the piston. And we haven't mentioned yet that in case of membran we don't know exactly when the membrane is opening and closing, in one case is asymmetric and in an other is symmetric (piston port), and so on.

By the way is there any advance concerning the system 24/7?

So I asked what I asked because of the abovementioned things, and I'd like to know if is there any principle to start-up? Or as a matter of fact it is all the same as long as the Helmholtz frequency is ideal? Can I apply the formula Jennings?

I have just read your writing 'Transfer Theory' and I have been very glad because I deal with the same thing for a while. Well, I have to confess that regretfully I have negligated this topic till now, although according to my experiences this is one of the most important part of a well constructed 2T engine.

Frits Overmars a écrit:

Jeram a écrit:

I am currently redrawing the Aprilia 125 cylinder designs into CAD so that I can then scale them up to 80x80mm bore and stroke. Currently, I am simply scaling up the cylinder from 125cc to 400cc, But I thought it would be best to approach both of you to see if you have any tips you could give me regarding anything I should watch out for when performing such an alteration of scale/size, and is there anything in general that should be optimised now that the engine size is larger.
I do hope that an essentially scaled-up RSA-400 cylinder will be a formidable beast!

It will certainly be a beast, in more ways than one. In my younger years (about 100 years ago) I raced a 370 cc two-stroke single. Racing was allright, but cruising was nearly impossible. To keep the engine from jerking, I had to keep it pulling all the time. But then it would go much too fast for public road conditions.
You made the right choice in selecting 80x80 mm bore and stroke dimensions; that should compare almost 1 on 1 with the 54x54.5 mm sizes of the RSA125.
The second important factor in scaling up is the mean piston speed. Considering the stroke ratios of your engine and the RSA, maximum power should be at 54.5/80 * 13000 = 8856 rpm and maximum revs at 9878 rpm.
Keep us imformed .

Thanks Frits, yes that is exactly where our peak power is to be with our current cylinder (ported OEM cylinder).
Although it would be lovely to yield astronomical power at such a high RPMs, I simply cannot afford to hire superman to perform connecting rod duties for long enough.

To Keep the piston speed at a safe level we plan to copy the RSA port timings, but control peak power RPMs solely with the pipe.

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?

Many Thanks.

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

If the piston ring dimensions, especially the radial depth of the ring, are scaled up along with the rest of the engine, ring life should not suffer. But I have no recent relevant experience.

Forgi a écrit:

Dear Frits, As I can see this post of mine is not gonna be your favourite!

Neither will my answer be yours, Forgi. I even considered not answering at all because I do not want to become rude. But I fear you will not give up that easily.

Citation :

I suggest to take the resonance Helmholtz as a constant, because it is hard to calculate

A refreshing approach. Then may I suggest you take the engine as a constant?

Citation :

Can I apply the formula Jennings?

Gordon Jennings published hundreds of articles containing countless formulae. And you expect me to guess which one you mean? I am physicist, not a clairvoyant.

Frits Overmars a écrit:

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

If the piston ring dimensions, especially the radial depth of the ring, are scaled up along with the rest of the engine, ring life should not suffer. But I have no recent relevant experience.

Thanks Frits, yes you are correct. I read the wrong dimension on the drawings. The aprilia exh is close to 70% as you stated and thus should be perfectly fine.

I will let you know how this project goes. It will certainly be interesting to compare different scales of the RSA cylinder to see a curve of diminishing returns for cc increases (50cc, 125 and 400cc)

Jan or Frits:
About the RSA engine, concerning the different transfer port timing of the A port and the B & C ports. Was this timing difference done by trail and error or was there some indication that this differencs was needed?
If this was done by trail & error what was your thinking for trying this? Or what was the indication to try this?

Frits Overmars a écrit:

Neither will my answer be yours, Forgi. I even considered not answering at all because I do not want to become rude. But I fear you will not give up that easily.

There is an idiom here in Hungary which says: "If he is thrown out through the door, he enters through the window."
Of course I can say it with all my respect.:)

Frits Overmars a écrit:

Gordon Jennings published hundreds of articles containing countless formulae. And you expect me to guess which one you mean? I am physicist, not a clairvoyant.

Well, you are right, I cite the relating part from Jennings' book:

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

Forgi a écrit:

I cite the relating part from Jennings' book...

Then let me cite this:
[Vous devez être inscrit et connecté pour voir ce lien]
It may give you an idea of the inadequacy of such simple Helmholtz-calculations.
And hopefully you will understand that you just cannot take the Helmholtz resonance as a constant;
it is an event of paramount importance in any engine.