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

We want ample angle.area for our transfer ports while at the same time keeping their height within limits, so we need all the transfer area we can get; we want to use as much as possible of the cylinder circumference.
The best way to utilize the available real estate would be to aim all transfers radially inward; that way the cross section widths of all ports would be equal to their chord widths and you can't get any better than that. All transfer streams would meet in the center of the cylinder, slow each other down and form a central column with only one direction to go: upwards, in the direction of the cylinder head.
But since you can't have a transfer port at the exhaust side of the cylinder, an imbalance would occur and that central column would be inclined (sic) to topple over towards the exhaust side of the cylinder. You don't want that because too much of the fresh charge would take the escape route into the exhaust duct without first scavenging the cylinder.

How do you prevent that central column from leaning towards the exhaust side? If you omit the transfer ports directly opposite the exhaust, you would restore the scavenging balance, but you would sacrifice too much valuable port area. There is a solution, but let me address some other scavenging aspects first.

We want as much transfer port area as possible, so it would make sense to have all transfer ducts enter the cylinder perpendicularly, right? Nope.
To begin with, most pistons are domed, so transfer flow entering the cylinder would collide with the dome. Aiming the transfer ducts axially at about the same angle as the piston dome, usually about 10°, will not cost any effective cross section area and it will noticeably improve the flow coefficient. Larger-than-zero axial angles at the port floors will also enable you to fit larger inner radii in the transfer ducts, another benefit for the flow.

Second: those transfer streams entering the cylinder and colliding in the center will convert kinetic energy into potential energy. In English: their flow velocities will slow each other down in the collision process and the static pressure in the middle of the resulting central column will be higher than the pressure in the transfer ducts.
That static pressure in the central column is a good thing: it will provide for a higher density of the fresh charge in the column and that helps to expel the hot, thin burnt gases from the previous combustion cycle. Think of it as using a jet of water to chase away smoke: that will work a lot better than the other way around (using smoke to chase away the water).

But the static pressure at the foot of the central column can also have adverse effects. Too high a static pressure will impair the flow, because the higher this pressure is, the smaller will be the pressure differential that accelerates the charge through the transfer ducts. Aiming the transfer ducts axially a little will improve the flow, just like it did because of the domed piston. Slightly axially-aimed transfer streams will provide for a less violent, not completely head-on collision. The central pressure can be controlled this way, and the transfer streams will keep the axial component of their velocity, so the central column does not need to begin its journey to the cylinder head with zero velocity. So the axial column speed can be controlled as well by the axial transfer angles.



Most two-stroke people define radial scavenging directions by quoting the distances where the ports would intersect the center line (the leading distance and trailing distance in the drawing below left). Gordon Blair used that notation in his publications, and 95% of us followed suit.
But there is a better, more universally applicable way.
I will explain with an example, not of scavenging directions but of port timing: I might say that a transfer port height of 13 mm is perfect for a racing engine. That may be true for a 125 cc engine but it would be nonsense for a 50 cc or a 500 cc.
But if I say that a transfer port timing of 130° is perfect for a racing engine, then that is valid for any engine, regardless of its cubic capacity. Absolute distance values (millimeters, inches etc.) are not suitable for universal guidelines. Degrees are, as are percentages of bore or stroke. Rpm values are not; mean piston velocities are.

I express transfer duct directions in degrees. Each duct has a leading flank and a trailing flank. Each flank intersects the bore at a point which I can define with a positional angle. And each flank hits the fore-aft center line of the bore with an included angle which I call the directional angle. The drawing below left may clarify what I mean. And the drawing on the right is an example of an existing cylinder.
Now we can express the radial characteristics of the transfer ports with positional and directional angles, regardless of bore and stroke.
And we can express the ports' axial characteristics with axial angles, but that only gives a 'universal value' for engines with identical bore/stroke-ratios.
We may quote a height H in the cylinder where the transfer port's roof would hit the opposite cylinder wall. But we need to express H as a percentage of the stroke. Then we will have a truly universal value.
Then we will also see that short-stroke engines require smaller axial angles.

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



As we are on the subject of scavenging angles, now would be a good time to say something about the axial angles of the A-transfers.
Surely a duct with an axial angle of over 20° offers a smaller cross-section to the flow than a duct that enters the cylinder perpendicularly?
Yes it does. But there are two good reasons to angle it upward anyway.

First, perpendicular mixture streams coming from the A-ports would collide and slow one another right down. The axial angles provide for less velocity losses and less pressure losses, so despite their smaller cross-section, upward ports may flow as much, if not more, than perpendicular ports.
(Now you may well ask why the B-ports do not get the same treatment. It is because the central scavenging column, resulting from all incoming scavenging streams together, must not have too much axial velocity, or the loop scavenging will result in a loop-loss into the exhaust).

Second, there is a thing called scavenging balance (I invented the word for my personal use, so this may well be the first time you ever saw it).
If you looked closely at the scavenging picture of the MB-cylinder I posted above, you may have noticed that the 'radial scavenging directional resultant' had a value of 101,045°.
90° would have meant 'straight up'; more than 90° indicates that the central scavenging column is leaning towards the exhaust side of the cylinder.
But we don't want that; it is bad for the scavenging of the rear part of the cylinder, and it is risky because it may provoke scavenging losses straight into the exhaust.


But how can we prevent a scavenging column from toppling over to the exhaust side like the leaning tower of Pisa? Not by pushing against its basis, but by pushing higher up. Hence the axial angle of the A-ports. The pictures will tell the story. (If only the Pisa architect had known a bit more about two-stroke scavenging....)

Transfer theory part 4
vectors

Let us assume that all transfer ports are of the same height. Let's also assume that a port with twice the cross-sectional width will give twice as strong an impulse (that is already doubtful; it presumes equal densities and equal flow velocities in all ducts, and as duct contents can have different inertias, their accelerations may differ, as will their flow velocities at any given moment).

If you accept these assumptions, you can resolve each transfer stream into an axial component, a fore-aft component over the piston, and a left-to-right component over the piston. The axial components all work in the same direction: towards the cylinder head. The left-to-right components will cancel each other out (if they don't the scavenging is asymmetric) while contributing to the pressure creation at the root of the central column (which in turn will accelerate the axial flow and thus enhance the axial vector), and the fore-aft components will result in a vector that may either point towards the rear side of the cylinder, be zero, or point towards the exhaust side.
This fore-aft vector together with the axial vector will give a resultant that will lean towards the rear of the cylinder, or point straight up towards the head, or lean towards the exhaust side.

What we want to achieve, is an axial column that clings to the rear of the cylinder, so it can wash away the spent gases with as little turbulence as possible. Turbulence will result in mixing of fresh charge and burnt gases, and we don't need that. And mixing will heat up the fresh charge, bringing it nearer to the detonation treshold. And we certainly don't need that!

I realize this is a crude way of describing a complicated flow dynamics event, but hopefully it will help you form a mental picture (no pun intended).

Merry Christmas and a Happy New Year

Merry christmas for you too !!

Thank you mr Santa Overmars (or Frits Claus if you prefer) . Vrolijk Kerstfeest!

Vagelis a écrit:

Thank you mr Santa Overmars (or Frits Claus if you prefer) . Vrolijk Kerstfeest!

Καλά Χριστούγεννα Vagelis!

Great Explanation.
I was not fully aware of this, I used to think that some losses throught the exhaust maybe return to the cylinder when rpm is on pipe.

So with this, we should always start with A-Ports a little higher than B ports, and C-Port a little lower than B-ports?

Like this, from Honda RS 97' Manual...I try to make timings more or less equal as this!

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

Merry Christmas

Thanks Frits for the great explanation of two stroke scavenging

Many years ago I only can dream with this kind of knowledge and explanations in motorbikes forums. Today it´s a reality. I´ll wait for the next chapters

Jan Thiel a écrit:

Marc a écrit:

Do Jan and Frits willl be "closed" for Christmas vacation?



Thanks so much for all.

No I will be here every day!

Then this is going to be no vacation for you two!!
Thank you all for this beautiful addiction!
Merry Chrismas to everybody!!

Merry Chrismas to everybody too , specially to Frits and Jan for the big pleasure that i have to read you here

[Vous devez être inscrit et connecté pour voir cette image]What can I say more? [Vous devez être inscrit et connecté pour voir cette image] Thank you for all your share knowledges[Vous devez être inscrit et connecté pour voir cette image]

Did you made a chapter about twice exhaust , Mr Overmars? sorry for this request.

Jan or Frits,

You have said one of the improvements was using the 120mm connecting rod. What was the length of the connecting rod used initially, 110mm?

What horsepower improvement was found by changing to the longer connecting rod?

Thank you

Michael

michaelten a écrit:

Jan or Frits, You have said one of the improvements was using the 120mm connecting rod. What was the length of the connecting rod used initially, 110mm?
What horsepower improvement was found by changing to the longer connecting rod?

Initially the rod length was 115 mm.
You cannot just change the rod length in an engine; you will also need to change the crankcase and/or the cylinder and/or the piston. And each change has an influence: on the crankcase volume, on the piston weight, on the passage from the inlet to the transfers, etc. And then the carburation and the ignition timing will have to be re-optimized, maybe even forcing a change to the compression ratio and/or the pipe dimensions. It would require a lot of development and testing, which might also have led to a power improvement with the original con rod still in place...

Below you'll find the power curves of the RSA and the RSW (and the reed valve Derbi 125 ridden by Jorge Lorenzo).
You will see that the difference in maximum power between the RSA and the RSW is nearly zero; the difference is in the shape of the power curves before and after the maximum and that is something you cannot express in a simple number.

You see, Michael, as is so often the case, there is no simple answer to your simple question: "What horsepower improvement was found...."

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

Sometimes I think why Honda keep 104mm rod for many years, and only changed to 109mm later.

Would it be a need of crankcase compression problem with this reed valve engine? Even Aprilia RS125, KTM 125 and Mito 125 have 110mm rod's..... And a lower piston and just some simple changes would keep crank volume very near the same.

I Know this is an Aprilia Topic, but this forum needs also a good KTM/Honda topic

romeuh80 a écrit:

I Know this is an Aprilia Topic, but this forum needs also a good KTM/Honda topic.

Maybe you can invite Harald Bartol .

HI.
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.

Hello, I'm Nikola Masic and I'm new on this forum. I accidntaly came accross this amazing and valuable discussion while digging for information on head design. I've been tuning vintage Yamaha, Suzuki and Vespa engines for 18 years.

I have a question regarding rod length: 120mm is very long, by conventional thinking. What was the reason for such a hight rod/stroke ratio, was it piston longevity, increased case volume or time-area consideration? Or was this lenght arrived at by simple trial and error on the dyno?

Thank you.

vespafiend a écrit:

Hello, I'm Nikola Masic and I'm new on this forum. I accidntaly came accross this amazing and valuable discussion while digging for information on head design. I've been tuning vintage Yamaha, Suzuki and Vespa engines for 18 years.

I have a question regarding rod length: 120mm is very long, by conventional thinking. What was the reason for such a hight rod/stroke ratio, was it piston longevity, increased case volume or time-area consideration? Or was this lenght arrived at by simple trial and error on the dyno?

Thank you.

120mm is NOT particularly long, it is quite a normal length.
After much testing 50cc's arrived at a best length of 85mm
85:39,5=2,15
An MBA 125/2 had a 90mm conrod.
90:41=2,19
So a 125 conrod with 120:54,5=2,20 is about the same!
Even calculated in the '50cc way' it would be 54,5X2,15=117!
So the conclusion is that 125's always used way too short conrods.
Why?
Probably to reduce total engine dimensions.
At Aprilia we started with 110, and finally arrived at 120.
Each time the rod was lengthened the power improved.
Also if we kept crankcase volume the same!
Making the crankcase volume bigger increased power even more!
A lot of time and money could have been saved if they had asked, and listened to,
to someone with 50cc experience!
Instead rod lengths of 110, 113, 115, 118 and finally 120 were tried!

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

Frits,

Thanks very much for the reply! It's amazing to hear the amount of work involved in developing the RSA.

The reed valve Derbi was certainly under-powered compared to the Aprilias

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..