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

Hi everyone, let me introduce myself:
I'm old 2-stroke engines fan, I own and drive a 125 gearbox kart, and recently I discovered this fantastic forum, and in particular this section where the most respected participants, Frits Overmars and Jan Thiele        (without taking anything away from the other very competent people of the forum).

I read very carefully the various posts related to the design of the Aprilia RSA 125 engine, and I came across, among the many other interesting things, on designing the cylinder head.

Since I'm trying to optimize my engine, I made a small spreadsheet to try to optimize the cylinder head, based on the data published by Frits, and unfortunately there are some data that do not seem correct to me (in the attached image there are mine calculations and alongside those of Frits).

In practice I calculated the volumes of the head by breaking down into the geometric parts that make up the cylinder head: the toroid, the cylinder inside the toroid and the spherical cap relative to the squish area and the squish volume.

There are significant differences in the overall volume of the head and the relative compression ratio.

Dear Frits, please, could you tell me if I did something wrong?
Did you add the volume to the inside of the spark plug or anything else?

If you want I can publish the excel file to see the formulas applied, but being basic geometry, I think I have applied them correctly.

Thank you very much for all the information that cannot be found elsewhere, and in advance for any response.

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

I'm curious for the formula you used to calculate the actual combustion chamber (the bath tub). you use "1/4 toroid volume", but if you did that, that can't be correct.

I searched quite some time myself to get the correct volume of that part of the combustion chamber.

my formula is =(PI()*(6*D19*X6^2+3*PI()*D19^2*X6+4*D19^3))/6000

in which D19 = Radius bowl and  X6 = radius flat roof combustion chamber.

in my spreadsheet, the squish velocity is spot on with Blair's, so my formula is correct.

free for anyone that want's to use it :
[Vous devez être inscrit et connecté pour voir ce lien]

is protected by pasword so you can't fuck it up. If you want to look inside, pasword = x

JanBros a écrit:

I'm curious for the formula you used to calculate the actual combustion chamber (the bath tub). you use "1/4 toroid volume", but if you did that, that can't be correct.

I searched quite some time myself to get the correct volume of that part of the combustion chamber.

my formula is =(PI()*(6*D19*X6^2+3*PI()*D19^2*X6+4*D19^3))/6000

in which D19 = Radius bowl and  X6 = radius flat roof combustion chamber.

in my spreadsheet, the squish velocity is spot on with Blair's, so my formula is correct.

free for anyone that want's to use it :
[Vous devez être inscrit et connecté pour voir ce lien]

is protected by pasword so you can't fuck it up. If you want to look inside, pasword = x

Hi JanBros,
first of all thanks for the reply and above all for the correct formula.

Meanwhile, as I read your answer, I immediately understood where I went wrong: I made the 1/4 section of the toroid, without considering that the external part obviously has a higher volume than the internal part    

I feel so stupid that I would like to hide myself ....

The formula used is this, and my mistake was to divide the result by 4:
V=2 x PI^2 x R x r^2 - formula for volume of the entire Toroid, where r is the radius of the toroid, and R is the distance of the toroid from the toroid center.

I don't know why but I can't download the excel file, I'm sent to an empty page, but it doesn't matter, I understood the principle of your formula, and as soon as I have a minute I insert it into my file.

Thanks again, I'm in debt with you

seems there are to many "t's" in http  

corrected the link in my post.

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

JanBros a écrit:

seems there are to many "t's" in http  

corrected the link in my post.

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

Many thanks

JanBros a écrit:

I'm curious for the formula you used to calculate the actual combustion chamber (the bath tub). you use "1/4 toroid volume", but if you did that, that can't be correct.

I searched quite some time myself to get the correct volume of that part of the combustion chamber.

my formula is =(PI()*(6*D19*X6^2+3*PI()*D19^2*X6+4*D19^3))/6000

in which D19 = Radius bowl and  X6 = radius flat roof combustion chamber.

in my spreadsheet, the squish velocity is spot on with Blair's, so my formula is correct.

free for anyone that want's to use it :
[Vous devez être inscrit et connecté pour voir ce lien]

is protected by pasword so you can't fuck it up. If you want to look inside, pasword = x

Dear JanBros, many thanks again: trying to understand your formula, (that give the same results as Frits and of my new calculation up to 3th decimal):

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

Where 6000 is 6 x 1000 used to transform in cc the volume, so we can momentary not consider it and use only 6, we can rewrite your formula as:

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


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


Doing so, we are considering only the external quarter of the toroid, that means if we want to consider the external half of the toroid, we have 2x 3,1416 x Sphere Volume with the same radius “r” of the toroid.

It’s interesting to note, and not so intuitive, that the external part of toroid Volume is bigger than the internal part of an amount of 3,1416 x a sphere with the same radius “r”.

I had not find anywhere this formula, so I try to verify it doing a data integration, in practice I use the same method used in Integral Formula (and also in FEM) in this way:
- I took the height of the toroid “r” and divided it in 1000 parts;
- Using sine function and the “Infinitesimal Height” I found the angle;
- I used this angle to find cosine and find the ring depth, so I can calculate the r and R of the ring;
- Then I can calculate the area of the ring, multiply that by the “Infinitesimal Height” and found the volume;
- Doing so for each of 1000 sections you can find the volume of the ¼ toroid (or any part of the toroid you want to use).

The precision is limited by the number of slices, but using 1000 slices, I found the same results as Frits did.

Not so efficient and elegant than your formula, but the potential advantages of this solution could be (changing only a bit the calculation formula):
- have a toroid "flatted" in the upper or lower part;
- use a toroid radius bigger than the height of the combustion chamber;
- use a different profile instead of round profile.

I don't know how to insert my excel file here, so If you want I can give you by mail

don't remember how I found the formula, searching or asked for a solution in a math's forum.

Frits Overmars a écrit:

In the Aprilia cylinders C is not higher than B, Ken. The A-ports have a timing of 130°; B and C both have 132°.
To my knowledge there has not been any new development since the RSA era worth mentioning.
Kart cylinder internals are usually copied from Aprilia or Honda cylinders; Honda had high A-ports, lower B-ports and an even lower C-port; what we called a Dalton-staggering.

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

Hi Frits,
in reference to what you have written above, I wanted to understand why having the A port lower than the B and C ports is advantageous compared to the Honda system that has overturned them.

As I wrote a few posts before, I drive a 125 gearshift kart and I love the 2-stroke engines: from the technical data sheets of all the gearshift kart engines it is clear that everyone uses the Honda scheme, or Dalton-staggering as you call it.    

As you know in karting we have an infinite number of limitations and practices:
- Use of Reed-valve motors only;
- Fixed ignition;
- Minimum head volume 11 cc;
- Maximum exhaust timing 199° (but usually is 194-196°);
- 30 mm Dell'Orto VHSH carburetor;
- Petrol from the Petrol Station, often 95 RON, in some races PANTA 102 RON;
- Counter-rotating engine, Silencers and Intake Filters cik-fia approved, and so on.

The questions I would like to ask you are these:
I have read quite carefully (or at least I hope) your post "Torre di Pisa", and basically the design of the scavenging done in that way helps the maximum filling and the discharge of the exhaust gases to have the maximum blowdown area; but I wonder why the Honda system can't do the same? What are the principles for which it is or is not possible?

I ask you this because I assume that kart engine designers, while not at your level, are not completely crazy, and if they all came to design that type of cylinder they will also have some reason, right?

Do you think it would be advantageous to use the Aprilia scheme even in an engine with these characteristics so different from the Aprilia engine?

I thank very much you and Jan for all the pearls you have given, and in advance for the answer to my questions.

Boa noite a todos.
Alguém que simplifique, por favor.
ATN?
Antecipadamente muito obrigado.

laranjateam a écrit:

Boa noite a todos.
Alguém que simplifique, por favor.
ATN?
Antecipadamente muito obrigado.

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

Scalvo a écrit:

Hi Frits,
in reference to what you have written above, I wanted to understand why having the A port lower than the B and C ports is advantageous compared to the Honda system that has overturned them.
As I wrote a few posts before, I drive a 125 gearshift kart and I love the 2-stroke engines: from the technical data sheets of all the gearshift kart engines it is clear that everyone uses the Honda scheme, or Dalton-staggering as you call it.    
As you know in karting we have an infinite number of limitations and practices:
- Use of Reed-valve motors only;
- Fixed ignition;
- Minimum head volume 11 cc;
- Maximum exhaust timing 199° (but usually is 194-196°);
- 30 mm Dell'Orto VHSH carburetor;
- Petrol from the Petrol Station, often 95 RON, in some races PANTA 102 RON;
- Counter-rotating engine, Silencers and Intake Filters cik-fia approved, and so on.
The questions I would like to ask you are these:
I have read quite carefully (or at least I hope) your post "Torre di Pisa", and basically the design of the scavenging done in that way helps the maximum filling and the discharge of the exhaust gases to have the maximum blowdown area; but I wonder why the Honda system can't do the same? What are the principles for which it is or is not possible?
I ask you this because I assume that kart engine designers, while not at your level, are not completely crazy, and if they all came to design that type of cylinder they will also have some reason, right?
Do you think it would be advantageous to use the Aprilia scheme even in an engine with these characteristics so different from the Aprilia engine?
I thank very much you and Jan for all the pearls you have given, and in advance for the answer to my questions.

Scalvo, those kart engine designers are certainly not crazy. Their main problem is that they are not allowed to use any electronics: no variable ignition, no variable power jet and no electronically controlled power valve. They compensate this by using a relatively short expansion pipe, combined with deliberate scavenging losses which lower the exhaust gas temperature in circumstances that require a lower exhaust pulse velocity. The scavenging is manipulated via higher A-ports that make the engine more sensitive to the blowdown time.area.
They also play with the width of the squish band; you may have noticed that kart engines usually have a smaller squish band than the 50% area of the RSA.
In short: at the lower end of the power band they accept scavenging losses as a means of simulating a longer expansion pipe; at the upper end of the power band the reduced squish effect leads to a lower burning speed which gives hotter exhaust gases.

Wow: thank you so much Frits, really thank you, what a great explanation!!!
In these only 6 lines there are a lot more information than I could ever hopep for, information that turned on the light bulb in my (small) brain and lead me to ask you other things, I hope not too much abuse your kindness ....

Frits Overmars a écrit:

those kart engine designers are certainly not crazy(….) They compensate this by using a relatively short expansion pipe, combined with deliberate scavenging losses which lower the exhaust gas temperature in circumstances that require a lower exhaust pulse velocity.
The scavenging is manipulated via higher A-ports that make the engine more sensitive to the blowdown time.area: this way the direction of the central scavenging column can be influenced by the revs.
In short: at the lower end of the power band they accept scavenging losses as a means of simulating a longer expansion pipe (….)

So, if I well understand, a relative short exhaust pipe generate the desired higher exhaust temperature (very interesting to know), but without electronics, power valve and power jet, and making the A-ports higher than B and C-ports, they use scavenging losses to lower the exhaust temperature in the lower of the power band to gaining some power at relatively low revs.

I suppose from this information that the central scavenging column is influenced by revs because al low revs you have a little short-circuit from A-port to exhaust port due to the exhaust “not in time” resonance pulse, and the central scavenging column is lightly shifted towards the exhaust port, while at high revs the “correct” exhaust pulse reset it the correctly, is it right?

Frits Overmars a écrit:

They also play with the width of the squish band; you may have noticed that kart engines usually have a smaller squish band than the 50% area of the RSA.
In short: (………) at the upper end of the power band the reduced squish effect leads to a lower burning speed which gives hotter exhaust gases.

This is exactly the next question that I would had ask to you: I ever wondering why the squish band is so little in kart engines, and I started my discussion here just for this; your explanations answer to everything!!!!

But since I’m very curious, and I’m trying to enhance my tought over, this open a lot of new considerations and questions....

Since two years I'm developing an Exhaust Flow Restrictor similar in the principle to yours, but using the design from Maurizio Cavaliere, as the attached images.
[Vous devez être inscrit et connecté pour voir cette image]

Being this restrictor located into the exhaust stinger, this is not subject of regulations, so we can assume it as regular solution. where is the point?

Using this restrictor I gain particulary in the upper gears transition (4th to 5th and 5th to 6th gear) but I had many detonation (or as Jan always say, pre-ignition) problems, partially resolved changing carburetion, but the balance between performance and detonation remain an open issue.

Now having your explanation on the squish effects on exhaust temperatures and burning speed, and knowing that the restrictor raise exhaust temperatures (I just know it before…), my questions to you are:

Wanting to use an exhaust restrictor in this case, could be a good idea combine it with an increase of the squish band?

If yes, doing a greater squish band, currently we only have about 35%, and having a lot of volume (11 cc), I must raise the plug-to-piston distance, could it have negative effects?
Thanks a lot again

Scalvo a écrit:

if I well understand, a relative short exhaust pipe generate the desired higher exhaust temperature

A short exhaust pipe does not generate a higher exhaust gas temperature by itself, but if the pulses traveling through the pipe have a shorter distance to travel, they arrive back at the cylinder sooner, which raises the pipe's resonant frequency and the optimum engine rpm. But this is only true for a constant exhaust gas temperature. If the temperature goes down, the speed of sound also decreases and the pulses travel slower.

Citation :

making the A-ports higher than B and C-ports, they use scavenging losses to lower the exhaust temperature in the lower of the power band to gaining some power at relatively low revs.

Scavenging losses consist mainly of the mixture that came out of the A-ports because those ports are closest to the exhaust port. In general there should be just enough blowdown time.area to let the cylinder pressure drop below the crankcase pressure before the transfer ports start to open.  But at higher revs the cylinder pressure may still be higher than the pressure in the transfer ducts when these start to open. Burnt gases will enter the highest transfer ports and the scavenging coming out of these portst no longer consist of fresh mixture, but of burnt gases that will not lower the temperature of the gases in the exhaust pipe so much, if they short-circuit into the exhaust port.
In short: at higher revs the gases in the pipe will have a higher temperature. That is exactly what we would achieve with a variable ignition timing, if that were allowed.

Citation :

Since two years I'm developing an Exhaust Flow Restrictor... Using this restrictor I gain particulary in the upper gears transition but I had many detonation problems... could it be a good idea to combine it with an increase of the squish band?

You can either use a restrictor that gives maximum power, or you can use a wider restrictor that will yield a lower average pressure and temperature in the pipe. That will cost some top power but it can improve power at the lower end of the power band, like it says in the exhaust pipe concept below. [Vous devez être inscrit et connecté pour voir cette image]

What you'd really need for karting is an end cone with a variable bleed area (below), but there will probably be a rule forbidding it. Anyway, I think the narrow squish band will still be necessary in a kart engine. [Vous devez être inscrit et connecté pour voir cette image]

Citation :

doing a greater squish band, currently we only have about 35%, and having a lot of volume (11 cc), I must raise the plug-to-piston distance, could it have negative effects?

I don't think the plug-to-piston distance in itself is critical, but it is important that there is clean, combustible mixture between the plug electrodes. You can promote that by modifying the spark plug. Below are two identical plugs, but the right one has some material removed in order to get more clean mixture between the electrodes, which results in an engine that is more willing to rev. You can easily try this yourself.  I hope it's not yet forbidden in karting...[Vous devez être inscrit et connecté pour voir cette image]

Thanks again Frits, your words are really useful.
Now I have to take some time to think about it ....  

Do you think that even a small increase in squish band could be not useful?
Even reducing the squish height might not be useful?
Now we have about 0.80 mm.
I can easily realize a new head with a little increments of some % points (for example from 35 to 40%) and with a little decrease the squish band from 0.80 to 0.70 mm.

The problem I would to solve is detonation.

Frits Overmars a écrit:

What you'd really need for karting is an end cone with a variable bleed area (below), but there will probably be a rule forbidding it. Anyway, I think the narrow squish band will still be necessary in a kart engine. [Vous devez être inscrit et connecté pour voir cette image]

Yes, I had read it a lot of posts ago, obviously it's not allowed, but the idea is really brilliant.

Frits Overmars a écrit:

I don't think the plug-to-piston distance in itself is critical, but it is important that there is clean, combustible mixture between the plug electrodes. You can promote that by modifying the spark plug. Below are two identical plugs, but the right one has some material removed in order to get more clean mixture between the electrodes, which results in an engine that is more willing to rev. You can easily try this yourself.  I hope it's not yet forbidden in karting...[Vous devez être inscrit et connecté pour voir cette image]

Even the spark plugs are regulated, this because some time ago some "genius" had used some spark plugs that protruded into the combustion chamber, all to subtract a few cents of cc and infinitely increase the compression ratio that is limited by regulation.
So today we also have a list of approved spark plugs, as well as "correct assembly" in the combustion chamber [/quote]

Scalvo a écrit:

Do you think that even a small increase in squish band could be not useful?
Even reducing the squish height might not be useful? Now we have about 0.80 mm. The problem I would to solve is detonation.

I dropped my crystal ball, so you will have to test those increases in squish band width yourself.
The detonation should disappear if you tighten the squish gap from 0,8 mm to 0,5 mm. However, the more violent turbulence will speed up the burning, lower the exhaust gas temperature and reduce the overrev capacity, unless you compensate the faster burning by retarding the ignition timing. That in itself will also lessen the deto risk.

Frits Overmars a écrit:

I don't think the plug-to-piston distance in itself is critical, but it is important that there is clean, combustible mixture between the plug electrodes. You can promote that by modifying the spark plug. Below are two identical plugs, but the right one has some material removed in order to get more clean mixture between the electrodes, which results in an engine that is more willing to rev. You can easily try this yourself.  I hope it's not yet forbidden in karting...

Scalvo a écrit:

Even the spark plugs are regulated, this because some time ago some "genius" had used some spark plugs that protruded into the combustion chamber, all to subtract a few cents of cc and infinitely increase the compression ratio that is limited by regulation.

I would not bother reducing the combustion chamber volume. A high compression ratio is good if you try to tune a foul-stroke engine. Two-strokes dont need it.

In karting rules, there is something important : "any Frits Overmars's idea will be automatically forbidden".

I also like the foul-stroke concept !!!

bentou a écrit:

In karting rules, there is something important : "any Frits Overmars's idea will be automatically forbidden".
I also like the foul-stroke concept !!!

Thank you Bentou

bentou a écrit:

.....
I also like the foul-stroke concept !!!

I like  it too  


quote " but it is important that there is clean, combustible mixture between the plug electrodes " , it also works in the foul strokes world , and pretty much



.

Hello, Frits.
I have a question I quote.
When I do the calculations for an exhaust to a 250cc engine based on its method the D3 section, D4 gets a huge diameter 167.8 mm will this be correct for a low-speed motor?

Thank you

laranjateam a écrit:

When I do the calculations for an exhaust to a 250cc engine based on its method the D3 section, D4 gets a huge diameter 167.8 mm. Will this be correct for a low-speed motor?

I did some reverse engineering on your data, Laranjateam.
In the exhaust concept that I posted here yesterday, we can see that:
D4 = 3,5 * X
X = (407 - exhaust timing angle) * SquareRoot (cylinder capacity *  rpm of maximum power) / 7100

With the values that you provided, we find:
D4 = 3,5 * X = 167,8
X = 167,8 / 3,5 = 47,9429
47,9429 = (407 - exhaust timing angle) * SquareRoot (250) * SquareRoot (rpm of max.power)  / 7100
rpm of max.power = [ 21528,44 / (407 - exhaust timing angle) ]²

I don't know the exhaust timing angle but let us assume that it is between 180° and 200°.
For 180° exhaust timing the rpm of max.power becomes   8994 rpm
For 190° exhaust timing the rpm of max.power becomes   9843 rpm
For 200° exhaust timing the rpm of max.power becomes 10816 rpm
Assuming 68 mm bore and stroke, the mean piston speed at the rpm of max.power will be between 20,4 m/s and 24,5 m/s
I would not call that a low-speed motor....

Thank you Frits.
A lot of health for you and your family.

. Below are two identical plugs, but the right one has some material removed in order to get more clean mixture between the electrodes, which results in an engine that is more willing to rev. You can easily try this yourself.  I hope it's not yet forbidden in karting...[Vous devez être inscrit et connecté pour voir cette image][/quote]

Cette modification des bougies ne revient elle pas à monter une bougie plus chaude? Il me semble qu’une électrode centrale proéminente est toujours préférable et qu’on n’utilise un indice thermique plus froid que si cela est nécessaire pour évacuer les calories ?

This was successfully tried at Aprilia about 20 years ago.
It worked VERY well!
But it was too much work to do this for all our riders....
As we would have to modify 100's of plugs
And if we asked the spark-plug factory to make their plugs that way we would not have any advantage over other factory's.
So it was never applied…...

Frits Overmars a écrit:

Scalvo a écrit:

Do you think that even a small increase in squish band could be not useful?
Even reducing the squish height might not be useful? Now we have about 0.80 mm. The problem I would to solve is detonation.

I dropped my crystal ball, so you will have to test those increases in squish band width yourself.
The detonation should disappear if you tighten the squish gap from 0,8 mm to 0,5 mm. However, the more violent turbulence will speed up the burning, lower the exhaust gas temperature and reduce the overrev capacity, unless you compensate the faster burning by retarding the ignition timing. That in itself will also lessen the deto risk.

I dropped mine too, this is the reason why I asked for yours

Frits Overmars a écrit:

I don't think the plug-to-piston distance in itself is critical, but it is important that there is clean, combustible mixture between the plug electrodes. You can promote that by modifying the spark plug. Below are two identical plugs, but the right one has some material removed in order to get more clean mixture between the electrodes, which results in an engine that is more willing to rev. You can easily try this yourself.  I hope it's not yet forbidden in karting...

Scalvo a écrit:

Even the spark plugs are regulated, this because some time ago some "genius" had used some spark plugs that protruded into the combustion chamber, all to subtract a few cents of cc and infinitely increase the compression ratio that is limited by regulation.

Frits Overmars a écrit:

I would not bother reducing the combustion chamber volume. A high compression ratio is good if you try to tune a foul-stroke engine. Two-strokes dont need it.

Thanks a lot Frits, next time I try a new Head combined with a retarded ignition timing

Good Morning Frits, if it's not too much trouble for you,
How do you calculate the speed at which the maximum power of a motor is achieved?
Thank you.