[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 4)

LucF sorry I have no weights, this is all data from the vehicle I worked on over several years

RAW a écrit:

LucF sorry I have no weights, this is all data from the vehicle I worked on over several years

Thanks RAW, I will try to figure out with the information I have in a simulation.

In 2011 minimum weight for top fuel was 2320 pounds. After the run the projectile had to be on or above the limit set by nhra.

Sanderhoutman a écrit:

In 2011 minimum weight for top fuel was 2320 pounds. After the run the projectile had to be on or above the limit set by nhra.

Ok Thanks Sander

LucF a écrit:

Sanderhoutman a écrit:

In 2011 minimum weight for top fuel was 2320 pounds. After the run the projectile had to be on or above the limit set by nhra.

Ok Thanks Sander

Google Luc. Sometimes works like a charm  

hi frits i have a question about this picture. with a cylinder that has A,B,C,D ports (A,B,C are in the side of the cylinder and D is in the rear), would it be best to have A,B,C ports aimed to the area where the black circle is ??

also do you have some advice for what upward angle the A,B,C ports should have  ? the piston crown is 7* angle just so you know.  thnx


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heres what i have right now

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brokedown a écrit:

hi frits i have a question about this picture. with a cylinder that has A,B,C,D ports (A,B,C are in the side of the cylinder and D is in the rear), would it be best to have A,B,C ports aimed to the area where the black circle is ?

More or less, yes. It's hard to say just looking at your sketch, with only an impression of the radial angles and without numerics. What also strikes me, is that the leading and trailing port flanks don't converge.
It also seems like you let a lot of real estate between C and D lie fallow. Why? Awkwardly situated ring pegs?

Citation :

also do you have some advice for what upward angle the A,B,C ports should have  ? the piston crown is 7* angle just so you know.

It's all in the picture, Brokedown. Look at the α up-values for the axial scavenging angles.

Frits Overmars a écrit:

What also strikes me, is that the leading and trailing port flanks don't converge.
It also seems like you let a lot of real estate between C and D lie fallow. Why? Awkwardly situated ring pegs?

after im done all the port angles will converge and aim to a location aprox at the black circle in the previous photo.

the ring pegs are near the D port and in a fine location. we can only assume all the dead space between C and D was just poor design on ktm's part.   

they didnt make good use of the available wall area but im going to fix that   

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the rear is welded so C port can be much wider

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also i wanted to test what happens when the exh floor is raised a few mm. this is just a rough idea of what i was thinking. lower rear portion of the aux exh ports were welded also which allows a better shape with alot less short circuit from the crankcase to the pipe when the piston hits TDC.

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Citation :

It's all in the picture, Brokedown. Look at the α up-values for the axial scavenging angles.

ok so if my piston dome is 7* i should have C port at 7*. A port should be around 25-30*. B port should be somewhere in the middle perhaps about 15* ?  thnx for help frits

Frits and Jan, I want to thank you for your previous replies. They have sure helped me and other people who admire your work a very big deal! Elsewhere it was said that blowdown should be as big as possible, but doesn't this make the engine's powerband to narrow? Should it really be as big as possible in all racing engines? Is it the same story as with the exh port timing of 190degrees: as long as you have a good pipe and a transmission to keep you in the powerband, it is good for a racing 2stroke?

Also I was wondering if I was right thinking that it wouldn't make sense to use a toroidal head on a 6speed gokarting engine, because they utilise a very small squishband to make the combustion process slower as they can not use variable ignition timing. And a toroidal head would only make the turbulence by the squishband bigger, so that both effects kind of equalize each other?

Senne s a écrit:

Elsewhere it was said that blowdown should be as big as possible, but doesn't this make the engine's powerband to narrow? Should it really be as big as possible in all racing engines? Is it the same story as with the exh port timing of 190degrees: as long as you have a good pipe and a transmission to keep you in the powerband, it is good for a racing 2stroke?

Blowdown depends on four factors: exhaust timing, transfer timing, total exhaust port width and exhaust flow coefficient.
I'll ignore the flow coefficient for now because that deserves a book in itself. And Jan should write it  .
 
Exhaust timing should be 180° effective; about 190° geometrical, for optimum resonance. And yes, as long as you have a good pipe and a transmission to keep you in the powerband, it is good for a racing 2stroke.

Transfer timing should be such that cylinder pressure has dropped below scavenging pressure by the time the transfer ports start to open.

Total exhaust port width: the more you utilize the total circumference of the cylinder bore, the more blowdown angle.area you'll get, but very wide exhaust ports increase the danger of losing fresh charge through short-circuiting.
 
The typical two-stroke powerband results from the pressure fluctuations in the exhaust pipe. When everything is optimal, around the rpm of maximum torque, the pipe will create a low pressure in the cylinder at around BDC in order to help scavenging, and a high pressure between transfer closure and exhaust closure in order to supercharge the cylinder with fresh mixture that got washed through earlier in the transfer phase.

At low revs, especially around 2/3 of maximum torque rpm, the high pressure from the pipe arrives too early at the cylinder, when the transfer ports are still wide open, and this pressure will push the fresh mixture from the cylinder back into the crankcase, causing a gap in the torque curve. The higher the transfer timing, the greater this unwelcome effect.

Citation :

Also I was wondering if I was right thinking that it wouldn't make sense to use a toroidal head on a 6speed gokarting engine, because they utilise a very small squishband to make the combustion process slower as they can not use variable ignition timing. And a toroidal head would only make the turbulence by the squishband bigger, so that both effects kind of equalize each other?

You may be right there. I haven't done much work on kart engines, amongst other things because I don't like the dumb rule of having to use a fixed ignition timing. It was once introduced to keep the sport affordable, but the effect has since long become negative. Every second-hand MX-engine has a variable ignition, but if you wish to use it on a kart you'll have to buy a new ignition with a fixed timing   .

Sujet: Re: [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 3) Aujourd'hui à 10:42
Senne s a écrit:
Elsewhere it was said that blowdown should be as big as possible, but doesn't this make the engine's powerband to narrow? Should it really be as big as possible in all racing engines? Is it the same story as with the exh port timing of 190degrees: as long as you have a good pipe and a transmission to keep you in the powerband, it is good for a racing 2stroke?
Blowdown depends on four factors: exhaust timing, transfer timing, total exhaust port width and exhaust flow coefficient.
I'll ignore the flow coefficient for now because that deserves a book in itself. And Jan should write it .

Exhaust timing should be 180° effective; about 190° geometrical, for optimum resonance. And yes, as long as you have a good pipe and a transmission to keep you in the powerband, it is good for a racing 2stroke.

Transfer timing should be such that cylinder pressure has dropped below scavenging pressure by the time the transfer ports start to open.

Total exhaust port width: the more you utilize the total circumference of the cylinder bore, the more blowdown angle.area you'll get, but very wide exhaust ports increase the danger of losing fresh charge through short-circuiting.

The typical two-stroke powerband results from the pressure fluctuations in the exhaust pipe. When everything is optimal, around the rpm of maximum torque, the pipe will create a low pressure in the cylinder at around BDC in order to help scavenging, and a high pressure between transfer closure and exhaust closure in order to supercharge the cylinder with fresh mixture that got washed through earlier in the transfer phase.

At low revs, especially around 2/3 of maximum torque rpm, the high pressure from the pipe arrives too early at the cylinder, when the transfer ports are still wide open, and this pressure will push the fresh mixture from the cylinder back into the crankcase, causing a gap in the torque curve. The higher the transfer timing, the greater this unwelcome effect.
Citation :
Also I was wondering if I was right thinking that it wouldn't make sense to use a toroidal head on a 6speed gokarting engine, because they utilise a very small squishband to make the combustion process slower as they can not use variable ignition timing. And a toroidal head would only make the turbulence by the squishband bigger, so that both effects kind of equalize each other?
You may be right there. I haven't done much work on kart engines, amongst other things because I don't like the dumb rule of having to use a fixed ignition timing. It was once introduced to keep the sport affordable, but the effect has since long become negative. Every second-hand MX-engine has a variable ignition, but if you wish to use it on a kart you'll have to buy a new ignition with a fixed timing .

Frits what if we road race our kart and have a variable ignition ? What is the generic effect of the toroidal head upon a 2 stroke ? Let's assume we've all got an rsw or rsa for ease of clarity

Thanks a lot Frits, I think I understand now.
I hope I can once reach the same level as yours!

RAW a écrit:

Frits what if we road race our kart and have a variable ignition ? What is the generic effect of the toroidal head upon a 2 stroke ?

There are two issues here: a variable (preferably prgrammable) ignition and the shape of the combustion chamber.  
Once you have a variable ignition, you no longer need to provoke a slow combustion in order to generate hotter exhaust gas at high revs; you can do that by retarding the ignition timing.
You want to keep the thermal load on the engine as low as possible, which means that you want the shortest possible combustion phase. This will limit the heat transfer of the combustion to the surrounding metal, improving both the thermal soundness of the engine and the efficiency of converting fuel energy into horsepower.
Squish helps a lot in spreading the flame and you can get the best squish with a head shape like this:

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Ah, so this is the reason for the sharp transition at the edge of the squish: to avoid the boundary layer following the surface of the head "around the corner"?

Frits Overmars a écrit:

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2 questions Frits :
- at point 2 : do you end the arc 90° after point 3, or do you end perpendicular on the piston crown ?
- when playing with different head volumes, the only thing you change is the diameter of the arc ?

JanBros a écrit:

at point 2 : do you end the arc 90° after point 3?

Yes.

Citation :

when playing with different head volumes, the only thing you change is the diameter of the arc ?

I usually keep the 50% squish area and the 90° and I vary the radius of the arc and the position of its center.

tnx !

GrahamB a écrit:

Ah, so this is the reason for the sharp transition at the edge of the squish: to avoid the boundary layer following the surface of the head "around the corner"?

There was no particular reason!
We tried a number of head designs, maybe 30-40 different ones.
And discovered that having no radius was better....

Mr Overmars, Mr thiel, have you a picture, or a drawing that explains "raising the duct floor" of the exhaust duct,
how much it s   good to raise this exhaust duct  

remonter le plancher du conduit d’échappement : c'est ce qu'a fait Brokedown
=>
regarde ses photos quelques messages avant le tien : on voit très bien qu'il a remonté le plancher grâce a de la soudure

le but étant de conserver suffisamment de vitesse aux gaz sortant, c'est pas essais successifs que tu pourras déterminer de combien il est bon de remonter le plancher (si c'est nécessaire)

merci stephane, car j avais cru comprendre que c etait pour creer un "mur" au gaz frais lors de la vidange!!! mon anglais est nul (comme google translate d ailleurs !!!)

hi pagi. im not sure what is the correct amount to raise the exh floor but i think frits said its good the have the exh outlet about the same area as the blowdown. thats how i understand him anyways   


i have worked on this alittle more but im not finished yet. the red line is original



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Mr overmars, Mr Thiel

what do you think about the location ( at this beginning) off the separation between the A and B ports, because i have see many options ?(like 500NSR beginning very late)

and what is the best radius for this separation ? (3,4 mm, more....)

What did you weld that with?

It looks like the stuff you weld with a propane torch... Low melting point.

williamsmotowerx a écrit:

What did you weld that with?

It looks like the stuff you weld with a propane torch... Low melting point.

Well, it certainly puddles nicely !