[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 3) (Locked)

hi frits. ive read several 2t books but im not sure i trust what they say because there seems to be conflicting information from different authors. for example heres what gordon jenning and graham bell wrote in their books.

jennings pg75: First, it should be clear to anyone who has made even a cursory study of the
various two-stroke engines' port timings that no close correlation between these timings
and engine speed exists.

grahm bell pg34: My theory is that an engine requires a certain
exhaust duration to attain a specific engine speed.

heres my question frits. i understand time area . but ive never found a good answer for what angle area meens. is there a simple explanation for the definition of angle area ? i thought it had something to do with crank angle but im not sure. thnx

The obvious guess would be that angle area would be the integral of port area with respect to crank angle, just as time-area is the integral with respect to time. And in fact since W rpm is (360W / 60) degrees / second, you should be able to convert directly from one to the other:

area.degrees = area.time x 6 x rpm

The obvious obvious advantage is that angle-area is independent of rpm and you can derive time area for any rpm of interest by inverting that formula.

I'm sure Frits will correct me if I'm wrong :)

Quote from: Nick on Today at 00:17:19
Quote from: Jamathi on November 25, 2013, 05:26:40
The 1968 Jamathi engine.
The bevel gears of the water pump drive never went much.
Again, the same gears are still there!

If you need to change more often. Will there be sufficient time between the switching times to raise? Exhaust temperature
The average close six container has around 13% between the resist so if you then switch remains more time to release. Outlet temperature back
I think you might exhausts were different then? Or is negligible this?

Sander

That is almost negligible Sander.
Since the 60s, there is not very much has changed. At the outlets
Really just detail work.
At the cylinders, the more!
The engines now have a much better power band.
A modern race2takt would be slower than with 12 or 14 gears with 6.
8 or 9 speed would be ideal, then you do have. No cassette tray
That would have saved over the years, a lot of money and untold work!

More often turn means a higher average temperature.
Too often turn causes a loss of time.
Short circuits you also to 5 speed enough.

Jan would you explain in greater detail the temperature & how it all works please

brokedown a écrit:

i understand time area . but ive never found a good answer for what angle area meens. is there a simple explanation for the definition of angle area ? i thought it had something to do with crank angle but im not sure.

OK, here goes.
In the beginning there is a cylinder. With just one rectangular port, to keep things simple. That port has three linear dimensions: the distance from cylinder top plane to the top edge of the port, the distance from top plane to bottom edge of port, and the port width. That's all.

Now we put that cylinder on a crankcase containing a crankshaft with a certain stroke, a con rod with a certain length, and a piston.
Rotate the crank and the piston will descend until it starts to open the port. Keep rotating the crank, and after a certain number of degrees the piston will close the port again. Now we can say that in addition to its linear dimensions the port has also a timing.

Next: angle.area.
Let's assume that the port starts to open at 90° before Bottom Dead Center. We rotate the crankshaft 1° further and then the port will be open a little bit, say x mm. The open area is now x mm high times the port width. And this piece of open area will stay open all the way from 89° before BDC right until 89° after BDC on the next upstroke. That little piece of open port area has an angle.area of 178° * x * width.

We rotate the crankshaft 1° further still, and the port will open a little bit more, say another y mm. This additional open area will stay open from 88° before BTC till  88° after BDC. So this second little piece of open port area has an angle.area of 176° * y * width. And so on.
When we add all these pieces of open port area times the number of degrees that they are open, we find the total angle.area of the port.

Finally: time.area.
Now we look at a running engine. The angle.area of a port is a fixed value, but the time.area is not. How much time it takes to rotate the crankshaft 1 degree, depends on the rpm, as Graham explained above. Double the rpm, and that time will be halved, and so will be the time.area.
Emptying and charging the cylinder requires a certain time.area. If the rpm rises too high, the engine will run out of breath because the available time for emptying and charging the cylinder will become too short.

thnx frits i understand. im sure many people in the future will find that explanation of great value also  .  the best part of all 3 segments of the rsa125 is all information from you and jan and other folks is put in practical terms which is easier to understand.  for instance the leaning tower transfer discussion. again thnx. the 3 rsa125 segments have been priceless info to me and far better than any book or other website

RAW a écrit:

Quote from: Nick on Today at 00:17:19
Quote from: Jamathi on November 25, 2013, 05:26:40
The 1968 Jamathi engine.
The bevel gears of the water pump drive never went much.
Again, the same gears are still there!

If you need to change more often. Will there be sufficient time between the switching times to raise? Exhaust temperature
The average close six container has around 13% between the resist so if you then switch remains more time to release. Outlet temperature back
I think you might exhausts were different then? Or is negligible this?

Sander

That is almost negligible Sander.
Since the 60s, there is not very much has changed. At the outlets
Really just detail work.
At the cylinders, the more!
The engines now have a much better power band.
A modern race2takt would be slower than with 12 or 14 gears with 6.
8 or 9 speed would be ideal, then you do have. No cassette tray
That would have saved over the years, a lot of money and untold work!

More often turn means a higher average temperature.
Too often turn causes a loss of time.
Short circuits you also to 5 speed enough.

Jan would you explain in greater detail the temperature & how it all works please

this is a quote from deraceheldenvanweleer translated by Google?
I dont have a problem with it but i looked at it and was like huh did i post this here?

G'day mate

hey frits just to be 100% sure i understand lets look at 2 engines of aproximatly 400cc to make it simple  . please correct me if my thinking is wrong

84 x 73= 404cc
80 x 80= 402cc

the over sqaure engine would offer less angle.area, and thus less power and less rpm before it runs out of breath ?. becasue even though the bore is bigger, the reduction of stroke reduces the port area proportionatly ? where as the bore increase is only area increased by pi ?

brokedown a écrit:

hey frits just to be 100% sure i understand lets look at 2 engines of aproximatly 400cc to make it simple  . please correct me if my thinking is wrong
84 x 73= 404cc
80 x 80= 402cc
the over sqaure engine would offer less angle.area, and thus less power  and less rpm before it runs out of breath ?. becasue even though the bore is bigger, the reduction of stroke reduces the port area proportionatly ? where as the bore increase is only area increased by pi ?

You grasped it, Brokedown: a short-stroke engine is not a good idea for a two-stroke.
With a four-stroke it is the other way around: the bigger the bore, the more room there is for big valve diameters.

Mic a écrit:

Question for Jan and Frits

I can see the exhaust of the lower cylinder of the RSW/RSA250 has a straight section first and then a quite sharp bend.

Is this better than a long smooth bend? It looks like there is enough space for a smoother shape but perhaps it's important to keep the first section as straight as possible?

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The first part of the header is very important, the straighter for longer the better the pipe works. A contact in the US did many tests with a 256 engine he placed a rear pipe onto the front cylinder and picked up a 1-2hp.

Conversely, I wonder if it would not be better to turn as close as possible of the cylinder, then try to get a pipe as straight as possible.
In order to get the best wave symmetry, to get the most effective return waves.
Each curve signifies a deviation waves and my feeling tells me that the deviation would be less harmful in a small diameter tube than in a large ???

fpayart a écrit:

Conversely, I wonder if it would not be better to turn as close as possible of the cylinder, then try to get a pipe as straight as possible.
In order to get the best wave symmetry, to get the most effective return waves.
Each curve signifies a deviation waves and my feeling tells me that the deviation would be less harmful in a small diameter tube than in a large ???

There are several aspects here.
A: if there must be a bend in the pipe, try to keep the radius as large as possible.
B: The higher the flow velocity (i.e: the smaller the diameter), the more important is it to keep the pipe straight.
These two aspects are somewhat contradictory; the resultant rule would be that the radius becomes smaller as the diameter increases.
I would not worry too much about wave symmetry. Waves can turn around remarkably tight corners without a noticeable effect; they are mainly susceptible to cross area changes.

Aprilia has spent so much time and money to test all solutions, this is probably the ones who are right.
As you say Frits, right at the exit and largest possible curves.  

hey frits i found on page 49 of segment 2 you explain very well angle.area. i read all that before but some how forgot it was there. sorry to ask the same question again  .   now to find angle .area of oddly shaped windows. what is your prefered method ?

brokedown a écrit:

hey frits i found on page 49 of segment 2 you explain very well angle.area. i read all that before but some how forgot it was there. sorry to ask the same question again  .   now to find angle .area of oddly shaped windows. what is your prefered method ?

The explanation in part 2, page 49 was specifically about the blowdown angle.area; the explanation above is about angle.areas in general.
I have no preferred method of dealing with non-rectangular ports. It has just been a lot of work to write a computer program that can deal with all imaginable port shapes.

your hiding us a secret computer program ?  

hi guys  . some pages back i seen this [Vous devez être inscrit et connecté pour voir ce lien] . can everyone recomend if i use it ? what do you say about it frits ?  

i had a pipe made for my engine. original the manifold is 45mm but i think that is to small for 500cc so i increase it to 50mm with manifold from different engine. but perhaps that is to small still. so maybe i need a new pipe again. is it mostly the amount of blowdown which determines starting pipe diameter ? thnx guys . dont laugh if you think its to many curves. its the only may to make it fit  


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brokedown a écrit:

hi guys  . some pages back i seen this [Vous devez être inscrit et connecté pour voir ce lien] . can everyone recomend if i use it ? what do you say about it frits ?

On the net you can find several pipe calculators, based on the 'fos exhaust concept' that I presented here some time ago. Not all of them are reliable, but this one seems to behave allright. 

Citation :

Is it mostly the amount of blowdown which determines starting pipe diameter ?

Nope; the blowdown does not come into it. All pipe diameters grow with cylinder capacity and rpm of max.power, and they become smaller as the exhaust timing gets longer.

Frits Overmars a écrit:

brokedown a écrit:

hi guys  . some pages back i seen this [Vous devez être inscrit et connecté pour voir ce lien] . can everyone recomend if i use it ? what do you say about it frits ?

On the net you can find several pipe calculators, based on the 'fos exhaust concept' that I presented here some time ago. Not all of them are reliable, but this one seems to behave allright. 

Citation :

Is it mostly the amount of blowdown which determines starting pipe diameter ?

Nope; the blowdown does not come into it. All pipe diameters grow with cylinder capacity and rpm of max.power, and they become smaller as the exhaust timing gets longer.

I checked out the pipe program and it does not ask for rod ratio; Does rod ratio change the pipe design (lengths, diameters and cone angles) it seems as though it would because of the longer dwell at TDC and BDC on a long rod verses less dwell time on a short rod combo..

Frits Overmars a écrit:

brokedown a écrit:

hi guys  . some pages back i seen this [Vous devez être inscrit et connecté pour voir ce lien] . can everyone recomend if i use it ? what do you say about it frits ?

On the net you can find several pipe calculators, based on the 'fos exhaust concept' that I presented here some time ago. Not all of them are reliable, but this one seems to behave allright. 

Citation :

Is it mostly the amount of blowdown which determines starting pipe diameter ?

Nope; the blowdown does not come into it. All pipe diameters grow with cylinder capacity and rpm of max.power, and they become smaller as the exhaust timing gets longer.

Hello Frits,

What is your opinion about the difusor option in the exhaust calculator?
i know that you have advised to take the simple route and play with the simple exhaust.
beeing a starting fabricator that i am i realise its important to keep things simple.

i had a little fun with the calculator and set horn coefficient at level 3.
this gives some nice creations.
Assuming i would evolve in life and start to grasp the fenomenoms that surround exhaust design.
What horn coëfficiënt would you choose for the number of difusor sections?

gasflow with a difusor angle more than a certain degree ( aproximatly 7/8 ish not sure?) means losing power.
so the combined number of sections can seem steeper but should always be kept at a optimum.
the horn coëfficiënt number and what it does in the calculator is just a number.

What should one do? play with the number until they reach the optimal difusor angle.
And what angle should one strive for? 

What i also would like to know is the updated way of compression ratio as you've descibed.
can your simplified formula for exhaust calculating be continued or do we need to alter some input for optimum result?

Regards
Sander

SPEEDSLUT a écrit:

I checked out the pipe program and it does not ask for rod ratio; Does rod ratio change the pipe design (lengths, diameters and cone angles) it seems as though it would because of the longer dwell at TDC and BDC on a long rod verses less dwell time on a short rod combo..[/i]

I repeat: these exhaust calculators were put on the internet by several people who based their programs on my 'simple exhaust concept'. I take no responsibily for these programs and any questions about them should be addressed to their makers, not to me.

The FOS exhaust concept is only meant to help beginning tuners on their way.
A lot of important factors, like compression ratio, ignition timing, type of fuel, carburettor diameter, crankcase volume and angle.areas, are not taken into account.
Instead of all those factors that I left out, I included one variable, the speed of sound. Starting with 550 m/s will get you in the right ballpark, after which you can vary this value according to your findings.
Final remark: the calculation of the tailpipe restrictor diameter is critical: you can only apply it to engines that are thermally sound. Air-cooled engines are not.

Sanderhoutman a écrit:

Hello Frits, what is your opinion about the difusor option in the exhaust calculator?

There is no 'diffusor option' in my exhaust concept, Sander. please take a look at my previous reaction, above.

Citation :

I had a little fun with the calculator and set horn coefficient at level 3. this gives some nice creations... What horn coëfficiënt would you choose for the number of difusor sections?

A horn coëfficient is a nice thing if you wish to design a horn. But a two-stroke exhaust pipe is not a musical instrument;
it is a gasdynamic device and the 'laws' (approximations would be a better word) of acoustics do not apply.

Citation :

What should one do? play with the number until they reach the optimal difusor angle. And what angle should one strive for?

There is no 'optimal diffusor angle'. In theory the angle varies according to its distance from the cylinder and according to several other factors.

Citation :

What i also would like to know is the updated way of compression ratio as you've descibed. Can your simplified formula for exhaust calculating be continued or do we need to alter some input for optimum result?

All you need to do is vary the speed-of-sound value. For a lower compression ratio you should use a higher speed value.

Frits Overmars a écrit:

SPEEDSLUT a écrit:

I checked out the pipe program and it does not ask for rod ratio; Does rod ratio change the pipe design (lengths, diameters and cone angles) it seems as though it would because of the longer dwell at TDC and BDC on a long rod verses less dwell time on a short rod combo..[/i]

I repeat: these exhaust calculators were put on the internet by several people who based their programs on my 'simple exhaust concept'. I take no responsibily for these programs and any questions about them should be addressed to their makers, not to me.

The FOS exhaust concept is only meant to help beginning tuners on their way.
A lot of important factors, like compression ratio, ignition timing, type of fuel, carburettor diameter, crankcase volume and angle.areas, are not taken into account.
Instead of all those factors that I left out, I included one variable, the speed of sound. Starting with 550 m/s will get you in the right ballpark, after which you can vary this value according to your findings.
Final remark: the calculation of the tailpipe restrictor diameter is critical: you can only apply it to engines that are thermally sound. Air-cooled engines are not.

I really just want to know if rod ratio affects the pipe design.. IE if you have a pipe that works well on a long rod ratio will it work on a short rod ratio . A short rod revs a bit lower so one would think a long rod ratio would need a bit shorter head pipe. Dont know that is why I ask..

SPEEDSLUT a écrit:

I really just want to know if rod ratio affects the pipe design.

Changing the rod ratio will change the angle.areas, so yes, it does affect pipe design. But without complete information about the engine I can't possibly say by how much.

Citation :

if you have a pipe that works well on a long rod ratio will it work on a short rod ratio.

Maybe, maybe not; see above.

Citation :

A short rod revs a bit lower so one would think a long rod ratio would need a bit shorter head pipe.

Shortening only the head pipe will change the various length ratios of the pipe elements; not a good idea in general, although it could work out positive if the pipe was not optimal to begin with.

Summary: I have some notion of physics and I have some notion of engines. I do not have a crystal ball .

Frits I tried your FOS pipe calculator using RSA125 values and ended up with this.

The total length looks rather small compared to the tubo 102 which had a total length of 803mm ( measured from piston )

Am I doing something wrong here?

EDIT: With speed of sound set to 587m/s the length fits with tubo 102 but are there some guidelines for estimating the correct value?
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hello

frits when you calculating the anglearea you take the effective area of ​​the channel.
and not the area on the circumference.? is there a regular relation in which the exhaust area should stand to inlet area and in which relation to the blowdown?

thanks Manuel