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

In my definition a dome or hemispherical head has an offset of zero whereas a toroidal head has a positive offset.
You cannot construct a dome combustion chamber with the same volume as a toroidal head while using the same combustion dome radius as in the toroidal head. But I think the dome is a superseded shape anyway.
Determining the various head shapes requires some mathematics and there is no easy way to explain them here.

Mr Overmars
if i drawn a toroidal head for a bore off 82 mm ( is a jet ski twin cylinder 800cc), i imagine my width off the offset has to be more important than the rsa head....??  could i keep the same  percentage off the rsa head , 22.47 mm offset is =  41.61 per cent off the bore...

Question Angel Nieto, going back to the bikes ??

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

pagi a écrit:

if i drawn a toroidal head for a bore off 82 mm ( is a jet ski twin cylinder 800cc), i imagine my width off the offset has to be more important than the rsa head....??  could i keep the same  percentage off the rsa head , 22.47 mm offset is =  41.61 per cent off the bore...

Pagi, you ask me to say something about an engine from which I only know the bore. My crystal ball is not that good.
You can use most of the same percentual dimensions as on the RSA but you'll need to choose a lower compression ratio for your 800 cc twin. By the way: the RSA-offset is not 22,47 mm.

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

sorry for the mistake !!!! (combustion dome offset ) , for sure i have choose a lower compression ratio(13.5 /1) , keeping 50 per cent for the widht off the squish band (programmable ignition)..

thank you

Frits Overmars: "In my definition a dome or hemispherical head has an offset of zero whereas a toroidal head has a positive offset."

Perfectly 're right.
I hope I will not be misunderstood , but I think, if we examine cylinder head with piston dome ,for example rsa cylinder head and piston, it is true that the cylinderead is zero offset, but on the really combustion chamber we have positive offset (toward the combustion chamber volume center).
I 'm trying to plan this in mind, the cylinder heads. Flame front smallest possible way to travel around: Reducing the expected burn time.
I saw audi engine factory, maybe by newest r5 tfsi (no serial production), for stroke with toroidal piston crown.
Sorry my bad english.

Frits,

I read earlier in another topic, you works on engmod2t powerband calculator. The program make a power curve definite integral, between used crankshaft revolution?
This is a great feature, several times tricky to compare the different power curves, accurately the difference curve between the engine power curves and the driving resistance.
I look forward to the results.

Frits, I'm confused. Your combustion chamber diagram shows X4 as zero and X1 as 54. Doesn't this mean a 108 bore?

gpracing a écrit:

Frits, I read earlier in another topic, you work on engmod2t powerband calculator. The program make a power curve definite integral, between used crankshaft revolution?
This is a great feature, several times tricky to compare the different power curves, accurately the difference curve between the engine power curves and the driving resistance.
I look forward to the results.

Yes, GPracing. I am happy to confirm that my Power Range concept will be incorporated in the next version of EngMod2T. And I am also happy to show the accompanying text and illustrations here on Pit-Lane.

Peak power alone does not say much about the usefulness of an engine. The combination of power curve and transmission, the sort of application that the engine will be used for, the abilities of the rider and the condition of the track, together define which is the optimum engine character.
An example: a 125 cc road race engine can always be kept between 10.000 and 14.000 rpm thanks to its six-speed gearbox. Whether this engine produces 2 hp or 20 hp at 6000 rpm, is unimportant.
But a kart engine with direct drive without a gearbox has trouble staying above 5000 rpm in slow corners; it only can manage to do so by means of a very short gearing that forces the engine to rev over 17.000 rpm on the straightaway. For such a kart it is imperative that acceleration out of slow corners does not cost too much time. There may be only one such corner on the whole circuit, but time lost there cannot be made up by a higher top speed on the straight.
Therefore a sensible tuner will not concentrate on peak power; he will make sure that the power is never really bad in the whole range from 5000 to 17.000 rpm (in this kart-example).

I prefer to work not with a power band but with a power range, which I defined as the highest rpm of a power curve, divided by its lowest rpm.
Experience has taught what kind of power range is needed for a certain application. Road racing calls for a range of about 1,4 . When a CVT is involved, I imagine 1,2 or even less might be enough. Motocross calls for something like 2. A touring bike needs at least 3 to be comfortable. And the direct-drive kart in the above-mentioned example needs about 3,4 (17.000/5000).

Let us assume that we have a measured power curve from 7000 to 14000 rpm. That gives a power range of 2.
Within this power curve all possible range values are investigated, from range=1,00; range=1,01; range=1,02; etc, up to range=2.
For each of these range values the whole power curve is examined in order to find which lower and upper rpm values yield the highest average power.

For range=1,5 for instance, we start with calculating the average power between 7000 and 10.500 rpm. Then we proceed with 7010--10.515; then with 7020--10.530; and so on, until the final possibility of 9333--14.000 rpm. And the highest value found is stored as the average power for range 1,5 , together with its corresponding lower and upper rpm limits.

All stored values for average power are displayed in range graphs. So when you are preparing an engine that will need a range of 1,6 , you can see at a single glance which engine delivers the best average power at range 1,6 . Furthermore you can see between which rpm values this engine is most effective. And you don't need to fall into the trap that all engine builders have fallen into at some time: attaching too much importance to peak power.

Maybe you are used to comparing the acceleration times of different engines on an inertial dyno. But that only makes sense if these engines are all run between the same initial rpm and the same final rpm, and all with the same gearing.
The range concept does not suffer from any of these limitations; it functions under all circumstances. And because you can compare the range graphs of all your engines, you will be able to distinguish much quicker which range is best for a specific application. That is an experience you would otherwise only gather after years of trial and error.


Suppose you are preparing an engine for an application that requires a power range of 1,5. Then which of the two power curves below should you pick? Hard to say, isn't it? But when you look at the power range curves, it is clear as day: the yellow curve wins.

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


Two more power curves. The yellow curve does not look very useful. But maybe it could work with a CVT.... The power range curve tells us what we need to know. If the CVT can keep within a 1,2 rpm range, the yellow curve is OK. And if the CVT can keep within a 1,1 rpm range, the yellow curve is a winner!

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


Another advantage of the power range curve: it can show you the rpm limits you should operate between.
At range 1,2 the yellow power curve works best between 9970 rpm (the blue curve) and 11.964 rpm (the white curve).

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

In the above Power Range text I mentioned average power. Average power is not the area under the power curve. A claim like this calls for an explanation, so here goes.

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

Take a look at the power curve above. It is a peculiar curve where at rpm-point E the power totally collapses, but then comes back. The reason I drew it like this will become clear in a minute.
We can look at the red area under the curve and convert that to a rectangle. The yellow rectangle in the picture below has the same area as the red areas under the original power curve. So you could say the average power between rpm-point A and rpm-point B has a magnitude equal to the height A--D. Or could you?

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

Let us put the engine with the peculiar red power curve in a vehicle and start accelerating, from rpm-point A to rpm-point B. How long will that take? It will take forever because the power at rpm-point E is zero, so acceleration at point E is zero; we will never get past that point. Acceleration from A to B will take an eternity which means that average power between A and B is zero!


Admittedly you won't encounter a power curve like the one in the first red picture very often. But the acceleration versus area argument holds for any power curve. Let's take a look at a 'normal' curve without a zero-power point. The curve below is split up into 9 equal rectangles, each with a horizontal dimension A--B that represents rpm, and a vertical dimension A--G that represents power.

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

When you convert the sum area of those 9 rectangles into a single rectangle with the same rpm spread A--F, it will have a height A--H that is 1,8 * A--G. Assuming A--G equals 1 HP, then the average power A--H would be 1,8 HP (picture below).

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


Now let us look at the same power curve and determine the average power via the acceleration approach (picture below). We will assume that acceleration from A to B with the available power A--G takes 1 second. Between B and C the power is twice as much, so acceleration from B to C will take ½ second. And so on.
Total acceleration from A to F takes 20/6 s. If this acceleration were to have a constant value, it would take 1/5 * 20/6 = 4/6 s from A to B. This would require a constant power of 6/4 * A--G. Assuming again that A--G equals 1 HP, then the average power would be 1,5 HP instead of the 1,8 HP that came out of the area-approach...

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

By the way, if you really want to grasp the difference between the 'area under the curve'-approach and the acceleration approach, put another 97 yellow rectangles on top of the 3 rectangles between C an D in the above picture, and repeat the area-calculation and the acceleration-calculation of average power.

lodgernz a écrit:

Frits, I'm confused. Your combustion chamber diagram shows X4 as zero and X1 as 54. Doesn't this mean a 108 bore?

This is the accepted way to quote X-coordinates for CNC-lathes: as diameters rather than radii. A lathe-operator only needs the coordinates shown in the drawing to produce a perfect copy of the RSA combustion chamber.

Frits Overmars a écrit:

lodgernz a écrit:

Frits, I'm confused. Your combustion chamber diagram shows X4 as zero and X1 as 54. Doesn't this mean a 108 bore?

This is the accepted way to quote X-coordinates for CNC-lathes: as diameters rather than radii. A lathe-operator only needs the coordinates shown in the drawing to produce a perfect copy of the RSA combustion chamber.

Oh. I see. All those sleepless nights for nothing...

Good articles, but missed a important thing: a bad driver factor, as me. :)
this requires more-more widht powerband :) :)
I have a little articles with same theme, might be of interest for pictures, but text i cannot translate to english:

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

gpracing a écrit:

Good articles, but missed a important thing: a bad driver factor, as me.

No worries, GPracing; I did not forget you:

Citation :

The combination of power curve and transmission, the sort of application that the engine will be used for, the abilities of the rider and the condition of the track, together define which is the optimum engine character.

Thank you Frits for sharing.

Frits Overmars a écrit:

The combination of power curve and transmission, the sort of application that the engine will be used for, the abilities of the rider and the condition of the track, together define which is the optimum engine character.

Yesterday it was too late , sorry for my inattention.
But I forgot one thing. A friend of mine, Excel Wizard Csathó Ákos, produced a driving dinamics xls.
Unfortunately only in Hungarian language, and difficulty in good parameterization, but useful for illustrating the thing. Count acceleration, top speed (with diagramm):

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

Hi,
does anybody know the usual oversize measurements (undersize sandcore for casting) for machining the final cylinder bore?

The final bore is:
54mm aluminium or gray cast
60mm aluminium or gray cast
67mm aluminium or gray cast

Regards
Michael

Frits,

I asked you in an earlier post and you did not answer me, so as i did not find the answer all along this topic, if you could explain me, us, what it the advantage to use a stringer in an exhaust pipe,? what it would improve compare to a basic pipe for ending the exhaust systme?

Thanks
Pierre

I think that the stinger isolate the expansion chamber from the outside, what is after the stinger is less important than on a conventional "tube de fuite", for instance a muffler or a longuer output.

pierre95 a écrit:

i read many time this topic and i did not find the answer to my question. So, can you explain me what is the advantage to use a stringer in the exhaust pipe, what does it improve ? Torque? Max power? Larger power band? all of that? or mainly one first ?

pierre95 a écrit:

I asked you in an earlier post and you did not answer me, so as i did not find the answer all along this topic, if you could explain me, us, what it the advantage to use a stringer in an exhaust pipe,? what it would improve compare to a basic pipe for ending the exhaust systme?

I have no idea what a stringer is, Pierre.

EDIT: The above post written by Bentou lets me assume that you meant stinger. I myself never use the word stinger; it is american for tailpipe: the basic pipe ending the exhaust system, as you describe it. Can you see why I did not understand your repeated question at first? You asked about the difference between two words that indicate the same part.

Maybe you were thinking about a restrictor and the end of the reflector cone; it serves to control the wave reflection in the exhaust pipe. I specified its dimensions in the FOS exhaust concept that I posted in this thread before.
Controlling the exhaust gas outflow via a restrictor is better than via a tailpipe because it is not so susceptible to dents, carbon build-up etc. Bentou was perfectly right with his reaction.
If the exhaust pipe outflow is controlled by a stinger, you can make the tailpipe as long as you wish, provided its diameter is clearly bigger than the restrictor diameter (this information too can be found in the FOS exhaust concept). And when you are free to choose the tailpipe length, you can put the silencer somewhere out of harm's way.
The restrictor contributes to the efficiency of the exhaust pipe; when all other engine parameters are correct, the restrictor improves all the factors you mentioned above.

question about that, in its concept fos claimed to be indifferent to the length of the tail pipe after the counter-cup, right now am developing this exhaust and after very many tests only in that part, have made significant changes in performance, changing length and diameter, including the silencer !, and viewing restriction option and more tube diameter has not been the best, in the end we opted to go by reducing the diameter at losing power after long tube that was cut to find the optimum point . I realize that the development of exhaust is very laborious, and although you can have a base through formulas, the end result is always the truth dyno.
[Vous devez être inscrit et connecté pour voir ce lien]

ktuningteam a écrit:

..although you can have a base through formulas, the end result is always the truth dyno.
[Vous devez être inscrit et connecté pour voir ce lien]

Nice pictures, Ktuningteam, but I missed one thing: a dyno power curve.

ktuningteam a écrit:

question about that, in its concept fos claimed to be indifferent to the length of the tail pipe after the counter-cup, right now am developing this exhaust and after very many tests only in that part, have made significant changes in performance, changing length and diameter, including the silencer !, and viewing restriction option and more tube diameter has not been the best, in the end we opted to go by reducing the diameter at losing power after long tube that was cut to find the optimum point . I realize that the development of exhaust is very laborious, and although you can have a base through formulas, the end result is always the truth dyno.
[Vous devez être inscrit et connecté pour voir ce lien]

I fully agree with you, the tailpipe diameter and length still make a considerable difference, as does the silencer!
Also bends etc make quite a difference, and should always be compared with a straight pipe.
Sometimes even a straight pipe is not the best!
And then there is the way of testing.....
It is all really quite complicated!
For example the way the pipe reaches it's working temperature going through the gears is not easy
to simulare on a dyno!

What is !! but the exhaust I have designed in conjunction with others, and they want to keep secret, I respect their decision, the championship is at stake, has many levels and do not want to give any information to rivals. 24cv to 13500 rpm. 80 cc (a wheel). I can tell you that my opinion is very low power and can reach much more, My purpose is to 27/28 hp at 15,000 rpm, but we are in first class last race did record time, so I guess the rest rivals are in the same or less power

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