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

Frits Overmars a écrit:

'Opening' the end of the reflector will cause a substantial weakening of the reflected pulse and thus less charging of the cylinder. Izze simple, no?

I think I am stuck on the "substantial". But then again I don't know about the area the engine is likely to auto-ignite. But I got the point now :-) Can you outline the area where auto-ignition occurs?

If one would combine the variable tailpipe idea with the idea I suggested the effect woud be even bigger, as you need more presssure in the crankcase and less in the exhaust pipe. Variable transfer timing would be nice also, if feasible.

Howard,

High octane fuel usually contains a lot of Toluene.
Once Honda had problems wit fuel ionisation in their Formula 1 engines.
They resolved it by pre-heating the fuel!

Haufen a écrit:

Variable transfer timing would be nice also, if feasible.

That would be my ideal. Lowering all the transfer roofs would shorten the transfer timing and lengthen the blowdown timing, so the cylinder pressure would drop further before the transfers would open. It would cure the hig revs/low power-detonation and it would improve the power band because a too early-returning exhaust pulse would have less opportunity to push the fresh cylinder contents back into the crankcase.
A controllable transfer height would even make a throttle valve unnecessary.
There's only the little problem of how to build it...

Haufen,

Auto-ignition usually occurs between 10 to 40% throttle at high revs.
In fast, non full throttle corners.

Retarding the ignition was tried to diminish power.
This makes the exhaust very hot.
Then, when you need full power, it is not there because the exhaust temperatures are wrong.
This takes a little time, when the engine is back to full power you are already at the end of the straight!
The same goes for water injection in the exhaust.
It was tried by Rotax about 25 years ago.
There was a LOT more power at low revs, so the rider had to take it easy when opening the throttle.
But the engine revved a little bit less, because the exhaust temperature did not recover at high revs.
And lap times became actually slower.
After a day of testing the system was switched off.
Lap times immediately improved!

A very important thing when accelerating is the power you have after changing gear.
Spark interruption may be not so good for this!
As I did not have the dyno I wanted this gearchange effect could not be tried on the dyno, very regrettably!
Retarding the ignition and weakening the mixture by powerjet can also have a negative effect on this.
The exhaust temperature should be 'Right' for the No. of revs after you change gear.
If the temperature is too high there will be less power!
So it is REALLY complicated!

Jan Thiel a écrit:

The exhaust temperature should be 'Right' for the No. of revs after you change gear.
If the temperature is too high there will be less power!
So it is REALLY complicated!

Could this not be partially fixed by using gear-dependent maps? I guess then the problem is if the rider wanted to use over-rev to avoid a gear-change before the next corner, the map would kill the over-rev Of course with GPS-dependent maps...

A really dumb question: when you went to the dyno with a new set-up (different pipe or cylinder for eg), how did you go about jetting it? Via detonation detector and EGT, or were you able to use a lambda of some sort that could resist the oil?

Top level bikes like the RSA even HRC A kit or copies uses ECU with a mapping that uses a gear sensor ......

Jan Thiel a écrit:

A very important thing when accelerating is the power you have after changing gear.
Spark interruption may be not so good for this!

The new "Seamless shift" gearbox with double dog rings used by Honda on their MotoGP bike would solve this problem as it doesn't need ignition cutting.

Seamless shift gearbox

It still involves the rpm changing. The point is that at high rpm you are using retarded ignition to increase exhaust temp to increase the speed of sound in the pipe, which re-tunes it for higher rpm. However when you change gear you'd like the exhaust temp to change instantly to suit the lower rpm... but it doesn't.

I'd have thought cutting the spark may actually help by flushing a bit of raw fuel into the pipe... assuming it didn't ignite.

Mic a écrit:

Jan Thiel a écrit:

A very important thing when accelerating is the power you have after changing gear. Spark interruption may be not so good for this!

The new "Seamless shift" gearbox with double dog rings used by Honda on their MotoGP bike would solve this problem as it doesn't need ignition cutting.

The system with double dog rings is from Zeroshift. Honda uses a different principle (I wrote something about the Honda-gearbox a while ago here on Pit-lane).
But you are right: a seamless gear shift system would make ignition cutting unnecessary and keep the gasdymnamics in the engine on full song all the time. I had some discussions with the Zeroshift people in 2007 but Aprilia's race director said he did not need such a system....

GrahamB a écrit:

Jan Thiel a écrit:

The exhaust temperature should be 'Right' for the No. of revs after you change gear.
If the temperature is too high there will be less power!
So it is REALLY complicated!

Could this not be partially fixed by using gear-dependent maps? I guess then the problem is if the rider wanted to use over-rev to avoid a gear-change before the next corner, the map would kill the over-rev Of course with GPS-dependent maps...

A really dumb question: when you went to the dyno with a new set-up (different pipe or cylinder for eg), how did you go about jetting it? Via detonation detector and EGT, or were you able to use a lambda of some sort that could resist the oil?

Gear dependent maps were already used in 1998.
We also had GPS dependent maps later.

When testing something new of course you start with a safe, rich, mixture.
Then you jet down until a certain limit is reached.
We mostly looked at the piston surface, looking through the sparkplug hole after each run.
Lambda sensors were not used.

GrahamB a écrit:

It still involves the rpm changing. The point is that at high rpm you are using retarded ignition to increase exhaust temp to increase the speed of sound in the pipe, which re-tunes it for higher rpm. However when you change gear you'd like the exhaust temp to change instantly to suit the lower rpm... but it doesn't.

I'd have thought cutting the spark may actually help by flushing a bit of raw fuel into the pipe... assuming it didn't ignite.

That could really be true!
But I had no way of knowing this because at the dyno no gearchanging could be done!
Ideally you would have a very quick-changing exhaust temperature.
I think it would be interesting to isolate the pipe by coating it , preferably on the inside!
By blowing some air on the outside of the pipe the engine would lose 2000-3000 rpm.
We could interrupt the spark on the dyno, with max. power carburation the engine would run on without spark!
This caused problems on-track of course: you could not change gear!
So you really could not use max. power carburation!
Gearchanges were done at 13250.

Jan Thiel a écrit:

I certainly thought about cutting sparks.
But remember: the problem was AUTO-ignition!

Yes I see but wondered if raw fuel would mitigate the effects. Seems best defence is attack with the right hand......."when in doubt go flat out"

Sorry to disturb you, gentlemen!

As you probably don't know, there is an automatic process to split a topic after 50 pages...

The first part of this gorgeous topic is there : [Vous devez être inscrit et connecté pour voir ce lien]

Whew! Thanks Marc. I thought I was going to have to sell copies..

guys, I would appreciate if u take a look at this topic:

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

any comments are welcome!

I have taken a look, but , so sorry, I cannot see the pictures.
Only a frog in a bloc of ice.
Did I do something wrong?

I want the thread that jan went to. I saw an engine on the one I follwed

it works for me jan? wax did you see the pics?

but i added the direct links so you can access them by clicking...already appreciate that you wanted to look at it jan!

Hi Jan,
I noticed that on the Aprilia cylinder drawings that Frits displayed, the auxiliary exhaust ports exit the cylinder approximately at 12.37 degrees to the centre line of the cylinder.
With the RSA auxiliary exhaust ports being wider, could you tell me what angle did you aim them at?
Was this angle chosen for the exit efficiency or charging efficiency?

The drawings show that the auxiliary exhaust ports opened fully at one timing but the available photos of the RSA port appear to be varied - is this right?

Many thanks for your answer

The auxiliary ports of the Aprilia RSW and RSA are exactly the same.
When developing the RSA engine at Derbi we used Aprilia cylinders.
We could do this because both Aprilia and Derbi were owned by Piaggio.
Later a new cylinder was made, with better cooling and a different distance of the fixing holes to the crankcase.
But all the transfer and exhaust ducts remained exactly the same.

The best exit angle was determined on the flowbench.
Several years before the RSA was made.
Around 1998 if I remember well.
After that it was never changed.
The angle was chosen for maximum exit efficiency.
The design publicized by Mr Overmars is somewhat older, about 1995 I think.

Hi Jan,
Thank you for answering so many of my questions. I appreciate very much your generosity in sharing information.
On the RSA, where the auxiliary exhaust ports join into the main exhaust port, they are nearly separate identities to the main exhaust port unlike many other motors in particular the Japanese GP and the earlier Rotax 256 designs where they rejoin the main port very quickly and at a greater angle.
Can you comment on the relationship between the flow in this area and what you found worked better– should they blend in at a low angle to avoid turbulence or kept separate as long as possible?

Thank you again for your insights into this area.

I tried to keep them separated by making the 'dividing wall' longer.
I did this mainly to keep it from cracking!
It resulted in power loss and a lot less max. revs
So I had to make the 'dividing wall' as short as possible.
Sometimes it cracked when a piston seized, then it was impossible to repair.
Enlarging the auxiliaries gave a big iprovement!
This was done in 1998 I think.
Later we started CNC machining the exhaust duct, around 2000.
And we made the main exhaust port a bit narrower, 40mm.
Widening the exhaust duct at the flange brought us 0,7HP
I first tested the cylinder at the dyno, took it off and went to the CNC workshop.
They machined it while I waited for it, in half an hour.
When I returned to the factory and retested the cylinder it gave 0,7HP more.
All done in less than 2hours!

Did you stop there or did you return to the CNC workshop for a little more widening
Will you tell us the final width and height of the RSA exhaust exit?

Many thanks for your answers Jan.
Talking to you and Frits is very much appreciated. I have thought about these questions I have asked you for many years and I have not had the resources available to pursue the answers. Knowledge is one of the best gifts to give away - again thank you.

Further widening gave no extra power.
I do not remember the exact height and width.
F. Payard will surely know this!
But I estimate a height of about 33 and a width of 46 or maybe 48.

Thanks Jan,
Do you think that the nearly round exhaust duct that the Japanese GP motors generally use is a mistake?