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

Jan Thiel a écrit:

This still is an unresolved problem! I was thinking about a way to reduce engine power without closing the throttle. But how can you do this? I did not find a solution before I retired.
And nobody else was really interested.

How about much larger travel on the exhaust power valve.

With a shorter exhaust port duration power is lower. And this is already controlled with the stepmotor over the ECU.

Mic a écrit:

Jan Thiel a écrit:

This still is an unresolved problem! I was thinking about a way to reduce engine power without closing the throttle. But how can you do this? I did not find a solution before I retired.
And nobody else was really interested.

How about much larger travel on the exhaust power valve.

With a shorter exhaust port duration power is lower. And this is already controlled with the stepmotor over the ECU.

This causes detonation (auto ignition)
The problem is that the burned gases do not exit the cilinder!
Retarding ignition also does not make sense.
As you have an AUTO-inition problem!
So the engine does not 'listen' to its ECU anymore!
What you would need is the same fresh gas flow, but with less HP!
Not easy to achieve!
A variable tailpipe might help.

i've seen variable motorised head dome prototype , less compression => less Hp and no pre ignition but more revs ....

Haufen,

We also had such a test bench at Aprlia.
The prototype of this test bench was developed by Apicom in collaboration wit Aprilia.
So we had it first, and now anyone can buy it.
It was helpful but not what I wanted.
A so-called step test.
And without the airbox!

Mic a écrit:

How about much larger travel on the exhaust power valve. With a shorter exhaust port duration power is lower. And this is already controlled with the stepmotor over the ECU.

Like Jan wrote, a shorter exhaust duration will worsen the detonation. What happens is this:
During normal operation, the blowdown time.area of the exhaust ports is sufficient to drop the cylinder pressure below the crankcase pressure before the transfer ports open, even at high rpm.
At part-throttle that cylinder pressure will drop to the same level, but now the crankcase pressure is much lower and exhaust gases will enter the transfer ducts, contaminating and heating the fresh mixture even before it enters the cylinder.
A theoretical solution would be a power valve that enlarges the normal exhaust timing instead of lowering it. But that is impractical as it would ruin the shape of the exhaust duct and it would cause cooling problems in the cylinder's exhaust area.
A variable tailpipe area, like Jan says, can be a more practical approach. I designed a simple solution, shown in the drawing below, but then two-stroke development at Aprilia was terminated because of Dorna's ban on two-strokes
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This could very well have been the solution.

Another way to lower HP without sacrificing piston cooling would be to richen the mixture when you want to lower the power. With a signal to a fuel enrichening solenoid you could achieve a power range. It would then be instantaneous and programmable. Not enviornmentally friendly but would work for racing. The mixture ratio difference from high power to low power would need to be just rich enough before a misfire and just lean enough for sustained high power.

The variable tailpipe idea will work but I suspect the pipe temperature would drop off and it would take several seconds to regain full power. But then again rich mixture may have the same problem.
Two strokes are like redheads. Hard to figure out and very tempermental. But when they are happy they are a lot of fun!
HG

I thought about that until I remembered my 1973 Honda CR125. Piston port motor, delivered jetted slightly rich, and no Keihin jets or needles available. The wrong combination of throttle and rpm resulted in a crankcase full of fuel which required a good minute or so to clear: sitting at the side of the track, throttle wide open and engine barely turning as great plumes of smoke billowed out of the pipe... "loading up" was the expression.

Give me a cold pipe any day

Just dream of an engine with all variable ; exhaust pipe with servos to modify lengh etc , servos for cylinder ports etc etc ... What a mess for the setting but probably a hudge torque curve !?

A completely different question: I notice that the RSA uses a mechanical water pump, whereas on Japanese bikes it became quite trendy to use a battery-powered electric pump instead, at least in national races. Are the losses in the Aprilia pump too small to worry about, or was there an issue with using batteries for the length of a GP?

I think I saw a photo of a KTM also with an electric pump...

Frits Overmars a écrit:

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Very clever idea Frits. Presumably there would also be a "sound meter option" that would put it in the full power position at part throttle....

GrahamB a écrit:

A completely different question: I notice that the RSA uses a mechanical water pump, whereas on Japanese bikes it became quite trendy to use a battery-powered electric pump instead, at least in national races. Are the losses in the Aprilia pump too small to worry about, or was there an issue with using batteries for the length of a GP?
I think I saw a photo of a KTM also with an electric pump...

You are right about the KTM (first picture) but I cannot recall any genuine Japanese racing bike with an electric pump.
The RSA125 was originally desiged to work with an electric pump. But as it turned out, none of the available pumps even came close to the flow Jan wanted, so a mechanical pump was fitted as an afterthought. Jan and I both feel that it is the ugliest aspect of the RSA engine. But it works...

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We use both the Bosch electric pump and a mechanical pump. The main reason is to have a means of cooling thev motor during a restart without the engine running. Also having both we found the engine runs 10 degrees cooler. We do have the advantage of having a heat exchanger that we fill with crushed ice before the race. The ice is replenished as we drive by the track throwing ice into it. Our engine runs at around 90-100 degrees farenheight. Once during as particularly wet and slushy race the engine temperature never went above 80 degrees. Dyno tests show best power at around 85-90 degrees. We have had temperatures as high as 140 degrees on a particularly cold and clean ice track. I have spoken with some kart racers that could only wish for that kind of cooling.

Jan Thiel a écrit:

The entering fresh charge is ignited by the remaining, hot, burned gases!
You can see very severe damage to the piston after maybe 10 seconds at 20% throttle.
This still is an unresolved problem! I was thinking about a way to reduce engine power without closing the throttle. But how can you do this? I did not find a solution before I retired.
And nobody else was really interested.

On aftermarket ignitions I use there is a mode I can switch on where a number of indivdual sparks are cut, depending on throttle position. It is now only configured for cutting 1 in every 3 sparks on over run(tps <10% with high rpm). Would that sort of system solve the part throttle detonation? If the number of sparks cut could be varied along with TPS for this to begin and end? If it would be helpful I'm certain the manufacturer would only need a software change to do this.

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

GrahamB a écrit:

Remember that retarding the ignition is used at high rpm to increase exhaust temps and effectively shorten the pipe. So it's likely to increase the heating of the front edge of the piston...

Indeed, retarding too much caused detonation!

GrahamB a écrit:

A completely different question: I notice that the RSA uses a mechanical water pump, whereas on Japanese bikes it became quite trendy to use a battery-powered electric pump instead, at least in national races. Are the losses in the Aprilia pump too small to worry about, or was there an issue with using batteries for the length of a GP?

I think I saw a photo of a KTM also with an electric pump...

The flow of the electric pump proved to be insufficient.
We needed about 4 times the flow of the electric pump!
This really made quite a difference to detonation.
And the power loss caused by the mechanical pump was very small.
Power was helped by the better cooling.
So the nett loss was quite small.

Howard Gifford a écrit:

We use both the Bosch electric pump and a mechanical pump. The main reason is to have a means of cooling thev motor during a restart without the engine running. Also having both we found the engine runs 10 degrees cooler. We do have the advantage of having a heat exchanger that we fill with crushed ice before the race. The ice is replenished as we drive by the track throwing ice into it. Our engine runs at around 90-100 degrees farenheight. Once during as particularly wet and slushy race the engine temperature never went above 80 degrees. Dyno tests show best power at around 85-90 degrees. We have had temperatures as high as 140 degrees on a particularly cold and clean ice track. I have spoken with some kart racers that could only wish for that kind of cooling.

Cooling with ice would be a dream come true!
Did you have a power loss below 80F (30C)?
And caused by what, in your opinion?
Diminishing bearing clearance maybe?
Or a combustion problem?
We tried from 40C to 90C, every 5C more caused 0,5HP power loss!

Institute of TwoStrokes a écrit:

Jan Thiel a écrit:

The entering fresh charge is ignited by the remaining, hot, burned gases! You can see very severe damage to the piston after maybe 10 seconds at 20% throttle.
This still is an unresolved problem! I was thinking about a way to reduce engine power without closing the throttle. But how can you do this?

On aftermarket ignitions I use there is a mode I can switch on where a number of individual sparks are cut, depending on throttle position. Would that sort of system solve the part throttle detonation?

As Jan pointed out, once you have auto-ignition, the engine does not listen to its ECU any more. So you would have to start skipping sparks well before the onset of detonation.
In a foul-stroke your proposed system does work, but a two-stroke would react far from linear. For example, if you skip 1 in 4 sparks, you will loose much more than 25% of engine power because that one missing spark will cause the gasdynamics processes to collapse. The main problem would be to realize a smooth transition from intermittent to full ignition.

Hi Jan. I suspect the power loss is due to incomplete atomization of the fuel at colder engine temperatures. High octane fuel (118 octane) will not even fire at cold temperatures so we have to keep the sleds warm just to start them. Also we suffer what we call "cold seizures" if the engine is too cold. Pistons grow but the cylinder doesnt causing a 4 point seizure. I found our engines make the biggest power at 85 degreesF ( 30C ) coolant temperature. Some of our race track ambient air temperatures can be down to -40C (same in celsius as farenheight) Power output increases quite dramatically with cold air like this but so does the risk of detonation. Air density sometimes is 110 or higher. It throws the jetting chart out the window!!!

"In a foul-stroke your proposed system does work, but a two-stroke would react far from linear."

Was that a Freudian slip Frits or just a misspelling? Im not a fan of foul (four) strokes either!!
They belong in rototillers

Howard Gifford a écrit:

Was that a Freudian slip Frits or just a misspelling?

Neither, Howard. It was quite deliberate.
I can do it in other languages as well: viestakt versus viertakt (dutch), falschtakt versus viertakt (german).
Other suggestions, anyone?

Jan Thiel a écrit:

Retarding ignition also does not make sense.
As you have an AUTO-inition problem!
So the engine does not 'listen' to its ECU anymore!
What you would need is the same fresh gas flow, but with less HP!
Not easy to achieve!
A variable tailpipe might help.

I think I expressed myself unclearly. What I meant was the following:
Imagine your engine with the throttle opened just above the auto-ignition range. Then you have sufficient transfer flow, but too much power. To lower the power, now retard the ignition. Then you still have sufficient transfer flow, but with less power.

I think Honda used auto-ignition to their advantage on two-strokes. As far as I remember they did it with a (very) variable exhaust power valve.

Frits Overmars a écrit:

Mic a écrit:

How about much larger travel on the exhaust power valve. With a shorter exhaust port duration power is lower. And this is already controlled with the stepmotor over the ECU.

Like Jan wrote, a shorter exhaust duration will worsen the detonation. What happens is this:
During normal operation, the blowdown time.area of the exhaust ports is sufficient to drop the cylinder pressure below the crankcase pressure before the transfer ports open, even at high rpm.
At part-throttle that cylinder pressure will drop to the same level, but now the crankcase pressure is much lower and exhaust gases will enter the transfer ducts, contaminating and heating the fresh mixture even before it enters the cylinder.
A theoretical solution would be a power valve that enlarges the normal exhaust timing instead of lowering it. But that is impractical as it would ruin the shape of the exhaust duct and it would cause cooling problems in the cylinder's exhaust area.
A variable tailpipe area, like Jan says, can be a more practical approach. I designed a simple solution, shown in the drawing below, but then two-stroke development at Aprilia was terminated because of Dorna's ban on two-strokes
[Vous devez être inscrit et connecté pour voir cette image]

I think I have not gotten behind the variable tailpipe idea, yet. What would you like to vary with it and to achieve which effects? At little throttle openings the pressure inside the exhaust pipe is already very close to atmospheric pressure (if not even) on most engines. And if you had say 100mbar inside the pipe at the critical throttle opening, then the engine might have had more power with a bigger tailpipe.

'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?