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

Thank you Howard for your response about the port shaping. Have you any idea how much gain you got from those changes?

About the drawings of the Aprila RSA 125. Looking over those drawings it seems some of the drawings are of a different engine or a different state of tune. In other words a couple of the drawings don't match other ones, and I don't see a sectional one of the boost port, (port c), in relation to the other transfer ports. Just wondering if I missed one somewhere.

Also was there any work done inside the air box to promote laminar air flow?

Other questions are what the deck height is at TDC?
Also about the head. I saw a picture of the head as a toroidal shape. How close was the plug to the piston head and what was the spuish band ratio used and was the head changed according to the race track?

jfn2 a écrit:

About the drawings of the Aprila RSA 125. Looking over those drawings it seems some of the drawings are of a different engine or a different state of tune. In other words a couple of the drawings don't match other ones, and I don't see a sectional one of the boost port, (port c), in relation to the other transfer ports. Just wondering if I missed one somewhere.

The 2D drawing from Frits was of the old APC cylinder which came around year 2000 on the RSW125/250.

The RSA125 used a much newer APF cylinder.

Howard Gifford a écrit:

Maximum speed is in the 165 KMH range dropping to 100 in the corners.

You can work out the max theoretical pressure benefit in Pa as
0.5*density*vel²
with SI units.
165km gets you 1.3kPa, so 1.3% more pressure. However you want density and the thermodynamics give you some temp along with the pressure... so multiply by about 2/3 to get the density increase, about 0.85%.

That's the same benefit as keeping the air colder by 2.3°C.

However I imagine there are carb icing issues on a snow mobile...

Frits, you and I think very much alike regarding the tail end of the pipe. I'd like to gather my thoughts before saying more.

On a different question more aimed at Jan, how do you go about choosing the deck height? That's the term I know but maybe you call it something else: where the case stops and the cylinder starts. I understand you have to clear the bearings and all, but where is the best place to design the split?

That is easy to answer Brian.

You have the crankshaft, and an inlet hole somewhere, on the side or behind.
And you need some thickness above it, about 10mm.
That's all!

Howard Gifford a écrit:

Frits i tried your idea of a connecting tube between the carbs and it resulted in a drastic loss of power and a reduction in both air flow and fuel flow. I suspect the connecting tube allows one crankcase to rob from the other.

My internet has been down for quite some time (long story), so only now can I answer.
The cross-over connecting tube is connected to both inlet tracts between the carb and the reed valve, so the reeds will make sure that one crankcase cannot steal directly from its neighbour. What it can do, is steal some of the ram pressure that occurs at the end of each inlet phase. On the other hand, breating will become much easier for each crankcase because the combined cross-area of both carbs is always available.
Air flow through each carb will become more uniform, so the suction signal at the carb's needle tube will be weaker. That means the mixture will become leaner unless you compensate with larger jetting. Did you work in that direction?

Hi Frits. I tried richer jets but the engine didnt like them. The bsfc went up but the power went down.. This is for a rotary motor remember, with a single inlet plate that controls both inlet ports. There is a substantial time that both inlet ports are open so I suspect the suctiion from one side reaches into the open crankcase of the other. Airflow went down to 165 cfm with the crossover and it is around 190 without it.

Jan or others, how did you control the tuned pipe wall temperature (or EGT directly?) when testing on dyno? This seemed the most difficult thing to mimic when testing either the GP engines at QUB or my R/C boat engines, in the lab. The inertial dyno or computer controlled brake seem to work best because we can match the test engine's acceleration with reality. On steady-state testing, EGT would simply climb 500, 600, 700, 750 °C until way past reality and the piston would seize.

This has always been one of Jan's greatest handicaps. He has asked for an inertial test bench over and over, but Aprilias race director Witteveen, or The Great Leader as we call him, never deemed it necessary....

When EGT goes up and up there should be some serious problem with the engine.
We never had pistons seize during our steady state tests.
Working on the dyno continuously 5 days a week!
I am 100% convinced our engine could have run for 6 hours at max power without seizing.
The problems arise when you close the throttle, or run part throttle!
The piston is mainly cooled by the transfer flow.
And at part throttle there is less transfer flow, causing detonation (auto ignition)
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.
At 100% throttle the engine was undestructible!
By making the transfer ports as wide as possible we had very good piston cooling.

As Frits has written, I would have liked to have also a dynamic testing possibility, with a flywheel.
In my opinion you should simulate a straight, starting at around 10.000 rpm, and shift through the gears
until you reach top speed. And with the airbox as used on the bike, with a ventilator that simulates the raising
air speed as it would be on track! Maybe it is interesting to know that without a ventilator the engine gave
less power with the airbox fitted.

I was told that such a testing system was too expensive.
And unnecessary as we won anyway!

I can also see a disadvantage of 'dynamic' testing.
Because the duration of the test is so short you can get away with very extreme (too extreme?) settings,
without damage.

Jan Thiel a écrit:

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.

Modern car engines reduce engine power by retarding the spark timing. Should work on a two-stroke also (probably within smaller boundaries as you don't want to burn into the exhaust port). Depending on how much (more) throttle opening you would need to avoid detonation, I think it could be possible to make the RSA engine bullet proof at part load also.

If you imagine a diagram showing throttle opening on one axis and rpm on the other axis, can you describe the area where the engine was prone to detonation?

Jan Thiel a écrit:

As Frits has written, I would have liked to have also a dynamic testing possibility, with a flywheel.
In my opinion you should simulate a straight, starting at around 10.000 rpm, and shift through the gears
until you reach top speed. And with the airbox as used on the bike, with a ventilator that simulates the raising
air speed as it would be on track! Maybe it is interesting to know that without a ventilator the engine gave
less power with the airbox fitted.

I was told that such a testing system was too expensive.
And unnecessary as we won anyway!

I can also see a disadvantage of 'dynamic' testing.
Because the duration of the test is so short you can get away with very extreme (too extreme?) settings,
without damage.

From what I heard Malossi has a testbench on which they can simulate the tracks they race on to finetune CVT and jetting on their racing scooters.

Reading the stories about the development of the RSA engine makes your results even more impressive to me. You had no indication system, no fuel balance, no exhaust gas analysis and no possibility to dynamically test the engine on a dyno. I guess at least Honda had almost all of that equipment if not all and used it regularly, and yet their engine could not keep up with yours! Hondas biggest mistake of that time probably was not to hire you, give you enverything you want (including tons of money and testing equipment) and let you do whatever you want.

So, a huge virtual hat's off from me to you for your achievements!

Car engine are able to turn off the fuel , and compress air flow only


(injection)

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...

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...

Yes I know, but part load egt is usually lower than full load egt. And I think most of us would prefer higher part load egt over part load detonation. Of course, how far one could go and how far one would need to go would have to be tested, and how much would be needed would depend on the engine.

Toopack a écrit:

Car engine are able to turn off the fuel , and compress air flow only


(injection)

Yes but 4 stroke doesn't need the fuel/oil mixture for lubrication like the 2 stroke does.

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...