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

Hello everyone :-)

I have a question about the different versions of the cylinders:
The drawings that where uploaded in this thread some time ago - which cylinder is this?
A "younger" one or an old version?

Thanks for answering!
Tim

Hello Mr. Jan,
looking at the drawings of the cylinder Aprilia gp, I was wondering how you have defined the horizontal and vertical angles of transfers (primary and secondary), and if tests have been made of various combinations and what results they have brought.

Thank you very much,Master.

Robertor a écrit:

Hello Mr. Jan,
looking at the drawings of the cylinder Aprilia gp, I was wondering how you have defined the horizontal and vertical angles of transfers (primary and secondary), and if tests have been made of various combinations and what results they have brought.

Thank you very much,Master.

have you not seen the drawings ? it is all very clear how the angles are defined

Today I tested different disk valve durations. The orange is 140/90, green is 145/95. This is all being done with the EFI system. The EFI continues to be of great success. It can instantly be tuned to affect change.

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Riley Will a écrit:

Today I tested different disk valve durations.  The orange is 140/90, green is 145/95.  This is all being done with the EFI system.  The EFI continues to be of great success.  It can instantly be tuned to affect change.

Don't you love it when you can change the inlet timing and the fuel amount stays as you programmed it, Riley?
Imagine doing this with a carburetter....

Frits,

This is the most fun I've had tuning in years! Keep making gains:)

Here is a curve from our Mule engine with VHSG43mm with PJ in Green vs the EFI in Red. Not apples to apples comparo, different cylinder, different days, but curves represent our optimum at the time. EFI curve is from today.

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If someone was to design a current engine based on the JBB 250cc V-twin design, what would you change?

Reverse Cylinders?
Dual Disk Valves?
Common crankcase compartment?
4 inlets?

This engine is very different and in some ways very simple. Less gears, less bearings, perfect balance, no need for a balance shaft...

For a kart (I suppose you are thinking about putting it on a Superkart) I would reverse both cylinders.
For a motorcycle I would reverse the vertical cylinder and rotate the whole crankcase assembly 45° forward.
I would not change anything else. Great engine concept!

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Frits, for a kart you say you would reverse the cylinders what is the firing phasing on this engine ?
Also what would you attempt as the optimum firing phasing for a twin kart engine given the rear drive grip a kart has

Frits,
Without wishing to denigrate the great work of JBB.
I do not understand what you find so great about the configuration of this engine.
Apart from compactness of width and length, and a perfect primary balance, what are the benefits?
I found against by a number of disadvantages: the lack of rigidity of the crankshaft? the difference in length of the intake ducts.
To fill my loopholes, could you comment on your arguments in favor of this configuration. You surely have good reasons that escape me.

@ Raw,
The firing phase is 90°, 270°.
Below, pictures of the crankshaft
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RAW a écrit:

Frits, for a kart you say you would reverse the cylinders what is the firing phasing on this engine ?

90°.

Citation :

Also what would you attempt as the optimum firing phasing for a twin kart engine given the rear drive grip a kart has

90°  . Both connecting rods are on the same axis and the angle between the cylinders is 90°.

EDIT: Francis, you were a little quicker than me .

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Frits,

Can you rotate the cylinder 180 degrees without any setbacks?

I remember Jan saying the the RSA250 lots power then they had to reverse crankshaft rotation due to new gearbox layout. This loss was then compensated with new cylinder.

So I thought that rotation and cylinder orientation was important.

fpayart a écrit:

Frits, Without wishing to denigrate the great work of JBB. I do not understand what you find so great about the configuration of this engine.
Apart from compactness of width and length, and a perfect primary balance, what are the benefits?
I found against by a number of disadvantages: the lack of rigidity of the crankshaft? the difference in length of the intake ducts.
To fill my loopholes, could you comment on your arguments in favor of this configuration. You surely have good reasons that escape me.

You already summed up some good reasons: compactness of width and length, and a perfect primary balance.
Further: simplicity (low number of parts) and hence low weight and low friction; reduced torque fluctuation during each crankshaft revolution thanks to the 90°/270° firing phase (more about this at [Vous devez être inscrit et connecté pour voir ce lien] This means less stress on clutch, transmission and tires, and the possibility to use softer, grippier tire compounds.
The air box can be smaller because both cylinders are not trying to suck air at exactly the same time. Regarding these last two items a tandemtwin engine is at a clear disadvantage.
You are right about the inlet tract lengths being unequal and about the crankshaft being less rigid, but on the other hand the crankshaft main bearings are less stressed because the connecting rod forces partly even each other out thanks to the common big end pin.
Maybe I forgot to mention some aspects, in which case I hope that Jean-Bertrand Bruneau will correct me  .

Francis, before I forget: if I ever need a superkart engine, I will come to you. Riley Will is not very interested in the European market and his engines will not respect the FIA/CIK homologation rules.

Mic a écrit:

Frits, can you rotate the cylinder 180 degrees without any setbacks?

Yes, if you don't forget to also rotate the pistons  .

Citation :

I remember Jan saying the the RSA250 lots power then they had to reverse crankshaft rotation due to new gearbox layout. This loss was then compensated with new cylinder. So I thought that rotation and cylinder orientation was important.

It is true that both RSA250-crankshafts rotated in opposite directions compared to the RSW250. But the power loss was caused by something else: the RSA had bigger big end pins, hence larger con rods, which were more in the way of the inlet flow, so the power advantage of the RSA250 over the RSW250 was less than expected.

Frits,

If the CIK gets themselves current then I would reconsider. If I was to race in Europe for 2014 I would support FPE too:happy: 

I like this JBB design. Something different than redoing the inline. Less parts and friction are a big deal for me!

Maybe I need to make another "Mule" of the JBB style?

Frits, sorry would you please confirm, when you say 270 / 90 firing sequence and the reduced stress to the tyre grip and componetry I assume you are talking about a bike ?
I suppose we have gone of topic here and should move this discussion to the relevant thread as we are discussing superkart use and related torque use abilities
I will continue my question in the relevant thread if you would be kind enough to follow
Sujet: Re: [Technique] BRC (Superkart)

RAW a écrit:

Frits, sorry would you please confirm, when you say 270 / 90 firing sequence and the reduced stress to the tyre grip and componetry I assume you are talking about a bike ?

Not just about bikes, RAW; I was talking about engines in general. Did you notice the above link to the Big Bang-topic?
The torque fluctuation has a distinct influence on anything with tires and I suppose it will have a similar influence on air and water propellers.

Riley Will a écrit:

If someone was to design a current engine based on the JBB 250cc V-twin design, what would you change?

Reverse Cylinders?
Dual Disk Valves?
Common crankcase compartment?
4 inlets?

This engine is very different and in some ways very simple.  Less gears, less bearings, perfect balance, no need for a balance shaft...

4 Inlets an EXACTWELD 250 with variable valve timing

Frits, so I understand, there seem to be two issues here:
1. The 90° cylinder axes which give
a) primary balance in the longitudinal-vertical plane;
b) minimal inertial torque, hence lower drive-train stress and tyre abuse;
c) not so much airbox depression due to reduced intake overlap compared to a tandem twin with simultaneous firing (but still worse than a 180° motor!);
2. The "almost single crank-pin" design that reduces vibration due to the rocking couple.

So the first part is an advantage of an 90° motor (even the viel-takt), and the second is specific to JBB?

Institute of TwoStrokes a écrit:

4 Inlets an EXACTWELD 250 with variable valve timing

The Exactweld tandem twin is a very nice engine, though I haven't seen one with variable valve timing yet.
But it is even more complicated than a normal tandem twin; the primary gears have to be small because of the inlet ducts, and the cranks must be contrarotating to keep the engine from vibrating itself to pieces. So there are more gears, and more losses.

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GrahamB a écrit:

Frits, so I understand, there seem to be two issues here:
1. The 90° cylinder axes which give
a) primary balance in the longitudinal-vertical plane;
b) minimal inertial torque, hence lower drive-train stress and tyre abuse;
c) not so much airbox depression due to reduced intake overlap compared to a tandem twin with simultaneous firing (but still worse than a 180° motor!);
2. The "almost single crank-pin" design that reduces vibration due to the rocking couple.

So the first part is an advantage of an 90° motor (even the viel-takt), and the second is specific to JBB?

The JBB has an 'almost single crank-pin'; a 90° foul-stroke twin has a truly single crank pin, hence even less rocking couple.
Re you point 1b: the issue is not inertial torque (the heavier the flywheels, the more the torque peaks would be smoothed down) but it is about the positive torque peaks from once cylinder coinciding with the negative torque peaks from the other cylinder, producing a smooth resultant torque.

Frits Overmars a écrit:

Mic a écrit:

Frits, can you rotate the cylinder 180 degrees without any setbacks?

Yes, if you don't forget to also rotate the pistons  .

Citation :

I remember Jan saying the the RSA250 lots power then they had to reverse crankshaft rotation due to new gearbox layout. This loss was then compensated with new cylinder. So I thought that rotation and cylinder orientation was important.

It is true that both RSA250-crankshafts rotated in opposite directions compared to the RSW250. But the power loss was caused by something else: the RSA had bigger big end pins, hence larger con rods, which were more in the way of the inlet flow, so the power advantage of the RSA250 over the RSW250 was less than expected.

Frits,

During the expansion stroke the rotation direction of the crank determines which side of the cylinder wall the piston is pushed towards.

Here I thought that you would see a difference between having the piston push toward the exhaust port where the big port gives less supporting surface area vs. towards the C port side where the wall surface area is much bigger.

Didn't seem to be a problem on the NSR500...

Mic a écrit:

During the expansion stroke the rotation direction of the crank determines which side of the cylinder wall the piston is pushed towards.

Mic, to be exact you should write: "which side of the cylinder wall the piston is pushed or pulled towards".
The con rod angle, together with the force in the con rod (push or pull) determines which side that is. And after reading the Big Bang story you may find that is is not the side you were originally expecting.

Citation :

Here I thought that you would see a difference between having the piston push toward the exhaust port where the big port gives less supporting surface area vs. towards the C port side where the wall surface area is much bigger.

You are right, but if I remember correctly, Jan Thiel's tests have not shown a distinct difference between the two situations. So the difference, if any, is small.