[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's statement about the hemispherical / toroidal head shape may need some clarification. It depends on what you call hemispherical.
My definition would be that the centre of the dome lies on the centre axis of the cylinder bore. As soon as the dome centre is offset from the bore axis, I call the resulting shape toroidal. The first picture below shows an example of this offset: the distance between points 3 and 4.

Such an offset may or may not lead to the centre of the combustion chamber intruding, bringing the spark plug closer to the piston, as in the third picture. Tests at Aprilia have shown the intruding shape to be inferior to the shape with a level area around the plug, probably because the intrusion causes a scavenging shadow behind it, leaving burnt gases in the combustion chamber.

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Yes indeed Frits, that is exacty wat it was.
The compression ratio used on the dyno was always 16:1.
There was a choice of different compression ratio's for use on track.

Maybe 100 different heads were tried.
A very important detail: even the smallest radius on the end of the squish-band gave a power loss.

Dernière édition par Howard Gifford le Mar 11 Sep 2012 - 16:21, édité 1 fois

Wow Frits that is a very clear description of the head design. Thank you... I call this design the "Bathtub" design versus the true hemispherical design. The combustion chamber transitions at a sharp edge 90 degrees to the squish band and has a flat roof. Sort of like an upside down bathtub. I will machine a set (scaled up to suit a 65mm bore) with these dimensions and test them this weekend. We have been using a true hemispherical head design basically cut with a 44.5 mm ball nose end mill to make the combustion chamber.
I see your squish band covers 50% of the bore area and the squish is 1.28% of the stroke. Is this pretty standard for most applications or do bigger bores and strokes change these numbers? 1.28% of my stroke is .842mm or .033". That is tighter than we have ever run!!!!

what stroke / bore do you use ?

My calculator say 65mm :)

So 0.7mm for 54mm stroke, nothing surprising then...

Mits46 on the Aprilia engines ?

Yes my stroke is 65.8mm.

Howard Gifford a écrit:: Wow Frits that is a very clear description of the head design. Thank you...

My pleasure, Howard.

Citation :: I call this design the "Bathtub" design versus the true hemispherical design.

You are welcome to call it anything you like, but unless you have a round bathtub it would not make much sense; the Aprilia head design is purely circular. That's what all the X- and Z-directions in the first picture are for: it can be produced on any CNC-lathe; no need for milling.

Citation :: I see your squish band covers 50% of the bore area and the squish is 1.28% of the stroke. Is this pretty standard for most applications or do bigger bores and strokes change these numbers? 1.28% of my stroke is .842mm or .033". That is tighter than we have ever run!!!!

It's pretty standard, regardless of bore and stroke. The 1.28% of stroke squish gap is even on the wide side. Provided your crankshaft and con rod are OK, you can use 1%.

Nombre de messages : 28 Localisation : Slovenia Date d'inscription : 19/03/2012

On our small engines I use 1% squish gap on the inside of the squish band (0,4mm), and an smaller gap on the outside(about 0,3mm). I did also try a -0,2mm squish gap (not intentionally, at first), and it actually did work very well... before it flushed away all the oil from the piston skirt under the piston ring gap. Then I oiled it with my finger, and power was over-average again.

You mean you actually hit and compressed the piston against the head !?

Nombre de messages : 28 Localisation : Slovenia Date d'inscription : 19/03/2012

GrahamB a écrit:: You mean you actually hit and compressed the piston against the head !?

That is right. I could clearly hear the piston hitting the head when releasing the gas. The piston top also had a clear squish "stamp" on it. The interesting thing I noticed was that there was no sign of detonations. Except there was a small channel dug out over the ring gap, it looked like it was sandblasted. And after some dynoruns, the piston skirt under the ring gap (under the C transfer) was completely oil free. The performance was good, it gained something everywhere, as far as I remember, but it was only experimented for a short time.

The 0,7mm squish was the minimum we could use with good power at high pm.
Less squish gave more power at low rpm, until 12000 rpm, but the engine would not rev.
It would just stop firing at about 13500
We tried everything we could to make the engine rev with a 0,55 squish.
Impossible! Even 0,65 was negative!
The piston would touch with 0,45.
So the 'real' squish at high rpm was 0,25

Experience with the 50cc Garelli in 1983 showed the less squish we used the less detonation we had.

Nombre de messages : 86 Localisation : Australia Date d'inscription : 05/05/2012

Dear Masters of 2strokes
I assume the new FPE inline twin engine ( 2 x 125cc barrels etc upon a common crankcase ) has been influenced by you both greatly, has this type of engine and it's intended use required a different approach than that of the RSA or has it been business as usual ?

The pipes upon this engine appear to differ in spec to that you have published in this forum for the RSA, is this due to an increase in loads the engine is operating with ? 1973 - [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked) - Page 10 2917

1973 - [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked) - Page 10 2917

Francis Payart has been building these tandem twin engines for a couple of years. They were based on the Rotax 256, and some development from the Aprilia RSW250 and RSA125 may have crept in.
I am not at liberty to tell you anything about his approach and about the pipes he is using. You will have to ask Francis himself about that.

@ Raw,
Correct.
We were very influenced by all Aprilia engines.
As you said, the biggest difference between a motorcycle and a superkart is the rate of engine load.
On a superkart we are often full load more than 80% and it is impossible to use the same carractéristiques that the RSA engine.
Especially the head volume which must be bigger.
The exhausts are also different. We try to achieve the same result with the front and rear.
Now, with the same carburetors setting, we get the same exhausts temperatures.

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Sujet: Many Thanks Jeu 13 Sep 2012 - 1:30

Thanks for the photos Francis - I love your work - it is true art and a great credit to you and Damian.
And not to forget the Masters too - Thanks Jan and Frits for sharing the mysteries of the two stroke with us. It is a pleasure to read your comments and think about these in light of our own knowledge - we are richer for your generous gift.

And many thanks to Marc for putting this all together 1973 - [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked) - Page 10 241515

1973 - [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked) - Page 10 241515

1973 - [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked) - Page 10 771973

1973 - [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked) - Page 10 809262

Best regards
Allan.

Hello Francis,

the escapes in photo are "inspired" of last RSA titanium version?

it's my impression, no dwell, repetitive lengthes of section, and CC (contre cône) in five parts... right ?

To keep the same temperature of functioning you use ceramic masks or you worked the lengthes of escapes or other thing ?

thank you for all sharings ! 1973 - [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked) - Page 10 241515

eric² a écrit:: Hello Francis,

the escapes in photo are "inspired" of last RSA titanium version?
Yes
it's my impression, no dwell, repetitive lengthes of section, and CC (contre cône) in five parts... right ?
The amount of sections of the rear cone is the result of the shape we want to obtain
To keep the same temperature of functioning you use ceramic masks or you worked the lengthes of escapes or other thing ?
We only use a protector on the front exhaust

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27 ans d'écart, le Nissag à Silverstone, (avec mes gamins), le pot avant était couvert aussi !

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Nombre de messages : 19 Localisation : England Date d'inscription : 07/09/2012

A question or two for Mr Thiel,

Could you outline the thinking behind the use of cylinderical roller bearings for the RSA crankshaft main bearings, are they historically something you've used in your designs or a recent inclusion?
How did you control crankshaft end float (were flanged inner races used)?
Did you test steel, hybrid and ceramic elements, if so, what criteria was used in the final choice of race/roller material?

Not long after the race introduction of the RSA there were reports of RV drive failures, could you describe how these failures manifested themselves and how you where able to identify the causes and what where the solutions required?

Regards

Before fishing Dutchmen you may want to introduce yourself, in the "presentation" area of the forum.

Thank you

1973 - [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked) - Page 10 980796

1973 - [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked) - Page 10 980796

dutch fisher a écrit:: A question or two for Mr Thiel,

Could you outline the thinking behind the use of cylinderical roller bearings for the RSA crankshaft main bearings, are they historically something you've used in your designs or a recent inclusion?
How did you control crankshaft end float (were flanged inner races used)?
Did you test steel, hybrid and ceramic elements, if so, what criteria was used in the final choice of race/roller material?

Not long after the race introduction of the RSA there were reports of RV drive failures, could you describe how these failures manifested themselves and how you where able to identify the causes and what where the solutions required?

Regards

The roller bearings were already there when I started working at Aprilia.
I do not know who introduced them.
Flanged inner races were used, the flanges broke often.
Later the flanges were separated from the inner race, which proved quite reliable.
End float was 0,4mm.

The RV drive failures mainly occurred after I retired.
So I do not know much about this!
Only steel rollers were used.

Hi Frits

Many thanks for the information that you provided so freely to others.

Regarding your toroidal combustion chamber drawing and dimensional information, am I correct in assuming that you allow a volume of 0.62cc for the volume inside the spark plug in the figure for your combustion chamber volume (8.6cc).

I have assumed that you carry out your upside down volume check with a spark plug installed?

Best Regards

Ian

You are right in assuming that the upside-down volume is checked with a spark plug installed, Ian, but its internal volume is disregarded in the calculation. The protruding isolator and electrodes more than make up for the volume inside modern racing plugs.
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Upside down volumes were never checked at Aprilia, not necessary with CNC-machined heads!
Volume checking of thousands of heads a year would have been too much work.

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