[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)

peter1962 a écrit:

[there I was thinking that Jorg Möller was the second best two stroke genius of that era   (after Jan of course)   but after 50 years we hear that he found the 2Stroke gospel by looking at yamaha.

When Yamaha first dominated two-stroke racing, Jörg was still a student. And shortly after he joined Van Veen, he had the opportunity to study the Yamaha technique. He would have been crazy not to take that opportunity.
Jan Thiel (Jamathi), Jörg Möller (Van Veen Kreidler) and Francisco Tombas (Derbi) were the protagonists in the 50cc field in the early 1970s, and all three had great respect for each other. And so have I.

Good evening Frits, while we are talking about Jörg Möller, you precised that he was a great crankshaft designer. I am a curious guy, and if you could share one of Jörg's inputs on this specific topic it would be great.

Frits Overmars a écrit:

peter1962 a écrit:

[there I was thinking that Jorg Möller was the second best two stroke genius of that era   (after Jan of course)   but after 50 years we hear that he found the 2Stroke gospel by looking at yamaha.

When Yamaha first dominated two-stroke racing, Jörg was still a student. And shortly after he joined Van Veen, he had the opportunity to study the Yamaha technique. He would have been crazy not to take that opportunity.
Jan Thiel (Jamathi), Jörg Möller (Van Veen Kreidler) and Francisco Tombas (Derbi) were the protagonists in the 50cc field in the early 1970s, and all three had great respect for each other. And so have I.

Christal clear, and not meant to demean anything.

carlovitch1 a écrit:

Good evening Frits, while we are talking about Jörg Möller, you precised that he was a great crankshaft designer. I am a curious guy, and if you could share one of Jörg's inputs on this specific topic it would be great.

I can't tell you much about it Carlo. I happen to know that Jötg had special bearings made, first at Hoeckle in Germany, and later also in Italy.
Maybe Jan Thiel can shed some more light on this.

The trick was to change the bearing balls for smaller ones to make them into C5 or C6.
We got those balls from Hoeckle, who also designed and made the crankshafts.....
I learned a lot about crankshaft design from them!
As did Jorg Moller...

Good morning Frits and Jan, that's interesting stuff to know.
If I understand correct, using smaller bearings and with more free play (on "normal" bikes you get C3, more rarely C4) was to reduce friction ?
I suppose this would mean a more frequent replacement (which for top level racing engine is no big deal)?

We used the same bearings all season..... Normal bearings with more free play.
Then there were about 11 GP-s a year.

Thanks Jan, this is indeed a significant lifetime for main bearings on a racing engine.
I would have bet they were wearing out quicker due to high solicitation, shall I understand that the fact that with a very well balanced crankshaft the vibes were so reduced that they were not damaging the bearings?

The balancing was done by Hoeckle.
The vibrations were typical of a parallel twin.
Once C3 bearings were tried, failed after 3 minutes....
We used C5, never had a bearing failure!

Good to know that only this change on main bearing free play can give such a result difference!
Thanks for the trick, Jan.

Hello Jan,

Jan Thiel a écrit:

The balancing was done by Hoeckle.
The vibrations were typical of a parallel twin.
Once C3 bearings were tried, failed after 3 minutes....
We used C5, never had a bearing failure!

Sorry, I have a little trouble following the conversation
From what engine are you talking about?

Thank you.
Francis.

@ Carlovitch1,

The functional clearance of the bearings, mounted in the engine, results from its own tolerance class and from the tolerances on the diameter of the bores in the cases and on the crankshaft axis.
And this at operating temperature.

Francis.

This makes sense, Francis, and if I understand correct, with a more powerful engine, you generate more heat then need more play.

No clearance should be determined for a given engine and for a given temperature.
Knowing that the ideal temperature is close to 50 °.
More heat means larger cases bores. And therefore, more final clearance.

Bonjour a Tous !
j'aurais une question pour Frits !
Sur la photo (du liens en bas) c'est le haut d'un port d'exaust de moteur de motoneige !
Dans le centre on voit qu'il y a une partie plus propre comparativement au coté ou il y a un peu de carbonne !
le centre du port est toujour plus haut alors j'imagine que c'est pour cette raison que c'est lavé comparativement au extrémité ?
Est ce qu'il y a quelque chose a lire ou a apprendre de cette photo pour amélioré la performance ?
je me disais que je pourrait peut etre monté legerement le port dans les extrémiter pour que ca s'ecoule plus uniformément !
merci

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

porttiming124 a écrit:

j'aurais une question pour Frits ! Sur la photo (du liens en bas) c'est le haut d'un port d'exaust de moteur de motoneige ! Dans le centre on voit qu'il y a une partie plus propre comparativement au coté ou il y a un peu de carbonne !
le centre du port est toujour plus haut alors j'imagine que c'est pour cette raison que c'est lavé comparativement au extrémité ?
Est ce qu'il y a quelque chose a lire de cette photo pour amélioré la performance ?
je me disais que je pourrait peut etre monté legerement le port dans les extrémiter pour que ca s'ecoule plus uniformément !
merci

There can be a several causes for the carbon pattern in your picture: a flow concentration that blows certain spots clean, hot spots that locally char the oil, and very hot spots that burn themselves clean.
Your assumption that the center of the port ceiling is clean because of the local flow concentration, is probably correct.
And you are also right in assuming that raising the port edges would improve the blowdown flow.  
But the port is elliptical instead of rectangular for a reason: this shape is necessary to keep the piston rings alive.
[Vous devez être inscrit et connecté pour voir cette image]

Je vais vérifier la forme de mon exaust comparativement à vos recommandations !
merci Beaucoup frits !
Toujours apprécié !

I have just finished reading this whole topic, best week of knowledge gaining in my life.
Thank you Jan and Frits for sharing your knowledge
Janbros, your spreadsheets are really good.

I have modified 2T engines on-and-off for 30yrs, starting with a Honda MT50. For the last 20yrs I have messed with twin cylinder jetskis. I have been used to dealing with the flat-sided transfer tunnels, knowing they weren't optimal for hp but everyone had the same problem.

I am hoping someone will advise me on this:
On aftermarket cylinders for a special class of competition there are two cylinders siamesed into one casting (595cc x 2), firing at 180. On a cylinder I have now, the transfer ducts on the two outer sides of the cylinder are curved, whereas on the side of the dividing wall between the ducts are mostly flat and narrower, especially the duct for the B, due to limited space between bores. It is an 8 port(2 Boosters) setup with axial angles of A=5 B=10 C=10 D=55
I did a crude transfer flow test, blowing air through from the intake manifold with the cylinder head off, and used a thin rod with some cotton attached to get an idea on flow symmetry.
The A ports short-circuit out the exhaust port, I imagine this will be worse with head on and exhaust pulses pulling the transfer flow.
The general scavenging flow is towards the sides of the cylinders sharing the dividing wall, this is obviously from the flat duct walls causing the flow to enter cylinder at a higher angle. I used wax to change the roof angles of the all the outer ports and the inside D ports to flow more vertically and the flow became more symmetrical.
To make the best of the cylinder, do I re-profile the ports so the flow is as symmetrical as possible or is there a trade-off point between symmetry and mass flow?

Welcome to this forum and thanks for the flowers, Carbonx2.
Mass flow of fresh mixture is useless if the mass short-circuits into the exhaust port. On the other hand, symmetry is desirable, but it should not be at the expense of mass flow.
Then again, lack of symmetry can lead to swirl, mixing fresh mixture with spent gases instead of expelling them.
In short, I don't have a ready-made answer to your problem. Maybe Jan can shine his light on it.

Thank you Frits for the quick response.
I am happy my thinking falls in line with yours. The answer might have to come from testing changes in stages, I am at least sure that the A-ports need increased axial angle to help stop the short-circuiting.

CARBONX2 a écrit:

Thank you Frits for the quick response.
I am happy my thinking falls in line with yours. The answer might have to come from testing changes in stages, I am at least sure that the A-ports need increased axial angle to help stop the short-circuiting.

Yes, most likely. And maybe you can also aim their radial direction angles further away from the exhaust port.
[Vous devez être inscrit et connecté pour voir cette image]

Frits.
I am going to map the radial angles accurately, a simple look shows the A-ports are aimed on the intake side of the centre of the piston but not as much as your layout. There are also recesses/pockets behind the A and C ports, I will fill those to make continuous reducing ducts.
On checking my written down numbers on axial angles vs what I thought they were, I found what I stated earlier was a bit off, it should be A=5 B=15 C=10 D=55. I think the A ports short-circuiting from the flow ricocheting off the B port flow. If I up the A port axial angle to 25, there should be significant improvement(Your calculations for a 5-port A port produce an angle of 27,5)

This article by Kevin Cameron may hint at the answer to my question:
[Vous devez être inscrit et connecté pour voir ce lien]
Which leads me to think I should concentrate on symmetry by checking/correcting the ports in the following order C, B, A

Things are aimed more at the inlet side than expected
Port mapping - radial angles
A Port
Leading = 37
Trailing = 50

B Port
Leading = 55
Trailing = 85

C Port
Leading = 90
Trailing = 118

D Port
Leading = 175
Trailing 179

[Vous devez être inscrit et connecté pour voir cette image]

CARBONX2 a écrit:

A=5 B=15 C=10 D=55. I think the A ports short-circuiting from the flow ricocheting off the B port flow. If I up the A port axial angle to 25, there should be significant improvement(Your calculations for a 5-port A port produce an angle of 27,5)

Those radial angles don't look bad at all. But your calculated axial A-port angle of 27,5° surprises me. It would mean that your 595 cc cylinders have an 87,5 mm bore and a 99 mm stroke. It's possible but I don't think it's likely.
You did not accidentally swap the bore and stroke values, did you? Then your engine would have a 95 mm bore and an 84 mm stroke,
and the axial A-port angle should become 22,1°

I must have calculated wrong, it is:
95mm bore
84mm stroke
Thanks for the correction