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

I have held them in my hands, at the Mahle research centre. But they were meant for turbocharged fourstroke diesels. Advantages: no thermal expansion and good dry-running properties. Disadvantages: lower mechanical strength so the wall thickness had to be three times that of alloy pistons which annihilated any weight advantage, and low thermal conductivity which absolutely ruled them out for twostroke applications.

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Hello all, I race on go-kart with 100cc direct drive engine, don't know if you have ever worked with them.
You talk about the number of port.
Do you know why in go-kart a lot of engines have 3 ports instead of 5 ?
And why engine rev to over 20k rpm with exhaust duration at 177° ? Is it because of the convergent is leaky ?

Thanks to Jan, Frits and other member for sharing your knowledge

Frits Overmars a écrit:

I have held them in my hands, at the Mahle research centre. But they were meant for turbocharged fourstroke diesels. Advantages: no thermal expansion and good dry-running properties. Disadvantages: lower mechanical strength so the wall thickness had to be three times that of alloy pistons which annihilated any weight advantage, and low thermal conductivity which absolutely ruled them out for twostroke applications.

Thanks for the info Frits,

The Honda piston rumor was simply that, just a rumor I heard when in an aprilia 125gp garage a few years ago. Purely hearsay.

Jeram a écrit:

Frits Overmars a écrit:

I have held them in my hands, at the Mahle research centre. But they were meant for turbocharged fourstroke diesels. Advantages: no thermal expansion and good dry-running properties. Disadvantages: lower mechanical strength so the wall thickness had to be three times that of alloy pistons which annihilated any weight advantage, and low thermal conductivity which absolutely ruled them out for twostroke applications.

Thanks for the info Frits,

The Honda piston rumor was simply that, just a rumor I heard when in an aprilia 125gp garage a few years ago. Purely hearsay.

Beryllium/Aluminium pistons were used in Formula One before being banned.BeAl alloys are typically 22% less dense than Aluminium,have a Modulus of Elasticity about the same as mild steel and have excellent thermal conductivity. I don't know if anyone has made two stroke pistons from this material.

CRECY a écrit:

Beryllium/Aluminium pistons were used in Formula One before being banned. BeAl alloys are typically 22% less dense than Aluminium,have a Modulus of Elasticity about the same as mild steel and have excellent thermal conductivity. I don't know if anyone has made two stroke pistons from this material.

LockAlloy (a Lockheed trade mark) pistons were designed by Dolph van der Woude, wo also designed the original Aprilia 250 cc racing V-twin. Weight saving was impressive: 68 g for the Lockalloy piston vs 127 g for the standard Aprilia piston. But the ban on GP-twostrokes stopped the experiment.

Cet alliage existe aussi sous la dénomination commerciale "Beralcast".
Ses avantages les plus importants sont bien sûr sa faible densité mais aussi
son module d'élasticité plus élevé que celui des alliages légers. Ces particularités
le prédestinait à faire des étriers de freins. Brembo en avait fait pour la F1.
Après les seules applications que je connaissent sont des applications en aéronautique
militaire, moulés à la cire perdue. Je ne sais pas pourquoi cet alliage prometteur
ne s'est pas plus développé ( je m'y étais intéressé il y a 15- 20 ans. Peut-être
que les risques sévères de bérylliose sont un début d'explications?...

Les raisons avançées, à ma connaissance, était la bérylliose et les coûts.

Pour la bérylliose, on peut imaginer des pistons qui se vaporisent dans un moteur risque d'arroser toute l'assistance, mais pour les étriers de freins ce n'est un risque que lors de fabrication... ce qui ajoute finalement au coût car obligation de plus de précautions lors d'usinage.

Mais c'est loin d'être le plus stupide d'interdictions de matériel, à mon avis.

Frits or Jan,were coatings such as the one from Emot used in the RSA?Were any other treatments used such nitriding, oil shedding,heat dispersants etc..And thanx for all your contributions to the advancement of 2 stroke knowledge.

Allow me to introduce myself. My name is Ken Seeber and I live in Perth, Western Australia. I have been interested in 2 stroke engines and motorcycle GP technology since forever. I was fortunate to work at the Orbital Engine Company for 25 years where a lot of time was spent on the design and manufacture and/or application of various 2 stroke engines, the key focus of this activity was the application of the Orbital 2 fluid direct fuel injection system. This offered many gains in terms of fuel consumption and emission reductions with production applications on various 2 stroke engines including Mercury Optimax outboards, Piaggio, Aprilia and other 50 cc scooters.
During this time, I built and raced a 125 cc bike, based on an early YZ125 (Ø56 * 50) engine which I converted to watercooling with a rotary disc valve using a magnesium 34 mm Mikuni as used on an RG500 engine.

Being a late/recent starter in getting on to the Pit-Lane 125 Aprilia forum, I got excited after reading Frits’s articles on transfer ports, particularly in Page 33 where he suggests the axisymmetric layout with all the transfer ports pointed radially inwards.
We built a cylinder of this type in around 1984, even using hydroformed pipes. The pictures tell the story. Perhaps a bit crude with handmade patterns, welded on water jacket etc, but as a homebuilt project, it was the only way at the time. How did it go? Not as good as the modified YZ cylinder, but in hindsight the port timings were far too mild. However, as a result, it idled extremely smoothly (reliably 2 stroking) indicating good cyclic stability of the scavenging process. If I did another one I would incorporate a downslope roof for the exhaust ports, upslope for the roof of the transfer ports plus more current (eg Aprilia RSA 125) port timings with exhausts to suit.

Didn’t go any further with it at the time due to wives, kids, house, career etc. However I do believe that Frits is absolutely correct in pointing to this layout offering a step increase in performance.
There are some downsides to such an engine layout:
1. Exhaust system. I obviously used 2 separate pipes, and this maybe the best for overall performance, although a single pipe system is possible, but for the fact that one may experience too much cooling of the exhaust gases if they were ducted into one outlet within a cast cylinder resulting in a loss of exhaust energy. Either way, packaging could become an issue, possibly practically limiting it to a one cylinder application.
2. Exhaust &fuel/air mixture gases short circuiting between exhaust and transfer ports. To minimise this, it would be necessary to provide a piston profile that is close fitting, particularly directly below the ring. However, this not to say that this is not important in any high performance 2 stroke engine with a close proximity or overlap between the exhaust and transfer ports.
3. Piston cooling. Approx 70% of the heat transferred to the piston is conducted away to the cylinder bore via the piston ring(s). The main moment for this to occur within the cycle is at TDC and BDC when the piston stops and provides the greatest time for the transfer. This is well recognised in most current performance 2 strokes by the fact that the coolant is introduced behind the cylinder wall under the exhaust port, at a level where the ring stops at BDC. However, when we surround the cylinder with transfer ports/passages, this is basically no longer easily possible.

We are still involved in 2 stroke engines with a business in Perth manufacturing pistons for Yamaha kart engines ( [Vous devez être inscrit et connecté pour voir ce lien] ).

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Hey Ken nice to see you here.

Jan has said many times RSA gets a lot of it's cooling from the intake charge clinging to the crown at BDC.

yeahhim a écrit:

Frits or Jan,were coatings such as the one from Emot used in the RSA?Were any other treatments used such nitriding, oil shedding,heat dispersants etc.

Probably yes to nitriding transmission parts (but I never really looked into that). No to all other treatments you mention.

Hi Ken, welcome here.

Ken Seeber a écrit:

...Piston cooling. Approx 70% of the heat transferred to the piston is conducted away to the cylinder bore via the piston ring(s).

How was this percentage established? Any links or pdf's maybe?

Welcome on board, Ken!

Frits Overmars a écrit:

Hi Ken, welcome here.

Ken Seeber a écrit:

...Piston cooling. Approx 70% of the heat transferred to the piston is conducted away to the cylinder bore via the piston ring(s).

How was this percentage established? Any links or pdf's maybe?

I find it difficult to believe that a 0,8mm pistonring would transfer more heat to the cylinder than a 57mm long piston!
And do not forget that both generate heat by friction against the cylinder!
So I am curious too!
One is never too old to learn.

Frits Overmars a écrit:

Every divider between two ducts acts as a heat exchanger: the mixture passing along them is heated by the heat in the metal.

Frits do you think that big divider at the base of the RSA cylinder not only gives direction to intake streams but the large thermal mass does not contribute to heating of the intake?

Institute of TwoStrokes a écrit:

Frits Overmars a écrit:

Every divider between two ducts acts as a heat exchanger: the mixture passing along them is heated by the heat in the metal.

Frits do you think that big divider at the base of the RSA cylinder not only gives direction to intake streams but the large thermal mass does not contribute to heating of the intake?

I don't think thermal mass plays a role here (except in the first moments after starting a cold engine).
Heat transport through aluminium can be a factor 100 better than heat transfer from a metal surface onto a passing gas mass.
Even the heat flow through the cross area of a divider only half as thick as the RSA's would be more than capable of keeping up with the heat transmission from the divider's external surface to the mixture flowing along it.

Besides, the temperature difference between that divider and the mixture coming from the crankcase is moderate, as the inside of the RSA's transfer ducts is watercooled.

Jan,

What experiments with ring tension did you find the lowest friction numbers without incurring blow-by and acceptable ring life?

With the RSA did you have a particular ring section with the 0.8 rings and any noteworthy problems with flutter?

What coatings where used at Aprillia on the rings and pistons?

Neil Hintz, New Zealand, just seeing how this forum works. Wow a forum dedicated to just twostrokes. Ken, nice cylinder, nice work. Good on you for giving it a go, too often people just give up at the idea stage. Espesially when other people say it won't work, usually they have no idea!!

Fritz & Jan,
Firstly, thanks for the interest. Secondly, I am age challenged, so must be able claim some leniency in terms of accuracy of the 70% heat transfer value.
However it was a figure that I always remember, and a quick search confirms this in some applications:
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In hindsight, this might have been more applicable to 4 stroke engine with more rings and a shorter piston skirt, compared to a 57 long piston with a 0.8 ring as Jan has rightfully questioned, particularly combined with the high rate of gas flow through the engine
However, I would think that heat transfer from a single ring in conjunction with a properly contacting top land would be well over 50%. Happily though, I stand to be corrected.
Still, the point I was trying to make is that it is important to maximise the cooling at levels for the ring and top land when they stop at TDC & BDC.
Thanks. Ken

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Just testing to see if I can post images, a vairable rotary valve housing.

dutch fisher a écrit:

Jan,

What experiments with ring tension did you find the lowest friction numbers without incurring blow-by and acceptable ring life?

With the RSA did you have a particular ring section with the 0.8 rings and any noteworthy problems with flutter?

What coatings where used at Aprillia on the rings and pistons?

We had a quite low ring tension.
The combustion itself presses the ring against the cylinder wall.
We had normal rectangular rings.
The rings were coated by the Japanese manufacturer.
Once the pistons had lead coating.
After this was prohibited zinc was used.
You had to be careful to polish the piston crown.
Because the zinc seemed to cause detonation.

uniflow a écrit:

[Vous devez être inscrit et connecté pour voir cette image]Just testing to see if I can post images, a variable rotary valve housing.

And you did't just use any old picture, did you, Uniflow? Maybe you can tell us a bit more about that valve cover and about yourself, while I too test to see if I still can post images .[Vous devez être inscrit et connecté pour voir cette image]

Frits' humour, I love it

Yes, I couldn't help myself. What to tell? I've tinkered with twostrokes for many years small road race ( 100cc ) and trail / MX bikes mostly singles. I've built lots of different things over the years for different engines, hobby, ( I don't make a living out of it ). This is a rotary valve housing to suit a Kawasaki 350 F9 that I'm campaining at the moment. Mostly using the F9 as a test hack. This housing is adjusted by an Ignitec ignition unit via an R1 servo. Changes with revs. Gives me good trail bike timing at low speed and good race timing at high speed. Seems to work well, athough I would say that anyway because I made it! The F9 is running EFI, transfer port / crank case injection. It's running on ethanol ( E90 ). Unfortunatly the cylinder is still the original with just a bit of a port job, this needs to be fixed if the power is to be increased any more. I do cast twostroke cylinders ( water cooled ) but have never cast an air cooled one. I am in the process of building up a new engine ( water cooled this time ) to fit in a modern YZF 250 frame. The new engine is 360cc vairable rotary valve, EFI, reverse turning crank with balance shaft. The cylinder is my own with blade type powervalve. If you are interested I could post a few more pictures of the EFI F9.