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

Howard Gifford a écrit:

Thanks Frits that's exactly what I thought. When designing the motor' is a square motor (bore vs stroke) the ideal or do you think slightly over square (longer stroke than bore) is better? The rules limit us to 460 cc and we can choose any bore / stroke combination as long as we don't exceed 460 cc. BTW what 50 mm carb would you recommend? Maybe a pair of Lectrons?

Howard, oversquare means: bore/stroke >1. For two-strokes an 'undersquare' engine, i.e. the bore smaller than the stroke, gives better angle.area values and a better combustion chamber shape.
We never got round to establishing the optimum bore/stroke-ratio but square or slightly undersquare should be good.
Don't worry about the higher piston speed resulting from the long stroke. A decent two-stroke engine will run out of breath before the piston speed can cause mechanical problems. The time.areas (i.e. the angle.areas divided by rpm) will form an effective rev limiter.

Lectron carbs will be fine, but for a set of 50 mm Lectrons you will need a watercooled creditcard...

Howard, what's the rpm at max power for your project moter ? It's for a sled I presume . Is midrange power important for this kind of racing ?
Have you had already a good look at the Yoshimura MJN carb design, I think this might be the best carb solution up to now. Are carbs mandatory ?
Ciao for now.

Hello everybody, I’m new on this blog.

My question is mostly addressed to Frits and Jan to check if they know, or if they have experimented some ways to boost the performances on 2 stroke engines using increased pressure resulting from an added expansion from the shock between 2 volumes of gas volumes with different temperatures. Nothing related with the new systems used by Husqvarna and Stihl (both garden tool engines) because their models only can perform well at constant engine speed. I’ve already checked the 5 forum parts on this topic, but if nothing has escaped me, I cannot see references about this technical feature in detail.

I explain the possible. During one research about how to increase the performances on 2 strokes crankcase scavenged without using added mechanical complexity (superchargers, blowers, blind counter-pistons, or moving walls), I came with the idea in using a part of one principle conceived by Guy Nègre (R.I.P. big genius) on his compressed air engine. Then I’ve figured that this is not new, and was experimented before in a rough manner. Currently I’m working on this in a private way experimenting to check basic results.

In one concept, Guy Nègre has used additional heated air and mix it on discharge chamber with released compressed air to achieve a greater expansion resulted from the thermal shock of volumes at different temperatures. Pure fluid mechanics.

I’ve heard that Suzuki in the sixties, and Aprilia most recently perhaps have experimented something similar, but till now I cannot get a confirmation by any way.

You both, do you know about what I am talking, or have experimented with something similar?
Thanks.

I am not familiar with what you describe, Iker. Can you point me to some more in-depth information?

Frits, sorry but presently I can’t disclose so much in public. Please understand it.

Like highlighted initially, I’ve used the feature of mixing a warm fluid to achieve an extra increase of pressure on other fluid volume at lower temperature and compressed partially. Not a copy, but based on Negre’s principle.

Then during extensive researches trying find the most info possible, was when I’ realized that something similar was already experimented some decades ago, but not diffused. In fact, the Inventor with which I’m currently in contact crossing info, has conceived the system with another target than the mine. He was mostly focused in decrease emissions.

By my side when I’ve started with this, the target was only to apply it on race engines, achieving better volumetric efficiency, more power on all rotational speeds, increased and flatter torque curve, surpassing the problem of fluid pumping at higher revs verified on common crankcase scavenged 2 strokes characterized by irregular power delivery.

A friend from long time, has commented me that may be something similar was already done in the past with racing engines (Suzuki? and Aprilia ??). This was the reason of my post, try obtaining all info that I can.

The basic principle of this tech feature is quite sharp to achieve a perfect equilibrium if applied in a rough manner. To do it perfectly is necessary develop a control system. To compare, is like commute with a bike, high power 2 strokes, but without exhaust valve. May become a nightmare (or crash), of course.

During my primary experiments with cheap 2 stroke engines, piston port intake, 42cc, after the 3rd modification, one has blow-up surpassing the 15000~15200 rpms, where before under OEM specs only can revs till 7600~7800. This was exceptionally out of my knowledge. Since near 25 years preparing and modifying a big range of engines, I never have seen something like this. Only on an old 250cc Rotax with rotary valve, in a Kram-it Enduro Bike, with other type of modifications, was where I’ve achieved an important added extra rpms.

Nowadays, I’m on process with a 100cc kart engine, piston port intake, to verify if with bigger volumes the equilibrium still being so sharp.

I’m only doing it during my afterhours, as a part time hobby, and I’m advancing slowly but without pauses to consolidate the basics (data recovering).

Sorry, but I can’t disclose more because this still not being totally in public domain. Please understand and don’t be upset. By my better intention, contacting the “gurus of 2 strokes” was only get the most info possible to support my research.

Are you ok ????

what your saying doesn't have any sens .....

I you want to be understood , BE CLEAR AND SPECIFIC

and if you dont, have a nice day


.

sounds a bit like the workings of a Ryger.
Iker, is that you Luk ?

Hi Frits and Jan,
I recently read a comment in a karting magazine and it warned that the Kart (two stroke) exhaust pipe should not be painted or plated as it will reduce performance. The article did not explain the reason for this loss of performance and left me wondering how this action could affect the tuning.

I understand that you have both commented previously about making exhaust pipes from different metals having an effect on the exhaust tuning and this is understandable given different material properties, but do you agree with not plating or painting a two stroke exhaust due to a performance issue?

Regards,
Allan.

GtG001 a écrit:

Hi Frits and Jan,
I recently read a comment in a karting magazine and it warned that the Kart (two stroke) exhaust pipe should not be painted or plated as it will reduce performance. The article did not explain the reason for this loss of performance and left me wondering how this action could affect the tuning.

I understand that you have both commented previously about making exhaust pipes from different metals having an effect on the exhaust tuning and this is understandable given different material properties, but do you agree with not plating or painting a two stroke exhaust due to a performance issue?

Regards,
Allan.

Just a small note, black painting absorbs heat from inside the pipe, metal plating do the reverse concentrating heat on pipe. So you can figure easily what is happening with the back pressure temperature entering the cylinder. Results depends from exactly what is needed.

JanBros a écrit:

sounds a bit like the workings of a Ryger.
Iker, is that you Luk ?

I don't work for Ryger, neither connected with them.
But technically I recognize very well what they are intendeed to do, and the principles of his model.

They should show clear results to make proof that his model is in line with the expected.

Sometimes theory not match well with reality, others may exceed.  Let's wait...

fd-racing a écrit:

Are you ok ????

what your saying doesn't have any sens .....

I you want to be understood , BE CLEAR AND SPECIFIC

and if you dont, have a nice day  


.

Sorry if someone is pissed about unclear explanation, but like explained this tech model presently still not being totally in public domain.

For someone expertised in fluid mechanics is more easy.
Was the only way that I've found to reach Frits and Jan, using this forum.

A nice day for you too.

Iker a écrit:

For someone expertised in fluid mechanics is more easy.

you're a funny guy

.

Iker a écrit:

I’ve used the feature of mixing a warm fluid to achieve an extra increase of pressure on other fluid volume at lower temperature and compressed partially.

Iker, my first reaction would be that the heat of the warm fluid would increase the temperature and the pressure of the cold fluid, but at the same time the warm fluid would cool down and loose an equal amount of pressure.
I understand that you cannot afford to be more specific here, but I'd like to know more....

[/quote] I understand that you cannot afford to be more specific here, but I'd like to know more....[/quote]

Frits, perhaps Jan is aware about this singular tech feature..?
About knowing more, side contact.., pls.

regarding squish : it is agreed that a minimum of 1% of stroke is safe. I suppose this is the limit for "normal revving engines" - by that I mean for example +/- 14;500 for a square 125?)
but what can be considered safe for an engine that hit's only about half it's "normal max rpm" ? talking about my Sky-MX (39x41.4) moped : tuned for max power at 7.500-8.000 rpm, and I don't think at a race it will ever reach 9.000, so that's about only half of the "normal max rpm" for 50cc.
Is my simple thinking correct that a squish of 0.4mm is over-safe and that 0.2mm would be ok (very simple thinking : half rpm's, half stretching/bending, half squish needed) ?

concernant squish : 1% de cours  est accepter comme le minimum. mais je suppose que c'est pour des moteurs qui atteints des tr/mins max normale (par example : 14500tr/min pout un 2-temps 125cc).
Mais comment pour un moteur qui n'atteint seulement la moitié des "tr/mins max normale" ? par example : le moteur de mon Honda Sky 50cc (39x41.4 , mob pour faire MX) est préparer pour avoir la puissance max à environ 7500-8000 tr/mins, et je ne crois pas qu'il va jamais atteindre les 9000 dans une course. donc c'est seulement la moitié des "tr/mins max normale" pour un 50cc.
Est ce que ma simple tête raissonne correct en penser +/- que moitié tr/mn = moitié de stretch = moitié de squish nécessaire pour être safe ?

JanBros a écrit:

regarding squish : it is agreed that a minimum of 1% of stroke is safe. I suppose this is the limit for "normal revving engines" - by that I mean for example +/- 14;500 for a square 125?). But what can be considered safe for an engine that hit's only about half it's "normal max rpm" ?
Is my simple thinking correct that a squish of 0.4mm is over-safe and that 0.2mm would be ok (very simple thinking : half rpm's, half stretching/bending, half squish needed)?

Jan, it is not agreed that a minimum of 1% of stroke is safe for a squish gap. This 1% is just a rule of thumb, and individual safe limits may vary, as I already wrote elsewhere ( [Vous devez être inscrit et connecté pour voir ce lien] ) .

I'm afraid your thinking may be a bit too simple: play in crankshaft main and big end bearings is independent of rpm, while bending and stretching forces rise with the square of rpm. So I suppose it will be a case of trial and error for your moped.

ok, tnx. I'll stick with .4mm (which has been ok up to now)

Frits I need some advice. I built a new motor with a 3 mm longer stroke using RSA clone cylinders. Bore and stroke are now 54 X 57.5. The port opening timings are set to 82.5 ATDC on the main exhaust, 85 on the aux exhaust, 114 on the B ports, and 115 on the A and C ports. RV inlet timing is 145-70. I plan to rev it at max rpm 10800 or so. I lowered all the transfers to match the now lower piston at BDC.
My question is, Should I also lower the exhaust port floor to match the piston or should I leave it slightly (1.5 mm) higher than the piston edge at BDC?

Howard, there is no need to lower the exhaust port floor. In the latest Aprilia cylinders (picture below) this floor was already well above BDC. You must have noticed this when you cloned an RSA cylinder.
But in a true clone you would also use an exhaust timing of 202° instead of your 195°. This makes a world of difference.
[Vous devez être inscrit et connecté pour voir cette image]
By the way, using RSA clone cylinders will not be a guarantee for success if you lower the piston speed that much.
The Aprilia RSA does 26,3 m/s at its maximum revs of 14.500 rpm; your engine will only do 20,7 m/s at 10.800 rpm.
The scavenging flow may be too fast for those revs, so you may need to apply different axial transfer angles. But that would definitely terminate the idea of an RSA clone.

Frits Overmars a écrit:

Howard, there is no need to lower the exhaust port floor. In the latest Aprilia cylinders (picture below) this floor was already well above BDC. You must have noticed this when you cloned an RSA cylinder.

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

Frits, I'm surprised: the "new" port is smaller and that "step" that burnt gases have to climb to enter the exhaust port at BDC ??? the "V" shape of the exhaust floor may help ...
Not logical for me but the "new" must be better than the "old"

It's quite simple really, Pascal. The majority of the burnt gases should already be gone by the time the transfer ports start opening, and that 'step' will prevent the fresh mixture from escaping directly into the exhaust.

That makes sens ! never saw that "step" before; is there other engines using that ? non energy saving engines ...

The old tuning tip using spacer under the cylinder and cutting the top of cylinder to raise the ports could match with this idea - without the V shaped port floor.

pascaltz a écrit:

That makes sens ! never saw that "step" before; is there other engines using that ? 

It has been in my FOS-design since 1983; the Aprilia RSA is the only other engine with this port shape that I know of.

Finally a Ryger engine has run on the SAME track, on the same day, with a TM
The TM was 2.5 seconds a lap faster.....
Observed by a very serious and capable person
Also the times between gearchanges were notably longer.
Power was estimated by the lap times as being around 32-33HP.....
More or less as expected, and in line with the simulation Vannik did with 2strokeEngMod.

Awaiting new comments.....

A 'so called' expert,
who just doesn't like big-mouthed people...

Finally I have read the whole thread!! Is like the final part of a peregrination, then I win the rigth to make an answer to the monks of knowledge

Well, my question is about the interpretation of the drawings that Jan and Frits share about the RSA cylinder. I found on a "Diagramma angolare delle luci" that the floor and roof of the A ports are parallel and the normal direction of the slope (marked as red arrow) points sligthly back of the middle of the cylinder.
[Vous devez être inscrit et connecté pour voir cette image]
On the other hand, there is a section of the cylinder in wich the normal directions of roof and floor of the A ports are not parallel and pointing slightly more close to the exhaust direction.
[Vous devez être inscrit et connecté pour voir cette image]
My question... what is the correct one? Or is just a problem of my interpretation of the projections on drawnings? Maybe the normal direction of the slope change among the heigth?
Help!

Thanks to everybody for the knowledge on this forum and specially to Frits and Jan!