[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 4)

Vortex a écrit:

Trying to understand the relation between different concepts in scavenging angles. [Vous devez être inscrit et connecté pour voir ce lien] As you can see 2 different cylinders but the same typ of engine. Do you think it is normal that on the model on right side the ports 1+2 can't join together in the middle?

With ports 1+2 you mean the A-ports and the B-ports? In your picture the flows do not join in the middle, but this may be a distorted view, depending on the axial angles. If the A-ports are steep enough, they will join the B-flow after all.
But I would prefer the left-side concept anyway.

Citation :

Here a picture from used piston with that configuration. [Vous devez être inscrit et connecté pour voir ce lien] . Are those marked points because there is no more flow coming from 1+2 at this point?

It looks that way. The A-ports in the right-side picture have a much smaller cross-sectional area than those in the left-side picture and leave a large part of the cylinder bore unscavenged.

Citation :

How to measure exactly the vertical angle of ports?

I assume that you mean the axial port angles (which are usually vertical, but not always - Kreidler!) The simplest way is to push a piece of soldering wire in the port roof and the cylinder bore, take it out carefully and lay it on a graduated arc.

Frits Overmars a écrit:

Vortex a écrit:

Trying to understand the relation between different concepts in scavenging angles. [Vous devez être inscrit et connecté pour voir ce lien] As you can see 2 different cylinders but the same typ of engine. Do you think it is normal that on the model on right side the ports 1+2 can't join together in the middle?

With ports 1+2 you mean the A-ports and the B-ports? In your picture the flows do not join in the middle, but this may be a distorted view, depending on the axial angles. If the A-ports are steep enough, they will join the B-flow after all.
But I would prefer the left-side concept anyway.

Citation :

Here a picture from used piston with that configuration. [Vous devez être inscrit et connecté pour voir ce lien] . Are those marked points because there is no more flow coming from 1+2 at this point?

It looks that way. The A-ports in the right-side picture have a much smaller cross-sectional area than those in the left-side picture and leave a large part of the cylinder bore unscavenged.

Citation :

How to measure exactly the vertical angle of ports?

I assume that you mean the axial port angles (which are usually vertical, but not always - Kreidler!) The simplest way is to push a piece of soldering wire in the port roof and the cylinder bore, take it out carefully and lay it on a graduated arc.

Very fast, thank you Frits,
Axial i measure the A ports with 32° and B ports 15°.

Hello to all,

Frits and Jan, if i have only 2 transferports (A ports), can i use the RSA radial- and axialangles and the positionangles? Or won`t that work because the scavenging with only 2 transfers is totally different?

And with only 2 transferports would it also be good to have auxiliary exhaustports and trying to achieve maximum possible blowdown area?

Are there "general rules" i have to follow when working with only 2 transfers in comparison to a 5 transfer scavenging like in the rsa?

thank you!

I read somewhere(maybe in this topic) that when you have only two transfers that you need to aim them more than usual to the back of the cylinder and also give more axial angle so it can promote a sort of loop scavenge.

In those cases too much exhaust port might be detrimental and destroy the loop scavenge.

Martin1981 a écrit:

if i have only 2 transferports (A ports), can i use the RSA radial- and axialangles and the positionangles? Or won`t that work because the scavenging with only 2 transfers is totally different?

It's not totally different, but if you use the RSA position angles of just the A-ports, you will get a very limited total port area.

Citation :

with only 2 transferports would it also be good to have auxiliary exhaustports and trying to achieve maximum possible blowdown area?

Auxiliary exhaust ports will allow you to use a low exhaust timing (190°) and still have sufficient blowdown angle.area for the rpm you are planning to run (which may be severely limited by those small A-ports). But in case this rpm will be so low that you can get sufficient blowdown with 190° timing and just one exhaust port, it is better to use a single port because that will expose less exhaust duct area to the hot exhaust gases.

Citation :

Are there "general rules" i have to follow when working with only 2 transfers in comparison to a 5 transfer scavenging like in the rsa?

I never really thought of it. Maybe you could combine the A- and B-ports, but the piston ring may not like the combined port width.

Okay so the Position Angles of the RSA Cylinder won`t work for my Application. I try to make the Ports 24mm wide if the Pistonring agrees. The Stock Exhaustport is 22mm wide and the Pistonring does not complain. The Bore Diameter is 38mm, Stroke is 44mm. 130° Transferduration means 10,4mm high Transferports. So the Ports would not be soooo small, would they?

Another Thing that i am interested in: A Longstroke Engine is better than a Shortstroke. With 38mm Bore and 44mm Stroke we certainly talk about a Longstroke Engine.

But i could easilie get a Crankshaft with 46,48 or even 54mm Stroke. But somehow i got the Feeling that it makes no Sense to build an Engine with 38mm Bore and 54mm Stroke. Is there a Limit or a Point at one could say, the Engine it is "too much longstroke"?

Thanks

Is that a Sachs engine?

No Senso, Simson. One could ask why i do not try to tune a watercooled Engine with 5 Port Scavenging. But i want to keep it simple and without Welding or Epoxying the Simson Cylinder does not offer the Room for 5 decent Ducts.

Martin1981 a écrit:

Is there a Limit or a Point at one could say, the engine it is "too much longstroke"?

Yes, there will be a limit, or rather an optimum. But I can't tell you what it is.

Martin1981 a écrit:

...without welding or epoxying the Simson Cylinder does not offer the Room for 5 decent ducts.

This one does. Interested? contact [Vous devez être inscrit et connecté pour voir ce lien] in Zwickau. Or better still, pay them a visit; you'll like it.
[Vous devez être inscrit et connecté pour voir cette image]

Can you measure the stud pattern, looks like it might be able to use sachs cylinders, and those can be drilled for extra transfer ports and exhaust boost ports.
This is one I have done a couple years ago, 38x44mm:

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

Senso adding 3rd exh ports and additional transfers is easily doable in simson , people do this here on daily basis. You just need to know "how" to do it. To install this "RSA look" cylinder cases must be welded preety good i think because stock gasket surface is very little. So this cylinder is 4 transfers and 1 exh piston ported and propably nicasil ? , I wonder how fast "tuners" here would grind those dividers to 2mm (or less) thickness

@@ Martin1981

here is info about 2 transfer angles and STA from another great 2 stroke tuner called Wobbly from NZ

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A pair of transfers is a nightmare.
I did an old 390 aircooled Husky and the only way to get any power at all was to reliner and divide the HUGE transfer pair in two.
And for sure you cannot even think about using Frits or my design styles for what is essentially a lawnmower due to the pipe overscavenging
the transfer system.
This means that at BDC a good diffuser ( read fat ) will create so much negative pressure ratio, most of the uncontrolled transfer streams will do a U turn and exit the cylinder.
When it comes to the piston port, the only approach I can see is to create a port as wide as you want, then use as little timing as is needed to match the STA
to the blowdown and transfer numbers.
These small size pistons can easily go 80% of bore with the Exhaust, so the limiting issue is going to be how much transfer you can create.
My first guess would be 60* back and 30* up, that at least will create some sort of loop effect for you.
Is it possible to get a reed into the inlet, then a boost port can be added and this works well, going all the way back to Kaden et al.

thanks for the fast response to Frits, Senso and Muciek! i REALLY appreciate that!

Langtuning.... crazy guys in a positive way :-) they married the rsa design with the simson cylinder. will this work without the c port as the inlet is piston controlled? 

when the transfers open the inlet is already closed i guess. so there is not so much to suck for the exhaust. so the crankcase should be very big particularly in piston ported inlet engines? 
Frits, can one say that in general the piston port inlet is worse than the reed and the rotary? and if so, how much is the power worse in %?


Muciek, i know that it is possible to cut b ports in a simson cylinder. but can one really call it "ports"?
mostly the holes lack on inner radius. but maybe for power it is better to have bad b ports than having none at all?


I am reading the ESE thread with great interest also. yesterday i reached page 1000. i understand what wobbly says. maybe in a big cylinder (okay, what is a big cylinder) it is better to have more than 2 ports. but some 50cc cylinders....

look at langtunings 50cc aircooled race engines..... Jan Schäffer claims they have 17 HP! With only 2 transferports and pistonport inlet! so if they really have that power, maybe Frits can confirm, i would be very happy with that for the beginning... And adding b ports would mean the engine having 20 HP or so? Hart to believe.

Another Thing i Think about: Wobbly says that is very important that transfer and blowdown area match "correctly". The langtuning 17 HP engine with only 2 transfers does have Aux Exhaustports. I would concern about the blowdown much too big for only 2 transfers. but obviously, if that engine really pumps 17 hp, it seams to work.

I know, my questions and thoughts do not match the RSA topic so much, as the RSA engine is a very modern engine with 5 port scavenging. But in spite of that i am curious what Jan and Frits think about the 2 transfer system.

When comes to B ports I would go on them as much as I can without making hole outside cylinder radiator and then use epoxy to make inner radius sticking epoxy to sleeve. I'm too very curious about 2 transfer system since I deal with engines like that here and adding 4th ports is not possible.

as far as i know all aircooled simson GS cylinders had only 2 transfers. and the bikes were not the slowest....

it is possible to add b ports in a simson cylinder. but it is a mess. and why bother with that when you can have 17 hp with only 2 transfers. these 2 are huge of course.


langtunings m541 rsa cylinder is e revolution, no question. but i guess it`s price will also be :-P

That sachs cylinder that I showed earlier has a reed valve intake, so it as a bit more flexibility regarding the boost ports than a piston port intake, none the less I still think that there is a lot to improve in that port-map, specially the exhaust boost ports are limited to a 5x6mm exit duct, and the original sachs exhaust duct has a huge diameter, 32mm if I'm not mistaken, being a lot bigger than the optimal for a 50cc engine.

There is also a little fail in the main transfers, they should be aimed much higher, as they are they enter the cylinder almost horizontally and that shows in the burns marks in the piston crown.

Never the less with a single point ignition the engine revs up to 14k rpm, but its dead below 8k rpm, sadly that was my best at the time, today I think I could do better.

Not to forget the kreidler van veen engine, that put out 21hp, that engine had 4 transfers(plus boost? ) but it had only a single oval exhaust port, the magic for me is still in the right axial and radial transfer angles, at least to my limited knowledge.

Martin1981 a écrit:

when the transfers open the inlet is already closed i guess. so there is not so much to suck for the exhaust. so the crankcase should be very big particularly in piston ported inlet engines? 
Frits, can one say that in general the piston port inlet is worse than the reed and the rotary? and if so, how much is the power worse in %?...... look at langtunings 50cc aircooled race engines..... Jan Schäffer claims they have 17 HP! With only 2 transferports and pistonport inlet! so if they really have that power, maybe Frits can confirm, i would be very happy with that for the beginning... And adding b ports would mean the engine having 20 HP or so? Hart to believe.

I can confirm the 17 HP for the aircooled Langtuning cylinder with piston port induction. The crankcase volume and the combustion volume are big indeed. Adding B-ports would improve the power, but it is difficult to say by how much because then the A-ports would have to become smaller. But 20 HP seems possible.
Not many people have experimented with piston port induction recently, but Jan Schäffer estimates that it is as good as reed valve induction. Rotary valve induction is still the winner of these three systems: it gives about 10% more power than a reed valve.

Frits, big combustion volume..... are you talking about a 9:1 compression ratio that once was tested in bernsgrün by langtuning, and some time ago talked about in this thread?

Jawohl

  thanks Frits. if i only was able to write or speak dutch as good as you German. :-P

Yes Senso, the Angles.... Sometimes i think, why bothering so much with the angles.

When the Exhaustport(s) open, the exhaustgas leaves the cylinder because it is under pressure and the pipe sucks it out. when the transfers open ALL the exhaustgas SHOULD have left the cylinder. (So i totally understand the raised exhaustport floor idea.) if this was the case the fresh charge is not responsable for SCAVENGING the exhaustgas out of the cylinder. it would only need to FILL the cylinder. so there was no need for axial angles. okay, the axial angle would be good to avoid the fresh charge colliding  with the domed pistoncrown. and why being worried about short circuiting because of no radial angles, if the fresh charge gets sucked into the pipe anyway? It "only" has to push it back in the cylinder later.....

but if all that was so easy, we already had no radial angles anymore i guess.

It would be nice if all the exhaust gas would have left the cylinder by the time the transfer ports open. But pipe suction can only drop the cylinder pressure to about 0,5 bar absolute, so at the beginning of transfer there is still 0,5 bar of exhaust gas in the cylinder. It is not dense anymore, but it is useless (no oxygen left) and it is hot, so it has to be scavenged by the fresh charge before the latter is sucked into the exhaust duct.

hm, okay. 0,5 bar. then one must hope that the crankcasepressure is more than 0,5 bar. hopefully the pressure is not too low if the crankcase is too big. ups, sorry i just recognized... ABSOLUTE.



Frits, regarding the raised exhaustfloor, there is still something i am "concerned" about.

the residual exhaust pulse we need for true resonance....

it is created by the pressurewave from the initial pulse that is reflected by the bafflecone and then bounced of the pistonskirt. but if we lift the floor until the transferports upper edge, there is no pistonskirt between bdc and transferports upper edge the pressure wave can bounce of. 

so would the the lifted floor weaken the residual pulse or influence it in a bad way?

but on the other hand, there shouldn`t be a pressure pulse that charges the cylinder before the transfers are closed. so in this case it does not matter if the floor is lifted until transfers upper edge or if it is not.

Martin1981 a écrit:

hm, okay. 0,5 bar. then one must hope that the crankcasepressure is more than 0,5 bar. hopefully the pressure is not too low if the crankcase is too big.

When the transfers open, the crankcase pressure is about 1 bar absolute, even without any  piston pumping effect, so no problems there.

Citation :

Frits, regarding the raised exhaustfloor, there is still something i am "concerned" about. the residual exhaust pulse we need for true resonance.... it is created by the pressurewave from the initial pulse that is reflected by the bafflecone and then bounced of the pistonskirt. but if we lift the floor until the transferports upper edge, there is no pistonskirt between bdc and transferports upper edge the pressure wave can bounce of. so would the the lifted floor weaken the residual pulse or influence it in a bad way?

If the system is functioning as it should, the residual exhaust pulse is not reflected by the piston skirt but by the rising pressure in the cylinder.

Citation :

but on the other hand, there shouldn`t be a pressure pulse that charges the cylinder before the transfers are closed. so in this case it does not matter if the floor is lifted until transfers upper edge or if it is not.

Right. Two-strokes are a nice brain puzzle, aren't they?

Frits or Jan,
I am going to raise the floor or the exhaust port on our tz350 2mm as the RSA.
Would changing the exhaust exit from round to oval as on RSA have the same benefits, even though it only is a single exhaust port design
many thanks
Paul

Paul Gane a écrit:

I am going to raise the floor or the exhaust port on our tz350 2mm as the RSA. Would changing the exhaust exit from round to oval as on RSA have the same benefits, even though it only is a single exhaust port design.

Hard to say, Paul. My first reaction is: don't bother. But then I've never tried it, so who knows.
A TZ350 dearly needs more blowdown angle.area but the cylinder studs prevent adding auxiliary exhaust ducts; maybe you can improve the exhaust outflow by radiusing the exhaust window's top edge.
In any case, do not go searching power through revs; revving it will only increase the thermal load.

Has it ever been tried to campfer/round of the piston edges in front of the transfer ports ? as the piston edge is almost all the time in front of the transfers, shouldn't the inflow off the gasses benefit from a rounded edge as oposed to a straight edge that should cause turbulence where it is not needed ?

If your answer is "go ahead and try it" , how much material would you recommend to remain above the top ring at least (I would try it on a 50cc moped)