[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 3) (Locked)

Vagelis, can you post stinguer sizes in a way they could be seen

Romeuh,

Stinger nozzles are like this in 21 & 102:
-- D1~D2/L
1) 25.5~23.3/8 mm
2) 23.3~23.3/10 mm
3) 23.3~24.5/14 mm

Stinger length is just an assumption, it's not 200mm.

In pipe 102 it's like this:
1) 23.3~23.3/10 mm
2) 23.3~24.5/14 mm
But this is also an assumption. based on the other two designs. Original design by Frits ends with the 51~23.3/50 cone.

romeuh80 a écrit:

Frits Overmars a écrit:

Vagelis a écrit:

Frits, would this pipe be an Aprilia pipe?.....It looks very much like the RSA pipe design!

It does, but it looks even more like an older Aprilia pipe, designated Tubo 55.
I suppose someone has hand-measured a Tubo-55 because the dimensions differ in some places, but only very slightly.
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I wonder if those tiny differences could be felt on dyno...

Tiny differences can certainly be felt on a dyno, even 1mm!

Hi all,

I just measured the crankcase volume of my new 80cm³ reed valve engine and factor 6 is what I got out, so it is really big.
Does somebody have experiences (and the kindness to share them :) ), if a crankcase can be "too big", so that it would cost power?

The engine will be on the dyno in a few weeks, I will show you the results then.

Regards,
Daniel

SB07 a écrit:

PVO a écrit:

Frits, can you tell me more about the additive manufacturing process you're going through to make cylinders? Who are you using for the work and can any type of aluminum be used? I've been researching this as a way to make some prototypes I want to hear your thoughts on it. Thanks.

Paul

Hi Paul!

for my cylinder...I have contacted Marchetto-group

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i sent them 3D drawing

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Hello i wonder how you like the cilinders. I am realy interested about this type of manufacturing.

Regards
Sander

Frits Overmars a écrit:

Vagelis a écrit:

About the A-tubo designs (21, 102 and now 55).. Were they part of a 100+ pipe testing procedure over Jan's years at Aprilia? Or they were code named after something?

Just a matter of evolution: after tubo # 1 came tubo #2, and so on.

Citation :

I take the liberty to post all three designs together. Hope you don't mind, since they were already in public

No problem.

And after No. 100 we restarted with No. 1

Daniel A. a écrit:

Hi all,

I just measured the crankcase volume of my new 80cm³ reed valve engine and factor 6 is what I got out, so it is really big.
Does somebody have experiences (and the kindness to share them :) ), if a crankcase can be "too big", so that it would cost power?

The engine will be on the dyno in a few weeks, I will show you the results then.

Regards,
Daniel

What do you mean by factor 6?
6 times cubic capacity?
In TDC or BDC?

I never experienced a crankcase 'too big'
And I am still curious!
Somewhere there must be a limit.

Yes, 6 times cubic capacity, so 480cm³. In TDC.

But didn't you write somewhere, that you extended the RSA conrod until you got the best curve? That would mean that with conrods longer than 120mm the power was worse, or maybe not?

The conrod in my engine is 95mm.

You once wrote, that a reed valve engine might need a smaller crankcase volume than a rotary valve engine. Do you suspect a reason for this?

However, yesterday I dissambled the engine to put some plugs into the balancing boreholes of the crankshaft, because somehow it disturbed me to know that it is really SO big. It will have ~465cm³ then (factor 5,83). Still the biggest crankcase I've heard of so far

By the way: It is as always an honor to be able to write with you about such things!

Regards,
Daniel

Sanderhoutman a écrit:

SB07 a écrit:

PVO a écrit:

Frits, can you tell me more about the additive manufacturing process you're going through to make cylinders? Who are you using for the work and can any type of aluminum be used? I've been researching this as a way to make some prototypes I want to hear your thoughts on it. Thanks.

Paul

Hi Paul!

for my cylinder...I have contacted Marchetto-group

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

i sent them 3D drawing

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

Hello i wonder how you like the cilinders. I am realy interested about this type of manufacturing.

Regards
Sander

The couple I've been involved with making ended up being cast so I can't comment directly. Hopefully Stefano will chime in as I believe his were done via sintering.

Paul

Daniel A. a écrit:

Yes, 6 times cubic capacity, so 480cm³. In TDC.

But didn't you write somewhere, that you extended the RSA conrod until you got the best curve? That would mean that with conrods longer than 120mm the power was worse, or maybe not?

The conrod in my engine is 95mm.

You once wrote, that a reed valve engine might need a smaller crankcase volume than a rotary valve engine. Do you suspect a reason for this?

However, yesterday I dissambled the engine to put some plugs into the balancing boreholes of the crankshaft, because somehow it disturbed me to know that it is really SO big. It will have ~465cm³ then (factor 5,83). Still the biggest crankcase I've heard of so far

By the way: It is as always an honor to be able to write with you about such things!

Regards,
Daniel

The longer conrod was used to improve inlet flow.
Still longer conrods were not tried.

About crankcase volume I can only say that there must be a limit somewhere.
But I never had the possibility to find it!
And my experience with reed-valve engines is relatively small!
Your conrod length seems about right.

Vagelis a écrit:

It would be interesting to hear about the pipe development story, if you or Jan have some spare time (and wish to spare it here). :)

Waiting for this

Jan Thiel a écrit:

About crankcase volume I can only say that there must be a limit somewhere.
But I never had the possibility to find it!
And my experience with reed-valve engines is relatively small!
Your conrod length seems about right.

I was thinking a lot about the crankcase volume too. The Derbi 50cc engines have a rather big crankcase volume and usually react positive if you reduce conrod length from 90mm to 85mm.

My theory is somehow that their is a limit and once it is reached the mass gets too inert. But I'm only guessing...

Hi,

I was just thinking for a while and this is my theory:

There are 2 factors causing the fresh charge to flow from the crankcase up to the cylinder:
Factor 1) the precompression, "pushing" the charge up
Factor 2) the sucking exhaust, "pulling" the charge up

On the one hand: The bigger the crankcase, the worse the precompression and the worse the "pushing up".

And on the other hand: The bigger the crankcase, the more volume to suck from. And now it seems logical, that there is only a certain volume needed to suck from and not an infinite volume, because you can not get an infinite mass through the transfers. If you have more than that certain volume, factor 2 will not improve power anymore, because the transfers can not carry more than that certain amount of charge.

BUT: Factor 1 makes the power worse if you still enlarge the volume more and more.

That would mean, that the optimal crankcase volume was depending on the transfers time area / flow efficiency. The crankcase volume needs to be as much as necessary to make factor 2 work perfectly, but should not be more than that because of factor 1.

It would also mean, that as long as enlarging the transfers results in more power (and Jan once said, that every mm² transfer area brought power), the crankcase is too big(!), hmm

What do you think? Maybe the RSA's crankcase IS already too big? And the longer conrod only improved power, because of the better flow under the piston?

Wild theory, I know

Best regards, and please don't offend me, if this might be complete nonsense

Daniel A. a écrit:

....it seems logical, that there is only a certain volume needed to suck from and not an infinite volume, because you can not get an infinite mass through the transfers. If you have more than that certain volume, factor 2 will not improve power anymore, because the transfers can not carry more than that certain amount of charge....
That would mean, that the optimal crankcase volume was depending on the transfers time area / flow efficiency.

During the transfer of mixture the crankcase pressure drops. The larger the crankcase volume, the smaller that pressure drop will be. That means, a large crankcase delivers its mixture with a higher average pressure, thus with a higher average density. And even if the flow volume were limited by time.area and flow efficiency, a denser mixture means more mixture mass and thus more combustion energy per cycle.

The great thing to note here is that the RSA is a rotary inlet engine and not reed valve inlet, those need some pressure difference to have good lift to achieve big inlet areas, if there is a small pressure difference there is little reed lift and a very bad flow obstruction.

A couple of friend of mine have already noted that with too big crankcase volume they get lazy engines that don't want to rev, the base are dt lc 50(5R2) that have 55mm italkits from the dt80 engines, and instead of using the 42mm stroke crank with 84mm con-rod they use 44mm stroke with cr80/85 94mm con-rods, that leaves them with a 8mm spacer, and has the cylinder has a super small intake there is also the need to mount 10mm spacers to mount the tzr 50 reed cages, the results they get is that the engine performs much better with some more grinding and 8mm spacers in the reeds, with the 10mm ones the engine doesn't want to rev and as a big loss of power, but well nothing is those engines or cylinders for the matter of fact is a good thing, the ducts are in the best case scenario 10mm wide, the crankcase duct ramps are very small and step, the intake is super small for a 100-105cc engine..

Frits Overmars a écrit:

Daniel A. a écrit:

....it seems logical, that there is only a certain volume needed to suck from and not an infinite volume, because you can not get an infinite mass through the transfers. If you have more than that certain volume, factor 2 will not improve power anymore, because the transfers can not carry more than that certain amount of charge....
That would mean, that the optimal crankcase volume was depending on the transfers time area / flow efficiency.

During the transfer of mixture the crankcase pressure drops. The larger the crankcase volume, the smaller that pressure drop will be. That means, a large crankcase delivers its mixture with a higher average pressure, thus with a higher average density. And even if the flow volume were limited by time.area and flow efficiency, a denser mixture means more mixture mass and thus more combustion energy per cycle.

Hi

But the pressure in the cylinder at the opening of transfers at 13k rpm would'nt be higher than a low cranck compression like the RSA?

Would the crankcase pressure of a considerable high crank compression engine making some real rpm have time to drop to the pressure present in the cylinder at the moment of tranfer closing?

Frits Overmars a écrit:

During the transfer of mixture the crankcase pressure drops. The larger the crankcase volume, the smaller that pressure drop will be. That means, a large crankcase delivers its mixture with a higher average pressure, thus with a higher average density. And even if the flow volume were limited by time.area and flow efficiency, a denser mixture means more mixture mass and thus more combustion energy per cycle.

I don't understand, how you conclude to the bold sentence.

I agree, that the larger the crankcase volume, the smaller the pressure drop will be. But in my mind this is, because the pressure in the crankcase right before transfer opening is already lower with a large crankcase than with a small crankcase.

In both cases ("large" and "small" crankcase) the pressure after the drop is atmospheric, or am I wrong? So why does a larger crankcase deliver its mixture with a higher average pressure?

The funny thing is: I believe, that you are right Frits. But of course I want to understand, why, too

Regards

romeuh80 a écrit:

But the pressure in the cylinder at the opening of transfers at 13k rpm would'nt be higher than a low cranck compression like the RSA?

The pressure in the cylinder at transfer opening is entirely dependent on the efficiency of the exhaust pipe.

Citation :

Would the crankcase pressure of a considerable high crank compression engine making some real rpm have time to drop to the pressure present in the cylinder at the moment of tranfer closing?

The pressure in any crankcase will be lower than the pressure in the cylinder at transfer closing because of the kinetic energy of the transfer flow. At the beginning of transfer this flow is accelerated and this acceleration continues as long as the transfer pressure is higher than the cylinder pressure. When both pressures are equal, the flow has reached its maximum velocity.

Next the crankcase pressure keeps dropping and the now increasing negative pressure differential will slow the flow down until it comes to a stop when the pressure in the cylinder is higher than the crankcase pressure by the same amount as it was lower than the crankcase pressure at the beginning of transfer.

Daniel a écrit:

I agree, that the larger the crankcase volume, the smaller the pressure drop will be. But in my mind this is, because the pressure in the crankcase right before transfer opening is already lower with a large crankcase than with a small crankcase.

If the crankcase would behave like a pump, you would be right. But when the engine is running in the power band, it is no longer a pump; it is a resonator.

Citation :

In both cases ("large" and "small" crankcase) the pressure after the drop is atmospheric, or am I wrong?

Yes, you're wrong; see my answer to Romeuh80 above.

Citation :

The funny thing is: I believe, that you are right Frits. But of course I want to understand, why.

That is exactly the same motivation that drives me.

Frits Overmars a écrit:

romeuh80 a écrit:

But the pressure in the cylinder at the opening of transfers at 13k rpm would'nt be higher than a low cranck compression like the RSA?

The pressure in the cylinder at transfer opening is entirely dependent on the efficiency of the exhaust pipe.

Citation :

Would the crankcase pressure of a considerable high crank compression engine making some real rpm have time to drop to the pressure present in the cylinder at the moment of tranfer closing?

The pressure in any crankcase will be lower than the pressure in the cylinder at transfer closing because of the kinetic energy of the transfer flow. At the beginning of transfer this flow is accelerated and this acceleration continues as long as the transfer pressure is higher than the cylinder pressure. When both pressures are equal, the flow has reached its maximum velocity.

Next the crankcase pressure keeps dropping and the now increasing negative pressure differential will slow the flow down until it comes to a stop when the pressure in the cylinder is higher than the crankcase pressure by the same amount as it was lower than the crankcase pressure at the beginning of transfer.

What a Knowledge

I think I understand.

So the size of the transferes (volume mostly from crank to cylinder) also plays a good role, if they are too big the gas would not reach very high speed, and we would have lower kinetic energy. Maybe it would carry more mass, but kinetic energy is (mass * speed^2)/2 , soo speed is somewhat more importante in the transfers for that effect to work best.

Sorry for the bad english

Thanks

Frits Overmars a écrit:

Daniel a écrit:

I agree, that the larger the crankcase volume, the smaller the pressure drop will be. But in my mind this is, because the pressure in the crankcase right before transfer opening is already lower with a large crankcase than with a small crankcase.

If the crankcase would behave like a pump, you would be right. But when the engine is running in the power band, it is no longer a pump; it is a resonator.

I know that it is more a resonator than a pump. But I thought that it is still "a bit" a pump (factor 1 in my "theory"). To be precise, I thought that it is a pump exactly until that point, where the transfers open. And so I thought, that the crankcase pressure was lower right before transfer opening with a larger crankcase, because the compression of the relatively small piston results in a higher pressure if acting on a smaller volume. Maybe I forget the influences of the previous cycle

All in all: You still think, the bigger, the better, Frits?

Daniel A. a écrit:

Frits Overmars a écrit:

If the crankcase would behave like a pump, you would be right. But when the engine is running in the power band, it is no longer a pump; it is a resonator.

I thought that it is a pump exactly until that point, where the transfers open.

That is right. But it is a very ineffective pump: its volume ratio is only between inlet closing and transfer opening.

Citation :

I thought, that the crankcase pressure was lower right before transfer opening with a larger crankcase. Maybe I forget the influences of the previous cycle

Yes, that may be the case.

Citation :

All in all: You still think, the bigger, the better, Frits?

A large crankcase volume is good but with a very large case the flow velocity inside the case will become so low that fuel and air separate. If we had direct fuel injection, I would definitely say 'the bigger the better'. In practice this means opening the crankcase to the atmosphere as soon as the pressure inside the case allows it. With a good engine running at rpm of maximum torque that would be about 15° after BDC. In theory that is also valid for an engine with a carburetter but in practice the pulse signal to the carburetter jets would be so weak that it would be difficult to achieve good carburation.

Okay, so no matter if the crankcase is "small" or "large", in the one case the pump is "very ineffective" and in the other case it is "very very ineffective". So regarding this the crankcase volume doesn't allow big differences.

But I still don't understand, why a smaller pressure drop means, that the mixture is delivered with a higher average pressure :/

Regards

Sorry to get in between your conversations but only for this simple mathematical idea

Inicial pressure - final pressure/2 = mean (average) pressure

If final pressure is near inicial pressure -> small pressure drop, mean (average) pressure value remains higher than if final pressure value is lower - > bigger pressure drop

Thank you Frits and Jan for this great topic that allowed me to confirm some ideas that were wrong in books but right for my brain

Regards

I asked this question on the kiwi biker forum but couldn't get an answer. On the rsa did you try running it with a wasted spark on the dyno and if so was there a gain to be had at all. I know on a two stroke there is gains to be had by indexing the plug to help reduce fouling of the plug. I am wondering the extra spark at bdc could help achieve the same thing