[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 1) (Locked)

if I have a sixth sense i think this 24/7 inlet shows its full potential with a very large crankcase volume, i was thinking about a high primary compression might help any mixture backflow that would eventually occour... the test michael did today was on an am6, which surely has a high compression because it has structural limits and is designed to run a 50cc...
so, i think i'm getting a longer conrod for my scooter engine ^^

Hi nasone, Frits told at me at page 29 the same thing
citation"The best way to adopt your engine to this system is probably to make the crankcase volume really large" . Also if I was building the valve I respectet that this 24/7 inlet system will need big crankcase volume. I choosen this Am6 engine because specally this one got a large crankcase volume:-) I was also thinking about to put a 5mm longer rod at the crankshaft, before doing the next tests.

Hi guys,
I love the way you work together
So I can retire and go live in a warm country without snow, like my old friend Jan Thiel did, hm ?

Citation :

this 24/7 inlet shows its full potential with a very large crankcase volume

could you explain why ?

of course.
in a normal reed caged engine, the reeds stay closed when the piston is around bdc. in this moment the pressure in the crankcase is higher than in the intake duct, so the closed reeds are preventing any mixture from escaping the crankcase.

we don't want to let it escape, we want it to stay near the tranfer ducts, ready to be sucked (or pumped, if we live in the '50 ) in the cylinder.

with the 24/7 inlet the mixture can theorically escape in any moment because the inlet is always open, we can reduce this to a minimum by working with waves, inertia forces (helmholtz resonance mentioned before) but another thing that we can do is preventing the pressure to go higher by increasing the volume, so lowering the crankcase primary compression.

also for the exhaust pipe is easier to suck from a large volume than from a small volume so this makes things even better.

I think the real problem is not the backflowing itself, but that once the mixture start to backflow you need to waste some energy later to stop it and make it go back in the crankcase.

Frits, thank you but you can not retire, there's still a lot of work to do here (just joking, you deserve some relax after all... but make sure you have an internet connection!)

Hello , and congratulation to Nasone and Michael for their job .

In our standard reed caged engines (CVT scooter ENGINES or in Mickael 6gears AM6 minarelli engines ) we use primary compression ratio such as 1.13:1 .
According to you Frits , with this kind of "revolutionnary system" , what kind of value could help the good job of this system ?
If in our engines the best is 1.13:1 with 90.32CC (it means arround 700cc ) .
But with our small crankcases , we will have , or to put smaller engine (cilinder/piston), or to put some very long conrod . 10MM of conrod will represent for exemple about 50CC more in primary compression ratio that is to say 1.12:1 .... is it enough ? Does a Long conrod will bring inconvenients ?
What do you think ?
For tests do you think that , because of our limitated standard crankcases , we will have to reduce the main CC of the engine ?

nasone32 a écrit:

I think the real problem is not the backflowing itself, but that once the mixture start to backflow you need to waste some energy later to stop it and make it go back in the crankcase.

Quite right. The ideal situation would be (and can be) that the inward flow from carburettor to crankcase just comes to a halt and then is accelerated again for the next intake phase, before it gets a chance to start flowing backwards.
We could try a combination of the 24/7-valve with a trombone-type intake duct. First the intake duct is long and the reed valve is closed. Then, as the engine comes into its power band, the reed valve is pushed open. Then the intake duct is gradually shortened, in accordance with the rising engine rpm.
Tests should tell us if this combination is sensible; maybe it is unnessary and we can stick to the simple valve.
I love simplicity (just compare the 24/7-valve with the complex gearing for the RSA-rotary valve).

Michael, do you think you can build and test such a combination before the weekend ?
Well, you will probably rely on your variomatic to keep the engine rpm constant. If variomatics had not been forbidden in motorcycle racing, I would already have used one a century ago

fab evospeed a écrit:

In our standard reed caged engines (CVT scooter ENGINES or in Mickael 6gears AM6 minarelli engines ) we use primary compression ratio such as 1.13:1.

That is really low. In the Aprilia RSA125 this ratio is 1,24 (650 cc at TDC; 525 cc at BDC). But maybe the crankcase volume still was not big enough. Jan Thiel always wanted to perform this test, but the technical director did not see the need.

Citation :

According to you Frits , with this kind of "revolutionnary system" , what kind of value could help the good job of this system ?

I don't know. We need a certain Helmholtz frequency, but there are several ways to achieve it:
via the volume, via the tract length and via the tract diameter.

Citation :

Does a long conrod bring inconvenients ?

I have never seen any negative effects with long conrods, only positive ones.

Citation :

For tests do you think that , because of our limitated standard crankcases , we will have to reduce the main CC of the engine ?

That may be a good idea. Not only because of the primary compression ratio, but also because of the time.areas for the ports. In most big-bore engines the time.areas are too small for the rpm that they are forced to run (forced by exhaust pipes with too high resonance frequencies).

Hello guys,
I have seen in some old books that yamaha had studied a small chamber joined to the intake duct to prevent these bad configurations with reeds valves (for the twin 350cc), and I think it a easy option to test too...if you want(or can)
don't stop the movement !! GAZ [Vous devez être inscrit et connecté pour voir cette image]

a longer conrod should help on the time area side too.
Fab I had a few am6, but never figured that they have such a big crankcase volume!
are you sure you can have 700cc in that 50cc engine? that's even more than RSA!
the best I've seen on a 70cc scooter engine even using a longer conrod, spacer under the reed cage and enlarged crankcase (around the crank webs) is something on 400cc...

nasone32 (Jeu 16 Déc - 17:46) a écrit:

a longer conrod should help on the time area side too.
Fab I had a few am6, but never figured that they have such a big crankcase volume!
are you sure you can have 700cc in that 50cc engine? that's even more than RSA!
the best I've seen on a 70cc scooter engine even using a longer conrod, spacer under the reed cage and enlarged crankcase (around the crank webs) is something on 400cc...

Exact Nasone32 , from my side , without working on this system but to preview increased cc parts such as the "big evo from polini" , i made some weld on the 50 cc crankcase . before this , I succeed in reaching (sorry if languages mistakes ) 420 cc in a piaggio engine with 50CC (1.12) .
I supose that with weld i have more than this now but i did not took the mesurement of it.For the AM6 i never worked on it if you compare to Mickael who made thousands of proves at dyno.


@Frits : thanks for replies .
Another question concerning this primary volume could be :Is it logical for you to obtain more volume under the cylinder than a long intake system ( i mean from valve to crankshaft for exemple) ?

fab evospeed a écrit:

Is it logical for you to obtain more volume under the cylinder than a long intake system ( i mean from valve to crankshaft for exemple) ?

Like I said, Fab: I don't know. We will have to test the various possibilities. And of course you can have a large volume and a long intake tract; then you would need an enormous carburettor to get the desired Helmholtz frequency.
All I can say now: I do not think that we need a smaller crankcase volume or a larger carburettor than the RSA has. Let us play with the length first; that is also the easiest thing to vary.

Marc (Jeu 16 Déc - 18:57) a écrit:

Frits Overmars (Jeu 16 Déc 2010 - 1:28) a écrit:

Hi guys,
I love the way you work together
So I can retire and go live in a warm country without snow, like my old friend Jan Thiel did, hm ?

Sorry, NO!

There is a lot of rain (and sometimes Tsunami) in Thailand...
It's better for that you stay in Nederland...... and connected!

at the end , if you are very fed up with Nederland , you can come at home : in Réunion island (indian ocean ) there is sun and mechanical sports

fab evospeed a écrit:

Marc (Jeu 16 Déc - 18:57) a écrit:

Frits Overmars (Jeu 16 Déc 2010 - 1:28) a écrit:

Hi guys,
I love the way you work together
So I can retire and go live in a warm country without snow, like my old friend Jan Thiel did, hm ?

Sorry, NO!

There is a lot of rain (and sometimes Tsunami) in Thailand...
It's better for that you stay in Nederland...... and connected!

at the end , if you are very fed up with Nederland , you can come at home : in Réunion island (indian ocean ) there is sun and mechanical sports

there's not much more a man could want, right? :)

yes except that for the moment the only real circuit is growing little by little and is only 1.3km long the place can be worst !
From my part every 2 stroke appasionated guy from this forum who wants to visit reunion island will be welcomed at home !!For sure :)

sorry for of topic .

Marc a écrit:

Frits Overmars (Jeu 16 Déc 2010 - 1:28) a écrit:

Hi guys,
So I can retire and go live in a warm country without snow, like my old friend Jan Thiel did, hm ?

Sorry, NO!

There is a lot of rain (and sometimes Tsunami) in Thailand...
It's better for that you stay in Nederland...... and connected!

+1 Marc, Merci beaucoup Mr Overmars pour toutes ces informations!
ce forum est vraiment top il faut le dire, c'est pas tous les jours qu'on discute avec des grands messieurs comme Eric Offenstadt ou Frits Overmars par exemple....

Merci et Bravo Marc!

Hi Guys,
sorry that I couldent pay attention to this topic today, because of testing the24/7valve I put way more important work by side and now I got to do this.......(if not my boss will killing me next week:-).

@Frits,
a 1000 times sorry but I could not work at all the next days on the projekt

I think it would be a good idea to test a variable intake dukt,but I think also before doing this test the crankcase volume should be optimal figured out at high rpm range , and than to extend the intake dukt to its optimal perfomance at low rpm range.If I got this data I only got to do a variable intake dukt construktion.If I will do this test at my engine like it is at the moment,with an crankcase volume wich is 100% to small!, this test wuold do no sence.
I will do my best to work a little bit this weekend on It but I can not promise anything.

we can easily imagine a helmholtz chamber on the intake duct, like yamaha YEIS but with control in the same time than reeds valve. And in this case have a smallest volume for the reed and a bigger one without.

i have make some test with my old 125DTR ( YPVS, YEIS ... )

with and without YEIS -> nothing remarkable

even to say, better answer without the suspicious device ...

pierre a écrit:

we can easily imagine a helmholtz chamber on the intake duct, like yamaha YEIS but with control in the same time than reeds valve. And in this case have a smallest volume for the reed and a bigger one without.

I can easily imagine a lot of confusion when you complicate the system with a YEIS-type chamber.
A system with only the crankcase volume, the 24/7-valve and the intake tract is already complicated enough.
You must consider that Michael has to start at zero with his research; he cannot look at previous solutions because there aren't any. So let's keep things simple; otherwise he might never find the right combination.

I thought not to complicate the system but make it more obvious / easy by operating with more volume.

with such a system you insert other resonances, if we need more volume in the intake we just use a bigger carb...

Frits, looking at some charts you posted before (about the symmetric engine) I noticed how the effective exhaust (and transfer also) phase starts later than geometrical. always known this but never known how to calculate it. I think it is related to inertial phenomena but not only... something like that until some port area has been uncovered the flow can not really start. I don't want any formulas but a little explanation would be really nice...
these things probably are really little details, but i think details are very important to have a full understanding of what happens in the running engine (and knowledge is power by the way )

nasone32 a écrit:

Frits, looking at some charts you posted before (about the symmetric engine) I noticed how the effective exhaust (and transfer also) phase starts later than geometrical. always known this but never known how to calculate it. I think it is related to inertial phenomena but not only... something like that until some port area has been uncovered the flow can not really start.

You noticed right, Nasone. The geometrical opening point of a port offers no occasion to start either a wave action or a mass motion. For this you need a combination of a pressure differential (which is certainly present at the exhaust port, but not at the transfer ports) and an open area. But one cannot quote a fixed value for this area. Specific time*area would be the closest suitable concept, but even this is not decisive.
All depends on the initial flow behaviour through the orifice, formed by port ceiling and piston edge, and this behaviour is one of the most difficult events to lay down in a mathematical model. You might call it the 'big bang' of gas dynamics.

Citation :

I don't want any formulas but a little explanation would be really nice... These things probably are really little details, but i think details are very important to have a full understanding of what happens in the running engine.

These 'little details' are fundamental, but I do not think this forum is the place to dive this deep in the theory. Besides, it would cost me more time than I have.

Citation :

Besides, it would cost me more time than I have.

I undestand that so i'm thanking you, your answer was enough to have a place to start some research.

nasone32 a écrit:

If anybody wants it just let me know.

Yes please, thank you very much