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

Hi Frits,
I am sorry about that - like most of the people here, it is hard not to ask questions when you have such knowledgeable people present and I do appreciate your time and patience.     

If I can leave you one last quote by Jan on this matter:
At high rpm there is some reverse flow, the transfer ducts becoming black, and even the
crankcase and carburettor bell mouth because of the entering burned gasses.
This happens because with raising rpm, the pre-exhaust flow becomes insufficient.
When rpm rises still more there is no more transfer flow and the engine stops completely.
So transfer flow depends completely from what is 'allowed' by the exhaust flow!

Regards and many thanks Fritz   
Allan.

Jan & Frits

The old 258 rotax is quoted as having a coolant flow rate of 55 L/pm @ 12,000 rpm
Given the advancements of the RSW & RSA engines do you know what the flow figures are upon these engines ?

Frits Overmars a écrit:

I think we should not dive too deep into this subject here, Allan, or we might loose all our readers. After all, this is a motorcycle forum, not a physics lecture.

Physics is what got me here in the first place

RAW a écrit:

The old 258 rotax is quoted as  having a coolant flow rate of 55 L/pm @ 12,000 rpm
Given the advancements of the RSW & RSA engines  do you know what the flow figures are upon these engines ?

If I remember correctly, the RSA125 flowed 60 liter per minute, more than twice the amount of the 250 cc Rotax 258.

teriks a écrit:

Physics is what got me here in the first place

Do you mean mummy & daddy-physics or something more technical?  .

Frits Overmars a écrit:

teriks a écrit:

Physics is what got me here in the first place

Do you mean mummy & daddy-physics or something more technical?  .

Well, both, but the former migh be classified as biology :)

GtG001 a écrit:

Hi Frits,
I am sorry about that - like most of the people here, it is hard not to ask questions when you have such knowledgeable people present and I do appreciate your time and patience.     

If I can leave you one last quote by Jan on this matter:
At high rpm there is some reverse flow, the transfer ducts becoming black, and even the
crankcase and carburettor bell mouth because of the entering burned gasses.
This happens because with raising rpm, the pre-exhaust flow becomes insufficient.
When rpm rises still more there is no more transfer flow and the engine stops completely.
So transfer flow depends completely from what is 'allowed' by the exhaust flow!

Regards and many thanks Fritz   
Allan.

At transfer open there will still be higher pressure above the piston than in the crankcase. Burnt fuel in the transfer ducts is a symptom of too little blowdown.

I would think that the pressure in the transfer passages would mainly be a function of the exhaust system, port timing and the rpm. I would suggest that if a well balanced (not crankshaft) engine was running at its optimum "tuned"speed that there would be no blowdown into the transfers and that no carbon would form. Outside the "tuned window" then it could be well possible, as evidence the formation of carbon. As always, I could be wrong though.
Ken

Ken Seeber a écrit:

I would think that the pressure in the transfer passages would mainly be a function of the exhaust system, port timing and the rpm.  I would suggest that if a well balanced (not crankshaft) engine was running at its optimum "tuned"speed that there would be no blowdown into the transfers and that no carbon would form.  Outside the "tuned window" then it could be well possible, as evidence the formation of carbon. As always, I could be wrong though.

Not this time Ken; you are quite right  .
But rev any engine high enough and the blowdown time.area will become too small. Or use 'normal' revs with the throttle almost closed and the crankcase pressure will be so low that even with ample blowdown the cylinder pressure will not drop below the crankcase pressure before the transfers open. The latter situation was notorious on the racing Aprilias: they never detonated under full power but they could detonate like hell at 10% throttle.

dear mr Frits
as all lovers of 2-stroke, i have read all the pages with a real interest. I have questions concerning the combustion chambers and transfert port
I see the combustion chamber have a squish area about 50 percent, did you try more ? do you think that is a valid value for largest bore (i think for example a bore about 82 mm) with unleaded 98 ?
And last questions    whats happens if theres chamfers at the end of the transfers ports?

sorry for my poor english (but is best than google translate   )

cordially

pagi a écrit:

I see the combustion chamber have a squish area about 50 percent, did you try more ? do you think that is a valid value for largest bore (i think for example a bore about 82 mm) with unleaded 98 ?

In the past, especially direct-drive kart engines used a lot more than 50%. And today, 6-speed kart engines use a lot less because this will reduce the squish effect which will in turn slow down the combustion. These engines are not allowed to use a variable ignition timing and slowing down the combustion is another way of generating hotter exhaust gases at high revs, which will improve overrev.
50% squish area should be a good value for any engine that has no ignition restrictions, irrespective of the bore size.

Citation :

whats happens if theres chamfers at the end of the transfers ports?

I assume that you are talking about the ports' upper ends, where they enter the cylinder. A chamfer there will make it easier for exhaust gas to enter the transfer ducts, which is undesirable.

thank you for the answer  

   
Thank you Frits!

quiko a écrit:

   
Thank you Frits!

A subject that gets no interest at all in our ignition discussions is the actual amount of ignition energy. Most probable cause is the apparent total lack of knwowledge about tis subject.
All ignition advance values dont mean a thing if not accompanied by the current thru the spark plug. Mentioning an amount of millijoules also does not say very much, was it the calculated amount (as more or less usual ) or the actual measured amount ? The only thing really that does the job of igniting the mixture is the current thru the spark plug, and nothing else, I am afraid.
Applying a huge amount of energy to the plug and consequently more current enables the tuner to run considerable richer mixtures and as a result of this a lower piston crown temperature with all the benefits of it. After all this piston crown is the ony real vulnerable part in all of our dum dum up and down piston engines.

PS If you are really interested I can have your ignition system evaluated about its energy quality, be however prepared the measuring results can be shocking.......

Tada for now

I have a question for Mr. Frits

what is the function of this type of exhaust? what are the differences with the classical model? Thank you


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

Frits Overmars a écrit:

pagi a écrit:

I see the combustion chamber have a squish area about 50 percent, did you try more ? do you think that is a valid value for largest bore (i think for example a bore about 82 mm) with unleaded 98 ?

In the past, especially direct-drive kart engines used a lot more than 50%. And today, 6-speed kart engines use a lot less because this will reduce the squish effect which will in turn slow down the combustion. These engines are not allowed to use a variable ignition timing and slowing down the combustion is another way of generating hotter exhaust gases at high revs, which will improve overrev.
50% squish area should be a good value for any engine that has no ignition restrictions, irrespective of the bore size.

Citation :

whats happens if theres chamfers at the end of the transfers ports?

I assume that you are talking about the ports' upper ends, where they enter the cylinder. A chamfer there will make it easier for exhaust gas to enter the transfer ducts, which is undesirable.

hi

Frits did the reduced squish area in the 6-speed kart engines has a variable effect over the rpm range? if yes how does it works that it doesnt make hotter gas in the low rpm?
other wise you can only change the ignition timing and you also get hotter
exhaust gases.

thanks Manuel

Manuel Rainer,
I believe Frits has already explained this elsewhere, here's what he said:

Is it useful to slow down combustion on some operating phases? Yes and no. I will try to explain
this in a little physics lesson.
No matter which way the piston is moving, as long as there is combustion, the pressure in the
cylinder keeps rising. Only after combustion is finished the expansion of the burnt gas can begin.
Slow combustion means that the piston is already well on its way down before expansion can begin;
it means less expansion for the burnt gas in the cylinder before the exhaust ports open.
Less expansion means less cooling down of the gas in the cylinder: it is still hotter when it enters
the expansion chamber. In hotter gas the speed of sound is higher and that means a higher resonance
frequency for the cylinder-pipe system, so it works better at high rpms.
But how do you slow the combustion speed down? Less squish? Mixture too rich? You do not want
to do that...
Fortunately there is a simpler solution. We do not slow combustion down; we just start it later: we
retard the ignition timing. As far as the exhaust gas temperature in the expansion chamber is
concerned, the effect is the same: the engine runs better at high revs.
That is the reason for programmable ignition systems.
Below the power band the ignition advance can be more than 30° so that there is a whole lot of
expansion; the burnt gas contains hardly any energy when it enters the exhaust pipe, so the exhaust
pulses that arrive at the wrong moments at low rpm, are weak and will not disturb the scavenging
too much.
At the rpm of maximum torque the ignition advance is about 14° (careful, a litte too much advance
here and you get detonation)

Hope I helped you there.

I have a small question myself too,
If you have a nikasil cylinder liner, and the ports have rather small chamfers.
Should I make the chamfers bigger, even if I then file off all the nikasil on the already existing liner?
So that I have a bare aluminium chamfer? Or is it better to keep the small (but nikasil coated) chamfer?

Senne s a écrit:

Hope I helped you there.

Thanks Senne

but i know how a variable ignition is working. my question was if a smaller squish area has a variable effect over the rpm range, or if it makes always a hotter exhaust gases.
i this case you can only change the ignition timing and you also get hotter exhaust gases.
but if it only makes it hotter in high rpm, how does it works that it doesnt make hotter gas in the low rpm?

it will know Frits

thanks Manuel

Not only the timing of the ignition, but also the advantage of a variable spark strength is described.
[Vous devez être inscrit et connecté pour voir ce lien]

In the link you can see the graph of the tests performed with a TZ 250:
[Vous devez être inscrit et connecté pour voir cette image]
The green graph is the stock Yamaha TZ250 CDI.
The blue graph is the same bike with the CDI Zeeltronic PDCI-25V with spark energy (62 mJ) constant at all rpm.
The red graph is the same bike with the CDI Zeeltronic PDCI-25V with 50% spark energy (about 30 mJ) at 12000 rpm.

I have to run some test with the motor 125 by varying the energy of the spark. For this reason I'm interested in this argument. I hope to find the time to perform the tests, the CDI is ready and wired.

Just use max energy and build a new curve !

{mRk} a écrit:

In the link you can see the graph of the tests performed with a TZ 250:
[Vous devez être inscrit et connecté pour voir cette image]
The green graph is the stock Yamaha TZ250 CDI.
The blue graph is the same bike with the CDI Zeeltronic PDCI-25V with spark energy (62 mJ) constant at all rpm.
The red graph is the same bike with the CDI Zeeltronic PDCI-25V with 50% spark energy (about 30 mJ) at 12000 rpm.

I have to run some test with the motor 125 by varying the energy of the spark. For this reason I'm interested in this argument. I hope to find the time to perform the tests, the CDI is ready and wired.

The red line with lower sparkenergy proofs that the exhaust is not controle the right rpm.
When the exhaust will be made at a higher rpm (which can be calculated too) you will need full sparkenergy again.
The new total curve will be significantly higher than the red line.
The explenation is described in the facebook note.

Thanks for the explanation.
I wanted a exhaust tuned to a lower revs. To improve the performance of the exhaust at high revs I wanted to vary the energy of the spark. This is a test I wanted to do. If I understand correctly, it is not recommended to do so.
For the TZ 250 used for the tests was made a new exhuast?

The idea is to extend the working range of the exhaust. For similar reasons I wanted to experiment with de Laval nozzles. I just got the tube 28x1 mm for changing the exhaust.

{mRk} a écrit:

In the link you can see the graph of the tests performed with a TZ 250:
[Vous devez être inscrit et connecté pour voir cette image]
The green graph is the stock Yamaha TZ250 CDI.
The blue graph is the same bike with the CDI Zeeltronic PDCI-25V with spark energy (62 mJ) constant at all rpm.
The red graph is the same bike with the CDI Zeeltronic PDCI-25V with 50% spark energy (about 30 mJ) at 12000 rpm.

I have to run some test with the motor 125 by varying the energy of the spark. For this reason I'm interested in this argument. I hope to find the time to perform the tests, the CDI is ready and wired.

Comment

You mention the amount of energy being 62 mJ. Is this a calculated value or a measured one and if so how was it measured.

Bob van der Zijden