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

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carlovitch1



Nombre de messages : 33
Localisation : Pays Catalan
Date d'inscription : 20/05/2018

MessageSujet: Re: [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)   Lun 9 Juil - 13:45

Yes, he did answer several times to amateurs, as we far away from being all professional people on this forum. I suggest you ask your question.
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Frits Overmars

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Nombre de messages : 2014
Age : 69
Localisation : Raalte, Holland
Date d'inscription : 12/10/2010

MessageSujet: Re: [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)   Mar 10 Juil - 9:45

You can ask any question you want, Reinan. I will make only two reservations.
The first: this is an open forum, not a physics discourse, so I cannot go into matters too deeply;
it must remain understandable and interesting for forum members without a physics background.
The second: please do not ask questions that have already been answered here.


Dernière édition par Frits Overmars le Mer 11 Juil - 16:42, édité 2 fois
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ReinanRacing



Nombre de messages : 7
Localisation : Japon
Date d'inscription : 24/06/2018

MessageSujet: Re: [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)   Mar 10 Juil - 12:42

OK. Here we go. Frits Overmars states that the timing of the outlet port should be 180 degrees to set up an optimal resonance in the expansion chamber. I have some questions about this phenomenon, but while writing up my question I may have answered it by myself. To check my understanding:

The outlet is piston ported; thus, the "Frits Overmars optimal" outlet would open at 90 degrees ATDC and would close 180 degrees later, i.e. at 270 ATDC (or 90 BTDC).

Suppose the exhaust pulse extends over D degrees crank angle, so the pulse enters into the expansion chamber between 90 ATDC and 90 + D ATDC degrees. The pulse then travels to the back of the chamber, reflects, and travels back to the cylinder. Then I guess the objective is to make the timing such that the port closes exactly when the tail of the pressure pulse has entered into the cylinder, so that the maximum amount of reflected pulse energy is added to the mixture in the cylinder. As a result of "trapping" the reflected high pressure pulse, a low pressure pulse will now start to travel from the cylinder into the chamber; when this low-pressure pulse echoes, it returns at the cylinder exactly at the moment that the exhaust opens again -> the exhaust gases expand into a low pressure, which helps with the scavenging.

Is the correct mental picture?
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Frits Overmars

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Nombre de messages : 2014
Age : 69
Localisation : Raalte, Holland
Date d'inscription : 12/10/2010

MessageSujet: Re: [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)   Mar 10 Juil - 14:45

ReinanRacing a écrit:
Frits Overmars states that the timing of the outlet port should be 180 degrees to set up an optimal resonance in the expansion chamber.... I guess the objective is to make the timing such that the port closes exactly when the tail of the pressure pulse has entered into the cylinder... As a result of "trapping" the reflected high pressure pulse, a low pressure pulse will now start to travel from the cylinder into the chamber; when this low-pressure pulse echoes, it returns at the cylinder exactly at the moment that the exhaust opens again -> the exhaust gases expand into a low pressure, which helps with the scavenging. Is the correct mental picture?
No it is not, Reinan. I did explain this before, but luckily I found the relevant text without too much of a search. Here it is once more.
======================================================
When the exhaust port opens, a pressure pulse starts moving through the exhaust pipe. It is reflected at the end cone and it should be back at the cylinder just before the exhaust port closes.
Next a part of this reflected pulse bounces off the partly-closed exhaust port and a residual pulse starts moving down the exhaust pipe. This residual pulse too is reflected by the end cone and starts moving back to the cylinder. Ideally it will arrive at the exhaust port just when the port opens again. Then the cylinder pressure and the pressure of the residual pulse combine their energy and the resulting pulse will be stronger than the pulse from the previous cycle. And the combined pulse from the next cycle will be stronger still, and so on; we have achieved true resonance.

Some may argue that we want a low pressure in the exhaust pipe when the port opens because then the spent gases will experience less resistance while leaving the cylinder. But that is not true. Gas flow depends on a pressure ratio. But once that ratio reaches 2, the flow velocity will reach Mach 1, the speed of sound. Raising the pressure ratio any further will not raise the flow velocity any further.
The cylinder pressure at exhaust opening can be as high as 11,7 bar and the pressure of the reflected pulse will then be about 2,6 bar. Thus the pressure ratio is well above 2, so lowering the pressure in the exhaust duct outside the cylinder will not do any good to the flow.

What has the exhaust timing got to do with the 'true resonance' I mentioned above?
The initial pulse starts moving at Exhaust Opening and it has to be back at Exhaust Closing, or a little earlier. This pulse travels with the speed of sound and its journey up and down the exhaust pipe will take t seconds.
The residual pulse starts moving at Exhaust Closing and it has to be back at the next Exhaust Opening. This pulse also travels with the speed of sound and its journey up and down the exhaust pipe will also take t seconds.
So from EO to EC takes t seconds and from EC to EO also takes t seconds. In English: the exhaust port should be open just as long as it should be closed.
Assuming that the crankshaft rotates with a uniform speed, this means that the crank angle during which the exhaust is open must be equal to the crank angle during which the port is closed. So both angles must be 180°.

I developed this line of thought some 40 years ago, but when I first published it in 1978 (in the motorcycle magazine Moto73 of which I was the technical editor) everybody called me crazy. Some people still do, but I got used to it .

Above I made a couple of assumptions. The crankshaft does not rotate with a uniform speed, but at high revs the deviation is negligible. In case you really want to know, I did the math for the Aprilia RSA125. At a nominal rpm of 13.000 the minimum rotation speed is 12940 rpm @ 110° after TDC and the maximum value is 13060 rpm @ 351° aTDC. What's more significant: the deviation in crankshaft position from truly uniform rotation is always less than 1°. So that really is negligible.

Second assumption: both the initial pulse and the residual pulse move with the speed of sound. Not true: the pulse pressures in exhaust waves are so high that acoustics rules do not apply any more. We are dealing with gas dynamics here and the stronger a pulse, the faster it moves. Since the residual pulse is weaker than the initial pulse, they move at different speeds. But we will leave this aside for now.

Third assumption: the initial pulse starts moving as soon as the exhaust port starts opening. More or less true, but we are not interested in the first weak appearance of the pulse; we want to know when the pulse reaches its maximum amplitude. And that requires a certain amount of open exhaust port area. It turns out that for our desired theoretical exhaust timing of 180° we will need a geometrical exhaust timing of about 190°, depending on the shape of the port: does it open gradually or does it open over its full width all at once.

The obvious question will be: why has the Aprilia RSA125 a geometrical exhaust timing of 202°? True, at 190° the maximum torque value would be higher, but the engine would not want to rev because the blowdown time.area would be too small.
The 202° are a compromise: a bit less torque and a bit more revs; as long as the torque decline is smaller than the rpm rise, we gain horsepower.


Dernière édition par Frits Overmars le Mer 11 Juil - 18:17, édité 1 fois
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Frits Overmars

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Nombre de messages : 2014
Age : 69
Localisation : Raalte, Holland
Date d'inscription : 12/10/2010

MessageSujet: Re: [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)   Mer 11 Juil - 6:13

Here is some more material on the subject, kindly provided by professor Neels van Niekerk, also known as Vannik, the creator of the EngMod2T engine simulation software. Thanks Neels .

EDIT: the forum software is playing tricks: it refuses to accept attachments. I'll try again later.
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ReinanRacing



Nombre de messages : 7
Localisation : Japon
Date d'inscription : 24/06/2018

MessageSujet: Re: [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)   Mer 11 Juil - 12:28

Thanks for your reaction. I had read your contribution before but apparently it did not register correctly. With respect to your citation, I would like to clarify two points:

- you claim "Gas flow depends on a pressure difference ratio. But once that ratio reaches 2, the flow velocity will reach Mach 1, the speed of sound." and "The cylinder pressure at exhaust opening can be as high as 11,7 bar and the pressure of the reflected pulse will be about 2,6 bar. Thus the pressure ratio is well above 2".

In this comment, you refer to a "pressure difference ratio" and a "pressure ratio". I have tried to use the text book of Gordon P. Blair to check your assumption, and I found (assuming air as the working fluid):

- pressure ratio P = p / p0
- pressure amplitude ratio X = P^(1/7) = (p / p0)^(1/7)
- local Mach number M = 5(X - 1) / X

Taking p/p0 = 2, I find X = 1.10409 and M = 0.471 => not Mach 1, as claimed.

Taking X = 2, I find M = 2.5 => not Mach 1, as claimed.

What am I doing wrong?
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Vannik



Nombre de messages : 1
Age : 61
Localisation : Centurion, South Africa
Date d'inscription : 15/09/2012

MessageSujet: Re: [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)   Lun 16 Juil - 10:36

Reinan,

Please do not mix pressure ratio with pressure differential. If the pressure ratio at one point is 1 and at the other 2, the differential is 1.
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http://www.vannik.co.za
Frits Overmars

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Nombre de messages : 2014
Age : 69
Localisation : Raalte, Holland
Date d'inscription : 12/10/2010

MessageSujet: Re: [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)   Lun 16 Juil - 18:15

Vannik a écrit:
Reinan, Please do not mix pressure ratio with pressure differential. If the pressure ratio at one point is 1 and at the other 2, the differential is 1.
Thanks Neels.
I still haven't managed to post the graph that you provided. In fact I haven't been able to attach any pictures at all lately.

@ Marc Seriau:
My problem with attaching pictures only occurs in Pit-Lane. Has there been a change in forum software recently?
Am I the only one experiencing difficulties in attaching pictures?
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ReinanRacing



Nombre de messages : 7
Localisation : Japon
Date d'inscription : 24/06/2018

MessageSujet: Re: [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)   Hier à 12:44

Vannik a écrit:
Reinan,

Please do not mix pressure ratio with pressure differential. If the pressure ratio at one point is 1 and at the other 2, the differential is 1.

I am confused. Can you (or Frits) provide an example calculation showing the "Mach 1" result?

As a new poster I don't want to be too rude, but can we also please make sure of the definitions?

A "differential" is a mathematical operator, so that we can have the differential of pressure with respect to time or space. A "pressure differential"?

If we are talking about the difference of pressure between two points, say p1 at point 1 and p2 at point 2, then p2 - p1 is the pressure difference.

In the book of Gordon P Blair (GPB), two quantities are defined:

Pressure ratio: p / p0 where p is the absolute pressure and p0 is the reference pressure (which can be chosen arbitrarily, as long as you choose the same reference value in all calculations)

Pressure amplitude ratio: X = (p / p0)^((gamma - 1) / (2 * gamma)); where p is the pressure, p0 is the reference pressure, and gamma is the ratio of the specific heats (approx. 7 / 5 = 1.4 for air)

In GPB, the Mach number is defined in equation (2.1.23) on page 60:

Me = ce / ae = G5 * (Xe - 1) / Xe

where G5 approx. 5 (for air), and Xe is is pressure amplitude ratio for the wave that we are currently investigating.

Taking the example of Frits Overmars with a pressure wave of amplitude p1 = 11.7 bar and a reflected pressure wave of amplitude p2 = 2.6 bar, I get the following (taking the reference pressure to be p0 = 1 bar):

M1 = 5 * (X1 - 1) / X1 = 1.4814 (air molecules moving from cylinder into the exhaust)
M2 = 5 * (X2 - 1) / X2 = 0.64 (air molecules moving from exhaust towards the cylinder)

Thus the velocity of the air molecules in this superposition would be M = 0.84 (from cylinder towards exhaust), nearly supersonic.

If the second pressure wave has a lower pressure, then the flow would become supersonic (?).

But I still don't understand the general observation that "a pressure ratio of 2 results in Mach 1". I would really appreciate the explanation. Does it mean that "X1 = 2 * X2" and the two waves are moving in opposite direction?
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Frits Overmars

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Nombre de messages : 2014
Age : 69
Localisation : Raalte, Holland
Date d'inscription : 12/10/2010

MessageSujet: Re: [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)   Hier à 13:12

Reinan, may I kindly draw your attention to the following quote?
Frits Overmars a écrit:
this is an open forum, not a physics discourse, so I cannot go into matters too deeply;
it must remain understandable and interesting for forum members without a physics background.
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ReinanRacing



Nombre de messages : 7
Localisation : Japon
Date d'inscription : 24/06/2018

MessageSujet: Re: [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)   Hier à 13:22

Frits Overmars a écrit:
Reinan, may I kindly draw your attention to the following quote?
Frits Overmars a écrit:
this is an open forum, not a physics discourse, so I cannot go into matters too deeply;
it must remain understandable and interesting for forum members without a physics background.

You certainly may. I found the remark quite discouraging the first time and even more discouraging the second time. I have a "physics background". I am interested in the answer. Does my physics background mean that I am not allowed to ask questions? Or does it mean that I am not worthy of a proper answer to an honest question? What is "to go into matter too deeply?" How can you know what is "understandable" for other forum members?

I have cited the relevant equations (at least I think those are the relevant equations); I am using a well-known text book and I have shown my (erroneous) analysis. I would like to hear from an expert how my analysis is wrong, so that in the future I will not make the same mistake.

If you cannot or will not answer the question, that is fine, then please just say so and I will not bother you again.
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ReinanRacing



Nombre de messages : 7
Localisation : Japon
Date d'inscription : 24/06/2018

MessageSujet: Re: [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)   Aujourd'hui à 13:32

I received a private message from: LucF.

LucF, thank you for your message. It clarifies a lot.

My number of messages is too low, I cannot answer to your private message.
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