AccueilCalendrierGalerieFAQRechercherS'enregistrerConnexionNouveaux messages depuis dernière visite
Rechercher
 
 

Résultats par :
 
Rechercher Recherche avancée
Qui est en ligne ?
Il y a en tout 27 utilisateurs en ligne :: 12 Enregistrés, 0 Invisible et 15 Invités :: 1 Moteur de recherche

alain combemale, BauGG, Dialmax, flatart, kaouet, Larbin, MacPepR, panerai, quadneufdocteur, riviere, singleton560, t2

Le record du nombre d'utilisateurs en ligne est de 187 le Lun 03 Déc 2012, 14:45
Derniers sujets
» [Oldies] Michel ROUGERIE, 30 ans déjà !
Aujourd'hui à 23:32 par singleton560

» [ oldies ] Angel Nieto Mike Hailwood ( Part 4 )
Aujourd'hui à 22:50 par Dukasuz

» [Oldies] Renzo Pasolini, photos insolites
Aujourd'hui à 22:34 par philippe7

» MILAN EICMA 2018
Aujourd'hui à 21:40 par denis boulom

» [WSBK] 2019
Aujourd'hui à 21:07 par t2

»  [Oldies] Les découvertes de Tonton Hubert (Part 4)
Aujourd'hui à 19:46 par Adco

» [GP] MotoE
Aujourd'hui à 19:05 par nard gilles

» [Oldies] Charles Krajka
Aujourd'hui à 18:43 par fmd

» [En construction] Pépé et les Kawas! (1/5)
Aujourd'hui à 18:36 par damienfollenfant

» [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 17:19 par Frits Overmars

» Salon Epoqu'Auto LYON 2018
Aujourd'hui à 16:01 par Dan42

» [MotoGP] GP 2018 de Valence au Circuit de Ricardo Tormo - Les 16 , 17 et 18 Nov 2018
Aujourd'hui à 15:57 par carlovitch1

» [Road racing] GP Macao 2018
Aujourd'hui à 08:48 par 2strokeman

» [Oldies] Le mystère des antipodes, ces usines à champions bien trempés
Hier à 19:18 par 88 daytona

» [Oldies] C'était le bon temps...
Hier à 18:59 par charlie grouniache

» [Oldies] Daytona 1973
Hier à 13:31 par Pierre"PhilRead"

» compte tours Motoplat
Hier à 13:28 par fullgazlolo

» [MotoGP] GP 2018 de Malaisie à Sépang - Les 2 , 3 et 4 Nov 2018
Hier à 13:25 par bartau

» [GP] Eurosport: du changement!
Hier à 10:32 par enndewell joel

» [GP] Chaises musicales ou resignature pour 2019.
Hier à 10:16 par EDOUARD Jean

» [Oldies] Les Aermacchi deux temps
Hier à 09:23 par Ala Zorro

» [Road racing] Saison 2018
Sam 10 Nov 2018, 21:01 par mickey

» Italian Meeting Montlhéry 2018
Sam 10 Nov 2018, 18:21 par Le Xav

» [Oldies] Quizz??????
Sam 10 Nov 2018, 17:54 par Mykeul

» [Oldies] Ils ont p'loté la belle à culbuter de Varese (2ème partie)
Sam 10 Nov 2018, 17:28 par Ala Zorro

» La moto racontée par C. Sarron
Sam 10 Nov 2018, 15:20 par bubu

» [Technique] Moteurs MotoGP
Sam 10 Nov 2018, 13:20 par BM1150R

» [MotoGP] Michelin
Ven 09 Nov 2018, 19:30 par Pasky

» [SORTIES] La 10ème Sunday Ride Classic – 23-24-25 Mars 2018 au Circuit Paul Ricard
Ven 09 Nov 2018, 07:21 par formi83

» Doutes concernant éditions passées du Grand Prix de France (et pas soulement...)
Ven 09 Nov 2018, 06:53 par Niggi46

La réclame...
Mots-clés
ducati HOnda 2013 yamaha zone coupe fourche france aermacchi 2018 metiss 1973 eurosport side icgp suzuki ROAD classic aprilia wanted geco rouge RACING moto motobecane triumph
Meilleurs posteurs
Marc
 
philwood
 
mickey
 
Pierre"PhilRead"
 
EDOUARD Jean
 
Jarno
 
Fügner
 
yves kerlo
 
janpol84
 
Dialmax
 
La réclame...

Partagez | 
 

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

Aller en bas 
Aller à la page : Précédent  1 ... 9 ... 15, 16, 17, 18, 19, 20  Suivant
AuteurMessage
carlovitch1



Nombre de messages : 133
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 09 Juil 2018, 15:45

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.
Revenir en haut Aller en bas
Frits Overmars

avatar

Nombre de messages : 2056
Age : 70
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 2018, 11:45

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 2018, 18:42, édité 2 fois
Revenir en haut Aller en bas
ReinanRacing



Nombre de messages : 46
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 2018, 14:42

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?
Revenir en haut Aller en bas
Frits Overmars

avatar

Nombre de messages : 2056
Age : 70
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 2018, 16:45

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 2018, 20:17, édité 1 fois
Revenir en haut Aller en bas
Frits Overmars

avatar

Nombre de messages : 2056
Age : 70
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 2018, 08:13

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.
Revenir en haut Aller en bas
ReinanRacing



Nombre de messages : 46
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 2018, 14:28

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?
Revenir en haut Aller en bas
Vannik



Nombre de messages : 12
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 2018, 12:36

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.
Revenir en haut Aller en bas
http://www.vannik.co.za
Frits Overmars

avatar

Nombre de messages : 2056
Age : 70
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 2018, 20:15

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?
Revenir en haut Aller en bas
ReinanRacing



Nombre de messages : 46
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 17 Juil 2018, 14:44

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?
Revenir en haut Aller en bas
Frits Overmars

avatar

Nombre de messages : 2056
Age : 70
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 17 Juil 2018, 15:12

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.
Revenir en haut Aller en bas
ReinanRacing



Nombre de messages : 46
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 17 Juil 2018, 15:22

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.
Revenir en haut Aller en bas
ReinanRacing



Nombre de messages : 46
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 18 Juil 2018, 15:32

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.
Revenir en haut Aller en bas
Virgil Tripp



Nombre de messages : 4
Localisation : usa
Date d'inscription : 06/12/2016

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)   Dim 22 Juil 2018, 00:20

This forum is a great source to learn from the experiences of others regarding the theory and operation of the highly developed 2 cycle racing engine. Reinan, when applying your understanding of theory, can you share some examples of expansion chambers designed/built and specific engine modifications done by you? Dyno charts and race results would be appreciated. I am always curious about projects others are working on.
Revenir en haut Aller en bas
ReinanRacing



Nombre de messages : 46
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)   Dim 22 Juil 2018, 03:15

Virgil Tripp a écrit:
Reinan, when applying your understanding of theory, can you share some examples of expansion chambers designed/built and specific engine modifications done by you? Dyno charts and race results would be appreciated. I am always curious about projects others are working on.

I have to disappoint you. I do not fabricate anything, I do not participate in races and I have nothing to share.

I have restored a Suzuki RG400. This was the first time for me to restore a motorcycle. I have rebuilt the engine from scratch and in that process I have read some of the literature about high performance 2T engines (of course, we can debate whether the Suzuki RG400 engine qualifies as "high performance" but I think it does). I was pleasantly surprised by the book "Design and simulation of 2-stroke engines" by Gordon P. Blair because it provides most of, if not all, the basic equations and analysis methods in a quantitative description (*) of the 2T engine.

I was pointed to the present forum discussion by a friend and I have read most of the contributions. I was very surprised about some strong assertions by Frits Overmars about the physical phenomena occurring in the 2T engine. I am a scientist and the basic rule is that strong assertions require strong evidence; on the contrary, that which is asserted without evidence can be dismissed without further discussion. Frits Overmars made statements on various occasions about the timing of the exhaust port ("the timing of the exhaust port must be 180 degrees open, 180 degrees closed"), the "Helmholtz-resonator" ("when the engine is running in the power band point it acts as a Helmholtz-resonator and no intake timing devices are needed"), the volume of the crank case ("bigger is better"), the occurrence of sonic flow ("if the pressure ratio exceeds 2 you reach Mach 1, supersonic flow") and several other assertions, but he did not provide any equations or theoretical explanations to support such assertions. I am a theoretical person so I like to hear the details, especially because I think that some of Frits Overmars' assertions physically do not make sense (the story about the Helmholtz-resonator), are not entirely true (crank case volume), or, in some cases, completely false (sonic flow). I am genuinely interested in the theoretical background of these assertions and I decided to become a forum member and ask some questions. Unfortunately, so far without too much result.

(*) in scientific or engineering discussion one must make a discussion between quantitative and qualitative descriptions. Most forum postings are a qualitative description ("the bigger the cylinder displacement, the larger the power of the engine"), but for design and analysis you need a quantitative description, with the exact equations describing the exact relations between the parameters, dependent and independent variables (**)

(**) parameters and variables. There is a distinction between independent variables (variables over which you don't have control, such as time and space), dependent variables (variables over which you have control, such as temperature, engine speed, crank angle as a function of time), and parameters (aspects of the engine which you can control but which are not freely variable, such as bore, stroke, compression ratio, etc). Note: with increased electronic control, many "parameters" can be changed into "variables", for example the parameter "exhaust port opening angle" can become a variable by the use of an exhaust valve.
Revenir en haut Aller en bas
ReinanRacing



Nombre de messages : 46
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)   Dim 22 Juil 2018, 05:39

To answer my own question, I refer to the book "Design and simulation of 2-stroke engines" by Gordon P. Blair. In section 2.2.4 GPB notes:

"In typical engine configurations it is rare for the magnitude of finite amplitude waves which occur to provide a particle velocity that approaches the sonic value."

and

"Even in high-performance racing engines a pressure ratio of an exhaust pulse greater than 2.2 atmospheres is very unusual, thus sonic particle velocity emanating from that source is not likely. What is a more realistic possibility is that a large exhaust pulse may encounter a strong oppositely moving expansion wave in the exhaust system and the superposition particle velocity may approach or attempt to exceed unity." (note: GPB means "superposition particle velocity may approach or attempt to exceed the local sonic velocity")

OK, taking the example of Frits Overmars: pressure wave p1 of absolute pressure 11.7 bar (1.17 MPa) and oppositely moving pressure wave p2 of absolute pressure 2.6 bar (0.26 MPa). Using the theory in GBP section 2.2.4, the resulting Mach number M = 0.8764, or about 88% of sonic velocity. With a margin of 12% to sonic velocity one can debate whether such a margin is "large" or "small".

Equation to evaluate Mach number in superposition of pressure waves (GPB section 2.2.4):

M = 5 * (X1 - X2) / (X1 + X2 - 1)

where X is the pressure amplitude ratio for the wave, i.e. for air, X1 = (p1 / p0)^(1/7), and p0 is the "reference pressure". Frits Overmars asserts

Frits Overmars a écrit:
Gas flow depends on a pressure ratio. But once that ratio reaches 2, the flow velocity will reach Mach 1 [...]

This assertion is not supported by the equation. Take X1 / X2 = 2 => X1 = 2 * X2, and substitute, then

M = 5 * X2 / (3 * X2 - 1)

To approach sonic velocity, we have M = 1 :

1 = 5 * X2 / (3 * X2 - 1) => X2 = -0.5

Since X2 >= 0 (always, because we cannot have negative pressures, the lowest possible pressure is zero), we find that a pressure amplitude ratio of 2 (that is, X1 / X2 = 2) NEVER results in Mach 1.

Thus I will state it clearly: Frits Overmars, __either__ your assertion is wrong, __or__ my analysis is wrong.

Please tell me and the other forum members who is wrong and who is right.
Revenir en haut Aller en bas
Virgil Tripp



Nombre de messages : 4
Localisation : usa
Date d'inscription : 06/12/2016

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)   Dim 22 Juil 2018, 05:54

The RG400 is an inspired design by any standard. The restoration of one is an admirable project.

Regarding the necessity of intake timing devices when the engine is in the power band (on the pipe), I offer a personal anecdotal experience from 1969. The subject kart engine was a McCulloch 100cc MC-7 converted to a dual reed cage/dual carb bottom feed configuration running on methanol.

We were dyno testing the effect of varying the ignition timing while under load in the power band. After one pull, we made a minor adjustment and attempted to restart. It would not restart. Inspection revealed that one reed pedal was missing in its entirety. The reed material was phenolic in nature and was consumed by the engine with no noticeable negative mechanical effect (compression was still good). It was unknown at which point it failed during the dyno pull. We had to conclude that, in this case, performance did not suffer with a missing reed after the engine was already running at 10,000 to 14,000 RPM.

Before this (about 1961) I observed a similar situation. Enduro kart racing was just getting started and Jim Akkerman (a NASA scientist and kart racer) was running his modified West Bend 820 (125cc) at an enduro (road race) event. He was easily lapping all other competitors. While running strong, he came in for fuel and it would not restart. After the race, I asked what happened and he said a reed broke.

These experiences have always been interesting to me but I never did anything to advance further
understanding.
Revenir en haut Aller en bas
ReinanRacing



Nombre de messages : 46
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)   Dim 22 Juil 2018, 11:30

I do not doubt the veracity of the claim that a 2T can run without timing devices.

When the exhaust port opens, a high pressure wave enters into the exhaust chamber, and the chamber is designed to send back "suction" waves. If these waves have a sufficiently low pressure, they will effectively "suck" the mixture through the transfer ports, also causing a sufficiently low pressure in the crank case to induct fresh mixture through the inlet tract.

Frits Overmars refers to such systems as "Helmhotz resonators". In fact, we would be dealing with two, coupled Helmholtz resonators, i.e. one resonator due to the volume of the crank case, connected to the atmosphere through the inlet tract; and one Helmholtz resonator consisting of the volume above the piston, and the exhaust system and its connection to the atmosphere. These two resonators are then coupled through the transfer ports. Helmholtz resonators are very peculiar devices with very particular properties. For example, one property of a Helmholtz resonator is its (static) volume, and in a 2T engine, the both the volume above the piston and below the piston are continuously changing. As such, such coupled Helmholtz resonators would have very interesting properties, about which I would like to know more.
Revenir en haut Aller en bas
JanBros



Nombre de messages : 218
Localisation : Belgique
Date d'inscription : 05/12/2011

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)   Dim 22 Juil 2018, 11:58

in  fact, there are 4 resonator's : you also have the airbox and the exhaust chamber.

if you have read this topic, you should have also read this :

"A Helmholtz resonator consists of a volume connected to a duct". That is what Wikipedia tries to tell you.
But that is a Helmholtz resonator in its simplest form; one that you won't find anywhere in an engine.
What you do find in an engine is an intake tract with a variable cross-sectional area. This tract is from time to time connected to a variable volume (the crankcase) through a very variable window (the intake port / reed valve / rotary inlet).
The crankcase is connected to a number of transfer ducts with variable cross-sectional areas, which are from time to time connected to a very variable volume (the cylinder) through a number of very variable windows (the transfer ports).
The cylinder is from time to time, through a very variable window (the exhaust port), connected to an exhaust pipe with a very variable cross-sectional area who at the same time doubles as a volume. This pipe volume is constantly connected to a big volume (the outside world) through a tailpipe with a constant cross-sectional area and constant entry and exit windows (thank God, finally someting that's not variable).
We call this a compound Helmholtz resonator .
The various papers also tell us how to calculate the resonator's frequency:
" frequency = speed of sound / (2*pi)* Sqr ( cross-sectional area of the neck / ( volume of the resonator * effective neck length ) ) ".
O yes, the speed of sound... It is dependent on temperature, which is not really constant in the intake tract and the crankcase, rather variable in the transfer ducts and very variable in the cylinder and the exhaust pipe.
Now the above frequency formula is not exact; it is an approximation that is usable as long as the volume of 'the' tract is very small compared to the resonator's volume.
So when engines are concerned, that formula goes very far out the window.


furthermore, Frits already answered your questions, in that he will not answer them as this forum isn't meant for scientists, but for enthusiast, tuners and DIY'ers. The only other answer he has forgotten to give you, is that Prof. Blair has done a lot of good for the 2-stroke , but he never built a winning 2-stroke GP engine.

Please stop "harrassing" the experts - who are so kind to share their knowledge with us- to insist that they should answer/solve your problem. THEY decide what they will say or answer  and we do not want them to leave because someone out of nowhere makes it a burden for them to come here.
Revenir en haut Aller en bas
ReinanRacing



Nombre de messages : 46
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)   Dim 22 Juil 2018, 12:28

JanBros a écrit:
Please stop "harrassing" the experts - who are so kind to share their knowledge with us- to insist that they should answer/solve your problem. THEY decide what they will say or answer  and we do not want them to leave because someone out of nowhere makes it a burden for them to come here.

I resent the notion that I am "harrassing" anybody. I am merely asking some questions, in the (naive ?) expectation that somebody will be able to answer those questions. I also strongly resent the idea that I should make some sort of evaluation about what kind of question is "allowed" to be put forward. I am not a mind reader so this requirement of self-censorship is nonsensical.

For a true expert, questions related to the area of expertise are easy to answer, or at least, questions should be easy to answer. If not, the person is not an expert.

That being said, I am completely aware of the fact that any person has the freedom to not answer my question. In that case, the person should say: "I will not answer your question".

Frits Overmars' assertion that my question is somehow "too difficult" is both an insult to me personally and an insult to the other forum members in general. A question can never be too difficult. Why should I inhibit myself to "easy" questions? How should I be able to determine the "difficulty" of a question? And the idea that Frits Overmars can decide what is "understandable" to other forum members is so astonishingly patronizing that I am surprised that anybody would defend that position.

And about the "burden" of being on a forum. Well. Anybody is free to ignore me. If the "experts" do not wish to be burdened by questions from mere amateurs, then those amateurs should stay in their Ivory Towers and hand down their wisdom in the form of text books or otherwise.

If Frits Overmars is unwilling (or uncapable) to discuss my question, then that is fine, but I would like him to say that directly, and not make obfuscated references about the "level of the discussion" etc.

Finally, I also strongly resent your opinion about Gordon P. Blair. As if "building a GP winning engine" is the only true proof of expertise. There are more engines in this world due to the work of Gordon P. Blair than due to the work of Jan Thiel or Frits Overmars.
Revenir en haut Aller en bas
ReinanRacing



Nombre de messages : 46
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)   Dim 22 Juil 2018, 13:38

JanBros a écrit:
if you have read this topic, you should have also read this :

"A Helmholtz resonator consists of a volume connected to a duct". That is what Wikipedia tries to tell you.
But that is a Helmholtz resonator in its simplest form; one that you won't find anywhere in an engine.
What you do find in an engine is an intake tract with a variable cross-sectional area. This tract is from time to time connected to a variable volume (the crankcase) through a very variable window (the intake port / reed valve / rotary inlet).
The crankcase is connected to a number of transfer ducts with variable cross-sectional areas, which are from time to time connected to a very variable volume (the cylinder) through a number of very variable windows (the transfer ports).
The cylinder is from time to time, through a very variable window (the exhaust port), connected to an exhaust pipe with a very variable cross-sectional area who at the same time doubles as a volume. This pipe volume is constantly connected to a big volume (the outside world) through a tailpipe with a constant cross-sectional area and constant entry and exit windows (thank God, finally someting that's not variable).
We call this a compound Helmholtz resonator .
The various papers also tell us how to calculate the resonator's frequency:
" frequency = speed of sound / (2*pi)* Sqr ( cross-sectional area of the neck / ( volume of the resonator * effective neck length ) ) ".
O yes, the speed of sound... It is dependent on temperature, which is not really constant in the intake tract and the crankcase, rather variable in the transfer ducts and very variable in the cylinder and the exhaust pipe.
Now the above frequency formula is not exact; it is an approximation that is usable as long as the volume of 'the' tract is very small compared to the resonator's volume.
So when engines are concerned, that formula goes very far out the window.

Yes, I did read that piece, and I noticed that it does not really explain anything. That's why I'm asking questions.

I tested with a PET bottle. V = 1000 ml, length of the neck approx. 35 mm, diameter of the neck approx. 21 mm. The WikiPedia entry about Helmholtz resonators then predicts a frequency of 156 Hz. The actual tone of the bottle is lower, but let's say that the theoretical value is not very wrong.

OK, now we go to an Aprilia RSA engine. We have been told that the crank case volume is approx 650 cm3, with an 38 mm carburettor and a length of the inlet tract of perhaps 8 centimeter. We find a resonance frequency of about 255 Hz, corresponding to 15,300 RPM. Much higher than the operating speed of the actual engine. But of course, the crank case volume varies during the operation of the engine. So take for example 525 cm3 (650 - 125) -> resonance frequency 283 Hz or nearly 18,000 rpm. Or take a crank case volume of 650 + 125 = 775 cm3, giving a resonance frequency of approx. 233 Hz or 13,980 RPM. All these results are above the operating range of the actual engine, but we have to take into account the fact that the "Wikipedia formula" may give an overestimation.

Whatever the actual value of the resonance frequency, the truth of the matter is that the volumes of the cavities are continuously changing when the engine is operating. Thus there is not "one" resonance frequency, but perhaps a "resonance range". I am sure that this is not the whole story, and I am interested in the details. I am not asking for a detailed answer __on this forum__; if someone can refer me to specialized text books or scientific papers (perhaps of the SAE?) then I would be perfectly happy.
Revenir en haut Aller en bas
JanBros



Nombre de messages : 218
Localisation : Belgique
Date d'inscription : 05/12/2011

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)   Dim 22 Juil 2018, 13:43

you completly missed my point.

I specificaly put "harrassing" between bracket's, doesn't that give a clue ?
You can ask any question you like, they can answer with anything they want. Sometimes you do not get the answer you were hoping for, sometimes you even do not get a reply. Than most of the time the problem lies within the question itself, or how it is asked.
Anybody is free : true. So Frits is also free to believe the question is too dificult or outside the purpose of this topic without having to do any explanation whatsoever. Frits "ignored" your question in a polite way  - even ging a short reason why- which is also his freedom as you say.
without amateurs, this topic would not excist so your claim that "amateurs experts  should stay in their Ivory Towers and hand down their wisdom in the form of text books or otherwise" has no real point. You read Blair's book, but you seem to have a problem : Blair isn't on this forum, so you can not ask him about it. An open forum seems a lot more interesting to me than a book that may pop up more questions to the reader without ever being able to get an answer. Here at least there is the possibility you get an answer, or that you are put on the right path to find out yourself.
how "directly" he says to you he will not answer your question is up to him, and people can have different interpretations of what directly means. to me it was "pretty direct" and even explaining why.

and than finaly : there is always a difference between theortics and the reality of practics. I didn't say a bad thing about  Blair, I (it was not actualy my quote but from Frits or Jan or Wobbly I believe), simply stated that using only Blair's book for building an engine will not make you win a  GP. Jan Thiel has said many times all development on cylinders at Aprilia was done simply by trying/doing stuff, not by entering numbers into formula's.

and here the discussion ends for me.


Dernière édition par JanBros le Lun 23 Juil 2018, 00:22, édité 1 fois
Revenir en haut Aller en bas
Frits Overmars

avatar

Nombre de messages : 2056
Age : 70
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)   Dim 22 Juil 2018, 14:16

Thank you Janbros
Revenir en haut Aller en bas
ReinanRacing



Nombre de messages : 46
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)   Lun 23 Juil 2018, 02:09

Too bad my sincere inquiries remain unanswered. I am hoping that perhaps someone else will be interested and able to answer my questions.
Revenir en haut Aller en bas
Jan Thiel



Nombre de messages : 469
Age : 78
Localisation : Bangkok
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 23 Juil 2018, 10:41

I bought all Blair's books, I used to buy all tuning books!
But for my work, looking for more power, they were mostly useless to me.
Except for 1 very good writer's articles: Kevin Cameron.
He made you THINK!
In my opinion thinking is far more important than calculating!


The Aprilia RSA inlet length was determined in a very simple and sure way.
It was made equal to the RSW's.
We tried lengthening and shortening 5mm, with distance pieces.
Both tests were negative.
It took us 1 hour.....
The crankcase volume must have been more than the RSW's 670cc, but I never measured it...
And the carburetor was 42mm.

You can calculate whatever you want.
But the only REAL result comes from dyno-testing, which you have to do anyway
So I preferred to leave out the calculations.....
But if I really needed some calculation I asked Frits, not very often.
His answers were always excellent!
And often confirmed already achieved results.


Dernière édition par Jan Thiel le Lun 23 Juil 2018, 11:32, édité 1 fois
Revenir en haut Aller en bas
LucF



Nombre de messages : 100
Age : 75
Localisation : Pays Bas
Date d'inscription : 25/05/2011

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 23 Juil 2018, 11:30

Quote from : Jan Thiel on October 18, 2012 , 06:17:55
Beautiful graphics Luc ! That 'red ' abilities are 100 % definitely achievable. It will , however, most of the time, and especially money, are missing.
To meet the RSA power was used 5 days per week by 6 people and with it the aid of the drawing room, and electronic engineers. And that over about 12 years is not that simple ! We started in 1995 with 46.5 hp, and ended in 2007 with 54hp, so 0.6 to 0.7 HP per year on average. Actually seems very little , but it was a lot of work ! We made about 300-400 cylinders per year. So to get that far, there are at least 3600 cylinders made. But maybe 4800 !, I did not count them .... Of course, a lot of cylinders were equal to each other ! But we surely tested 30 different types of flushing channels, and also exhaust conduit 20 different types. Estimated that another 100 different cylinder heads and of course lots of exhaustpipes!
Overall it was pretty special, and will not arise so quickly again!

Quote from Jan Thiel November 19, 2015
Calculations with a specific program are certainly very useful!
I wish I had something at Aprilia, then I had definitely become further!

So answers at certain questions can be very usefull !
Revenir en haut Aller en bas
http://www.geocities.ws/lucfoekema/indexNL.html
Contenu sponsorisé




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)   

Revenir en haut Aller en bas
 
[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)
Revenir en haut 
Page 16 sur 20Aller à la page : Précédent  1 ... 9 ... 15, 16, 17, 18, 19, 20  Suivant
 Sujets similaires
-
» WANTED TONY "Barbe-rousse"
» Traduction, help wanted
» Wanted Kader
» Bannière Wanted
» Graphiste Wanted

Permission de ce forum:Vous ne pouvez pas répondre aux sujets dans ce forum
 :: ACTUALITES :: [GP] :: [GP125 (et 250 Snif!)]-
Sauter vers: