Calculating exhaust duration

[Truls221]

Been reading some chapters from blair's book which I found on the net.

Discovered some nice formulas, which can be used together to calculate the exhaust duration.

What you need to know is expansionchamber tuned length in mm (measured from piston to stinger), max power (kW) and rpm, and swept volume in cc.

The formula for calculating the length of your expansionchamber;

Lt = (83,3 * v * d)/rpm

where d is exhaust duration and v is speed of sound in the expansion chamber.

v is calculated using a constant k = 401,8 times exhaust temperature Tex in Kelvin.

v = (401,8*Tex)^0,5

Tex is calculated using the formula for BMEP = (6,0 *10^5*kW)/(cc*rpm)

Tex is = 534,67e^0,0485*BMEP

Put all these confusing formulas together, and you can find the exhaust duration. Useful if you see a dynograph from an engine which you know the expansionchamber tuned length of.

d = (Lt*rpm)/(83,3*(401,8*(534,67e^(0,0485*((6,0 *10^5*kW)/(cc*rpm))))))

Don't know if the the formula is very useful, but it's interesting to see how the exhaust duration differs from engines otherwise quite similar.

[Truls221]

To let people better understand this formula, I´ll be posting an Excel-sheet as soon as I get a webspacing.

[gravedigger]

könnte man nicht diese topic umwandeln in ein "how to build" dingens.

tipps rund ums berechnen und fertigen der puffis?

was passiert, wenn ich welche geometrie am puffi ändere?

welches material?

welche werkzeuge?

welche rohrbögen für den krümmer?

wie genau den endämpfer basteln?

wie abwickeln?

wo gibt es nette proggis?

ABER kein topic daraus machen, der GSF auspuff muss genau SO aussehen!!!

eines kann ich schon beisteuern, mit inventor 7 und 8 kann man keine asymetrischen kegelabschnitte abwickeln. :veryangry:

dafür lassen sich schön konische krümmer aus geraden segmenten konstruieren.

Bearbeitet 19. Dezember 2003 von gravedigger

[gertax]

wieso sollte man das in ein auspufftopic umwandeln? Hier gehts darum wie man auf die Auslasszeit rückschließen kann.

Truls, have you ever tried to calculate average egt going the Blair way? Might be interesting as far as precision is concerned. I'm quite pesimistic on that one.

Anyway, I'm looking forward to your Excel sheet, should be great help trying to find out how precise exhaust duration can be calculated, e.g. if the outcome is only one or two degrees off or if you get a range of let's say 10 degrees, then calculationg things would be useless because on most engines you know that range before anyway.

[beodeo]

....eines kann ich schon beisteuern, mit inventor 7 und 8 kann man keine asymetrischen kegelabschnitte abwickeln. :veryangry:

dafür lassen sich schön konische krümmer aus geraden segmenten konstruieren.

*switch scooterforum to CADforum*

ja das hat auch bei mir ein bischen gedauert bis ich das hinbekommen habe

1. keine Kreise sondern Polygone zeichnen

2. anzahl der ecken nicht zu hoch (36 oder je nach rechner vielleicht 48 oder mehr)

3. Eckpunkte der beiden Polygone deckend übereinander

4. Ecken des Körpers verrunden (Inventor kann nur abwickeln wenn Rundungen da sind - bis version 6 zumindest , 7 o. 8 hab ich nicht)

dann geht das .... vielleicht is halt sehr maschinenbaulastig das programm :grr:

[Truls221]

For example I could guess with a little help from the formula that carsten fichtls expansion chamber is between 1047-1051 mm long from piston to stinger..............

[Truls221]

here it is,

http://home.no/trulsherland/duration_calc.xls

Bearbeitet 21. Dezember 2003 von Truls221

[gertax]

not working? If I alter values, nothing happens... ?!

[kan_Kuma]

The spreadsheet is working fine with me.

@Truls

I wouldn't like to discourage you. This formula works nicely and accurate in theory, but in real life, the values for the exhaust timing one gets would differ greatly from the actual values. My first guesstimation would be in the range of +/- 5° if not more, and this result will render the calculation futile.

It would be interesting to get proof. Do you have a calculation handy, where you know the exact values? Although I doubt that anyone could deliver a sincere horspower figure

Btw. Where'd you get those Blair pages from the net? I'd be interested in having a read myself.

Cheers

Chris

Bearbeitet 22. Dezember 2003 von kan_Kuma

[Truls221]

The point is that the values you input are closely related to each other. If you take maximum horsepower at the relevant rpm from a dynograph, and capacity,

then put this into the formula these are the determining factors for the result. The tuned lenght are more like a constant in this matter.

[Truls221]

here are the sources:

http://www-staff.lboro.ac.uk/~elvpc/progs/expch.html

http://members.bellatlantic.net/~vze2p5sj/expcham.htm

[f_lammy]

Vielleicht hilft dieses fur was Anderungen machen an einer Auspuff

Frank