Air flow and Pressure Ratio Calcs
Turbo Mini Forums

Here is my method of calculating the above.

Comments would be appreciated.

Please use with caution.

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Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

maths! yuck lol

What a mixed up mess of units !!!
I can do degrees C to degrees K in my head, but cc to cubic feet...had to keep looking up conversion factors....
You've assumed adiabatic in the compressor so it will be slightly out but I can follow it all and I think I agree with it all (although I haven't checked all of the actual maths) until the last change from 183CFM to 12.62lbs/min.
What value for density have you used at this point?
Rod.

Schrödinger's cat - so which one am I ???

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

The method is the one detailed in the Hugh MacInnes book "Turbochargers" that I've developed to include Intercooling.

I was just discussing this book with my wife and we agreed that I must have bought it in 1978 because I read it on holiday in Bournemouth that year. It has only took me thirty years to put it into practise!

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

I can follow everything up until the intercooler bit. How did you get 12.62 lb/min coming out of the intercooler with 10.9 lb/min going in? The volumetric flow will change but the mass should be the same should it not?

Edited by Paul S on 8th Apr, 2008.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Ahh, I stand corrected then!

Schrödinger's cat - so which one am I ???

Yes, the compressor efficiency on a map is just the adiabatic efficiency.

I do have a spreadsheet, but I'm reluctant to publish it as, in the wrong hands, it could be misinterpreted.

I would rather those interested study the method and develop their own spreadsheet.

If someone just wants answers and is not interested in the method then there are several web based calculators that they can use.

I would be happy to help develop our own TurboMinis web based calculator.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Schrödinger's cat - so which one am I ???

''Because you cool the charge with the intercoller, thereby reducing the density,''
??

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

Picky :)

Ok, what I meant was reducing the volume for a given mass.

On 7th Apr, 2008 robert said:
''Because you cool the charge with the intercoller, thereby reducing the density,''
??

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

I'll take a look at my stuff later - as/when i find any kind of time. :(

Thanks Paul for posting this guide. I have been meaning to check my setup against the compressor map for months now but never got round to it. Then I found myself in work today with all my work until the end of the month complete. After annoying everyone requesting more work I decided to be properly productive and work out the airflow for my engine.

My assumtions followed Pauls as I don't know enough about most of the subjects to change them.

Engine Capacity: 1150 cc (1098 +0.060")
Ambient Pressure: 1000 mbara (live beside the sea)
Ambient Temperature: 25 deg C (Pauls)
Intercooler Efficiency: 80 % (Pauls)
Compressor Efficiency: 71 % (Pauls)
Engine Volumetric Efficiency: 85%
Pressure Loss through Air Filter: 50 mbarg (Pauls)
AFR 12.50 (Pauls)
BSFC 0.50 Lb/hp/hr (Pauls)
Boost: 700 mbar
EDIT: Peak BHP @ 5800 using an SW5-07

Final figures turned out as:
Air Flow: 11.269 lbs/min
Pressure ratio: 1.789

http://www.turbobygarrett.com/turbobygarre...44_454083_2.htm

It seems to land right in the peak efficiency island. Hopefully I didn't screw up on any of the few things I had to decide for myself :(

Edited by GaryOS on 29th Jul, 2009.

On 12th Nov, 2009 Paul S said:

I think Gary OS has taken over my role as the forum smart arse

That is a good choice, if you never, ever would consider anything larger than your current engine.
If you *think* wou might do a 1275 in the future, I'd jump all over the next one up. It really doesn't look that much different - but you will gain a lot of flow through the turbine.

Thanks for the input Dave. I really have no intentions of changing engine in the next 5 years minimum. I have spent way too much time, effort and money to even think about other builds once this is done. I'm burned out on this one

EDIT: Just to clarify, the next one up is the same turbo that Turbo Tel uses, isn't it?

Edited by GaryOS on 29th Jul, 2009.

On 12th Nov, 2009 Paul S said:

I think Gary OS has taken over my role as the forum smart arse

Yes. I've got one as well. Looks night and day superior to a T2.

Ok, Could someone please explain something for me? Where does the assumtion of 71% efficiency for the compressor come from? Also, where is it applicable?

On 12th Nov, 2009 Paul S said:

I think Gary OS has taken over my role as the forum smart arse

Well, it looks like I have found the answer. It appears to be straight off the compressor map. If that is the case, why is it listed in the assumptions before any calculations begin? Is it an estimate or a goal? Also, I don't remeber using it to calculate pressure ratios of air flows so is it needed?

On 12th Nov, 2009 Paul S said:

I think Gary OS has taken over my role as the forum smart arse

Just had a look at these to graphs:




If assume Paul's torque curve remains flat beyond 5630 rpm (I can see no reason why it wouldn't) and extrapolate the power curve out to 6500 rpm you get 121 BHP at 6500 rpm, which is exactly the same as calculated above.

Well done, I'm seriously impressed.

Edited by Rob H on 2nd Mar, 2010.

I just need to improve the head to make it flow a bit more, add a GT1752 and we could see around 130hp !

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Paul, I'm stuck with the increase in temperature, where does the [(n-1)/n] come from and why does n = 1.4? when I pump the numbers in your example into the formula for temperature increase I seem to get an output temperature that's cooler than the input temperature which is blatantly wrong.

Paul, don't answe that just yet, I think I've figured it out, just need to re-crunch the numbers.

EDIT:

Had to look at my old A-Level physics books and played around with my calculations at lunch but I'm happier now. It would appear that I needed to raise the pressure ratio to the power of (n-1)/n not the output pressure which I had previously raised to the power of (n-1)/n.

Also figured out where the (n-1)/m n came from, simply taken from the ideal gas law with n being the heat capacity ratio which is 1.4 for diatomic gases.

Simples

Edited by Rob H on 3rd Mar, 2010.

Don't worry he's out on business today and won't be able to get on TM. I think he'll be getting withdrawal symptoms about now :)