I/C Temperature Math - update

["David G. Lawson" <dbldmnd@compuserve.com>]

A few days ago I went through the baseline MC case which DeWitt throughly
covered in his 'search for a baseline' post. But one thing caught my eye,
both Scott M. and I got 80-82% efficiency numbers where DeWitt was showing
73-75% efficiency numbers. Scott and I used degrees F for our calculations
and DeWitt used degrees C. Keep in mind that the data published in the SAE
report 860103 which we have all mentioned is in degrees C. I think I found
out the reason for the difference.

Myself, and I think Scott, took the intake manifold temp and converted it
to degF and then took the delta temp which the intercooler produced and
converted it to deg F and then added them together to get the turbo outlet
temp. I have included both the degC and degF calculations below.

If you use deg C
Tm = intake manifold temp = 38 degC (published Audi data)
Dic = intercooler delta temp = 52 degC (published Audi data)
Tco = turbo compressor outlet temp = Tm + Dic = 38 + 52 = 90 deg C = 194
degF

If you use deg F
Tm = intake manifold temp = 100.4 degF
Dic = intercooler delta temp = 125.6 degF
Tco = turbo compressor outlet temp = Tm + Dic = 100.4 + 125.6 = 226 degF

Obviously different answers which vary by 32 degF.

The difference comes about because when working with the i/c delta temp, we
shouldn't be adding in the 32 degrees which you would normally do when
converting from degC to degF, i.e. degF = degC*1.8 + 32

So if we choose to do our math in degF, we should only multiply the
intercooler delta temp by 1.8 but not add 32. So the degF example becomes

Tm = intake manifold temp = 100.4 degF
Dic = intercooler delta temp = 93.6 degF
Tco = turbo compressoroutlet temp = Tm + Dic = 100.4 + 93.6 = 194 degF

For the tests Audi performed the ambient temp was 20 degC = 68 degF

So the intercooler efficiency calculation becomes

Eic = (Tco-Tm)/(Tco - Ta) = (194 - 100.4)/(194-68) = .743 = 74.3%
efficiency

This difference only manifested itself when the intercooler delta temp was
converted from degC to degF like it is usually done. If Audi had published
compressor outlet temps, we would not have noticed this difference.

Now with Scott M.s data for his 1.8 bar modification which he presented
last wed, his temps were actual numbers before and after the intercooler so
the efficiency for his data shows

Tm = intake manifold temp = 85 degF
Tco = turbo compressor outlet temp = 210 degF
Dic = intercooler delta temp = 210-85 = 125 degF
Ta = ambient temp = 43 degF

Eic = (Tco-Tm)/(Tco - Ta) = (210-85)/(210-43) = .749 = 74.9% efficiency

So we now have some baseline numbers for the stock 2 pass intercoolers,
in the stock system with 1.42bar, the intercooler is 74.3% efficient and in
a 1.8bar modified system, the same intercooler is 74.9% efficient. Real
data producing real efficiency numbers here.

We are on the way to building a database of the stock 2 pass intercooler
efficiency relative to increased boost pressure mods. Now if Scott M. could
swap the 1.8 prom with a 1.95 prom and run the same set of tests, we could
see if this real world in car test data would match the other numbers
presented by Eric F. 

I'm taking some time off, keep these tech discussions moving and play nice.
-
Dave Lawson