Re: S.P.E.A.R.C.O.intercooler guys??

[Robert Myers <rmyers@inetone.net>]

>I have to think about this for a minute, but doesn't temperature and
>pressure completely define the density of a gas in terms of variables? 
>Bob Myers?  PV=nRT  so I think so.

Hey Y'all,

If your eyes tend to glaze over when discussing gas behavior in
mathematical terms, it's time to hit <Delete> right now.

Graydon,

Under the KISS principle I tend to stick with treating air as an "ideal
gas".  They ain't no sech animal but it makes an acceptable first
approximation.

n in the above equation is the number of moles of gas in the sample which
equals the weight (g) of the sample divided by the molecular weight (M).  

n = g/M

Substitute this in PV=nRT and solving for g/V which is the density you get:

Density = g/V = PM/RT

Since we are dealing with the oxygen component of the air and since oxygen
has a constant Molecular weight (as do all other gases just not the same
constant) both M and R are constants for this situation.  Likewise air is
always the same percentage oxygen and its mole fraction (X) is also
effectively a constant.  That being the case you can describe another
constant K as K = MX/R.  (Note that in the ideal gas equation, as commonly
seen, we are assuming that the gas sample is a single component and that
its mole fraction is 1.  That 1 appears with the n as nX but since it is 1
we don't write it since 1n = n.  When we extend the ideal gas law to cover
multiple component mixtures of gases then we must include the mole fraction
term.)

Thus the above equation becomes

D = K * P/T

>From this, Density is seen to increase when Pressure is increased and to
decrease when Temperature is increased.  

Remember that the Temperature term requires that you use one of the
absolute temperature scales, commonly the Kelvin scale in which 0 Celsius =
approx 273 K is used.  Any temperatures measured must be converted to K
temperatures by the equation K = C + 273.  If you measure temperatures in
Fahrenheit degrees then you must convert to C and then K before you do the
rest of the math.

Note that these same calculation can be done using weights in pounds rather
than grams and cubic feet instead of liters and degrees F rather than
degrees C but then you convert to the Rankin temperature scale.  The
results will be the same but R will have a different value related to the
different measuring scales.  However, the ratios will remain the same.
What do you say we KISS.  Keep it simple, Stupid.

A common error is to measure two temperature such as say 100 F and 200 F
and say that one temperature is twice the other or since 100/200 = .5 to
say that the temperature has been reduced by 50%.  This is not correct.
100F = 37.8C = 310.8K while 200F = 93.3C = 366.3K  Given this relationship
going from 200F to 100F is a ratio of 310.8/366.3 = 0.848 or about a 15%
decrease in temperature.

If you are still with me here you are very patient.  I was a bit sloppy in
some of the derivation above but it helps to KISS.

HTH

Humidity will tend to lower the density of the air mixture and will thus
tend to decrease the amount of oxygen in the cylinder when everything else
is kept constant.  However, water vapor also appears to have some sort of
catalytic effect which results in a greater combustion efficiency which
would tend to increase the energy available from the engine.  This is why
some use a direct water injection into the fuel mixture.


___
   Bob
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