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Old 10-07-2012, 01:54 PM   #121
TerraPhantm
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Join Date: Feb 2008
Location: Mountain Top
Posts: 5,898
My Ride: 2005 M3 Coupe
Quote:
Originally Posted by mkodama View Post
lol, your posts are much longer than they need be.

So for your ideal gas law, you are only acknowledging the air within the expansion tank, and your variables are:

P=variable you're solving for
V=?
n=?
R=?
T=coolant temperature

I think we are all in agreement that if you completely top off the expansion tank with coolant, you will get the 2bar relief pressure on the expansion tank cap?
R is the gas constant. You don't need to solve for R. You can also treat n as constant since it wouldn't change in a closed system (which it is until it starts venting).

If you set up an equation for both states, you get the following:
P1V1 = nRT1
P2V2 = nRT2

P1V1/T1 = nR
P2V2/T2 = nR

P1V1/T1 = P2V2/T2

Now you need to make some assumptions. For our purposes, it's reasonable to assume that starting temperature and pressure will be 20C and 1atm respectively. We know the final temperature gets to about 95C. Now if we assume that the system does indeed reach 3 bar, we can calculate how much the volume changes on relative terms.

V1/V2 = P2T1/P1T2
V1/V2 = 3 * 293 / 368
V1/V2 = 2.38 and V2/V1 = ~0.42

That means for the pressure to reach 3 bar, the volume of air must decrease to 42% of its starting value. If we know the volume of air in the ET when cold, we can verify whether or not this actually happens by calculating how much coolant expands during operation. If it's enough to create 3 (or more) bar, then we know the cap does something.
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Last edited by TerraPhantm; 10-07-2012 at 04:23 PM.
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