10-06-2012, 11:05 PM
Join Date: May 2007
My Ride: 330i
EDIT: I was wrong. You need to assume an initial and final temperature to calculate the expansion (see my calculation below). However, in order to calculate the pressure, the system we are dealing with is simple enough that everything can be expressed with state functions. Just to recap:
Originally Posted by WDE46
Him assuming initial values is the best way to do this kind of thing.
1. Only the knowledge of temperature of the coolant is needed to know the pressure.
2. Amount of water vapor in the ET is negligible in the calculation of the pressure.
3. Since fluids are incompressible, it follows that the gas pressure of the air in the ET is also the overall pressure in the system.
4. In order to calculate the expansion of the coolant in the ET accurately, we need to know the cold fill level, which should be very easy to find out.
[EDIT]: Apparently we also need to know some sort of an expansion coefficient, as well as a temperature change (I was wrong in assuming this being a state function. Pressure is, but expansion requires initial and final temperatures. The latter also requires the fill level though, which I will assume to be between 8 and 11 liters. You can do the extrapolation).
Now water has an expansion coefficient of 0.000214, and etylene glycol has 0.00057.. Since these two form a homogenous mixture, for a 50-50 mixture, the expansion coefficient will be average of the two: 0.000392.
Assuming a fill level of 8 liters, the maximum expansion will be 235 mL between 20-95 degrees. For a fill level of 11 liters, the expansion will be around 325 mL. Not too far from Terra's numbers.
I don't know how this will drive the pressure to near 3 bars provided the system is not filled above the max fill level [/EDIT].
My feeling is that system will NEVER operate near the 3 bar maximum pressure even after the expansion is taken into account provided the system is filled between min and max levels.
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Last edited by SeanC; 10-07-2012 at 08:40 AM.