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Old 10-04-2012, 08:30 PM   #55
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Join Date: May 2007
Location: TX
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Originally Posted by lcoleman View Post
Thinking off the top of my head here...pv=nrt would probably only work for a static system. The entire cooling system has a pressure differential (consider the points directly before and directly after the water pump) which causes flow. Also, the volume of the system is fixed, and the volume of a given amount of air changes depending on pressure, no?

Regardless of the science behind it, I agree with TxZHP04 in post #13; all this modification will do is introduce a leak as the "mean system pressure" in the ET exceeds the limits of the lower cap. Changing the cap will not change the operating pressure of the system, as it simply bleeds excessive pressure (which only happens if the T gets too high, right?). Think more like underdrive pulley if you want to reduce the pressure...and have a head gasket handy.

Long story short: waste of time.
It's not a waste of time and there is a very simple justification for why ideal gas law should work here.

First of all, we are only interested in the pressure that the gas exerts onto the cap in the expansion tank. This gas doesn't circulate the system. Coolant does. So it doesn't have anything to do with the system being static.

Also, volume of air is fixed if it is contained, as in the case of ET (since the coolant expands a little, the volume of the gas actually decreases by the same amount, but this is negligible). In a system with a fixed volume, increasing temperature increases the Pressure only. Directly seen from the ideal gas law.

Second, while the coolant reaches temperatures above the 96 degree celcius in the engine block, it is around 96 degree celcius in your lower radiator hose. It is likely to be higher by 10-20 degrees in the ET, but that doesn't affect the conclusion (10-20 degrees more will increase the pressure by a fraction of a bar).

At thermal equilibrium (e.g. at operating temperature), the coolant and the gas in the ET have the same temperature.

That's pretty much it.

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Last edited by SeanC; 10-04-2012 at 08:35 PM.
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