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Old 10-06-2012, 09:27 PM   #10
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Originally Posted by WDE46 View Post
I trust my math more than the intuition here. The math says that the system shouldn't experience any more than about 1.4 bar absolute pressure. That means the ET will experience a 0.4 bar maximum of pressure exerted on its inner surface. That is 1.6 bar below the cap's crack pressure. This never approaches 10-12 psi. Unless we are severely miscalculating something, I don't see why we have the 2 bar caps.

The only thing we haven't taken into account with the pressure calculation is water vapor in the ET air and the coolant expansion. The water expands by 3.8% (about 1/3 of a quart) when heated from 20C to 100C, so that could conceivably almost fill the ET depending on its size. It could be a very major factor. I need its dimensions, specifically air volume at min and max. We could find this, by topping off the ET with distilled water, then removing it into a measuring device until the max level, take a reading, then continue to the min. That is the easiest and most accurate way. If it so happens that the air volume is cut in half by the expansion of water, then we will find the system actually runs at around 2 bar. If this is the case, then I would recommend not filling the ET to maximum to reduce system pressure and extend ET life.

The most complex part of all this is the mixtures that are present. We have 50/50 water and antifreeze for our fluid mixture and a mix of water vapor/air/antifreeze vapor in the ET buffer volume.
The amount of water vapor in the expansion tank (hence the system) will not be more than 4% (even 100% humidity). It is negligible.

Originally Posted by TerraPhantm View Post
Well... Assuming you start at 20C and end up at 95C the final volume would have to be ~42% of the starting volume to reach 3 bar. So if the coolant expanding by about 300mL causes the system to reach that point, that would mean the air space when cold is about 515mL. Which I suppose sounds reasonable enough.

So assuming the system is capable of reaching 3 bar - the question would now be why BMW wants such high pressures. Maybe it's due to a potential for cavitation, maybe it's due to a possibility of boiling coolant at localized hot spots. Or maybe it's something I can't think of.

But assuming any of those risks are legitimate, a better solution might be to use a lower temperature thermostat in conjunction with a 1-bar or 1.4 bar cap. From what I've read, the higher operating temperatures were chosen for emissions purposes.
What is the total coolant fill up to the min level for a well bled system? If we knew this, we could calculate the expansion of the coolant using 3.8% expansion rule much more accurately.

Not saying your numbers are not reasonable, but biasing the results to initial values doesn't seem too justifiable to me. We should be able to express things as state functions, meaning depending on the properties of the system at a given instant.

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