Interesting question
@deQuincey ; I do not have the answer.
I do like to brawl about things , and dump my thoughts anywhere however
(Note that the 1.0 bar purge pressure is actually 2.0 bar atmosferic pressure; it is 1.0 bar OVER atmosferic pressure. )
I would think the system would operate during normal driving condition at a near zero state of pressure for the following reason: I would consider the system to be closed; closed as there is no addition of air or coolant during driving. There are thus only two factors that can create pressure: 1 thermal expansion of the media: coolant and the air in the tank, and 2. gas formation.
- The coolant expansion is a linear thing, and even from freezing point (0) to 99 degree Celsius the volume chance effect is small: (99*the factor of 0.00021 = 0,02 thus 5 liters will become 5,1 Liters (gallons) volume increase (Wikipedia: for every 1 degree Celsius increase in temperature, water will expand by 0.00021 times its original volume)
If the pressure in the air volume in the tank is ambient (1bar) at the begin, and the airvolume was 1 liter, then the 0.1 liter of fluid expension will increase the pressure by + 10% going to 1.1 bar.
- To the second point; Gas formation. IF it occurs, it increases the volume by a factor of ~1600, so that would indeed seriously build pressure. But I doubt that gas formation occurs during normal operation, as the system tries to limit the max temp to 95 or 97 Celsius, doesn't it? (correct me if i'm wrong, isn't that what the temperature sender does; open the cooling circuit to push the temp down? ) If that is correct, then gas will not form throughout the system. This leaves gas formation close to the cylinder wall: for this I think the speed at which the coolant passes through the cylinder head is too high for the coolant to heat up beyond the boiling point.
But for this last thought i have little argumentation to defend it, it's just a thought.