Quote:
Originally Posted by johnnyrod
.... Embee am I right in saying that one advantage of water is you can get boiling at very hot spots which relieves the heat, even though it does temporarily take up more room in the cooling system?
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Yes. What is referred to as "nucleate" (i.e. at a point) or "film" (on a surface) boiling is a very useful way of removing a lot of heat (latent heat of vapourisation) from a local hot spot, an exhaust port for example. The key is to have sufficient flow speed for the vapour to be rapidly carried away from the hot spot so fresh liquid can repeat the process, which is partly why you see holes placed strategically in the head gasket or castings just at the exhaust port or spark plug boss etc.
Boiling will absorb a lot of heat without increasing the temperature, and as long as the vapour gets whisked away and can condense again very rapidly in the slightly cooler liquid the volume increase is minimal.
A coolant system is typically designed to be able to de-gass a certain amount if there is any gas leakage across the head gasket (from combustion), there will be either a gas volume above the liquid header in a hot-bottle design or a collector feature at the radiator cap in a cold bottle system. Local boiling should be re-condensed within the flow so shouldn't usually reach the de-gass feature. A cold bottle system will usually recover any vapour which does get that far, a hot bottle system will lose coolant if it vents so it is avoided.