Back in my youth, when pressurized cooling systems with overflow tanks were introduced, it was sometimes said the radiator caps had "thermostatic valves" that opened when cold, to expel air, but closed when hot, to control boiling.
I've been looking at caps ever since, hoping to discern just how this trick was accomplished. I've never seen anything resembling a bimetal or wax capsule but admisttedly have never dissected a cap. There just didn't seem any space for such a gadget. All I could find was an inlet check valve to refill the cooling system as it cooled after shutdown and the spring loaded relief valve that vented pressure at setpoint.
Nonetheless, every liquid-cooled engine I've dealt with has kept its radiator full to the absolute top of the radiator filler neck, so the scheme somehow manages to expel air very effectively. This seems to work even when the coolant doesn't get hot enough to come near the relief pressure.
Does anybody happen know how this is accomplished? Does mere expansion of the trapped air and coolant produce enough pressure to force the relief valve open? Clearly, expansion of the liquid coolant can accomplish the task, but that would allow an air pocket to persist at sufficient volume, at least till the system started to boil. At that point steam would expel the air and the system would refill next time it cooled.
Or, is there some invisible heat-sensitive element hidden in the radiator cap?
Thanks for reading,