Expansion tank
Jaguar: 420 and Daimler Sovereign (1966-69)
engine cooling system. Ultra-light aluminium version of the original tank
Modern cooling systems work mostly with relatively high pressure, so the cooling water temperature can attain a higher temperature to prevent evaporation.
The original solution allowed the precious (see below) cooling water to escape directly through a small hose at the filler neck onto the road. Expansion tanks take the high pressure coolant and pass it back to the system after it has been cooled down thus avoiding the need to keep topping up with fresh water. It is much better to have water without any residual oxidation potential in a long-running system! This modification results in a pressure rise curve which is flat and smooth and the system is not constantly under severe pressure as the expansion volume in the radiator itself is only minimal.