Expansion tank became widespread in the engine cooling system when they began to use antifreeze fluid, such as antifreeze or cooling fluid. The basis of such fluids is ethylene glycol solution, the thermal expansion coefficient of which is greater than that of water. Pouring the liquid into the cooling system into the upper tank of the radiator, when the engine warms up, it expands and is forced out through the radiator cap relief valve pressure. If the car engine is turned off and cools down, the volume of fluid is gradually reduced and in the cooling system there is a vacuum. At the same time the valve cover is actuated. This results in air suction into the radiator. This can lead to air pockets in the coolant jacket when the engine is on that breaks the process of heat transfer.

In order to prevent overheating of the engine air entering must be replaced with liquid entering. Thus, there is a need for additional reservoir, which would have contained the liquid. Expansion tank was connected to the radiator filler neck with a hose. In addition, it was installed so that its middle was almost at the top of the radiator. As a consequence, excess heated fluid is pressed out into the tank. When the engine cools down, the liquid again fills the system, thus eliminating the ingress of air. This is not the only function the expansion tank fulfils. Due to this part it became possible to create more streamlined cars.

When the expansion tank appeared, the whole scheme of the cooling system changed. So, now coolant circulates from right to left or vice versa, rather than top down.