Surplus coolant is not lost during warming-up and after shutting down the hot engine, but, is collected in a reservoir-tank (figure) and when cooling is necessary, is supplied to the cooling system again. In this process, gaseous portions can also escape from the cooling system and cause no further damage. The waterpump uses less energy because it must not always prime the pump anew. Gaseous coolant (Cavitation) on the intake side is avoided.Especially important is, that the coolant temperature, through high pressure, can reach possibly up to 120°C without reaching its boiling point. The engine can be operated at a higher operating temperature, which causes an increase in efficiency.

The reservoir-tank is only partly filled with coolant. When the engine is cold, the coolant level should lie between the min. and max. marking. The space above the max. mark fills partly with coolant at high temperatures. Gases escape through a pressure relief valve in the screw-top. There another valve is also often found, which allows air, when cooling down, into the system again, thus preventing the development of a vacuum in the reservoir-tank. 06/09