The water pump is driven by a V-belt in a 4-cylinder engine, and by a toothed belt in 5- and 6-cylinder engines.
The path of the coolant depends on its temperature at a given moment in time:
If the engine has not yet reached operating temperature, the coolant flows through a small circuit that passes through the engine and heat exchanger. In the four-cylinder engine with the Mono-Motronic injection system, the intake manifold is connected to the small coolant circuit in addition.
When the temperature reaches approximately 87°C, the thermostat opens a large circuit that also turns on the radiator. If the coolant temperature rises further, so that even in the radiator the temperature is above 92°C, the radiator fan starts working.
Cooling system designed to operate under excess pressure
In the cooling system, high pressure prevails at operating temperature (9.2-1.5 bar) which increases the boiling point of the coolant significantly above 100°C. This creates a certain reserve. The engine can reach operating temperatures exceeding 100°C, and there is no risk of boiling in the cooling system.
The correct pressure in the system is created thanks to the threaded cap of the expansion tank, which is equipped with a safety valve (1.2-1.5 bar) and a vacuum valve (0.06-0.1 bar). The vacuum valve lets air in when the coolant gets colder, thus taking up less space.
(The original article is available on the website: «AudiManual»)