Engine cooling system (Audi 80 B4)

When the engine is running, the water pump constantly supplies coolant to the cooling jackets, which removes the heat generated as a result of combustion of the air-fuel mixture.

The water pump is driven by a V-belt in four-cylinder engines, a toothed belt in five-cylinder engines, and a V-belt in six-cylinder engines. The water pump of diesel engines is driven by a V-belt.

Before the engine warms up to operating temperature, the coolant circulates in the so-called small circuit, which is formed by the cylinder head, engine block and heater. The small circulation circuit also includes the intake manifold.

When the coolant temperature reaches approximately 87°C, the thermostat opens the large circulation circuit, which includes the radiator. If the coolant temperature continues to increase and even reaches over 92°C in the radiator, the radiator fan is turned on.

In the cooling system, at the operating temperature of the engine, a pressure of approximately 1.2-1.5 bar is created, which helps to increase the boiling point of the coolant. The pressure in the cooling system is maintained at the proper level by a pressure-reducing (outlet) valve designed for a pressure of 1.2-1.5 bar and an inlet valve of 0.06-0.1 bar, installed on the filler cap of the expansion tank. The inlet valve lets in air; the coolant cools and decreases in volume.

(The text is based on materials from the website «AudiManual»)