If the engine is not yet warm, then the coolant flows through «small contour», which goes through the engine and the heating heat exchanger.
Above about 92°C, the thermostat opens «big outline», in which the radiator is connected. There, the liquid is cooled by the oncoming air flow and the air flow created by the radiator fan.
Pressurized cooling system
The cooling system, depending on the engine, contains a different amount of coolant:
This rather small amount of coolant would not be enough for fast driving on the autobahn or going up the pass if the cooling system was not under pressure. An overpressure of approximately 1.2-1.5 bar dominates in the cooling system at operating temperature. This raises the boiling point of water from 100°C to about 125°C. Thus, a certain reserve is created. Thanks to this, the engine can reach operating temperatures in excess of 100°C, and at the same time there is no «liquid boiling hazard» in the cooling system.
Good «working climate» for the engine provide:
- A cap on the expansion tank that regulates the pressure.
- A thermostat that, after starting a cold engine, allows coolant to flow through the radiator not immediately, but as the temperature rises.
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