Engine Thermostat
There are two types of thermostats in BMW cars, the mechanical type, and the type that is electrically heated. Both are designed to bring the temperature of the coolant up to operating temperature quickly.
When the engine is cold, the thermostat is closed; in the closed position, it sends coolant from the engine back to the engine, through a bypass; as a result, the coolant temperature rises quickly. As the coolant reaches the rated temperature, the thermostat gradually opens, to allow coolant to flow from the engine to the radiator.