Function Unit: Notes

The so-called function unit is located in the active reservoir. It has the external appearance of a surge chamber and accommodates a heater, filter and a level sensor. The delivery unit is attached to it.

Unlike a surge chamber in the fuel tank, the lower section of the function unit has slots. This chamber creates a smaller volume in the reservoir that scarcely mixes with the urea-water solution outside the chamber.

There is a PTC heating element (positive temperature coefficient) in the base of the chamber that can heat up this smaller volume at a relatively fast rate. The intake line is also heated. In this way, the liquid urea-water solution can be made available for vehicle operation even at the lowest temperatures.

Fig 1: Identifying Function Unit

Courtesy of BMW OF NORTH AMERICA, INC.

The heating element in the chamber is connected to the heater for the intake line to form one heating circuit. A power semiconductor supplies the current for this heating circuit. The power semiconductor is controlled by the DDE. The DDE can determine the current that flows across the heating elements and can therefore monitor their operation.

The temperature sensor provides the signal for the heating control system. It is designed as an NTC sensor (negative temperature coefficient). The temperature sensor is integrated at the bottom end of the level sensor.

Fig 2: Cutaway View Of Function Unit

Courtesy of BMW OF NORTH AMERICA, INC.