A heated surface of the hot-film air mass sensor in the flow of intake air is controlled to a constant temperature with respect to the intake air. The intake air flowing past this surface cools this heated surface and thus changes its resistance. The heating current which is necessary in order to maintain the constant temperature is the measurement variable for the air mass drawn in. The DME control unit uses this variable to calculate the injection timing.
Important advantages:
The hot-film air mass meter renders unnecessary clear-burning of the sensor after the engine has been shut down. Any dirt deposits on the surface do not influence the sensor signals directly since the protective film cleans itself due to the constant overtemperature.