Air Flow Meter/Sensor: Description and Operation

Hot Wire Air Mass Sensor:

PURPOSE
The Air Mass Meter measures the amount of air entering the intake manifold, providing the Digital Motor Electronic (DME) control unit with precise "air mass" information.

OPERATION
During operation, a plastic-coated heated platinum wire is subjected to the flow of intake air in the inner tube of the hot-film Mass Air Flow Sensor (MAF). Heat is dissipated from the hot wire by the flow of air. This heat is compensated by controlling the heating current. At the same time, the supplied current also flows via a precision resistor whose voltage drop represents a direct measure for the air mass which is drawn in. Air-temperature fluctuations are detected by way of a compensation resistor and taken into account in the measurement.

The advantages offered by the air mass sensor can be found in the fact that, in this case, the air mass is measured and not the air flow (volume). Fluctuations in the air density (different air pressure due to differences in altitude and weather situation) are already taken into account during initial measurement. In addition, less resistance is offered to the flow of air in the air mass sensor since it is not equipped with an air sensor plate which needs to be pressed down.

The voltage value of the air mass sensor in conjunction with the engine speed is a direct measure for the engine load.