What is a Mass Flow Sensor?

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Mass Flow Sensor:

A mass (air) flow sensor (MAF) is a sensor used to determine the mass flow of air entering a fuel-injected combustion engine.

The atmosphere information is important for the engine control unit (ECU) to balance and deliver the right fuel mass to the engine. Air changes its density with temperature and pressure. In automotive applications, air density varies with the ambient temperature, altitude, and therefore the use of forced induction, which suggests that mass flow sensors are more appropriate than volumetric flow sensors for determining the number of intake air in each cylinder.

There are two common styles of mass airflow sensors in use on automotive engines. These are the vane meter and therefore the hot wire. Neither design employs technology that measures the atmosphere directly. However, with additional sensors and inputs, an engine’s ECU can determine the mass flow of the intake air.

Both approaches are used almost exclusively on electronic fuel injection system (EFI) engines. Both sensor designs output a 0.0–5.0 volt or pulse-width modulation (PWM) signal that’s proportional to the atmospheric state rate, and both sensors have an intake air temperature (IAT) sensor incorporated into their housings for many post-on-board diagnostics (OBDII) vehicles. Vehicles before 1996 could have MAF without an IAT. An example is the 1994 Infiniti Q45.

When a MAF sensor is employed in conjunction with an oxygen sensor, the engine’s air/fuel ratio may be controlled very accurately. The MAF sensor provides the open-loop controller predicted airflow information (the measured airflow) to the ECU, and also the oxygen sensor provides closed-loop feedback so as to create minor corrections to the expected atmospheric state. Also, see the manifold absolute pressure sensor (MAP sensor).

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