A mass (air) flow sensor
(MAF) is used to find out the mass flowrate
entering a fuel-injected internal combustion engine
The air mass information is necessary for the engine control unit
(ECU) to balance and deliver the correct fuel mass to the engine. Air changes its density as it expands and contracts with temperature and pressure. In automotive applications, air density
varies with the ambient temperature
and the use of forced induction
, which means that mass flow sensors are more appropriate than volumetric flow
sensors for determining the quantity of intake air in each cylinder. (See stoichiometry
and ideal gas law
There are two common types of mass airflow sensors in use on automotive engines. These are the vane meter and the hot wire. Neither design employs technology that measures air mass directly. However, with additional sensors and inputs, an engine's ECU can determine the mass flowrate
of intake air.
Both approaches are used almost exclusively on electronic fuel injection
(EFI) engines. Both sensor designs output a 0.0-5.0 volt or a pulse-width modulation
(PWM) signal that is proportional to the air mass flow rate, and both sensors have an intake air temperature (IAT) sensor incorporated into their housings for most post OBDII vehicles. Vehicles prior to 1996 could have MAF without an IAT. An example is 1994 Infiniti Q45.
When a MAF sensor is used in conjunction with an oxygen sensor
, the engine's air/fuel ratio can be controlled very accurately. The MAF sensor provides the open-loop controller
predicted air flow information (the measured air flow) to the ECU, and the oxygen sensor provides closed-loop
feedback in order to make minor corrections to the predicted air mass.