A mass air flow sensor is used to find out the mass of air 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,
altitude and use of a turbocharger and this is an ideal application for
a mass sensor. (See stoichiometry and ideal gas law.)
There are two
common types of mass airflow sensors in usage on automotive engines. These are
the vane meter and the hot wire. Neither design employs technology that measures
air mass directly. However, with an additional sensor or two, the engine's air
mass flow rate can be accurately determined.
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.
When
a MAF 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
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.
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