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. Also see MAP sensor.
Saturday, May 22nd, 2010 AT 1:42 PM