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

The mass air flow (MAF) sensor detects the amount of air drawn into the engine and relays a signal to the engine control unit (ECU). The signal is used to calculate the proper amount of fuel to deliver to the engine. Air changes its density as it expands and contracts with temperature and pressure.

Air flow sensors measure either the volume or the mass of air flowing in a channel. In an automobile, the primary use of an air flow sensor is to determine the amount of air drawn into the engine through the intake manifold. There are two main types of air flow sensors: volume air flow sensors, which measure the effect of moving air on a pinwheel or deflecting plate, and mass air flow sensors, which measure the mass of air as it passes through the sensor. As the load on an internal combustion engine varies, it is necessary to determine how much air is entering the cylinders in order to determine how much fuel to inject and how to adjust the engine timing. Because the stoichiometry of the air-fuel reaction depends fundamentally on the mass (proportional to the number of molecules) of the air, rather than the volume, the use of mass air flow sensors (MAFs) is much more widespread.

The first airflow sensors to appear in cars measured the deflection of a plate placed in the airflow path. Later, another style of volume air flow sensor, the Karman Vortex airflow sensor, was introduced. This sensor determined the velocity of the air flow by monitoring the frequency of an artificially induced turbulence using optical sensors. The ECM combines volume airflow information with air temperature measurements to calculate the mass air flow.

There are several types of mass air flow sensors. The most common designs use a thin resistive wire (usually platinum) that hangs down into the air flow. This wire is heated to a specified temperature, usually 100's of degrees Fahrenheit above the ambient temperature detected by the intake air temperature (IAT) sensor. As the air moving through the sensor cools the hot wire, the current flow through the wire changes. This current is detected by the sensor, which then relays this information to the ECM. Other types of mass airflow sensors include "cold-wire" sensors (which detect the varying inductance of a circuit connected to thin metallic strips that vibrate as the air goes by) and membrane sensors (MEMS devices that use the change in temperature of a heated membrane to detect air flow in a manner similar to the "hot wire" sensor).
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