Automotive mass airflow sensors

A mass airflow sensor is used to determine the mass of air entering the engine. The air mass information is necessary to calculate and deliver the correct fuel mass to the engine. Air is a gas, and its density changes as it expands and contracts with temperature and pressure. In automotive applications, air density varies with the vehicle's operating environment, and is an ideal application for a mass sensor. (See stoichiometric, ideal gas law, and density.)

There are two common types of mass airflow sensors in usage on gasoline engines. They 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 gasoline burning, EFI (electronic fuel injection) engines. Both sensor designs output a 0 - 5.0 volt signal that is proportional to the air mass flow rate, and both sensors have an IAT sensor (intake air temperature) incorporated into their housings.

When a MAF is used in conjunction with an exhaust gas 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 engine's ECU, and the EGO sensor provides closed-loop feedback in order to make minor corrections to the predicted air mass.

(Also see stoichiometric and MAP sensor.)


Vane meter sensor

A vane, or paddle, projects into the engine’s intake air stream on a spring-loaded arm. The vane moves in proportion to the airflow, and a voltage is generated in proportion to the distance the vane moves. The vane measures air volume, not mass, however by measuring the air temperature and pressure to determine air density, a true mass airflow calculation can be achieved. (See ideal gas law.)

The vane meter approach has some drawbacks:

it restricts airflow which limits engine output
its moving electrical contacts can wear
finding a suitable mounting location within a confined engine compartment is problematic
the vane has to be oriented with respect to gravity

Hot wire sensor (MAF)

A hot wire mass airflow sensor determines the mass of air flowing into the engine’s air intake system. This is achieved by heating a wire with an electric current that is suspended in the engine’s air stream, not unlike a toaster wire. The wire's electrical resistance increases with the wire’s temperature, which limits electrical current flowing through the circuit. When air flows past the wire, the wire cools decreasing its resistance, which in turn allows more current to flow through the circuit. As more current flows, the wire’s temperature increases until the resistance reaches equilibrium again. The amount of current required to maintain the wire’s electrical resistance is directly proportional to the mass of air flowing past the wire.

If air density increases due to a pressure or temperature change, but the air volume remains constant, the denser air will remove more heat from the wire indicating a higher mass airflow. Unlike the vane meter's paddle sensing element, the hot wire responds directly to air density. This sensor's capabilities are well suited to support the gasoline combustion process which fundamentally responds to air mass, not air volume. (See stoichiometric.)

Some of the benefits of a hot-wire MAF compared to the older style vane meter are:

responds very quickly to changes in air flow
low airflow restriction
smaller overall package
less sensitive to mounting location and orientation
no moving parts improve its durability
less expensive
separate temperature and pressure sensors are not required (to determine air mass)
There are some drawbacks:

dirt and oil can contaminate the hot-wire deteriorating its accuracy
installation requires a laminar flow across the hot-wire