Types of Mass Airflow Sensors
There are two main types of mass airflow sensors in use on cars today. They are the vane meter and the hot wire. The sensors do not measure air mass directly. With the input of other sensors, the computer can accurately determine the air mass flow rate.
Both types of sensors are used almost all applications on fuel injected engines. Both sensor designs output a 0.0–5.0 volt or a pulse-width modulation signal that is proportional to the air mass flow rate, and both sensors have an intake air temperature sensor incorporated into the airstream.
When the mass airflow sensor signal is combined with the oxygen sensor, the engine’s air/fuel ratio can be controlled very precisely. The MAF sensor provides the open-loop predicted air flow information (the measured air flow) to the engine computer, and the oxygen sensor provides feedback in order to make minor corrections to the predicted air mass.
Vane meter sensor
The Vane meter sensor measures the air flow into the engine with a spring-loaded air flap attached to a variable resistor. The vane moves in relation to the airflow, and a voltage is generated proportionally to the distance the vane moves. Therefore the movement of the vane controls the amount of fuel injected.
Many Vane meter airflow sensors have an air-fuel adjustment screw, which opens or closes a small air passage on the side of the sensor. This screw controls the air-fuel mixture by letting a metered amount of air flow past the air flap, thereby, leaning or richening the mixture. This allows for calibration of the sensor. By turning the screw clockwise the mixture is enriched and counterclockwise the mixture is leaned.
The vane moves because of the drag force of the air flow against it, it does not measure volume or mass directly. The drag force depends on air density, velocity and the shape of the vane. As a result, many Vane Airflow Sensors include an intake air temperature sensor (IAT sensor) to allow the engines PCM to calculate the air density, and the fuel delivery accordingly.
Hot wire sensor
General Motors was the first car company to use the hot wire sensor, and is the most common sensor in use today. This is achieved by heating a wire with an electric current that is suspended in the engine’s air stream, like a hair dryer. The wire’s electrical resistance increases as the wire’s temperature increases, 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 temperature is directly proportional to the mass of air flowing past the wire. The integrated electronic circuit converts the measurement of current into a voltage signal which is sent to the PCM.
If air density increases due to pressure increase or temperature drop, 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 combustion engine process which fundamentally responds to air mass, not air volume.
A hot wire burn-off cleaning circuit is employed on some of these sensors. A burn-off relay applies a high current through the platinum hot wire after the vehicle is turned off for a second or so, thereby burning or vaporizing any contaminants that have stuck to the platinum hot wire element.
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
separate temperature and pressure sensors are not required (to determine air mass)