Volume Air Flow:
The VAF (volume airflow) sensor measures the airflow into the engine with a spring-loaded air vane (flap/door) attached to a rheostat (potentiometer). The vane moves in proportion to the airflow. A voltage is applied to the potentiometer and a voltage appears on the output terminal of the potentiometer proportional to the angle the vane rotates, or the movement of the vane may directly regulate the number of fuel injected, as within the K-Jetronic system.
Many VAF sensors have an air-fuel adjustment screw, which opens or closes a little duct on the side of the VAF sensor. This screw controls the air-fuel mixture by letting a metered amount of airflow past the air flap, thereby leaning or richening the mixture. By turning the screw clockwise the mixture is enriched and counterclockwise the mixture is leaned.
The vane moves thanks to the drag force of the airflow against it; it doesn’t measure volume or mass directly. The drag force depends on air density (air density, in turn, depends on air temperature), air velocity, and also the shape of the vane, see drag equation. Some VAF sensors include a further intake air temperature sensor (IAT sensor) to permit the engine’s ECU to calculate the density of the air, and also the fuel delivery accordingly.
The vane meter approach has some drawbacks:
- it restricts airflow which limits engine output
- its moving electrical or mechanical contacts can wear
- finding an acceptable mounting location within a confined engine compartment is problematic
- the vane has got to be oriented with relation to gravity.
- in some manufacturers, fuel pump control was also a part of the VAF internal wiring.
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