The amount of fuel needed for each engine cycle varies, depending on pressure changes in the manifold. That means manifold pressure needs to be measured. One way to do this is to measure airflow.
Measuring airflow allows for changes in an engine that can affect volumetric efficiency, such as wear, combustion chamber deposits and valve clearance.
The vane-type air-flow sensor is located between the air filter and the intake manifold. All the air drawn in by the engine is measured by this sensor. It’s the main measurable variable for fuel control and metering.
The sensor measures the force from the air-stream, on an airflow sensor flap. It overcomes the opposing force of a coil spring on the flap, and deflects it. As the air flow increases, so does the deflection of the flap, and its angle. A potentiometer converts the angular position of this flap into a corresponding voltage. This voltage becomes a signal, and it is transmitted to a control unit.
The sensor operates in a wide range of conditions. At light throttle openings, only a small amount of air passes, but the sensor must be able to measure it.
The engine’s intake strokes can cause the sensor to oscillate. This is counter-acted by a compensation flap. It operates in a dampening chamber and moves in unison with the air sensor flap. Since intake manifold pulsations have the same effect on both flaps, the forces cancel each other out, and measurement isn’t affected.
An adjustable bypass diverts a small quantity of air past the sensor flap. It is used to adjust the air-fuel mixture at idle speeds.
