The air flow meter is one of the most important sensors because it is used in L-type EFI to detect the intake air mass or volume. Signal of the intake air mass or volume is used to calculate the basic injection duration and basic ignition advance angle. The air flow meter is largely classified into two types, mass air flow meters that detect the intake air mass, and volume air flow meters,intake air mass, and volume air flow meters, respective types include the following.
Mass air flow meterHot-wire type Volume
air flow meterVane type anoptical Karman vortex type
Currently, most models use the hot-wire type of air flow meter because it has superior measurement accuracy, lighter weight, and better durability.
REFERENCE
Vane Type
The vane type air flow meter is composed of many components, as shown in the illustration. When air passes through the air flow meter from the air cleaner, it pushes open the measuring plate until the force acting on the measuring plate is in equilibrium with the return spring. The potentiometer, which is connected coaxially with the measuring plate, converts the intake air volume to a voltage signal (VS signal) which is sent to the engine ECU.
REFERENCE
Optical Karman Vortex Type
This type of air flow meter directly senses the intake air volume optically.Compared to the vane type air flow meter, it can be made smaller and lighter in weight. The simplified construction of the air passage also reduces intake air resistance. A pillar (called the vortex generatorplaced in the middle of a uniform flow of air generates a vortex called a Karman vortexdown-stream of the pillar. As the generated Karman vortex frequency is proportional to the air flow speed, the air flow volume can be calculated by measuring the vortex frequency. Vortexes are detected by subjecting the surface of a piece of thin metal foil (called a mirror) to the pressure of the vortexes and optically detecting the vibrations of the mirror by means of a photocoupler (an LED combined with a phototransistor). The intake air volume (KS) signal is a pulse signal like that shown in the illustration. When the intake air volume is low, this signal has a low frequency. When the intake air volume is high, this signal has a high frequency. (1/1)
Hot-wire type
(1) Construction
As shown in the illustration, the construction of the hot-wire type of air flow meter is very simple. The compact and lightweight mass air flow meter shown in the illustration at left is a plug-in type that is installed onto the air passage,and causes part of the intake air to flow through the detection area. As shown in the illustration, a hot-wire and thermistor,which are used as a sensor, are installed in the detection area. By directly measuring the intake air mass, detection precision is improved and there is almost no intake air resistance. In addition, since there are no special mechanisms, this meter has excellent durability. The air flow meter shown in the illustration also has a built-in intake air temperature sensor.
(2)Operation and function
ccurrent flows to the hot-wire (heater) causing it to be heated. When air flows around the wire, the hotwire is cooled corresponding to the intake air mass. By controlling the current flowing to the hot-wire in order to keep the hot-wire temperature constant, that current becomes proportional to intake air mass. Intake air mass can then be measured by detecting that current. In case of hot-wire type air flow meters, this current is converted into a voltage that is then output to the engine ECU from the VG terminal.
(3)Inner circuit
In an actual air flow meter, as shown in the illustration, a hot-wire is incorporated into the bridge circuit. This bridge circuit has the characteristic of the potentials at point A and B being equal when the product of resistance along the diagonal line is equal ([Ra+ R3]R1=RhR2).
When the hot-wire (Rh) is cooled by intake air, resistance decreases resulting in the formation of a difference between the potentials of points A and B. An operational amplifier detects this difference and causes a rise in the voltage applied to the circuit (increases the current flowing to the hot-wire (Rh)). When this is done, the temperature of the hot-wire (Rh) again rises resulting in a corresponding increase in resistance until the potentials of points A and B become equal (the voltages of points A and B become higher).
By utilizing the properties of this type of bridge circuit, the air flow meter is able to measure intake air mass by detecting the voltage at point B.