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How Does the Air Flow Meter Work?
2024-10-09
The air flow meter is a key component in the automotive engine management system. Its working principles are mainly as follows:
1. Hot wire type: A very thin metal wire (hot wire) made of platinum or tungsten is placed in the air flow path. When air flows through the hot wire, the hot wire cools down due to the air cooling effect, and the ECU monitors the change in the resistance of the hot wire because the resistance is directly related to the temperature. As the air flow increases, the hot wire cools more and the resistance decreases, and the ECU adjusts the fuel injection amount accordingly. However, the hot wire is susceptible to contamination and needs to be cleaned or replaced regularly.
2. Hot film type: The working principle is similar to the hot wire type, but a film coated with a heat-sensitive material is used instead of a thin wire. The film changes in resistance due to air flow cooling, and the ECU determines the air flow by monitoring this change. Its structure is more solid and durable, but the cost is relatively high and the response speed is slightly slow.
3. Vane type: It consists of two parts: an air flow meter and a potentiometer. The air drives the turbine blades to rotate, and the number of impeller revolutions is proportional to the volume of air passing through.
4. Karman vortex type: When a cylindrical or triangular object is placed in the fluid, two rows of vortices with opposite rotation directions and alternating appearance will be generated downstream of the object. The air flow rate is measured by detecting the relevant changes generated by the vortex.
5. Optical Karman vortex: In the process of generating Karman vortex, the air pressure on both sides of the vortex generator changes, and the metal foil is vibrated through the guide hole. The light of the light-emitting diode shines on the vibrating metal foil. The light reflected by the metal foil received by the phototransistor is the light modulated by the vortex. The output is demodulated to obtain a frequency signal representing the air flow rate.
6. Ultrasonic Karman vortex: Ultrasonic transmitting probes and receiving probes are installed on both sides of the downstream pipeline of the Karman vortex generator. Due to the influence of Karman vortex on air density, the time for ultrasonic waves to travel from the transmitting probe to the receiving probe will be later than that without vortex, resulting in a phase difference. The phase signal is processed to obtain a vortex pulse signal.
In short, the air flow meter converts the inhaled air flow into an electrical signal and sends it to the electronic control unit (ECU) as one of the basic signals for determining fuel injection, thereby helping the engine to accurately control fuel injection and optimize engine performance and emissions. Regular inspection and maintenance of the air flow meter is essential to keep the vehicle in optimal operating condition.
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