The flow sensor is one of the key components of the EFI system. It directly affects the normal running of the vehicle. The air flow sensor used in the car is a hot wire flow meter. Its function is to measure the air flow through the sensor within a certain period of time. The air flow signal is transmitted to the ECU. The ECU Monitors the operating condition of the engine based on the signal and calculates the fuel supply amount. A large air flow indicates that the engine is accelerating; a small air flow indicates that the engine is decelerating or idling.
Working principle: When the air entering the throttle body flows through the sensor, it takes away some of the heat. The larger the air flow, the more heat is taken away. In order to maintain the temperature of the sensor sensing element at a constant temperature, additional current is required to heat the sensing element. The sensor determines the amount of air flow by measuring the voltage of the current.
The hot wire in the air flow sensor is made of metal platinum wire and extends into the bypass air passage of the throttle body. The air flow sensor adopts the Wheatstone bridge principle, and the electric heating line placed in the air flow changes the resistance value due to the cooling effect of the air flow, and the bridge is thus out of balance, and the control circuit automatically increases the voltage and increases the flow. The current of the hot wire increases the resistance of the hot wire as the temperature increases, and the bridge regains equilibrium. During the adjustment process, the voltage signal transmitted by the air flow sensor to the engine electronic control module (ECU) changes with the change of the air flow, and a compensation resistance wire is also provided in the air flow of the near hot line to prevent the electric air from changing due to the change of the air temperature. The bridge is out of balance.
After the hot line of the air flow sensor is fouled, the voltage signal transmitted to the ECU will be inaccurate. At this time, the dirt will affect the radiation and the cooling effect will be reduced. When the air flow rate increases, the temperature of the hot wire decreases slowly, and the amount of change in the resistance value decreases accordingly. Therefore, the current of the voltage and the flow heat line cannot be correspondingly increased, so that the signal voltage transmitted to the ECU is low, causing the mixture to be too lean. . Although the hot wire air flow sensor is equipped with a burning circuit, that is, the ECU will automatically heat the hot wire at 1000 ° C for 1 s each time to stop the dirt and dust on the hot wire, but in some areas, especially China In the remote areas, due to the low fuel quality, the intake pipe is tempered, causing excessive impurities and carbon deposits to be bonded to the metal platinum wire. Therefore, the purification device for the single heating line is difficult to remove. Therefore, the air flow sensor must be removed and sprayed directly to restore its normal function.
Finally, the air flow sensor measures the air flow into the engine in g/s. The air flow measurement is a measure of the engine load throttle opening and air volume and is similar to the engine load, intake manifold absolute pressure sensor or vacuum sensor signal. The air flow signal should remain relatively constant when the car is at a constant speed, with the throttle opening gradually changing and changing dramatically during sudden acceleration. The powertrain control module PCM uses the information of the air flow to control the fuel supply. The sensor produces a frequency signal that is not easily measured during the test, 32 to 150 Hz. The signal of this change will be proportional to the air flow.
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