The air flow sensor converts the inhaled air into an electrical signal and sends it to the electronic control unit (ECU) as one of the basic signals for determining the fuel injection. It is a sensor that measures the air flow drawn into the engine. In order to obtain the optimal concentration of the mixture under various operating conditions, the electronically controlled gasoline injection engine must correctly measure the amount of air sucked into the engine at each moment, which serves as the main basis for the ECU to calculate (control) the amount of fuel injected. . If the air flow sensor or line fails, the ECU cannot obtain the correct intake air amount signal, and the fuel injection quantity control cannot be performed normally, which will cause the mixture to be too rich or too lean, and the engine may not operate normally. There are many types of air flow sensors for electronically controlled gasoline injection systems. The current common air flow sensors can be divided into blade (wing), core, hot wire, hot film, Karman scroll, etc. according to their structure. Several. 1. Structure and working principle of vane air flow sensor The traditional Bosch L-type gasoline injection system and some mid-range models use this type of vane air flow sensor, such as Toyota CAMRY car, Toyota PREVIA car, Mazda MPV multi-purpose car. Its structure is shown in Figure 1, consisting of two parts: an air flow meter and a potentiometer. The air flow meter has a rotary vane (measuring piece) that can swing around the axis in the intake passage. As shown in Fig. 2, the coil spring acting on the shaft can close the intake passage of the measuring piece. When the engine is running, the intake airflow pushes the deflection of the measuring piece through the air flow meter to turn it on. The opening angle of the measuring piece depends on the balance between the thrust of the intake air flow on the measuring piece and the elastic force of the coil spring on the measuring piece shaft. The amount of intake air is changed by the driver operating the throttle. The larger the intake air amount, the larger the thrust of the airflow to the measuring piece, and the larger the opening angle of the measuring piece. Connect a potentiometer to the measuring spindle, as shown in Figure 3. The sliding arm of the potentiometer rotates coaxially with the measuring piece, and converts the change in the opening angle of the measuring piece (ie, the change in the amount of intake air) into a change in the resistance value. The potentiometer is connected to the ECU through wires and connectors. The ECU measures the intake air amount of the engine based on the amount of change in the potentiometer resistance or the amount of change in the voltage acting thereon, as shown in FIG. In the vane air flow sensor, there is usually an electric gasoline pump switch, as shown in FIG. When the engine is started, the measuring piece is deflected, the switch contact is closed, and the electric gasoline pump is energized; after the engine is turned off, the measuring piece is turned to the closed position, and the electric gasoline pump switch is turned off. At this time, the electric gasoline pump does not operate even if the ignition switch is in the open position. There is also an intake air temperature sensor in the flow sensor for measuring the intake air temperature and temperature compensation for the intake air amount. The vane air flow sensor wire connector typically has seven terminals, such as 39, 36, 6, 9, 8, 7, 27 in FIG. However, after the electric gasoline pump control contact switch inside the potentiometer is cancelled, it becomes 5 terminals. Figure 6 shows the "marks" of the Nissan and Toyota vane air flow sensor wire connector terminals. Its terminal "mark" is generally marked on the jacket of the connector. 1. Structure and working principle of Karman vortex air flow sensor The structure and working principle of the Karman vortex air flow sensor are shown in Figure 11. In the middle of the intake pipe, there is a first-class linear or triangular vortex generator. When the air flows through the vortex generator, a series of asymmetrical but very regular Kalman vortex is generated in the airflow at the rear. Air vortex. According to the Karman vortex theory, this vortex row is turbulently moved in the flow direction of the airflow in turn, and the speed of its movement is proportional to the air flow velocity, that is, the number of vortices passing through a point behind the vortex generator in a unit time is proportional to the air flow velocity. Therefore, the air flow rate and flow rate can be calculated by measuring the number of eddy currents per unit time. There are two methods for measuring the number of vortices per unit time: the mirror detection type and the ultrasonic detection type. Figure 12 shows a mirror-detected Karman vortex flow sensor with a light-emitting diode and a phototransistor. The light beam emitted by the LED is reflected by a mirror to the phototransistor to turn on the phototransistor. The mirror is mounted on a very thin metal reed. The metal reed generates vibration under the pressure of the vortex of the intake air flow, and its vibration frequency is the same as the number of vortices generated per unit time. Since the mirror vibrates with the reed, the reflected beam also changes at the same frequency, causing the phototransistor to turn on and off at the same frequency as the beam. The ECU calculates the amount of intake air based on the frequency at which the phototransistor is turned on and off (Figure 11). The Lexus LS400 sedan uses this type of Karman scroll air flow sensor. Figure 13 shows an ultrasonically detected Karman scroll air flow sensor. On both sides of the rear half there is an ultrasonic transmitter and an ultrasonic receiver. While the engine is running, the ultrasonic transmitter continuously emits ultrasonic waves of a certain frequency to the ultrasonic receiver. When the ultrasonic waves pass through the intake airflow to the receiver, the phase of the ultrasonic waves changes due to the influence of the vortex in the airflow. The ECU calculates the number of vortices generated per unit time based on the frequency of the corresponding change measured by the receiver, thereby obtaining the air flow rate and flow rate, and then determining the reference air amount and the reference ignition advance angle based on the signal. Hot wire air flow sensor inspection 1, structure and working principle The basic structure of the hot wire air flow sensor consists of a platinum hot wire (platinum wire) that senses air flow, a temperature compensating resistor (cold wire) that is corrected based on the intake air temperature, a control circuit board that controls the hot wire current and produces an output signal, and an air flow rate. It consists of components such as the housing of the sensor. According to the different installation positions of the platinum hot wire in the housing, the hot wire air flow sensor is divided into two types of structure: mainstream measurement and bypass measurement. Figure 18 is a block diagram of a hot wire air flow sensor using a mainstream measurement method. It has a metal protective net at both ends, and the sampling tube is placed in the center of the main air passage. The sampling tube is composed of two plastic sheaths and a hot wire support ring. A platinum wire (RH) with a hot wire diameter of 70 μm is placed in the support ring and its resistance varies with temperature and is an arm of the Wheatstone bridge circuit (Fig. 19). A platinum film resistor is installed in the plastic sheath at the front end of the hot wire support ring. The resistance varies with the temperature of the intake air. It is called the temperature compensation resistor (RK) and is the other arm of the Wheatstone bridge circuit. A precision resistor (RA) is attached to the plastic sheath at the rear end of the heat wire support ring. This resistor can be trimmed with a laser and is also an arm of the Wheatstone bridge. The voltage drop across the resistor is the output signal voltage of the hot wire air flow sensor. The Wheatstone bridge also has an arm resistor RB mounted on the control board. The working principle of the hot wire air flow sensor is that the hot wire temperature is kept by the hybrid integrated circuit A and its temperature is different from the intake air temperature. When the air mass flow rate is increased, the hybrid integrated circuit A increases the current through the hot wire, and vice versa. Then decrease. Thus, the current through the hot line RH is a single function of the mass flow of the air, i.e., the hot line current IH increases as the air mass flow increases, or decreases as it decreases, typically varying between 50-120 mA. The Bosch LH petrol injection system and some high-end cars use this air flow sensor, such as Buick, Nissan MAXIMA (Maxima), Volvo and so on. 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