Analysis of air flow sensor structure and working principle

The air flow sensor converts the intake air into an electrical signal and sends it to the electronic control unit (ECU) as one of the basic signals to determine the fuel injection. It is a sensor that measures the air flow into the engine. In order to obtain the best concentration of air-fuel mixture under various operating conditions, the electronically controlled gasoline injection engine must accurately measure the amount of air drawn into the engine at each instant, which is used as the main basis for the ECU to calculate (control) the fuel injection amount. . If the air flow sensor or circuit fails, and the ECU cannot obtain the correct intake air quantity signal, it will not be able to control the fuel injection quantity normally, which will cause the mixture to be too rich or too lean, causing the engine to run abnormally. There are many types of air flow sensors for electronically controlled gasoline injection systems. At present, common air flow sensors can be divided into blade (wing plate) type, core type, hot wire type, hot film type, Karman scroll type, etc. 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 vane type air flow sensor, such as Toyota CAMRY car, Toyota PREVIA passenger car, Mazda MPV utility vehicle, etc. Its structure is shown in Figure 1, which consists of two parts, an air flow meter and a potentiometer. The air flow meter has a rotating fin (measuring piece) that can swing around the shaft in the intake channel. As shown in Figure 2, the coil spring acting on the shaft can make the measuring piece close the intake path. When the engine is working, the intake airflow is deflected by the air flow meter to push the measuring piece to turn on. The opening angle of the measuring piece depends on the balance between the thrust of the intake air flow to 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 greater the thrust of the airflow to the measuring piece, and the greater the opening angle of the measuring piece. A potentiometer is connected to the measuring piece shaft, as shown in Figure 3. The sliding arm of the potentiometer rotates synchronously with the measuring piece coaxially, which converts the change of the opening angle of the measuring piece (that is, the change of the intake air amount) into the change of the resistance value. The potentiometer is connected to the ECU through wires and connectors. The ECU measures the intake air amount of the engine according to the change in the resistance of the potentiometer or the change in the voltage acting on it, as shown in FIG. 4.

In the vane type air flow sensor, there is usually an electric gasoline pump switch, as shown in Figure 5. When the engine starts to run, the measuring piece deflects, the switch contact is closed, and the electric gasoline pump is energized. After the engine is turned off, the measuring piece turns off the electric gasoline pump switch while turning to the closed position. At this time, even if the ignition switch is in the on position, the electric gasoline pump does not work.

There is also an intake air temperature sensor in the flow sensor, which is used to measure the intake air temperature and compensate the intake air temperature.

The blade type air flow sensor wire connector generally has 7 terminals, as shown in 39, 36, 6, 9, 8, 7, 27 in Figure 5, but there are also after the cancellation of the electric gasoline pump control contact switch inside the potentiometer, It becomes 5 terminals. Fig. 6 shows the "marks" of the wire connector terminals of the vane air flow sensor for Nissan and Toyota vehicles. The 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. There is a first-class linear or triangular vortex generator in the middle of the intake pipe. When the air flows through the vortex generator, a series of asymmetric but very regular columns called Carmen vortex will be continuously generated in the airflow behind it Air vortex. According to Karman's vortex theory, this vortex train is turbulently moving in the direction of air flow in sequence, and its moving speed is proportional to the air velocity, that is, the number of vortices passing a certain point behind the vortex generator in unit time is proportional to the air velocity. Therefore, the air velocity and flow rate can be calculated by measuring the number of vortices 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 Carmen vortex flow sensor with mirror detection, which contains a light-emitting diode and a phototransistor. The light beam emitted by the light-emitting diode is reflected on the phototransistor by a reflector, and the phototransistor is turned on. The reflector is mounted on a 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 light beam also changes at the same frequency, so that the phototransistor also turns on and off at the same frequency with the light beam. The ECU can calculate the intake air volume based on the frequency of the on and off of the phototransistor (Figure 11). The Lexus LS400 car uses this type of Karman scroll air flow sensor.

Figure 13 shows an ultrasonic detection type Karman scroll air flow sensor. There are an ultrasonic transmitter and an ultrasonic receiver on both sides of the rear half. When the engine is running, the ultrasonic transmitter continuously sends ultrasonic waves of a certain frequency to the ultrasonic receiver. When the ultrasonic wave reaches the receiver through the intake airflow, the phase of the ultrasonic wave 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, so as to obtain the air flow rate and flow rate, and then determines the reference air volume and the reference ignition advance angle based on the signal.

Inspection of hot wire type air flow sensor

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 the air flow, a temperature compensation resistor (cold wire) that is corrected according to the intake air temperature, a control circuit board that controls the hot wire current and generates an output signal, and the air flow The sensor housing and other components. According to the installation location of the platinum hot wire in the shell, the hot wire air flow sensor is divided into two structural forms: mainstream measurement and bypass measurement. FIG. 18 is a configuration diagram of a hot-wire type air flow sensor adopting a mainstream measurement method. It has metal protective nets at both ends. The sampling tube is placed in the center of the main air channel. The sampling tube is composed of two plastic sheaths and a hot wire support ring. The platinum wire (RH) with a hot wire diameter of 70 μm is arranged in the support ring, and its resistance value changes with temperature. It is an arm of the Wheatstone bridge circuit (Figure 19). A platinum thin film resistor is installed in the plastic sheath at the front end of the hot wire support ring, and its resistance value changes with the intake air temperature. It is called a temperature compensation resistor (RK), which is the other arm of the Wheatstone bridge circuit. A precision resistor (RA) is bonded to the plastic sheath at the rear end of the hot wire support ring. This resistor can be trimmed with a laser and is also an arm of the Wheatstone bridge. The voltage drop across this 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 circuit board.

The working principle of the hot wire air flow sensor is: the temperature of the hot wire is kept by the hybrid integrated circuit A from the temperature of the suction air. When the air mass flow increases, the hybrid integrated circuit A increases the current flowing through the hot wire, and vice versa. Then decrease. In this way, the current through the hot wire RH is a single function of the air mass flow, that is, the hot wire current IH increases as the air mass flow increases, or decreases as it decreases, generally varying between 50-120mA. Bosch LH gasoline 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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