Overhead wires in the wild will whine and make a sound when they are blown by the wind. The higher the wind speed, the higher the sound frequency, which is caused by the vortex formed by the airflow flowing through the wire. This phenomenon will occur in liquids, gases and other fluids, and vortex street flow sensors can be made by using this phenomenon. Two rows of eddy currents are formed after the cylinders are arranged in the pipeline, and the flow can be measured according to the frequency of the eddy currents. Because the vortex appears in two parallel rows, alternating from left to right, similar to street lamps on both sides of the street, it is called vortex street. Because this phenomenon was first discovered by Kaman, it is also called Kaman vortex ultrasonic Kaman vortex air flow sensor. The ultrasonic air flow sensor has two air inlet channels, a main channel and a bypass channel, and the detection part of the air inlet flow is located on the main channel. The purpose of setting the bypass channel is to adjust the flow rate of the main channel and make the detection characteristics of the main channel in an ideal state. That is to say, for engines with different displacement, by changing the cross section of the air flow sensor channel, one specification of air flow sensor can cover multiple engines. The triangular column on the main channel and several vortex amplification plates constitute the Kaman vortex generator. An ultrasonic transmitter and an ultrasonic receiver belonging to an electronic detection device are arranged on both sides of the place where Carmen Vortex Street is generated, and these two components can also be classified as sensors. The electrical signals generated by these two electronic sensors are shaped and amplified by the control circuit (hybrid integrated circuit) of the air flow sensor, and then input into a microcomputer. In order to check the eddy current with ultrasonic waves, sound-absorbing materials are pasted on the inner wall of the eddy current channel to prevent the irregular reflection of ultrasonic waves.
Pressure change detection type Kaman vortex air flow sensor
Vortex appears alternately from both ends of the vortex generator, so the pressure at both ends of the vortex generator changes alternately. This pressure change is guided to the mirror cavity through the pressure guide hole on the conical column on the downstream side of the vortex generator. The mirror in the mirror cavity is tensioned with a very thin tension band, so the tension band is twisted and vibrated. In addition, the leaf spring is used to apply appropriate tension to the tension belt, so that in addition to vibration and eddy current pressure,
The pressure caused by eddy current flows into the reflector cavity through the pressure guide hole, which is synchronous with the pressure change in the reflector cavity, and the reflector forms torsion and vibration on the tension band. The reflector is very light and will work even under the condition of low flow and small pressure change. On the upper part of the reflector, a light sensor consisting of a light emitting diode and a phototransistor is correspondingly arranged. When the light emitted by the diode is reflected by the reflector and incident on the phototransistor, it will become current and be output through the waveform circuit. When the throttle valve turns from closed to fully open within 30 seconds, that is, it opens quickly, the response characteristics of the sensor are shown in the figure. The curve below is the output characteristic of Kaman vortex street air flow sensor after F/V conversion, and the curve on the graph is the opening characteristic of throttle valve. It can be seen from the figure that the air flow sensor can accurately reflect (1~45 ms) the relationship between air flow and frequency for the change of flow in the throttle valve: under a wide flow,
Ultrasonic Kaman vortex air flow sensor with differential pressure sensor;
The characteristics of Kaman vortex street air flow sensor are high precision, long service life and high reliability. And high-performance engines, that is, engines that further reduce fuel consumption and improve output power, also need to expand the detection range of intake air. However, the old ultrasonic Kalman vortex air flow sensor will produce overmodulation in high flow area. Due to this factor, this sensor has the disadvantage of insufficient measuring range. Therefore, an air flow sensor with differential pressure sensor is developed.
1. Use a vortex generator with small pressure loss: the function of the vortex generator is to form a stable vortex in the whole flow range.
2. Pipeline structure with small pressure loss.
3. Measure the tiny eddy current pressure.
4. Air flow sensor with differential pressure sensor.
Structure of hot wire air flow sensor;
As a heating element, the hot wire is made of platinum wire with a diameter of 70um, and is installed in the pipeline in tension mode, so that the specific air temperature is designed to be high 120 degrees. There is also an air temperature compensation resistor in the temperature sensor. It is composed of platinum film printed on alumina ceramic substrate and installed in the pipeline together with precision resistor. In order to prevent performance degradation caused by dust attached to the hot wire, a dust combustion circuit is provided. When the ignition switch is turned off, under certain conditions, the hot wire is heated to above 1000 degrees and kept for about 1 second to burn off dust and other attachments. Because platinum wire is used as heating element, it has good responsiveness.
Similarly, there is a hot film wire air flow sensor (H/F). Similar to H/W sensor, H/F also uses planar thin film resistor as heating element. The manufacturing method is as follows: the platinum film evaporated on the alumina substrate is patterned to form a comb resistor, and then the required resistance value is adjusted, and then the protective film is manufactured, and then the electrode leads are connected. Compared with the hot-wire heating element, the hot-film heating element has a slightly worse response, but because it is made by graphic method, the resistance value is higher and the current consumption is small, so it can be small and light. In addition, because its heating element is flat, the projected area can be made as small as possible from the upstream, so that when it is arranged in the metering channel, it can reduce adhesion, that is, improve the anti-pollution ability. The air flow sensor (MAF) calculates the amount of air entering the engine by sensing the heat taken away by the air entering the engine. The powertrain control module (PCM) uses the mass air flow to monitor the actual amount of air entering the engine and calculate the main fuel supply. When the amount of air entering the engine is large, the value sensed by the air flow sensor is large, indicating that the engine is in the state of acceleration or high load, and vice versa, indicating that the engine is in the state of deceleration or idling.
Long-term/short-term fuel regulation is to change the pulse width of the injector through PCM, so that the air-fuel ratio of the engine is as close as possible to 14.7∶ 1 (the best ratio). Both short-term and long-term fuel regulation data can be detected by automobile diagnostic instruments. The important difference between short-term fuel adjustment and long-term fuel adjustment is that the former represents a small change in a short time, while the latter represents a big change in a long time.
Short-term fuel regulation is a part of electronic control system of automobile engine. When the engine is in a closed-loop state, short-term fuel adjustment will make a small and temporary correction to the air-fuel ratio. Short-term fuel regulation takes 0.45V as the reference point and continuously monitors the output voltage of oxygen sensor. When the engine is in a closed-loop state, the signal voltage of the oxygen sensor should change within a constant range of 0.1~ 0.9V. When the voltage of the oxygen sensor monitored by PCM changes stably near the reference point of 0.45V, PCM continuously adjusts the fuel supply to ensure that the air-fuel ratio of the engine is as close as possible to14.7:1. The short-term fuel adjustment value is expressed as a percentage between-100% and+100%, and the middle point is 0%. If the short-term fuel adjustment value is 0%, it means that the air-fuel ratio is the ideal value 14.7: 1, and the mixture is neither too rich nor too thin. If the short-term fuel adjustment shows a positive value higher than 0%, it means that the mixture is lean and PCM is adjusting the fuel supply system to increase the fuel injection. If the short-term fuel adjustment shows a negative value below 0%, it means that the mixture is too rich, and PCM is adjusting the fuel supply system to reduce the fuel injection. If the mixture is too thin or too rich, which is beyond the scope of short-term fuel adjustment, long-term fuel adjustment is needed at this time.
The long-term fuel adjustment value is obtained from the short-term fuel adjustment value and represents the long-term correction value of fuel deviation. If the long-term fuel adjustment shows 0%, it means that the fuel supply is just right in order to maintain the air-fuel ratio controlled by PCM; If the long-term fuel adjustment shows a negative value below 0%, it means that the mixture is too rich and the fuel injection quantity is decreasing (the fuel injection pulse width is decreasing); If the long-term fuel adjustment shows a positive value higher than 0%, it indicates that the mixture is too lean, and PCM is compensating by increasing the fuel supply (increasing the fuel injection pulse width). The long-term fuel adjustment value can indicate how much the power control module has compensated. Although short-term fuel adjustment can make a large range of small adjustments to the fuel supply more frequently, long-term fuel adjustment can show the trend that short-term fuel adjustment is lean or rich. Long-term fuel adjustment can obviously change the fuel supply to the required direction after a long time.