On the electronically controlled fuel injection device, the sensor that measures the amount of air sucked by the engine, that is, the air flow sensor, is one of the important components that determines the control accuracy of the system. When the control accuracy of the air-fuel ratio (A/F) of the air and mixed gas sucked by the engine is specified to be ±1.0, the allowable error of the system is ±6[%]~7[%], and this allowable error is allocated to the system The allowable error of the air flow sensor is ±2[%]~3[%].
The ratio max/min between the maximum and minimum air flow intake of a gasoline engine is 40-50 in a natural intake system, and 60-70 in a supercharged system. In this range, the air flow The sensor should be able to maintain a measurement accuracy of ±2~3[%]. The air flow sensor used in the electronic control fuel injection device should not only maintain the measurement accuracy over a wide measurement range, but also have excellent measurement response and can be measured. For pulsating air flow, the processing of the output signal should be simple.
According to the different characteristics of the air flow sensor, the fuel control system is divided into L-type control for direct measurement of intake air volume and D-type control for indirect measurement of intake air volume (according to intake manifold negative pressure and engine The speed indirectly measures the intake air volume. The microcomputer ROM in the D-type control mode stores in advance the intake air volume under various conditions based on the engine speed and the pressure in the intake pipe. The intake air pressure and speed in the state, refer to the intake air volume memorized by ROM, can calculate the fuel volume. The air flow meter used in the L-type control is basically the same as the general industrial flow sensor, but it can adapt to the harsh environment of the car, but it is not The response to the sudden change in flow rate that occurs when the accelerator is throttled and the requirement that the uneven air flow caused by the shape of the intake manifold before and after the sensor can be detected with high accuracy.
The original electronic fuel injection control system did not use a microcomputer. It was an analog circuit. At that time, a valve-type air flow sensor was used, but as the microcomputer was used to control fuel injection, several other air flow sensors appeared.
The structure of the valve type air flow sensor.
The valve type air flow sensor is installed on the gasoline engine, installed between the air filter and the throttle. Its function is to detect the air intake of the engine, and convert the detection result into an electrical signal, and then input it into the computer. The sensor is composed of an air flow meter and a potentiometer.
First look at the working process of the air flow sensor. The air sucked in by the air cleaner rushes to the valve, and the valve turns to the position where the intake air volume is balanced with the return spring and stops, that is, the opening of the valve is proportional to the air intake volume. A potentiometer is also installed on the rotating shaft of the valve. The sliding arm of the potentiometer rotates synchronously with the valve. The voltage drop of the sliding resistance is used to convert the opening of the measuring piece into an electrical signal, which is then input to the control circuit.
Kaman scroll air flow sensor
In order to overcome the shortcomings of the valve type air flow sensor, that is, under the premise of ensuring the measurement accuracy, expanding the measurement range and eliminating the sliding contact, a small and lightweight air flow sensor has been developed, that is, the Karman vortex air flow sensor. The Karman vortex is a physical phenomenon. The detection method of the vortex and the electronic control circuit have nothing to do with the detection accuracy. The air passage area and the size change of the vortex generating column determine the detection accuracy. And because the output of this sensor is an electronic signal (frequency), when inputting a signal to the control circuit of the system, the AD converter can be omitted. Therefore, from an essential point of view, the Karman vortex air flow sensor is a signal suitable for microcomputer processing. This sensor has the following three advantages: high test accuracy, can output linear signals, and simple signal processing; long-term use, performance will not change; because it detects volume flow, it does not require correction of temperature and atmospheric pressure.
When a Karman vortex is generated, it follows the change of speed and pressure. The basic principle of flow detection is to use the change of speed. The signal is a square wave, digital signal. The more air intake, the higher the frequency of the Kaman vortex, and the higher the frequency of the output signal of the air flow sensor.
The temperature, temperature and pressure compensation air flow sensor is mainly used for the flow measurement of industrial pipeline fluids, such as gas, liquid, vapor and other media. It is characterized by small pressure loss, large measuring range, and high accuracy. It is hardly affected by fluid density, pressure, temperature, viscosity and other parameters when measuring volume flow under working conditions. There are no movable mechanical parts, so the reliability is high and the maintenance is small. The instrument parameters can be stable for a long time. This instrument adopts piezoelectric stress sensor with high reliability and can work in the working temperature range of -10℃～+300℃. There are analog standard signals and digital pulse signal outputs, which are easy to use with digital systems such as computers. It is a relatively advanced and ideal flow.
The biggest advantage of the air flow sensor is that the meter coefficient is not affected by the physical properties of the measuring medium, and can be extended from a typical medium to other mediums. However, due to the large difference in the flow rate range of liquid and gas, the frequency range is also very different. In the amplifier circuit that processes the vortex signal, the passband of the filter is different, and the circuit parameters are also different. Therefore, the same circuit parameter cannot be used to measure different media.
Post time: Jan-06-2022