What is a car air flow meter
The air flow sensor, also known as the air flowmeter, is one of the important sensors in electronic fuel injection engines. It converts the inhaled air flow into an electrical signal and sends it to the electronic control unit (ECU), which serves as one of the basic signals for determining fuel injection and is a sensor for measuring the air flow inhaled into the engine.
On an electronically controlled fuel injection device, the sensor that measures the amount of air inhaled by the engine, namely the air flow sensor, is one of the important components that determine the control accuracy of the system. When the control accuracy of the air-fuel ratio (A/F) of the air and mixture sucked in by the engine is specified as ±1.0, the allowable error of the system is ± 6% to 7%. When this allowable error is distributed to each component of the system, the allowable error of the air flow sensor is ± 2% to 3%.
The ratio of the maximum to the minimum intake air flow of a gasoline engine, max/min, is 40 to 50 in a naturally aspirated system and 60 to 70 in a turbocharged system. Within this range, the air flow sensor should be able to maintain a measurement accuracy of ±2 to 3[%]. The air flow sensor used in the electronically controlled fuel injection device should not only maintain measurement accuracy over a wide measurement range, but also have excellent measurement response, be capable of measuring pulsating air flow, and the processing of the output signal should be simple.
According to the different characteristics of the air flow sensor, the fuel control system is classified into L-type control that directly measures the intake volume and D-type control that indirectly measures the intake volume based on the measurement method of the intake volume. The intake volume is indirectly measured according to the negative pressure of the intake manifold and the engine speed. In the D-type control mode, the microcomputer ROM pre-stores the intake air volume under various states with the engine speed and the pressure in the intake pipe as parameters. Based on the intake pressure and speed measured in each operating state and referring to the intake air volume remembered in the ROM, the microcomputer can calculate the fuel consumption. The air flowmeter used in the L-type control is basically the same as that of a general industrial flow sensor. However, it can adapt to the harsh environment of automobiles, but it also has the requirement of responding to the sharp changes in flow when the accelerator is pressed and the requirement of high-precision detection in the uneven airflow caused by the shape of the intake manifolds before and after the sensor.
The initial electronic fuel injection control system did not use microcomputers. Instead, it was an analog circuit. At that time, a valve type air flow sensor was used, but as microcomputers were applied to control fuel injection, several other types of air flow sensors also emerged.
The structure of the valve type air flow sensor.
The valve type air flow sensor is installed on the gasoline engine, between the air filter and the throttle. Its function is to detect the intake air volume of the engine and convert the detection results into electrical signals, which are then input into the microcomputer. This sensor is composed of two parts: an air flowmeter and a potentiometer.
First, let's look at the working process of the air flow sensor. The air drawn in by the air filter rushes towards the valve. The valve stops at the position where the intake volume is balanced with the return spring. That is to say, the opening degree of the valve is directly proportional to the 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 utilized to convert the opening degree of the measuring plate into an electrical signal, which is then input into the control circuit.
Kaman vortex air flow sensor
To overcome the shortcomings of the valve type air flow sensor, that is, to expand the measurement range while ensuring measurement accuracy and eliminate sliding contacts, a small and lightweight air flow sensor, namely the Karman vortex air flow sensor, has been developed. 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 at all. The area of the air passage and the size change of the vortex generating column determine the detection accuracy. Also, because the output of this type of sensor is an electronic signal (frequency), when inputting signals to the control circuit of the system, an AD converter can be omitted. Therefore, in essence, the Karman vortex air flow sensor is a signal suitable for microcomputer processing. This sensor has the following three advantages: high testing accuracy, the ability to output linear signals, and simple signal processing; The performance will not change even after long-term use. Since it is for detecting volumetric flow rate, there is no need to correct for temperature and atmospheric pressure.
When a Karman vortex is generated, it changes with the variation of speed and pressure. The basic principle of flow detection is to utilize the change in speed within it. The signals are square waves and digital signals. The more the intake volume, the higher the frequency of the Karman vortex, and the higher the frequency of the output signal of the air flow sensor.
The temperature and pressure compensation air flow sensor is mainly used for the flow measurement of various media in industrial pipelines, such as gas, liquid, steam, etc. Its features include low pressure loss, wide measurement range, high precision, and it is almost unaffected by parameters such as fluid density, pressure, temperature and viscosity when measuring the volume flow rate under working conditions. There are no moving mechanical parts, so it has high reliability and requires little maintenance. The instrument parameters can remain stable for a long time. This instrument adopts piezoelectric stress sensors, which are highly reliable and can operate within a working temperature range of -10℃ to +300℃. It has both analog standard signals and digital pulse signals output, making it easy to be used in conjunction with digital systems such as computers. It is a relatively advanced and ideal flow rate.
The greatest advantage of air flow sensors is that the instrument coefficient is not affected by the physical properties of the measured medium and can be extended from one typical medium to other media. However, due to the significant difference in the flow rate ranges of liquid and gas, the frequency ranges also vary greatly. In the amplifier circuit for processing vortex street signals, the passband of the filter is different, and so are the circuit parameters. Therefore, the same circuit parameter cannot be used to measure different interfaces
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