Air flow sensor

1 Overview Editing

Electronically controlled gasoline injection engines must be able to accurately measure the amount of air drawn into the engine at each moment in order to obtain the best concentration of the mixture under all operating conditions. This is the main basis for the ECU to calculate (control) the amount of fuel injected. . If the air flow sensor or the line fails, the ECU cannot obtain the correct intake air quantity signal, and the control of the fuel injection amount cannot be normally performed, resulting in the mixture being too rich or too lean and causing the engine to operate abnormally. There are many types of air flow sensors for electronically controlled gasoline injection systems. Common types of air flow sensors can be divided into vane (flap) type, quantum core type, hot line type, hot film type, Karman vortex type according to their structural types. Species.

Principle 2 Structure editor

In an electronically controlled fuel injection device, a sensor that measures the amount of air sucked into the engine, that is, an air flow sensor, is one of important components that determines the accuracy of the system control. When the control accuracy of air/fuel ratio (A/F) of the air and air-fuel mixture sucked by the engine is specified to be ±1.0, the allowable error of the system is ±6[%]~7[%], and the allowable error is assigned to the system. The allowable error of the air flow sensor is ±2[%]~3[%] for each component.

The ratio max/min of the maximum and minimum values ​​of the air flow sucked by the gasoline engine is 40 to 50 in the natural intake system, and 60 to 70 in the supercharged system. In this range, the air flow rate The sensor should be able to maintain the measurement accuracy of ±2~3[%]. The air flow sensor used on the electronically controlled 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. The pulsed air flow and output signal processing should be simple.

According to the characteristics of the air flow sensor, the fuel control system can be divided into direct type air intake L type control and indirect air intake type D type control (according to the intake manifold negative pressure and engine The rotational speed indirectly measures the amount of intake air. In the microcomputer ROM in the D-type control method, the amount of intake air in various states based on the engine speed and the pressure in the intake pipe is preliminarily stored. The microcomputer operates according to the measured operation. In the state of the inlet pressure and speed, referring to the ROM's stored air intake, it can be calculated that the air flow meter used in the L-type control of fuel is basically the same as the general industrial flow sensor, but it can adapt to the harsh environment of the car, but The response to the sudden change in the flow rate at the time of the throttle and the requirement for highly accurate detection in the inhomogeneous airflow caused by the shape of the intake manifold before and after the sensor.

The first electronic fuel injection control system used was not a microcomputer. Instead, the analog circuit was a gate type air flow sensor, but as the microcomputer was used to control the fuel injection, several other air flow sensors appeared.

The structure of the gate type air flow sensor.

The valve type air flow sensor is installed on the gasoline engine and is 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 electric signal and input it into the microcomputer. The sensor consists of an air flow meter and a potentiometer.

Look at the working process of the air flow sensor. The air sucked by the air cleaner rushes toward the shutter, and the shutter is stopped at the position where the intake air amount is balanced with the return spring, that is, the opening degree of the shutter is proportional to the intake air amount. The rotary axis of the valve is also equipped with a potentiometer, the sliding arm of the potentiometer rotates in synchronization with the valve, and the opening of the measuring piece is converted into an electric signal by using the voltage drop of the sliding resistor and then input into the control circuit.

Kalman 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, to expand the measurement range, and to eliminate the sliding contact, there has been developed a small and lightweight air flow sensor, namely Karman scroll type air flow sensor. The Karman vortex is a physical phenomenon. The vortex detection method, electronic control circuit and detection accuracy are not related at all. The accuracy of the detection accuracy is determined by the passage area of ​​the air and the dimensional change of the vortex generation column. Since the output of this sensor is an electronic signal (frequency), the AD converter can be omitted when inputting a signal to the control circuit of the system. Therefore, by nature, Karman scroll air flow sensor is a signal suitable for computer processing. This sensor has the following three advantages: high test accuracy, can output linear signal, signal processing is simple; long-term use, performance will not change; because it is to detect the volume flow, so do not need to modify the temperature and atmospheric pressure.

The principle circuit of the flow detection of this air flow sensor is shown in the figure. When a Karman vortex is generated, the basic principle of the flow detection is to use the change of the speed as the speed and pressure change. The signal waveform of the air flow sensor output to the control module is shown in the figure. Signals are square waves, digital signals. The more the intake air amount, the higher the Karman vortex frequency and the higher the frequency of the air flow sensor output signal.

The temperature and pressure compensation air flow sensor is mainly used for the flow measurement of medium fluids in industrial pipelines, such as gases, liquids, vapors and other media. Its characteristics are low pressure loss, large range, high precision, and almost no influence of parameters such as fluid density, pressure, temperature, and viscosity when measuring volume flow. No moving mechanical parts, so high reliability and low maintenance. Instrument parameters can be stable over a long period of time. The instrument uses a piezoelectric stress sensor, high reliability, can work in the operating temperature range of -10 °C ~ +300 °C. There are analog standard signal and digital pulse signal output. It is easy to use with digital systems such as computers. It is a relatively advanced and ideal flow. [1]

The biggest advantage of the air flow sensor is that the meter factor is not affected by the physical properties of the measured medium and can be extended from a typical media to other media. However, due to the wide range of fluid and gas flow rates, the frequency range also varies greatly. In an amplifier circuit that processes vortex signals, the passbands of the filters are different and the circuit parameters are also different. Therefore, the same circuit parameters cannot be used to measure different media.

3 Measurement range editing

The instrument flow and measurement range of the air flow sensor are shown in the following table:

Instrument path

Code

Standard range

Code

Extended range

DN25 (1")

S

2.5-25 m3/h

W

4-40 m3/h

DN40 (1.5")

S

5-50 m3/h

W

6-60 m3/h

DN50 (2")

S1

6-65 m3/h

W1

5-70 m3/h

S2

10-100 m3/h

W2

8-100 m3/h

DN80 (3")

S1

13-250 m3/h

W1

10-160 m3/h

S2

20-400 m3/h

W2

DN100 (4")

S1

20-400 m3/h

W1

13-250 m3/h

S2

32-650 m3/h

W2

DN150 (6")

S1

32-650 m3/h

W1

80-1600 m3/h

S2

50-1000 m3/h

W2

DN200 (8")

S1

80-1600 m3/h

W1

50-1000 m3/h

S2

130-2500 m3/h

W2

DN 250 (10")

S1

130-2500 m3/h

W1

80-1600 m3/h

S2

200-4000 m3/h

W2

DN 300 (12")

S

200-4000 m3/h

W1

130-2500 m3/h

Detection principle

The overhead wires in the field will make a noise when they are blown by the wind. The higher the wind speed is, the higher the sound frequency is. This is due to the turbulence caused by the air flowing through the wire. This phenomenon occurs in fluids such as liquids and gases. This phenomenon can be used to make a scroll-type flow sensor. Two columns of vortices are formed after the columns are arranged in the pipeline, and the flow rate can be measured according to the frequency of vortices. Because vortexes form two parallel lines and appear alternately on the left and right, similar to the street lights on both sides of the street, there is a name for vortex street. Because this phenomenon was first discovered by Karman, it is also called Karman vortex street. Ultrasonic type Karman vortex air flow sensor Ultrasonic air flow sensor with two inlet channels, main channel and bypass channel, detection of intake air flow The part is located on the main channel. The purpose of setting the bypass channel is to be able to adjust the flow of the main channel so that the detection characteristic of the main channel is in an ideal state. In other words, for engines with different displacements, by changing the size of the air flow sensor channel section, it is possible to cover a variety of engines with one type of air flow sensor. The triangular column on the main channel and the several vortex amplification plates constitute the Karman vortex generator. On both sides where the Karman vortex is generated, ultrasonic transmitters and ultrasonic receivers belonging to the electronic detection device are relatively disposed, and these two components can also be classified as sensors, and the electric signals generated by the two electronic sensors are transmitted through the air. The flow sensor's control circuit (hybrid integrated circuit) is shaped and amplified to an ideal waveform and then input to the microcomputer. In order to use the ultrasonic inspection vortex, sound-absorbing material is adhered to the inner wall of the vortex channel in order to prevent irregular reflection of the ultrasonic wave.

Pressure change detection Karman scroll air flow sensor

Eddy currents are generated alternately from both ends of the vortex generator so that the vortex generators are alternately generated at both ends, so that the pressure at both ends of the vortex generator is also alternately changed. Such pressure changes through the downstream side of the vortex generator. The pressure hole in the column guides into the mirror cavity. The mirror in the mirror cavity is tensioned with a very thin tension band. Therefore, the tension and vibration appear on the tension band. In addition, the plate spring is used to give tension. By adding a proper tension to the tight band, it is difficult to influence the pressure change other than the vibration and the swirl pressure, so that stable torsional vibration can be obtained.

The pressure formed due to the vortex appears through the pressure-guiding hole into the mirror cavity, which synchronizes with the pressure change in the reflective cavity, and the mirror forms torsion and vibration on the tension band. The mirror is very light and it will act even under low flow and very small pressure changes. In the upper part of the reflector, a light sensor composed of a light emitting diode and a phototransistor is correspondingly configured. The light emitted by the diode is reflected by the reflector and is emitted to the phototransistor and becomes a current, which is output via the waveform circuit.

characteristic

When the throttle is closed and fully opened within 30 seconds, ie, the quick opening, the response characteristics of this sensor are as shown in the figure. The curve below is the output of the F/V converted Kalman scroll air flow sensor. Characteristics, the curve on the graph is the opening characteristics of the throttle. It can be seen from the figure that the air flow sensor can accurately reflect the change of the flow rate in the throttle (1~45 milliseconds). The relationship between the air flow rate and the frequency is very wide. There is a linear relationship between the air flow velocity and the vortex frequency within the flow rate range.

Ultrasonic Karman Scroll Air Flow Sensor with Differential Pressure Sensor:

Karman vortex air flow sensor features: high precision, long life and high reliability. However, high-performance engines, which further reduce fuel consumption and increase output power, are also required to expand the detection range of the intake air amount. However, the old ultrasonic type Karman vortex air flow sensors will produce an over-modulation phenomenon in high flow areas. Due to the influence of this factor, this sensor has the disadvantage of insufficient measurement range. For this purpose, air flow sensors with differential pressure sensors have also been developed.

1. Using a vortex generator with low pressure loss: The function of the vortex generator is to create a stable vortex throughout the entire flow range.

2. Pipeline structure with less pressure loss.

3, measure the tiny vortex pressure.

4. Air flow sensor with differential pressure sensor.

Hot wire air flow sensor structure:

As a heating element, the hot wire is made of platinum wire with a diameter of 70 μm, and it is tightly installed inside the pipe, and it is designed to be 120 degrees higher than the intake air temperature. The temperature sensor also has an air temperature compensation resistor. It is formed from a platinum film printed on an alumina ceramic substrate, which is placed inside the pipe together with a precision resistor. In order to prevent performance degradation caused by dust adhered to the hot wire, a dust burning circuit is provided. When the ignition switch is placed in the off position, under certain conditions, the hot wire is heated to over 1000 degrees for about 1 second. Burns dust and other attachments. Because platinum wire is used as a heating element, its responsiveness is good.

Similar to this is the hot-film air flow sensor (H/F). Similar to the H/W sensor, the H/F also uses a flat-film resistor as a heating element. The manufacturing method is as follows: A thin film of platinum evaporated on an alumina substrate is formed into a comb-like resistor by patterning and then adjusted to a desired resistance value. Thereafter, a protective film is formed, and an electrode lead is further attached. Compared with the hot wire type, the thermal film type heat generating element has slightly poorer responsiveness, but because it is formed by a pattern method, the resistance value is high and the consumed current is small, and it can be made small and light. In addition, since the heat generating element is of a planar type, when viewed from the upstream, it is possible to make the projected area small so that it is possible to reduce the adhered matter, that is, to improve the stain resistance, when disposed in the metering passage.

Throttle valve

The throttle sensor is used to convert the throttle opening into a voltage signal and control the amount of fuel injected through the ECU. There are two kinds of switch-type throttle position sensors and linear throttle sensors. The switch-type throttle position sensor has a simple structure, but its output is not continuous. In addition to the above three, sensors used for electronic control of automotive engines include pressure sensors, oxygen sensors, temperature sensors, knock sensors, crankshaft position sensors, and rotational speed sensors. Any modern automobile system or device that uses electronic control cannot be separated from sensors such as automatic transmissions, anti-lock brake systems, and anti-skid systems. Especially in recent years, there are more and more vehicle electronic devices, such as security alarm devices, communication devices, entertainment devices, and assisted driving devices used for improving comfort and reducing fatigue. Of course, home-made cars have only just started in electronic control technology, mainly focused on the electronic control of the engine. Because of this, automotive sensor pressure switches will have more room for development in China.

Air flow

A very important control content in the air flow sensor engine electronic control system is the best air fuel control. To achieve this goal, an accurate measurement of the intake air flow of the engine must be made. Commonly used air flow sensors include air flow meter, Karman vortex air flow meter, hot air flow meter and hot film air flow meter. The damper type air flowmeter has a simple structure and high reliability, but has a large intake resistance, a slow response and a large volume; and the hot wire type air flowmeter has not only reliable operation but also fast response due to no moving parts. The disadvantage is that the flow velocity distribution is not The average error is larger. Although the working principle of the hot-film air flow meter is similar to that of the hot-wire air flow meter, since the hot-film type sensor does not use the platinum wire as the hot wire, the hot wire resistance, the compensation resistance, etc. are made by the thick film process, and the same ceramic base is used. On the chip, the heating element does not directly bear the force generated by the flow of air, thereby increasing the strength of the heating element, not only making the reliability of the air flowmeter further improved, but also reducing the error and improving the performance of the sensor.

Air flow injection volume

The air flow sensor (MAF) calculates the amount of air entering the engine by sensing the heat of the air entering the engine to calculate the amount of air entering the engine. The power system control module (PCM) uses air mass flow to monitor the amount of air actually entering the engine and calculates Oil supply. The amount of air that enters the engine is large, and the air flow sensor senses a large value, which means that the engine is in an accelerated or high-load condition, and vice versa, it indicates that the engine is in a deceleration or idling state.

The long-term/short-term fuel adjustment is to change the pulse width of the injector through PCM to keep the air-fuel ratio of the engine as close to 14.7:1 (optimum ratio) as possible. Both the short-term fuel adjustment and the long-term fuel adjustment data can be detected by an automotive diagnostic instrument. The important difference between the short-term fuel adjustment and the long-term fuel adjustment is that the former represents a small change in a short period while the latter represents a large change in a long period of time.

Short-term fuel adjustments are part of the electronic control system for automotive engines. When the engine is in a closed-loop state, short-term fuel adjustments will make small, temporary corrections to the air-fuel ratio. Short-term fuel adjustment continuously monitors the output voltage from the oxygen sensor and uses 0.45V as a reference point. When the engine is in a closed-loop state, the signal voltage of the oxygen sensor should be changed within a constant range of 0.1 to 0.9V. When the oxygen sensor voltage monitored by the PCM steadily changes near the reference point 0.45V, the PCM continuously adjusts the fuel supply amount so as to ensure that the air-fuel ratio of the engine is as close as possible to 14.7:1. The value of the short-term fuel adjustment is expressed as a percentage between -100% and +100%, and the intermediate point is 0%. If the value of the short-term fuel adjustment is 0%, it means that the air-fuel ratio is an ideal value of 14.7:1, and the mixture is neither too strong nor too dilute. If the short-term fuel adjustment shows a positive value above 0%, it means that the mixture is dilute, and the PCM will adjust the fuel injection system to increase the fuel injection. If the short-term fuel adjustment shows a negative value less than 0%, it means that the mixture is rich, and the PCM will adjust the fuel supply system to reduce the amount of fuel injected. If the mixture is too thin or too thick to exceed the short-term fuel adjustment range, then long-term fuel adjustments are required

The long-term fuel adjustment value is obtained from the short-term fuel adjustment value and represents the long-term correction value of the fuel deviation. If the long-term fuel adjustment shows 0% indicates that in order to maintain the air fuel ratio controlled by the PCM, the fuel supply is appropriate; if the long-term fuel adjustment shows a negative value less than 0%, it indicates that the mixture is too rich and the injection quantity is decreasing. (Injection pulse width decreases); If the long-term fuel adjustment shows a positive value above 0%, it means that the mixture is too lean and the PCM is compensating by increasing the fuel supply (injection pulse width increase). The value of the long-term fuel adjustment can indicate how much the power control module has compensated. Although short-term fuel adjustments can make more frequent small adjustments to the fuel supply, long-term fuel adjustments can indicate the tendency for short-term fuel adjustments to shift to thin or thicker directions. Long-term fuel adjustments can change the fuel supply significantly in the required direction over a longer period of time.

4 maintenance essentials editor

The air flow signal is one of the main signals used by the engine electronic control unit (ECU) to control the concentration of the mixed gas. If the intake air volume increases, the ECU controls the amount of fuel injected, and vice versa.

Impact on the car

Although the abnormality of the air flow sensor will not cause the engine to fail to start, it will definitely affect the engine's power performance, such as idling instability, poor acceleration, intake pipe “tempering” and black smoke emitted from the exhaust pipe. At the same time, excessive exhaust emissions will be caused.

(1) Caused engine acceleration failure

A Passat GLi sedan, which has a mileage of 45,000 km, will accelerate the engine to 4200 r/min, and then step on the accelerator pedal, the engine speed will drop instead. Detected with VAG1551 fault diagnostic instrument, no fault code stored. Reading data flow at 4200r/min, found that the air flow data can only reach 1.1 ~ 1.3g / s, and can not change with the throttle opening and closing. After replacing the air flow sensor, the fault is eliminated. The reason. The deviation of the output signal of the air flow sensor is not enough for the electronic control unit (ECU) to record the fault code, but because the air flow signal can not accurately reflect the actual intake air amount, the amount of fuel injected by the ECU accordingly is less, so the engine speed Do not rise and fall.

(2) Caused the intake pipe to “temper”

A Jetta sedans. Engine idle jitter occurs and the intake pipe “tempered” during rapid acceleration. Check the air intake system and see no air leaks. Replace the fuel filter and clean the four injectors. It is invalid. Check fuel pressure, normal when idling and accelerating. Remove the air flow sensor connector test, the failure phenomenon has improved. Measure the resistance value of each terminal of air flow sensor, normal. Finally, it was found that dust is sticking to the hot film resistors of the air flow sensor. After removing dust with carburetor cleaning agent, the fault was eliminated.

For an EFI engine using a hot-film type air flow sensor, it is applied to both ends of the hot film (resistor) with a constant voltage, so that the resistor heats up and its temperature is controlled by the circuit. The ECU judges the amount of intake air based on the current flowing through the thermal membrane resistance, and determines the amount of fuel injection to suit the needs of different operating conditions of the engine. If the thermal film is dusty and a thermal insulation layer is formed, when the intake air amount becomes larger, the temperature change thereof is slowed down, the required current becomes smaller, and the injection amount determined by the ECU accordingly is reduced. At this time, the actual intake air volume is relatively large, which leads to excessive dilution of the mixed gas, eventually causing engine idle jitter, and rapid acceleration “tempering” and other failure phenomena.

(3) The automatic transmission cannot lift into the overdrive

If the air flow sensor is short-circuited to ground, the mixture will be too lean and the output power of the engine will drop, which will cause the automatic transmission to fail to enter the overdrive. The air flow sensor should be replaced at this time.

Preliminary judgment

There are two types of air flow sensor faults. One is that the signal is out of the specified range, indicating that the air flow sensor has failed. Modern electronically controlled vehicles have a fail-safe function. When the signal of a certain sensor fails, the electronic control unit (ECU) replaces it with a fixed value, or replaces the signal of the faulty sensor with the signals of other sensors. After the air flow sensor fails. The ECU uses the signal from the throttle position sensor instead. The other is inaccurate signals (ie, performance drift). Inaccurate air flow sensor signals may be more hazardous than no signal. This is because. Since the signal does not exceed the specified range. The electronic control unit (ECU) will control the fuel injection according to this inaccurate air flow signal, so it often causes the mixture vapor to be too thin or too thick. If there is no air flow signal, the ECU will use the throttle position sensor signal instead, the engine's idle speed is rather stable.

Using this feature, you can judge the performance by unplugging the air flow sensor connector.

1 If the fault does not change, the air flow sensor is damaged. This is because the ECU has confirmed the failure of the air flow sensor and has used the throttle position sensor signal instead. At this point there is no result of the air flow sensor is the same, so the failure phenomenon does not change.

2 If the fault phenomenon is reduced, the performance of the air flow sensor drifts and the signal is biased. Because the air flow signal is within the effective range, the ECU controls the fuel injection according to the distorted signal, causing obvious failure phenomena. After unplugging the connector of the air flow sensor, the ECU considers that the air flow sensor has completely failed and instead uses the throttle position sensor signal instead, so the engine's working condition has improved.

3 If the failure phenomenon has deteriorated. The air flow sensor is normal. This is because the ECU controls the fuel injection according to the normal air flow sensor signal before removing the plug. After unplugging the plug, the ECU uses the throttle position sensor signal to control the injection. Because the latter's control accuracy is not as high as the former, the fault phenomenon has deteriorated.

In addition, because the air flow sensor signal is the main basis for controlling the air-fuel ratio, an infrared exhaust gas analyzer can be used to measure the exhaust gas composition during engine idle conditions and 2000 r/min stable operating conditions. If the difference from the standard value is too large, it may be a malfunction caused by poor performance of the air flow sensor.

Fault code characteristics

(1) After the failure of the hot-film type air flow sensor (G70), the electronic control unit (ECU) does not directly give the fault code of the air flow sensor, but shows it through other fault codes, usually "00561" (mixed steam adjustment value exceeds Adjust the limit) or "17916" (the theoretical limit of the idling system is reached).

(2) Misalignment of other parts of the engine may record the fault code of the air flow sensor. In the maintenance practice, it is common to record the fault code of the air flow sensor in the following situations.

1 Throttle dirty, may record the air flow sensor's fault code. A Polaris 1.8T sedan. In normal driving. Sometimes the ASR (drive anti-skid control) indicator light on the instrument panel suddenly lights up. Pressing the ASR light switch does not work. Only turning off the ignition switch and restarting the engine will the ASR light be extinguished. Connect the trouble shooting system VAS5051 to detect and read out the “Less air flow sensor G70 signal value displayed in the engine system” error message. The resistance of each terminal of G70 is not more than 1.5Ω. Observe the G70 waveform, normal. Replacement of G70 is invalid. The car uses CAN-BUS multi-channel information transmission system. The ABS/ASR control unit communicates with the engine control unit via the CAN-BUS bus. On the one hand, when the throttle body is dirty, the throttle opening value increases, and the actual intake air amount does not increase. The signal that causes the G70 does not match the opening of the throttle valve, and thus records the fault message that the signal value of the air flow sensor G70 displayed in the engine system is too small. On the other hand, the anti-skid control of the ASR system is achieved by reducing the engine speed to adjust the output torque of the engine. Therefore, although the fault phenomenon is manifested in the brake system, the root cause of the fault is in the engine. When the throttle opening signal and the air flow signal deviate (the throttle opening is large, and the actual air intake measured by the G70 is small), the ABS/ASR control unit considers that the engine has reduced the power output and is driving the anti-skid Control then lights up the ASR indicator.

2 Throttle position sensor is malfunctioning and it is possible to record the fault code of the air flow sensor. A Jetta sedan is detected by a fault diagnostic device and reads the unreasonable fault code of the air flow sensor signal. Replace air flow sensor but it is invalid. The so-called "unreasonable" is compared with the signals of related sensors. In fact, the ECU determines the engine load based on the engine speed, the throttle position signal, and the air flow signal. The throttle position sensor was further checked and it was found that the maximum learning value and the minimum learning value did not agree with the prescribed values, and the basic setting could not be performed. After replacing the throttle assembly (including the throttle position sensor) and performing basic settings, the fault is eliminated.

3 Oxygen sensor is damaged. The fault code of the air flow sensor may be recorded. When the Oxygen sensor of Jetta's car is damaged, it will memorize the fault code of the air flow sensor. The principle is: due to "cylinder missing" and other reasons, the fuel combustion is not complete, beyond the adjustment range of λ, causing the signal of the oxygen sensor to be inaccurate. The engine ECU then continuously adjusts back and forth between the lean and too rich mixture. The air flow signal received by the ECU is inconsistent with the oxygen sensor signal. However, from a practical point of view, the failure of the oxygen sensor can not be adjusted and the serious drift of the air flow sensor signal is the same, and the ECU memorizes the fault code of the air flow sensor according to the priority principle.

Data flow analysis

(1) The unit and range of the air flow sensor signal parameters depend on the type of air flow sensor. The parameter units for the wing-type, hot-line, and hot-film air flow sensors are "V" and range from 0 to 5V. The size of this parameter is generally inversely proportional to the amount of intake air, that is, when the intake air amount increases, the output voltage value decreases, and “5V” indicates that the intake air amount is the minimum. "0V" indicates the maximum amount of intake air.

The unit of the signal parameter of the vortex air flow sensor is "Hz" or "ms", and its variation range is 0-1600Hz or 0-625ms. The value at idle is 25 to 50 Hz, and the value at 2000 r/min is 70 to 100 Hz. If the value under different conditions differs from the standard value very much or does not change. The air flow sensor is faulty.

(2) By analyzing the data flow of the air flow sensor, it can be judged whether there is a leakage phenomenon in the engine air intake system. Under normal conditions, the air flow signal data at idle speed is about 2.5 g/s. If it is less than 2.0 g/s, there is air leakage in the air intake system: if it is greater than 4.0 g/s, there is an additional load on the engine.

An Audi A6 1.8T sedan is equipped with a manual transmission. When the engine is running, it shakes every 2 to 3 minutes. However, the engine starts and accelerates normally. Connect the diagnostic instrument VAS5052 to 01-08-02 to read the data stream. The air flow data displayed in the 4th area is periodically and frequently jittered between 0.3 and 3.5 g/s. Examination revealed. The clamp at the lower part of the connection between the air cleaner housing and the air intake hose is not installed, causing air leakage. After the leak has been treated. Troubleshooting.

(3) During the inspection process, the maintenance personnel will find that after disconnecting the connector of the air flow sensor, the data of the air flow can still be seen in the data stream, and within the normal range, the rapid acceleration response is also sensitive. It is actually the failure mode of the control system. It is the air flow backup data given by the engine ECU based on signals such as speed and load.

Maintenance points

(1) Cleanliness after Hotline and Hot Film Dirty

If the engine has a "tempering" fault, it is often a serious hazard to the air flow sensor. As the engine's airflow reverses (ie "tempers") in the intake manifold, which contains carbon particles, these carbon particles tend to adhere to the air flow sensor's sensing element and have the following consequences: At idle, the air flow sensor The signal is large, but the signal is small during acceleration and heavy load.

Whether the hotline has the ability of self-cleaning is checked by removing the air filter and observing the hotline from the inlet of the air flow sensor. If the engine is off for 5 seconds, the hotline will not emit a reddish glow for about 1 second. The self-cleaning ability has been lost.

After the hot wire (hot film) is contaminated, it can be in a hot engine or idle state. The filter of the air cleaner is removed and the carburetor cleaning agent is used to directly spray the hot wire or hot film to remove the coke adhered thereto.

(2) Treatment after damage of hot film air flow sensor

Many models use thermal film air flow sensors manufactured by BOSCH, whose core consists of an integrated circuit (digital-to-analog conversion circuit) and a Wheatstone bridge, without a voltage regulator circuit. Therefore, when the power supply voltage is too high or there is a momentary high voltage, this hot film air flow sensor is prone to burnout. The reason why the peak voltage of the circuit is too high (more than 16V) is that the battery is heavily sulphated and its capacity is reduced, and the peak voltage of the generator cannot be absorbed. Therefore, the curing of the battery is one of the causes of the damage of the hot-film type air flow sensor. The solution is to install a 7812 three-terminal voltage regulator integrated circuit at the front end of the hot film air flow sensor.

(3) Blocking air from entering without measurement

These irregularities include: rupture of the intake pipe, loosening of the vacuum hose, and tight sealing of the intake manifold and the cylinder head. If the above situation exists. Some of the air will directly enter the cylinder without being metered by the air flow sensor, eventually resulting in engine misalignment.

(4) Replacement of Volkswagen Vehicle Air Flow Sensor after Damage

The Volkswagen's air flow sensor is a malfunctioning part. After the air flow sensor is damaged, if the original parts are not found for a moment, the general interchangeability of parts will be faced. If the engine is equipped with different types of air flow sensors, the control of the injection quantity will be inaccurate. In the open-loop control phase, the fuel consumption of the engine may increase, and the temperature of the three-way catalytic converter may be too high. In the closed-loop control stage, the oxygen sensor will continuously correct the concentration of the mixed gas and cause the air-fuel ratio to change frequently, resulting in an engine Unstable work.