A method for urea crystallization diagnosis in exhaust aftertreatment system based on ultrasonic echo signal response characteristics

Abstract

The present invention belongs to the technical field of motor vehicle emission control, and specifically relates to a method for diagnosing urea crystallization in an exhaust aftertreatment system based on the response characteristics of an ultrasonic echo signal. The present invention determines the target crystallization site, reasonably arranges the positions of the ultrasonic transmitter and the signal collector, calibrates the screening curve for different microphone echo amplitude identification, and combines the ultrasonic echo signal collected by the microphone array during the actual diagnosis process to confirm the crystallization state of the target microphone corresponding area. Then, the size and thickness of the urea crystallization area are determined by the external expansion envelope algorithm and the time difference between the crystallization reflection echo signal received by the microphone and the signal emission, and finally the current degree of crystallization of the system is estimated. The present invention can be used for all models equipped with the injection-mixing system, and has good universality.

Description

Technical Field

[0001] The invention belongs to the technical field of motor vehicle emission control, and in particular relates to a urea crystallization diagnosis method for an exhaust gas aftertreatment system based on ultrasonic echo signal response characteristics. Background Art

[0002] Engine exhaust contains harmful substances such as nitrogen oxides (NOx for short), the main components of which are NO and NO2. NOx is the reaction product of N2 and O2 in the air sucked into the cylinder by the engine at high temperatures. Motor vehicle emission regulations limit the emission of NOx and set different limits for different models at different stages.

[0003] Urea selective catalytic reduction technology (Urea-SCR technology for short) is the main technology for controlling NOx emissions from engines. The most common form of this technology is: using urea aqueous solution to decompose and produce ammonia, and under the action of the SCR catalyst, ammonia and NOx undergo a selective catalytic reduction reaction to generate nitrogen and water, which are then discharged into the atmosphere. By injecting different amounts of urea into the exhaust of the diesel engine, the NOx emissions can be effectively controlled.

[0004] Figure 1 This is a schematic diagram of the existing Urea-SCR system structure. Urea crystals (also known as urea deposits) are a common problem faced by Urea-SCR systems in actual applications. The accumulation of urea crystals will cause the exhaust back pressure to increase, the catalyst efficiency to decrease, and the engine economy and emission characteristics to deteriorate. In severe cases, it may even block the exhaust pipe, causing the engine to fail to work properly.

[0005] Currently, there are no corresponding sensors or detection devices on vehicles that can monitor the occurrence of urea crystallization. The urea crystallization problem is often not discovered until the degree of urea crystallization has become serious enough to affect the vehicle's power or emission characteristics. At this time, the engine has been running for a long time in a state of poor economy and emissions. Summary of the invention

[0006] The purpose of the present invention is to provide a method for diagnosing urea crystallization in an exhaust gas aftertreatment system based on ultrasonic echo signal response characteristics, thereby solving the problem that it is difficult to accurately diagnose the degree of urea crystallization in the prior art. The specific technical solution is as follows:

[0007] A method for diagnosing urea crystallization in an exhaust gas aftertreatment system based on ultrasonic echo signal response characteristics includes the following steps:

[0008] Step (1) confirming the crystallization high-risk area of ​​the target SCR system. Specifically, by conducting a bench urea crystallization risk test on the target engine-aftertreatment system, artificially creating urea crystallization conditions to confirm the crystallization high-risk area of ​​the target aftertreatment system, and defining this as the target crystallization site (generally located at the boundary of the mixer, the angle area between the wall and the mixer);

[0009] Step (2) emits ultrasonic waves of a specific frequency for diagnostic purposes through a certain device. Preferably, the signal generating device used generates a sound wave signal of a certain frequency through a piezoelectric chip or a piezoelectric ceramic transducer, which is an ultrasonic directional generating device with adjustable emission direction and multiple target frequencies; the directional sound wave generating device can both quickly and continuously emit pulses and scan their reflected echoes. The directional sound wave generating device is arranged on the extension line of the crystal stacking direction, and emits ultrasonic waves directly at the target crystallization site. The directional sound wave generating device needs to ensure a certain spatial isolation distance from the target crystallization site, and the spatial separation distance should be at least greater than 2 times the characteristic diameter of the pickup array.

[0010] Step (3) receives sound waves in the form of a microphone array to achieve signal acquisition. According to the structural characteristics of the target crystallization site, a corresponding microphone array is arranged to receive the sound wave signal through a near-field model; the main microphone and the signal generating device can be of the same structure, and when the ultrasonic wave acts on the wafer, the deformation caused by the forced vibration of the wafer can be converted into a corresponding electrical signal. When urea crystals exist at the target crystallization site, the emitted ultrasonic signal passes through the air, is reflected by the urea crystals, and reaches the main microphone through the shortest path.

[0011] Since the signal generator is close to the target crystal point, the acoustic field model uses a near-field model to simulate the sound wave as a spherical wave. The vibration from the sound source is related to the amplitude difference between the signals received by the microphone array unit. By receiving the sound wave through the array uniform square array model, the planar array can obtain the horizontal azimuth and vertical azimuth information of the signal.

[0012] Step (4) collects the echo signal data obtained by each microphone when crystallization exists and when system crystallization does not exist, and calibrates a time-varying echo amplitude identification screening curve for each microphone. The calibration method for a single microphone is: assuming that the echo amplitude when there is no crystallization is: A Norm , assuming that the echo amplitude when crystals exist is: A Deposit , select (A Norm +A Deposit ) / 2 is used as the characteristic amplitude of the echo amplitude identification screening curve. The time interval between the transmission time and the echo reception time is obtained by correcting the basic time by temperature.

[0013] In the process of actual vehicle diagnosis, step (5) each pickup uses the signal obtained in step (3) and the echo amplitude identification screening curve of step (4) to distinguish the reflected sound wave signals from the metal surface and the crystalline surface.

[0014] Taking a signal transmission-reception cycle as an example, the identification screening curve is explained. Since the sound wave signal received by the microphone contains the vibration signal that is directly transmitted from the sound source through the air to the target microphone, it is necessary to calculate the time when the first sound wave reaches the microphone based on the local sound speed and the distance from the microphone to the sound wave emission center. During this period of time after the signal is emitted, a filter amplitude is set. Since the sound wave signal reaches the microphone almost lossless, its amplitude is not much different from the amplitude when the sound wave is emitted, so the filter amplitude needs to be set higher than the emission amplitude.

[0015] After the ultrasonic wave is transmitted to the target crystallization site, if there is no crystal at the crystallization site, the ultrasonic wave is reflected on the metal surface. After a period of time, the pickup receives a vibration signal with a slightly reduced amplitude. When there is urea crystal at the target crystallization site, the ultrasonic wave is reflected on the crystal surface, and the proportion of vibration energy absorbed is significantly greater than when it is reflected on the metal surface. After a period of propagation time, the pickup receives a vibration signal with a significantly reduced amplitude.

[0016] When the amplitude of the echo signal exceeds the amplitude threshold corresponding to the identification screening curve, it is identified as an echo reflected by the metal wall, and the point corresponding to the pickup does not enter the crystallization area estimation module. When the amplitude of the echo signal is lower than the amplitude threshold corresponding to the identification screening curve, it is identified as an echo reflected by urea crystals, and the point corresponding to the pickup will enter the crystallization area estimation module.

[0017] Step (6) uses the acoustic wave recognition results of the microphone array at different arrangement positions and refers to the spatial arrangement coordinate information of the acquisition system to confirm the size of the crystallization area. The main reflected ultrasonic signals received by the microphones at different positions correspond to different reflection positions. When the recognition result of the microphone receiving signal at this position is crystallization, it is considered that urea crystals exist in the area corresponding to the reflection position. Since crystallization is usually continuous, when the recognition results of multiple microphone receiving signals are detected as crystallization, the size of the crystallization area is calculated using the external expansion envelope algorithm, thereby determining and confirming the nominal crystallization area.

[0018] Step (7) uses the time difference between the crystal reflection echo signal received by the microphone and the signal transmission time to confirm the nominal crystal thickness. When the crystal thickness increases, the return time of the signal received by the main microphone will be shortened, indicating that thicker crystals exist in the system. Since the time it takes for the sound wave to hit the metal wall and reflect back to the main microphone after propagating through the air mainly depends on the propagation distance and air temperature, when the air temperature increases, the propagation speed of the sound becomes faster. The return time of the signal received by the main microphone will also be shortened, so temperature correction is required. The measured echo signal return time is t m , propagation distance S m =c*t m , assuming that the shortest distance from the ultrasonic emission point to the metal surface is S0, then (2S0-S m ) / 2 is the urea thickness.

[0019] Step (8) estimates the degree of crystallization. Based on the calculated crystallization area and crystal thickness, the volume of the crystals is obtained, and the current crystallization risk level can be confirmed.

[0020] The present invention provides a urea crystallization diagnosis method for an exhaust aftertreatment system based on ultrasonic echo signal response characteristics. The method determines the target crystallization site, reasonably arranges the positions of an ultrasonic transmitter and a signal collector, calibrates the screening curve for different microphone echo amplitude identification, and confirms the crystallization state of the target microphone corresponding area in combination with the ultrasonic echo signal collected by the microphone array during the actual diagnosis process. Then, the size and thickness of the urea crystallization area are determined by an external expansion envelope algorithm and the time difference between the crystallization reflection echo signal received by the microphone and the signal emission, and finally estimates the current crystallization degree of the system.

[0021] The target crystallization site involved in the present invention is obtained through a real urea crystallization test of a target engine-aftertreatment system. It is the area where crystallization first occurs when the injected urea cannot be completely decomposed. As a key site for urea crystallization diagnosis, it can be used for all vehicle models equipped with the injection-mixing system and has good universality. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the structure of the existing Urea-SCR system;

[0023] Figure 2 The present invention is a flow chart of a method for diagnosing urea crystals based on the response characteristics of ultrasonic echo signals;

[0024] Figure 3 It is a schematic diagram of the signal receiving situation of the microphone matrix at the target crystal site under different crystallization states;

[0025] Figure 4 The schematic diagram for distinguishing the reflecting medium based on the echo amplitude identification screening curve;

[0026] Figure 5 This is the calculation principle diagram of the nominal crystal area of ​​urea;

[0027] Figure 6 This is the calculation principle diagram of the nominal crystal thickness of urea. DETAILED DESCRIPTION

[0028] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] The functional flow chart of the online diagnosis of urea crystallization risk of the exhaust gas after-treatment system in this embodiment is as follows: Figure 2 The following diagnostic method is completed within a period of time after the engine is stopped.

[0030] (1) Identify the high-incidence crystallization areas of the target SCR system. By conducting a bench urea crystallization risk test on the target engine-aftertreatment system, identify the high-incidence crystallization areas of the target aftertreatment system and define them as target crystallization sites. For different target crystallization sites, ultrasonic transmitting and receiving systems are arranged in a targeted manner.

[0031] (2) The ultrasonic emitting device is used to emit ultrasonic waves of a specific frequency for diagnostic purposes and is arranged on the extension line of the crystal stacking direction to emit ultrasonic waves directly at the target crystal site.

[0032] (3) The ultrasonic receiving system receives sound waves in the form of a microphone array to achieve signal acquisition. By artificially controlling the generation of urea crystals, the echo amplitude characteristics of the ultrasonic waves reflected by the crystals and the echo amplitude characteristics reflected by the exhaust pipe wall are collected respectively, and the vibration amplitude dissipation characteristics of the ultrasonic waves under different exhaust temperature conditions are confirmed.

[0033] (4) After the diagnosis begins, the ultrasonic transmitter emits ultrasonic waves at a certain time interval, and the microphone returns the received echo signal, the current temperature signal and the position ID to the data processing unit. For each microphone at the position ID, the data processing unit generates a calibrated echo amplitude identification screening curve. The screening curve is compared with the real echo signal after temperature correction, and it can accurately determine whether urea crystals exist in the area corresponding to the target microphone.

[0034] (5) The nominal crystal area can be calculated by summarizing the judgment results of all microphone arrays, and the nominal crystal thickness can be calculated by summarizing the return time of the echo amplitude of all microphones that receive the attenuated signal. The nominal crystal volume can be calculated based on the nominal crystal area and nominal crystal thickness, and the crystal risk level of the system can be confirmed by comparing it with the crystal risk volume threshold.

[0035] Specific instructions such as Figure 3 The schematic diagram of the signal reception of the microphone matrix at the target crystallization site under the same crystallization state is shown. Based on the spatial position relationship between the sound source and the target microphone, the target diagnosis area corresponding to the target microphone is relatively fixed. When the target microphone receives the corresponding echo signal, the crystallization state of the target diagnosis area is determined based on the echo amplitude identification screening curve corresponding to the microphone. When there are a large number of crystals, the corresponding part of the microphones in the microphone matrix will receive echo signals with obvious amplitude attenuation. When there is no urea crystal in the system, the echo signals received by all microphones should not have obvious attenuation.

[0036] Figure 4 This is a schematic diagram of the principle of distinguishing reflective media based on the echo amplitude identification screening curve; the identification screening curve is explained using a signal transmission-reception cycle as an example. Since the sound wave signal received by the microphone contains the vibration signal that is directly transmitted from the sound source through the air to the target microphone, it is necessary to calculate the time when the first sound wave arrives at the microphone based on the local sound speed and the distance from the microphone to the sound wave emission center. During this period of time after the signal is emitted, a filtering amplitude is set. Since the sound wave signal arrives at the microphone almost losslessly, its amplitude is not much different from the amplitude when the sound wave is emitted, so the filtering amplitude during this period of time needs to be set higher than the emission amplitude.

[0037] After the ultrasonic wave is transmitted to the target crystallization site, if there is no crystal at the crystallization site, the ultrasonic wave is reflected on the metal surface. After a period of time, the pickup receives a vibration signal with a slightly weakened amplitude. When there is urea crystal at the target crystallization site, the ultrasonic wave is reflected on the crystal surface, and the proportion of vibration energy absorbed is significantly greater than when it is reflected on the metal surface. After a period of propagation time, the pickup receives a vibration signal with a significantly weakened amplitude.

[0038] Therefore, the amplitude of the echo amplitude identification screening curve in the echo reflection period needs to be accurately set, usually set to the middle value between the metal wall echo amplitude and the urea crystal echo amplitude, so as to effectively distinguish the reflected sound wave signals from the metal surface and the crystal surface. When the amplitude of the echo signal exceeds the amplitude threshold corresponding to the identification screening curve, it is identified as the echo reflected by the metal wall, and the point corresponding to the pickup does not enter the crystallization area estimation module; when the amplitude of the echo signal is lower than the amplitude threshold corresponding to the identification screening curve, it is identified as the echo reflected by the urea crystal, and the point corresponding to the pickup will enter the crystallization area estimation module.

[0039] Figure 5 The figure is a schematic diagram of the calculation principle of the nominal crystal area of ​​urea. In this embodiment, the size of the crystal area is confirmed by using the acoustic wave recognition results of the microphone array at different arrangement positions and referring to the spatial arrangement coordinate information of the acquisition system.

[0040] The main reflected ultrasonic signals received by the microphones at different positions correspond to different reflection positions. When the identification result of the signal received by the microphones at this position is crystallization, it is considered that urea crystals exist in the area corresponding to the reflection position. Figure 5 The calculation method of the crystallization area is explained by taking the following as an example, where: A is the nominal crystal radius of the center pickup; r B is the nominal radius of the inner pickup; r C is the nominal radius of the outer ring pickup. Let the total number of inner ring pickups be n 总 , where the number of pickups receiving the attenuated signal is n1, and the total number of outer ring pickups is m 总 , where the number of microphones receiving attenuated signals is m1, if the center microphone receives an attenuated signal, then c=1, and if it receives a normal signal, then c=0. k is the area deformation coefficient, which is related to the distance between the microphone and the crystallization target position and the sound wave emission angle. The estimated nominal crystallization area A can be calculated by the following formula: A=k*[IIr A 2 *c+(IIr B 2 -IIr A 2 )*n1 / n 总 +(IIr c 2 -IIr B 2 )*m1 / m 总 ].

[0041] Figure 6: is a calculation principle diagram of the nominal crystal thickness of urea. In this embodiment, the crystal thickness is calculated by using the time difference between the time when the microphone receives the crystal reflection echo signal and the time when the reference echo returns.

[0042] The reference echo return time is obtained by measuring the signal emission time and the echo signal received by the metal wall at the target crystal site in a non-crystallized state. When the crystal thickness increases, the signal return time received by the main pickup will be shortened, indicating that the system has thicker crystals. Since the time it takes for the sound wave to hit the metal wall and reflect back to the main pickup after propagating through the air mainly depends on the propagation distance and air temperature, when the air temperature increases, the propagation speed of the sound becomes faster, and the reference echo return time at this time also needs to be corrected accordingly. The signal return time received by the main pickup will also be shortened. The specific calculation method is as follows:

[0043] The measured return time of the echo signal is t m ;

[0044] Propagation distance S m =c*t m ;

[0045] Assuming that the shortest distance from the ultrasonic emission point to the metal surface is S0, then (2S0-S m ) / 2 is the nominal crystal thickness of urea.

[0046] The above describes in detail the preferred implementation of this patent, but this patent is not limited to the above implementation. Various changes can be made within the knowledge scope of ordinary technicians in this field without departing from the purpose of this patent.

Claims

1. A method for diagnosing urea crystallization in an exhaust gas aftertreatment system based on ultrasonic echo signal response characteristics, characterized in that: The steps include: Step (1) identifying a high-incidence crystallization region of a target SCR system; Step (2) emits ultrasound waves of a specific frequency for diagnostic purposes through a signal generating device; Step (3) receiving sound waves in the form of a microphone array to achieve signal collection; Step (4) by collecting echo signal data obtained by each microphone when the system crystallization exists and when the system crystallization does not exist, a time-varying echo amplitude identification screening curve is calibrated for each microphone; Step (5) During the actual vehicle diagnosis process, each pickup uses the signal obtained in step (3) and the echo amplitude identification screening curve in step (4) to distinguish the reflected sound wave signals from the metal surface and the crystal surface; Step (6) using the acoustic wave recognition results of the microphone array at different arrangement positions, referring to the spatial arrangement coordinate information of the acquisition system, confirming the size of the crystallization region to obtain the nominal crystallization area; Step (7) using the time difference between the crystal reflection echo signal received by the microphone and the signal transmission time to confirm the nominal crystal thickness; Step (8) estimates the degree of crystallization. Based on the calculated crystallization area and crystal thickness, the volume of the crystals is obtained, thereby confirming the current crystallization risk level.

2. The diagnostic method according to claim 1, characterized in that: The step (1) specifically involves conducting a bench urea crystallization risk test on the target engine-aftertreatment system, artificially creating urea crystallization conditions to identify high-incidence crystallization areas of the target aftertreatment system, and defining them as target crystallization sites.

3. The diagnostic method according to claim 1, characterized in that: The signal generating device in step (2) is an ultrasonic directional generating device which generates a sound wave signal of a certain frequency through a piezoelectric chip or a piezoelectric ceramic transducer and has an adjustable emission direction and a plurality of target frequencies.

4. The diagnostic method according to claim 3, characterized in that: There is a certain spatial separation distance between the ultrasonic directional generating device and the target crystallization site, and the spatial separation distance should be at least greater than 2 times the characteristic diameter of the pickup array.

5. The diagnostic method according to claim 1, characterized in that: In step (4), the calibration method of a single microphone is: assuming that the amplitude of the echo when there is no crystal is: A Norm , assuming that the echo amplitude when crystals exist is: A Deposit , select (A Norm +A Deposit ) / 2 is used as the characteristic amplitude of the echo amplitude identification screening curve; the time interval between the emission time and the echo reception time is obtained by correcting the basic time by temperature.

6. The diagnostic method according to claim 1, characterized in that: In the step (5), when the amplitude of the echo signal exceeds the amplitude threshold corresponding to the identification screening curve, it is identified as an echo reflected by the metal wall, and the point corresponding to the pickup does not enter the crystallization region area estimation module; when the amplitude of the echo signal is lower than the amplitude threshold corresponding to the identification screening curve, it is identified as an echo reflected by urea crystals, and the point corresponding to the pickup will enter the crystallization region area estimation module.

7. The diagnostic method according to claim 1, characterized in that: The nominal crystal thickness in step (7) is calculated as follows: Crystal thickness = (2S0-S m ) / 2, where the shortest distance from the ultrasonic emission point to the metal surface is S0, and the propagation distance is S m =c*t m , c is the ultrasonic propagation velocity, t m It is the measured return time of the echo signal.

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