System and method for judging urea crystallization of national sixth diesel vehicle and national sixth diesel vehicle

Abstract

The invention relates to the field of vehicle after-treatment, and particularly discloses a system and a method for judging urea crystallization of a National VI diesel vehicle. According to the National VI diesel vehicle, an upstream NOx sensor is arranged between an engine and a DOC system and is used for detecting the concentration of nitric oxide in waste gas discharged by the engine; the downstream NOx sensor is arranged on a tail gas pipe of the SCR system and is used for detecting the concentration of nitrogen oxide in tail gas discharged by the SCR system; when the vehicle enters a regeneration state, the crystallization detection controller detects whether the difference value of the nitric oxide concentrations detected by the upstream NOx sensor and the downstream NOx sensor exceeds a crystallization threshold value or not, if yes, it is judged that urea crystals are generated, and alarm prompting is conducted. The urea crystallization state can be timely and accurately detected.

Description

Technical Field

[0001] The present invention relates to the field of vehicle post-processing, and in particular to a urea crystallization judgment system and method for a National VI diesel vehicle, and a National VI diesel vehicle. Background Art

[0002] With the upgrade of emission regulations, heavy-duty diesel vehicles that meet the National VI standard use the SCR (Selective Catalytic Reduction) technology route to control nitrogen oxide emissions in diesel engine exhaust. The use of SCR requires the active addition of urea to the exhaust system as a reductant necessary for the SCR reaction. Although at the current technical level, the urea injection system has been able to maximize the urea decomposition efficiency and reduce the crystallization problem of urea droplets on the surface of system components such as the mixer during normal engine operation, there is still a greater risk of urea crystallization when the diesel engine is in low-speed and low-load conditions for a long time. Usually, when the engine control system determines that crystallization exists, it will remove the crystallization by adjusting the engine operating conditions and increasing the exhaust temperature. However, there is currently no effective method to directly determine urea crystallization, and failure to clean up urea crystals in time will affect the vehicle's power performance and emission performance, and even affect the user's driving experience.

[0003] Currently, there are two methods for judging whether urea crystals exist: direct detection and estimation based on empirical models. The direct detection method means that when a crystallization problem occurs, driving performance is affected, and the driver sends the car to a service station. Professionals can visually check whether there are internal crystals by disassembling the post-processor. This method cannot identify crystallization failures in a timely manner, and the impact on engine performance and emission performance has already occurred, affecting user use. The estimation method based on empirical models refers to evaluating the current NOx sensor model, analyzing the model's NH3 emissions, SCR conversion efficiency (or nitrogen oxide conversion efficiency), etc., to determine whether urea crystals exist. This type of method is limited by operating conditions, such as engine reverse drag conditions, which can easily cause certain deviations and misjudgments. Summary of the invention

[0004] In order to solve the above problems, the present invention provides a urea crystallization judgment system and method for a National VI diesel vehicle, and a National VI diesel vehicle, which can realize timely and accurate detection of the urea crystallization state.

[0005] In the first aspect, the technical solution of the present invention provides a urea crystallization judgment system for National VI diesel vehicles, comprising: Upstream NOx sensor: installed between the engine and the DOC system to detect the concentration of nitrogen oxides in the exhaust gas emitted by the engine; Downstream NOx sensor: installed in the tailpipe of the SCR system to detect the concentration of nitrogen oxides in the tail gas emitted by the SCR system; Crystallization detection controller: It is electrically connected to the upstream NOx sensor and the downstream NOx sensor respectively. When the vehicle enters the regeneration state, it detects whether the difference in nitrogen oxide concentration detected by the upstream NOx sensor and the downstream NOx sensor exceeds the crystallization threshold. If it exceeds, it is determined that urea crystals have occurred and an alarm is issued.

[0006] In an optional implementation, the crystallization detection controller is an engine ECU.

[0007] In a second aspect, the technical solution of the present invention provides a method for determining urea crystallization of a National VI diesel vehicle, which is implemented based on any of the above-mentioned systems and comprises the following steps: Determine the crystallization threshold based on bench testing; Obtain vehicle regeneration state parameters to determine whether the vehicle has entered the regeneration state; If the regeneration state is entered, the nitrogen oxide concentration collected by the upstream NOx sensor and the downstream NOx sensor is obtained in real time; Detecting whether the difference in nitrogen oxide concentration collected by the upstream NOx sensor and the downstream NOx sensor exceeds a crystallization threshold; If so, it is determined that urea crystals have been produced and an alarm is issued.

[0008] In an optional implementation, the alarm prompt is performed, specifically including: Give an alarm prompt through the instrument panel or diagnostic system.

[0009] In an optional embodiment, the crystallization threshold is determined according to a bench test, specifically comprising: Steps to build the experimental setup; a step of forming urea crystals; Steps for monitoring NOx concentration and determining crystallization threshold during regeneration.

[0010] In an optional embodiment, the step of building a test device specifically includes: Build a National VI diesel vehicle after-treatment system, including the engine, DOC system, DPF system and SCR system; the DOC system and DPF system can be regenerated and trigger regeneration high temperature; the SCR system includes a catalyst, a mixing chamber, and a urea injection system that can manually control the injection amount; A test upstream NOx sensor is set between the engine and the DOC system, and a test downstream NOx sensor is set on the tailpipe of the SCR system; An exhaust temperature sensor is provided to monitor the exhaust temperature in the SCR system during regeneration.

[0011] In an optional embodiment, the step of forming urea crystals specifically includes: Setting injection conditions, including manually controlling the urea injection system to inject urea at a fixed ammonia-nitrogen ratio and controlling the injection duration under normal engine operation conditions; By continuously running the engine, the SCR inlet temperature is kept below the temperature threshold, thereby causing incomplete hydrolysis of urea to produce crystals; After shutdown, disassemble the mixing chamber of the SCR system and weigh the weight of the crystallized material inside it; If the weight of the crystalline material does not meet the test requirements, continue to form urea crystals until the test requirements are met.

[0012] In an optional embodiment, the steps of monitoring the nitrogen oxide concentration and determining the crystallization threshold during the regeneration process specifically include: Manually stop urea injection, increase the engine load or manually control the regeneration conditions to raise the exhaust temperature in the SCR system to the regeneration temperature to allow the crystals to thermally decompose; During the regeneration phase, the nitrogen oxide concentration data collected by the test upstream NOx sensor and the test downstream NOx sensor are recorded in real time; The crystallization threshold is determined based on the nitrogen oxide concentration data recorded for both.

[0013] In an optional embodiment, the crystallization threshold is determined based on the recorded nitrogen oxide concentration data of the two, specifically including: Obtaining a number of upstream nitrogen oxide concentration data and downstream nitrogen oxide concentration data recorded periodically; Check whether the downstream nitrogen oxide concentration value in each record exceeds the upstream nitrogen oxide concentration value; If the exceeding time is longer than the preset time, the time between the beginning and the end of the exceeding time will be recorded as the target time period; Calculate the difference between each downstream nitrogen oxide concentration value and the corresponding upstream nitrogen oxide concentration value during the target period; The average of all the differences was calculated and recorded as the crystallization threshold.

[0014] In a third aspect, the technical solution of the present invention provides a National VI diesel vehicle, which is equipped with any of the systems described above and executes any of the methods described above.

[0015] The system and method for judging urea crystallization of a National VI diesel vehicle and the National VI diesel vehicle provided by the present invention have the following beneficial effects compared with the prior art: by monitoring the change in NOx sensor readings of urea crystallization in the SCR mixing chamber during the regeneration process, the significant difference in NOx concentration between the upstream and downstream sensors is used to determine whether urea crystallization exists in the SCR system. This innovative method based on NOx concentration difference solves the problem that it is difficult to directly detect urea crystallization in the prior art. The comparative analysis method of sensor readings does not require the addition of additional sensor equipment, and can realize the function of crystallization detection on the basis of existing hardware, avoiding the traditional disassembly and inspection process, reducing maintenance costs and vehicle downtime. By monitoring the feedback information of upstream and downstream NOx sensors, it is automatically determined whether urea crystallization exists in the SCR mixing chamber. When urea crystallization is detected, an alarm is fed back to the user, thereby realizing timely and accurate detection and alarm of urea crystallization. At the same time, it can be widely used in various types of SCR systems and diesel engines under different working conditions, thereby enhancing adaptability and practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the embodiments of the present invention or the technical solutions of the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0017] Figure 1 The present invention provides a schematic block diagram of the structure of a urea crystallization judgment system for National VI diesel vehicles.

[0018] Figure 2 It is a flow chart of a method for determining urea crystallization of a National VI diesel vehicle provided by an embodiment of the present invention.

[0019] Figure 3 This is a schematic diagram of the data recorded by the experimental NOx sensor. DETAILED DESCRIPTION

[0020] In order to enable those skilled in the art to better understand the scheme of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than 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.

[0021] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0022] The key terms appearing in the present invention are explained below.

[0023] DOC: Diesel Oxidation Catalyst, diesel oxidation catalyst.

[0024] DPF: Diesel Particulate Filter, diesel particulate filter.

[0025] SCR: Selective Catalytic Reduction, selective catalytic reduction.

[0026] ECU: Electronic control unit, electronic controller.

[0027] Figure 1 This is a schematic block diagram of the structure of a urea crystallization judgment system for a National VI diesel vehicle provided by an embodiment of the present invention. This system can judge crystallization by the abnormal increase in the reading of the downstream NOx sensor during the engine regeneration process. The principle is that during the engine regeneration process, the exhaust temperature rises, and the crystalline substance mainly composed of cyanuric acid is thermally decomposed at high temperature to produce a large amount of ammonia (NH3). Since the NOx sensor cannot distinguish between NOx and NH3, the large amount of ammonia released by cyanuric acid will cause the downstream NOx sensor reading to increase abnormally. After receiving the measurement signal, the engine ECU makes a judgment, and after determining that there is a crystallization fault, it promptly warns the user to handle it to avoid the impact of crystallization on the vehicle's power performance and exhaust performance.

[0028] Based on the above principle, the present embodiment provides a National VI diesel vehicle urea crystallization judgment system including an upstream NOx sensor, a downstream NOx sensor and a crystallization detection controller.

[0029] Upstream NOx sensor: installed between the engine and the DOC system to detect the concentration of nitrogen oxides in the exhaust gas emitted by the engine.

[0030] Downstream NOx sensor: installed in the exhaust pipe of the SCR system to detect the concentration of nitrogen oxides in the exhaust gas emitted by the SCR system.

[0031] Crystallization detection controller: It is electrically connected to the upstream NOx sensor and the downstream NOx sensor respectively. When the vehicle enters the regeneration state, it detects whether the difference in nitrogen oxide concentration detected by the upstream NOx sensor and the downstream NOx sensor exceeds the crystallization threshold. If it exceeds, it is determined that urea crystals have occurred and an alarm is issued.

[0032] It is understandable that the upstream NOx sensor detects the nitrogen oxide content in the exhaust gas discharged by the engine without post-treatment, and the downstream NOx sensor can be set at the end of the exhaust pipe of the SCR system to detect the nitrogen oxide content in the exhaust gas after post-treatment.

[0033] In an optional implementation, the crystallization detection controller may be an engine ECU, which gives an alarm prompt through a dial or a diagnostic system.

[0034] This system makes judgments based on the DPF regeneration time of heavy-duty vehicles. When the DPF is regenerated at high temperature, the high-temperature gas passes through the mixing chamber. The NOx concentration measured by the tail NOx sensor is higher than a specific value, and it is determined that crystallization has occurred in the mixing chamber. The NOx sensor signal is fed back to the ECU to prompt crystallization. The system can quickly, accurately and timely determine the crystallization of the mixing chamber.

[0035] An embodiment of a National VI diesel vehicle urea crystallization judgment system is described in detail above. Based on the National VI diesel vehicle urea crystallization judgment system described in the above embodiment, an embodiment of the present invention also provides a National VI diesel vehicle urea crystallization judgment method corresponding to the system.

[0036] Figure 2 It is a flow chart of a method for determining urea crystallization of a National VI diesel vehicle provided in an embodiment of the present invention. According to different requirements, the order of the steps in the flow chart can be changed, and some can be omitted.

[0037] like Figure 2 As shown, the method includes the following steps.

[0038] S1, Determination of crystallization threshold based on bench test.

[0039] In this embodiment, whether urea crystallization exists is determined by whether the difference between the data collected by the upstream NOx sensor and the downstream NOx sensor exceeds the crystallization threshold. Therefore, it is necessary to determine the crystallization threshold first. To improve the effectiveness of the crystallization threshold, this embodiment determines the crystallization threshold in advance through a bench test, and stores the determined crystallization threshold in the engine ECU. The engine ECU makes a judgment based on the crystallization threshold.

[0040] S2, obtaining vehicle regeneration state parameters to determine whether the vehicle enters the regeneration state.

[0041] This embodiment detects whether urea crystals are generated in the regeneration state, and determines whether the vehicle enters the regeneration state by acquiring the vehicle regeneration state parameters in real time.

[0042] S3: If the regeneration state is entered, the nitrogen oxide concentration collected by the upstream NOx sensor and the downstream NOx sensor is obtained in real time.

[0043] In this embodiment, after the vehicle enters the regeneration state, the upstream NOx sensor and the downstream NOx sensor are controlled to operate, and the values ​​collected by the two sensors, ie, the nitrogen oxide concentration, are obtained in real time.

[0044] S4, detecting whether the difference between the nitrogen oxide concentrations collected by the upstream NOx sensor and the downstream NOx sensor exceeds a crystallization threshold.

[0045] This embodiment determines whether urea crystallization occurs according to a pre-configured crystallization threshold value, calculates the difference between the data collected by the two sensors in real time, and determines whether the difference exceeds the crystallization threshold value.

[0046] S5: If yes, it is determined that urea crystals are generated and an alarm is issued.

[0047] In this embodiment, if the difference between the data collected by the two sensors exceeds the crystallization threshold, it means that urea crystals are generated, that is, an alarm is given, and an alarm can be given through the instrument panel or the diagnostic system, suggesting that the SCR system be cleaned and maintained. In an optional embodiment, the crystallization threshold determines whether a certain amount of urea crystals have been generated. When the difference exceeds the crystallization threshold, it means that a certain amount of urea crystals have been generated. Different amounts of urea crystals correspond to different crystallization thresholds, and the user can obtain and configure the corresponding crystallization threshold as needed.

[0048] For example, 500 ppm is used as the crystallization threshold value. 500 ppm is a given fluctuation allowable range based on sensor error, air flow distribution unevenness, etc. This value can also be adjusted according to actual needs.

[0049] a) Normal without crystallization: If the downstream NOx sensor reading is basically consistent with the upstream reading during regeneration or only fluctuates slightly (<500 ppm), it can be considered that there is no obvious crystallization in the SCR mixing chamber.

[0050] b) Crystallization exists: If the downstream NOx reading during regeneration is significantly higher than the upstream NOx (>500 ppm) and there is a NOx peak, it can be determined that crystallization exists.

[0051] This embodiment determines the crystallization threshold through a bench test, which specifically includes the following steps.

[0052] Step 1, steps for setting up the experimental device.

[0053] First, build the National VI diesel vehicle after-treatment system, including the engine, DOC system, DPF system and SCR system, that is, the engine + DOC + DPF + SCR system.

[0054] DOC system and DPF system: can regenerate normally and trigger regeneration high temperature.

[0055] SCR system: The test is carried out on a test bench with a complete National VI SCR system, including catalyst, mixing chamber, and urea injection system. The urea injection system should be able to manually control the injection amount.

[0056] NOx sensor: Install NOx sensors upstream and downstream of the SCR system to measure the NOx concentration entering and exiting the SCR system. This includes setting up a test upstream NOx sensor between the engine and the DOC system, and setting up a test downstream NOx sensor on the exhaust pipe of the SCR system.

[0057] Exhaust temperature sensor: used to monitor the exhaust temperature in the SCR system during regeneration to ensure that the regeneration process reaches the high temperature required for thermal decomposition.

[0058] Step 2, the step of forming urea crystals.

[0059] (1) Setting the injection conditions, including manually controlling the urea injection system to inject urea at a fixed ammonia nitrogen ratio and controlling the injection duration under normal engine operating conditions.

[0060] Under normal engine operation, the urea injection system is manually controlled to inject urea at a fixed ammonia nitrogen ratio, and the injection time is controlled to gradually form urea crystals in the mixing chamber. The ammonia nitrogen ratio is defined as the molar ratio between ammonia (NH3) and nitrogen oxides (NOx) entering the SCR catalyst. When the ammonia nitrogen ratio is 1, NH3 and NOx react completely; when the ammonia nitrogen ratio is greater than 1, there is a risk of crystallization of excess urea. In this step, it is necessary to promote crystallization. Therefore, the ammonia nitrogen ratio can be fixed to any value greater than 1. It is recommended to set it to 2, which is conducive to the rapid formation of crystals.

[0061] It should be noted that the ammonia nitrogen ratio can be adjusted to other values, such as 1.8, 2.5, etc., according to different engine emission characteristics or the working conditions of the urea injection system, to simulate the formation and detection process of urea crystals under different injection conditions. This change can adapt to more types of SCR systems and emission control requirements, and further verify the possibility of crystallization under different ammonia nitrogen ratio conditions.

[0062] In an optional embodiment, in addition to adjusting the ammonia nitrogen ratio, different urea injection doses can also be used for testing. In different test stages, the urea injection amount is changed (for example, higher or lower than the standard dose) to accelerate or delay the formation of crystals. This method helps to understand the effect of different injection doses on the crystal formation rate and broaden the scope of application of the detection method.

[0063] (2) By continuously running the engine, the SCR inlet temperature is kept below the temperature threshold, thereby promoting incomplete hydrolysis of urea and producing crystals.

[0064] By running the engine continuously, the SCR inlet temperature is kept low (e.g. below 250°C), which promotes incomplete hydrolysis of urea to produce crystals. The running time needs to be long enough (e.g. several hours) to ensure that a certain amount of urea crystals accumulate in the mixing chamber.

[0065] It should be noted that different exhaust temperatures can also be used for the regeneration process. In the above scheme, the regeneration temperature is set to 500°C-600°C, while in an alternative scheme, higher or lower regeneration temperatures (such as 400°C-500°C) are tried to observe the effect on the thermal decomposition of crystallization. This will help determine a wider regeneration temperature range and optimize the crystallization detection process.

[0066] (3) After shutdown, disassemble the SCR system mixing chamber and weigh the weight of the crystallized material inside it.

[0067] (4) If the weight of the crystalline material does not meet the test requirements, continue to form urea crystals until the test requirements are met.

[0068] After shutdown, disassemble the SCR mixing chamber and weigh the mass of the crystals inside it to ensure that the accumulated urea crystals meet the experimental requirements (for example, 50g, the specific amount can be determined according to the actual system capacity and test conditions).

[0069] Step 3, the step of monitoring the nitrogen oxide concentration and determining the crystallization threshold during the regeneration process.

[0070] (1) Manually stop urea injection and increase the engine load or manually control the regeneration conditions to raise the exhaust temperature in the SCR system to the regeneration temperature to allow the crystals to thermally decompose.

[0071] Start the regeneration process: manually stop urea injection, increase the engine load or manually control the regeneration conditions to gradually increase the exhaust temperature in the SCR system to the regeneration temperature (500-600°C), which is sufficient to thermally decompose crystals (such as cyanuric acid, etc.).

[0072] (2) During the regeneration phase, the nitrogen oxide concentration data collected by the upstream NOx sensor and the downstream NOx sensor are recorded in real time.

[0073] Initial stage monitoring: real-time recording of concentration data of the NOx sensors upstream and downstream of the test, Figure 3 This is a schematic diagram of the test NOx sensor recording data. If the downstream NOx concentration significantly exceeds the upstream NOx concentration, and a significant peak appears at the beginning of regeneration, it means that the crystalline material in the SCR system is decomposing and releasing ammonia, causing the NOx sensor to mistakenly detect a high concentration of NOx.

[0074] During the regeneration process, the exhaust temperature is increased to decompose the urea crystals in the mixing chamber, releasing ammonia, which is then misread as NOx by the NOx sensor, indirectly detecting the presence of crystals. This method of triggering detection using thermal decomposition reactions effectively utilizes the regeneration function of the existing system, does not require additional sensor equipment, and greatly reduces the complexity and cost of the detection system.

[0075] This embodiment achieves accurate accumulation and detection of urea crystals in the SCR mixing chamber by controlling the manually injected urea ammonia nitrogen ratio and combining it with the concentration monitoring of the NOx sensor. This invention simulates the urea injection and crystal formation process in actual working conditions by controlling the ammonia nitrogen ratio, ensuring the reliability of the experimental method and the repeatability of the results.

[0076] (3) Determine the crystallization threshold based on the recorded nitrogen oxide concentration data of the two.

[0077] a. Obtain some upstream nitrogen oxide concentration data and downstream nitrogen oxide concentration data recorded periodically.

[0078] b. Check whether the downstream nitrogen oxide concentration value in each record exceeds the upstream nitrogen oxide concentration value.

[0079] c. If the exceeding time lasts longer than the preset time, the time between the beginning and the end of the exceeding time will be recorded as the target time period.

[0080] d. Calculate the difference between each downstream nitrogen oxide concentration value and the corresponding upstream nitrogen oxide concentration value during the target period.

[0081] e. Calculate the average of all differences and record the calculated average as the crystallization threshold.

[0082] In the presence of urea crystals, the test downstream NOx sensor reading abnormally exceeds the test upstream NOx sensor reading and lasts for a period of time. Based on this, when the downstream nitrogen oxide concentration value exceeds the upstream nitrogen oxide concentration value, this embodiment simultaneously detects whether the exceeding state is maintained for a certain period of time. If it is, the average of the difference during this period of time is calculated as the crystallization threshold. In an optional implementation, the crystallization threshold is set to 500ppm (for 10 consecutive seconds), that is, when the downstream NOx sensor reading minus the upstream NOx sensor reading is greater than 500ppm, it is judged that crystallization exists.

[0083] It should be noted that the present embodiment uses a standard NOx sensor. As an alternative, a dual-channel sensor or other sensors capable of distinguishing NH3 from NOx, such as an ammonia sensor (NH3 sensor) or a new composite NOx sensor, may be used to improve the ability to distinguish NH3 from NOx. This will further improve the accuracy of determining the presence of crystals, especially when a large amount of ammonia is released during the decomposition of urea crystals.

[0084] This embodiment provides a National VI diesel vehicle, which is equipped with the National VI diesel vehicle urea crystallization judgment system of the above embodiment, and executes the National VI diesel vehicle urea crystallization judgment method of the above embodiment.

[0085] The above disclosure is only a preferred embodiment of the present invention, but the present invention is not limited thereto. Any non-creative changes that can be thought of by a person skilled in the art, as well as several improvements and modifications made without departing from the principle of the present invention, should fall within the protection scope of the present invention.

Claims

1. A urea crystallization judgment system for National VI diesel vehicles, characterized in that: include, Upstream NOx sensor: installed between the engine and the DOC system to detect the concentration of nitrogen oxides in the exhaust gas emitted by the engine; Downstream NOx sensor: installed in the tailpipe of the SCR system to detect the concentration of nitrogen oxides in the tail gas emitted by the SCR system; Crystallization detection controller: It is electrically connected to the upstream NOx sensor and the downstream NOx sensor respectively. When the vehicle enters the regeneration state, it detects whether the difference in nitrogen oxide concentration detected by the upstream NOx sensor and the downstream NOx sensor exceeds the crystallization threshold. If it exceeds, it is determined that urea crystals have occurred and an alarm is issued.

2. The urea crystallization judgment system for National VI diesel vehicles according to claim 1 is characterized in that: The crystallization detection controller is the engine ECU.

3. A method for determining urea crystallization in a National VI diesel vehicle, characterized in that: The system according to claim 1 or 2 is implemented, comprising the following steps: Determine the crystallization threshold based on bench testing; Obtain vehicle regeneration state parameters to determine whether the vehicle has entered the regeneration state; If the regeneration state is entered, the nitrogen oxide concentration collected by the upstream NOx sensor and the downstream NOx sensor is obtained in real time; Detecting whether the difference in nitrogen oxide concentration collected by the upstream NOx sensor and the downstream NOx sensor exceeds a crystallization threshold; If so, it is determined that urea crystals have been produced and an alarm is issued.

4. The method for determining urea crystallization of a National VI diesel vehicle according to claim 3, characterized in that: Prompt an alarm, including: Give an alarm prompt through the instrument panel or diagnostic system.

5. The method for determining urea crystallization of a National VI diesel vehicle according to claim 3 or 4, characterized in that: The crystallization threshold is determined based on bench tests, including: Steps to build the experimental setup; a step of forming urea crystals; Steps for monitoring NOx concentration and determining crystallization threshold during regeneration.

6. The method for determining urea crystallization of a National VI diesel vehicle according to claim 5, characterized in that: The steps to build the test device include: Build a National VI diesel vehicle after-treatment system, including the engine, DOC system, DPF system and SCR system; the DOC system and DPF system can be regenerated and trigger regeneration high temperature; the SCR system includes a catalyst, a mixing chamber, and a urea injection system that can manually control the injection amount; A test upstream NOx sensor is set between the engine and the DOC system, and a test downstream NOx sensor is set on the tailpipe of the SCR system; An exhaust temperature sensor is provided to monitor the exhaust temperature in the SCR system during regeneration.

7. The method for determining urea crystallization of a National VI diesel vehicle according to claim 5, characterized in that: The steps of forming urea crystals specifically include: Setting injection conditions, including manually controlling the urea injection system to inject urea at a fixed ammonia-nitrogen ratio and controlling the injection duration under normal engine operation conditions; By continuously running the engine, the SCR inlet temperature is kept below the temperature threshold, thereby causing incomplete hydrolysis of urea to produce crystals; After shutdown, disassemble the mixing chamber of the SCR system and weigh the weight of the crystallized material inside it; If the weight of the crystalline material does not meet the test requirements, continue to form urea crystals until the test requirements are met.

8. The method for determining urea crystallization of a National VI diesel vehicle according to claim 5, characterized in that: The steps for monitoring the nitrogen oxide concentration and determining the crystallization threshold during the regeneration process include: Manually stop urea injection, increase the engine load or manually control the regeneration conditions to raise the exhaust temperature in the SCR system to the regeneration temperature to allow the crystals to thermally decompose; During the regeneration phase, the nitrogen oxide concentration data collected by the test upstream NOx sensor and the test downstream NOx sensor are recorded in real time; The crystallization threshold is determined based on the nitrogen oxide concentration data recorded for both.

9. The method for determining urea crystallization of a National VI diesel vehicle according to claim 8, characterized in that: The crystallization threshold is determined based on the nitrogen oxide concentration data recorded for both, including: Obtaining a number of upstream nitrogen oxide concentration data and downstream nitrogen oxide concentration data recorded periodically; Check whether the downstream nitrogen oxide concentration value in each record exceeds the upstream nitrogen oxide concentration value; If the exceeding time is longer than the preset time, the time between the beginning and the end of the exceeding time will be recorded as the target time period; Calculate the difference between each downstream nitrogen oxide concentration value and the corresponding upstream nitrogen oxide concentration value during the target period; The average of all the differences was calculated and recorded as the crystallization threshold.

10. A National VI diesel vehicle, characterized in that: A system as claimed in claim 1 or 2 is configured to execute the method as claimed in any one of claims 3 to 9.

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