Method for diagnosing failure of nox sensor downstream of scr

Abstract

The application provides a SCR downstream NOx sensor failure diagnosis method, comprising the following steps: 1) confirming that the SCR downstream NOx sensor, the ammonia sensor and the ECU are in normal communication; installing a PEMS device and confirming that the PEMS device test data is accurate; 2) starting the vehicle to be tested, and performing PEMS emission tests under three working conditions of urban area, suburb and highway; 3) performing data elimination on the PEMS device, aligning the remaining data by working condition, then comparing and obtaining errors under different working conditions, and obtaining a total error; 4) if the total error is greater than the set threshold, the SCR downstream NOx sensor installed on the vehicle to be tested has failed; or if the total error is less than or equal to the set threshold, the SCR downstream NOx sensor installed on the vehicle to be tested is normal. The application can accurately judge the failure of the SCR downstream NOx sensor, has the advantages of scientific rationality, high reliability and wide application range, and can provide a theoretical basis for the normal operation of the vehicle.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of exhaust treatment of internal combustion engines, and particularly refers to a method for diagnosing failure of a downstream NOx sensor of SCR. BACKGROUND

[0002] Downstream NOx sensor x As an important part of the aftertreatment assembly, the downstream NOx sensor can detect the value of nitrogen oxides (NOx) at the outlet of SCR. This value not only reflects the emission level of the vehicle, but also can be used to calculate the emission model and the efficiency of SCR, and to correct the urea injection coefficient in a closed loop. This is of great significance for optimizing calibration parameters and improving emission levels.

[0003] Damage to the downstream NOx sensor can cause the diesel engine control system to enter an open loop state, making it impossible to accurately feedback emission data, and thus making it difficult for the engine control unit to accurately adjust emissions. In this case, the emissions of the vehicle will exceed the specified standards, and the service life of the exhaust emission treatment equipment may be shortened. As an important part of the electronically controlled urea injection system, the downstream NOx sensor plays a key role in the normal operation of the engine and effective control of exhaust emissions. Once the downstream NOx sensor fails, it may cause the engine operating conditions to deteriorate, resulting in symptoms such as idle stall, engine misfire, and power loss.

[0004] Therefore, there is an urgent need for a method that can accurately determine the failure of the NOx sensor, providing a theoretical basis for the normal operation of the vehicle. SUMMARY

[0005] To overcome the shortcomings of the above-mentioned technologies, the purpose of the present application is to provide a method for diagnosing failure of a downstream NOx sensor of SCR. By comparing the test data of the downstream NOx sensor and the ammonia sensor of SCR with the test data of PEMS equipment, the failure of the NOx sensor can be accurately determined, providing a theoretical basis for the normal operation of the vehicle.

[0006] To achieve the above-mentioned purpose, the technical solutions adopted by the present application are as follows:

[0007] A method for diagnosing failure of a downstream NOx sensor of SCR, comprising the following steps:

[0008] 1) Confirm that the downstream NOx sensor and the ammonia sensor of SCR are in normal communication with the ECU; install the PEMS equipment and perform a pre-test gas check to confirm that the test data of the PEMS equipment are accurate and reliable;

[0009] 2) Start the vehicle to be tested and perform a PEMS emission test to obtain data during the test; the PEMS emission test is performed under three working conditions of urban, suburban and highway;

[0010] 3) The PEMS device is used to remove the data before the dew point of the SCR downstream NOx sensor, and the remaining data is time-aligned by working condition. Then, the error under different working conditions is obtained by comparison, and the total error is obtained based on the error under different working conditions;

[0011] 4) The total error is compared with the set threshold value to determine whether the SCR downstream NOx sensor is failed: if the total error > set threshold value, the SCR downstream NOx sensor installed in the vehicle to be tested is failed; or if the total error ≤ set threshold value, the SCR downstream NOx sensor installed in the vehicle to be tested is normal.

[0012] Preferably, the installation positions of the SCR downstream NOx sensor and the ammonia sensor are at the SCR outlet, and the sampling position of the PEMS device is at the exhaust pipe outlet.

[0013] Preferably, in the step 2), the vehicle to be tested is an N2 vehicle, and the test time proportions of different working conditions in the PEMS emission test are respectively: 45% for urban area, 25% for suburban area, and 30% for highway, and the total test time is ≥ 3h, and the vehicle speeds under urban area, suburban area and highway working conditions are respectively 15-30km / h, 45-70km / h and > 70km / h.

[0014] Preferably, in the step 2), the data includes the total content of NOx and NH3 measured by the SCR downstream NOx sensor, the content of NH3 measured by the ammonia sensor, and the content of NOx measured by the PEMS device; and the data of the SCR downstream NOx sensor and the ammonia sensor are obtained through the ECU.

[0015] Preferably, the step 3) specifically includes:

[0016] 3.1) The NOx data measured by the PEMS device within the time before the dew point of the SCR downstream NOx sensor is removed;

[0017] 3.2) The remaining data after data removal of the PEMS device is time-aligned with the data measured by the SCR downstream NOx sensor and the ammonia sensor;

[0018] 3.3) The difference value is obtained by subtracting the value measured by the ammonia sensor from the value measured by the SCR downstream NOx sensor after time alignment, and the average value of the difference value is obtained; the average value of the data measured by the PEMS device after time alignment is obtained; the two average values are compared to obtain the error, and the absolute value of the error is calculated, and the total error is obtained based on the absolute value.

[0019] Preferably, in the step 3.2), the time alignment is performed under different working conditions, and the data obtained under the urban, suburban and highway working conditions are time-aligned respectively; the specific operation of the time alignment is that, in the starting stage of any working condition, the time difference between the peak position of the value measured by the SCR downstream NOx sensor and the peak time of the NOx value measured by the PEMS device is obtained, and the time alignment is performed according to the time difference; the data measured by the SCR downstream NOx sensor and the ammonia sensor have no time difference, and thus the time alignment is not needed.

[0020] Preferably, in the step 3.3), the error calculation is performed under different working conditions, so as to obtain the absolute values of the errors under different working conditions, and the weighted average of the absolute values of the errors under different working conditions is obtained by introducing weights for different working conditions.

[0021] Preferably, the distribution of the weights is as follows: 0.2 for the urban area, 0.3 for the suburban area, and 0.5 for the highway.

[0022] Preferably, the error is a relative error.

[0023] Preferably, in the step 4), the threshold value is set to 10%.

[0024] Compared with the prior art, the present application has the following advantages:

[0025] The present application provides a SCR downstream NOx sensor failure diagnosis method, which combines the data measured by the SCR downstream NOx sensor and the ammonia sensor, compares the sensor data and the PEMS device data after time alignment, and calculates the difference. In particular, considering that the PEMS test is performed under the urban, suburban and highway working conditions, corresponding weights are introduced for different working conditions, the comprehensive error of the values measured by the sensor and the values measured by the PEMS device is calculated by using the weighted average method, and a threshold value is set. When the comprehensive error exceeds the set threshold value, it can be determined that the sensor fails. The present application can accurately judge the failure of the SCR downstream NOx sensor, has the advantages of scientific rationality, high reliability and wide application range, and can provide a theoretical basis for the normal operation of the vehicle. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 The present application provides a flowchart of a SCR downstream NOx sensor failure diagnosis method. DETAILED DESCRIPTION

[0027] In order to better explain the present application, the main content of the present application is further illustrated by the following specific embodiments, but the content of the present application is not limited to the following embodiments.

[0028] The application provides a SCR downstream NOx sensor failure diagnosis method, mainly including the following steps. Figure 1 The application method specifically includes the following steps:

[0029] 1) Before the formal PEMS test, it is necessary to confirm that the SCR downstream NOx sensor and the ammonia sensor are in normal communication with the ECU and can read the NOx and NH3 values from the ECU; the PEMS device is installed and checked with standard gas before the test to ensure that the PEMS device test is accurate.

[0030] 2) A complete road emission test, i.e. PEMS test, is performed according to the regulations, and is carried out in urban, suburban and highway conditions. The data measured by the SCR downstream NOx sensor and the ammonia sensor are recorded by the ECU, and the tail emission data are recorded by the PEMS device during the test. The recorded data are processed in the next step. The test vehicle of the specific embodiment is a N2 vehicle, and the time proportions of urban, suburban and highway conditions are 45%, 25% and 30% respectively, and the total test time is not less than 3h. The vehicle speeds in urban, suburban and highway conditions are 15-30km / h, 45-70km / h and >70km / h respectively.

[0031] 3) The PEMS device data are removed, and the remaining data are time-aligned according to the conditions, then the relative errors in different conditions are obtained by comparison, and the total error is obtained based on the relative errors in different conditions.

[0032] 3.1) The PEMS device records the tail emission data from the beginning of the test, while the SCR downstream NOx sensor needs to pass through the dew point before it starts to have data. In actual calculation, the data of the downstream NOx sensor recorded by the PEMS device before passing through the dew point are removed, and the data after passing through the dew point are compared with the sensor data.

[0033] Three groups of data are obtained during the test: the SCR downstream NOx sensor data, the ammonia sensor data and the PEMS device test NOx data. Since the SCR downstream NOx sensor needs to pass through the dew point to measure the data, while the PEMS device starts to measure the tail emission concentration from the beginning of the test, the PEMS device measured NOx data before passing through the dew point are removed, and the removed data are compared.

[0034] 3.2) The installation position of the SCR downstream NOx sensor and the ammonia sensor is at the outlet of the SCR, and the sample analyzed by the PEMS device is extracted from the tail end of the exhaust pipe. The time lag is much longer than the sensor, and the time alignment needs to be performed before the error calculation. There is a peak in the tail exhaust when the vehicle starts. The time difference between the NOx peak measured by the sensor and the PEMS device is observed to obtain the corresponding time difference, and the time alignment is performed. The installation position of the ammonia sensor and the SCR downstream NOx sensor is similar, and they are almost at the same cross section in the exhaust flow direction. It is considered that the test data of the ammonia sensor and the SCR downstream NOx sensor are consistent in time, and the time alignment of the PEMS device is also considered consistent with the SCR downstream NOx sensor. Due to different working conditions and different exhaust flow rates, the time passing through the exhaust pipe is also different, and the lag of the PEMS device relative to the sensor is different (maximum in urban area, second in suburban area, and minimum in high speed). The working conditions can be divided into urban area, suburban area, and high speed when performing time alignment.

[0035] 3.3) The SCR downstream NOx sensor cannot distinguish NOx and NH3 in the exhaust gas. The data measured by the SCR downstream NOx sensor is the sum of the NOx value and the ammonia leakage value, and the NOx measured by the PEMS device is pure nitrogen oxide. Therefore, when comparing the data measured by the SCR downstream NOx sensor with the data measured by the PEMS device, the difference value obtained by subtracting the data measured by the ammonia sensor from the data measured by the SCR downstream NOx sensor is used to compare with the NOx data measured by the PEMS device.

[0036] The difference value between the data measured by the SCR downstream NOx sensor and the ammonia sensor is divided into working conditions to obtain the average value, and the PEMS device data after dew point removal and time alignment is divided into working conditions to obtain the average value. The relative error of the two average values is calculated, and the absolute value of the relative error is obtained, which is the absolute value of the relative error of the three working conditions in urban area, suburban area, and high speed.

[0037] The relative error in each working condition is calculated as follows:

[0038] The relative error = the difference between the two average values / the average value of the PEMS device data;

[0039] Taking the urban working condition as an example, the calculation method of the relative error is as follows: assuming that the average value of the difference between the data measured by the SCR downstream NOx sensor and the ammonia sensor in the urban working condition is A, and the average value of the PEMS device data is B, the relative error of the urban working condition is (A-B) / B. At this time, the absolute value of the relative error in the urban working condition is |(A-B) / B|. The calculation of other working conditions is referred to the urban working condition.

[0040] Different working conditions will not only result in different exhaust flow rates, affecting time alignment, but also result in different exhaust flow rates, affecting the mixing uniformity of nitrogen oxides in the exhaust gas. The greater the exhaust flow rate, the better the exhaust mixing uniformity, and the more accurate the data measured by the NOx sensor, ammonia sensor and PEMS device downstream the SCR. In view of this, different weight factors are introduced for different working conditions, and the city, suburb and highway are 0.2, 0.3 and 0.5 respectively. The absolute values of the relative errors of the above three groups are weighted and averaged to obtain the total error, that is:

[0041] Total error = 0.2 x absolute value of relative error under urban working condition + 0.3 x absolute value of relative error under suburban working condition + 0.5 x absolute value of relative error under high-speed working condition.

[0042] 4) Set the preset threshold to 10%, compare the total error with the preset threshold, if the total error > preset threshold, it is considered that the downstream NOx sensor has failed, otherwise it is considered that the sensor is normal.

[0043] Finally, it should be pointed out that: the above only for the preferred embodiments of the present application, and is not intended to limit the present application, although the foregoing embodiments of the present application are described in detail, for those skilled in the art, it still can be modified, or part of the technical features of the equivalent replacement. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application, should be included in the protection scope of the present application. Other unexplained parts belong to the prior art.

Claims

1. A method of diagnosing failure of a NOxsensor downstream of an SCR, characterized in that: The method comprises the following steps: 1) confirming that the NOx sensor downstream of the SCR, the ammonia sensor and the ECU are in normal communication; installing a PEMS device and checking the calibration gas before the test to confirm that the test data of the PEMS device are accurate and correct; 2) starting the vehicle to be tested and performing a PEMS emission test to obtain data during the test; the PEMS emission test is performed under three working conditions of urban, suburban and highway; 3) removing the data before the dew point of the NOx sensor downstream of the SCR from the PEMS device, and time-aligning the remaining data under different working conditions, then comparing and obtaining the error under different working conditions, and obtaining the total error based on the error under different working conditions; The step 3) specifically comprises: 3.1) removing the NOx data measured by the PEMS device within the time before the dew point of the NOx sensor downstream of the SCR; 3.2) time-aligning the data measured by the NOx sensor downstream of the SCR and the ammonia sensor after data removal of the PEMS device; the time alignment is performed under different working conditions, and the data obtained under three working conditions of urban, suburban and highway are time-aligned respectively; the specific operation of the time alignment is: in the starting stage of any working condition, the time difference between the wave peak position of the data measured by the NOx sensor downstream of the SCR and the wave peak time of the NOx data measured by the PEMS device is obtained, and the time alignment is performed according to the time difference; the data measured by the NOx sensor downstream of the SCR and the ammonia sensor have no time difference, and do not need to be time-aligned; 3.3) subtracting the data measured by the ammonia sensor from the data measured by the NOx sensor downstream of the SCR after time alignment to obtain a difference value, and averaging the difference value; averaging the data measured by the PEMS device after time alignment; comparing the two average values to obtain the error, calculating the absolute value of the error, and obtaining the total error based on the absolute value; the error calculation is performed under different working conditions to obtain the absolute value of the error under different working conditions, and a weight is introduced for different working conditions to obtain a weighted average value of the absolute value of the error under different working conditions, thereby obtaining the total error; 4) comparing the total error with a set threshold value to determine whether the NOx sensor downstream of the SCR is failed: if the total error > the set threshold value, the NOx sensor downstream of the SCR installed in the vehicle to be tested is failed; or if the total error ≤ the set threshold value, the NOx sensor downstream of the SCR installed in the vehicle to be tested is normal.

2. The SCR downstream NOxsensor failure diagnosis method according to claim 1, characterized by: The installation positions of the NOx sensor downstream of the SCR and the ammonia sensor are at the outlet of the SCR, and the sampling position of the PEMS device is at the outlet of the exhaust pipe.

3. The SCR downstream NOxsensor failure diagnosis method according to claim 2, characterized by: In the step 2), the vehicle to be tested is an N2 vehicle, and the test time ratio of different working conditions in the PEMS emission test process is: urban 45%, suburban 25% and highway 30%, and the total test time is ≥ 3h, and the vehicle speeds under the working conditions of urban, suburban and highway are 15-30km / h, 45-70km / h and > 70km / h respectively.

4. The SCR downstream NOxsensor failure diagnosis method according to claim 3, characterized by: In the step 2), the data include the total content of NOx and NH3 measured by a NOx sensor downstream of the SCR, the content of NH3 measured by an ammonia sensor, and the content of NOx measured by a PEMS device; the data of the NOx sensor and the ammonia sensor downstream of the SCR are obtained by an ECU.

5. The SCR downstream NOx sensor failure diagnosis method according to claim 1, characterized by: The distribution of the weights is as follows: 0.2 for urban area, 0.3 for suburban area, and 0.5 for highway.

6. The SCR downstream NOx sensor failure diagnosis method according to claim 1, characterized by: The error is a relative error.

7. The SCR downstream NOx sensor failure diagnosis method according to claim 6, characterized in that: In the step 4), the threshold is set as 10%.

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