| In recent years,with the implementation of the National VI emission regulations,higher requirements have been placed on the on-board diagnostic system(OBD)of the SCR system,and the robustness is an important indicator to measure the uncertainty of the OBD system fault detection.Therefore,it is the great significance to study the high robustness OBD method strategy that meets the requirements of the current SCR system.This paper takes the OBD diagnosis strategy of the diesel engine SCR system of the National VI road vehicle as the research object.A high robustness OBD method strategy is formulated for the fault diagnosis of the key functions of the FAI＿SCR urea injection system and the fault diagnosis of the overall efficiency of the SCR system.Firstly,the demand analysis and fault classification are carried out for the key functional fault diagnosis of the FAI＿SCR urea injection system.Using the special structure of the FAI metering unit and the T3 theory,the fault diagnosis strategy and robustness analysis are formulated.The factors that may affect the T3 parameter(system voltage,urea temperature,urea concentration,injection flow and product stability)are tested,and the results show that the T3 parameter is less affected by the above factors and is much smaller than the T3 change when the system fails.The OBD system has a low risk of false alarms.By comparing the T3 parameters of the FAI＿SCR urea injection system when it is normal and faulty,the OBD critical value is calibrated to ensure that the critical value and the normal value have a greater degree of distinction and reduce the risk of system false alarms.The test was carried out on a Yunnei China VI 2.5L heavy duty diesel engine bench,and the OBD diagnosis strategy is verified in the form of fault simulation and fault parts.The results show that the diagnostic strategy performs well and is sensitive to the fault type.Secondly,the SCR efficiency monitoring system model is built based on MATLAB/Simulink software.Limit the release of efficiency monitoring by setting multiple monitoring release conditions based on the calibrated MAP to eliminate system uncertainty.By setting the open and adjustable monitoring integration time length and monitoring integration time interval to adapt to the requirements of different monitoring release rates.Parameter identification and software simulation of the model are carried out according to the bench test data.Calibration was carried out on a bench equipped with a Yunnei China VI 2.5L heavy-duty road diesel engine,and the efficiency monitoring system was verified with the WHTC test cycle as the core.Real vehicle verification was carried out on a Dayun brand vehicle equipped with a Yunnei Six 2.5L heavy-duty road diesel engine.The results show that the SCR efficiency monitoring system has high sensitivity to the diagnosis of low catalyst efficiency.The evaluation and verification of the discrimination between the OBD critical value and the normal value in the whole stage of the SCR efficiency monitoring system are carried out.The results show that there is always a discrimination degree of more than 0.2 between the critical value of fault judgment(efficiency limit)and the normal catalyst efficiency,which can effectively reduce false alarms.OBD strategy has high robustness. |
