Study On Monitoring Of The NO_x Sensor Signal Plausibility And Aging SCR Catalyst Of Heavy-duty Diesel Engine

As the advantage of higher power performance and lower oil consumption, dieselengines are widely applied in medium/heavy trucks and passenger carriages. With theaggravation of environmental pollution caused by vehicles, a series of legislation areput out to reduce emissions from automobile engines and more stringent. But onlyrelying on In-cylinder purge without other aftertreatment technology, it have becomeincreasingly difficult for meet the increasingly stringent emission regulations. Atpresent, the Selective Catalytic Reduction(SCR), using urea as reducing agent,because of the advantage of low oil consumption and strong resistibility, is presentlyconsidered the mainstream technique for the removal of nitrogen oxides from theexhaust of heavy-duty diesel vehicles.As an important technique for on board detection of vehicle emissions, On-BoardDiagnosis(OBD), which is in line with the automotive industry development trends insafety, energy saving and environmental protection, has been introduced worlewide.For State IV and higher emissions regulations in China, the application of SCRaftertreatment technology must be equipped with OBD systems for heavy-duty dieselengines. And based on the current mainsteam SCR system, four parts are deeplystudied in this paper inclouding the SCR diagnosis functional requirements, faultsdiagnosis and management, plausibility check of the NOx sensor signal and themonitoring of aging SCR catalyst.Based on HJ437-2008Technical specification for on-board diagnostic (OBD)system of compression ignition and gas fuelled positive ignition engines of vehicles,the diagnosis functional requirements of emissions-related components in SCRsystem are established. Combination of SCR system structure in diesel engine, weselect a configuration which OBD function is embedded in SCR dosing control unit tomake use of the SCR dosing control unit resources to the best advantage. The configuration related to the diagnosis function of OBD system is constructed on theSCR aftertreatment system of diesel vehicle, for giving the control unit of SCRsystem relevant OBD function to meet the requirements of stringent emissionregulation.For a SCR system with a mass of controllable parameters and complex andchangeable action, a faults diagnosis method based sensor signal character isadvanced. Based on understanding profoundly inherent characteristic of various typesof signal, this method processs time series parameters which uses the principles offinite state machine, with combining analysed and generalized characteristic withexperience and common sense. The purpose of the fault management is informationfusion and output information which meeting the OBD regulations.According to the working environment of the vehicle NO_Xsensor, fault types ofNO_Xsensor are analysed, and corresponding diagnostic strategies are constitutedincluding circuit connection diagnosis and plausibility diagnosis of the NO_Xsensorsignal. As the NO_Xsensor using in the paper can make self-diagnosis function,plausibility diagnosis of the NO_Xsensor signal is especially studied, and thecorresponding diagnostic models are builded by Simulink simulation software.To the problem of catalyst aging during operation, a diagnostic strategies basedon NO_Xconversion efficiency of an SCR system is designed, and the correspondingdiagnostic models are built according to the diagnostic strategies. Through monitoringof NO_X conversion efficiency, the diagnosis of catalyst aging and OBD legislationrequirements about NO_Xemission threshold value can be achieved.Through bench experiments sampling data, a off-line simulation is validatedabout the fault diagnosis models of NO_Xsensor and aging catalyst. The resultsindicated that the monitoring strategy of the OBD system could meet with therequirements and ensure the control measures of NO_Xemission work normally.