Research On Failure Diagnosis And Tolerant Control Method For Urea-SCR Catalyst Convertor On A Diesel Engine

The vehicle ownership in China is significantly increasing in recent years due to the rapid industrialization in China.As a kind of efficient and economic internal-combustion engine,the diesel engine is widely applied in various commercial vehicles and some passenger vehicles.However,the high NO_x and PM emission of the diesel engine will lead to serious environmental problems.Urea-SCR technology is regarded as the most promising and efficient technique for NO_x removal.As the main functional component,SCR catalyst will be aging when it operates in a harsh environment for a long time.In addition,the performance of Urea-SCR system will be significantly deteriorated due to the SCR catalyst aging effect.Aiming at this problem,this thesis proposed catalyst failure diagnosis system to real-time monitor the aging condition of catalyst.Moreover,a Lyapunov-based MRAC controller was designed to ensure great performance of Urea-SCR system when SCR catalyst was aging.Firstly,this thesis took the Urea-SCR control system as the research object and proposed the ammonia-coverage-ratio-based closed-loop control strategy as the baseline control strategy.The control models of Urea-SCR system were established,including the control strategy model,the SCR model and the NO_x sensor model.Then,in order to make the control models of Urea-SCR system accurately reflect the state of Urea-SCR control system,the non-linear least square method was utilized to identify the unknown parameters in the kinetics model of chemical reactions in catalytic convertor,the temperature model of catalytic convertor and the NO_x sensor model.The verification results showed that the Urea-SCR system model established in this thesis is valid.Next,owing to the fact that the performances of Urea-SCR system were significantly influenced while the SCR catalyst was in-service aging,this thesis defined the aging factor to quantitatively reflect the aging degree of SCR catalyst according to the failure mechanism of SCR catalyst.The relationship between the aging effect and the performances of a SCR system under the ammonia-coverage-ratio-based closed-loop control was studied by means of the SCR aging model under various aging factors.The results indicated that performances of the SCR control system may be significantly deteriorated due to the SCR catalyst aging effect.Finally,for ensuring emission performances of the Urea-SCR system,EKF-based catalyst failure diagnosis system and Lyapunov-based MRAC were designed to achieve the SCR aging failure-tolerant control strategy.The simulation results indicated that SCR aging failure-tolerant control system performed well against SCR catalyst aging effect.This thesis quantitatively described aging degree of SCR catalyst and studied the aging effect on the performance of Urea-SCR system when the SCR catalyst was under various aging conditions by means of simulation method.Furthermore,the diagnosis system and fault-tolerant control system were integrated into SCR control system against SCR catalyst aging effect.This study can be conductive to understanding of SCR catalysts aging mechanism and aging effect on the performance of Urea-SCR system.Meanwhile,it offers theoretical basis and technical support for SCR catalyst failure diagnosis and tolerant control.In addition,this thesis provides an original idea to investigate the failure of SCR catalyst.