Model-based Control And Obd Strategies For SCR System

In the recent years,with the rapid improvement of the living quality of the people and development of the city,the issues of the exhaust emissions of internal combustion engine becomes increasingly prominent.The Selective Catalytic Reduction(SCR)after-treatment technique for diesel engine is studied in this paper.The model of SCR catalytic is established by investigating its chemical reaction mechanism systematicly.Then,the SCR control strategy and the On-Board Diagnostics(OBD)diagnosis strategy are designed,which have been verified on the engine European Steady Cycle(ESC)and European Transient Cycle(ETC)operating condition,respectively.The main contributions are listed as followings:(1)The SCR catalyst model is established.Firstly,the chemical reaction mechanism of SCR catalyst is investigated systematicly.In order to establish the model conveniently,the SCR catalyst is divided into several ideal SCR cells along the direction of the airflow axis.Based on the Continuous Stirred Tank Reactor(CSTR)method,the models of SCR catalyst are proposed by combining the Arrhenius theorem and the law of conservation of mass.The stability and observability of the chemical reaction kinetic model of SCR catalyst are analyzed theoretically.Subsequently,the model are validated under the engine ESC and ETC conditions,respectively.(2)The state observer for SCR catalyst is designed.After a briefly introduction of the adaptive sliding mode observer,a state observer based on adaptive sliding mode observer is provided for the SCR catalyst.And its stability and robustness are proved by the Lyapunov stability theory.Finally,the reliability of the state observer is verified under the engine ESC and ETC conditions.(3)The model-based control strategy for SCR system is developed.A model-based closed-loop feedback control strategy with the structure of combination the feed-forward controller and SCR ammonia storage state control is proposed.The theoretical requirements of the urea solution is injected by the feedforward control which adopts the controller structure of combining the MAP calibration and model calculation and obtains a fast response.According to the distribution law of the ammonia storage state of the SCR catalyst along with the axial direction of the air flow and noticing the structural characteristics of the catalyst model,the SCR ammonia storage state controller is designed by the backstepping method,hence,the distribution state of SCR ammonia storage in the direction of the airflow is controlled.Therefore,the SCR ammonia storage state control can implement the objectives of which the Nitrogen Oxide(NOx)emission is reduced and the secondary pollution caused by ammonia leakage is avoided simultaneously.The experimental results of engine ESC and ETC conditions demonstrate that the proposed model-based closed-loop control strategy can effectively reduce the NOx emission while reduce the amount of ammonia leakage.(4)The model-based OBD diagnosis strategy of SCR system is proposed.Some common fault types of SCR system are analyzed firstly.Then,the model-based OBD fault diagnosis strategy is provided for several typical fault types.When there occur four most representative failure modes of which the downstream temperature sensor and NOx sensor of SCR catalyst are broken-down,the catalyst is aged and the Urea nozzle is stoppage,the performance of the model-based OBD strategy and the closed-loop control strategy are verified,respectively.The verification resluts show that the proposed model-based SCR control strategy and OBD strategy are effective whether the SCR system encounter faults.