Study On Control Strategy Of Selective Catalytic Reduction System In Diesel Engine

Due to the special combustion process,diesel engines generate more particulate matter and nitrogen oxide(NO_x).The air pollution is becoming more and more serious and the nitrogen oxide emission regulations are becoming increasingly strict worldwide.It is necessary to adopt correct exhaust treatment technologies to deal with the nitrogen oxide emission.The selective catalytic reduction(SCR)systems are one of the effective methods which utilize aqueous urea solution(Adblue)to catalytically convert nitrogen oxide in the engine exhaust.In order to convert the nitrogen oxide as much as possible,enough Adblue should be injected and controlled accurately.However,the tailpipe ammonia slip will exceed the restrictions and lead to secondary pollution if the Adblue is injected excessively.In addition,the internal working conditions of the catalyst cannot be measured by sensors,so it is needed to design an observer to estimate them.Therefore,based on the GT-Power and Matlab/Simulink,the research has been done aimed at the estimation of the internal working conditions and the control for Adblue injection.The main works in this thesis are as follows:(1)Research on the chemical kinetics model of SCR systemBased on the analysis of the structure and operating principle of the SCR system,the chemical kinetics model of SCR system is established for the design of observer and control of the SCR system.Besides,the observability and controllability are analyzed.(2)Research on the estimation of ammonia concentration in the SCR catalyst based on Luenberger sliding mode observerConsidering that the internal working conditions of the catalyst cannot be measured by sensors,a Luenberger sliding mode observer based on the two-cell SCR system model is established in this thesis.This observer uses the measurements of the upstream and downstream nitrogen oxide sensors,ammonia sensors and temperature sensors to estimate the ammonia concentration in the SCR catalyst.Then the stability of the observer is analyzed.The simulation results show that the proposed observer has good tracking performance in the case of different temperature and different gas composition.(3)Research on the Backstepping control for the SCR system based on Luenberger sliding mode observerBased on the aforementioned two-cell SCR system model,a Backstepping control for the SCR system based on Luenberger sliding mode observer is designed.The whole SCR system is decomposed into two subsystems,and then the Lyapunov functions are designed for the two subsystems separately.Firstly,a virtual control law is designed to ensure the downstream subsystem is convergent and the emissions limits are not exceeded.Then this virtual control law is set as the tracking target of the upstream subsystem.Finally,the control law of the whole SCR system can be obtained.The simulation results show that the Backstepping control for the SCR system based on Luenberger sliding mode observer has smaller overshoot and shorter setting time compared with traditional PID control.(4)Research on the neural network model predictive control for an SCR systemAiming at the the nonlinearity,time delay and input and output constraints of the SCR system,BP neural networks are utilized to estimate numerical model for their better nonlinear fitting capacity.Then the control input can be got from the combinations of the numerical models and model predictive control.Besides,the stability of the neural network model predictive controller is analyzed.The simulation results show that the neural network model predictive control has better control performance with zero-overshoot and shorter setting time compared with traditional PID control and Luenberger sliding mode observer based Backstepping control.