Urea Dosing Optimization And Control Strategies Of Urea-SCR System In Diesel Engine

Due to the tightening emission regulations and the worsening air pollution,there is an increasing emphasis on the progress of advanced post treatment technologies for control of vehicle emissions.Selective catalytic reduction?SCR?technology is a promising diesel after-treatment technology due to its high efficiency,high selectivity,high economy and sulfur resistance.Clearly,the urea dosing control dominates the overall SCR system performances;its insufficient injection would result in the inadequate input of NH₃ and being inaccessible to optimize NOx reduction;on the other hand,its excess injection would give rise to high urea consumption and undesirable NH₃slip to the tailpipe.However,the optimal dosage of urea in SCR as one major control objective to greatly reduce NOx emissions and ensure achievement of the acceptable NH₃ slip level is still challenging due to the complexity of the SCR operating conditions and the trade-off between NOx reduction and NH₃ slip.This paper focuses on the optimization and control strategy of Urea-SCR system to achieve maximum NOx conversion and NH₃ slip shall be limited below 10 ppm.The main contents are as follows:?1?The control based SCR system modelingThe SCR system is regarded as a continuous stirred tank reactor and a three state control based model is established according to the mass conservation law.Taking into account the different time scale within Urea-SCR system,a one-state model is further applied with the aim of capturing the essential behavior of the system with a low computational burden.Moreover,the proposed model is verified and analyzed by the steady state data of engine.?2?The model based NSGA-? for optimization of Urea-SCR systemA model based NSGA-? method is proposed for the optimization of Urea-SCR system.The optimal urea dosage and ammonia coverage ratio,the corresponding NOx conversion efficiency and the ammonia slip can be obtained and the optimal control target of Euro VI can be achieved.For the purpose of illustrating the dependence of the optimized solution on kinetic model a sensitivity study is carried out and the optimized solutions obtained by three-state nonlinear model and reduced order one-state model respectively are compared.In addition,the engine full load optimal maps are obtained by proposed method,which can act as reference values or a benchmark of the control system.?3?The state observer of SCR systemA nonlinear state observer is designed to obtain the ammonia coverage,NOx concentration,NH₃ concentration.Two different initial values of ammonia coverage are selected in the observer to analyze the effect of the initial values on the state estimation accuracy.The results show that the initial error of the observer state estimation will increase with the different initial values of ammonia coverage compared with the same initial values of ammonia coverage,but the observer designed in this paper is still the same.It can ensure that the estimated value converges to the model value in limited time.The ETC test cycle is used to verify the estimation effect of the observer and the errors between the estimated value and the model value are quantitatively analyzed.The nonlinear state observer has high estimation accuracy and can be applied to the closed-loop control strategy of Urea-SCR system.For the sake of obtaining unknown state parameters of the system,high gain observers are designed to obtain the ammonia coverage,NOx concentration,NH₃concentration,ammonia dosing rate and ammonia storage capacity in the catalyst,which can be regarded as the feedback control parameters for NMPC and adaptive control strategy.?4?The model predictive control and adaptive control strategyThe model predictive control is proposed and the control objective is to achieve maximum amount of NOx emissions reduced and ammonia slip to the acceptable extend using the inlet concentration of ammonia as the manipulated variable.The standard European steady-state cycle?ESC?is performed for data generation from a diesel engine equipped with a commercial SCR and to verify the effectiveness of the designed control strategy.Simulation results over entire ESC demonstrate that optimal NOx conversion efficiency fits well with various engine operating modes and NOx emissions requirement is satisfied?above 300?the desired NOx conversion efficiency>80%?during most part of the cycle with NMPC controller.In addition,the ammonia concentration at the catalyst outlet don't exceed the 10 ppm limit.Furthermore,considerably changing of the ammonia storage capacity occurs with sharply change in temperature,NMPC still has a good performance to keep ammonia slip below a defined limit.Furthermore,the deviations between the actual ammonia dosing rate and the control demand are considered and an adaptive control strategy is adopted to correct the actual ammonia dosing rate.