Research On Design Method Of The Urea-SCR Control System For Diesel Engines

The Urea-SCR system has become one of the most promising technologies to reduce NOx emissions for the diesel engine, due to the advantages of higher NOx reduction effi-ciency and insensitiveness to the sulfur. Urea-SCR system can achieve higher conversion efficiency and limit the ammonia slip through controlling the urea dosing precisely. But the on-board SCR system operates in the complex condition with a lot of external inter-ferences, the map-based control method cannot meet the increasingly stringent emission regulations. This paper concentrates on the design method research of the Urea-SCR con-trol system, and proposes the ammonia coverage ratio tracking control strategy based on feedforward-feedback form and direct optimization control strategy based on nonlinear model predictive control method respectively.In this paper, the Urea-SCR system used in the 11.596L displacement diesel engine is selected as the research object, and the system simulation model is established according to the system mechanism. The SCR system is divided into two units:the upstream and downstream unit, each unit has the uniform state. When establish the simulation model, not only the gaseous ammonia, gaseous nitrogen oxides and adsorbed ammonia (ammonia coverage) but also the internal temperature are considered. And the validity of the model is verified by the functional simulation. The simulation model can describe the distributed parameter characteristic effectively of the SCR system but also the model is brief relativelyFor engineering application, the exhaust aftertreatment problem is converted into the ammonia coverage ratio tracking control problem. An ammonia coverage tracking controller is designed based on differential flat feedforward controller and H∞ feedback controller. Firstly, the Urea-SCR system is described as a lumped parameter model, and a simplified control-oriented model is deduced. Secondly, the flatness property of the Urea-SCR system model is certified based on the differential flat flatness theory, and the feedforward control input is got. On this basis, considering feedforward control input as the balance point, linearize the control-oriented model by Taylor formula, then design an ammonia coverage ratio tracking feedback controller which meet the H∝ performance. The model error, parameter drift and the unmodeled dynamic in the system are considered as the amplitude bounded disturbance when design the feedback controller. Finally, the tracking performance and robust performance of the controller are demonstrated under ETC test cycle. Besides, based on the existed optimal ammonia coverage ratio, the effect of the Urea-SCR tracking control strategy is verified, and experiment results show that the control strategy can guarantee system NOx conversion efficiency average to 71.2159%, the maximum to 91.4531%, at the same time, limit the NH3 slip average to 9.2231ppm, the maximum to 29.8777ppm.Because the lumped parameter model cannot describe the distributed parameters characteristics accurately, while the optimal ammonia coverage ratio obtained off-line will lose reference value when system parameters drift. So it’s difficult to meet the increas-ingly stringent emission regulations using the ammonia coverage ratio control strategy. Therefore, a direct optimization strategy is proposed based on the nonlinear model pre-dictive control(NMPC) algorithm in this paper. Firstly, a new two unit control-oriented model is deduced to describe the distributed parameter characteristic, and the ammonia coverage ratio and temperature in the two units are chosen as state variables only, so a forth order control-oriented model are obtained. Then the accuracy of control orient-ed model is verified through ESC and ETC test cycle simulations. Secondly, a NMPC based direct optimization controller is designed. When design the controller, the NOx zero emission is chosen as the target optimization, the ammonia slip is regarded as the system output constraint, urea nozzle injection capacity is regarded as control constraint and get prediction model of the system through discretizing the control-oriented model using Euler formula. And the nonlinear constrained optimization problem is solved by sequential quadratic programming method ultimately. Finally, the performance of direct optimization control system is verified under ESC and ETC test cycles. In ESC test condition, the NOx conversion efficiency can reach 85% or more, the ammonia slip can be restricted with 30ppm, except for the idling condition; In ETC test condition, the NOX average of conversion efficiency is 86.8647% and the maximum value is 99.4567%, the average value of NH3 slip is 12.3022ppm and the maximum value is 29.5372ppm. So the direct optimization control strategy has higher conversion efficiency compared with the ammonia coverage ratio tracking strategy.In this paper the ammonia coverage tracking control strategy and direct optimal control strategy were respectively applied to design the Urea-SCR control system, and some achievements are made in this field, but there are still some further researches should be studied:(1) The online calculation workload is huge when NMPC method is used to optimize the SCR system directly, so it is difficult to apply that in the automotive control systems. Therefore, the quick solution method of NMPC should be explored; (2) The aftertreatment problem in idle condition is ignored in this paper, so it still requires further consideration; (3) The above mentioned methods were verified in offline experiments, future research could be completed in bench test.