Study On Control Strategy Of SCR System In Diesel Engine

As the increasingly stringent emission regulations,the additional exhaust after-treatment techniques is necessary for the diesel engines to meet with the No.4 phase of Chinese emission standards.Among exhaust after-treatment techniques for diesel engines,the selective catalytic reduction is presently considered as one of the most promising way.The reducer flow control is particularly important when use the SCR technique to reduce NO_x in exhaust gas from mobile diesel.In order to establish reasonable reducer control strategy,the emission performance of diesel engine along with the ammonia storage characteristic and the conversion efficiency characteristic of catalyst must be considered.Aiming at the ammonia storage characteristic of SCR catalyst,ammonia storage functions lied on different exhaust temperature and space velocity were obtained through tests.The ammonia storage correction strategy was established based on steady-state control strategy.Moreover,according to the characteristic that the conversion efficiency of catalyst varied with temperature,conversion efficiency map of the catalytic converter was drawn and control strategy based on real-time temperature was set up.The control strategy of SCR system was evaluated according to GB 17691-2005 requirements through ESC tests and ETC tests.The conclusion was drawn that the engine with the steady-state control strategy embedded the ammonia storage correction could meet the No.4 phase emission standards.The temperature control strategy needed to be further validated in tests.In order to instruct the calibration of SCR system for diesel engine and improve the efficiency of gaining appropriate control parameters of reducer flow,the relationship between NO_x conversion and molar ratio of ammonia to nitrogen oxides was obtained by simplifying the chemical reaction process of SCR system.Based on this relationship a method to predict NO_x emission from diesel engines under different reducer flow control parameters was established.The test results showed that the method could predict accurately and provide reference for the calibration of SCR system.In view of the requirements of OBD system in GB17691-2005,this thesis analyzed the influence of abnormal operation on the NO_x emission and the conclusion was drawn that the change of reducer concentration or flow had the same influence on NO_x conversion efficiency under different conditions.Then worked out a method to estimate NO_x emission level of ETC tests base on the NO_x emission of only one steady condition,and established monitoring strategy of the OBD system. The tests in China Automotive Technology & Research Center showed that the monitoring strategy of the OBD system could meet with the requirements of OBD1 and ensure the control measures of NO_x emission work normally.