Research On Urea Injection Control Mechanism Of Diesel Engine SCR System

As energy conservation and emission reduction targets have increased and emission regulations have become more stringent,the China?Emissions Regulation has further tightened the limits of diesel engine nitrogen oxides?NOx?emissions,the use of efficient after-treatment technology combined with advanced internal purification technology is a necessary technical means for diesel engine emission control.Selective Catalytic Reduction?SCR?is an efficient diesel NOx after-treatment technology and a key technology for diesel engines to meet China?and more stringent emission regulations in the future.The SCR catalyst model and the urea injection control strategy are the core of the SCR system,and they are critical to ensuring NOx conversion efficiency and controlling NH₃ slip.Through in-depth analysis of the structure of the SCR system,the internal reaction mechanism,and the influencing factors of the conversion efficiency of the SCR system,the SCR catalyst model and the urea injection closed-loop control strategy model were constructed by MATLAB/Simulink software,the key control parameters MAP were calibrated.and the model and control strategy are simulated and verified.The main research is as follows:?1?The development trend of current emission regulations,the formation mechanism of diesel engine NOx and the after-treatment technology of diesel engine NOx emissions were analyzed.The structure and operating principle of the SCR system and the main reactions occurring inside the SCR catalyst were introduced.The factors affecting the catalytic conversion efficiency of SCR,such as catalyst inlet temperature,catalyst ammonia coverage,exhaust mass flow rate,ammonia-nitrogen ratio,exhaust gas composition and catalyst aging,were systematically analyzed.?2?According to basic theories of thermodynamics,chemical reaction kinetics and law of conservation of mass,the SCR catalyst model was established by one-dimensional finite element method.The catalytic converter model contains several functional modules,including:mathematical model of chemical reaction inside the catalytic converter,calculation model of NOx original emission in the upstream of SCR,calculation model of NO₂ emission in the upstream of SCR,calculation model of catalyst ammonia coverage rate,catalyst temperature calculation model catalyst aging calculation model,SCR system overall efficiency calculation model.?3?Based on catalyst ammonia coverage a closed-loop urea injection control strategy was established and the corresponding standard quantity in the control system was calibrated.The diesel engine original NOx emission was used as the input of the control model,and the simulation verification of the control model was performed according to the WHSC and WHTC test cycles of China?.The simulation results show that the SCR closed-loop control strategy based on ammonia coverage control can effectively reduce NOx emissions.The average conversion efficiency of NOx in steady-state test cycle reaches 81%,and the average conversion efficiency of transient test cycle NOx reaches 72%,The slip amount of NH₃ is controlled within 10 ppm.