Model-Based Two-Stage SCR Urea Injection Strategy for Diesel Engine to Meet Low NO_X Emission Regulation

Diesel engine emission regulations are becoming stringent regarding nitrogen oxide (NO_X) limits. The challenge is to improve the NO_X conversion efficiency of the after treatment system under low temperatures. Although the two-stage selective catalytic reduction (SCR) has become a hot research topic, developing the urea injection strategy faces two challenges of accurately predicting ammonia (NH₃) coverage and the complexity of two-stage SCR co-control. Therefore, first, control-oriented close-coupled SCR (ccSCR) and SCR models were established based on chemical reaction kinetic mechanism, mass conservation, and continuous stirred tank reactor law. The integrated model was established by coupling them with a control-oriented diesel oxidation catalyst model. Second, the effects of temperature, exhaust mass flow rate, oxygen concentration, NO_X concentration, and nitrogen dioxide ratio on the NH₃ coverage target value were investigated based on the integrated model. Third, a two-stage SCR co-control strategy was proposed. The NH₃ coverage initial value and the threshold temperature for ccSCR to stop urea injection of the control strategy were optimized under the world harmonized transient cycle (WHTC). The results show that the NH₃ coverage target value is most sensitive to temperature and decreases with increasing temperature. The composite tailpipe NO_X under the WHTC is as low as 0.2 g/(kW·h).