Simultaneous estimation of ammonia injection rate and state of diesel urea-SCR system based on high gain observer

Selective catalytic reduction (SCR) controller achieves high NO_x conversion and low NH₃ slip primarily by means of controlling the rate and timing of injection of urea solution. No any low-cost commercial sensor can directly measure the actual ammonia (NH₃) injection rate currently; thus, it is necessary to propose a numerical method to estimate the actual NH₃ injection rate, which helps stabilization of emission control performances of a diesel SCR system during its service life cycle. A three-state nonlinear model was established and a method for designing high-gain observer (HGO) was studied so that those state variables such as NH₃ injection rate, NO_x concentrations, NH₃ concentrations and NH₃ coverage ratio can be estimated simultaneously. Such design method only requires bounded unknown input dynamics rather than any assumption of the fact how the unknown input changes. The simulation verification was performed by the ETC test cycles and qualitative and quantitative analysis of estimation errors were carried out. Compared with the extended Kalman filter (EKF) observer, the proposed high-gain observer has a higher estimation accuracy and better target traceability.