Sliding-mode observers for urea selective catalytic reduction system state estimations based on nitrogen oxide sensor measurements

The state estimations for urea-based selective catalytic reduction systems have received growing interest in the past few years because of their importance to selective catalytic reduction controls and onboard diagnostics, as well as the high cost and difficulties of direct sensor measurements. Most of the existing estimation techniques suffer because of the ammonia cross-sensitivity from the nitrogen oxide sensor and require an ammonia sensor to be available downstream of the selective catalytic reduction, which may increase the costs of production urea-based selective catalytic reduction systems. The purpose of this study is to design observers for estimating the key states (the nitrogen oxide concentration, the ammonia concentration, and the ammonia coverage ratio) of the urea-based selective catalytic reduction system at the selective catalytic reduction outlet in various situations, using only a nitrogen oxide sensor. The sliding-mode observers were designed to ensure high robustness against the ammonia cross-sensitivity from the nitrogen oxide sensor and uncertainties from the selective catalytic reduction inputs. Simulation verification results under the Federal Test Procedure 75 test cycle showed that the proposed observers were able to estimate the states accurately, in hot and cold exhaust conditions, for both single-cell selective catalytic reduction systems and two-cell selective catalytic reduction systems. The proposed observer-based estimation technique has the potential for reducing the cost of advanced selective catalytic reduction control systems and for applying it to onboard diagnostics.