Cycle-Driven Optimization of a Fixed-Structure Controller for Urea Dosing in a Mobile SCR System

A model-based urea-dosing controller has been developed for the selective catalytic reduction (SCR) units on a diesel engine exhaust aftertreatment system (EATS). The SCR units consist of an integrated SCR-coated filter and then followed by a flow-through SCR catalyst. The controller was developed based on an analysis of the data generated from a Millbrook London Transport Bus (MLTB) test cycle fed into a validated model of the SCR-filter and SCR units. The critical system parameters that showed strong correlation with outlet nitrogen oxides (NO_x) and ammonia (NH₃) emissions were first identified, and then the sensitivity of those parameters was analyzed. The most sensitive system parameters were configured as the controller gain parameters. A proportional controller based on the key parameters with optimized gains settings for the MLTB cycle delivered over a 10% reduction in cumulative NO_x emission over the cycle compared to a fixed NH₃/NO_x ratio (ANR) controller. The controller was shown to be robust to errors in sensor measurement in that it delivered significantly less variation in performance compared to a fixed ANR controller.