Cycle driven optimisation 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 after-treatment system. The SCR units consist of an integrated SCRcoated filter followed by a flow-through SCR catalyst. The controller was developed based on 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 with strong correlation with outlet NOx and NH3 emissions were first identified, then the sensitivity of those parameters was analysed. The most sensitive system parameters were configured as the controller gain parameters. A proportional controller based on the key parameters with optimised gains settings for the MLTB cycle delivered over 10% reduction in cumulative NOx emission over the cycle compared to a fixed NH3/NOx 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.