Ammonia coverage ratio and input simultaneous estimation in ground vehicle selective catalytic reduction (SCR) systems

Abstract

In this work, we aim to design an observer which can estimate the state and the input simultaneously for a ground vehicle diesel engine selective catalytic reduction (SCR) system. The observer design method is based on the system model, derived from the main chemical reactions. By assuming that the states of the SCR system are homogenous inside and the SCR cell is a continuous stirred tank reactor, a three-state nonlinear model is obtained. To estimate the ammonia coverage ratio and input simultaneously for the nonlinear SCR system, the high-gain observer method is adopted in this paper. The available measurements of the SCR system are the ammonia (NH3) concentration and the NOx concentration. As the NOx sensor is cross-sensitive to the NH3 concentration, the NOx sensor reading is corrected with the precise NH3 sensor measurements. It is further assumed that the engine-out NOx concentration is available since an engine-out emission map can be established and calibrated. With the three physical variables, the ammonia input to the SCR and the ammonia coverage ratio in the SCR are both estimated. The designed observer is applied to a medium-duty diesel engine equipped with an SCR system. It shows from the simulation results that the designed observer works well and the estimation error is small.