Automotive Selective Catalytic Reduction System Model-Based Estimators for On-ECU Implementation: A Brief Overview

The amount of ammonia stored on the walls of the catalyst (or ammonia storage) is a parameter with significant impact on NO_x reduction efficiency and undesired ammonia slip of Selective Catalytic Reduction catalysts. This makes the ammonia storage interesting for utilization in urea injection control. However, ammonia storage is not directly measurable onboard vehicles, it can only be estimated. Model-based online estimation requires models that are capable of capturing the main phenomena of the SCR and at the same time can be computed onboard vehicle. While the modeling of SCR and model-based control is well present in the literature, it is apparent that few attempts of implementing the models on production ECUs were published. This paper reviews literature on ammonia storage, outlet NH₃ and NO_x concentration estimation in SCR and SCR/DPF systems-including the estimation of NO_x sensor cross-sensitive to NH₃-in order to present the state of the art. The discussion includes applications of estimators (virtual sensors), accuracy of estimation, required inputs, computational demands, robustness and sources of error.