Adaptive SCR Model for MPC Control Including Aging Effects 2015-01-1045
The focus in the development of modern exhaust after treatment systems, like the Diesel Oxidation Catalyst (DOC), the Diesel Particulate Filter (DPF) and Selective Catalytic Reduction (SCR), is to increase on one hand the oxidation rates of Carbon monoxide (CO), HC (Hydro Carbons) and NO (Nitrogen Oxide) and on the other hand the reduction rates of Particulate Matter (PM) and the NOx emissions to fulfill the more and more restricting requirements of the exhaust emission legislation.
The simplest, practical most relevant way to obtain such a dosing strategy of a SCR system is the use of a nonlinear map, which has to be determined by extensive calibration efforts. This feedforward action has the advantage of not requiring a downstream NOx sensor and can achieve high conversion efficiency under steady-state operating conditions for nominal systems. However, such an open-loop approach leads to suboptimal conversion efficiency during transient operation condition and in the presence of system variations (e.g. aging, doser variation). Hence, feedback information and an adaptation criterion for an appropriate SCR life time control is an essential feature to achieve the demanding legislation limits.
Against this background, this proposal presents an ammonia dosing control by an adaptive model predictive controller. The proposed SCR-model estimates the surface coverage of the SCR and the controller calculates the appropriate Adblue dosing. To address variations due to a model/plant mismatch, caused by aging effects an adaptive identification of the model is proposed. To avoid a complex additional identification and required excitation, only a few adaptation parameters, mainly responsible for the aging effects of the SCR are identified. Finally this approach is compared to the nominal dosing for a SCR in simulation. The purpose is to underline the advantages of such an easy adaptable approach concerning a balanced NH3 slip and NOx reduction for aged SCR converters.