Control-Oriented Modeling of a LNT-SCR Diesel After-Treatment Architecture
Résumé
Lean NOx trap (LNT) and Selective Catalytic Reduction catalysts (SCR) are two leading candidates for Diesel NOx after-treatment. Each technology exhibits good properties to reduce efficiently Diesel NOx emissions in order to match the forthcoming EURO 6 standards.NOx reduction in LNT is made through a two-step process. In normal (lean) mode, Diesel engine exhausts NOx is stored into the NOx trap; then when necessary the engine runs rich during limited time to treat the stored NOx. This operating mode has the benefit of using onboard fuel as NOx reducer. But NOx trap solution is restrained by limited active temperature windows. On the other hand, NH3-SCR catalysts operate in a wider range of temperature and do not contain precious metals. However, NH3-SCR systems traditionally use urea-water solution as reducing agent, requiring thus additional infrastructure to supply the vehicles with enough reducer. These pros and cons are quite restrictive in classical LNT or NH3-SCR architecture.The present paper presents an after-treatment architecture combining a NOx trap and a passive NH3-SCR. Synergy of the two systems is possible if the SCR takes advantage of the LNT ability to produce Ammonia (NH3). Indeed, during the rich phases (purges), small amounts of Ammonia are formed as by-product, which can be used in the downstream catalyst as the NOx reducing agent.A major difficulty to operate the proposed architecture is the real-time management of the NOx purge: for future control and diagnosis applications, it is crucial to have accurate but low-complexity models. A complete reduced model of the physicochemical phenomena involved is proposed in the present paper. Based on simplified chemical assumptions and time scale separation, the latter is suitable for on-board diagnostics and model-based control. Validation has been achieved through extensive experiments.