The leading aftertreatment technologies for NO{sub}x removal from the exhaust gas of lean burn engines, Diesels in particular, are urea based Selective Catalytic Reduction (SCR), Lean NO{sub}x Traps (LNT) and Active Lean NO{sub}x Catalysts (ALNC). It is generally believed that the SCR technique has the potential of providing the best NO{sub}x conversion efficiency relative to the other techniques. Nonetheless, it is crucial that the high conversion efficiencies be achieved with a minimum slippage of unreacted ammonia as tail pipe emissions. This necessitates a precise control over the urea injection process. The complex behavior of the catalyst substrate with respect to adsorption and desorption of ammonia in conjunction with a lack of "stored ammonia" sensing capabilities makes the control problem challenging. In this paper we present a model-based control design approach using a lumped parameter model of an SCR system that includes the essential dynamics of the plant. The model includes the adsorption, desorption and surface coverage dynamics, along with the NO{sub}x reduction and ammonia oxidation dynamics based on the relevant chemical reaction rates.
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