In this paper we tackle issues relevant to model based control design for a Urea based Selective Catalytic Reduction (SCR) process relevant to automotive applications. A three state, control oriented, lumped parameter model of the system is used to investigate essential controllability and observability properties of the Urea-SCR plant. Results from the controllability and observability analysis of both nonlinear and linearized models are shown to have realistic implications. Observer design for predicting gas phase ammonia slip is outlined and results presented. An altered definition of the catalyst efficiency is used in control design. It is shown that this altered definition lends itself readily to control synthesis in the Sliding Mode framework while satisfying the dual control objectives of maximizing $NOx$ reduction and minimizing ammonia slip.

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