Staircase ammonia coverage ratio profile control for Diesel engine two-cell selective catalytic reduction systems

This paper presents a Diesel engine selective catalytic reduction (SCR) ammonia surface coverage ratio profile control design for a two-cell SCR system, which enables a staircase ammonia coverage ratio profile for simultaneous high NO_x conversion efficiency and low tailpipe ammonia slip. The novel two-cell SCR architecture greatly increases the system complexity, and controlling ammonia surface coverage ratios of two cells at different levels by a single control input is not trivial. To address these challenges, a nonlinear backstepping based control law is proposed to regulate the ammonia coverage ratio of the upstream cell to a desired (high) value in order to maintain high SCR NO_x conversion efficiency, and to keep the ammonia coverage ratio of the downstream cell below a low upper limit for high ammonia adsorption capability. Simulation studies based on the first phase of the FTP75 cycle showed that the proposed controller can regulate the ammonia coverage ratios in each cell well and can satisfy the control requirements.