A robust ammonia coverage ratio control method for a two-cell selective catalytic reduction system in low temperature operations

The control of Diesel engine selective catalytic reduction (SCR) systems during low temperature operation is of great challenge due to the complicated urea-to-ammonia conversion process. The existing model-based SCR controls may fail to achieve their control objectives in the situation since most of them adopted SCR models with the assumption of complete urea decomposition at the front portion of catalyst. To ensure low tailpipe NO_x emissions and ammonia slip, a nonlinear backstepping and nonlinear damping based robust controller was proposed. The control objectives are to maintain a low ammonia loading and a medium ammonia loading in the upstream cell and downstream cell, respectively. Simulation results under the FTP-72 cycle demonstrate high robustness of the proposed controller in regulating the ammonia coverage ratios against the uncertainties from the delayed urea-to-ammonia conversion process. Comparison with a previously reported controller shows the advantage of the robust controller in limiting the tailpipe ammonia slip without significant loss of deNO_x capability.