Modeling and Control of a Urea-SCR Aftertreatment System
A dynamic system model for simulating the transient performance of a NOx aftertreatment system using Selective Catalytic Reduction with urea as a reductant (urea-SCR) was developed, calibrated for a heavy-duty engine application, and used to develop a closed loop self-tuning control strategy. The closed loop controller was able to reduce the FTP cycle NOx emissions from a Cummins heavy-duty engine by 84% while maintaining the mean ammonia slip below 7 ppm and the peak ammonia slip below 55 ppm. The peak ammonia slip occurred during the LA Freeway phase of the FTP cycle. Components of the urea-SCR aftertreatment system model include a urea dosing system, an exhaust pipe and a fresh vanadia-based SCR catalyst. The urea dosing system model incorporates the evaporation, thermolysis and hydrolysis stages in the conversion of urea to ammonia in the exhaust pipe and on the catalyst.