Modeling of a DOC SCR-F SCR Exhaust Line for Design Optimization Taking Into Account Performance Degradation Due to Hydrothermal Aging

摘要

With the upcoming Euro 6c emission regulations, the performance of Diesel exhaust lines needs to be improved to meet NO_X and soot emission targets. A promising exhaust line architecture to reach these requirements is the association of a Diesel Oxidation Catalyst (DOC), a Selective Catalytic Reduction coated on a particulate filter (SCR-F) and a Selective Catalytic Reduction (SCR) catalyst. To develop this system, the car manufacturers have to face several challenges. One of the first is the design of the exhaust line volumes, which has a strong impact on the light-off temperatures of the catalysts and so on system performance. Then, urea injection has to be optimized with an adapted control system to maximize NOx reduction while keeping low tailpipe ammonia emission. Moreover, performance degradation of catalysts due to harsh exhaust conditions during vehicle life time have to be detected by OBD system. To meet these challenges, system simulation approaches allow to create virtual exhaust lines and perform, in a quite short time and with reduced cost, a large number of tests over a wide range of operating conditions. In this work, an exhaust line composed of a DOC, a close coupled SCR-F and an underfloor SCR has been modeled using both synthetic gas bench and engine bench tests data. From this reference, hydrothermal aging impact on the SCR catalysts has been studied on synthetic gas bench using several aging conditions. Using these experimental data combined with results from previous work on catalyst aging modeling, a hydrothermal aging model has been developed to represent the evolution of the SCR catalyst performance. This model has then been validated on data from a complete exhaust line having aged SCR catalysts. To finish, different changes in the exhaust line geometry have been investigated by simulation, moving the under-floor SCR to a close coupled position with volume variations. The impact of these modifications on the exhaust line performance has then been confirmed using test bench results.This methodology has proven to be very efficient in modeling a complete Diesel Euro 6c after-treatment exhaust line, allowing to easily take into account the impact of exhaust line geometry or catalyst aging in order to optimize this complex system.