Control of a Combined SCR on Filter and Under-Floor SCR System for Low Emission Passenger Cars

Similar to single-brick SCR architectures, the multi-brick SCR systems described in this paper require urea injection control software that meets the NOx conversion performance target while maintaining the tailpipe NH3 slip below a given threshold, under all driving conditions. The SCR architectures containing a close-coupled SCRoF and underfloor SCR are temperature-wise more favorable than the under-floor location and lead to significant improvement of the global NOx conversion, compared to a single-brick system. But in order to maximize the benefit of close-coupling, the urea injection control must maximize the NH3 stored in the SCRoF. The under-floor SCR catalyst can be used as an NH3 slip buffer, lowering the risk of NH3 slip at the tailpipe with some benefit on the global NOx conversion of the system. With this approach, the urea injection strategy has a limited control on the NH3 coverage of the under-floor SCR catalyst. To take more advantage of the under-floor SCR catalyst for improving the NOx conversion, the NH3 coverage of the under-floor SCR must be taken into account, and therefore a combined control of both catalysts is required. This paper presents a control strategy proposal for such multi-brick SCR systems: the NH3 coverage of the SCRoF is optimized while taking into account the current states of front and rear SCR catalysts. The strategy focuses on achieving highest NOx conversion efficiency while preventing excessive tailpipe NH3 slip, even in the worst driving conditions, such as sudden full-load acceleration.