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

<div class="section abstract"><div class="htmlview paragraph">Similar to single-brick SCR architectures, the multi-brick SCR systems described in this paper require urea injection control software that meets the NO<sub>x</sub> conversion performance target while maintaining the tailpipe NH<sub>3</sub> slip below a given threshold, under all driving conditions.</div><div class="htmlview paragraph">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 NO<sub>x</sub> conversion, compared to a single-brick system. But in order to maximize the benefit of close-coupling, the urea injection control must maximize the NH<sub>3</sub> stored in the SCRoF.</div><div class="htmlview paragraph">The under-floor SCR catalyst can be used as an NH<sub>3</sub> slip buffer, lowering the risk of NH<sub>3</sub> slip at the tailpipe with some benefit on the global NO<sub>x</sub> conversion of the system. With this approach, the urea injection strategy has a limited control on the NH<sub>3</sub> coverage of the under-floor SCR catalyst.</div><div class="htmlview paragraph">To take more advantage of the under-floor SCR catalyst for improving the NO<sub>x</sub> conversion, the NH<sub>3</sub> 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 NH<sub>3</sub> 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 NO<sub>x</sub> conversion efficiency while preventing excessive tailpipe NH<sub>3</sub> slip, even in the worst driving conditions, such as sudden full-load acceleration.</div></div>