Passive TWC+SCR Systems for Satisfying Tier 2 Bin2 Emission Standards on Lean-Burn Gasoline Engines

Joseph  Theis, Ford Motor Company

A laboratory study was performed to assess the potential capability of passive TWC+SCR systems to satisfy the Tier 2 Bin 2 emission regulations on lean-burn gasoline applications.  After accounting for cold-start emissions, stoichiometric emissions, and engineering tolerances, target levels for HC, CO, and NOx during lean/rich cycling were established.  30 s lean/10 s rich tests were performed on a reduced-OSC Pd/Rh TWC operating at 550 C and different volumes of a Cu/zeolite SCR catalyst operating at 330 C, where the NO concentration was held at 220 ppm during the lean periods and varied from 500 to 2000 ppm during the rich periods.  The NOx slip target was satisfied with the equivalent of 3.3L of SCR volume and 1500 ppm NO rich.  Reducing the OSC in the TWC would increase its NH3 yield, but some OSC is needed for stoichiometric NOx lightoff, warmed-up 3-way activity, steam reforming of the HC during the rich purge periods, and diagnostic capabilities.  The optimum temperature range of the TWC was 600-650 C due to the short delay in NH3 generation, high NH3 yields, and good steam reforming capability.  SCR samples aged at 800 C on a durability schedule that alternated between stoichiometric, rich, and lean operation had low NH3 storage capacity, but samples aged on the same schedule at 700 C maintained high NH3 storage capability.  The addition of an underbody TWC improved the steam reforming capability during the rich purges.  A close-coupled (CC) TWC + underbody (U/B) TWC + SCR catalyst system satisfied the NOx slip target with lean times up to 60 seconds and lean NO levels up to 600 ppm.  A two-step purge strategy reduced the CO slip and allowed the NOx, HC, and CO targets to be met.  Sulfur poisoning decreased the NH3 yield and steam reforming capability of the TWC, but the performance was recovered after a rich desulfation at 700 C.  The SCR catalyst was relatively robust to the effects of sulfur poisoning.  It was concluded that periodic exposure of the CC TWC and U/B TWC to high temperatures, such as those that occur during the US06 test, would minimize the accumulation of sulfur and maintain good NH3 yield and steam reforming capability.  As a result, the CC TWC + U/B TWC + SCR catalyst system should maintain good performance without the need for active desulfations.

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