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

A laboratory study was performed to assess the potential capability of passive TWC+SCR systems to satisfy the Tier 2, Bin 2 emission standards for lean-burn gasoline applications. In this system, the TWC generates the NH₃ for the SCR catalyst from the feedgas NO_x during rich operation. Therefore, this approach benefits from high feedgas NO_x during rich operation to generate high levels of NH₃ quickly and low feedgas NO_x during lean operation for a low rate of NH₃ consumption. It was assumed that the exhaust system needed to include a close-coupled (CC) TWC, an underbody (U/B) TWC, and an U/B SCR converter to satisfy the emission standards during the FTP and US06 tests while allowing lean operation for improved fuel economy during select driving conditions. Target levels for HC, CO, and NO_x during lean/rich cycling were established. With a 30 s lean/10 s rich cycle and 200 ppm NO lean, 1500 ppm NO rich and the equivalent of 3.3 L of SCR volume were required to satisfy the NO_x target. The ability of the CC TWC and U/B TWC to promote steam reforming of the HC during the rich periods was crucial for maintaining the HC slip below the target level. A study on the effects of the A/F ratio and temperature on the NH₃ production from the CC TWC revealed that a temperature of 600°C generated high NH₃ yields while providing good steam reforming activity. A two-step purge strategy generated high NH₃ yields while minimizing the CO slip. The two-step purge and a CC TWC + U/B TWC + SCR catalyst system satisfied the HC, CO, and NO_x slip targets. Sulfur poisoning decreased the steam reforming activity of the TWC as well as its NH₃ yield, although the NO_x conversion of the SCR catalyst was not significantly affected by the sulfur. It was assumed that the CC TWC would remain naturally desulfated with routine exposure to hot rich conditions during cold starts, accelerations, and high load operation, thus eliminating the need for an active desulfation strategy.