Advanced Urea SCR System Study with a Light Duty Diesel Vehicle

U.S. federal vehicle emission standards effective in 2007 require tight control of NOx and hydrocarbon emissions. For light-duty vehicles, the current standard of Tier 2 Bin 5 is about 0.07 g/mi NOx and 0.09 g/mi NMOG (non-methane organic gases) at 120,000 mi. However, the proposed future standard is 0.03 g/mi for NMOG + NOx (~SULEV30) at 150,000 mi. There is a significant improvement needed in catalyst system efficiencies for diesel vehicles to achieve the future standard, mainly during cold start. In this study, a less than 6000 lbs diesel truck equipped with an advanced urea Selective Catalytic Reduction (SCR) system was used to pursue lower tailpipe emissions with an emphasis on vehicle calibration and catalyst package. The calibration was tuned by optimizing exhaust gas recirculation (EGR) fuel injection and cold start strategy to generate desirable engine-out emissions balanced with reasonable temperatures. Major catalyst components were diesel oxidation catalysts (DOCs) followed by SCR catalysts, SCR-coated filter and ammonia slip catalyst. The upstream DOC package options were explored to more effectively eliminate engine-out hydrocarbons while generating heat for the downstream catalyst system. Cu/zeolite SCR formulations were used on traditional flow-through cordierite substrates as well as on high porosity SiC filters. Key system variables included the cold-start strategy, catalyst placement in the system, and substrate design, all having impact on the aftertreatment performance.