Effect of Biodiesel Blends on Urea Selective Catalytic Reduction Catalyst Performance with a Medium-Duty Engine

Testing to investigate biodiesel's impact on the performance of a zeolite-based selective catalytic reduction (SCR) system was conducted. The tests employed a 2004 compliant Cummins ISB with common rail fuel injection, EGR, and variable geometry turbo. This 5.9L, 300HP engine was retrofitted with a Johnson-Matthey DPF + SCR (SCRT^™) system. Testing was conducted over eight steady-state engine operating modes which provided a wide range of exhaust temperature and exhaust chemistry conditions. Fuels tested were a 2007 certification quality ultra-low sulfur diesel (ULSD), as well as a soy derived biodiesel in a B20 blend. B20 produced slightly lower catalyst temperatures and higher NO₂:NO_x ratios relative to ULSD, but no measureable difference in the overall NO_x conversion over the SCR system. The dominant variable influencing SCR performance is the catalyst space velocity, which is unchanged with the use of B20. Testing to show how the soot load on the diesel particulate filter (DPF) could influence NO₂:NO_x and hence NO_x conversion in the SCR revealed a very slight reduction in NO₂ levels for a heavily loaded filter, but showed no change in NO_x conversion. Both engine-out and tailpipe NO_x levels were slightly higher for B20; however urea injection rates were fixed at optimal values for ULSD.