The challenge for decreasing the emissions of compression ignition engines now remains mainly on NOx control. If the Lean NOx Trap (LNT) and Selective Catalytic Reduction by Urea (Urea-SCR) are very efficient, their extra-cost and management are a major issue for the OEMs. In that context, the selective catalytic reduction by hydrocarbons (HC-SCR) appears to be an interesting alternative solution, with a more limited NOx conversion efficiency but an easier packaging (diesel fuel as a reductant) and a limited price (reasonable coating cost / no PGM).
In the framework of the RedNOx project, a prototype catalyst made of 2% silver on Alumina coated on cordierite was manufactured and tested on a synthetic gas bench. In parallel, an exhaust implementation study has been led to ensure the most suited conditions for injection. Thanks to SGB and simulation results, adapted engine tests have been designed and performed.
If the results on SGB are consistent with the literature with a maximum NOx conversion reaching about 70% and an operating range between 300 and 500°C, first engine results were very disappointing showing efficiencies lower than 15% for all the operating points. Three guidelines for improving these results were identified and studied:
The way the reductant is introduced in the exhaust, especially the discontinued HC delivery due to the injection pattern
The gas velocity: unusual small GHSV values for automotive applications are needed to ensure a good NOx conversion rate.
The nature of the used reductant. Diesel fuel (ULSD) compared with decane lowers the efficiency, but synthetic Diesel fuel (SD) and, moreover, ethanol, largely improve it, especially for high GHSV.
