An investigation into the post-LNT NH3 generation and NOx slip over an LNT was completed for the purposes of determining the proper calibration for NOx reduction over a downstream SCR. Experiments were conducted in an engine dynamometer on the DW12 2.2L diesel engine that was calibrated for rich operation.
The catalyst system was evaluated at 325°C for three different engine speed/load points. Engine speeds of 1750, 2000, and 2500 rpm were chosen, resulting in a LNT space velocities of 35,000 - 68,000 hr-1. Increased lean times were tested, which resulted in NOx loadings on the LNT that ranged from 0.10 - 0.50 grams NOx/L catalyst. In addition, varied degrees of richness were tested, corresponding to lambda sensor readings of 0.90, 0.92, and 0.94.
LNT performance was evaluated based on it's ability to provide a stoichiometric ratio of reactants to the SCR for NOx conversion. Gross and net NOx conversion was evaluated for a LNT-SCR system. Optimal NOx conversion for this type of system required a balance between providing sufficient reductant for NH3 generation without poisoning the SCR from HC. System performance was controlled by calibration of NOx loading and degree of richness.