Nitrogen Selectivity in Lean NOx Trap Catalysis with Diesel Engine In-Cylinder Regeneration

NOx emissions have traditionally been difficult to control from diesel engines; however, lean NOx trap catalysts have been shown to reduce NOx emissions from diesel engines by greater than 90% under some conditions. It is imperative that lean NOx traps be highly selective to N₂ to achieve the designed NOx emissions reduction. If selectivity for NOx reduction to NH₃ or N₂O is significant then, ultimately, higher levels of pollution or greenhouse emissions will result. Here studies of the N₂ selectivity of lean NOx trap regeneration with in-cylinder techniques are presented. Engine dynamometer studies with a light-duty engine were performed, and a lean NOx trap in the exhaust system was regenerated by controlling in-cylinder fuel injection timing and amounts to achieve rich exhaust conditions. NH₃ and N₂O emissions were analyzed with FTIR spectroscopy. Both engine and bench experiments show that excess reductant delivery during regeneration leads to high NH₃ emissions and poor N₂ selectivity. Specific design of in-cylinder regeneration techniques that minimize excess reductant or allow O₂ purge can optimize N₂ selectivity of the lean NOx trap catalyst.