Post-combustion emission controls

Nitrogen selectivity in lean NO_x trap catalysis with diesel engine in-cylinder regeneration

Authors
J.E. Parks II, S.P. Huff, J.A. Pihl, J.-S. Choi, B.H. West

Publication
SAE Paper No. 2005-01-3876, 2005 SAE Powertrain & Fluid Systems Conference and Exhibition.

Abstract
NO_x emissions have traditionally been difficult to control from diesel engines; however, lean NO_x trap catalysts have been shown to reduce NO_x emissions from diesel engines by greater than 90% under some conditions. It is imperative that lean NO_x traps be highly selective to N₂ to achieve the designed NO_x emissions reduction. If selectivity for NO_x 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 NO_x trap regeneration with in-cylinder techniques are presented. Engine dynamometer studies with a light-duty engine were performed, and a lean NO_x 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 NO_x trap catalyst.