Ammonia Generation and Utilization in a Passive SCR (TWC+SCR) System on Lean Gasoline Engine

Journal Article · Tue Apr 05 00:00:00 EDT 2016 · SAE International Journal of Engines

DOI:https://doi.org/10.4271/2016-01-0934· OSTI ID:1261347

Prikhodko, Vitaly Y. ^[1]; Pihl, Josh A. ^[1]; Toops, Todd J. ^[1]

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1. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)

Lean gasoline engines offer greater fuel economy than the common stoichiometric gasoline engine, but the current three way catalyst (TWC) on stoichiometric engines is unable to control nitrogen oxide (NOX) emissions in oxidizing exhaust. For these lean gasoline engines, lean NOX emission control is required to meet existing Tier 2 and upcoming Tier 3 emission regulations set by the U.S. Environmental Protection Agency (EPA). While urea-based selective catalytic reduction (SCR) has proven effective in controlling NOX from diesel engines, the urea storage and delivery components can add significant size and cost. As such, onboard NH₃ production via a passive SCR approach is of interest. In a passive SCR system, NH₃ is generated over a close-coupled TWC during periodic slightly rich engine operation and subsequently stored on an underfloor SCR catalyst. Upon switching to lean operation, NOX passes through the TWC and is reduced by the stored NH₃ on the SCR catalyst. In this work, a passive SCR system was evaluated on a 2.0-liter BMW lean burn gasoline direct injection engine to assess NH₃ generation over a Pd-only TWC and utilization over a Cu-based SCR catalyst. System NOX reduction efficiency and fuel efficiency improvement compared to stoichiometric engine operation were measured. A feedback control strategy based on cumulative NH₃ produced by the TWC during rich operation and NOX emissions during lean operation was implemented on the engine to control lean/rich cycle timing. At an SCR average inlet temperature of 350 °C, an NH₃:NOX ratio of 1.15:1 (achieved through longer rich cycle timing) resulted in 99.7 % NOX conversion. Increasing NH₃ generation further resulted in even higher NOX conversion; however, tailpipe NH₃ emissions resulted. At higher underfloor temperatures, NH₃ oxidation over the SCR limited NH₃ availability for NOX reduction. At the engine conditions studied, greater than 99 % NOX conversion was achieved with passive SCR while delivering fuel efficiency benefits ranging between 6-11 % compared with stoichiometric operation.

• Research Organization: Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Fuels, Engines and Emissions Research Center (FEERC); Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). National Transportation Research Center (NTRC)
• Sponsoring Organization: USDOE Office of Energy Efficiency and Renewable Energy (EERE)
• Grant/Contract Number: AC05-00OR22725
• OSTI ID: 1261347
• Journal Information: SAE International Journal of Engines, Vol. 9, Issue 2; ISSN 1946-3936
• Publisher: SAE International
• Country of Publication: United States
• Language: English

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