Modelling of passive ammonia selective catalytic reduction system for lean-burn spark ignition direct injection engines

Lean-burn spark ignition direct injection (SIDI) engines offer potential fuel economy savings; however, the lack of simple and cost-effective lean nitrogen oxide (NO _x ) aftertreatment systems hinder their broad application. Lean NO _x traps (LNT) and urea selective catalytic reduction (SCR) catalysts have been widely investigated as possible lean gasoline aftertreatment technologies, but with limited acceptance.

Passive ammonia SCR operation is a promising low-cost alternative for lean NO _x aftertreatment with SIDI engines. The aftertreatment system includes close-coupled three-way catalyst (TWC) converters and one or more under-floor SCR catalysts. Ammonia (NH₃) is formed on the TWC with short pulses of rich engine operations and the generated NH₃ is then stored on the under-floor SCR catalysts. During subsequent lean operations, the NO _x that breaks through the TWC converters is reduced to nitrogen by NH₃ stored on the SCR catalysts.

This paper provides an overview of system level analysis as a complement to experimental testing in the development of a passive NH₃ SCR-based aftertreatment system. Various aftertreatment architectures (SCR type, the order, and location of the catalysts) were efficiently evaluated and screened, to achieve the best trade-off between engine back-pressure, catalyst warm-up, emission constraints, and acceptable thermal ageing under high-speed/load conditions.