Browse Publications Technical Papers 2010-01-0366
2010-04-12

Passive Ammonia SCR System for Lean-burn SIDI Engines 2010-01-0366

Lean-burn Spark Ignition Direct Injection (SIDI) engines offer potential fuel economy savings, however, lack of cost-effective lean NOx aftertreatment systems has hindered its broad application. Lean NO Trap (LNT) and Urea Selective Catalytic Reduction (SCR) technologies have been widely investigated as possible solutions, but they both have considerable drawbacks. LNT catalysts suffer from high Platinum Group Metals (PGM) cost, poor thermal durability, sulfur poisoning and active SO regeneration requirements. Urea SCR systems require a secondary fluid tank with an injection system, resulting in added system cost and complexity. Other concerns for urea SCR include potential freezing of the urea solution and the need for customers to periodically fill the urea reservoir. In this paper we report a low-cost, high efficiency concept that has the potential to be a key enabler for lean-burn gasoline engines. The aftertreatment system includes a close-coupled three-way catalytic converter (TWC) and one or more underfloor SCR catalysts. NH₃ is formed on the TWC during short periods of rich engine operations and the generated NH₃ is then stored on the underfloor SCR catalysts. During the subsequent lean operations, the NO that breaks through the TWC converter is converted by the NH₃ stored on the SCR catalysts. Test results during the New European Driving Cycles (NEDC) showed that very high (≻85%) lean NO conversion efficiencies were achieved with aged converters. The potential and remaining issues of this new concept will be discussed. HC emission reduction is challenging due to the low exhaust temperatures. Further improvements in SCR catalyst technologies are required for high speed (≥100 km/h) lean operations. In summary, passive NH₃ SCR has been demonstrated as an efficient and low cost lean NO aftertreatment technology for stratified gasoline engines.

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