Aftertreatment Options and Challenges for Lean-Burn Gasoline Engines

Robert  McCabe, Ford Motor Company

The quest for improved fuel economy has rekindled interest in lean-burn gasoline engines.  A direct injection engine with stratified-charge capability can provide leaner A/F ratios and consequently better fuel economy than those obtainable with a lean-burn port fuel injected (PFI) engine.  Nevertheless, challenges remain to obtain benefits in emissions and/or fuel economy relative to those achievable with alternative combustion strategies such as EGR and variable cam and/or valve timing.  In regards to emission control, the principal challenges are high engine-out NOx and HC emissions, the thermal durability requirements, and the need to achieve high NOx efficiencies over a broad temperature window.  In this talk, we consider both the emission and fuel economy consequences of several potential aftertreatment options for lean gasoline engines, including lean NOx traps (LNT), urea-SCR, and LNT+in-situ SCR (i.e., LNT+SCR).  While the main focus is on NOx emissions, we also briefly consider the challenges associated with hydrocarbon (HC) and particulate matter (PM) emissions – all with an eye toward comparing and contrasting lean gasoline emission challenges with those of diesel engines.  Lean NOx traps do not require an extra reductant on-board, but the temperature window can be limited and the fuel penalty associated with the rich purges can cut significantly into the fuel economy benefits of lean operation.  Urea-SCR technology eliminates the need for the rich purges and can potentially offer a broader temperature window, but it requires an additional reductant on-board (e.g., urea).  Also, relative to its use on diesel engines, the application on lean-burn gasoline engines presents new challenges in the dynamic control of urea injection as well as SCR catalyst durability.  The LNT+SCR concept eliminates the need for the additional reductant on-board and can expand the temperature window relative to the LNT alone, but the purge fuel penalty still reduces the fuel economy benefit of lean operation.  The strong interactions between lean combustion characteristics, engine controls, and aftertreatment system durability and efficiency point to a need for model-driven system selection and optimization.

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