Passive SCR for lean gasoline NO_X control: Engine-based strategies to minimize fuel penalty associated with catalytic NH₃ generation

Lean gasoline engines offer greater fuel economy than common stoichiometric gasoline engines. However, excess oxygen prevents the use of the current three-way catalyst (TWC) to control nitrogen oxide (NO_X) emissions in lean exhaust. A passive SCR concept, introduced by General Motors Global R&D, makes use of a TWC that is already onboard to generate NH₃ under slightly rich conditions, which is stored on the downstream SCR. The stored NH₃ is then used to reduce NO_X emissions when the engine switches to lean operation. In this work, the effect of engine parameters, such as air-fuel equivalence ratio and spark timing, on NH₃ generation over a commercial Pd-only TWC with no dedicated oxygen storage component was evaluated on a 2.0-liter BMW lean burn gasoline direct injection engine. NO_X reduction, NH₃ formation, and reductant utilization processes were evaluated, and fuel efficiency was assessed and compared to the stoichiometric engine operation case. Air-fuel equivalence ratio was found to be one of the most important parameters in controlling the NH₃ production; however, the rich operation necessary for NH₃ production results in a fuel consumption penalty. The fuel penalty can be minimized by adjusting spark timing to increase rich-phase engine out NO_X emissions and, thereby, NH₃ levels. Additionally, higher engine out NO_X during engine load increase to simulate acceleration resulted in additional fuel savings. A 10% fuel consumption benefit was achieved with the passive SCR approach by optimizing rich air-fuel equivalence ratio and spark timing while also utilizing acceleration load conditions.