Ammonia Generation over TWC for Passive SCR NO_x Control for Lean Gasoline Engines

摘要

A commercial three-way catalyst (TWC) was evaluated for ammonia (NH_3) generation on a 2.0-liter BMW lean burn gasoline direct injection engine as a component in a passive ammonia selective catalytic reduction (SCR) system. The passive NH_3 SCR system is a potential low cost approach for controlling nitrogen oxides (NO_x) emissions from lean burn gasoline engines. In this system, NH_3 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, NO_x passes through the TWC and is reduced by the stored NH_3 on the SCR catalyst. NH_3 generation was evaluated at different air-fuel equivalence ratios at multiple engine speed and load conditions. Near complete conversion of NO_X to NH_3 was achieved at λ=0.96 for nearly all conditions studied. At the λ=0.96 condition, HC emissions were relatively minimal, but CO emissions were significant. Operation at AFRs richer than λ=0.96 did not provide more NH3 yield and led to higher HC and CO emissions. Results of the reductant conversion and consumption processes were used to calculate a representative fuel consumption of the engine operating with an "ideal" passive SCR system. The results show a 1-7% fuel economy benefit at various steady-state engine speed and load points relative to a stoichiometric engine operation.