Ammonia Generation over TWC for Passive SCR NO _X Control for Lean Gasoline Engines

A commercial three-way catalyst (TWC) was evaluated for ammonia (NH₃) 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 NH₃ SCR system is a potential low cost approach for controlling nitrogen oxides (NO_X) emissions from lean burn gasoline engines. In this system, NH₃ 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 NH₃ on the SCR catalyst.

NH₃ generation was evaluated at different air-fuel equivalence ratios at multiple engine speed and load conditions. Near complete conversion of NO_X to NH₃ 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.