Passive SCR Performance Under Pseudo-Transient Cycle: Challenges and Opportunities for Meeting Tier 3 Emissions

The passive ammonia (NH₃) selective catalytic reduction (SCR) system is a potential approach for controlling nitrogen oxide (NOx) emissions from lean-burn gasoline engines based on utilizing NH₃ generated by a three-way catalyst (TWC) during brief periods of fuel-rich engine operation. NH₃ generated by the TWC is stored and available to reduce NOx on a downstream SCR catalyst during subsequent periods of lean engine operation. Control of the overall passive SCR process can be more challenging than current urea-based approach because it depends explicitly on fuel-rich engine operation, which must be implemented in the context of transient engine operation. Under transient engine operation, significant variation in SCR temperatures is also to be expected. With NH₃ storage capacity highly dependent on exhaust temperatures, proper system architecture and engine operating strategy are needed for effective NH₃ utilization over the SCR catalyst. In this study, the performance of a passive SCR system was evaluated on a 2.0-l BMW lean-burn gasoline direct injection engine under 6-mode pseudo-transient cycle. The aim of this work is to understand how various engine operating strategies change the dynamics of NH₃ generation and utilization, and NOx reduction in the passive SCR system. A 5.9% fuel economy improvement relative to stoichiometric-only operation with 0.018 g/mi of NOx + NMHC emissions were demonstrated over 6-mode pseudo-transient cycle; however, CO emissions were twice the emission standard. The results of this work reveal challenges and opportunities for meeting Tier 3 emissions and improving fuel savings benefits.