Abstract
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.
