Effects of NO_X Storage Component on Ammonia Formation in TWC for Passive SCR NO_X Control in Lean Gasoline Engines

A prototype three-way catalyst (TWC) with NO_X storage component 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 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. Adding a NO_X storage component to a TWC provides two benefits in the context of a passive SCR system: (1) enabling longer lean operation by storing NO_X upstream and preserving NH₃ inventory on the downstream SCR catalyst; and (2) increasing the quantity and rate of NH₃ production during rich operation. Since the fuel penalty associated with passive SCR NO_X control depends on the fraction of time that the engine is running rich rather than lean, both benefits (longer lean times and shorter rich times achieved via improved NH₃ production) will decrease the passive SCR fuel penalty. However, these benefits are primarily realized at low to moderate temperatures (300-500 °C), where the NO_X storage component is able to store NO_X, with little to no benefit at higher temperatures (>500 °C), where NO_X storage is no longer effective. This study discusses engine parameters and control strategies affecting the NH₃ generation over a TWC with NO_X storage component.¹