Influence of HCCI and SACI Combustion Modes on NH ₃ Generation and Subsequent Storage across a TWC-SCR System

Advanced engine combustion strategies, such as HCCI and SACI, allow engines to achieve high levels of thermal efficiency with low levels of engine-out NO_x emissions. To maximize gains in fuel efficiency, HCCI combustion is often run at lean operating conditions. However, lean engine operation prevents the conventional TWC after-treatment system from reaching legislated tailpipe emissions due to oxygen saturation. One potential solution for handling this challenge without the addition of costly NO_x traps or on-board systems for urea injection is the passive TWC-SCR concept. This concept includes the integration of an SCR catalyst downstream of a TWC and the use of periods of rich or stoichiometric operation to generate NH₃ over the TWC to be stored on the SCR catalyst until it is needed for NO_x reduction during subsequent lean operation.

A laboratory study was performed on a gasoline engine to evaluate the NH₃ generation over a TWC and NH₃ storage over the SCR catalyst for a TWC-SCR system during lean to rich cycling of advanced combustion and for the purposes of comparison, conventional combustion. The results of this study show that the TWC-SCR after-treatment concept results in NH₃ generation and storage for advanced and conventional combustion. However, the following challenges were observed for the TWC-SCR system when used with advanced combustion: 1) NH₃ slip upon entering lean HCCI combustion is relatively high despite the lower catalyst temperature, 2) the time in rich engine operation before NH₃ formation is observed is higher for rich SACI as compared to rich SI, and 3) NH₃ generation over the TWC during rich SACI operation is lower per unit fuel due to the low levels of engine-out NO_x.