Influence of HCCI and SACI Combustion Modes on NH
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 NOx 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 NOx 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 NOx 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 NOx.