Low temperature and dilute homogenous charge compression ignition (HCCI) and spark-assisted compression ignition (SACI) can improve fuel efficiency and reduce engine-out NOx emissions, especially during lean operation. However, under lean operation, these combustion modes are unable to achieve Environmental Protection Agency (EPA) Tier 3 emissions standards without the use of lean aftertreatment. The three way catalyst (TWC)-SCR lean aftertreatment concept investigated in this work uses periodic-rich operation to produce NH3 over a TWC to be stored on a selective catalytic reduction (SCR) catalyst for NOx conversion during subsequent lean operation. Experiments were performed with a modified 2.0 L gasoline engine that was cycled between lean HCCI and rich SACI operation and between lean and rich spark-ignited (SI) combustion to evaluate NOx conversion and fuel efficiency benefits. Different lambda values during rich operation and different times held in rich operation were investigated. Results are compared to a baseline case in which the engine is always operated at stoichiometric conditions. SCR system calculations are also presented to allow for comparisons of system performance for different levels of stored NH3. With the configuration used in this study, lean/rich HCCI/SACI operation resulted in a maximum NOx conversion efficiency of only 10%, while lean/rich SI operation resulted in a maximum NOx conversion efficiency of 60%. If the low conversion efficiency of HCCI/SACI operation could be improved through higher brick temperatures or additional SCR bricks, calculations indicate that TWC-SCR aftertreatment has the potential to provide attractive fuel efficiency benefits and near-zero tailpipe NOx. Calculated potential fuel efficiency improvement relative to stoichiometric SI is 7–17% for lean/rich HCCI/SACI with zero tailpipe NOx and −1 to 5% for lean/rich SI with zero tailpipe NOx emissions. Although the previous work indicated that the use of HCCI/SACI increases the time for NH3 to start forming over the TWC during rich operation, reduces NH3 production over the TWC per fuel amount, and increases NH3 slip over the SCR catalyst, if NOx conversion efficiency could be enhanced, improvements in fuel efficiency could be realized while meeting stringent tailpipe NOx standards.

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