Three Way Catalyst-Selective Catalytic Reduction Aftertreatment System Evaluation for a Lean Burn Gasoline Engine Operating in Homogenous Charge Compression Ignition, Spark-Assisted Compression Ignition, and Spark-Ignited Combustion Modes

Low temperature and dilute homogenous charge compression ignition (HCCI) and spark-assisted compression ignition (SACI) can improve fuel efficiency and reduce engine-out NO_x 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 NO_x 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 NO_x 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 NO_x conversion efficiency of only 10%, while lean/rich SI operation resulted in a maximum NO_x 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 NO_x. Calculated potential fuel efficiency improvement relative to stoichiometric SI is 7–17% for lean/rich HCCI/SACI with zero tailpipe NO_x and −1 to 5% for lean/rich SI with zero tailpipe NO_x 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 NO_x conversion efficiency could be enhanced, improvements in fuel efficiency could be realized while meeting stringent tailpipe NO_x standards.