Study of an Aftertreatment System for HLSI Lean-burn Engine 2018-01-0945

Lean-burn is an effective means of reducing CO₂ emissions. To date, Homogenous Lean Charge Spark Ignition (HLSI) combustion, which lowers emissions of both CO₂ and NOx, has been studied. Although HLSI realizes lower emission, it is a major challenge for lean-burn engines to meet SULEV regulations, so we have developed a new aftertreatment system for HLSI engines. It consists of three types of catalysts that have different functions, as well as special engine control methods. As the first stage in achieving SULEV emissions, this study focused on enhancing performance under lean conditions.

HLSI engine exhaust gases contain high concentrations of hydrocarbons, including a large amount of paraffin, which are difficult to purify, rather than low concentrations of NOx. Therefore, the key point in low emissions is to purify not only NOx, but also high concentrations of paraffin at the same time. Other issues include maintaining high performance under stoichiometry operation and reducing N₂O emissions.

To resolve these issues, it is important to focus on the division of catalyst roles and their arrangement, and the modification of catalyst material. In this study, an aftertreatment system with three catalysts was developed. TWC was applied for the first catalyst to purify stoichiometry exhaust gases, and a new type of catalyst was used for the second and third catalysts in order to purify hydrocarbon and NOx under lean conditions. The new catalyst was an improvement based on a lean NOx trap catalyst, and was added to a paraffin purification material that highly enhances PGM activity by suppressing oxygen poisoning in PGMs, which hinders paraffin oxidation. The approach to enhancing NOx purification and reducing N₂O emissions is to arrange the second and third catalysts with the optimal temperature properties in each position. This layout covers a wide temperature range for NOx performance and reduces N₂O emissions.

This system was evaluated on an engine bench using a steady lean-rich cycle test. High performance under lean conditions was confirmed.