Browse Publications Technical Papers 2018-01-5040
2018-10-24

Study of an Aftertreatment System for Homogeneous Lean Charge Spark Ignition (HLSI) Lean-Burn Engine 2018-01-5040

Lean-burn is an effective means of reducing CO2 emissions. To date, homogeneous lean charge spark ignition (HLSI) combustion, which lowers emissions of both CO2 and NOx, has been studied. Although an HLSI combustion mode can result in lower in-cylinder emissions, 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.
Although an HLSI engine exhaust gas realizes low concentrations of NOx, it also contains high concentrations of hydrocarbons, including a large amount of paraffin, which are difficult to purify. 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 lower emissions under stoichiometric 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. A three-way catalyst (TWC) was used in the position nearest to the engine to remove NOx, CO, and HC during stoichiometric operation, 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 purification performance under lean conditions was confirmed.

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