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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.

Engine technologies using efficient combustion and down-sizing turbo have become important in order to reduce automotive CO2 emissions. However, the exhaust gas temperature also becomes lower by these technologies. As a result, the catalyst performance becomes lower. Therefore it is necessary to develop low temperature active catalysts to reduce emissions. This research was focused on Pd/CeO2, and it’s able to oxidize CO at low temperatures. In order to increase the catalyst activity, the addition of some elements to the CeO2 was studied. Zn addition was found to have an advantage to reduce the CO light off temperature by 60 °C. Then, we tried to clarify the cause of improvement. As a result, it made clear that the Zn addition promotes the active oxygen release from the CeO2 surface. However, repeated engine exhaust gas tests indicated a decline in purification performance.

Recently, Spark-Assisted HCCI has been developed to improve combustion robustness. On the other hand, there needs to be an aftertreatment system which can purify both stoichiometric and lean exhaust gas to meet stringent emission regulations. The difficulty of exhaust aftertreatment under lean conditions is not only NOx conversion but also HC conversion. This is because in addition to the lower exhaust gas temperature there is more paraffin in the HCCI gas. A new catalyst system which includes TWC at the close-coupled position and a lean catalyst at the underfloor position was developed. The underfloor lean catalyst has the ability of HC conversion (especially paraffin) and of NOx conversion. This concept for lean exhaust from Spark-Assisted HCCI can achieve high NOx and HC conversion efficiencies.