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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.