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Homogenous Charge Compression Ignition (HCCI) combustion offers efficiency improvements compared to conventional gasoline engines. However, due to the nature of HCCI, traditional HCCI combustion can be realized only in a limited operating range. The HCCI operation at high load is limited by excessive combustion noise. In order to maximize the fuel economy benefits of HCCI its operating range needs to be extended to higher loads. In particular, one immediate benefit of an increased load range on the NEDC driving cycle is the avoidance of transitions between SI operation and HCCI operation. In this research a detailed investigation of the fundamental reasons for high combustion noise was performed. Spark-assisted HCCI combustion was found to be a key factor to reduce combustion noise at high load condition.

Engines operating in Homogeneous Charge Compression Ignition (HCCI) combustion mode offer significant benefits of high fuel economy and low engine-out NOx emissions over the conventional spark ignition (SI) combustion mode. However, due to the nature of HCCI combustion, traditional HCCI combustion can be realized only in a limited operating range. High load is limited by the trade-off between ringing (combustion noise) and stability (COV of IMEP). Low load is restricted by the trade-off between NOx emissions and combustion stability (standard deviation of IMEP). The present research is focused on the extension of lo w load limit of HCCI combustion by developing HCCI idle operation. The main obstacle in developing HCCI idle combustion is lack of available thermal energy necessary for successful auto-ignition.

As vehicle fuel economy requirements continue to increase it is becoming more challenging and expensive to simultaneously improve fuel consumption and meet emissions regulations. The Passive Ammonia SCR System (PASS) is a novel aftertreatment concept which has the potential to address NOx emissions with application to both lean SI and stoichiometric SI engines. PASS relies on an underfloor (U/F) SCR for storage of ammonia which is generated by the close-coupled (CC) TWCs. For lean SI engines, it is required to operate with occasional rich pulses in order to generate the ammonia, while for stoichiometric application ammonia is passively generated through the toggling of air/fuel ratio. PASS serves as an efficient and cost-effective enhancement to standard aftertreatment systems. For this study, the PASS concept was demonstrated first using lab reactor results which highlight the oxygen tolerance and temperature requirements of the SCR.