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The purpose of this study was to investigate the combustion characteristics of a direct-injection-type homogeneous charge compression ignition (HCCI) engine. From this experimental study, we found that the diesel HCCI combustion phenomenon occurred in two stages of a combustion pattern, which are the cool flame and the hot flame. To investigate the combustion and emission characteristics of the HCCI engine, we evaluated the influence of intake air temperature, pressure, and an additive on HCCI combustion and emission performance characteristics; in particular, we focused on those characteristics of the cool and hot flame, the auto-ignition time, and the indicated mean effective pressure (IMEP) under various engine running conditions. This research showed that, as the intake temperature was increased and the additive was used, the onset angle of cool and hot flames and the starting time of auto-ignition were advanced; moreover, the influence of intake conditions (pressure, temperature) affected the cool flame and the hot flame simultaneously, whereas the additive mainly affected the cool flame more than the hot flame. In the higher-speed regions, the rate of the hot flame varied according to the air:fuel ratio; yet, in the lower-speed regions, an inverse trend occurred. This result was determined based on the time needed to reach a critical temperature for H2O2 decomposition. In the rich-mixture region, the ignition delay was inversely proportional to the intake temperature; however, in the lean-mixture region, an inverse trend occurred. An advancement of the auto-ignition time increased the HCCI engine output; however, excessive advancement decreased the IMEP and also increased the NO x emissions, because of knocking.

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