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To achieve carbon-neutrality, internal combustion engines need to further improve their thermal efficiency to reduce CO2 emissions. To accomplish this, it is necessary to quantify and enhance five factors that control indicated thermal efficiency: compression ratio, specific heat ratio, combustion duration, combustion timing, and heat transfer to wall. In this work, quantitative targets for each factor were defined, which were derived from a simulation that considered the influence of heterogeneity of diesel combustion on thermal efficiency. The simulation utilized a two-zone combustion model. In particular, the targets for the combustion duration, combustion timing and heat transfer to wall were increased significantly compared to those for a conventional engine, in anticipation of an expansion of the load range of premixed charge compression ignition (PCI) combustion to higher loads.

The combustion phenomena in an IDI diesel engine having a swirl chamber were detected by optical methods. The spatial and temporal distribution of the natural emission of OH radical was measured through the band pass filters and those of KL factor and flame temperature were obtained by the two-color method. As a consequence, the combustion phenomena are made clear by these experimental results.