Characteristics of HCCI Diesel Combustion Operated with a Hollow Cone Spray

This paper presents two factors for improving the performance and emissions characteristics in HCCI diesel combustion, one is reducing compression ratio and another is changing the injector position. In a previous study, it was shown that HCCI diesel combustion could be realized by utilizing a hollow-cone spray with normal injection pressure. However there remained two major problems of engine instability and increase in BSFC (decrease in brake thermal efficiency). By reducing the compression ratio from 18.8 to 16.8, the engine stability was much improved to the level of conventional diesel combustion and the increase in BSFC became almost half, which was mainly due to the change of combustion phasing. In addition to this, application of 5 mm inside position of the injector realized almost no penalty of BSFC at higher load condition. At around 15 deg BTDC, small increment of pressure rise was observed, which must be due to the occurrence of cool flame, and the heat released during this stage did not vary with load. Then this cool flame energy is shown to be another important factor for the increase in BSFC at lighter load.