Spray characteristics of diesel fuel injection system is one of the most important factors in diesel combustion and pollutant emissions especially in HSDI (High Speed Direct Injection) diesel engines where the interval between the onset of combustion and the evaporation of atomized fuel is relatively short.

An investigation into various spray characteristics from different holes of VCO (Valve Covered Orifice) nozzles was performed. The global characteristics of spray, including spray angle, spray tip penetration, and spray pattern were measured from the spray images, which were frozen by an instantaneous photography with a spark light source and ICCD. These spray images were acquired sequentially from the first injection to fifth injection to investigate injection-to-injection variation. For better understanding of spray development and their internal structures, a long-distance microscope was used to get magnified spray images at the vicinity of the nozzle hole with a laser sheet illumination. Also backward illuminated images with a spark light source were taken at various points of the spray field including vicinity of the nozzle hole to understand surface structures and breakup process of dense spray from VCO nozzle incorporated with common-rail injection system.

As injection pressure increases interaction between spray and ambient air becomes important to liquid penetration and spray angle. Macroscopic spray angle increases due to air entrainment as injection pressure increases though spray angle near the hole seems independent from injection pressure. Liquid penetration is initially affected by injection rate increase as needle is moving upward and liquid penetration increase rate is in accordance with injection pressure. After this stage, air entrainment and high potential of evaporation makes the increase rate slower and this tendency is more obvious for higher injection pressure. Microscopic images taken at the vicinity of the nozzle hole exit reveal that central dense region consists of thick ligaments or membranes and most of the liquid droplets are formed at the tip of ligaments from spray surface due to the waves developed on it. Some smaller liquid droplets seem to be generated from the bubble or membrane breakup process. Droplet sizing was performed from the microscopic images, which were frozen by spark light source that has light duration of 10ns and high-resolution CCD camera equipped with long distance microscope whose magnification factor is more than six. Fuel particle sizes, described as SMD (Sauter Mean Diameter) in many points, decreased during injection durations and higher injection pressure induced smaller value.

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