The diesel engine commonly introduces high boost pressure to achieve high engine efficiency. This extra air is supposed to significantly influence liquid vaporization. In experiment, surrounding gas conditions of 7-46.8 kg/m 3 were prepared by rapid compression and expansion machine RCEM. The nozzle hole length to diameter L/D ratio of 2.77, 3.73, 4.44 and 6.94 were used corresponding to orifice diameter of 0.072-0.180 mm. The close-up region from nozzle tip to 20 mm downstream was focused to simultaneously capture the vapor and liquid phases using shadowgraph and light scattering technique respectively. The information extracted from the images was then used for estimation of fuel mixture in the nearfield spray incorporated with a simple 1D jet model. The result showed that liquid length was dominated by both gas density including gas temperature and rate of fuel injection. The vapor cone angle showed tendency to increase with L/D ratio decreased. The widest vapor cone angle was found at L/D ratio of 2.77 which corresponded to achievement of the highest mass of fuel in vapor phase. It was an evidence that atomization is also one of the essential factors to improve vaporization. At the same gas temperature of 890 K, the fuel mass in liquid phase considerably reduced with increasing gas density. An increase in gas density resulted in substantially increase in entrainment. With identical orifice diameter of 0.180 mm, the smaller L/D=2.77 reported better atomization, shorten liquid length and enhanced entrainment compared with L/D=4.44.

The diesel engine commonly introduces high boost pressure to achieve high engine efficiency and reduce soot emission. This extra air is supposed to significantly influence mixing process and improve combustion efficiency. In experiment, surrounding gas densities of 11.7 -46.8 kg/m 3 were prepared by rapid compression and expansion machine RCEM. The nozzle hole length to diameter L/D ratio of 2.77 -6.94 were used corresponding to orifice diameter of 0.072 -0.180 mm. The high-speed imaging with soot and NOx measurement were arranged. Flame temperature and KL factor were analyzed based on two-color method. The result showed that with identical orifice size nozzle of D=0.180 mm, the shorter hole length nozzle of L=0.5 mm (L/D=2.77) provided higher entrained gas amount and higher vapor fuel/air mixture was achieved at near-field region. This near-field mixture was found to consistently behave throughout the injection period as it was responsible for shorter ignition delay and combustion drastically promoted with shorter combustion duration. At high boosted gas condition, the lower amount of soot produced by the shorter hole length nozzle was prominently exhibited due to strong combustion resulting in higher flame temperature and soot oxidation performed intensively during late combustion phase. Additionally, NOx emission was found to be a function of equivalence ratio of vapor fuel/air mixture at upstream of ignition and strongly related with flame temperature. The combustion and emission showed significant correlation with near-field spray characteristics.