2021 年 12 巻 2 号 p. 62-71
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/m3 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.
