Abstract: The near-field spray cone angle is one of the significant indicators that evaluate the primary atomization quality in the near-field area, the dynamic characteristics of which has a remarkable impact on the subsequent secondary atomization and combustion performance.However, the variation of near-field spray cone angle in the whole spray process is rarely studied.In order to make up for this work and provide a new reference for improving atomization performance, the characteristics of near-field spray cone angle of diesel jets 4 mm away from the nozzle tip under different injection pressures, back pressures and nozzle diameters were experimentally investigated by photography technique with the help of backlighting.An ultra-high speed digital camera of the Photron Fastcam SAZ type was employed with the maximum shooting frequency of 210000 fps (frames per second), and a fixed resolution of 384×160 pixels and a pixel size of 20 μm × 20 μm were obtained; and the exposure time of 250 ns was selected, which greatly reduced the trailing phenomenon.In addition, the high-resolution long distance microscope with the working distance ranging from 56 to 140 cm was employed.And a high pressure diesel fuel pump driven by a converter motor provided a controllable fuel pressure up to 160 MPa to the common rail.The injection pulse width was set as 1500 μs in the experiment and 3 different injection pressures were chosen, namely, 40, 60 and 80 MPa.Besides, the nozzle diameters were 0.12, 0.16 and 0.18 mm respectively and 4 back pressures were selected (0.1, 2, 3 and 4 MPa).To thoroughly investigate the variation characteristics of near-field spray cone angle, the whole spray process was divided into 3 stages, i.e., the beginning stage, the steady stage and the ending stage based on the needle valve lift curve while the injection pressure was 80 MPa.The results indicated that near-field spray cone angle presented a three-stage development, and there were 2 peaks (the beginning-period peak and the ending-period peak) and a period of stable fluctuation.During the beginning stage of injection, the beginning-period peak appeared earlier with the higher injection pressure, its value became higher, and simultaneously the duration of this stage was shorter, indicating the better starting-injection performance.However, the back pressure had a little influence on the near-field spray cone angle, and the development profiles of near-field spray cone angle under various back pressures were nearly superimposed during a period of time before and after the peak when the internal flow of the nozzle dominated the spray process; and then the distinction among different curves was gradually evident as the aerodynamic force played a major role in the spray development.Furthermore, the starting-injection performance was not affected by the back pressure.During the ending stage of injection, with the increase of the injection pressure and back pressure, the ending-period peak appeared later, however, the value of the peak was larger.Meanwhile, the fuel injection duration was longer as the injection pressure and back pressure increased when the injection pulse width was set as 1500 μs.During the steady stage of injection, a rise appeared in the figure for near-field spray cone angle with the increase of the back pressure.Nevertheless, the spray cone angle showed a decreasing trend with the increase of the nozzle diameter, and as for this phenomenon, there were 2 reasons to explain, one was the hydraulic flipping regime led to complete detachment of liquid from the hole wall, and the other was the small aperture spray was susceptible to the influence of small scale air entrainment and had the large surface curvature which would result in the bigger cone angle value for the small aperture spray.Taking the fact into consideration that the aerodynamic force governed the spray process in steady stage, the latter explanation was supposed to be the more reasonable one.Consequently, the injection pressure has more significant influence on the near-field spray than the back pressure, and the nozzle hole with smaller diameter can obtain the better atomization quality.