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To clarify the detailed structure of high pressure fuel spray, 2-D sectional images of non-evaporating fuel sprays in a high pressure vessel were observed by using the laser light sheet of a copper vapor laser. By this system, many sectional and continuous photographs of the same spray were obtained, and were very effective for the detailed observation of the spray inner structure and its developing process. The spray inner structure was very complicated, and its fuel density distribution was very heterogeneous. And for its developing process, the spray advances straight immediately after injected, then meanders, and deforms into a branch-like structure. Advancing downstream, these branches distribute complicatedly and heterogeneously with low density droplets. The heterogeneity is owing to these branches. And, the developing process is divided into four regions. Further, the effects of some parameters on this process were investigated.

It is well known that increasing the fuel injection pressure is effective for improving the diesel engine combustion. While studying the characteristics of the high pressure fuel spray which is injected in a high pressure vessel, the authors found weak shock waves generating around the fuel spray. To investigate the shock waves effect on the fuel spray the authors measured the propagation speed and pressure amplitude as functions of the injection pressure and ambient pressure. The results indicate that shock waves are generated when the fuel injection speed exceeds the ambient sonic speed. Also it was found that the pressure amplitude of shock wave is approximately 10 % of the ambient pressure and the shock waves spread at a sonic speed. The above results make us think that, isn't it possible to use shock waves for combustion improvement.