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Diesel engine performance and emissions are strongly coupled with fuel atomization and spray processes,which in turn are strongly influenced by injector flow dynamics.Modern engines employ micro-orifices with different orifice designs.It is critical to characterize the effects of various designs on engine performance and emissions.In this study,single injectors which have different nozzle size and wall thickness are employed.The size distribution of spray was conducted with the Phase Doppler Particle Analyzer(PDPA)and the combustion characteristics of these nozzles were experimentally investigated in the constant volume chamber.Time-resolved image of the natural luminosities(indicator of soot)from the combustion process were captured by high speed camera and combustion pressure was also acquired by pressure transducer mounted at the top of the chamber.Results indicate that the Sauter Mean Diameter(SMD)decreases significantly whereby enhancing injection pressure.The SMD decreases with the increase of the length-diameter ratio for the same wall thickness and the SMD decreases with the increase of the length-diameter ratio arranged from5.38 to7.69 with respect to 0.13mm orifice.The way that decreases SMD reduces greatly soot and improved heat release ratio.This paper explains the effect of different nozzle geometry on droplet size distribution and combustion and provides the theory basis for injector manufacture.