This study focused on the primary breakup of spray at the initial injector opening stage at injection to
atmospheric conditions. A real-time mass flow rate instrument was first used to study the injection characteristics
at the injector opening stage. A long distance microscope together with an ultrahigh speed CCD
camera was then employed to study the primary breakup of diesel spray by photography technique with
the help of backlighting. The mechanism of the formation of the mushroom shaped spray head was discussed
and the laminar flow regime was expected to be the main reason for the resultant shape. During
the initial spray stage, the mushroom spray head penetrated almost linearly with respect to time. The
quantification of the fuel mass/spray area ratio suggested that the dispersion of the spray is greatly
improved within a very short time, which is assumed to be caused by the further opening of injector.