One of the five jets issued from a five-hole injector used for gasoline direct injection（GDI） engines was carefully studied with the phase Doppler particle analyzer（PDPA） and high-speed imaging techniques to investigate the effect of back pressure on a fuel jet development. The tests were carried out in a constant volume vessel with the absolute ambient pressure ranging from 0.05 to 0.50 MPa. The result shows that the trajectory deviation of the target jet is observed under different back pressures

which is due to the pressure difference between the inner and outer sides of the target jet. The trajectory deviation affects the droplet size distribution. In addition

the spray at the sub-atmospheric or atmospheric ambient pressures exhibits more complicated process than that at elevated ambient pressure. Under the sub-atmospheric and atmospheric conditions

the spray tip becomes obtuse and the droplet velocity at different radial positions shows a “double peak” distribution inside the spray within a certain time

rather than a “single peak” distribution at elevated ambient pressure. It is believed that the ambient pressure imbalance caused by high-speed jet and the nozzle exit velocity distribution resulting from lower vapor pressure of gasoline could account for the difference.