Tiny pilot injection quantity strategy is usually applied under the multi-pilot injection condition to restrain the uncontrollability of the ignition timing of homogeneous charge compression ignition(HCCI)combustion concept.Meanwhile,the single-hole Diesel injector is originally applied in fundamental research to provide insights into the spray characteristics.However,the spray emerging from a realistic multi-hole injector approaches the practical engine operation situation better.In the current paper,the spray characteristics injected by a multihole nozzle and a referential single-hole nozzle for Diesel engines under very small injection amount and different rail pressure conditions were investigated in conjunction with a series of experimental and computational methods.High-speed video observation was conducted to visualize different spray properties.The images of sprays from each nozzle were freeze-captured at 10000 and 100000 fps under the condition of 0.3 mm3/hole injection quantity,80,120,180 MPa rail pressure,1.5 MPa ambient pressure,room temperature,and air environment.The experimental results revealed that the multi-hole nozzle had lower injection rate,longer injection duration,shorter spray tip penetration,wider spray angle and spray cone angle compared to those of the single-hole nozzle.The sensibility of the injection processes and spray behaviors of different nozzles to the rail pressure variation was quite different.Moreover,the CFD simulation was also conducted under the baseline condition to simulate the internal flow and fuel spray transients under the multi-pilot injection conditions,which provided additional discussion and explanations for the mechanisms behind the spray behaviors.