Diesel spray characteristics of multi-hole injectors under geometrical similarity condition

Small-bore engines are popular due to low CO2 emissions and low fuel consumption. To simplify the design steps for various engine sizes, similarity theory was applied to engines. In this paper, the spray characteristics of non-evaporating sprays for multi-hole diesel injectors under the geometrical similarity condition were investigated by diffused background illumination (DBI). The similarity theory is based on Chikahisa theory, Wakuri momentum theory, and Inagaki analysis of geometrical similarity. Under the similarity condition, the injectors with hole diameters of 0.07mm, 0.101mm, and 0.133mm were used at injection pressures of 45 MPa, 93 MPa, and 160 MPa. In addition, the injection pressure of 160 MPa was set as the same injection pressure condition for comparison. The results show that under the similarity condition and the same injection pressure conditions, spray tip penetration, spray angle, and other parameters follow the same trend of variation of hole diameters, except for the air-fuel ratio. The scaled spray tip penetration and the scaled spray cone angle (the angle at 30 times the hole diameters from the nozzle tip of the injector) of the three injectors under the similarity condition are well matched. However, for the injector with a hole diameter of 0.133mm, the scaled spray angle (the angle at half of the spray tip penetration) is larger than others at the start of the injection, and the scaled air-fuel ratio is larger throughout the process. The difference in internal flow is responsible for the variation of the scaled spray parameters of the injector with 0.133mm hole diameter compared to the others.