Abstract: In a constant volume combustion bomb, three different background gases (He, N₂, CO₂) are used to study the influence of background gas density and fuel temperature on the spray characteristics of a five-hole gasoline direct injection (GDI) injector under non-flash-boiling conditions by high-speed photography. The results show that background gas density is the key factor affecting spray collapse. With the increase of background gas density, the spray collapse shows a trend of strengthening first and then stabilizing, with the turning point around 4 kg·m^-3. When the background gas density is lower than 4 kg·m^-3, the spray plumes would become wider and resultantly the gap between jets becomes narrower with the increase of background gas density, further preventing the external gas from entering the enclosed low-pressure zone. The internal and external pressure difference of spray increases, resulting in the increase of spray collapse degree. When the background gas density is higher than 4 kg·m^-3, the gap between jets disappears, and the spray external and internal gas exchange is blocked. The internal and external pressure difference of spray tends to be stable, which leads to the stability of spray collapse. The change of fuel temperature has little effect on spray width

Key words: GDI spray, non-flash-boiling, background gas density, spray characteristics