Optical investigation on impacts of ambient pressure on macroscopic spray characteristics of ducted fuel injection under non-vaporizing conditions

•DFI and free spray under different ambient pressures were compared to each other.•Advantages of DFI over free spray under higher ambient pressure is more significant.•Mushroom-shaped spray of DFI spray under different ambient pressure was investigated. Ducted fuel injection (DFI) which mimics Bunsen burner, is a cutting-edge technology for reducing the soot formation of heavy-duty diesel engines. In this work, the Mie-scattering optical method was used to visualise impacts of ambient gas pressure on DFI and free spray characteristics with a single-hole injector. Ambient gas pressure varies from 3 to 12 MPa and diesel injection pressure from 40 to 120 MPa. Spray morphology, tip penetration, cone angle and area were discussed. The results indicated that DFI spray has a distinct velocity advantage over free spray, especially under high ambient pressure. As ambient pressure increases, DFI spray has better cone angle and area advantages than those of free spray. Increasing injection pressure helps to promote these advantages. Additionally, when the ambient pressure is high enough, a mushroom-shaped spray appeared at the duct outlet, diffusing actively around and forming a large-scale vortex. All of these characteristics of DFI spray can be interpreted as better serving the fuel-gas mixing as ambient pressure increases.