Visualization study on the impacts of duct length on macroscopic spray characteristics of ducted fuel injection under nonvaporizing conditions

Ducted fuel injection (DFI) is a cutting-edge technology that can significantly alter the spray pattern and boost the air-fuel mixture quality of internal combustion engines. In this work, diesel spray characteristics of ducted fuel injection with different ducts and free spray were investigated in a constant volume chamber under very high back pressure of 6 MPa via Mie-scattering optical technique. The ducts have the same inner diameter of 2 mm but different lengths that range from 6 mm to 18 mm. A single-hole injector with orifice diameter of 0.12 mm was employed. The injection pressure varied from 40 to 120 MPa. Analysis of macroscopic characteristics from three aspects, namely, spray tip penetration, spray cone angle, and spray area were performed. Results showed that a DFI spray with long duct has better spray characteristics among DFI sprays and free spray. The advantage is more pronounced as the injection pressure increases. Spray morphology close to the duct outlet showed that long ducts are more likely to form a mushroom-shaped head. The disappearance of the mushroom-shaped head forms large-scale vortex flow, which is beneficial to fuel-gas mixing.