Optical investigation of ambient temperature effects on spray behavior of ducted fuel injection under evaporative conditions for marine methanol–diesel dual-fuel engines

Ducted fuel injection (DFI) stands out as a promising spray optimization technology to address the poor pilot diesel mixing issue plaguing methanol–diesel dual-fuel marine engines, capable of strengthening in-cylinder turbulence and air entrainment to achieve uniform fuel–air premixing and deliver high-efficiency, low-pollution combustion. This paper presents an optical comparative experiment between conventional free spray and two typical DFI layouts (ST2 and CD4.5) under a constant ambient pressure of 40 bar and three high-temperature evaporative ambient conditions (623, 723, 823 K). Macroscopic spray indicators including spray morphology, axial and radial diffusion features were quantitatively characterized to reveal the coupling mechanism between ambient thermal conditions and DFI spray atomization as well as fuel vaporization performance. The experimental results reveal that rising ambient temperatures significantly accelerate fuel evaporation, enlarge spray cone angles and reshape spray penetration evolution laws. The CD4.5 duct configuration greatly facilitates radial spray dispersion, raising the maximum spray cone angle by 75.68% at the cost of slightly weakened spray tip penetration; whereas the ST2 duct dominates axial spray extension and exhibits superior penetration performance under high temperature environments. The comparative results fully validate the powerful tuning capacity of DFI geometry to customize spray spatial distribution. By rationally selecting duct structures to match diverse thermal operating conditions, the pilot diesel spray can be flexibly regulated to form well-mixed methanol–diesel–air mixtures, which effectively promotes combustion and suppresses pollutant generation for dual-fuel engines. This work provides solid optical data support and structural design guidance for the engineering deployment of DFI technologies in low-carbon methanol–diesel dual-fuel marine power systems.