Spray and combustion characteristics of methanol dual-fuel engine for marine application

Abstract

Methanol is attracting increasing attention as a low-carbon fuel for marine engines; however, its low cetane number and high latent heat of vaporization pose challenges for stable ignition and combustion. In this study, the spray, ignition, and combustion characteristics of a diesel–methanol dual-fuel injection system were experimentally investigated using a marine-scale multi-hole injector in a constant-volume combustion chamber under engine-relevant conditions. High-speed optical diagnostics were employed to analyze spray development, spray cone angle (SCA), ignition behavior, and heat release characteristics. The spray experiments showed that methanol consistently exhibits a larger SCA than diesel under all tested ambient pressures. At identical injection timings, the time-averaged SCA of methanol was approximately 0.2–13.4% larger than that of diesel, mainly due to its lower surface tension, which promotes enhanced breakup and radial dispersion despite its higher viscosity. In contrast, diesel exhibited up to 10% longer spray tip penetration owing to its higher density and momentum flux. Combustion experiments demonstrated that dual-fuel ignition and heat release behavior are strongly governed by the dwell time between diesel pilot and methanol injections. Overlapping methanol injection with early diesel ignition increased the peak heat release rate by up to 8%, whereas delayed methanol injection resulted in smoother heat release and extended combustion duration. Ignition delay showed only minor variation across most conditions, indicating that diesel pilot injection controls global combustion initiation, while increasing methanol injection duration extended the overall combustion duration by up to 2 ms. These results provide quantitative insight into spray–combustion interactions in diesel–methanol dual-fuel systems and highlight the importance of injection strategy optimization for stable and efficient low-carbon marine engine operation.