An Optical Investigation of the Effects of Diesel Injection Pressure and Intake Air Temperature in an Ammonia-Diesel Dual-Fuel Engine under Low-Load Conditions

This study investigates the combustion and flame development characteristics of an ammonia-diesel dual-fuel (ADDF) engine under low-load conditions, comparing it with conventional diesel combustion (CDC) mode. An optical engine system equipped with high-speed natural luminosity imaging and post-processing techniques was used to analyze the effects of diesel injection pressure and intake air temperature on combustion dynamics and flame propagation in the ADDF concept. Results indicate that under low-load conditions, the ADDF mode exhibits delayed ignition, lower peak in-cylinder pressure, and decreased peak apparent heat release rate (AHRR) compared to CDC with combustion efficiency decreasing from 82.63 % to 65.55 %. In the CDC mode, the blue premixed flame envelops the bright yellow diffusion flame. In contrast, the early combustion stage in ADDF shows fewer ignition kernels and develops primarily into a more dispersed yellow-green premixed flame of diesel-ignited ammonia, with lower intensity and larger unburned regions. Increasing the diesel injection pressure from 80  MPa to 120  MPa effectively increases the proportion of diesel premixed combustion, favoring the peak AHRR and improving combustion efficiency to 82.41 %. This results in earlier ignition kernel formation in the ADDF mode, creating a broader, continuous yellow-green premixed flame distribution and reducing soot radiation areas. Elevating the intake air temperature from 313.15  K to 393.15  K decreases ignition delay and improves combustion efficiency and stability. Higher temperatures facilitate faster ignition, increasing the number of ignition kernels and accelerating flame development. Overall, flame intensity is enhanced, and both the diesel-ignited ammonia premixed and diffusion flame areas are expanded. Overall, for a low-load condition with a 60 % ammonia energy ratio in the ADDF mode, the collaborative optimization of diesel injection pressure and intake air temperature can achieve a combustion efficiency of 92.15 % and a flame distribution that nearly fills the entire combustion chamber. These findings provide new insights into the optimization of the ADDF combustion mode under low load conditions and highlight the potential for sustainable and efficient dual-fuel engine applications.