A Numerical Investigation of the Effects of the Fuel Injection Pressure and Nozzle Hole Diameter on Natural Gas–Diesel Dual-Fuel Combustion Characteristics

Natural gas–diesel dual-fuel (NDDF) engines can reduce harmful emissions while maintaining diesel-like efficiency. However, under low-load conditions, they suffer from high methane (CH4) emissions, reduced combustion stability, and lower thermal efficiency. To address and improve these issues, this study numerically investigates the effects of the injection pressure (32, 50, 90, and 126 MPa) and nozzle hole diameter (NHD, 110–230 μm) on dual-fuel combustion. A total of 25%, 50%, and 75% natural gas energy fraction (NGEF) conditions are simulated for dual-fuel cases, and fully diesel-fueled conditions are also studied. The results at 50% and 75% NGEF indicate that increasing the injection pressure significantly improves thermal efficiency while reducing CH4 and soot emissions. Furthermore, at 75% NGEF, NHD reduction from 230 to 150 μm provides more stable combustion rates, higher thermal efficiency, and lower CH4 emissions. At 75% NGEF, the combination of 126 MPa of injection pressure and 150 μm of NHD reduces CH4 emissions by 77% and increases thermal efficiency by 9.8% compared to the baseline case (32 MPa and 230 μm). This study demonstrates that optimal combinations of injection pressure and NHD can significantly improve low-load issues in NDDF engines.