With developing in computer technology, three-dimensional (3D) computational fluid dynamics (CFD) internal combustion (IC) engine simulations, which generally use reduced chemical kinetic mechanisms and simplified combustion models, can provide more accurate results along with less initial investment and calculation costs compared to experimental setup. In this study, a heavy-duty diesel engine effects on performance, combustion, and emission characteristics by spraying natural gas from the intake port and transforming it into a spark-ignition engine were investigated through 3D ansysfortecfd program. The spark time was accepted as 0.5 °CA before top dead center (bTDC), which was the start of injection time for the diesel injector. Analyzes were carried out at 2300 rpm, full load, 17.5: 1 of high compression ratio, constant air/fuel ratio. Six different global mesh sizes were used in the converted engine model. Performance, in-cylinder combustion, and emission values were examined for these six different global mesh sizes and the most suitable one was tried to be found. As a result of the global mesh size study, it was concluded that the most suitable size was 2.25 mm. In terms of performance, when the data obtained with usage of natural gas were compared to that of diesel fuel, the gross indicated power (GIP), indicated mean effective pressure (IMEP), and indicated thermal efficiency (ITE) values were increased by 12.02%, 8.93%, and 8.7%, respectively, while the gross indicated specific fuel consumption (GISFC) value was decreased by 9.78%. When the emission values were examined, it was seen that the engine met the stage IIIB norms without usage of selective catalytic reduction (SCR), diesel particulate filter (DPF), and diesel oxidation catalyst (DOC) under the conditions.