In this study, a diesel tractor engine is numerically modeled to run entirely on natural gas by installing a natural gas fuel injector on the intake manifold and assembling a spark plug in lieu of a diesel injector. In the numerical study, the methane chemical kinetic mechanism representing natural gas, Reynolds-averaged Navier–Stokes (RANS) k–e turbulence, and the G-equation combustion model were used. The spark-ignition time was assumed to be 719.5 crank angle degree (CAD), which was the start of the diesel injection time. Analysis was carried out at 2300 revolutions per minute (rpm), at a high compression ratio of 17.5:1, at a fixed spark-ignition time, and at eight different equivalence ratios under full load. The equivalence ratio was changed by keeping the air mass constant and reducing the mass of the fuel. The effects of the obtained equivalence ratio on engine performance, combustion characteristics, and emission values were investigated. The results revealed that natural gas could be used up to Φ = 0.60 without affecting performance, increasing emissions, or exceeding the knock limit compared with the diesel cycle. In addition, it was observed that ultralow (below 10 particulate per million (ppm)) emission values could be obtained by further reducing the equivalence ratio, providing a uniform thermal field.