The main goal of the present study was to explore the impacts of various fuel injection strategies in a heavy-duty Direct Injection (DI) diesel engine operating under diesel-syngas combustion conditions computationally using CONVERGE Computational Fluid Dynamic (CFD) code. The SAGE combustion model coupled with a chemical kinetic n-heptane/toluene/PAH (Poly-Aromatic Hydro-carbons) mechanism that consisted of 71 species and 360 reactions were used to simulate the diesel-syngas combustion process and the formation and oxidation of emissions, e.g., Nitrogen Oxides (NOx), Particulate Matter (PM), Carbon Monoxide (CO), and Unburnt Hydro-Carbons (UHC). The separate effects of main (8 to 18 Crank Angle (CA) Before Top Dead Center (BTDC) with 2 CA steps) and post-injection (35 to 55 CA (After Top Dead Center) ATDC with 5 CA steps) timing of diesel fuel on the combustion characteristics and exhaust gas emissions were investigated under diesel-syngas combustion conditions. The numerical achievements revealed that the substitution part of the diesel with a CO–H2 gaseous mixture led to a considerably lower PM and UHC emissions in the exhaust gases with a CO penalty rate. Maximum Combustion Temperature (MCT) and Heat Release Rate Peak Point (HRRPP) were increased as Main-Injection Timing (MIT) was advanced. Also, advancing MIT led to a considerably higher level of NOx emissions but lower PM formation. Moreover, compared to baseline engine operating conditions, post-injection of diesel at 35 CA ATDC reduced both PM and UHC emissions simultaneously by nearly 26.5 and 89%, respectively.