In this computational research, the separate and simultaneous impacts of diesel direct injection timing, fuel spraying cone angle, and hydrogen gas addition on combustion characteristics, output emissions, and performance in a single-cylinder direct injection diesel engine was studied. In order to conduct the simulations, valid and reliable models for combustion, break-up, and for turbulence was used. The effects of fifteen fuel injection strategies based on characteristics such as time of fuel spraying (-15, -10 CA BTDC, and TDC) and nozzle cone angle (105, 115, 125, 145, and 160 degrees) under neat diesel combustion and diesel-hydrogen combustion engine operations conditions were explored. The obtained results indicated that the addition of H2 due to significant heating value has increased indicated power and improved indicated specific energy consumption at the expense of NOx emissions but considerably decreased CO and soot emissions simultaneously. By advancing injection timing, maximum pressure peak point, maximum temperature peak point, and maximum heat release rate peak point have increased and caused lower indicated specific energy consumption. However, using a wide spray angle (e.g., 160 cone degrees), resulted in lower indicated power and higher indicated specific energy consumption due to more fuel could spray in regions with lower oxygen concentrations compared to baseline operation case.