A mathematical model of combustion process in a diesel engine has been developed. The combustion process is considered to have both the premixed and diffusion flame. The combustion of fuel vaporized during the self-ignition delay period is modeled according to the conditions of premixed flame. A kinetic differential equation has been formulated for modeling this kind of combustion. The combustion of fuel during the injection process is modeled according to the theory of diffusion flames. This process is strongly influenced by processes of fuel injection, vaporization and diffusion. The atomization process is taken into account by means of the Sauter mean diameter (SMD) of fuel droplets. The instantaneous vaporization rate is defined by the current values of temperature, pressure, concentration of fuel vapors and the mean fuel droplet size in terms of the SMD. The mathematical model includes differential equations describing the processes of fuel injection, vaporization, heat transfer and combustion in both the premixed and diffusion flame that takes place in the engine cylinder. The above equations are solved together with the differential equation of the first law of thermodynamics expressing the energy conversion process in the cylinder of diesel engine. The fourth-order Runge-Kutta method is applied for obtaining numerical solution of the system of differential equations. The analysis is performed on a PC using FORTRAN 90. The results have been simulated for a marine direct injection (DI) diesel engine (Model Silzer 6RLB-66) having a cylinder bore diameter of 0.66 m, and stroke of 1.4 m. The amount of fuel used in this engine during the experiments is 0.03785 kg per cycle per cylinder. The numerical experiments have been carried out for the effect of duration of fuel injection and the beginning of fuel injection (expressed in terms of degrees of crank angle before TDC) on the subsequent combustion parameters and the integral indicator parameters of the engine.