A mathematical model of combustion process in a diesel engine has been developed according to the theory of chain reactions for the higher hydrocarbon compounds. The instantaneous rates of fuel vaporization and combustion are defined in terms of the current values of temperature, pressure, concentration of fuel vapors, overall diffusion rate, fuel injection rate, and mean fuel droplet size in terms of the SMD. Numerical experiments have been carried out for investigating the interdependency between various combustion-related parameters. Specifically, the effect of fuel droplet size (in terms of SMD) on the subsequent combustion parameters, such as, pressure, temperature, thermodynamic properties of air/gas mixture, heat transfer, fuel vaporization, combustion rate, current A/F ratio and gas mixture composition. In addition the integral indicator parameters of the engine, such as, mean indicated pressure, peak pressure, compression pressure have been analyzed.