A computer program, which applies the group combustion theory with a comprehensive treatment of thermal radiation to diesel engines, has been developed. A quasi-steady simulation including gas and liquid droplets flows, gas-to-droplets and droplets-to-droplets interactions, turbulence, finite chemical rate, and radiation heat transfer was made. The fuel droplets' penetration, evaporation, mixing with air, and combustion can be clearly simulated.Four distinct regions were identified in the spray plume: namely, preheat, incipient combustion, intense combustion and diffusion zones. Group combustion numbers that account for the most significant in-cylinder combustion factors, such as fuel/air motion, fuel/air ratio, droplet size, droplet spacing and droplet number density, possess decisive influence on the sizes of these zones. To design a smart diesel engine of better combustion efficiency, lower peak flame temperature and smaller engine size, one needs to select an optimum group combustion number to minimize the preheat and diffusion zones and maintains proper incipient and intense combustion zones.Coupling of thermal radiation and combustion has a significant effect on diesel engine combustion processes: the concentration distributions of soot and combustion gases, which are required to quantify thermal radiation, can be obtained from combustion calculations, while, the enhanced radiation heat transfer from these soot and combustion gases has a great effect on combustion calculations.