An approach is proposed for assessing the exhaust smoke level based on mathematical modeling of processes of formation and gasification of soot in the cylinder of a diesel engine using engine indicator diagrams and the results of testing fuels in conditions of a laminar diffusion flame. It is shown that the physical properties of a fuel affect the degree of combustion according to a diffusion mechanism responsible for sooting. New indexes must be used to determine the degree of sooting and exhaust smoke - the diffusion flame blackness index and the soot gasification index. The validity of the approach is confirmed by the good agreement of the results of the calculation with the mathematical model with the experimental data obtained on a diesel engine setup.