More and more requirements are imposed to engines of internal combustion due to the passage of time. These requirements are caused by the necessity to save fuel, to reduce emissions of pollutants into the atmosphere, to improve operational reliability, to reduce size and cost of spent materials, weight, noise level, to simplify manufacturing and operational processes. Use of gas as a fuel in the cities where number of engines is extremely large can significantly reduce environmental pollution. Some countries have separate environmental programs that encourage transfer of engines from gasoline to gas. At present, however, the use of gas engines in road transport is limited due to a number of technical and operational problems. Mathematical calculations and simulations are applied in order to solve this task and prevent possible problems in a manufactured. As part of the research to establish an influence of the diesel fuel ignition portion value in a gas-diesel engine on parameters of its operational process, it is necessary to develop an appropriate calculation method. In this regard, an analysis of methods and programs for calculation of the operational engine process has been carried out, and a method for calculation of an operational process for a gas-diesel engine has been developed in the paper. A computational study has been made in accordance with the developed methodology. The paper has revealed an influence of the diesel fuel ignition portion value on effective and environmental performance of an engine operation. The calculation has been performed for a nominal mode of the engine operation, gas fuel – propane-butane. Design parameters of a ГД-243-engine have been taken as initial data for the calculation. The following dependences have been established: as a diesel fuel ignition portion is increasing (replacement proportion of gas fuel with diesel), there is some decrease in engine power, and under the accepted conditions, there is an increase in CO2 with a decrease in CO and NO.