Ship propulsion systems are facing increasing demands in terms of energy efficiency and environmental friendliness. In order to meet the set requirements, various measures or technologies are applied to reduce the emission of harmful substances that occur during the combustion process in the engine cylinder. The paper analyzes the impact of primary measures that have a direct impact on the combustion process, as well as on the formation of NOX. An appropriate computer program has been developed to conduct research using a numerical model of engine operation. The verification of the numerical model and the computer program was carried out using the data provided by the manufacturer and the data obtained by measurements during sea trials. Using a numerical model of a diesel engine, a study of the impact of later onset of injection, earlier closing of the intake valve (Miller process) and split injection on efficiency and NOX emission was conducted. The impact of these measures on engine operation was analyzed with the aim of optimizing the operation of a mediumspeed diesel engine in the diesel-electric propulsion system of the ship. The application of a moderate later start of fuel injection leads to a reduction in NOX emissions with a relatively small reduction in efficiency and a slight increase in exhaust temperature. Earlier closing of the intake valve with a corresponding increase in the pressure in the intake manifold allows a significant reduction in NOX emissions without reducing efficiency. The results of the study showed that the application of an appropriate split fuel injection scheme allows to reduce NOX emissions with a relatively small reduction in efficiency. Comparison of the obtained results with the basic engine shows that the simultaneous application of these measures can achieve a significant reduction in NOX emissions with minimal impact on efficiency, which was the goal of the work.