The paper presents the results of research performed on an internal combustion diesel engine operating on a dual-fuel strategy. Fuels used during the study were diesel as a reference fuel, pure hydrogen, and ammonia as a hydrogen carrier. Fuel mixtures of diesel/hydrogen and diesel/ammonia were added to the engine, at energetic shares of 8, 12, 22, and 32% to maintain the constant engine load compared to the engine fueled with diesel. A comparison of the engine performance and the combustion process was made. Under the test conditions for all mixtures, the indication of the combustion pressure was made. The indication results were used to calculate engine performance parameters, such as IMEP, engine power, COVIMEP, and thermal efficiency. They were also used to describe the combustion process, including the determination of parameters like combustion stages, 50% MFB position, and HRR. The analysis of the results shows that adding hydrogen or ammonia as a main fuel causes a much faster combustion process compared to a diesel fuel engine. With an increase in energetic share, the combustion characteristic of the diesel/hydrogen engine shows two maxima in the HRR curve, besides 32% where there is only one peak. In contrast, for the diesel/ammonia engine, there is only one maximum, where an increase in the value of the kinetic combustion phase and the delay of ignition occur.