The diesel engine with direct injection of hydrogen gas has clear advantages over the hydrogen engine with forced ignition of a hydrogen-air mixture. Despite of this, the concept of hydrogen-diesel engine has not investigated until now. In the paper, a detailed study of the working process of hydrogen-diesel engine carried out for the first time. Based on the results of the experimental studies and mathematical modeling, it has established that the behavior of thermo-physical processes in the combustion chamber of hydrogen-diesel engine, in a number of cases, differs fundamentally from the processes that take place in the conventional diesel engines. There have been identified the reasons for their difference and determined the values of the operating cycle parameters of hydrogen diesel engine, which provide the optimal correlation between the indicator values and the environmental performance. For a single-cylinder hydrogen-diesel engine MAN (S/D= 300/240 mm/mm) the concentration of nitrogen oxides in combustion products is:[NOx]= 920 ppm, the mean effective pressure pe,= 9.0 bar, and the indicator efficiency of hydrogen diesel engine ηi= 0.48. Overall, it can be concluded that the implementation of the concept of hydrogen diesel engine, which has been left without due attention today, is quite real and achievable, and problems hindering the development of an efficient and environmentally friendly working cycle of hydrogen-diesel engine, according to the results of this study, can be successfully solved.