Abstract: To improve total efficiency of a single hydraulic free piston diesel engine, the energy saving methods were studied. The power transmission process was analysed. The relation between input, lost and output energy were calculated. The energy saving approach for system components was analysed with experimental results. The results indicate that the system efficiency reaches about 38% in theory. The measured system efficiency of the prototype is about 27.5%. The theory and experimental results indicate that the system efficiency is higher than that of the conventional assemble of internal-combustion engine and hydraulic pump. The indicated thermal efficiency of the internal-combustion engine part is closed to the high level of the conventional. Higher dynamic response valves can improve the pump chamber efficiency of the hydraulic pump part. Smaller throttle effects can increase the circular efficiency of the high-pressure and compression chamber.