In the present paper the issue of external heat balance in the diesel engine is presented. The methodology of heat balance estima- tion, as well as the comparison of the researches results concerning engine with the common rail injection system, which is fed by mineral diesel oil and BMD biofuel has been presented. The impact of biofuel on the nature of changes in the heat balance has been estimated. The researches were performed on the measurement station based on the 13 points ESC test. It has been stated that the use of fuel assigned in the ESC test in the engine fed by biofuel (BDM) slightly increases in comparison to the engine fed by mineral diesel oil (ON). However, there can be noticed the change of the flow of energy input in the heat balance of the engine fed by both types of fuel.