Grounds for the use of the bond graph method in the modeling of internal combustion (IC) engines, which are energy sources in systems consisting of different physical nature elements have been presented. The model of hybrid vehicle, as an example of such system, has been presented. Examples of definitions of the main engine parameters, which follow the established BG convention have been given. A proposal of formalization of the IC engine characteristics, which follows the selected method of modeling, has been presented as well as an analytic form of the characteristic as the multidimensional vector function. Example of the engine characteristic approximation with the use of the “Spline” functions and author’s computer programs has been given.