The new method for calculation of engine performance characteristics under variable loading is developed. It is shown, that the increase in values of the torque fluctuations results in corresponding increase in the brake specific fuel consumption (BSFC) and reduction of the engine effective power. Also, the regulator characteristics for both normal and extreme regimes of the engine operation are obtained. These regimes are characterized by the values of the torque variation coefficient 15% and 30%, respectively. The difference between regulator characteristics under variable loading and those obtained within the tests bed runs under constant loads are illustrated.The method for development of the relative operation characteristic plotted in a non-dimensional coordinates is proposed. It is useful for comparison between the performance characteristics of different engines, since their operation characteristics are plotted onto the same diagram.The appropriate equations for calculation of the BSFC increase and the engine effective power reduction with variation of the torque variation coefficient within the test bed runs are derived.