In this paper experimental data for different diesel engine types are described. The influence of load and speed on piston temperature is discussed. The problem whether higher speeds or higher mean effective pressures are preferable for a given output and for its increase are studied with the aid of experimental curves. Dependence of relative piston temperature on fuel consumption is shown for various engines. The role of engine speed in this dependence was found to be complicated. Piston temperature factors were introduced for practical purposes.