## 1 Introduction  
The measurement of exhaust emissions produced by the internal combustion piston engine is monitored and regulated significantly during road vehicles' driving and approval tests. Particular attention has recently been paid to the amount of CO\({}_{2}\) emissions. These depend on the overall efficiency of the ICE, which is determined by the thermal efficiency of the working cycle and the mechanical efficiency of the ICE. Therefore, increasing the thermal efficiency of the ICE and reducing mechanical losses in the engine is still a very topical task in the research and development of piston combustion engines (also due to the considered limits given by the future EURO VII standard). A frequently used procedure for determining the effect of various measures on individual subfunctional groups, measuring mechanical losses by motoring the engine at a test bench with a dynamometer or at a special test station, is used to reduce ICE mechanical losses. In both cases, it is necessary to equip the test equipment with a very accurate sensor for measuring torque, stabilizing operating fluid temperatures (coolant, lubricating oil) of the engine and automated recording and possibly also evaluation of monitored quantities. The shortcoming of today's way of measuring mechanical losses in an engine by motoring is the big difference between the pressure in the cylinder when the engine is motoring and the pressure in the cylinder during standard engine operation (i.e., with the combustion of the mixture) [1,2]. Another way to detect ICE mechanical losses is, for example, to calculate the mean effective pressure and mean indicated engine operating pressure from a thermodynamic analysis of the high-pressure engine indication or to extrapolate the fuel consumption pattern from the load characteristics. These methods provide reliable data on total mechanical losses in the engine when measurements are made correctly. However, to find new technical solutions leading to a reduction in mechanical losses in the ICE, such measurements are relatively expensive in terms of time and operation. Since friction losses in the piston group (piston + sealing piston rings) account for the largest share (about 40 to 50%) of mechanical losses in the ICE, much attention is paid to the impact of the construction and material performance of the piston group on mechanical losses in the ICE. The research is based on theoretical investigations and model calculations of the friction mechanism of piston rings on the cylinder wall and experiments [3,4]. A simplified view of the main items of lost mechanical energy in ICEs and their share of the total loss of mechanical energy in engines is shown in Tables 1 and 2[1]. The data on mechanical losses in the ICE are shown in the tables below. They indicate the predominant proportion of friction losses in the piston group and engine bearings. The main potential for finding possible solutions to reduce friction in the ICE is mainly the friction of piston rings. The movement and load of the piston ring in the engine cylinder are very variable during the stroke. The piston rings must minimise the penetration of lubricating oil from the crankcase into the cylinder and the antiblow function of the cylinder charge into the engine crankcases. It is relatively challenging to optimise the piston ring preload, and the shape of the friction surfaces on the piston rings to reduce friction losses. The value of the coefficient of friction can be reduced through coating technologies on the piston rings and improved lubrication technologies.