The work of the thesis has been focused on the study of the PCU system, which consists of piston, piston rings and liner, of a 12,9 L 6-cylinders in-line natural gas internal combustion engine for heavy duty application. The final goal is the evaluation of the friction of the PCU system and blow-by flowrate through a predictive model implemented in the program GT Power representative of a single crank. The thesis is organized starting from the current scenario about air pollution and emission regulations with a theoretical description of the phenomenon and the part of the engine contributing to friction and blow-by, with a focus on the part modeled in GT Power.