Paper presents results of modelling gas engine thermal cycle using AVL FIRE and KIVA 3V software. There are described three combustion models used in software. KIVA and FIRE software are used in Institute of Internal Combustion Engines and Control Engineering for analysis of thermal cycle of IC engines. FIRE software gives many possibilities with many combustion models. All used combustion models are dependent on turbulence of flow field before ignition. Comparison of modelling results of thermal cycle of IC engine is presented in paper. Pressure, temperature, heat release and turbulence parameters variations in function of crank angle as well as spatial distribution of above mentioned quantities at selected crank angles were determined.