Temperature and air–fuel ratio dependent specific heat ratio functions for lean burned and unburned mixture

The most important thermodynamic property used in heat release calculations for engines is the specific heat ratio. The functions proposed in the literature for the specific heat ratio are temperature dependent and apply at or near stoichiometric air–fuel ratios. However, the specific heat ratio is also influenced by the gas composition in the engine cylinder and especially becomes important for lean combustion engines.In this study, temperature and air–fuel ratio dependent specific heat ratio functions were derived to minimize the error by using an equilibrium combustion model for burned and unburned mixtures separately. After the error analysis between the equilibrium combustion model and the derived functions is presented, the results of the global specific heat ratio function, as varying with mass fraction burned, were compared with the proposed functions in the literature. The results of the study showed that the derived functions are more feasible at lean operating conditions of a spark ignition engine.