A contribution has been given to the thermodynamics approach usually used for analyzing the combustion process in IC engines on the basis of cylinder pressure data reduction. A survey of heat release type combustion models and of their calibration methods has first been carried out with specific attention paid to the bulk gas-wall heat transfer correlations used. Experimental results have given evidence that most of these correlations are incapable of predicting the phase shift occurring between the gas-wall temperature difference and the heat transfer during the engine compression and expansion strokes, owing to the transient properties of the fluid directly in contact with the wall. This work develops and applies a refined procedure for heat release analysis of cylinder pressure data including the unsteadiness effects of the convective heat transfer process. The new heat release model is based on a two-zone description of the cylinder contents with thermodynamic properties of both reactants and products evaluated by polynomial curve fits and a multiple species equilibrium composition calculation performed for the burnt zone. A complex Nusselt number model is proposed for surface-averaged instantaneous heat-flux evaluation. Comparisons between the model predictions and measured data are reported for different engines and operating conditions. The model is finally applied for the analysis of the pressure data taken in the pent-roof combustion chamber of a multivalve SI engine.