The heat release analysis has proved to be a powerful diagnostic tool for the analysis of the combustion process in spark ignition engines. Still, a fine tuning of the heat transfer correlations embedded in the heat release models is necessary for a correct diagnostic analysis of the pressure signal. To that end, a new methodology has been developed and assessed to properly locate the end of combustion on the basis of the heat release intensity. The results produced by the proposed method have been compared to those obtained by applying different methodologies available in the literature. The newly developed method has proved to be accurate and consistent and has allowed a reliable estimation of the end of combustion on a cycle-by-cycle basis. An extensive burn rate analysis has also been accomplished by means of a heat release model previously developed and purposely modified to embed the new end of combustion detection procedure. The main combustion related quantities have been considered for the experimental investigation to appropriately quantify the engine cyclic variability as a function of the relative air-to-fuel ratio. The experimental tests have been performed on a naturally aspirated 2 L engine featuring a fast-burn combustion chamber and running on gasoline and natural gas as well as on a 1.2 L turbocharged natural gas engine displaying a disk shaped combustion chamber. The diagnostic tool has proved to properly match the nonlinear behavior of the quantities related to the combustion duration in the cycle-resolved analysis and a general good agreement with previous works has emerged as far as the coefficient of variations of the main combustion parameters are concerned. Moreover, thanks to the automatic facet the proposed methodology retains, it is strongly recommended when an extensive cycle-by-cycle and cylinder-to-cylinder analysis needs to be performed. Finally, regardless of the considered fuel, the heat release model embedding the EOC detection procedure proved to be capable of properly detecting the combustion features induced by a fast-burn combustion chamber with respect to a traditional one. As a matter of fact, smaller Delta 0(10-90%) values and an overall reduced cyclic dispersion were highlighted for the 2 L engine. (c) 2012 Elsevier Ltd. All rights reserved.