The burnt fraction f of Wiebe equation has been shown to be dependent only on the newly defined parameter 'combustion burn factor (C-i)'; and the benefits of expressing heat release rate with respect to C-i have been presented. The errors associated with the determination of apparent heat release rate (Ahrr) and the cumulative heat release (Cum.Hrr) from the measured cylinder pressure data and the assumed specific heat ratio (gamma) was determined and compared. The gamma affected the calculated Ahrr more than the cylinder pressure. Overestimation of gamma resulted in an underestimation of the peak value of the Ahrr and vice versa, this occurred without any shift in the combustion phasing. A new methodology has been proposed to determine the instantaneously and mean value of gamma for a given combustion. This new methodology has been applied to determine gamma for a wide range of engine operating conditions and for different fuels. (C) 2014 Elsevier Ltd. All rights reserved.