A new method has been proposed to determine the values of the form factor (m) and efficiency factors (a) of the Wiebe equation. This is achieved by developing a modified form of Wiebe equation with only one constant. The modified version of Wiebe equation facilitates the determination of constants accurately, which enhances the accuracy of evaluating the burnt fraction. The error induced on the burnt fraction f with respect to the values of constants a and m obtained through different methods are discussed and compared. The form factor affects the burnt fraction significantly compared to the efficiency factor. A new non-dimensional parameter 'combustion burn factor (Ci)' has been identified in the modified Wiebe equation. The burn fraction f was found to be a function of Ci only, thus the benefits of expressing heat release rate with respect to Ci have been presented. (C) 2013 Elsevier Ltd. All rights reserved.