In this paper, formulation of a mathematical model for flame propagation in a spark ignited reciprocating engine has been described. As against the common practice of assuming a laminar flame propagation model with a suitable multiplying factor for turbulence effects, a more logical approach has been adopted bringing in the concept of an engine Reynolds parameter as the criterion to determine the combustion acceleration due to turbulence. The laminar burning velocity is calculated from Semenov's thermal model for the instantaneous unburned gas condition during combustion and is augmented for engine gas turbulence through an empirical function of the engine Reynolds parameter assuming wrinkled flame model. The turbulent flame velocity thus obtained is then corrected for flame transportation due to the expansion of the burning gases. The empirical constants in the formula have been evaluated in correlation to results of extensive experimentation on a single cylinder variable compression ratio research engine.