Numerical simulations of spray autoignition and flame propagation are presented in this paper, with emphasis on the chemical reaction aspect. The mathematical models and the computer code are verified by comparing the numerical simulation results with experimental data for a spray injected into a cold environment. The turbulent autoignition problem is that of the injection of liquid fuel into initially stagnant, hot air at high pressure, i.e., the conditions are similar to those found in the cylinder of a Diesel engine. Since autoignition is a highly transient phenomenon, in the modelling a time-like variable, denoted by ct is introduced to characterize the transient chemical reaction during the turbulent autoignition process. The model successfully covers the period from early autoignition to flame propagation. A balance equation for the Favre ensemble mean (C) over tilde(+) is derived from basic principles. The numerical results show good qualitative agreement with experimental data obtained by the Naples (Italy) combustion group.