The motivation for the present study is the attempt to enhance the understanding of the influence of inhomogeneous turbulence influence upon autoignition processes, which are of interest for practical applications such as the Diesel engine. Special attention is paid to the modelling of chemical source term closure and its coupling with inhomogeneous turbulence. Detailed two-dimensional simulations of turbulent autoignition with flamelet source term closure have been performed using a laminar flamelet presumed pdf model (pdf== probability density function). The transient autoignition calculations of the laminar flamelet are based on a reduced 4-step chemical kinetic mechanism of heptane combustion in air. New ways of modelling turbulent chemical source term closure are investigated. Specifically, an ignition time variable is introduced to characterize the aspect of transient chemical reaction during the turbulent autoignition process. The model successfully covers the period from early autoignition to flame propagation