The autoignition delay time and location of a n-heptane fueled high pressure and high temperature spray combustion chamber under Diesel engine conditions has been investigated numerically. The conservation equations for the fluid dynamics of sprays have been solved using the KIVA-II code with its standard spray models. A detailed chemical mechanism of 81 elementary reactions and 37 chemical species has been applied to describe the ignition and combustion of n-heptane. The coupling between complex chemistry and turbulence is treated by employing the Representative Interactive Flamelet (RIF) concept. Unsteady flamelets are computed using a separate flamelet code that interacts with the CFD solver at each time step. The scalar dissipation rate, which is an important parameter for the flamelet, has been studied numerically under different conditions. More than one RIF has been used to resolve the spray region in order to account for the non-uniformity of the scalar dissipation rate distribution. The effect of the number of RIFs on the results of ignition delay and location is discussed. Calculations with different injection velocities have been performed to study the dependence of the scalar dissipation rate on the injection velocity and thus the influence of injection velocity on the ignition delay time and location. Numerical results of the ignition delay times for different air temperatures agree well with experimental data.