All previous correlations of the ignition delay (ID) period in diesel combustion show a positive activation energy, which means that shorter ID periods are achieved at higher charge temperatures. This is not the case in the autoignition of most homogeneous hydrocarbons-air mixtures where they experience the NTC (Negative Temperature Coefficient ) regime in the intermediate temperature range, from about 800 K to 1000 K). Here, the autoignition reactions slow down and longer ID periods are experienced at higher temperatures. Accordingly the global activation energy for the autoignition reactions of homogeneous mixtures should vary from positive to negative values. In this paper, the discrepancy in the autoignition of a homogeneous charge and a heterogeneous charge is investigated. 3-D computer simulations of the physical and chemical processes in the combustion of fuel sprays in a diesel engine showed that this discrepancy is caused by the variations in the equivalence ratio of the first ignitable site in the spray with charge temperature. This finding has been validated by the simulation of autoignition of a homogeneous mixture at different equivalence ratios.