In this paper, we computationally investigate the fundamental aspects of autoignition and subsequent combustion phenomena in low temperature combustion (LTC) engine environments using direct numerical simulations (DNS). In particular, the effects of thermal and equivalence ratio stratification on the autoignition and subsequent front propagation in high pressure and stratified hydrogen-air turbulent mixtures are studied using detailed chemistry. Depending on fuel injection timing, exhaust gas recirculation, and wall heat loss, different correlations between temperature (T) - equivalence ratio (phi) fields can exist prior to the major heat release event. Here, we investigate three cases with different initial T-phi correlations: (A) a baseline case of a uniform composition with temperature inhomogeneities only, (B) uncorrelated T-phi fields, and (C) negatively-correlated T-phi fields. Numerical diagnostics are developed based on an appropriately defined Damkohler number to distinguish different modes of heat release. It is found that the majority of heat release in the baseline case and the uncorrelated case occurs during the front propagation in the form of both spontaneous ignition fronts as well as deflagration waves, whereas the negatively correlated case ignites predominantly homogeneously. Published by Elsevier Inc. on behalf of The Combustion Institute.