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Although HCCI engines are distinguished by mixture homogeneity, some degree of stratification always appears inside a combustion chamber. It applies especially to residual effected engines, utilizing negative valve overlap. Mixture stratification originates from imperfect mixing of fresh air with trapped residuals. As a result, temperature within the combustion chamber is uneven as well, and additionally influenced by heat transfer to the combustion chamber walls. Thermal stratification affects auto-ignition timing and combustion evolution to a high extent. The purpose of this study was to evaluate a degree of thermal stratification in HCCI engine utilizing negative valve overlap. Investigations were performed using three-dimensional CFD model of the combustion system, made by using AVL FIRE software. Simulations were realized for variable valve timings. It was found that that intake valve timing had significant effect on thermal stratification, whereas it did not affect amount of aspirated air to a high extent. This finding confirmed experimental observations, that the intake valve timing had a non-monotonic effect on auto-ignition timing.