Comprehensive low-temperature oxidation mechanisms are needed to accurately predict fuel auto ignition properties. This paper studies the effects of a previously unconsidered third O-2 addition reaction scheme on the simulated auto-ignition of n-alkanes. We demonstrate that this extended low-temperature oxidation scheme has a minor effect on the simulation of n-pentane ignition; however, its addition significantly improves the prediction of n-hexane auto-ignition under low-temperature rapid compression machine conditions. Additional simulations of n-hexane in a homogeneous charge compression ignition engine show that engine-operating parameters (e.g., intake temperature and combustion phasing) are significantly altered when the third O-2 addition kinetic mechanism is considered. The advanced combustion phasing is initiated by the formation and destruction of additional radical chain-branching intermediates produced in the third O-2 addition process, e.g. keto-dihydroperoxides and/or keto-hydroperoxy cyclic ethers. Our results indicate that third O-2 addition reactions accelerate low-temperature radical chain branching at conditions of relevance to advance engine technologies, and therefore these chemical pathways should also be considered for n-alkanes with 6 or more carbon atoms. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.