It is well established that the blending of ethanol with gasoline and primary reference fuels (PRFs) causes non-linear octane response. The chemical effect possibly responsible for the non-linear behavior remains to be understood. Previously, experiments were performed on Cooperative Fuel Research engine. Ethanol was added in volume percentage of 2-20% to five base fuels: three FACE (Fuels for Advanced Combustion Engines) gasolines, more specifically FACE I, J and A and their primary reference fuels (iso-octane/n-heptane). The engine was operated in HCCI mode for four different sets of speed and intake temperatures to obtain four HCCI numbers and their corresponding blending octane numbers. It was found that base fuel composition and octane number had an important role in the octane enhancement behavior of ethanol and methanol. In this present study, using experimental conditions, HCCI engine simulations with detailed chemical kinetics are performed for the already tested blends for FACE A and PRF 84. Previously, simulations were performed on PFR 70/ethanol blends and will be used for comparison. The geometrical data and the intake valve closure conditions of the experiment were used to replicate the measured start of combustion (SOC). The simulated compression ratios for the PRF mixtures were used to obtain the transfer function for the four HCCI conditions. The transfer function and the simulated compression ratio for the ethanol/PRF 84/FACE A gasoline blends were further used to obtain simulated blending octane numbers. The heat release analysis provided good understanding into the effect of ethanol addition on Low Temperature Heat Release (LTHR). A strong correlation of the low temperature oxidation reactions of base fuels with ethanol was found to be responsible for the observed blending effects.