As a sustainable biofuel, n-butanol can be used in conventional spark ignition (SI) and compression ignition (Cl) engines in order to reduce the dependence on fossil fuel. Homogeneous charge compression ignition (HCCI) is a novel combustion to improve the thermal efficiency of conventional SI engines at part loads. To understand the effect of alcohol structure on HCCI combustion under stoichiometric conditions highly diluted by exhaust gases, the combustion characteristics of n-butanol, ethanol and their blends with gasoline were investigated on a single cylinder port fuel injection gasoline engine with fixed intake/exhaust valve lifts at the same operating conditions in this study. The results show that autoignition timing for alcohol-gasoline blends is dependent on alcohol types and its concentration in the blend, engine speed and intake valve opening (IVO)/exhaust valve closing (EVC) timing. In the operating conditions with the residual gases more than 38% by mass in the mixture, alcohol-gasoline blends autoignite more easily than gasoline. Autoignition timing for n-butanol-gasoline blend is earlier than that for ethanol-gasoline blend with the same alcohol volume fraction at 1500 rpm in most cases while the autoignition timings for the blends with alcohol are relatively close at 2000 rpm at the same IVO/EVC timing. Combustion stability is improved with advanced EVC timing at a fixed IVO timing, which is benefit for the improvement in the thermal efficiency in the case of alcohol-gasoline blends. In addition, n-butanol-gasoline blends autoignite earlier than their ethanol-gasoline counterparts with identical oxygen mass content in the blend at the same IVO/EVC timings regardless of engine speeds. For alcohol-gasoline blends, the effect of IVO/EVC timing on the thermal efficiency of the HCCI engine is dependent on autoignition timing, fuel types and engine speeds. However, advanced autoignition timing for the blends with alcohol worsens the thermal efficiency of the HCCI engine in most cases. (C) 2015 Elsevier Ltd. All rights reserved.