NOVELTY - Fuel separation for flexible fuel vehicle capable of using ethanol-blended gasoline fuel comprising non-aromatic high-volatile gasoline component, non-aromatic low-volatile gasoline component, aromatic gasoline component and ethanol component, in which auxiliary fuel (15) is separated from ethanol-blended gasoline fuel, comprises e.g. (a) when ethanol concentration of ethanol-blended gasoline fuel stored in main fuel tank (T1) is within first ethanol concentration range, separating ethanol-blended gasoline fuel with polar separating membrane member at first separation temperature.    USE - The method is useful for fuel separation for flexible fuel vehicle capable of using ethanol-blended gasoline fuel.    ADVANTAGE - The method increases the octane value of ethanol-blended gasoline fuel and thus minimizing knocking in the high rotation/high load operational range of the internal combustion engine and allowing the engine compression ratio to be increased.    DETAILED DESCRIPTION - Fuel separation for a flexible fuel vehicle capable of using an ethanol-blended gasoline fuel comprising a non-aromatic high-volatile gasoline component, a non-aromatic low-volatile gasoline component, an aromatic gasoline component and an ethanol component, in which an auxiliary fuel (15) is separated from the ethanol-blended gasoline fuel, comprises: (a) when the ethanol concentration of the ethanol-blended gasoline fuel stored in a main fuel tank (T1) is within a first ethanol concentration range, separating the ethanol-blended gasoline fuel with a polar separating membrane member at a first separation temperature to obtain, at the permeation side of the membrane, a first auxiliary fuel that is enriched with the aromatic gasoline component and the ethanol component in comparison with the ethanol-blended gasoline fuel; and (b) when the ethanol concentration of the ethanol-blended gasoline fuel stored in the main fuel tank is within a second ethanol concentration range which is higher than the first ethanol concentration range, separating the ethanol-blended gasoline fuel with the polar separating membrane member at a second separation temperature which is lower than the first separation temperature to obtain, at the permeation side of the membrane, a second auxiliary fuel that is enriched with the non-aromatic high-volatile gasoline components in comparison with the ethanol-blended gasoline fuel.    DESCRIPTION OF DRAWING(S) - The figure shows a device for carrying out the fuel separation method.    Main fuel tank (T1)    Sub fuel tank (T2)    Separator (12)    Auxiliary fuel (15)    Internal combustion engine (16)