As one example, a fuel rail assembly for supplying pressurized fuel to a plurality of cylinders of an engine is provided. The fuel rail assembly includes a fuel rail housing defining an internal fuel rail volume having at least a first region and a second region; a fuel separation membrane element disposed within the fuel rail housing that segregates the first region from the second region. The membrane element can be configured to pass a first component of a fuel mixture such as an alcohol through the membrane element from the first region to the second region at a higher rate than a second component of the fuel mixture such as a hydrocarbon. The separated alcohol and hydrocarbon components can be provided to the engine in varying relative amounts based on operating conditions.