A fuel injector for delivering fuel to an engine in which a housing of the injector has an internal fuel chamber and at least one exhaust port in fluid communication with the fuel chamber. A valve member is moveable relative to the housing between a closed position in which fuel within the fuel chamber is inhibited against exhaustion from the housing, and an open position in which fuel is exhaustable from the housing. An ultrasonic waveguide is separate from the housing and valve member, with substantially the entire ultrasonic waveguide disposed within the fuel chamber to ultrasonically excite fuel within the fuel chamber prior to the fuel exiting through the at least one exhaust port in the open position of the valve member. An excitation device is operable in the open position of the valve member to ultrasonically excite the ultrasonic waveguide.