An impeller for an axial fan of the type adapted to be pressed onto a rotating rotor comprising a number of blades. Each of the side panels has a bottom side, a top side and two lateral sides. The valve needle has a hollow portion connected to the first fuel passage and a plurality of fuel apertures connecting the hollow portion and the second fuel passage, a head portion disposed to be seated on or unseated from the valve seat thereby intermittently injecting fuel through the nozzle hole. The multilayer material is disposed between the first pin and the second pin. The second temperature may be greater than the first temperature.