Fuel injection nozzle
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention refers to a fuel injection nozzle for internal combustion engines, especially Diesel engines, comprising a spring-loaded valve needle and a displacer piston sliding with the needle along the same axis against the force of the valve needle spring, in which nozzle the displacer piston and the valve needle are connected with the fuel pressure chamber of an injection pump. 
     2. Description of the Prior Art 
     A device according to the above-mentioned type can be taken from DE-OS No. 34 09 924, for example. Providing a displacer piston, which is mounted parallel to the valve needle with regard to the admission of fuel, serves to divide the injection process into a pre-injection and a main injection. For this purpose the valve needle is first lifted against the force of the valve needle spring during a pressure build-up in the fuel pressure line, through which the injection process is initiated. Thereafter also the displacer piston is shifted against the force of the valve needle spring because of the increase of pressure in the pressure line, through which first the force acting on the valve needle spring is increased and second a short decrease of pressure occurs because of the opening displacing volume, which decrease leads to a short shutting of the valve needle. The continuously rising pressure lifts the valve needle once more against the force of the prestressed valve needle spring, thus beginning the main injection. 
     With the known devices of the initially mentioned type the displacer piston was arranged in a separate component, where this additional component was housed in a series of high-pressure-components being clamped with each other. The additional components resulted in an increase of the length of the nozzle construction and regarding an unfavourable ratio of the aperture cross-section of the piston relative to the guiding cross-section of the piston the pressure relieve taken place relatively slowly with the known devices, especially in the higher speed range. Providing a longer displacer piston would require greater dimensions and would additionally enlarge the inertia of the construction. To limit the height of construction a relatively small ratio of the guiding dameter relative to the guiding length of the displacer piston had to be observed, through which yet again quantities of overflow oil occured and the risk of a tilting and blocking of the displacer piston existed. 
     Especially when using a monobloc injection pump and nozzle the additional amount of height of construction represents a big disadvantage, the addition height resulting from such an intermediary part. 
     SUMMARY OF THE INVENTION 
     The invention aims now at providing a device of the initially mentioned type with which the constructive expenditure is lowered and with which wear and the risk of a tilting of the displacer piston are reduced because of lesser height. For solving this task the arrangement according to the invention is essentially characterized in that the displacer piston is arranged in the valve needle spring chamber and overlaps the valve needle spring in axial direction. Because the displacer piston is arranged in the valve needle spring chamber a separate component for housing the displacer piston can be dropped and because the displacer piston overlaps the valve needle spring in axial direction, a large guiding length can be assured, which counteracts tilting and premature wear of the displacer piston. 
     The arrangement is advantageously chosen in such a way that the area of the displacer piston overlapping the valve needle spring is at least equal to double the diameter of the displacer piston. 
     For limiting the stroke of the displacer piston, projecting parts can be arranged in the valve needle spring chamber. In an especially simple way, the nozzle body, following in the direction to the injection bores, can serve as a stop for the displacer piston which results in a very simple construction where no expensive working steps are necessary. The valve needle spring requires always a certain amount of axial length and the necessary space must in no way be enlarged by arranging the displacer piston in the valve needle spring chamber. Altogether, the total height of construction of an additional component for housing the displacer piston is eliminated in this way, through which a compact and simple construction is achieved. 
     Because of the improvement of the guiding of the displacer piston on the basis of a substantially increased guiding length, a substantially lower leakage is observed. 
     To assure that oil eventually leaking via the displacer piston does not impair the functioning of the displacer piston, the arrangement is advantageously chosen such that the envelope of the displacer piston overlapping the valve needle spring comprises at least one radial opening in alignment with an overflow oil bore, wherein advantageously the radial opening opens into a circumferential groove at the outer surface of the displacer piston. 
     For adjusting a favourable cross-sectional ratio of the aperture cross-section of the piston relative to the guiding cross-section of the displacer piston, the construction is advantageously chosen such that the displacer piston is conical at the side facing the fuel pressure chamber and cooperates with a face wall of the valve needle spring chamber, the wall being shaped as a hollow cone and comprising a central bore. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following the invention is explained in greater detail with reference to embodiments shown in the drawing. In it 
     FIG. 1 shows an arrangement according to the invention of a displacer piston in a pump-nozzle-unit and 
     FIG. 2 shows a fuel injection nozzle with the arrangement of the displacer according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1 the plunger of a pump-nozzle-unit is designated with 1. During the compression stroke the plunger produces the fuel pressure necessary for the injection in the barrel 2, which fuel reaches the seat of the valve needle 5 via a transverse groove 3 and a bore 4. After reaching a predetermined pressure the valve needle 5 may take off the seat against the force of the valve needle spring 6, with which the pre-injection is started. After a further length of stroke of the plunger 1 the pressure is further rising, after which the built-up pressure moves the displacer piston downwards surmounting the force of the valve needle spring 6. Because of the substantial increase of the diameter between the inlet diameter d and the diameter D of the displacer piston suddenly a large displacing volume is at disposal, which volume is limited by the displacing stroke h E . Simultaneously with the compression of the spring 6 a sudden pressure relief takes place and the valve needle reaches the seat, closing the injection bores. The closing movement of the valve needle 5 being initiated by the pressure decrease in the bore 4 is aided by the additional biasing of the spring 6. 
     After covering the stroke h E  no further displacing volume is at disposal and the pressure can increase once more until it again surpasses the opening pressure of the valve needle 5. In the following, the main injection takes place after the renewed opening of the valve needle 5. 
     Leaking oil eventually entering the valve needle spring chamber 8 must be removed to maintain the reliable functioning of the displacer piston 7. For this, the part overlapping the valve needle spring 6 of the envelope of the hollow displacer piston 7 comprises a bore 9 which opens into an annular groove 10 at the outer surface of the displacer piston. The bore 9 as well as the annular groove 10 are in alignment with an overflow oil bore 11 so that with arbitrary angular position of the displacer piston 7 a removing of the leaking oil is assured. 
     The prestress of the valve needle spring can be adjusted by inserting distance plates 12 into the displacer piston 7. At the end reaching into the valve needle spring chamber 8 the valve needle 5 is provided with a spring plate 13 to be supported against the force of the valve needle spring 6. 
     The maximal opening stroke of the valve needle is limited by a stop and is designated h. The maximal displacing stroke h E  of the displacer piston 7 is limited by a stop 14 at the bottom of the valve needle spring chamber 8. 
     The guiding length of the displacer piston 7 is larger than double the guiding diameter D and is positioned to overlap the valve needle spring 6, through which a large change of the cross-section is achieved during opening the displacer piston. In this way also for high speeds an exact separation of pre-injection and main injection is effected. The mass of the piston remains small due to its form of a thin-walled hollow cylinder so that its inertia remains small. The enlarged guiding diameter and the large sealing length simplify the fabrication and improve the high-pressure sealing and the guiding of the displacer piston 7. 
     In the embodiment according to FIG. 2 is shown an injector built separately from a pump. The displacer piston 7 is placed in a screwed element 15 and overlaps again the valve needle spring 6. Because of the small number of partition lines between parts being fastened by the screw connection 16 and the screwed element 15 the risk of setting is lowered.