Fuel injectors for internal combustion engines

Abstract: In a multi-hole fuel injector of the VCO (Valve Covered Orifice) type the holes (2) are larger at their inner ends than their outer ends, i.e. they taper in some way, to achieve a bushy spray without making the wall of the nozzle too thin. They can best be formed by electro-erosion machining.

Description: This invention relates to fuel injectors for internal combustion engines, and in particular to the design of the nozzles of multi-hole injectors. The overall design of fuel injectors has remained constant over many years but there have been a number of detail improvements in the search for improved spray patterns, and to suit different combustion chambers. The tip of the nozzle of the normal multi-hole injector is of roughly hemispherical shape with two, three, four or more radially directed drilled holes of constant cross-section. On completion of the injection process there is still an appreciable quantity of fuel left inside the tip of the nozzle, known as the sac, and in the holes themselves, and this fuel, emerging subsequently during the cycle when the pressure in the cylinder has fallen, is not burnt or is only partially burnt and finds its way into the exhaust gases, with a consequent adverse effect on emission quality. Accordingly, in the interests of emission control, in particular in keeping down HC emissions, efforts have been made to reduce the volume of the sac by making it internally partially conical so that the tip of the needle largely fills it, or in the so-called VCO nozzle by moving the positions of the holes from the sac to the conical part of the wall so that the needle covers their inner ends and the sac is not only small but is anyway cut off from the spray holes when the valve is closed. The spray pattern is dependent on the ratio of the diameter to the length of the hole; a short hole will result in a bushy spray of lower penetration than a longer hole of the same diameter. The degree of penetration required is, however, dictated by the shape and size of the combustion chamber in which the injector is to be used. To achieve a bushy spray one can shorten the holes by reducing the wall thickness of the nozzle but there is a strict limit to how far this can be carried in the case of VCO nozzles, on grounds of strength; with the high injection pressures involved, there is a danger of the tip of the nozzle being blown off if it is of inadequate strength. In practice the wall thickness must be 1mm or at the very least 0. 8mm. In VCO nozzles it has been proposed in the past to countersink the outer ends of the holes with a view reducing their length for a given wall thickness but this was not adopted as it was found that carbon deposits soon built up in the countersink. The primary aim of the invention, therefore, is to allow a given design of VCO injector, i.e. one with a nozzle of a given wall thickness, to be produced with varying penetration qualities according to requirements, and in particular to allow the effective length of the holes to be reduced (i.e. for bushier spray patterns) without weakening the nozzle unacceptably. In the known fuel injector of FR-A-2 399 551, the injector being of the valve covered orifice type, the body of the injector terminates in a nozzle. The nozzle has two delivery holes therein, each of them having a substantially round cross-section and having an inner end portion inside the nozzle and an outer end portion outside the nozzle. The inner ends of the delivery holes are substantially larger than the outer ends, the inner ends being flared, the depth of the flare being desirably about the same as the width of the delivery hole. The nozzle also comprises a valve member being moveable in the body between a first position in which the inner end portions of the holes are uncovered and a second position in which the valve member substantially covers the inner end portion of the holes. The degree of penetration and kind of spray pattern produced is dependant on the angle of convergance of the nozzles. According to the invention, a fuel injector for an internal combustion engine, the injector being of the valve covered orifice type, comprises a body terminating in a nozzle and the nozzle having a plurality of delivery holes therein, each of the holes having a substantially round cross-section and having an inner end inside the nozzle and an outer end outside the nozzle, the inner end of each hole being substantially larger than the outer end, and a valve member being moveable in the body between a first position in which the inner end of each hole is uncovered and a second position in which the valve member substantially covers the inner end of each hole, characterised in that each hole has, starting from the inner end, a diametrical cross-section which decreases in a uniform conical convergent taper from a relatively larger inner end diameter to a relatively small outer end diameter. Bearing in mind the very small diameter (of the order of 0.2mm) of the holes the most practical way of forming them is by electro-erosion machining. Some examples of nozzles of fuel injectors are illustrated in the accompanying drawing, in which:- Figure 1 shows a typical VCO injector to which the invention may be applied; and Figure 2 shows a nozzle with tapered holes. In Figure 1 is illustrated the lower end of a typical VCO injector. The injector body 1 terminates in a conical tip having a number, usually at least three, of circumferentially spaced inclined spray holes 2 through which the fuel is sprayed under high pressure into the cylinder of an internal combustion engine in which it is mounted. The lower end of the body defines a sac 3 but in contrast to the layout in earlier forms of nozzle, the VCO nozzle has the inner ends of the holes opening not into the sac but into a conical portion of the inner wall of the body which is engaged by the conical tip 4 of the valve. Hence the name VCO, which stands for Valve Covered Orifice. The taper on the tip of the valve may differ very slightly from that of the internal surface of the body. As explained above, for reasons of strength, one cannot, with present technology, reduce the thickness of the wall of the injector body in this region below 0.8mm at the very minimum. A hole of this length and of a typical diameter of the order of 0.2mm will produce a plume of fuel of substantial penetration, more than may be desirable for a given set of engine requirements. Accordingly, as shown in Figure 2 we shape the holes to have a larger cross-section at the inner end then at the outer end, and we find that this achieves the effect of a shorter hole, yet without weakening the wall of the injector body. To form the tapered holes shown in Figure 2 the hole is first made through the wall of the nozzle by the known technique of electro-erosion using an electro-machining electrode in the form of a wire 5; the wire is then tilted at an angle to the axis of the initial hole and rotated about that axis, precessing about the point X, and the angle being increased as necessary to produce the required taper. It would in theory be possible to form the initial hole by another method, for example using a twist drill, but then the alignment of the electrode with the hole to the necessary high degree of precision presents problems.

Claims: A fuel injector for an internal combustion engine, the injector being of the valve covered orifice type, comprising a body (1) terminating in a nozzle and the nozzle having a plurality of delivery holes (2) therein, each of the holes having a substantially round cross-section and having an inner end inside the nozzle and an outer end outside the nozzle, the inner end of each hole being substantially larger than the outer end, and a valve member (4) being moveable in the body between a first position in which the inner end of each hole is uncovered and a second position in which the valve member substantially covers the inner end of each hole, characterised in that each hole has, starting from the inner end, a diametrical cross-section which decreases in a uniform conical convergent taper from a relatively larger inner end diameter to a relatively small outer end diameter. A method of forming a fuel injector according to claim 1 in which the holes (2) are formed by electro-erosion machining from outside the nozzle using an electrode which is precessed to describe a conical path about the axis of the hole.

Inventors: GASKELL DAVID JOHN, GASKELL, DAVID JOHN
Applicants: LUCAS IND PLC, LUCAS INDUSTRIES PUBLIC LIMITED COMPANY
Application year: 1989
Main classification: F02M61
IPC codes: B23H9, F02M61