A fuel injector for internal combustion engines

Abstract

 A fuel injector comprising an axially displaceable slide (10) and an annular chamber (26) between a section (11) of the slide (10) and a surrounding valve guide (5). In the closed position that chamber (26) is isolated from the forward part (27-29) of the fuel duct (19-24) by means of a valve consisting of a conical face (38) on the slide (10) and an opposed seat face (39) on the valve guide (5). The valve remains closed until the fuel pressure exceeds a predetermined percentage of that pressure at which the slide (10) has moved so far backward that it uncovers the atomizer holes (9) and, correspondingly, the valve closes, at decreasing fuel pressure, only some time after the slide (10) has obturated the atomizer holes (9).

Description

BACKGROUND OF THE INVENTION

[0001] This invention relates to a fuel injector for internal combustion engines, comprising a valve guide the forward end of which is formed as an atomizer closed at its front end and having one or more transverse atomizer holes for the injection of fuel, a slide axially displaceable in the guide for opening and closing the atomizer holes, a supply duct for supplying pressurized fuel to the atomizer, and a closing spring acting on the slide against the fuel pressure in the duct for retaining the slide in its closed position which is determined, by the abutment of opposed abutment surfaces on the slide and the valve guide, such that the atomizer holes are uncovered only after a certain travel of the slide from its closed position.

[0002] From the published Swedish patent application No. 74 16 296 there is known an injector of this kind, in which the abutment surfaces between the valve guide and the slide are located behind - as seen from the atomizer - a pair of cooperating, tight-fitting cylindric guiding and sealing surfaces on the slide and the valve guide, respecetively. An annular chamber defined between the slide and the valve guide in front of said cylindric surfaces communicates with the forward end of the fuel supply duct. The fuel pressure prevailing in the annular chamber is thus effective to open the atomizer holes and initiate the injection at increasing pressure in the fuel supplied, and when subsequently the pressure decreases the closing spring moves the slide back to its closed position, thereby terminating the injection.

[0003] It has been found that under certain conditions, especially at low engine loads, there can occur after-dripping of non-atomized fuel through the atomizer holes of the known injector. The exhaust gases of the engine will then contain unburnt or incompletely burnt fuel which leads to pollution of the environment and which furthermore results in a deterioration of the engine's fuel economy.

SUMMARY OF THE INVENTION

[0004] The invention aims at remedying this drawback and according to the invention said abutment surfaces,which determine the closed position of the slide, are formed as cooperating valve seat faces for controlling a flow passage between two successive sections of the fuel supply duct, said sections being located upstream and downstream, respectively, of said surfaces.

[0005] The invention is based on the assumption that the after-dripping from the known injector is due to the fact that at the termination of the injection period the slide is subjected to a relatively high force exerted by the powerful closing spring, while at the same time the opposed fuel pressure decreases rapidly. The two abutment surfaces will therefore impinge so hard on one another that the slide recoils, possibly several times, thereby temporarily uncovering the atomizer holes so that fuel at the reduced pressure can drip through the holes. The assumption is strengthened by the fact that the tendency towards after-dripping is the more pronounced the more abruptly the pressure relief in the fuel duct occurs.

[0006] In the injector according to the invention the risk of after-dripping is eliminated or at least substantially reduced in consequence of the functioning of said abutment surfaces as a valve which does not close until some time after the forward portion of the slide has covered the atomizer holes. Fuel, which is being pressed rearwardly from the atomizer during the subsequent final part of the closing movement, flows through the-open valve, and this rearward flow of fuel dampens the movement of the slide thereby ensuring a soft engagement between the abutment surfaces without danger of the slide recoiling and again uncovering the atomizer holes. The invention has the further advantage that the abutment surfaces remain in engagement during that period of the working cycle of a directly connected fuel pump, in which the fuel pressure rises to the value at which it can overcome the force exerted on the slide by the closing spring, and during that period the valve isolates the supply duct from the cylindric sealing surfaces between the slide and the atomizer. The risk of fuel leaking out between those surfaces prior to the intended injection period is thus prevented.

[0007] Preferably the dimensions of the closing spring, which determine inter alia its spring rate, are adapted to the travel of the slide from its closed position to its open position, i.e. the position in which it uncovers the atomizer holes, in such a way that the fuel pressure necessary for overcoming the spring force in the closed position, is less than one half, preferably 30 to 35% of the fuel pressure at which the slide uncovers the atomizer holes. When the amount of fuel present downstream of the valve seat faces after the slide has closed the atomizer holes can expand to said relatively low pressure before the valve seat faces come in engagement, leakage of fuel between the mating cylindric sealing surfaces of the slide and the atomizer after the atomizer holes have been covered by the slide, is prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The invention will now be described in more detail with reference to the accompanying drawings which rather schematically illustrate an embodiment of the fuel injector according to the invention, whereby

Fig. 1 shows a fractional view of the injector in axial section,

Fig. 2 shows, on a larger scale, the forward portion of the injector, in particular the slide proper and the surrounding valve guide together with the atomizer in the closed position of the slide, and

Fig. 3 is a view corresponding to Fig. 2 and on the same scale, showing the slide in its fully open position.

DETAILED DESCRIPTION

[0009] The fuel injector illustrated in the drawings comprises an elongate outer housing 1, which at its rear end has an enlarged head 33, by means of which the injector is mounted, as known in the art, in a cylinder cover (not shown) of an internal combustion engine, especially a Diesel-engine, and connected to a fuel pump (not shown). In head 33 there is a fuel oil inlet 34 in flow connection with a duct 2 extending through a central thrust tube 3 the front end of which is, under the influence of compression springs (not shown) acting on the external end face of head 33, kept in sealing engagement with the rear end of a thrust member 4. A forwardly oriented shoulder on thrust member 4 abuts against a tubular valve guide 5. Valve guide 5 fits tightly in housing 1 at its rear end and is prevented from rotating in the housing by means of a pin 6 engaging in a longitudinal groove in the forward end of the guide. Between the cylindric mating surfaces of the valve guide and the housing there is provided a sealing ring 7.

[0010] To the front end of valve guide 5 an atomizer 8, the forward end of which protrudes into the combustion chamber of the engine cylinder (not shown), is secured by suitable means, e.g. by electron beam welding or shrinking. Atomizer 8 is closed at its forward end and spaced behind that end it has one or more atomizer holes 9 in its wall. The above mentioned spring force on head 33 keeps, through parts 3, 4, and 5, a forwardly oriented conical face 35 on the atomizer in sealing engagement with a conical internal face on housing 1.

[0011] A slide generally designated by 10 is axially displaceable internally of valve guide 5 and comprises a middle longitudinal section 11, a cylindric front section 12 protruding with a tight fit into atomizer 8 for controlling the opening and closing of holes 9, and a rearwardly oriented end section 13 having radial clearance in valve guide 5. Slide 10 is biased in forward direction to its closed position as shown in Figs. 1 and 2, in which its front section 12 obturates the atomizer holes 9, by means of a closing spring 14, see Fig. 1, the rear and front ends of which abut against a washer 15 on thrust tube 3 and against a bushing 16, respectively. Bushing 16 is displaceable along the thrust tube and serves to transmit the spring force to a slit thrust bushing 17, the front end face of which abuts against the rear face of slide 10. In Figs. 2 and 3 the thrust bushing has been omitted for the sake of clarity. Details of the described structure for transmitting the closing force of spring 14 to slide 10 are shown in British patent specification No. 1,346,501.

[0012] The fuel supply duct 2 continues in a central duct 18 extending through thrust member 4 to a central duct 19 in a valve member 20 which is axially displaceable with a tight fit within the central section 11 of slide 10. Duct 19 is closed at the forward end of valve member 20 and communicates, through transverse bores 21, with a chamber 22 which internally of slide 10 adjoins the conical forward seat face 36 of the valve member which face is, under the influence of a relatively weak compression spring 23, maintained in engagement with a conical seat face 37 surrounding the opening of a short duct 24 in the slide.

[0013] Duct 24 is also closed at its bottom and communicates, through transverse bores 25, with a chamber 26 which adjoins a conical seat face 38 located at the transition between sections 11 and 12 of the slide. Said conical seat face 38 defines, together with an opposed conical seat face 39 on valve guide 5, the closed position of slide 10, as shown in Fig. 2, which the slide assumes under the influence of closing spring 14.

[0014] Downstream of said conical seat faces 38 and 39 an annular chamber 27 is defined between slide section 12 and the surrounding portions of valve guide 5 and atomizer 8. Through transverse bores-28 in slide section 12 chamber 27 communicates with a central duct 29 which opens into the front end of slide section 12 and which is closed at its rear end behind bores 28.

[0015] As explained in the above mentioned British patent specification No. 1,346,501 it is possible to maintain, during the intervals between the fuel injection periods, a constant circulation of fuel, if desired preheated, through a substantial part of the fuel injector by means of a priming pump which delivers fuel at a low pressure which is insufficient for overcoming the force of spring 23. From duct 18 the fuel then flows through a transverse bore 30 in the forward part of thrust member 4 to the annular space 31 internally of housing 1 behind valve guide 5 and from space 31 through a return outlet (not shown) back to the primary pump.

[0016] When the fuel pump is operative, the fuel pressure,which in chamber 22 acts on the forward face of valve member 20, will, at a certain time during the pump cycle, exceed said relatively low pressure whereby member 20 is pressed rearwardly to close bore 30. The fuel can then flow through the passage between faces 36 and 37 and through duct 24 to chamber 26 at the front side of the middle section 11 of slide 10. The slide remains, however, closed until the pressure in chamber 26 has risen to a value at which it can overcome the force exerted by closing spring 14. During this initial phase of building-up the pressure the fuel has therefore no access to the cylindric sealing faces between the front section 12 of the slide and the atomizer 8. The component parts of the injector are preferably dimensioned such that the slide starts its rearward movement from the closed position shown in Fig. 2, thereby permitting the flow of fuel onwards through the passage between seat faces 38 and 39 to chamber 27 and duct 29, only at a fuel pressure which is about 30 to 35% of the intended injection pressure at which the slide has moved so far backwards that its front end face uncovers the atomizer holes 9, as shown in Fig. 3, whereby the injection of atomized fuel starts.

[0017] When subsequently the fuel pressure decreases the slide starts moving towards the left thereby first obturating atomizer holes 9 when there is still a passage from chamber 27 to chamber 26 between the opposed valve seat faces 38 and 39 on the forward end of middle section 11 of the slide and on valve guide 5, respectively. That amount of fuel,which is being displaced from the interior of atomizer 8 in response to the forward movement of slide 10, can therefore flow backwards through duct 29, bores 28, and chamber 27 to chamber 26 and from there onwards to chamber 22, the volume of which increases in response to the movement of the slide since valve member 20 is retained in its right hand end position by the fuel pressure acting against the relatively weak spring 23. As mentioned above this rearward fuel flow results in a damping of the closing movement of slide 10 and thus ensures a suitable soft impingement between the conical valve seat faces 38 and 39 intermediate chambers 27 and 26.

[0018] In order to prevent corrosive and erosive attacks on atomizer holes 9 it is expedient to ` manufacture atomizer 8 of a material which is particularly resistant against those attacks, e.g. a cobalt alloy, and different from the material, normally steel, of which the valve guide is made. Corresponding attacks on the front end of slide section 12 which opens and closes the atomizer holes 9, can be prevented in a similar manner by welding a separate ring 32 of corrosion and erosion resistant material onto the end of the slide, as shown.

Claims

1. A fuel injector for internal combustion engines, comprising a valve guide (5) the forward end of which is formed as an atomizer (8) closed at its front end and having one or more transverse atomizer holes (9) for the injection of fuel, a slide (10) axially displaceable in the guide (5) for opening and closing the atomizer holes, a supply duct (2, 18-19, 21-22, 24-29) for supplying pressurized fuel to the atomizer,and a closing spring (14) acting on the slide against the fuel pressure in the duct for retaining the slide in its closed position which is determined, by the abutment of opposed abutment surfaces on the slide and the valve guide, such that the atomizer holes are uncovered only after a certain travel of the slide from its closed position, characterized in that the abutment surfaces (38, 39) are formed as cooperating valve seat faces for controlling a flow passage between two successive sections (26, 27) of the fuel supply duct located upstream and downstream, respectively, of said surfaces (38, 39).

2. A fuel injector as claimed in claim 1, characterized in that the dimensions of the closing spring (14) are adapted to the travel of the slide (10) from its closed position to the position in which it uncovers the atomizer holes (9), in such a way that the fuel pressure necessary for overcoming the spring force in the closed position, is less than one half, preferably 30 to 35%, of the fuel pressure at which the slide uncovers the atomizer holes.

3. A fuel injector as claimed in claim 1 or 2, characterized in that the valve seat faces (38, 39) are conical.

Drawing