Electromagnetically Actuated Fuel Injection Valve For Internal Combustion Engines

Abstract

An electromagnetically actuated fuel injection valve for internal combustion engines, the valve needle of which, guided in a bore of the valve housing, is connected to the armature of an electromagnet and is lifted from a valve seat when the electromagnet is energized, against the force of a return spring and against the direction of flow of the fuel which is fed under pressure from an external pressure source, thereby establishing communication between a pressure chamber adjacent the valve seat and a discharge nozzle outlet is described, wherein the valve needle is coupled to a compensating piston which is guided coaxially with the valve needle and the armature, in a fluid tight manner in the said bore. The compensating piston has a first frontal face which is pressure-relieved and a second frontal face which is exposed to the fuel pressure, and a cross sectional area which is so large that, when the injection valve is closed, the hydraulic forces acting on the valve needle in the directions of opening and of closing, are equal or at least approximately equal.

Description

BACKGROUND OF THE INVENTION

This invention relates to an electromagnetically actuated fuel injection valve for internal combustion engines, the valve needle of which is guided in a valve housing and is connected with the armature of an electromagnet; upon energization of the electromagnet, this valve needle is lifted from its valve seat against the force of a return spring and against the direction of flow of fuel which is supplied with a predetermined fuel pressure from a fuel pump or the like pressure source, thereby establishing communication between a pressure chamber adjacent the valve seat and a nozzle discharge outlet.

In fuel injection valves of this type which have been described in German patent 483,101 and in German Offenlegungsschrift 1,526,635, which have an inwardly opening valve needle and a valve seat controlled thereby, there always occurs in the closed injection valve a hydraulic force generated by the fuel pressure from the pressure source, which force tends to hold the valve closed and is proportional to the fuel pressure and to the valve seat diameter. In view of the limited space available about the engine for installations, the size of the electromagnet and consequently its force of attraction are limited; it is, therefore, necessary to keep the valve seat diameter in these valves very small to be suitable for high injection pressures. However, this has the drawback that the amount of fuel to be injected will be strongly throttled at the valve seat thus causing a loss of fuel pressure even before the fuel reaches the nozzle outlet of the valve. As the valve seat diameter must be kept within very narrow limits due to the required injection conditions, and as the same applies to the force of the magnet for the above-stated reasons, the possibilities for using such injection valves are unduly limited.

For these reasons, the known directly acting electromagnetically actuated fuel injection valves in which the valve needle is directly coupled with the armature of the electromagnet, can only be used for a limited pressure range in the fuel injection of diesel engines.

A further drawback of these valves resides in the fact that the opening and closing times of the valve depend on the fuel pressure; for, at a constant magnet energizing time, the opening movement of the valve needle is more or less delayed, or its closing movement more or less accelerated, depending on the level of the fuel pressure, whereby the amount of injected fuel is subject to variations.

It is also well known that, in the above-described fuel injection valves having an inwardly opening valve needle, the area exposed on the valve needle to the fuel pressure in the opening direction is increased when the electromagnet is energized and when the valve needle is lifted from its valve seat. The additional exposed area, which was covered while the valve was closed, depends on the seat diameter and on the type of valve construction. In the case of valves having a nozzle with a throttling pin-controlled orifice, of the type described in German Offenlegungsschrift 1,526,635, the added area effective as an annular area which is limited in radial direction externally by the valve seat diameter and internally by the diameter of a plug portion of the valve needle. When this annular area is very narrow, the increase in area after opening of the valve will be small and will cause hardly any drawback. However, if it is large in relation to the seat area which corresponds to the valve seat diameter, or if the valve described hereinbefore is of the type equipped with a nozzle having at least one orifice downstream of the valve seat, which has been described in German patent 483,101, then the area being added is relatively large. This will cause the added disadvantage that the force needed for closing the valve will be greater than the force required for opening the same. This drawback causes a slow, delayed closing of the valve needle and and favors "sticking" of the armature, i.e. the well-known effect caused by remanent magnetism in the electromagnet after its deenergization.

OBJECT AND SUMMARY OF THE INVENTION

It is the object of this invention to avoid the above-mentioned drawbacks of the known electromagnetically actuated fuel injection valves of the types described hereinbefore, and to provide a valve which is independent, in practice, from the fuel pressure so that it can be used with diesel engines even for injection pressures as high as 200 to 800 bar or more.

This object is attained by a coupling of the valve needle with a compensating piston which is guided coaxially with the valve needle and with the armature in a guiding bore of the valve housing; a first frontal face of this piston is pressure-relieved, while its other frontal face is exposed to the fuel pressure, and its cross sectional area is so large that, in the closed injection valve, the hydraulic force acting upon the valve needle in the direction of opening is equal or at least approximately equal to the acting thereon in the direction of closing the valve.

The above-mentioned hydraulic force which tends to hold the valve closed can be completely balanced by the compensating piston, thus achieving a static balance of forces. Small excess forces may be desired in order to accelerate the opening and the closing of the valve, in which case an approximate balancing of forces only may be produced.

In a particularly advantageous embodiment of the invention, the sole connection between a pressure chamber adjacent the valve needle and a fuel feeding chamber which faces toward the armature and is limited by the second frontal face of the compensating piston, is established by a fuel passage having a throttle means therein. The throttle means inserted in the fuel passage causes a pressure drop in the fuel which counteracts the additional hydraulic force which would occur without the provision of the throttle, and which would increase the hydraulic force acting in the opening direction when the valve is open. A complete or near complete compensation of this additional force can be achieved by providing a throttle means having a throttling cross sectional area reducing the pressure in the pressure chamber sufficiently to make the force acting in the pressure chamber on the valve needle in the direction of opening, when the injection valve is open, approximately equal to the force acting in the same chamber in the same direction when the valve is closed, whereby a dynamic balance of forces is achieved besides the static balance mentioned earlier.

In connection herewith, it is advantageous when the cross sectional area corresponding to the guiding diameter of the guided portion of the valve needle in the bore of the housing is at least ten times as large as the cross sectional area of flow through the valve seat, for, if this is the case, then the pressure drop produced by the throttle means will amount to maximally one tenth of the fuel pressure.

In order to prevent transverse forces from diminishing the effect of the compensating piston, a preferred embodiment of the invention provides for a coupling of the compensating piston to the valve needle via an elastically bendable connecting member, thereby extending the valve needle-and-armature unit, and for a diameter of the compensating piston which is equal to, or at least approximately equal to the diameter of the valve seat.

In this case, the adjustment of the fuel injection valve is facilitated by having a fuel supply duct extend essentially within and along the longitudinal axis of the valve needle, and by providing an exchangeable screw member which is inserted in an enlargement of the duct and which contains the throttle means, and by having a guiding bore for the piston disposed in an exchangeable guiding sleeve which is firmly mounted in a duct of the valve housing leading to a return fuel line.

In another advantageous embodiment of the valve according to the invention, the compensating piston is arranged intermediate the valve needle and the armature, and is connected with the armature and coupled to the valve needle for force transmission via a rigid connecting member; and the cross sectional area corresponding to the diameter of the compensating piston is smaller, by the cross sectional area corresponding to the diameter of the valve seat, than the cross sectional area corresponding to the guiding diameter of the valve needle, whereby only forces resulting from fuel pressure are effective between the compensating piston and the valve needle, thus increasing the safety of the operation.

A static and dynamic balancing of forces can also be achieved in yet another advantageous embodiment of the valve according to the invention by guiding the valve needle, downstream of the valve seat, in a fluid-tight manner by means of a guiding plug in a bore provided in the end face of the valve housing facing toward the combustion space, and by having the compensating piston serve as a connecting member between the armature of the electromagnet and the valve needle, the diameter of the piston being at least approximately equal to the diameter of the said guiding plug and to the diameter of the valve seat; advantageously, the guiding plug has the shape of a plug nozzle tip and is provided with a cavity; this cavity is closed toward the end of the valve needle remote from the valve seat and has the nozzle aperture opening into the combustion space, in a manner known per se, as well as at least one connecting bore leading to an annular channel provided in the mantle surface of the guiding plug and controlled by the valve seat.

In a further embodiment of the valve according to the invention the valve needle is provided with a guiding piston upstream of the valve seat and a spraying plug located downstream of the valve seat and penetrating into the nozzle aperture; and the compensating piston serves as a connecting member between the armature of the electromagnet and the valve needle, the piston diameter being at least approximately equal to the diameter of the valve seat; in a particularly preferred embodiment of this valve, a narrow annular gap is formed between the spray plug and the nozzle aperture which gap is laid out only for the passage of a partial amount of the fuel quantity to be injected for operation under full load, and the spray plug has at least one additional spray bore, controllable after a preliminary stroke, to permit passage of the total quantity of fuel to be injected under full load, or for a quantity of fuel to be injected under partial load which latter quantity is larger than the quantity of fuel injectable through the annular gap, whereby there will prevail the most favorable injection conditions for any partial amount of fuel to be injected as well as for the injection of the quantity required for full load operation. It is also possible to control the injection of a preliminary amount of fuel by means of this arrangement.

The invention will be better understood and further objects and advantages will become more apparent from the ensuing detailed specification of preferred but merely exemplary embodiments taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partly schematical sectional view of a first embodiment of the injection valve, in closed position;

FIG. 2 is a sectional view similar to that of FIG. 1, of the same injection valve in open position;

FIG. 3 is a sectional view of a second embodiment of the valve according to the invention;

FIG. 3a is an enlarged partial sectional view taken from FIG. 3, in the region of the valve seat;

FIG. 4 is a sectional view of a particularly illustrative portion of a third embodiment, taken in the region of the valve needle;

FIG. 5 is a sectional view of a particularly illustrative portion of a fourth embodiment, taken in the region of the valve needle and the compensating piston;

FIG. 6 is a sectional view of a similar region as shown in FIG. 5, but of a fifth embodiment; and

FIG. 7 is a sectional view similar to that of FIG. 6, but of a sixth embodiment of the valve according to the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring to the FIGS. 1 and 2, the first embodiment of an electromagnetically actuated fuel injection valve 10 comprises a valve housing 9 which has been shown schematically as an integral piece, and a valve needle 12 which is guided in a housing bore 11 by means of a guiding portion 13 thereof, the guiding diameter of which has been designated by D.sub.F1 (FIG. 2). The valve needle 12 is stepped and bears at the downstream end of its reduced diameter portion 17 a valve cone 14 which is seated on a valve seat 15 and thereby obturates the flow of fuel to the nozzle apertures 16, when the fuel injection valve is in closed position (FIG. 1). As the diameter D.sub.S1 of valve seat 15 which is equal to the diameter of the reduced diameter portion 17 of valve needle 12, is smaller than the diameter D.sub.F1 of the guiding portion 13 of valve needle 12, an annular beveled shoulder 18 is formed in the outside wall of valve needle 12 on which shoulder there is exerted the fuel pressure p.sub.Z of the fuel which is supplied from a pressure source 19 via a fuel supply line 21, when the injection valve is in closed position, and which forms a portion of the internal wall of pressure chamber 22 located upstream of and adjacent valve seat 15.

Pressure source 19 as well as the structural elements pertaining thereto are well known per se and are therefore represented only schematically. The pressure source may, for instance, be an engine-driven gear pump or a piston pump, the output pressure is held, by means of a pressure-regulating valve 23, at a desired pressure p.sub.Z, for instance in the order of 200 bar or 400 bar. In order to equalize fluctuations in fuel pressure, the pressure regulating valve 23 may be combined with a pressure reservoir (not shown); the construction of this reservoir and also that of the pressure-regulating valve 23 are well known.

The fuel which is supplied via the fuel supply line 21 flows past an electromagnet 25 the solenoid of which is cooled, and into a fuel supply chamber 28 via a fuel inlet duct 27. The fuel supply chamber 28 is connected with the pressure chamber 22 via a central, fuel-filled recess or bore 29 in valve needle 12 which recess extends substantially along the longitudinal axis of the valve needle and communicates with pressure chamber 22 via a throttle passage 31 having a throttling diameter F.sub.D which is shown in FIGS. 1 and 2 as an oblique reduced diameter bore opening out of the beveled wall of the shoulder 18 of valve needle 12.

The fuel pressure p.sub.Z prevailing in fuel supply chamber 28 is exerted, on the one hand, on the upstream annular end wall 32 of valve needle 12 remote from valve cone 15, and, on the other hand, on an armature 33 which is in contact with fuel on at least two opposite sides thereof (FIGS. 1 and 2), so that no excess hydraulic force will act on the armature 33 in the direction in which the valve opens. The armature 33 is guided with only a small tolerance in the electromagnet 25 in order to maintain gap losses as low as possible.

The armature 33 and the valve needle 12 are articulatedly connected with one another by means of a connecting member 34, whereby any axial misalignment between armature 33 and valve needle 12 occurring during manufacture will be compensated. A return spring 35 urges the valve cone 14 of valve needle 12 on to the valve seat 15 and thereby holds the injection valve 10 in the closed position shown in FIG. 1.

Parallel to fuel supply line 21, there is connected to fuel supply chamber 28 a central bore 36 through the upstream part of valve housing 9, remote from the nozzle apertures 16, to which bore 36 there is connected a fuel return line 37 which is free from pressure or is pressure-relieved. This fuel return line 37 leads to a fuel tank 38 from which the pressure source 19 takes in fuel via a suction line 39. Between bore 36 and the connection thereof to fuel return line 37, a compensating piston 41 is interposed, which piston is housed with a sealing fit in a guiding bore 42 being an enlarged zone of bore 36. The compensating piston 41 thus seals off bore 36 and fuel supply chamber 28 against the fuel return line 37.

The compensating piston 41 has a diameter D.sub.K1 which is approximately equal to the diameter D.sub.S1 of valve seat 15. A first frontal face 43 of piston 41 facing toward return line 37 is pressure-relieved as described above, i.e., it is exposed only to atmospheric pressure, apart from possible small amounts of leaking fuel that may leak through to return line 37. The opposite frontal face 44 of the compensating piston 41 is exposed to the fuel pressure p.sub.Z from the pressure source 19. On this side, the piston 41 is coupled to the armature 33 and also via an elastically bendable connecting member 45, which preferably consists of steel spring wire, to the connecting member 34 and by means of the latter to the valve needle 12.

FIG. 2 shows the injection valve 10 in its open position. When the electromagnet 25 is energized, the armature 33 is attracted and moves the valve needle 12 by means of connecting member 34 and against the force of return spring 35 to a position in which valve seat 15 is held open so that fuel can flow from the supply line 21 via inlet duct 27, fuel supply chamber 28 and central recess 29 and from there through throttle passage 31 and the pressure chamber 22 past the open valve seat 15 to the nozzle apertures 16, through which the fuel is injected into the combustion space (not shown) of the engine, in a manner known per se.

A second embodiment of a fuel injection valve 50 according to the invention, which has been represented in FIG. 3 and is destined for being attached to a diesel engine, comprises a first housing part 51 and a nozzle body 53 inserted in a stepped bore 52 therein, as well as a spring casing 54. In axial alignment with the first housing part 51, there are arranged an electromagnet 55 and a second housing part 56 which are screwed together, to form a fluid-tight unit, with the aid of the compressing force of a compression nut 57, sealing rings and the like sealing means, which have not been designated further, being interposed between the component parts of the unit where needed. An apertured connecting part 58, bearing a female plug member 60 to be connected to a source of electric current for the electromagnet 55, and an annular connecting piece 59 having a radially extending socket 61 for connection to a fuel supply line 21' are mounted on the second housing part 56 and secured therein against rotation by means of a pin 62, and by tightening the whole assembly by means of a nut 63. The parts 59 and 61 and the nut 63 are centered on a tubular socket part 64 of valve housing part 56. Socket part 64 has a central axial bore 65 which is pressure-relieved and is provided with an inner threading for connection to a fuel return line 37' leading to a fuel reservoir, in the same manner as shown for line 37 in FIG. 1. Similarly, line 21' is connected to a pressure source as shown with regard to line 21 in FIG. 1.

A valve needle 67 is inserted fluid-tight and guided by means of its cylindrical guiding part 68 in a bore 66 of nozzle body 53. The diameter of guiding part 68 has been designated by D.sub.F2. A pressure chamber 72 is arranged in nozzle body 53 adjacent a valve seat 71 disposed upstream of a nozzle orifice 69; as shown in detail in FIG. 3a, on an enlarged scale, a valve cone 73 of valve needle 67 is seated fluid-tight on the valve seat 71, having the diameter D.sub.S2, and obturates nozzle orifice 69. Valve cone 73 bears a throttling pin 74 which penetrates into the nozzle orifice 69. A nozzle orifice controlled in this manner is generally known used in pin-controlled nozzles. The slope of the conical bore forming the valve seat 71 in the nozzle body 53 is conventionally steeper than the slope of the valve cone 73 of the valve needle 67, in order to obtain an exactly defined valve seat as indicated by the diameter D.sub.S2 in FIG. 3a.

Fuel if fed to the pressure chamber 72 adjacent valve seat 71 from the fuel supply line 21' via connecting socket 61, a bore 75 in apertured connecting part 58, a channel 76 in the second housing part 56, an annular chamber 103 surrounding the solenoid 102 of electromagnet 55, and a duct 105 through the electromagnet to a spring chamber 77 in spring casing 54, and a flow duct provided in the form of an axial bore 78 in the valve needle 67, which bore 78 is in turn connected via slanted traverse bores 79 with the pressure chamber 72. In an enlarged zone 80 of the said needle bore 78, an exchangeable screw member 81 is inserted, in which there is provided a throttling passage 82 having a throttling diameter designated as F.sub.D.

An armature 83 of electromagnet 55 is guided in the magnet with only a small tolerance and is coupled with radial play by means of a coupling disc 84 to a connecting rod 85 one end of which is connected to the armature 83, while the other end of rod 85 is connected to the valve needle 67 by means of an articulated joint 86. This type of connecting means between the valve needle and the armature 83 permits a prescribed narrow guidance of the armature 83 in the magnet 55, whereby a maximum of magnetic force can be attained in a minimum of space without any impairing influence of guiding and frictional force on the operation of the valve.

The articulated joint 86 consists of a rod head 87 which is screwed on the connecting rod 85 and has a transverse bore 88 being deeply beveled at both ends thereof, and of a pin 89 which is inserted into a transverse bore 91 of the valve needle 67 and into the aforesaid transverse bore 88. As the rod head 87 has radial play relative to the enlarged zone 80 of the axial valve needle bore 78 and as the transverse bore 88 in the rod head 87 is beveled over about half its total length, an articulated joint is effected between the connecting rod 85 and the valve needle 67. Deviations of the longitudinal axes of valve needle 67 and armature 83 are thereby prevented from exercising a disadvantageous influence on the operation of the injection valve.

As an extension of the unit comprising valve needle 67 and armature 83, a compensating piston 93 is coupled to the end of the connecting rod 85 bearing the armature 83, by means of an elastically bendable, longitudinally rigid connecting member 92. The compensating piston 93 is slidably fit into a guide bore 94 in a guide sleeve 95, which latter is in turn exchangeably inserted in the second housing part 56 and is held therein by means of a screw 96. The flexibly elastic connecting member 92 is made advantageously from spring steel wire in order to avoid transverse forces caused by misalignment. The exchangeable guide sleeve 95 and the replaceable screw member 81 facilitate adjustment of the injection valve to suit the various engine characteristics to which, moreover, the valve seat diameter and the shape of the valve needle must be individually adapted in a manner known per se.

As in the case of compensating piston 41 in FIG. 1, the diameter D.sub.K2 of the compensating piston 93 is equal, or at least approximately equal, to the diameter D.sub.S2 of the valve seat 71, in order to balance, or at least to balance approximately, the hydraulic forces which are exerted upon the valve needle 67 when the injection valve 50 is in closed position. Similary to the compensating piston 41 in FIG. 1, the compensating piston 93 has a first end face 97 and a second end face 98 which latter is exposed to the fuel pressure p.sub.Z of the pressure source. This second end face 98 is in communication, via a central bore 99 in the second housing part 56 and a central bore 101 of electromagnet 55, with the spring chamber 77 which is in turn connected to the fuel supply line 21' in a manner described hereinbefore. The compensating piston 93 bears in its periphery indented grooves 93a which form a labyrinth seal so that the amount of fuel returned through leakage via the fuel return line 37' to the reservoir is kept small. Intermediate the bore 75 in the second housing part 56 and the spring chamber 77, the admitted fuel is led about the solenoid 102 of electromagnet 55.

A chamber 103 surrounding solenoid 102 communicates with the bore 75 via a channel 104, and further via a slanted duct 105 with the central bore 101 of the electromagnet 55 and with the spring chamber 77; the fuel flowing through chamber 103 cools the solenoid 102 of electromagnet 55.

Spring chamber 77, central bore 101 of the electromagnet 55 and the central bore 99 in the second housing part 56 constitute a fuel access path 106 from which fuel pressure p.sub.Z of the pressure source can be exercised simultaneously on the second end face 98 of the compensating piston 93 as well as on a stepped end face 107, facing toward the spring chamber 77, of the valve needle 67. Of course, when the valve is closed, the same feeding pressure p.sub.Z also prevails in the axial bore 78 of valve needle 67 and in the pressure chamber 72, for, when there is no flow of fuel, the throttling passage 82 has no effect.

Valve needle 67 is held in the shown closure position by a return spring 108 which is supported, on the one hand, on an annular shoulder 109 in spring chamber 77 and, on the other hand, on an annular shoulder 111 of valve needle 67. It is a function of the compensating piston 93, to be explained in detail hereinafter, to provide for a balance of the forces caused by the fuel pressure p.sub.Z and acting on the valve needle 67 when the injection valve 50 is in closed position. To achieve this end, it is advantageous so to adjust the force of return spring 108 in relation to the force of the electromagnet 55 that a desired closing and opening speed valve needle 67 is attained. Moreover, the return spring 108 serves to overcome the force, acting in the opening direction in the injection valve 50, of a very weak holding spring 112 which merely retains the armature 83 in position and urges it against the coupling disc 84.

A third embodiment of a fuel injection valve, designated by 120, of which only the nozzle part turned toward the combustion space has been shown in FIG. 4, is in particular distinguished from the second embodiment, shown in FIG. 3, by the fact that it comprises a nozzle having a cavity with radial orifices downstream of the valve seat.

A valve needle 67' has a guiding portion 68' and adjacent thereto a reduced diameter needle portion 121 provided with a valve cone 73'. In the position shown, the valve cone 73' obturates a valve seat 71' in a nozzle body 53' which is fastened fluid-tight under tension in a known manner to a housing part 123 of the injection valve 120 by means of a coupling nut 122. Viewed in the direction of fuel flow, the nozzle body 53' has downstream of valve seat 71' a closed bottom cavity 124 which communicates with the combustion space (not shown) of the engine through radial nozzle orifices 125 provided in wall of nozzle body 53' surrounding the cavity. A fuel flow duct 78' which corresponds to the axial valve needle bore 78 in FIG. 3 and is provided with transverse bores 79', connects a spring chamber 77' with a pressure chamber 72' adjacent the valve seat 71'. Fuel flow duct 78' is located only in the guiding portion 68' of valve needle 67' so as not to weaken the structure of needle portion 121. A screw member 81' provided with a throttle passage 82' is screwed into flow duct 78'. In the spring chamber 77' there is housed a return spring 108' similar to that in injection valve 50 of FIG. 3, and a connecting rod 85' connects the valve needle 67' with the armature (not shown) of an electromagnet and with a compensating piston (not shown). The parts not shown in this figure correspond essentially to those of the second embodiment shown in FIG. 3.

The fourth embodiment of an injection valve 130 the lower portion of which, essential to the illustration of the invention, has been represented in FIG. 5 has a valve needle 132 which is guided with its guiding portion 131 in a nozzle body 133 and possesses a slim needle portion 132a similar to that of valve needle 67' in FIG. 4. By means of a valve cone 134 borne by this needle portin 132a at its downstream end, there is obturated a valve seat 135 having the seat diameter D.sub.S3. This valve seat 135 controls the flow of fuel from a pressure chamber 136 located adjacent the valve seat 135, to nozzle orifices 137. The upstream part of fuel injection valve 130 is constructed similar to that of the injection valves 10 and 120 in FIGS. 1, 2 and 4.

The essential differences between the injection valve 130 of FIG. 5 and the injection valves of FIGS. 1 to 4, described hereinbefore, resides in the arrangement of a compensating piston 138 between the valve needle 132 and the armature (not shown) of an electromagnet of the type described further above. The diameter of this compensating piston has been designated by D.sub.K3. The piston 138 has a first frontal face 141 which faces downstream toward valve needle 67' and forms one side wall of a fuel storing chamber 142 which is pressure-relieved and which is connected via a leak fuel duct 143 to a fuel return line (not shown) leading to a fuel reservoir (not shown).

The chamber 142 and the duct 143 correspond to the pressure-relieved duct 65 in FIG. 3. Fuel from the pressure source (not shown) has access to the second frontal face 144 of compensating piston 138 and to a fuel supply duct 145, so that both these parts are exposed to the fuel pressure p.sub.Z. The fuel supply duct 145 is provided in a housing part 147 of a valve housing 148 and extends parallel to the chamber 142 and to a guiding bore 146 which houses the compensating piston 138. In duct 145 there is inserted a throttle means in the form of a throttle bore 151 in a transverse plate 149. The part of the duct 145 located downstream of throttle bore 151 has been designated as 145a and opens into the pressure chamber 136. In the latter, the fuel acts upon an annular shoulder 152 of valve needle 132, which shoulder is limited externally by the guiding portion 131, having the diameter D.sub.F3, of valve needle 132, and internally by the cross sectional area F.sub.S3, corresponding to the diameter D.sub.S3, of the valve seat 135. The shoulder 152, therefore, represents an annular surface when seen in vertical projection.

The fuel supply duct 245 has its end remote from the pressure chamber 136 a zone slanted toward the valve axis, 145b, by which it is connected to a fuel supply chamber 153. The part of this fuel supply chamber 153 which has been illustrated in FIG. 5 is delimited, on the downstream side, by the second frontal face 144 of compensating piston 138 and is formed by a part of the central guiding bore 146 and by the internal recess of an electromagnet (not shown) which corresponds to the recess designated by 101 in FIG. 3. Fuel supply chamber 153 is exposed to the fuel pressure p.sub.Z from the pressure source.

The compensating piston 138 is connected via a connecting rod 154 to the armature (not shown) of the electromagnet, and the piston is coupled with force-transmission to the valve needle 132 via a rigid connecting member 155. The cross sectional area F.sub.S3 which corresponds to the diameter D.sub.K3 of compensating piston 138 is smaller, by the cross section area F.sub.S3 corresponding to the diameter D.sub.S3 of the valve seat 135, than the cross sectional area F.sub.F3 which corresponds to a diameter D.sub.F3 of the guiding portion 131 of valve needle 132, so that the hydraulic forces exerted on the valve needle in closed position are the same in the opening as well as in the closing direction.

The rigid connecting member 155 is provided in its middle region with a collar 157 serving as an abutment for a return spring 156; the collar 157 bears a spherically shaped face 158 for a spring seat disc 159 which supports one end of return spring 156. This spring seat disc 159 is adapted to the spherical shape of face 158 and thus permits its position to be adapted to a possible inclined positioning of the return spring 156. The ends of the connecting member 155 are shaped as spherical heads 161 and 162, and each of these spherical heads is supported in a correspondingly spherically shaped recess 163 and 164, respectively, of the compensating piston 138 and of the valve needle 132. In order to avoid tilting and transverse forces as far as possible, the centers of the corresponding recesses are located at least approximately on the central axes of the compensating piston 138 and the valve needle 132, respectively.

In the embodiments of FIGS. 4 and 5, the arrangement of the electromagnet, its solenoid and its armature, is similar to that shown in FIG. 3, the electromagnet being mounted on the upstream face of valve housing part 123 (in FIG. 4) or 147 (in FIG. 5) and fastened to these parts by a compression nut as shown in FIG. 3 or the like known means.

A fuel injection valve 170 shown in FIG. 6 as a fifth embodiment comprises a valve needle 174 guided by means of a hollow guiding plug portion 171 thereof in an axial bore 172 provided in a nozzle housing 173. The axial bore 172 extends through the nozzle housing 173 in the region of its end face 175 facing toward the combustion space (not shown), and has the guiding plug portion 171 fit fluid-tight therein. There are attached to the nozzle housing 173, in axial upstream direction, an intermediary cylindrical part 176 and an electromagnet casing or stator 177 constituting together with an armature 179 and a solenoid 181 the electromagnet 178. The armature 179 is housed with play in an axial bore 188 of the electromagnet casing 177. The nozzle housing 173, the intermediary cylindrical part 176 as well as the electromagnet casing 177 are coupled together by means of a coupling nut 182 to form a fluid-tight unit constituting the valve housing 183.

The electromagnet casing 177 and the coupling nut 182 are only partially illustrated. On the part of valve housing 183 which has not been illustrated, connecting means for the fuel supply line from a pressure source and for the fuel return line are provided in a manner known per se.

A compensating piston 184 having the diameter D.sub.K4 is connected positively, or made integral with, the armature 179 and is coupled to the valve needle 174 by means of a transverse pin 185. It thus serves as a connecting member between the armature 179 and the valve needle 174. The compensating piston 184 is ground fluid-tight into a guiding bore 186 of the intermediary cylindrical part 176 and thus seals off a pressure chamber 187, located between the axial bore 172 in nozzle housing 173 and an enlarged downstream zone 186a of guiding bore 186, against an internal chamber 188a which is pressure-relieved and which is a downstream part of axial bore 188. Pressure chamber 187 is exposed to the fuel pressure p.sub.z in a manner to be explained further below. A fuel return line for returning leakage fuel to a reservoir is connected to the upstream end of axial bore 188 in a manner not shown; the pressure chamber 178 is connected via ducts 189 in electromagnet casing 177 and ducts 191 and 192 in the intermediary cylindrical part 176 to the pressure source.

The hollow guiding plug portion 171 of valve needle 174, having the diameter D.sub.F4, has the shape of a nozzle tip having a closed bottom cavity 194 and radial nozzle orifices 195 from he cavity to the outside of the valve 170. The cavity 194 is closed off toward the interior of the valve needle by means of a plug 196 and is provided with connecting bores 197 inclined upwardly and outwardly and communicating with an annular groove 198 in the external wall of the guiding plug portion 171.

Adjacent the annular groove 198, a valve cone 199 is formed by the frustoconical valve needle portion intermediate the plug portion 171 and a cylindrical portion 174a of larger diameter, forming the upstream end of valve needle 174. The upstream end of bore 172 toward the pressure chamber 187 forms a valve seat 201 engaged by valve cone 199 when valve 170 is closed, as shown in FIG. 6. By designing this valve seat 199 to have a sharp peripheral edge, the valve seat diameter D.sub.S4 is equal to the diameter D.sub.F4 of the valve needle guiding plug portion 171. As, in practice, the valve seat 201 is always slightly blunted, i.e., it has a radial width of one or two tenths of a millimeter, the valve seat diameter D.sub.S4 can be considered as being at least approximately equal to the diameter D.sub.F4 of the guiding plug portion 171. The diameter D.sub.K4 of the compensating piston is at least approximately equal to the diameter D.sub.S4 of the valve seat 201 and, as the internal chamber 188a of the electromagnet casing 177 is pressure-relieved, there has been achieved, in this construction of the fuel injection valve 170, a balance of forces when the valve is closed as well as when it is open, which can be considered as near perfect.

The sixth embodiment of a fuel injection valve, 210, as shown in FIG. 7, is distinguished from the fifth embodiment described in the foregoing, merely by a different construction of the valve needle and of the nozzle housing in the region of the valve seat.

The injection valve 210 comprises a valve needle 6 174' being guided in an axial bore 212 in a nozzle housing 173', by means of a valve needle portion shaped as a guiding piston 211. Downstream of the guiding piston 211, the valve needle 174' has a short needle part 213 of reduced diameter bearing a frustoconical part serving as a valve cone 199', which obturates a valve seat 201' as shown in FIG. 7. This valve seat 201' is formed by the edge between a nozzle orifice 215 being an axial bore in the bottom end of nozzle housing 173', and a conical seat 218 in the nozzle housing 173'. A spraying plug 214 is connected to the valve cone 199' downstream of and coaxially with valve needle 174'. This spraying plug 214 extends with play through the nozzle orifice 215, leaving in the latter an annular gap which is dimensioned for the passage of only a partial amount Q.sub.T of the total fuel amount Q.sub.V which is to be injected when operating the engine under full load. The spraying plug 214 has an axial spray bore 216 which opens toward the combustion space of the engine, and which is in communication with the aforesaid annular gap via a transverse bore 217. Thereby, depending on the width of the annular gap which is formed when valve cone 199' is raised from the valve seat 201' and after a preliminary stroke H.sub.V of the valve needle 174', there can be injected the total amount Q.sub.V required under full load, or an amount Q.sub.X required by operation under partial load, the latter being larger than the partial amount Q.sub.T injectable through the annular gap in bore 215. The valve seat diameter D.sub.S5 of valve seat 201' is equal to, or at least approximately equal to the diameter D.sub.K5 of the compensating piston 184'.

All remaining parts of the injection valve 210, e.g., the armature 179 of electromagnet 178, are identical with the parts as shown in the injection valve 170 of FIG. 6, and have therefore not been designated in FIG. 7.

OPERATION OF THE EMBODIMENTS

The operation of the above-described embodiments of the fuel injection valve according to the invention will now be explained.

In the case of injection valve 10 shown in FIGS. 1 and 2, the return spring 35 holds the valve needle 12 in the closed position when the electromagnet 25 is deenergized. In this state, valve cone 14 obturates the valve seat 15 and prevents the flow of fuel present in pressure chamber 22 under the pressure p.sub.Z, to the nozzle apertures 16. When the valve is closed, this fuel pressure also prevails in the fuel supply chamber 28. The following forces then act on the valve needle 12:

a. a force in the direction of closing needle 12,

P.sub.a = P.sub.F + p.sub.Z .sup.. F.sub.F1,

wherein P.sub.F is the force of the return spring and F.sub.F1 is the cross sectional area subject to fuel pressure and corresponding to the diameter D.sub.F1 of the guiding portion 13 of valve needle 12;

b. a force in the direction of opening needle 12,

P.sub.b = p.sub.Z .sup.. (F.sub.F1 - F.sub.S1 + F.sub.K1)

simultaneously counteracts the force P.sub.a so that the effective obturating force is

P.sub.A = P.sub.a - P.sub.b = P.sub.F + p.sub.Z .sup.. (F.sub.S1 - F.sub.K1) (c)

wherein F.sub.S1 is the cross sectional area corresponding to the seat diameter D.sub.S1 of the valve seat 15, and F.sub.K1 is the cross sectional area corresponding to the diameter D.sub.K1 of compensating piston 41.

As has been explained hereinbefore in connection with the first embodiment, when the diameter D.sub.K1 of the compensating piston 41 is equal to the diameter D.sub.S1 of the valve seat 15, the hydraulic forces due to the feeding pressure p.sub.Z compensate each other, and the return spring 35 holds the valve needle 12, solely due to the spring force P.sub.F, in the closing position shown in FIG. 1.

Due to the diameters of the compensating piston 41 and the valve seat 15 being equal, there has been achieved a complete balancing of hydraulic forces in the closed injection valve.

However, it may also be desirable to have, for instance, a slight excess force in the closing direction to support the closing movement of the valve needle 12, in order to increase the closing speed which would otherwise depend exclusively on the spring force P.sub.F of the return spring 35. In this case, the hydraulic forces acting on the valve needle 12 in the opening and in the closing direction must be made only approximately equal, i.e., the diameter D.sub.K1 of the compensating piston 41 and, consequently, its cross sectional area F.sub.K1, must be slightly smaller than the diameter D.sub.S1 of the valve seat 15 and correspondingly, the cross sectional area F.sub.S1 of the latter.

When energized, the electromagnet 25 must initially overcome the force P.sub.A, which means that the attracting force P.sub.M of the electromagnet 25 need only be larger than P.sub.F, when F.sub.S1 is equal to F.sub.K1. It will be seen from the foregoing that, in the case of a complete balance of forces by the compensating piston 41, the fuel pressure p.sub.Z has no influence of the attracting force P.sub.M of the electromagnet 25 which is required to open te injection valve.

However, in the valves described in connection with FIGS. 1, 2 and 4, the above-explained balance of forces will be disturbed as soon as the valve needle 12 has been lifted from its valve seat 15, for, the valve seat area F.sub.S1, hitherto covered by the valve cone 14, then forms an additional surface area exposed to the pressure of the fuel, whereby an additional force is exerted on the valve needle 12 in the direction of opening. Depending on the size of the additional surface F.sub.S1 which corresponds to the valve seat 15, and depending on the closing force of return spring 35 and on the fuel pressure p.sub.Z, this additional force may cause the valve needle 12 to remain in the open position, or at least cause the delay of the closing movement of the valve needle 12. The resulting slower obturation will also be further delayed by the well known sticking of the armature 33 in the electromagnet 25 caused by remanent magnetism in the electromagnet.

In the injection valve 10 shown in FIGS. 1 and 2, the throttle passage 31 opening out of the central needle recess 29, will cause a pressure drop .DELTA..sub.p between the fuel supply chamber 28 and the pressure chamber 22 such that the hydraulic forces acting upon the valve needle 12 will be equal in the opening as well as in the closing direction, when the valve is open as shown in FIG. 2. Owing to the throttle passage 31 and to the pressure drop .DELTA..sub.p caused therein, a fuel pressure reduced by .DELTA..sub.p, i.e., p.sub.D1 = p.sub.Z - .DELTA..sub.p, will prevail in the pressure chamber 22 when the injection valve is open. The throttle cross sectional area F.sub.D of the throttle passage 31 must be so dimensioned that the pressure p.sub.D1 in the pressure chamber 22 is sufficiently reduced with the injection valve 10 being open, that the force P.sub.open acting in this pressure chamber 22 on the valve needle 12 in the direction of opening will be equal to the force P.sub.closed acting in the same opening direction and in the same pressure chamber 22 on the valve needle 12 when the injection valve is closed. If slight excess forces are desired in the one or in the other direction, then the forces P.sub.open and P.sub.closed should be balanced only approximately.

Now, if P.sub.open is equal to P.sub.closed, as has been stated above, then it will only be necessary to consider the pressure conditions prevailing in the pressure chamber 22 at a given moment, for the areas F.sub.F1 and F.sub.K1 and the pressure p.sub.Z in the fuel supply chamber 28 will always be the same regardless of whether the injection valve is open or closed.

Therefore, the following equation will describe these conditions:

P.sub.open = P.sub.closed .fwdarw.p.sub.D.sup.. (F.sub.F - F.sub.S + F.sub.V) = p.sub.Z .sup.. (F.sub.F - F.sub.S) (d)

wherein F.sub.V is the valve opening cross sectional area of the injection valve. Moreover, in the first embodiment according to FIGS. 1 and 2 the cross sectional area F.sub.V1 is equal to the valve seat cross sectional area F.sub.S1, so that the following equation applies to the first embodiment:

P.sub.D1 .sup.. F.sub.F1 = p.sub.Z .sup.. (F.sub.F1 - F.sub.S1 ). (e)

It will be seen from this equation that the pressure p.sub.D1 in pressure chamber 22 which has been reduced by the throttling passage 31, will become smaller by the same ratio as the area on the valve needle 12 exposed to the fuel pressure in this pressure chamber 22 will become larger.

The cross sectional area F.sub.D1 of the throttling passage 31 which produces the required pressure drop .DELTA.p in a given fuel pressure p.sub.Z is also effective with variable fuel pressures and fuel quantities, for, expressed in a simplified manner, in the case of a smaller or larger fuel pressure p.sub.Z the required pressure drop .DELTA.p is also correspondingly smaller or greater. This assertion is valid subject to two conditions, namely that the respective throttle cross sectional areas of the throttling passage 31 and in the nozzle orifices 16 and the valve seat 15 are small relative to the chambers 28 and 22, respectively, located upstream thereof and that the pressure outside the injection valve, i.e. in the combustion space of the engine, is small relative to the fuel pressure p.sub.Z and the pressure p.sub.D1. For, in that case, a simple transformation of the equation of flow valid for both flow passages, namely the throttle cross sectional area F.sub.D1 in the throttling passage 31 and the cross sectional area of flow in the nozzle orifices 16 affords the equation

p.sub.Z - p.sub.D1 = K.sup.. p.sub.Z, (f)

wherein K is a constant representing the flow characteristics and cross sectional areas of the throttling passages which remain always constant.

The second embodiment according to FIG. 3 functions generally in the same manner as the first embodiment. One important distinction should be taken into account, namely that, in the open injection valve, the valve needle 67 will not free the entire valve cross sectional area corresponding to the valve seat diameter D.sub.S2 but that only a partial, annular valve cross sectional area F.sub.V2 will be opened by the valve needle 67 as the latter is provided with the throttle plug 74. This valve cross sectional area F.sub.V2 which is subject, in pressure the 72, to the pressure p.sub.D2 which has been reduced by the throttling passage 82 when the injection valve 50 is in open position, is a rule considerably smaller than the cross sectional area F.sub.S2 of the valve seat 71, which corresponds to the valve seat diameter D.sub.S2. Therefore only a very small pressure drop .DELTA.p need be generated by the throttling passage 82. If the difference between the diameter D.sub.S2 of the valve seat 71 and the diameter of the throttle plug 74 is very small, then it is possible, in particular in the case of low injection pressures, to omit throttling by means of the throttling passage 82 entirely.

A complete static and dynamic compensation of forces is attained in injection valve 50, shown in FIG. 3 when the diameter D.sub.K2 of the compensating piston 93 is equal to the diameter D.sub.S2 of the valve seat 71, and when the throttling passage 82 is so dimensioned that the pressure p.sub.D2 in pressure chamber 72 is sufficiently reduced, in the case of the injection valve 50 being open, so that, as a result, the following equation is valid for injection valves 10, 50 and 120 (FIGS. 1 to 4):

p.sub.D .sup.. (F.sub.F - F.sub.S + F.sub.V) = p.sub.Z hu . (F.sub.F - F.sub.K). (g)

When in the fuel injection valve 50 shown in FIG. 3, the diameter D.sub.K2 of the compensating piston 92 is equal to, and not only approximately equal to, the diameter D.sub.S2 of the valve seat 71, then, the following special equation applies for this case:

p.sub.D2 .sup.. (F.sub.F2 - F.sub.S2 + F.sub.V2) = p.sub.Z .sup.. (F.sub.F2 - F.sub.S2). (h)

The third embodiment, namely, the fuel injection valve 120 according to FIG. 4, functions in the same manner as the first embodiment shown in FIGS. 1 and 2. By means of its valve cone 73' arranged at the end of needle portion 121, the valve needle 67' controls the valve seat 71' which prevents the flow of the fuel present in pressure chamber 72' to the nozzle openings 125 when the injection valve is closed. When the injection valve 120, and therefore valve seat 71' are open, a pressure drop controlled by the throttling passage 82' will take place in pressure chamber 72', similar to that occurring in injection valve 10 as shown as in FIG. 2, which pressure drop will compensate the hydraulic forces acting upon the valve needle 67' even when injection valve 120 is open, in the same manner as has been described in connection with FIG. 2.

The fuel injection valve 130 the essential inventive features of which have been illustrated in FIG. 5, is fuel-pressure-relieved in the closed position shown, i.e., the hydraulic forces acting upon the valve needle 132 in the positions of opening and closing due to the fuel pressure p.sub.Z are equal. This pressure balance is attained by the fact that the cross sectional area F.sub.K3 corresponding to the diameter D.sub.K3 of the compensating piston 138 is equal to the cross sectional area engaged at the valve needle 132 in the pressure chamber 136 in the direction of opening. In this case, there applies:

F.sub.K3 = F.sub.F3 - F.sub.S3, (i)

wherein F.sub.F3 is the cross sectional area, corresponding to the diameter D.sub.F3, of the guiding portion 131 of valve needle 132, and F.sub.S3 is the cross sectional area of the valve seat 135 corresponding to the diameter D.sub.S3.

When a fuel injection is to take place, the electromagnet (not shown) which is then energized, attracts the compensating piston 138 via the connecting rod 154, and simultaneously therewith the valve needle 132 which is coupled with force transmission via the connecting member 155 and which is subject to the fuel pressure on the shoulder 152, will follow this movement. Thereby, the valve cone 134 of the valve needle 132 is lifted from the valve seat 135, and the fuel present in the pressure chamber 136 prior to injection under the fuel pressure p.sub.Z is injected through the nozzle orifices 137 into the combustion space of the engine.

During the injection, a pressure p.sub.D3, reduced by .DELTA.p due to the throttle bore 151 prevails in pressure chamber 136, and causes the hydraulic forces acting upon the valve needle 132 to be balanced even when this injection valve 130 is in the open position. This is the case, when the force acting in the fuel supply chamber 153 on the compensating piston 138, in the direction of closing movement of the valve needle 132, is equal to the force acting in pressure chamber 136 upon the valve needle 132 in the direction of its opening movement.

In the case of fuel injection valves which have a compensating piston 138 arranged between the armature of the electromagnet and the valve needle 132, as is the case in the ejection valve 130 of FIG. 5, a complete balance of hydraulic forces can be attained even when the injection valve is open, if the dimensioning of the throttling bore 151 is based on the following equation:

p.sub.D3.sup.. (F.sub.F3 - F.sub.S3 + F.sub.V3) = p.sub.Z.sup.. F.sub.K3, (j)

wherein F.sub.V3 is the cross sectional area of the valve opening.

As, in the case of fuel injection valve 130 according to FIG. 5, the flow cross sectional area F.sub.V3 is equal to the cross sectional area F.sub.S3 of the valve seat, (which is true also in the embodiments of FIGS. 1, 2 and 4) the preceding equation is simplified as follows:

p.sub.D3 .sup.. F.sub.F3 = p.sub.Z .sup.. F.sub.K3. (k)

From this equation it is easy to calculate p.sub.D3 as well as .DELTA.p = p.sub.Z - p.sub.D3.

In the case of fuel injection valve 170 shown in FIG. 6, the fuel fed from a pressure source which has not been illustrated further, flows via ducts 189, 191 and 192 into the pressure chamber 187 which partially surrounds the valve needle 174 as well as the compensating piston 184. As the diameter D.sub.K4 of the compensating piston 184 is equal to the guiding diameter D.sub.F4 of the guiding plug 171 and is also equal to the valve seat diameter D.sub.S4 of the valve seat 201, in accordance with the invention, and as the internal space 188 of the electromagnet 178, surrounding the armature 179 is pressure-relieved, the forces acting upon the valve needle 174 in the closed position, shown in FIG. 6, of the injection valve 170 are equal in the opening as well as in the closing direction, whereby a static balance of forces is attained. Therefore, in the case of this injection valve 170, the equation

D.sub.K4 = D.sub.S4 = D.sub.F4 applies. (l)

An almost complete balance of forces is achieved when these diameters are at least approximately equal to one another.

When the electromagnet 178 is energized, the armature 179 attracts the valve needle 174 via the compensating piston 184 and the connecting rod 185, and the valve cone 199 thereof clears the valve seat 201. The fuel present under fuel pressure p.sub.Z in the pressure chamber 187 can now flow through the annular groove 198 and the connecting bores 197 into the cavity 194 wherefrom it is injected through nozzle orifices 195 into the combustion space of the engine.

The fuel injection valve 210 according to FIG. 7 functions in the same manner as that of FIG. 6, with the exception that the equation D.sub.K5 = D.sub.S5 applies, whereby the desired balance of forces is achieved in the closed valve.

When injection valve 210 is open, an additional force acts on the valve needle 174' in the direction of opening, which force is proportional to the fuel pressure and to the annular area which is limited externally by the valve seat diameter D.sub.S5 and internally by the diameter of the spraying plug 114. This force can be neglected when the injection pressure is relatively low and the diameter of the annular area is narrow; in the case of very high injection pressures and a larger ring area diameter, this force can be balanced by means of throttling between spaces located above and below the guiding piston 211 (not shown) in a similar manner as in the case of the injection valves according to FIGS. 1 to 4.

Claims

What is claimed is:

1. In an electromagnetically actuated fuel injection valve of the type that has

a. a valve housing having a bore, a valve seat, a pressure chamber adjacent said valve seat, and at least one nozzle discharge outlet,

b. a valve needle guided in said bore,

c. return spring means for said needle,

d. an electromagnet having an armature, and

e. connecting means for connecting said armature to said valve needle,

wherein, upon energization of said electromagnet, said armature lifts said valve needle from said valve seat against the force of said return spring means and against the direction of flow of fuel under pressure into said pressure chamber, thereby establishing communication between the latter and said nozzle discharge outlet, the improvement comprising a compensating piston guided in said bore in a fluid-tight manner and substantially coaxially with said armature and said valve needle, coupling means for coupling said piston to said valve needle, said piston having a pressure-relieved first frontal face and a second frontal face being exposed to fuel pressure and having a cross sectional area sufficiently large so that the hydraulic forces acting on said valve needle in the direction of its opening and in the direction of its closing, when said injection valve is closed, are at least approximately equal.

2. The improvement described in claim 1, further comprising a fuel supply chamber in said housing facing toward said armature and being limited by said second frontal face of said compensating piston, a fuel supply duct being the sole connection between said fuel supply chamber and said pressure chamber, and throttle means in said fuel supply duct.

3. The improvement as described in claim 2, wherein said throttle means has a throttling cross sectional area whereby, when the injection valve is open, the pressure in the pressure chamber is reduced so that the force acting in said pressure chamber on the valve needle in the direction of opening is at least approximately equal to the force acting in the same chamber on the valve needle in the opening direction when the injection valve is closed.

4. The improvement as described in claim 1, wherein the cross sectional area corresponding to the guiding diameter of said valve needle in said bore is at least ten times as large as the cross sectional area of fuel flow through the valve seat.

5. The improvement as described in claim 1, wherein said coupling means comprise an elastically bendable connecting member.

6. The improvement as described in claim 5, wherein said compensating piston and connecting member are arranged upstream of said armature and substantially coaxially aligned with said armature and said valve needle.

7. The improvement described in claim 6, wherein the diameter of the compensating piston is at least approximately equal to the diameter of said valve seat.

8. The improvement described in claim 5, wherein said connecting member is made of spring steel wire.

9. The improvement as described in claim 2, wherein said fuel supply duct extends essentially along the valve needle axis and has an enlarged portion, and said throttle means comprise a screw member having a throttling passage therein through which said enlarged duct portion communicates with said pressure chamber.

10. The improvement as described in claim 1, further comprising a guiding sleeve in said bore guiding, said compensating piston and socket means adapted for connection to a fuel return line, said socket means being located at the end of said bore remote from said valve needle.

11. The improvement as described in claim 1, wherein said compensating piston is arranged between said valve needle and said armature and is coupled to the latter, said coupling means to said valve needle comprising a rigid connecting member.

12. The improvement as described in claim 11, wherein the cross sectional area corresponding to the diameter of said compensating piston is smaller by the cross sectional area corresponding to the valve seat, than the cross sectional area corresponding to the guiding diameter of said valve needle in said bore.

13. The improvement described in claim 11, wherein said rigid connecting member has a collar acting as an abutment for said return spring.

14. The improvement described in claim 13, wherein said collar bears a spherically shaped supporting means and said return spring means comprises a return spring and a spring seat disc supporting one end of said return spring.

15. The improvement described in claim 11, wherein said rigid connecting member bears at each end thereof a spherical head, said compensating piston has a spherically shaped recess for receiving therein one of said heads, and said valve needle has a spherically shaped recess for receiving therein the other head, of said rigid connecting member.

16. The improvement described in claim 15, wherein the centers of said spherical heads and of said spherical recesses are located at least approximately on the central longitudinal axes of portions of said compensating piston and of said valve needle, respectively, which portions are guided in said bore.

17. The improvement described in claim 1, wherein said connecting means of said armature to said valve needle comprise a connecting rod having radial play relative to said armature.

18. The improvement described in claim 17, wherein said connecting means further comprise articulated joint means disposed between said connecting rod and said valve needle and comprising a pin extending through said valve needle and through said connecting rod traversely to the valve needle axis, said connecting rod having a traverse bore therethrough, both ends of which are beveled.

19. The improvement described in claim 1, further comprising a hollow guiding plug mounted fluid-tight in said discharge outlet, said plug having a cavity therein for guiding an end portion of said valve needle downstream of said valve seat, and wherein said compensating piston serves as said connecting means between said valve needle and said armature and has a diameter which is at least approximately equal to the diameter of said guiding plug and to the diameter of said valve seat.

20. The improvement described in claim 19, wherein said cavity in said guiding plug is closed toward the end of said valve needle facing toward said compensating piston, said plug having at least one substantially radially extending discharge orifice from said cavity to the outside of said valve, an annular groove about the periphery of said plug in the region of said valve seat and controllable by the latter, and at least one duct through the wall of said plug for connecting said cavity and said annular groove.

21. The improvement described in claim 1, further comprising a spray plug guided in said discharge outlet and being connected to the end of said valve needle downstream of said valve seat, and wherein said valve needle has a piston portion upstream of said valve seat and guided in said bore, and wherein said compensating piston serves as connecting said means between said valve needle and said armature and has a diameter which is at least approximately equal to the diameter of said valve seat.

22. An electromagnetically operable fuel injection valve as described in claim 21, wherein said spray plug forms a narrow annular gap together with said discharge outlet, which gap is laid out for only a partial amount of the total fuel quantity to be injected during operation under full load, and wherein said spray plug has at least one spray bore, fully cleaned by said valve seat after a preliminary stroke and laid out for the fuel amount to be injected under full load, or for a fuel amount to be injected under partial load, which latter fuel amount is larger than the partial amount injectable through said annular gap.