US3680535A

US3680535A - Fuel injection system for combustion engines

Info

Publication number: US3680535A
Application number: US81221A
Authority: US
Inventors: Konrad Eckert; Heinrich Knapp; Reinhard Schwartz; Gregor Schuster
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1969-12-01
Filing date: 1970-10-16
Publication date: 1972-08-01
Anticipated expiration: 1989-08-01
Also published as: AT306449B; GB1336606A; NL162993C; NL7017464A; SU491239A1; CH524062A; DE1960144A1; PL82919B3; FR2072632A5; SU491239A3; BE759677A; CS150645B2; ES386026A1; PL73432B1; JPS4946669B1; SE361335B

Abstract

In an external-ignition combustion engine operating with compression of the fuel mixture and continuous injection into the suction duct fuel is measured off by a valve by means of a slider actuated by a suction-responsive plate against a return force provided by fuel pressure and subject to control in dependence on operating characteristics of the engine. A pressure-control valve may be subject to a spring bias which is varied by a surface cam in response to throttle position and to pressure downstream of the throttle in the suction duct. A further control in response to engine temperature may be provided.

Description

United States Patent Eckert et al. [4 1 Aug. 1, 1972 [54] FUEL INJECTION SYSTEM FOR 2,591,356 4/1952 Howe ..-.251/50 A COMBUSTION ENGINES 3,182,974 5/1965 Hill ..26-1/50 A 3, $222,32111122222112312, 521/2211 i s i "ggz gg gfimg $33: 3,301,536 12/1967 Swatman et al ..261/50 A 3,539,159 ll/197O Handtmann ..261/50 A ga1t-S1llenbuch, Gregor Schuster, Stuttgart, all Of Germany FOREIGN PATENTS OR APPLICATIONS AssigneeI Robert Bosch GmbH, Stuttgart, 1,281,746 /1968 Germany ..261/50 many 1 [22] Filed. Oct. 16 1970 Primary. ExaminerLaurence M. Goodridge Assistant Examiner-Cort Flint PP 81,221 Attorney-Edwin E. Greigg Foreign Application Priority Data [57] ABSTRACT Dec 1 1969 Germany P19 60144 6 In an external-ignition combustion engine operating with compression of the fuel mixture and continuous [52] U s C] 123/119 R 261/ A 123/139 Aw injection into the suction duct fuel is measured off by F02m69/00 a valve by means of a slider actuated by a suction- [581 F1610;66111713115511166511666, 146111, plate against a e Provided by 123/140 MC, 119 139; 261/39 69 50 A, fuel pressurel and sub ect tofcontrol in dependence on 50 AA 44 52 operating c aractensticso the eng ne. pressurecontrol valve may be sub ect to a spring bias which is varied by a surface cam in response to throttle posi- [56] References cued tion and to pressure downstream of the throttle in the UNITED STATES PATENTS suction ctluct. A farther coingol in response to engine em ra re r0 1 2,318,216- 5/1943 Garretson ..261/50 A u may e p v 6 2,574,670 11/1951 Sweeney ..261/50 A 21 Claims, 8 Drawing Figures 67 BE 72 I 7 Z 7 I 76 s 77 20 67 22 r e, 1 m 73 37 38 x d 52 0 76 I, 74 L 69 2 b '2 82:5 3 5 {8 2.7 E 72 m 9 47 3 77 7 4 40 [I 270 E 1 36 f 22c e 750 75 ,b b 4 d 1 I J PATENTEDMIB H9 3.680.535

' SHEET 2 UP 4 Fig.2

Fig3

FUEL INJECTION SYSTEM FOR COMBUSTION ENGINES FIELD OF THE INVENTION The invention relates to a fuel-injection system for an external-ignition combustion engine operating with compression of the fuel mixture and continuous injec tion into the suction duct.

BACKGROUND OF THE INVENTION It is known to provide in such engines a sensor and an arbitrarily adjustable throttle in longitudinally spaced arrangement in the suction duct, the sensor being deflected against a substantially constant return force in proportion to the rate of air flow to control the movable member of a fuel valve for measuring off an amount of fuel proportional to the amount of air.

In this type of fuel-injection system, it is desirable for the deflection of the sensor to be as far as possible proportional to the rate of air flow, so that a proportional amount of fuel can be measured ofl by the fuel valve without complicated constructional features having to be provided to assure the proportionality. This requires that the return force of the sensor be as constant as possible.

In the prior fuel-injection system of this type according to German Pat. No. 1,281,746, a spring is provided to create the return force. To obtain as accurate as possible a measurement of the rate of air flow in the suction duct, the deflection of the sensor must, however, be as large as possible. On the other hand, the return force of a spring varies with the length of the spring, and in order to provide reasonable constancy, the spring must be made very weak and of great length, i.e. have a flat characteristic. This has the disadvantage that the total size of the injection system becomes comparatively large. Also, the force of the spring may vary owing to transverse vibrations brought about by the engine. On the other hand, it is impossible to provide a constant return force by means of a short spring having a steep characteristic.

OBJECT AND SUMMARY OF THE INVENTION It is an object of the invention to provide a fuel injection system which does not exhibit the disadvantages referred to above.

This is achieved according to the invention by the use of pressure liquid for creating the return force, the liquid being supplied continuously at substantially constant pressure via a pressure line to a movable member coupled to the sensor.

If it is now required to vary the fuel-to-air ratio, for instance, in order to provide a richer mixture for starting the engine, this can be achieved by a control of the return force on the sensor in dependence on some operating characteristic of the engine. It is known, for instance, from U. S. Pat. No. 2,583,408 relatinG to a fuel-injection system having a spring for exerting a return force on the sensor, to vary the bias of the spring in dependence on temperature. In this connection, also, the use of a spring for the return force has various drawbacks. For instance, the force is not a linear function of displacement. Also, the spring must be comparatively long and weak in order to provide under normal operating conditions a substantially constant return force, but this, on the other hand, requires the displacement for varying the force of the spring to be correspondingly large. The size of the arrangement becomes correspondingly large even at small energy levels. Furthermore, the transmission of the engine characteristic to the spring requires complicated mechanical constructions.

In a favorable embodiment of the invention, at least one pressure control valve is provided downstream of the control member for varying the liquid pressure of the return force in response to operating characteristics of the engine.

Such an arrangement is advantageous in that the variation of the liquid pressure does not have to occur in the vicinity of the throttle but can be translated to any desired point by means of conduits. If it is desired to control in dependence on throttle position, the pressure control valve can be positioned close to the throttle without the latter having to be close to the sensor, which could lead to undesired aerodynamic interaction between these two units. Thus, it becomes possible to place the sensor with the measuring valve close to the air filter, which is practical in view of their appreciable size, whereas the throttle can be located immediately in front of the main part of the suction duct, from which connections are branched off to the cylinders. A further advantage is the incompressibility of the liquid, which makes for a more exact control.

According to a further embodiment, the movable member is coupled at least indirectly to the movable member or slider of the fuel valve, the slider operating in a cylinder which is connected to the fuel line via a channel having a restriction provided therein. This damps the control movements of the slider and provides more uniform engine operation.

In accordance with a still further embodiment, the liquid pressure is controlled in response to throttle position and/or suction duct pressure downstream of the throttle and/or at least one temperature characteristic. A twoor three-dimensional cam or electric circuit may serve to transmit such characteristics to the fuel valve. The liquid pressure may be derived from the fuel supplied to the fuel valve, preferably from a point longitudinally spaced therefrom. In the latter alternative, -a pressure-reducing valve may be inserted between the fuel valve and the point in question, thereby providing a pair of hydraulic circuits, one for measuring and one for the return function. A pressure fluctuation in one circuit, e.g. owing to fuel flow, then has little influence on the precision of the control action of the other circuit.

The invention will be explained in more detail with reference to embodiments thereof shown in the drawinG.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows an embodiment of the entire fuel injection system;

FIGS. 2, 3 and 4 show different embodiments of pressure control valves;

FIG. 5 shows the arrangement of a pressure-reducing valve in the fuel circuit;

FIG. 6 shows an hydraulic damping means for the moveble member of the measuring valve;

FIG. 7 is a section taken on line VII-VII of FIG. 8 through a pressure-reducing valve operating with a temperature-responsive member which simultaneously controls a bypass of the suction duct; and

FIG. 8 is a section taken on line VIII-VIII of FIG. 7.

Throughout the figures, identical parts are designated with identical reference numerals. Primed reference numerals indicate modified forms.

DESCRIPTION OF EMBODIMENTS In the fuel injection system shown in MG. 1, combustion air flows in the direction of the arrows through an air filter 2 lodged in a housing 1, a section 3 of a suction duct having a sensor 4 of a measuring device therein, a connecting tube 5 and a section 6 of the duct having an arbitrarily adjustable throttle 7, to one or more cylinders of an internal combustion engine (not shown). Sensor 4 consists of aplate mounted perpendicularly to the direction of flow but could also be a piston slidable perpendicularly to the said direction or a shutter rotatable on a spindle traversing the duct. In any case, sensor 4 moves in section 3 according to a substantially linear function of the rate of air flow, the pressure in the region between sensor 4 and throttle 7 being substantially constant for a constant return force exerted on sensor 4 and a constant air pressure ahead of sensor 4.

Sensor 4 immediately controls a measuring distributor valve 8. To transfer the movements of sensor 4, there is provided an arm 10 attached thereto and pivoted with a minimum of friction on a pin 9. Provided on arm 10 is a boss 11 for controlling the movable valve member or slider 12 of measuring valve 8. The end face 13 of slider 12 remote from boss 11 is subject to the pressure of a liquid serving as a return force for the sensor 4. The inoperative position of arm 10 is defined by an adjusting screw 14.

Fuel is supplied by a fuel pump 16 driven by an electric motor 17 and drawing fuel from a container 18 and supplying it through a fuel line 19 to measuring valve 8. From line 19 there branches off a return line 20 having a pressure-limiting valve 21 provided therein. The fuel flows from line 19 into a channel 22 in the housing of measuring valve 8. Channel 22 ends in an annular notch 220 provided in the housing and from which borings 22b lead to a cylinder 23 having a slider 12 fitting tightly therein. Borings 22b terminate at a point of cylinder 23 at which slider 12 has a peripheral notch 24 which is therefore connected without any intermediate restriction to annular notch 22a. Notch 24 partially overlaps a pair of control slots 25 to a greater or lesser extent depending on the position of slider 12. From control slots 25 fuel flows into channels 26 connecting with the injection valves (not shown) in the suction duct of the engine. From annular notch 22a the fuel not flowing into channels 26 flows into a channel 22c and reaches an annular notch 22d from which it passes through borings 22e into a line 27 leading to a pressure control valve 28 and from which a connection exists to cylinder 23 at the end face 13 of slider 12. In this passage, a restriction 23a is provided for damping the movements of slider 12. Also provided in channel section 220 is a restriction for reducing as much as possible the influence of pressure variations in the fuel measuring system on the slider return system. Pressure control valve 28 is a diaphragm valve 27a of the flat seat type with a diaphragm 29 as the movable valve member. Already a minimal movement of diaphragm 29 is sufficient to open the full annular valve area. The overflowing fuel then flows without pressure through a line 30 back into fuel container 18. Diaphragm 29 is biased by a spring 31, the force of which is adjustable in response to a control efl'ect derived in dependence on engine operation. To this end, there is provided a surface carn 32 which is rotatable jointly with throttle 7 and axially displaceable in dependence on he degree of suction or subpressure downstream of throttle 7 in the suction duct;

Surface cam 32 is axially displaceable on the shaft 33 of throttle 7, which is arbitrarily adjustable with the aid of a bar 34. Rotation of shaft 33 is transferred by an angular catch member 35 to surface cam 32, which is journaled at one end thereof on a diaphragm 36 of a suction chamber 37. Chamber 37 is connected via a line 38 to a point in the suction duct downstream of the throttle. If sufficient subpressure is present, cam 32 is displaced axially by diaphragm 36 against the force of a return spring 39. The cam surface is sensed by a pin 40, the movement of which is transferred by a spring disc 41 on spring 31, the bias of which determines the pressure for the return force on sensor 4.

Branching ofi from line 27 is a further line 67 connecting with a second pressure control valve 68, from which it is returned without pressure via a line 69 to fuel container 18. This valve controls the pressure for the return force of slider 12 and sensor 4 in dependence on engine temperature and comprises a flat-seat valve 70, the rate of flow of which is controlled by a diaphragm 71, which is biased in the closing direction by a spring 72. The fuel therefore flows through line 67 and valve 70 into a space 73 and thence without pressure via return line 69 to container 18.

Container 18 is subject to atmospheric pressure and approximately the same pressure is therefore present also in space 73. The space 74 separated from space 73 by diaphragm 71 and having spring 72 mounted therein forms part of a

bypass

75, 75a for the throttle of the suction duct and of which only the junctions with the suction duct and with valve 68 have been shown. Provided in space 74 is a slider 76 for controlling the bypass area and also serving as a spring washer for spring 72.

Slider 76 is displaced by a temperature-responsive member, such as a thermalexpansion device, 78 causing spring 72 to be less compressed and

bypass

75, 75a to be more open when the engine is cold than when it is warm. Therefore, in a cold engine more liquid flows through valve 70, the liquid pressure causing the return force to be correspondingly lower and the fuel-to-air ratio of the injected mixture higher.

The operation of the fuel-injection system for an engine that has reached its normal operating temperature is as follows:

With the engine running, pump 16 is driven by motor 17 to draw fuel from container 18 and supply it via line 19 to measuring valve 8. Simultaneously herewith, the engine draws air through suction duct 3, 5, 6 and causes sensor 4 to be deflected from its inoperative position.

Corresponding to this deflection of sensor 4, slider 12 is displaced by arm and uncovers a larger area of control slots 25. The quantity of fuel supplied to the engine therefore varies in dependence on the control effect derived from sensor 4. From annular notch 24 the remaining fuel flows to the terminal side of slider 12 and therefrom to

pressure control valves

28 and 68.

The direct coupling of sensor 4 with slider 12 results in a constant fuel-to-air ratio if the characteristics of these two members are sufficiently linear, which is the desired type of operation. The fuel-to-air ratio would then be constant over the entire operating range of the engine. As was indicated above, however, it is desirable to make the fuel mixture richer or thinner according to the section of the operating range of the engine. This is achieved, according to the invention, by varying of the return force acting on sensor 4.

As quantities that are indicative of engine loading and rpm there are available the throttle position and the suction duct subpressure and it is therefore convenient to derive the return force from these. This is achieved by means of a variation of the force of spring 31 of pressure control valve 28 occasioned by a corresponding rotational or axial displacement of surface cam 32 in dependence on the position of throttle 7 and the pressure in the suction duct, respectively. For instance, if the throttle at full load is in a position in which the suction duct is fully opened, it is desired to have the highest output power, i.e. a relatively rich mixture. Since the bias of spring 31 of control valve 28 determines the pressure acting on the end face 13 of slider 12, the return force acting upon sensor 4 must be reduced by a small amount to cause displacement of slider 12 to a position in which control slots 25 are more open and a correspondingly larger amount of fuel is injected. Contrarily, in the section of the operating range corresponding to a partially loaded state, the relatively higher pressure on the end face 13 of slider 12 results in a smaller deflection of sensor 4 and a correspondingly thinner mixture.

On the other hand, in the passive part of the operating range, with the engine being pushed by the vehicle, a strong subpressure is present in the suction duct and surface cam 32 is pushed far ahead against spring 39 causing a strong bias to be exerted by spring 31 of the pressure control valve. This increases correspondingly the return force on sensor 4, whereby in spite of the small amount of leakage air flowing around the closed throttle, there is no deflection of sensor 4 and no injection of fuel.

As long as the engine is cold, pressure control valve 68 causes an enrichment of the fuel mixture, since the pressure from which the return force is derived is now lower. Owing to the fact that a portion of the air flows through

bypass

75, 75a, sensor 4 is deflected more than what corresponds to the throttle position, so that already on account of this deflection a greater amount of fuel is measured ofi.

The pressure of the fuel acting on end face 13 of slider 12 and causing the return force on sensor 4 is therefore maintained inherently constant and is varied only in dependence on engine characteristics, such as in the embodiment shown, the position of the throttle, i.e. in dependence on the load, as well as the subpressure in the suction duct, i.e. in dependence on the rpm.

FIG. 2 shows the portion of the system comprising the pressure control valve. In this embodiment, the bias of spring 31 of control valve 28 is controlled by a cam 43, which is displaceable only axially in response to the pressure obtaining downstream of the throttle in the suction duct. To this end, cam 43 is mounted on rollers 44. The control movement is transferred in this case also to spring washer 41 by a pin 40. The throttle therefore is now in no way coupled to the cam. The adjustment of the bias of spring 31 and therefore the return force on sensor 4 is therefore dependent on rpm and load.

In the FIG. 3 embodiment, on the other hand, pin 40 is controlled by a cam 46 which is mounted on shaft 33 of throttle 7, whereby upon shaft 33 being rotated by operation of bar 34, pin 40 is displaced by cam 46. In this case therefore, there is a variation of the return force on sensor 4 in dependence only on the load.

In the FIG. 4 embodiment of the pressure control valve, the bias of spring 31 is varied by a surface cam 32, just as in FIG. 1. However, there is a difference in that there is provided in diaphragm 29' an aperture 48, through which fuel can flow from line 27 and the space 49 delimited by diaphragm 29 into the space 50 in which spring 31 is mounted and from there via line 30 back to container 18. This causes the pressure of container 18 to be present on both sides of diaphragm 29, whereby a possible fluctuation in the pressure of the fuel container has no control effect on the diaphragm.

In order to handle larger corrections by meanS of the pressure control valve, there is provided in the FIG. 5 embodiment of the measuring valve 8 in channel section 22c a pressure reducinG valve 52 shown as a simple diaphragm valve, the diaphragm 53 of which is subject on one side to the supplied fuel and on the other side to a spring 54 in combination with the atmospheric pressure entering through an aperture 55.

To prevent the slider of the measuring valve from being pushed back and forth by possible pressure shocks which might be transferred to it from the suction duct via sensor 4 and lead to fluctuations in the rate of injection and jerky behavior of the vehicle, FIG. 6 shows an embodiment 8" of the measuring valve in which the movement of slider 12" and therefore also that of sensor 4 is braked and damped by means of a disc 57 placed on the end face of slider 12".

To this end, slider 12" extends with its end face 13 into a cylindrical space 58 of approximately the same diameter as disc 57 and which is divided thereby into

spaces

58a and 58b connected with each other by means of damping

borings

23a and 60 to channel 220. Since the surface of disc 57, which is subject to the fuel pressure in space 58a, is larger than the surface facing space 58b,

borings

23a and 60 must be proportioned to have equal damping effects in both directions of slider 12".

In the embodiment shown in FIGS. 7 and 8,

pressure control valves

28 and 68 of FIG. 1 have been constructionally integrated into a pressure control system 28". In this case also, spring 31' can be controlled by means of a surface cam 32'. In addition, however, there is provided for transmitting the engine temperature a thermal expansion regulator 62 for controlling spring 31'. Regulator 62 acts via slider 63 on one arm of a twoarmed lever 64, the pivot point 64a of which is coupled to spring 31', and the other arm of which is in engagement with surface cam 32'. Lever 64 is guided by means of a link 65 attaching it to the housing. Slider also controls an idling channel 66 which, similarly to FIG. 1, connects a point upstream of the throttle in the suction duct with a point downstream thereof. With increasing temperature, slider 63 is pushed towards a position in which the bias of spring 31' and therefore also the return force on sensor 4 increases.

The invention can be used in any type of system where a suction duct draws in an amount of air, to which there is to be admixed a corresponding amount of a liquid, for instance, in chemical apparatus, gasoline heating systems and power turbines.

That which is claimed is:

1. In a fuel injection system for an extemal-ignition combustion engine operating on fuel injected into the suction duct and including an arbitrarily operable butterfly valve disposed in said suction duct, said system being of the known type that includes (a) a fuel valve for metering the fuel quantities supplied to said engine, (b) a sensor member forming part of an air sensor means and being positioned in said suction duct spaced from said butterfly valve, said sensor member being movable by and as a function of the air flowing in said suction duct, (c) arm means forming part of said air sensor means and being attached to said sensor member and engaging said fuel valve for transmitting the motion of said sensor member to said fuel valve to ensure the metering of fuel quantities proportionate to the air quantities flowing in said suction duct and (d) means for applying a substantially constant return force to said sensor member, the improvement comprising A. a movable member being in engagement with said air sensor means and following the movements thereof and B. means, including a pressure line, for continuously supplying liquid under constant pressure to said movable member for urging the latter against said air sensor means with a constant force having a magnitude independent from the position of said air sensor means and the position of said movable member, said means for continuously supplying liquid under constant pressure and said movable member constituting said means for applying a substantially constant return force to said sensor member.

2. A fuel-injection system accordinG to claim 1, comprising at least one pressure control valve responsive to an engine operating characteristic for controlling the pressure of said liquid.

3. A fuel-injection system according to claim 2, in which said pressure control valve is disposed in said pressure line downstream of said movable member.

4. A fuel-injection system according to claim 1, in which said movable member is coupled to said fuel valve.

5. A fuel-injection system according to claim 1, in which said movable member comprises a slider operating in a cylinder in a fluid-tight manner, said cylinder 6. A fuel-injection system according to claim 2, comprising means for controlling the pressure of said pressure control valve in response to deflection of said sensor-member.

7. A fuel-injection system according to claim 2, comprising means for controlling the pressure of said pressure control valve in response to the pressure in said suction duct downstream of said sensor member.

8. A fuel-injection system according to claim 2, comprising means for controlling the pressure of said pressure control valve in response to at least one temperature characteristic of said engine.

9. A fuel-injection system according to claim 6, comprising a cam movable in response to an engine operating characteristic to control said pressure control valve.

10. A fuel-injection system according to claim 6, comprising a surface cam movable in response to an engine operating characteristic for controlling said pressure control valve.

11. A fuel-injection system according to claim 2, comprising electric circuit means for transmitting an engine operating characteristic to said pressure control valve.

12. A fuel-injection system according to claim 1, comprising means for supplying fuel from said fuel valve as the liquid to said pressure line.

13. A fuel-injection system according to claim 12, in which said movable member responds to fuel pressure in said pressure line at a point spaced in the direction of flow from said fuel valve.

14. A fuel-injection system according to claim 13, in which a pressure-reducing means is provided between said fuel valve and said point.

15. A fuel-injection system according to claim 5, in which said movable member is a slider movable in a cylinder of said fuel valve and having an annular notch cooperating with control slots provided in said cylinder for measuring off fuel, said slider having an end face extending into a chamber to respond to said liquid pressure and a disc on said end face for damping movements of said slider.

16. A fuel-injection system according to claim 15, in which spaces are prOvided on both sides of said disc connected via individual restriction channels with said pressure line.

17. A fuel-injection system according to claim 2, in which said pressure control valve is a flat-seat valve having a biasing spring, said biasing spring being responsive to an engine operating characteristic for varying the bias thereof.

18. A fuel-injection system according to claim 17, in which the movable valve member of said pressure control valve is a diaphragm.

19. A fuel-injection system according to claim 18, in which both sides of said diaphragm are subject to equal fuel pressures.

20. A fuel-injection system according to claim 2, comprising: a bypass to said suction duct around said sensor member, a temperature-responsive control element coupled to a pressure control valve and to a closure member inserted in said bypass for closing the same in response to the normal operating temperature of said engine.

21. A fuel injection system according to claim 2, comprising 9 10 a first pressure control valve responsive to the posiwith said first pressure control valve and respontion of said arbitrarily adjustable throttle member sive t th ng n t p ratur said first and said and to the pressure downstream thereof in said Second Pressure. Valves y the liquid suction duct and pressure generating said return force. a second pressure control valve connected parallel 5

Claims

1. In a fuel injection system for an external-ignition combustion engine operating on fuel injected into the suction duct and including an arbitrarily operable butterfly valve disposed in said suction duct, said system being of the known type that includes (a) a fuel valve for metering the fuel quantities supplied to said engine, (b) a sensor member forming part of an air sensor means and being positioned in said suction duct spaced from said butterfly valve, said sensor member being movable by and as a function of the air flowing in said suction duct, (c) arm means forming part of said air sensor means and being attached to said sensor member and engaging said fuel valve for transmitting the motion of said sensor member to said fuel valve to ensure the metering of fuel quantities proportionate to the air quantities flowing in said suction duct and (d) means for applying a substantially constant return force to said sensor member, the improvement comprising A. a movable member being in engagement with said air sensor means and following the movements thereof and B. means, including a pressure line, for continuously supplying liquid under constant pressure to said movable member for urging the latter against said air sensor means with a constant force having a magnitude independent from the position of said air sensor means and the position of said movable member, said means for continuously supplying liquid under constant pressure and said movable member constituting said means for applying a substantially constant return force to said sensor member.

5. A fuel-injection system according to claim 1, in which said movable member comprises a slider operating in a cylinder in a fluid-tight manner, said cylinder being connected to said pressure line via a connecting channel having a restriction provided therein, said slider having a radial face exposed to said constant pressure.

6. A fuel-injection system according to claim 2, comprising means for controlling the pressure of said pressure control valve in response to deflection of said sensor member.

21. A fuel injection system according to claim 2, comprising a first pressure control valve responsive to the position of said arbitrarily adjustable throttle member and to the pressure downstream thereof in said suction duct and a second pressure control valve connected parallel with said first pressure control valve and responsive to the engine temperature; said first and said second pressure control valves vary the liquid pressure generating said return force.