GB2043777A - Fuel Injection System for Internal Combustion Engines - Google Patents

Abstract

The duration of injection by the injection valve (10) Fig. 1, fed from a source (13) of constant pressure is limited in dependence upon the actual quantity of fuel injected by a free loo piston arrangement (31) which covers at each injection operation a metering stroke (H1) substantially proportional to the quantity of fuel injected and, when it has completed its metering stroke (H1), actuates an activating member (39) via sensor 36 of the control device (23) which initiates the closing movement of the valve needle (17). Throttle line 35 permits return spring (34) to return piston to its starting position by the return spring (34) during the intervals between the injection operations. Alternatively the activating member may comprise a push rod 97 (Fig. 6) or a limit switch (Fig. 4 not shown) to actuate a control valve effecting the termination of injection; the starting position of the piston 93 being variable by means 101 to determine the metered amount of fuel. <IMAGE>

Description

SPECIFICATION Fuel injection System for Internal Combustion Engines The invention relates to a fuel injection system for an internal combustion engine, and more particularly one of the type described in Swiss Patent Specification No. 447 714. In this fuel injection system, the fuel located in the pressure chamber of the injection valve is permanently subjected to the pressure constantly supplied by the fuel pump, and the injection time is determined by the duty ratio, i.e. the ratio of "on" time to "off" time of the valve, of at least one electromagnetic control valve. In this instance, a free piston which is fitted in the pressure line, and which, at each injection operation, covers a stroke.

dependent upon the quantity of fuel injected, acts as a safety device and shuts off the supply of fuel to the injection valve if the quantity of fuel injected exceeds the maximum admissible quantity by a specific amount upon, for example, faulty switching of the solenoid control valve or upon sticking of the valve needle. In common with all other fuel injection systems, such as mechanically-hydraulically controlled fuel injection systems in which the duration of the injection of the fuel supplied under constant high pressure to the nozzle needle, is determined by the duty ratio of the control valves, this known fuel injection system has the disadvantage that the commencement of injection and the termination of injection, that is to say, the period of injection, are dependent upon the switching time of the control valve which has to be accurately complied with.Difficulties then arise particularly in the case of high-speed internal combustion engines; and the object of the present invention is to provide a system in which the control of the commencement of injection is separated from that of the termination of injection and in which the quantity of fuel injected at the high-pressure side is accurately metered.

Moreover, the free piston used in the known fuel injection systems also limits the quantity of fuel injected during idling; and, during part load, always limits it only to an injected fuel quantity which exceeds, by a specific amount, the maximum admissible quantity of fuel injected, that is to say, the engine can accelerate in a nonregulated manner despite the safety device.

There is provided by the present invention a fuel injection system for an internal combustion engine, having a fuel pump which feeds fuel under constantly high pressure by way of a pressure line to a pressure chamber, adjacent to the valve seat, of an injection valve, and a free piston arrangement which is fitted in the pressure line and whose free piston, displaceable in a cylindrical bore against the force of a return spring, covers, during each injection operation, a stroke substantially proportional to the quantity of fuel injected and is urged back to its starting position by the return spring during the intervals between the injection operations, which free piston arrangement has a throttle line interconnecting the two end faces of the free piston, and the fuel injection system has a control device which effects the opening and closing movement of a valve needle pressed against the valve seat by a closing means, wherein the free piston acts as a metering member of the control device and, upon attaining a metering stroke which is dependent upon the quantity of fuel to be injected and which, taking into account the leakage losses in the throttle line is substantially proportional to the quantity of fuel injected, actuates an activating member of the control device which initiates the closing movement of the valve needle.

In contrast to the known system, the fuel injection system in accordance with the invention has the advantage that only the closing movement of the valve needle is controlled in dependence upon the injected quantity of fuel by the activating member of the control device which is actuated by the free piston serving as a metering member, whilst, independently thereof, the commencement of injection can be initiated in a known manner by a signal generator for the commencement of injection.

The present invention can also be used in a particularly advantageous manner in a fuel injection system whose control device includes an electrical adjusting member, preferably a solenoid, which determines the duration of injection of the injection valve and which is controlled by a signal generator for commencement of injection, if the working stroke of the electrical adjusting member effecting the termination of the injection is arranged to be triggered by the activating member. Thus, in accordance with the invention, control of the commencement of injection is separated from that of the termination of injection in the fuel injection system, which preferably is equipped with a solenoid injection valve.

In a fuel injection system of the type defined, having a pilot-controlled injection valve in which the valve needle has a pressure shoulder which is disposed in the pressure chamber and which is acted upon in the opening direction by the fuel pressure, and in which that end of the valve needle remote from the valve seat has a control piston whose control-pressure chamber is alternately subjectable by at least one control valve of the control device to a control pressure fed by way of a control-pressure line for the purpose of actuating the valve needle, or is connectible to a low pressure chamber by way of a relief line, if the control valve is switched to initiate termination of injection by means of the activating member, there results a particularly advantageous and operationally reliable design of the control device, irrespective of whether the control device is controlled electrically or mechanically-hydraulically; and in a fuel injection system in which the control pressure chamber subjected to control pressure during the injection operations is connectible to the relief line by a first control valve for the purpose of initiating the commencement of injection, and is connectible to the control-pressure line by a second control valve in order to terminate the injection operation, the advantages sought in accordance with the invention are achieved in that only the second control valve is acted upon by the activating member which is actuable by the free piston. The above features can be applied in an advantageous manner to electrically controlled fuel injection systems as well as mechanically-hydraulically controlled fuel injection systems.

In electrically controlled or regulated fuel injection systems, the activating member can be equipped in a preferred manner with an electrical, non-contacting, position sensor or limit switch. A reliable system needing only a relatively slowly operating control circuit ensues by providing the activating member with an electrical, preferably inductive, limit switch which is actuable by the free piston upon attaining the metering stroke, and by having the position of the free piston fixed at the commencement of the stroke by means of a controllable or regulatable stroke stop which is adjustable in the direction of the stroke of the free piston and co-operates with that end face of the free piston which is remote from the return spring.

In fuel injection systems in accordance with the type defined, and which are equipped with a pilot controlled injection valve, a reliable control device provided with low-frequency operating control members can also be realised if upon the commencement of the stroke, the position of the free piston is fixed by a controllable or regulatable stroke stop which is adjustable in the direction of the stroke of the free piston and which cooperates with that end face of the free piston remote from the return spring and, upon completing its metering stroke, the free piston is arranged to actuate, by way of a push rod, the second control valve which is fitted in the controlpressure line, which, when in its starting position, closes the control-pressure line with the exception of a throttling cross-section permitting a small leakage flow and which, when in its end position assumed by the action of the free piston, keeps open the full flow-through cross-section of the control-pressure line. The controllable or regulatable stroke stop used in this construction requires a relatively slowly operating position control, and the push rod acting as the activating member actuates the second control valve purely mechanically without additional auxiliary forces or auxiliary means, so that the entire circuit is rendered easy to inspect, simple, and subject to very little trouble. It will be appreciated that the first control valve must operate rapidly and close before the second control valve.This is unnecessary in the fuel injection system if it is constituted so that the second control valve is in the form of a slide valve whose valve spool has, in addition to a first control point controlling the inflow from the control-pressure line to the control pressure chamber, a second control point which at the termination of injection closes the passage through the relief line from the controlpressure chamber to a chamber of low pressure, and the first control valve is fitted in line with the second control point in the relief line connecting the second control valve to the chamber of low pressure.Thus, by using the second control point which closes the passage through the relief line from the control-pressure chamber to a chamber of lower pressure upon the termination of injection, the first control valve has to close only before the commencement of the following injection operation. In this case, all the control members can be of mechanical-hydraulic construction with the exception of optionally electrically controllable regulation of the stroke stop, thus resulting in an inexpensive and reliably operating fuel injection system. It will be appreciated that the first control valve can also be in the form of a solenoid valve in control devices constructed in this manner.

Since, in accordance with the invention, the termination of injection or the closing movement of the valve needle is initiated by the activating member actuated by the free piston, the invention can be applied to any known type of fuel injection system in which operation is effected with a uniformly high system pressure, or a system pressure which is regulated to a desired value, and which exists in the pressure chamber at the valve needle.

Five embodimentts of the present invention with additional variants of the construction of the free piston arrangement, are illustrated in the accompanying drawings, in which: Figure lisa simplified illustration of the first embodiment of a fuel injection system having a pilot-controlled injection valve, Figure 2 shows the second embodiment in which at least two injection valves are provided with a single free piston arrangement, Figure 3 shows the third embodiment having a double nozzle, Figure 4 shows a variant of the construction of the free piston arrangement having a regulatable stroke stop, Figure 5 shows a further variant of the free piston arrangement, Figure 6 shows the fourth embodiment having a mechanically actuated second control valve, and Figure 7 shows the fifth embodiment also having a mechanically actuated second control valve and a first control valve which is also mechanically actuated.

The first embodiment, illustrated in a simplified form in Figure 1, has a pilot-controlled injection valve 10 to which the fuel, subjected to a uniformly high pressure, is fed from a fuel pump 13 by way of a reservoir 11 and a pressure line 12. With appropriate value of the injection pressure, and appropriate design of the lines, the pressure line 12 can also assume the function of the reservoir 11.

The fuel fed under injection pressure by the fuel pump 13 flows from the pressure line 12 into a pressure chamber 1 5 which is adjacent to a valve seat 14 and into which extends a pressure shoulder 1 6 of a valve needle 1 7 which is in the form of a differential piston and which, in a known manner, is subjected to the pressure of the fuel acting upon the pressure shoulder 1 6 in a direction to effect the opening movement of the valve needle 17.When the valve needle 1 7 is in its illustrated closed position, the valve needle 1 7 is urged towards the valve seat 14 in the closing direction by means of an intermediate piston 1 8 which is acted upon by pressure and which, in the present instance, acts as a closing means, and which has a larger external diameter than that of the valve needle 1 7. The closing pressure required is produced by the fuel subjected to injection pressure and which, in the present case, is fed to a control-pressure chamber 1 9 above the intermediate piston 1 8 which is also to be regarded as a control piston.The control-pressure chamber 19 communicates by way of a control pressure line 21 with the pressure line 12 subjected to injection pressure, although, when the intermediate piston 1 8 is of correspondingly large piston surface area, the pressure line 12 can have the pressure reduced by a pressure-reducing valve (notjllustrated). It will be appreciated that, alternatively, a separate source of fuel pressure or oil pressure can be connected to the controlpressure line 21 in a known manner.

A control valve 24 forming part of a control device 23 is fitted in the control-pressure line 21 in the head 22a of the housing 22 of the injection valve 10 and, when in its illustrated starting position effected by a return spring 25, connects the pressure line 12, acting as a source of closing pressure in the present instance, to the controlpressure chamber 1 9. The control valve 24 is in the form of a three port, two position solenoid valve and is provided with a solenoid 26 and, when in its second switching position assumed when the solenoid 26 is energized, connects the control-pressure chamber 1 9 to a relief line 27 which leads to a fuel tank 28, to be considered as a source of low pressure, from which the fuel required is also drawn by the fuel pump 13; the latter being in the form of a high-pressure feed pump.

A free piston arrangement 31 is incorporated in the pressure line 12, and a free piston 33 is slidingly guided in the cylindrical bore 32 of the arrangement 31. The free piston 33 is held in its illustrated initial position by a return spring 31 during the intervals between the injection operations and, during each injection operation, covers a stroke H dependent upon the quantity of fuel injected and is returned to its starting position by the return spring 34 during the intervals between the injection operations. This is rendered possible by a throttle line 35 which interconnects the two end faces of the free piston 33. The throttle line 35 can be of very narrow construction since, particularly in diesel engines, the ratio of the injection interval to the injection time is at least 35:1.Thus, the effect of the throttle line 35 is so small that, in the following text, the stroke of the free piston 33 can be considered to be proportional to the quantity of fuel.

The stroke H of the free piston 33 is detected by a position sensor which is in the form of an inductive displacement pick-up and which in the present instance is indicated only by an external pick-up coil 37. The free piston 33 acts, in conjunction with the position sensor 36, as a metering member of the control device 23, since, upon attaining a metering stroke H, proportional to a predetermined injection quantity of fuel, the displacement signal Sw detected by the position sensor 36 is fed to a control circuit 38 which transmits to the solenoid 26 a control signal Sl which triggers the closing movement of the valve needle 1 7. The inductive displacement pick-up and the corresponding portion of the control circuit 38 then act as an activating member 39, determining the termination of injection, of the control device 23.

In the illustrated arrangement, the electromagnet 26 is de-energised in order to control the termination of injection, and the control-pressure chamber 1 9 is subjected to the fuel pressure effecting the closing movement of the valve needle 1 7. The commencement of injection is triggered cyclically by a signal generator 41 for commencement of injection which is driven in dependence upon the angle of the engine crankshaft, the associated control signal S, of the control circuit 38 which energizes the solenoid 26 being correctible in dependence upon further operating data B1, B2, for example, in dependence upon engine speed and/or load.

Alternatively, the control valve, illustrated in Figure 1, in the form of an electro-magnetically driven three port, two position valve and which acts as an electrically operated control member, can be replaced by two two port, two position valves (not illustrated), of which the first control valve controls the connection, effecting the commencement of injection, between the controlpressure chamber 19 and the relief line 27, and the second control valve, actuated by the activating member 39, only determines the termination of injection by connecting the controlpressure chamber 1 9 to the control-pressure line 21. The free piston arrangement 31 having the associated activating member 39, can also be used to control a solenoid injection valve whose valve needle is actuated directly by the coil of a solenoid against the force of a closing spring, as in the case of petrol injection systems. With a solenoid injection valve of this kind, the control device constituted by the free piston arrangement can contribute to the simplification of the electronic regulation (not illustrated).

Figure 2 shows that part of the second embodiment which differs from the fuel injection system of Figure 1 and in which, as in the following examples, the parts which are the same as those shown in Figure 1 are provided with the same reference numerals, and parts which function in the same manner, but which are of different construction, are provided with an index mark.

Since the time available for the return stroke of the metering piston 33 is substantially longer than the injection time, approximately 35 times longer, it is possible to connect at least two injection valves 10' and 10" to a common pressure line 12, control-pressure line 21 and relief line 27, only one free piston arrangement 31' for controlling the termination of injection being required for the two injection valves 10' and 10". In this instance, the free piston arrangement 31' is equipped with a free piston 33' which carries a pick-up pin 46 screwed into a central screw-threaded bore 45.The pick-up pin 46 extends within a cylindrical chamber 47, which accommodates the return spring 34 and which is formed by the cylindrical bore 32, into a pick-up coil 37' arranged in the same cylindrical chamber 47 and, together with the coil 37', forms an inductive displacement pick-up 36' which, together with the control circuit 38, forms part of an activating member 39'. The control circuit 38 produces both a first control signal S" determining the termination of injection by the injection valve 10' and a second control signal S12 actuating the solenoid 26 of the control valve 24 of the second injection valve 10". The function and mode of operation of the fuel injection system are otherwise the same as those of the first embodiment illustrated in Figure 1.The first injection valve 10' differs from the injection valve 10 illustrated in Figure 1 only by virtue of the fact that the intermediate piston 1 8 acting as a closing means is not provided, and, in Figure 2, the portion 1 7a' of the valve needle 1 7' which is adjacent to the control-pressure chamber 19, and which is acted upon by the pressure in this chamber, acts as a control piston applying the closing force and thus acts as a closing means. To ensure reliable and rapid closing when the valve needle 17' is open, a flow throttle 48 is incorporated in that portion 1 2a of the pressure line 12 which extends within the valve housing 22 and is designed such that it produces the pressure drop towards the pressure chamber 1 5 which is required for a sufficiently rapid closing movement of the valve needle 17'.As an alternative to this, a closing spring 49 acting as a closing means is fitted within the control-pressure chamber 19 in the second injection valve 10", thus rendering it unnecessary to provide a flow throttle in the pressure line 1 2.

The commencement of injection by the two injection valves 10' and 10" is initiated, at a correspondingly different angular position of the crankshaft, by the slightly modified signal generator 41 for the commencement of injection by way of the control circuit 38, and the associated termination of injection is controlled by the metering piston 33' which is effective for the two valves and whose throttle line 35' enabling the return stroke is arranged off-centre and, in conformity with the more rapid return time, is dimensioned differently from the throttle line 35 in Figure 1. In order to adjust the displacement signal Sw taken from the pick-up 37' of the position sensor 36', the position of the pick-up pin 46 within the free piston 33' can be adjusted and then fixed by means of a lock nut 51.

In the case of slow-running engines, at least one further injection valve can be actuated by the control device 23' comprising the activating member 39' and the control valves 24, as is indicated by extensions (indicated by dash-dot lines) of the pressure line 12, of the controlpressure line 21, and of the relief line 27.

In the third embodiment of Figure 3, the injection valve 60 in the form of a double nozzle is pilot controlled in the same manner as the previously described injection valves, for which purpose the two valve needles 61 and 62 are each controlled by a respective control valve 24.

The two valve needles 61 and 62 control the fuel to be injected from the common pressure chamber 1 5 fed by way of the pressure line 12, only the single free piston arrangement 31 being effective for the two valve needles 61 and 62.

Thus, advantageously, there is no abrupt jump in the quantity of fuel in the case of valve needles 61 and 62 which are triggered successively to effect an opening movement, since the injection quantity is dependent upon the speed of stroke of the free piston, and the termination of injection is dependent upon the termination of injection signal produced by the activating member 39. In a known manner, the two valve needles 61 and 62 can be actuated successively to form the injection characteristic, or alternatively, commonly to control the cross-section.

The free piston arrangement, which is only indicated in Figure 3, can be constructed in conformity with the free piston arrangement of Figures 1 or 2, or, alternatively, it can be constructed as illustrated in Figures 4 and 5.

The free piston arrangement 63 illustrated in Figure 4 includes a free piston 64 which, when in its illustrated starting position effected by the return spring 34, is retained by a controllable or regulatable stroke stop 65. In the intervals between the injection operations, that end face 64a of the piston 64 which is remote from the return spring 34 abuts against the stroke stop 65 (only indicated in the present instance) which is adjustable by means of an adjusting device (indicated by an arrow 66) in the direction of the stroke of the free piston 64 in conformity with the quantity of fuel to be injected. The termination of the stroke of the free piston 64 is detected by a pick-up coil 67a of an inductive position sensor 67 acting as an electrical limit switch and is fed to a control circuit 38'. Thus, in this arrangement, the metering stroke H2 determining the quantity of fuel injected is determined by the controllable and regulatable starting position of the stroke stop 65 and the end position of the free piston 64 which is constant in any given arrangement and which is detected by the limit switch 67. The activating member 68, comprising the limit switch 67 and the control circuit 38' in the present instance, can be used instead of either of the activating members 39, 39' illustrated in Figures 1 to 3. The construction of the associated injection valves remains unchanged.

Figure 5 shows a variant 71 of the free piston arrangement 63, illustrated in Figure 4, in which the starting position of the free piston 34 is controlled or regulated by an adjusting pin 72 acting as a stroke stop. The adjusting pin 72 is fitted in a readily displaceable and sealed manner in a guide bore 73 incorporated in a wall 76, separating a pressure chamber 75 from a cylindrical chamber 74 accommodating the free piston 64, of a housing 77 of the free piston arrangement 71. The position of the adjusting pin 72 which determines the starting position of the free piston 64 and thus the quantity of fuel to be injected, is determined by the adjustable initial stressing force of an adjusting spring 78 manufactured with an accurately defined spring rating, and by the hydraulic pressure in the pressure chamber 75 which is controlled or regulated by a pressure-regulating valve 79.In the present embodiment, the pressure chamber 75 is connected to the pressure line 12 of the fuel injection system by way of the pressureregulating valve 79. It will be appreciated that, alternatively, another pressure source can be used in the present instance. In the present arrangement, an inductive displacement pick-up 82 equipped with a pick-up coil 81 serves to feed back the starting position of the free piston 63 determined by the stroke stop 72, and at the same time acts as an electrical limit switch for the end position of the free piston 64 which initiates the termination of injection. The inductive displacement pick-up 82, at the same time acting as a limit switch, thus forms part of an activating member 83 (not further illustrated).The regulating intervention at the pressure-regulating valve 79 can be effected in a known manner and is only indicated by an arrow 84 in the present instance. In order to avoid inadmissible oscillations in the injection system and at the free piston 64, a non-return throttle valve 85 is fitted in that portion of the pressure line 12 which leads to the injection valve, and a damping throttle 87 is incorporated in a line 86 which connects the pressure-regulating valve 79 to the pressure chamber 75.

The fourth embodiment illustrated in Figure 6 also shows a pilot-controlled injection valve 90 which is connected to the pressure line 12, the control-pressure line 21 and the relief line 27. The injection valve 90 operates like the injection valve 10' in Figure 2 and its housing 91 also accommodates a free piston arrangement 92 whose free piston 93, acting as a metering member, has its starting position, to which it is urged by return spring 34, determined by the controllable or regulatable stroke stop 72 in the same manner as the free piston 64 in Figures 4 and 5. A first control valve 94 fitted in the relief line 27 is in the form of a two port, two position valve and, when in its activated state, establishes communication between the control-pressure chamber 19 and the relief line 27 and thus initiates the injection operation.The valve 94 can be controlled electro-magnetically or, alternatively, mechanically, by an actuating member 95 and must shut off communication with the relief line 27 shortly before the termination of injection.

A slide valve 96 acting as a second control valve is provided for controlling the termination of injection and, at the end of the metering stroke covered by the free piston 93, its valve spool 100 is displaced from its illustrated position by a push rod 97, acting as an activating member, against the force of a return spring 98 into a switching position in which a control point 99 in the form of an annular groove in the valve spool 100' connects the control-pressure line 21 to the control-pressure chamber 19, whereby the full pressure of the fuel is admitted into the controlpressure chamber 1 9 and presses the valve needle 17' against its valve seat 14 and thus terminates the injection operation. The stroke control device for the stroke stop 72 is indicated at 101 and is not further illustrated.Even when the control valve 94 is closed and the valve spool 100 has returned to its illustrated position, the control point 99 has a throttling cross-section 99a which permits a small leakage flow and by means of which the required closing pressure is maintained until the commencement of the following injection operation.

The injection valve 110 illustrated in Figure 7 has similar components and functions substantially in the same manner as the injection valve 90 of Figure 6. In addition to the coaxial arrangement of the valve needle 1 7, free piston arrangement 92 and the second control valve, designated 111 in the present instance, the injection valve 110 differs from the injection valve 90 particularly by virtue of a second control point 112 which is also in the form of an annular groove in the valve spool 11 3 and which, when the valve spool 11 3 is in its illustrated starting position, maintains communication between the control-pressure chamber 19 and the relief line 27, but which, when in its displaced position, which it assumes upon the termination of injection, shuts off this communication.At the termination of injection, the free piston 93 displaces the valve spool 11 3 by way of the push rod 97, acting as an activating member, into its other end position in which the annular groove 99, in this instance acting as a first control point, establishes communication between the controlpressure line 21 and the control-pressure chamber 1 9. The injection operation is thereby terminated by actuating the valve needle 1 7 by way of the intermediate piston 1 8. In this instance, in contrast to the embodiment illustrated in Figure 6, the first control valve 94 can maintain its open position (not illustrated) initiating the commencement of injection, until the control point 112 of the valve spool 113 again establishes communication with the relief line 27 during the return stroke of the valve spool 113. Thus, the control valve 94 can operate more slowly than that described with reference to Figure 6 and, as is indicated in the drawings, can be driven mechanically. In order to achieve accurate control, the control valve 94 (illustrated only symbolically) can be in the form of a linear spool provided with oblique edges, a piston valve provided with control grooves, or a rotary spool which has an adjustable control sleeve and thus renders it possible to vary the commencement of injection. It will be appreciated that the control valve 94 can also be actuated electromechanically. However, the fifth embodiment as illustrated in Figure 7 operates with mechanicalhydraulic actuating means and then, by virtue of the components in accordance with the invention and the use of the free piston 93 acting as a metering member, renders it possible accurately to control the quantity of fuel to be injected.

Claims (12)

Claims

1. A fuel injection system for an internal combustion engine, having a fuel pump which feeds fuel under constantly high pressure by way of a pressure line to a pressure chamber, adjacent to the valve seat, of an injection valve, and a free piston arrangement which is fitted in the pressure line and whose free piston, displaceable in a cylindrical bore against the force of a return spring, covers, during each injection operation, a stroke substantially proportional to the quantity of fuel injected and is urged back to its starting position by the return spring during the intervals between the injection operations, which free piston arrangement has a throttle line interconnecting the two end faces of the free piston, and the fuel injection system has a control device which effects the opening and closing movement of a valve needle pressed against the valve seat by a closing means, wherein the free piston acts as a metering member of the control device and, upon attaining a metering stroke which is dependent upon the quantity of fuel to be injected and which, taking into account the leakage losses in the throttle line is substantially proportional to the quantity of fuel injected, actuates an activating member of the control device which initiates the closing movement of the valve needle.

2. A fuel injection system as claimed in claim 1, whose control device has an electrical adjusting member, preferably a solenoid, which determines the duration of injection by the injection valve which is triggered by a signal generator for commencement of injection, wherein the working stroke of the electrical adjusting member which effects the termination of injection is triggerable by the activating member.

3. A fuel injection system as claimed in claim 1, having a pilot-controlled injection valve in which the valve needle has a pressure shoulder which is disposed in the pressure chamber and which is acted upon in the opening direction by the fuel pressure, and in which that end of the valve needle remote from the valve seat has a control piston whose control-pressure chamber is alternately subjectable by at least one control valve of the control device to a control pressure fed by way of a control-pressure line for the purpose of actuating the valve needle, or is connectible to a low pressure chamber by way of a relief line, wherein the control valve is switchable into its switching position, initiating the termination of injection, by means of the activating member.

4. A fuel injection system as claimed in claim 3, in which the control-pressure chamber subjected to control pressure during the intervals between the injection operations is connectible to the relief line by means of a first control valve for the purpose of initiating the commencement of injection, and is connectible to the controlpressure line by a second control valve for the purpose of terminating the injection operation, wherein only the second control valve is actable upon by the activating member actuable by the free piston.

5. A fuel injection system as claimed in any of the claims 2 to 4, wherein the activating member includes an electrical position sensor which detects the metering stroke of the free piston.

6. A fuel injection system as claimed in claim 5, wherein the position sensor is an inductive displacement pick-up.

7. A fuel injection system as claimed in any of the claims 2 to 4, wherein the activating member is provided with an electrical, preferably inductive, limit switch which is actuable by the free piston upon attaining the metering stroke, and the position of the free piston is fixed at the commencement of the stroke by means of a controllable or regulatable stroke stop which is adjustable in the direction of the stroke of the free piston and co-operates with that end face of the free piston which is remote from the return spring.

8. A fuel injection system as claimed in claim 5, 6 or 7, wherein the electrical position sensor or limit switch of the activating member has a pickup coil which is arranged in the cylindrical chamber accommodating the return spring and into which a pick-up pin, adjustably mounted in the free piston, enters during the metering stroke of the free piston.

9. A fuel injection system as claimed in claim 7, wherein the position of the stroke stop is determinable by an adjustable adjusting spring and a hydraulic pressure, controllable by means of a pressure-regulating valve, in a pressure chamber separated from the cylindrical chamber of the free piston.

10. A fuel injection system as claimed in claim 4, wherein upon the commencement of the stroke, the position of the free piston is fixed by a controllable or regulatable stroke stop which is adjustable in the direction of the stroke of the free piston and which co-operates with that end face of the free piston remote from the return spring and, upon completing its metering stroke, the free piston is arranged to actuate, by way of a push rod, the second control valve which is fitted in the control-pressure line, which, when in its starting position, closes the control-pressure line with the exception of a throttling cross-section permitting a small leakage flow and which, when in its end position assumed by the action of the free piston, keeps open the full flow-through cross-section of the control-pressure line.

11. A fuel injection system as claimed in claim 10, wherein the second control valve is in the form of a slide valve whose valve spool has, in addition to a first control point controlling the infiow from the control-pressure line to the control pressure chamber, a second control point which at the termination of injection closes the passage through the relief line from the controlpressure chamber to a chamber of low pressure, and the first control valve is fitted in line with the second control point in the relief line connecting the second control valve to the chamber of low pressure.

12. A fuel injection system substantially as hereinbefore described with reference to Figure 1, 2,3,4,5,6 or 7 of the accompanying drawings.