GB2092223A

GB2092223A - Fuel Injection System

Info

Publication number: GB2092223A
Application number: GB8136947A
Authority: GB
Original assignee: Nissan Motor Co Ltd
Current assignee: Nissan Motor Co Ltd
Priority date: 1980-12-27
Filing date: 1981-12-08
Publication date: 1982-08-11
Also published as: DE3150805A1; FR2497294A1

Abstract

A fuel injection system includes at least one fuel injection valve (10) of the type injecting fuel when the pressure in the fuel is sufficient to lift its valve member (28) off the associated valve seat (24) against the combined force due to a spring (42) and the pressure of hydraulic fluid in a chamber (50). The hydraulic pressure is controlled in accordance with engine operating conditions e.g. engine speed, injection timing, engine and ambient temperature and humidity, to vary the opening pressure of the fuel injection valve. <IMAGE>

Description

SPECIFICATION Fuel Injection System This invention relates to a fuel injection system for use in compression-ignition or spark-ignition internal combustion engines and, more particularly, to such a fuel injection system having at least one fuel injection valve through which fuel is injected into the engine when the pressure in the fuel is sufficient to lift its valve member off the assiciated valve seat against the force of a load spring.

With conventional fuel injection valves, such as the pintle type comprising a valve needle having its tip portion forced tightly against a valve seat by a load spring to tightly close the nozzle orifice, injection commences when the pressure in the fuel is sufficient to overcome the pressure in the combustion chamber and ceases when the fuel pressure falls somewhat below the opening pressure. The load spring is selected suitably to have a strength sufficient to rest the valve needle tip portion on the valve seat under the combustion chamber pressure. Although the value of the opening pressure can be adjusted by altering the strength or initial compression of the load spring, it cannot be changed after the fuel injection valve is once installed on the engine.

It is the current practice to supply fuel to such fuel injection valves by employing a distributor type fuel injection pump having a single pumping element which serves all cylinders. Distribution to the individual fuel injection valves is made through a rotary distributor having the required number of outlet ports. This ensures uniform fuel delivery to all fuel injection valves and accurate uniform intervals between successive injections.

However, such a fuel injection pump has a tendency to cause rapid combustion leading to engine knock which gives rough and noisy running of the engine at idling speed during which the pressure in the fuel delivered from the fuel injection pump is relatively high and the rate of fuel injection into the engine is relatively high. In order to suppress such idling knock, the need has been recognized for an improved fuel injection system capable of lengthening the injection period whilst reducing the rate of fuel injected into the engine during idling.

Furthermore, the operating conditions of internal combustion engines particularly for use in automotive vehicles frequently vary considerably.

It is therefore desirable, by controlling the rate of fuel injected into the engine in accordance with varying engine operating conditions, to suppress diesel knock or rough engine running and reduce unwanted exhaust emissions. It may be considered to employ, as fuel injection valves, solenoid valves whose operational timing can readily be controlled in accordance with engine operating conditions. However, such solenoid valves will have a large solenoid valve for lifting the valve member off the valve seat against a large resilient force of the nozzle spring to permit fuel injection under a pressure sufficient to overcome the combustion pressure which is normally as high as 100 kg/cm2. This results in large and heavy fuel injection valves having a slow response time.

The present invention provides a new and improved fuel injection system including at least one fuel injection valve the opening pressure of which can be varied, in the course of the fuel injection, in accordance with engine operating conditions.

There is provided, in accordance with the present invention, a fuel injection system for use in an internal combustion engine which comprises a hydraulic pressure source for generating a predetermined hydraulic pressure, at least one fuel injection valve including a valve member biased by a load spring to rest against a valve seat, the fuel injection valve being adapted to inject fuel into the engine when the pressure in the fuel is sufficient to lift the valve member off the valve seat against the force of the load spring, and a pressure controller. The fuel injection valve includes a hydraulic actuator responsive to the hydraulic pressure delivered thereto from the hydraulic pressure source for applying a force to the valve member.The pressure controller controls the hydraulic pressure delivered from the hydraulic pressure source to the hydraulic actuator in accordance with engine operating conditions.

The hydraulic actuator comprises a pressure chamber formed in the fuel injection valve and communicated with the hydraulic pressure source, and a piston adapted to reciprocate within the pressure chamber. The piston is drivingly associated with the valve member for applying a force on the valve member in response to the hydraulic pressure charged in the pressure chamber.

Preferably, the pressure controller is associated with a lift sensor adapted to sense actual commencement and termination of fuel injection in response to movement of the valve member.

The lift sensor may comprise a piezo-electric element which provides a voltage signal when the load spring is compressed by movement of the valve member from the valve seat.

The pressure controller comprises a control unit adapted to provide a command signal in accordance with engine operating conditions, and a solenoid valve responsive to the command signal from the control unit for controlling the hydraulic pressure to the hydraulic actuator. The control unit may have inputs from a crankshaft position sensor adapted to provide a series of crankshaft position electrical pulses of a repetition rate directly proportional to engine speed and a reference pulse generator adapted to produce a reference electrical pulse at a predetermined number of degrees of rotation of the engine. The control unit provides, to the solenoid valve, a pulse signal corresponding to engine speed when a reference pulse signal occurs, thereby decreasing the hydraulic pressure to the hydraulic actuator with increases in engine speed.

The present invention will be described in greater detail by reference to the following description taken in connection with the accompanying drawings, in which: Fig. 1 is a longitudinal sectional view of a preferred embodiment of a fuel injection system constructed in accordance with the present invention; Fig. 2 is a sectional view showing a distributor type fuel injection pump for use in the fuel injection system of Fig. 1; Fig. 3 is a diagram showing a second embodiment of the fuel injection system of the present invention; Fig. 4 is a longitudinal sectional view showing a fuel injection valve used in the fuel injection system of Fig. 3; Fig. 5 is a longitudinal sectional view showing a modified form of the fuel injection valve of Fig.

4; Fig. 6 is a dragram showing an alternative circuit for controlling the operation of the fuel injection valves; and Fig. 7 is a longitudinal sectional view showing a conventional fuel injection valve.

With reference now to the conventional fuel injection valve shown in Fig. 7, there is shown a pintle-type fuel injection valve comprising a valve body A and a valve needle B which has its tip forced tightly against a valve seat C in the valve body by a compression coil spring D, thereby tightly closing the valve opening E. If the pressure in the fuel supplied through the inlet fuel passage F is above the opening pressure, the valve needle B will lift from the valve seat C against the force of the compression coil spring D to permit fuel injection through the valve opening E. The opening pressure of the fuel injection valve is determined by the strength of the compression coil spring D and cannot be changed in the course of fuel injection.

Turning now to Fig. 1 , there is illustrated one embodiment of a fuel injection system made in accordance with the present invention. The fuel injection system includes a fuel injection valve, generally designated at 10, which comprises an outer body or housing 12 within which an internal nozzle body 14 is mounted. The internal nozzle body 14 may be secured in position by a threaded fitment 1 6 so that a fuel passage 1 8 of the nozzle body 14 is disposed in communicating relation with an inlet fuel passage 20 of the fitment 1 6.

Within and coaxially with the internal nozzle body 4 is formed a cylindrical bore 22 for the reception of a valve member 28 slidable toward and away from a valve seat 24 which is communicated through the fuel passage 18 and the inlet fuel passage 20 with a fuel injection pump (not shown). The valve seat 24 is generally frusto conical and is intersected at its lower end by a nozzle orifice 26. The valve member 28 has a frusto-conical tip-portion 30 which is operable to matingly and conformingly engage the valve seat 24, in the seated position of the valve member 28, and close the nozzle orifice 26.

The valve member 28 has an extension 32 projecting upwardly therefrom as shown in Fig. 1, through an annular valve stop 34 into a cavity 36 formed in the fitment 16. The valve stop 34 is operable to lirnit and define the uppermost or fully open valve position. The extension 32 may abuttingly engage an inverted, mushroom-shaped fitment 38 which is biased toward this extension by a load spring 42 placed in the cavity 36between the fitment 38 and an annular abutment 44. The strength or initial compression of the load spring 42 should be selected to permit the valve member 28 to lift off the valve seat 24 when the fuel pressure is not less than the pressure developed in the combustion chamber.A vent 46 may be provided in the fitment 1 6 which communicates with the cavity 36 for removing fuel which leaks around the portion of the valve member 28 received within the cylindrical bore 22.

The inverted, mushroom-shaped fitment 38 has a connection rod 40 extending upwardly therefrom through the annular abutment 44 into a pressure chamber 50 is closed by an end cap 52 and opening upwardly, as shown in Fig. 1. The presure chamber 50 is closed by an end cap 52 through a sealing joint 60 placed between the end cap and the fitment 1 6. A conduit 54 extends through the end cap 52 into the pressure chamber 50 for the introduction of hydraulic pressure into the pressure chamber 50. A piston 56 is placed reciprocably within the pressure chamber 50. The piston 56 is abuttingly pressed against the upper tip end of the connection rod 40 by a compressed coil spring 58 placed between the piston 56 and the end cap 52 within the pressure chamber 50. The compressed coil spring 58 is effective to prevent the piston 56 from chattering.

The hydraulic pressure conduit 54 connects the pressure chamber 50 to a hydraulic pressure source 100 from which a constant hydraulic pressure is developed. The hydraulic pressure in the conduit 54 is controlled through a pressure controller 210 by a control unit 200 in accordance with engine operating conditions. The pressure controller 210 may be in the form of a solenoid valve although there is no intention to be limited thereto. The resultant force of the resilient force of the load spring 42 and the force resulting from the product of the hydraulic pressure charged in the pressure chamber 50 and the effective area of the piston 56 acts on the valve member 28 to rest it on the valve seat 24. The valve member 28 can be lifted off the valve seat 24 when the pressure in the fuel supplied through the fuel passages 1 8 and 20 reaches the opening pressure which creates, on the valve member tip portion 30, a force sufficient to overcome the resultant force. The fuel injection valve will not close until the fuel pressure has fallen to somewhat below the opening pressure.

One example, and without intention or inference of a limitation thereto, of a hydraulic pressure source suitable for use with the system of the present invention includes a distributor type fuel injection pump.

Referring to Fig. 2, the distributor type fuel injection pump is designated generally at 11 0.

The fuel injection pump includes a vane pump 112 secured on a drive shaft 114. The drive shaft is drivingly connected to an engine crankshaft and is rotated at one-half crankshaft speed. When the engine is operating, the vane pump 112 rotates to draw fuel through a filter 11 6 from a fuel tank 118 and discharge pressurized fuel through a pressure regulator 120 into a pump housing 122.

It will be understood that the pressure in the fuel charged in the pump housing 122 is proportional to the speed of rotation of the engine.

The fuel injection pump 110 also includes a plunger-pump 130 which is comprised of a cylinder 132 and a plunger 134 placed therein for reciprocating and rotating movement within the cylinder 132. The cylinder and the plunger define a pump chamber 136 into which the fuel charged in the pump housing 122 is introduced through a conduit 124. The conduit 124 is provided therein with a cut-off solenoid valve 138. The plunger 134 is secured at its one end to a disc cam 150 which is drivingly connected through a joint 1 52 to the drive shaft 114. The disc cam 1 50 has thereon face cams numbered in accordance with the number of engine cylinders.The face cams successively ride on a roller 1 54 provided on a roller ring 156 with rotation of the drive shaft 11 4. As a result, the plunger 1 34 rotates and reciprocates within the cylinder 132 to distribute pressurized fuel through outlet ports 1 58 to the individual fuel injection valves 10.

A governor mechanism 1 60 is drivingly connected through a drive gear 1 62 to the drive shaft 114. The governor mechanism 1 60 controls the amount of fuel to the individual outlet ports 1 58 by adjusting the position of a control sleeve 142 with respect to the metering port 144 formed in the plunger 134. The reference numeral 1 70 designates a timing control mechanism which is operable to control the timing of fuel injection by adjusting the angular position of the roller ring 1 56.

Referring back to Fig. 1, a part of the fuel discharged from the vane pump 11 2 is introduced through a conduit 180 into the hydraulic pressure conduit 54. The conduit 180 has therein a pressure regulator 1 82 and a metering orifice 1 84 which are operable to maintain the hydraulic pressure from the vane pump 112 at a predetermined value. The pressure in the fuel discharged from the vane pump 112 is about 2.5 kg/cm2 at engine idling speed and is about 8 kg/cm2 at high engine speeds. Thus, the pressure regulator 1 82 and the orifice 1 84 may be selected to maintain the hydraulic pressure to be introduced into the pressure chamber 50 at 2.5 kg/cm2.The hydraulic pressure conduit 54 is connected to the return side of the fuel injection pump 110 through a return passage 1 86 in which the solenoid valve 210 is placed. The control unit 200 is responsive to signals indicative of various engine operating conditions for controlling the mean time of opening of the solenoid valve 210 to control the hydraulic pressure to be introduced into the fuel injection valve pressure chamber 50.

Assuming now that the piston 56 placed in the pressure chamber 50 has a diameter of 20 mm and thus an effective area of 3.14 cm2, the hydraulic force acting on the piston 56 is 7.8 kg when a 2.5 kg/cm2 hydraulic pressure is introduced into the pressure chamber 50. For fuel injection valves currently used in automotive vehicles, this hydraulic force produces an increase of 30 kg/cm2 of the pressure under which the valve member is seated on the valve seat.

Consequently, if the load spring 42 is selected to produce a 100 kg/cm2 load pressure under which the valve member 28 is seated on the valve seat 24, the fuel injection valve is held closed until the fuel pressure acting on the valve member tip portion 30 reaches 1 30 kg/cm2. If the hydraulic pressure is released from the pressure chamber 50, the fuel injection valve will open when the fuel pressure is above 100 kg/cm2.

It can be seen that the fuel injection system of the present invention can vary injection characteristics such as injection commencement timing and injection period, even in the course of fuel injection, by controlling the level of the hydraulic pressure introduced into the pressure chamber 50 to vary the opening pressure above which the valve member 28 is lifted off the valve seat 24. This permits a desired fuel injection mode such as to effect preliminary injection of a small amount of fuel a desired time before the main fuel injection. Furthermore, the injection period can be lengthened at low engine speeds and low engine loads for the purpose of suppressing idling knock which causes noisy and rough running of the engine and can be shortened at high engine speeds and high engine loads for the purpose of improving the combustion efficiency.

Although the hydraulic pressure source 100 has been described as discharging a constant hydraulic pressure of 2.5 kg/cm2, it is to be noted that another type of hydraulic pressure source may be employed which discharges a constant hydraulic pressure of 10 kg/cm2. This permits a reduction of the effective area of the piston 56 to one-fourth and an increase of the response speed.

Referring to Figs. 3 and 4, there is illustrated a second embodiment of the present invention. Like reference numerals have been applied to Figs. 3 and 4 with respect to the equivalent components shown in Fig. 1. The fuel injection system shown has four fuel injection valves 10 for association with the respective cylinders of a four cylinder engine. The particular fuel injection system shown is only for illustrative purposes and the structure of this invention could be readily applied to any engine structure.

Three of the fuel injection valves are the same as shown in Fig. 1 and the remaining fuel injection valve is generally the same as shown in Fig. 1 except that a lift sensor 70 is provided for sensing the position of the valve member with respect to the valve seat. The lift sensor 70 is placed between the annular abutment 44 and a spring seat 72 on which one end of the load spring 42 is seated. A lead wire 74 extends through the fitment 1 6 from the lift sensor 70 to an output terminal 76. The lift sensor 70 may be comprised of a piezo-electric ceramic element adapted to produce a voltage thereacross when exposed to a pressure. When the valve member 28 is lifted off the valve seat 24 thus compressing the load spring 42, a pressure is exerted on the lift sensor 70 which thereby provides a voltage signal.The leading edge of the voltage signal represents the actual commencement of fuel injection. When the valve member 28 is seated on the valve seat 24 and the load spring 42 is returned to its initial state, the pressure on the lift sensor 70 is released and the voltage signal is diminished. The traiiing edge of the voltage signal represents the actual termination of fuel injection.

The fuel injection system includes a distributor type fuel injection pump. The fuel injection pump is the same as described for Fig. 2 and a detailed description will be omitted to avoid duplicity. A part of the fuel discharged from the vane pump 112 of the fuel injection pump 110 is introduced through a conduit 180 into the hydraulic pressure conduit 54. The conduit 180 is provided therein with a pressure regulator 182 and a metering orifice 1 84 which are operable to maintain the hydraulic pressure from the vane pump 112 at a predetermined value. The hydraulic pressure conduit 54 is connected to the return side of the fuel injection pump 110 through a return passage 186 having therein a pressure controller 210. The pressure controller 210 may be taken in the form of a solenoid valve.

The pressure controller 210 is controlled by a control unit 200 which has inputs from the lift sensor 70, a crankshaft position sensor (CPS) 222, and a reference pulse generator (RPG) 224.

The crankshaft position sensor 222 produces a series of crankshaft position electrical pulses, each corresponding to one or one-half degree of rotation of the engine crankshaft, of a repetition rate directly proportional to engine speed. The reference pulse generator 224 produces a reference electrical pulse at a predetermined number of degrees of rotation of the engine crankshaft, for example, every 180 degrees of crankshaft rotation for four cylinder engines. The reference electrical pulse represents the timing with which fuel is required to be injected. The control unit 200 may have inputs from other sensors adapted to sense engine operating parameters indicative of engine temperature, ambient temperature, ambient humidity, mass air flow into the engine, and perhaps others.The control unit 200 varies the hydraulic pressure to be introduced through the conduit 54 into the pressure chambers of the fuel injection valves by controlling the mean value of the opening duration of the solenoid valve 210 in accordance with the sensed engine operating conditions.

When the engine is operating, a regulated hydraulic pressure is charged through the conduit 54 into the pressure chamber 50 of each fuel injection valve to exert a hydraulic force on the piston 56 which thereby presses the valve member 28 against the valve seat 24. As a result, the valve member is seated against the valve seat under the resultant force of the hydraulic force and the load spring force. At a proper point of time in each cycle of operation, pressurized fuel is supplied from the fuel injection pump 110 into the fuel injection valve to exert a fuel pressure on the valve member tip portion 30. If the fuel pressure reaches an opening pressure determined by the resultant force of the hydraulic force and the load spring force, the valve member 28 will be lifted off the valve seat 24 and fuel injection will commence.

The control unit 200 is responsive to a reference electrical pulse from the reference pulse g'enerator 224 for opening the solenoid valve 210 for a time period proportional to the speed of rotation of the engine measured from the crankshaft position electrical pulses from the crankshaft position sensor 222. As a result, the hydraulic pressure charged in the pressure chamber 50 and thus the opening pressure will decrease with the engine speed increases.

Consequently, the rate of fuel injected into the engine decreases to slow down combustion speed at low engine speeds and increases to avoid afterburning at high engine speeds. This gives suppression of engine noise at low engine speeds and at the same time achieves fuel economy at high engine speeds. The control unit 200 may be designed to provide a pulse signal having a pulse width proportional to engine speed when a reference electrical pulse occurs.

Alternatively, the control unit 200 may be constructed to provide a pulse signal having a constant pulse width and a frequency proportional to engine speed.

Furthermore, the control unit 200 has an input from the lift sensor 70 which provides a signal indicative of the time at which fuel injection actually commences and the time at which fuel injection actually ceases for feedback control of the solenoid valve 210. This also permits control of injection characteristics such as fuel injection timing and injected fuel rate in the course of fuel injection.

Referring to Fig. 5, there is illustrated a modified form of the fuel injection valve wherein a spring 80 is interposed between the piston 56 and the upper end portion of the connection rod 40 in order to permit movement of the valve member 28 even in the course of control of the hydraulic pressure in the pressure chamber 50.

This arrangement eliminates the need for control of the solenoid valve 210 in synchronism with fuel injection and simplifies the structure of the control unit 200.

Although a single solenoid valve is employed for all of the fuel injection valves in the fuel injection system as shown in Figs. 3 to 5, it is to be noted that a solenoid valve 210 may be provided for each of the fuel injection valves 10, as shown in Fig. 6.

Although the hydraulic pressure source shown utilizes the hydraulic pressure discharged from a vane pump included in a distributor type fuel injection pump, it is to be realized that it may utilize the hydraulic pressure discharged from a separately provided pump or other suitable pump included in a lubricating system, a steering system or a torque converter. It is also to be realized that other forms of pressure controller 210 such as a servo valve, a high-speed solenoid valve, a motor driven rotary valve, or the like may be used and such other valves are intended to be included in the present invention.

There has been provided, in accordance with the present invention, a fuel injection system wherein the opening pressure of fuel injection valves can be controlled, in the course of fuel injection, in accordance with engine operating conditions. This can improve fuel economy and engine drivability and reduce unwanted exhaust emissions and combustion noise. While this invention has been described in connection with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art.

Accordingly, it is intended to embrace all alternatives, modifications and variations that fall within the scope of the appended claims.

Claims

1. A fuel injection system for use in an internal combustion engine, comprising: (a) a hydraulic pressure source for generating a predetermined hydraulic pressure; (b) at least one fuel injection valve including a valve member biased by a load spring to rest against a valve seat, said fuel injection valve being adapted to inject fuel into said'engine when the pressure in the fuel is sufficient to lift said valve member off said valve seat against the force of said load spring, said fuel injection valve including a hydraulic actuator responsive to the hydraulic pressure delivered thereto from said hydraulic pressure source for applying a force to said valve member; and (c) a pressure controller for controlling the hydraulic pressure delivered from said hydraulic pressure source to said hydraulic actuator in accordance with engine operating conditions.

2. The fuel injection system as claimed in claim 1, wherein said hydraulic actuator comprises a pressure chamber formed in said fuel injection valve and communicated with said hydraulic pressure source, and a piston adapted to reciprocate within said pressure chamber, said piston being drivingly associated with said valve member for applying a force on said valve member in response to the hydraulic pressure charged in said pressure chamber.

3. The fuel injection system as claimed in claim 1, wherein said pressure contrnlleris associated with a lift sensor adapted to sense actual commencement and termination of fuel injection in response to movement of said valve member with respect to said valve seat.

4. The fuel injection system as claimed in claim 3, wherein said lift sensor comprises a piezoelectric element for providing a voltage signal when said load spring is compressed by movement of said valve member from said valve seat.

5. The fuel injection system as claimed in claim 1, wherein said pressure controller comprises a control unit responsive to engine operating conditions for providing a command signal, and a solenoid valve responsive to the command signal from said control unit for controlling the hydraulic pressure to said hydraulic actuator.

6. The fuel injection system as claimed in claim 5, wherein said control unit has inputs from a crankshaft position sensor adapted to provide a series of crankshaft position electrical pulses of a repetition rate directly proportional to engine speed and a reference pulse generator adapted to produce a reference electrical pulse at a predetermined number of degrees of rotation of said engine, said control unit providing, to said solenoid valve, a pulse signal corresponding to engine speed when a reference pulse signal occurs, thereby decreasing the hydraulic pressure to said hydraulic actuator with the engine speed increases.

7. A fuel injection system for use in an internal combustion engine, comprising: (a) a hydraulic pressure source for generating a predetermined hydraulic pressure; (b) at least one fuel injection valve including a valve member biased by a load spring to rest against a valve seat intersected by a nozzle orifice, said fuel injection valve being adapted to inject fuel through said nozzle orifice into said engine when the pressure in the fuel is sufficient to lift said valve member off said valve seat, said fuel injection valve being formed therein with a cavity having a bored partition dividing said cavity into a spring chamber and a pressure chamber, said pressure chamber communicating with said hydraulic pressure source, a piston placed in said pressure chamber for reciprocating movement therewithin, a connection member extending through said partition into said pressure chamber and having thereon a spring rest placed in said spring chamber, said connection member abutting at its one end on said valve member and at the other end on said piston, said load spring being placed between said spring rest and said partition, said piston applying a force on said connection member in response to the hydraulic pressure charged in said pressure chamber; and (c) a pressure controller for controlling the hydraulic pressure supplied from said hydraulic pressure source to said hydraulic actuator in accordance with engine operating conditions.

8. The fuel injection system as claimed in claim 7, wherein said fuel injection valve includes a spring placed in said pressure chamber for preventing said piston from chattering.

9. The fuel injection system as claimed in claim 7, wherein said fuel injection valve includes a spring disposed between said piston and said connection member.

10. The fuel injection system as claimed in claim 7, wherein said fuel injection valve includes a spring rest on which one end of said load spring is seated, and a lift sensor sandwiched between said partition and said spring rest for detecting actual commencement and termination of fuel injection in response to movement of said valve member with respect to said valve seat.

11. The fuel injection system as claimed in claim 10, wherein said lift sensor comprises a piezo-electric element for providing a voltage signal when said load spring is compressed by movement of said valve member from said valve seat.

1 2. The fuel injection system as claimed in claim 7, wherein said pressure controller comprises a solenoid valve provided in a return passage communicating with said pressure chamber, and a control unit for controlling the means value of the opening duration of said solenoid valve in accordance with engine operating conditions.

13. The fuel injection system as claimed in claim 12, wherein said control, unit has inputs from a crankshaft position sensor adapted to provide a series of crankshaft position electrical pulses of a repetition rate directly proportional to engine speed and a reference pulse generator adapted to produce a reference electrical pulse at a predetermined number of degrees of rotation of said engine, said control unit providing, to said solenoid valve, a pulse signal corresponding to engine speed when a reference pulse signal occurs, thereby decreasing the hydraulic pressure to said hydraulic actuator with the engine speed increases.

14. A fuel injection system for use in an internal combustion engine substantially as described with reference to, and as illustrated in, Figs. 1 and 2, or Figs. 3 to 5, or Fig. 6 of the accompanying drawings.