GB2132704A - Fuel injection systems - Google Patents

Abstract

A pump injector for an I.C. engine fuel injection system comprises an injector body having a bore 4 accommodating a plunger 5. The plunger delivers fuel from a metering and injection space 4a to an injection nozzle via an outlet 6 when acted on by pressure fluid in an actuating space 4. The pressure space 4 communicates with a supply port 1 connected to a timed source of high pressure fluid and the metering space 4a communicates with a metering passage 3 which includes a non-return valve 10 and which communicates with a pressurised fuel supply. The plunger has a pressure release passage 37, 7 which communicates with the metering space whilst the injector body has first spill passages 9, 8 which is connected to a low pressure supply of the fluid and a second spill passage 8 which is connected to a fuel spill space. The spill passages communicate with the bore at axially spaced points so that when the plunger performs a delivery stroke the pressure space 4 is brought into communication with the first spill passage 9 and the metering space 4a is brought into communication with the second spill passage 8 simultaneously. <IMAGE>

Description

SPECIFICATION Fuel injection systems The present invention relates to pump injectors for fuel injection systems for internal combustion engines, particularly compression ignition, e.g. diesel, engines of small, high speed type and to fuel injection systems incorporating one or more such injectors. An important factor in the development of such engines has been the development of fuel injection pumps and fuel injection atomisersto give reliable characteristics over a wide range of engine speeds and fuel delivery rates. These have been developed interactively with various forms of combustion chambers having differing swirl characteristics.

Conventionally, fuel injection systems use a fuel injection pump driven in rotation by the engine and having means whereby the quantity of fuel locked up by the pumping elements and injected into the engine cylinders can be varied to suit the desired engine output The timed and metered output from the pump is transmitted by small bore high pressure pipes to the individual fuel injectors and nozzles associated with each of the engine cylinders.

In broad terms, such fuel injection pumps are one of two different types. The first uses a separate pump element for each cylinder ofthe engine, the plungers being in line and each individual plunger operated by its own cam, adjacent cams being phase to have the correct injection intervals to suitthefiring order of the engine to which the injection system is fitted. The plungers are rotatable and have lands attheirtop which may have a helical top, a helical bottom, or a combination of both which overrun suction and spill ports in their associated barrels. The angular position ofthe plungers determines the phasing and quantity of fuel injected.

The second type uses a single plunger ortwo or more opposed plungers acting in a single barrel actuated by a single cam orcam ring. Atthe appropriate time a rotating plate-valve head associ atedtheplungerdeliveringfuelwith each cylinder in turn.Thequantitydelivered per cycle is usually, but not invariably, controlled by throttling to a greater or less extentthe entry ofthe fuel into the pump barrel which is moderately pressurised by the engine's fuel pump which is usually incorporated in this type of high pressure fuel injection pump.

The nominal fuel injection rate i.e. the quantity of fuel displaced by the plungerfor each degree of rotation ofthe crankshaft is controlled bythe lift eharacteristics of the cam andthe diameterofthe associated plunger. Most diesel engines use a con stant start of injection timing at a particular operating Speed so that as the quantity of fuel injected is increased the end of injection timing is progressively extended by the helix at the bottom of the control land and the degree of plunger rotation. The actual injection timing is optimized experimentally by the phasing ofthe pump drive relative to the crankshaft.In practice, the optimum start of injection varies with operating speed so that either a compromisefixed time is used or a suitable speed sensitive phase changing device is introduced between the pump and its drive.

As mentioned above, the fuel discharged by the pumpissuppliedtothefuelinjectorsbysmall bore pipes. The injectors each comprise a nozzle holder, to the end of which a nozzle is clamped by a cap-nut, containing a stiff spring and a rod for transmitting the spring load to the "needle" fitted within the nozzle.

Means are provided to adjust the spring load on the nozzle. The nozzle needle is spring loaded onto a seating at the discharge end which is exposed to the gases within the combustion space of the associated cylinder. The nozzle needle has a differential area at its lower end, near to its seating, which is surrounded by an annular space into which the fuel from the injection pump is fed via suitable drillings. When the pump delivers fuel into the annular space the fuel pressure builds up until the productofthe pressure and the exposed differential area produces a sufficient force to overcome the spring load at which point the injector needle lifts permitting fuel to be discharged into the cylinder in afinelyatomisedform.

Two main types of injector nozzles are known.

These are the "pintle" type and the plain hole type.

Briefly, in the pintle type nozzle the fuel passes around the lifted needle, which has a shaped end protruding into the combustion chamber, through an annular orificeformed bythe needle andthe hole in theface of the nozzleto discharge axially into the combustion chamber. The spray angle is determined bythe shape ofthe needle end and the nozzle has an area which varies with lift. The plain hole nozzle may bea single plain central hole about three diameters long drilled axially giving a single axial spray or, as is more usual, it may have a small sac volume below the end ofthe conical needle into which one or more-- usually four in the case of small high speed direct injection engines - holes are drilled.These are at an angle to the needle axis and usually equally spaced circumferentially. In the plain hole nozzlethefuel flow area varies initially with the needle lift but very quickly the limiting area becomes that ofthe individual hole areas combined.

Generally, the pintletype nozzle is used in engines having various types of precombustion chamber having compression generated high swirl, or rotary air motion, within the prechamber. The multi-hole plain hole nozzle is generally used in direct injection engines which have a medium swirl ratio generated about the cylinder axis during the suction stroke and augmented during the compression stroke by compressing the swirling charge into a smaller diameter central combustion chamberformed in the piston crown. The injector is fitted as centrally as possible in the cylinder head between the valves.

By way of illustration, a small high speed turbocharged direct injection (D.l.) engine might have a bore of 80 mm and a stroke of 85 mm and run up to 4500 rev/min. At full load and speed about 25 mm3 of fuel must be injected into each cylinderthrough four 0.24 mm diameter nozzle holes in a period of 25 crankshaft, i.e. in about 1 millisecond. The nozzle needle spring loading corresponds to a nozzle opening pressure of 250 bar. Using the time and total flow figures and the total hole area a theoretical pressure difference of around 200 bar is required across the nozzle holes. At idling loadsthefuel quantity and necessary pressure are considerably reduced.

Once the nozzle needle has lifted the pressure difference required is dependent on the fuel quantity to be injected and the injection period available having regard to combustion considerations.

With the conventional types offuel injection pumps supplying the fuel through a length of small bore piping to the injector, the variation ofthe fuel volume contained in the high pressure part ofthe system will vary with the pressure due to the small but significant compressibility ofthe fuel on the one hand and the expansion of the fuel pipe and other system elements, on the other hand. At the same time hydraulic pressure waves are generated in the system. This leads to variations of the fuel pressure at the nozzle end ofthe system during the injection period which leads to fluctuations in the instantaneous rate of fuel injection during the injection period and also to injection continuing afterthe desired end of injection which results in exhaust smoke.

It is therefore desirable to make the actual injection at the nozzle more definitive, i.e. with a sharp beginning and end, with a more accurately controlled rate of fuel delivery during the desired injection period. One known approach is to combinethe individual injection plungers with the fuel injectors thereby eliminating or substantially reducing the length ofpipeworkandthevolume offuel between the injection pump and the fuel injectors. This tends to be expensive and bulky, requiring additional driving cams and in some cases push rods and rockers for operation.

Another known approach mostly used for large engines, isto pump thefuel up to a high pressure in a "common-rail" pipe connected to each fuel injector which is fitted with a mechanically operated shuttlevalve to allowthe high pressure fuel to flow into and through the injection nozzle into the engine cylinder forthe required injection period. Apartfrom being expensive the fire risk is increased, as the result of the presence of a pipe line continuously containing high pressure fuel, should a pipe crack in service.

Afurthersystem which may be regarded as intermediate, or a combination of, the above two systems is disclosed in a paper No. 650432 entitled "A new servo-operated fuel injection system for diesel engines" published in 1965 bythe Society of Automotive Engineers. In this, an engine driven servo pump unit raises the pressure offuel in an accumulator to a high value. The outlet of the accumulator is controlled by a timed rotaryfacesealingvalvewhich connects the accumulator to the various pump injectors successively byway of high pressure piping. Each injector has a cylinder containing an upwardly biassed piston whose upper surface is acted on bythe pressure in an inlet passage communicating with the accumulator.

Beneath the piston is a spill space communicating with a spill passage which is connected to the low pressure side ofthe servo pump. The piston is connected to a plunger below which is a metering space which communicates with an injector nozzle and with the inlet passage via a metering passage which includes a non-return valve. When the injector is connected to the accumulator, the piston is pushed downwardly and thus the plunger also thereby injecting the fuel trapped in the metering space. The rotary valve in the accumulatorthen connects the inlet passageto athrottled output control portwithinthe accumulator unit and the pressure above the piston thus falls away at a rate determined bythe throttle.

The piston thus moves upwardly undertheactionof its biassing spring, and the plunger thus rises also drawing in fuel displaced by the piston through the non-return valve from the inlet passage. After a predetermined time, measured in degreesthe rotary valve in the accumulator prevents any further back flow in the inlet passage. The distance that the piston has moved up determines the volume offuel below the plungerwhich will be injected through the nozzle in the next cycle. The spill passage serves merelyto return anyfuel which leaks around eitherthe piston or the plunger.

lnthissystem,thetermination of injection is controlled atthe end of fuel supply line remote from the injector nozzle and in view ofthefactthatthefuel pressure in the supply passage only decays gradually, at a rate determined by the throttling, the termination of injection through the nozzle is not precise. Accordinglythis system still suffers from the disadvantages referred to above of exhaust smoking and pressure transients.

According to afirst aspect ofthe present invention a pump injectorfor a fuel injection system for an internal combustion engine comprises an injector body connected to a nozzle which is controlled bya valve member, the injector body affording a bore accommodating a plunger which is reciprocable within the bore, the plunger and the body together defining an actuating pressure space at one end ofthe plunger and a metering and injection space at the other end of the plunger, the pressure space communicating with asupply port which, in use, is connected to a timed source of high pressurefluid and the metering space communicating with the nozzle' and with a metering passage which includes a first non-return valve and which, in use, communicates with a pressurisedfuel supply, the injector further including pressure release means which, when the plunger moves in the direction in which the volume of the actuating pressure space is increasing and that of the metering space decreasing, ventsthe pressures in the pressure and metering spaces down to substantially reduced values substantially.-s multaneously.

According to a further aspect ofthe present invention a pump injectorfor a fuel injectiQn system for an internal combustion engine comprises an injector body connected to a nozzle which is controlled by a valve member,the injector body affording a bore accomodating a plungerwhich is reciprocablewithin the bore, the plunger and the bodytogether defining an actuating pressure space at one end of the plunger and a metering and injection space at the other end of the plunger, the pressure space communicating with a supply portwhich, in use, is connected to a timed source of high pressure fluid and the metering space communicating with the nozzle and with a metering passage which includes a first non-return valve and which, in use, communicates with a pressurised fuel supply, the plunger affording a pressure release passagewhich communicates with the metering space and terminates in a release port in the side surface of the plunger, the injector body affording a first spill passage which communicates with the bore and, in use, is connected to a low pressure supply of the said fluid and a second spill passage which communicates with the bore and, in use, is connected to a fuel spill space, the first and second spill passages communicating with the bore at points which are so spaced apart in the direction of reciprocation ofthe plungerthat, when the plunger moves in the direction in which the volume of the pressure space is increasing and that ofthe metering space decreasing, the pressure space is brought into communication with the first spill passage and the metering space is brought into communication with the second spill passage via the pressure release passage substantially simultaneously.

Thus in the construction ofthe present invention, the termination ofthe fuel injection is not controlled at the high pressure pump as previously but instead is controlled within the pump injector which permits a more precise and accurate control. Of particluar importance is that as the plunger reaches the desired end of its injection stroke the pressure and metering spaces are simultaneously connected to a low pressurethereby immediately venting them and terminating the injection of fuel through the nozzle valve immediately. Fuel injection therefore does not continue beyond the time at which its termination is desired and exhaust smoking is thus eliminated.After the termination of injection, fuel flows through the metering passage via the non-return valve and returns the plungerto its desired position to start the next injection cycle which is initiated bytheapplication of a high fluid pressure pulse at the supply portthereby raising the pressure of the trapped fuel and expelling it from the metering space through the injection nozzle.

It is of course usually desirable to be able to vary the volume offuel that is injected on each stroke of the injector plunger and the injector may therefore include an adjustable mechanical stop, which may be mechanically linked to the enginethrottle, and which limits the maximum volume ofthe metering space. At the end ofthe injection stroke, both the pressure and metering spaces are vented to a low pressure and the metering space is subsequently refilled by the pressure ofthe fuel within the metering passage. If the injector includes an adjustable mechanical stop the pressure and metering spaces may be both connected at the end ofthe injection stroke to the same low but yet superatmospheric pressure and thus the metering passage may communicate with the second spill passage.Alternatively, the second spill passage may be connected, in use, to a space at substantially atmospheric pressure, e.g. the engine fuel tank.

For manufacturing reasons it may be convenientfor the plunger to comprise two separate members acting together and it may be desirableforthat portion ofthe plungerwhich partially defines the pressure space to have a greater area than that portion which partially defines the metering space since this results in the fuel injection pressure being greater than the pressure appliedto the plunger in the pressure space.

It is usual in pump injectorsforthe injector plunger to be actuated by high pressurefuel. However, in view ofthefact that the pressure space never communicates with the metering space it is possibleforthe plungerto be actuated by a high pressure fluid other than fuel, e.g. a conventional hydraulic fluid. However, a certain amount of leakage may occurfrom the pressure space to the metering space and for the sake of convenience it is preferred that, in use, the plunger is actuated by high pressure fuel. In this event it is convenientforthe firstspill passage to communicate with the second spill passage.

In one embodiment of the invention, the pressure release passage in the plunger communicates with a peripheral groove in the plunger so positioned that it communicateswiththemetering passage down- stream of the first non-return valve at substantially the sametimeasthe pressure and metering spaces come into communication with the first and second spill passages, and the metering passage includes a further non-return valve adjacent the point at which it communicates with the metering space.In this construction, any ofthe pressurised fuel which leaks from either the pressure or metering space around the plunger will accumulate in the peripheral groove and be discharged at the end of each injection cycle into the metering passage which is at a relatively low pressure. It is preferred that, as conventional,the valve member is biased into the closed position by a spring accommodated within a spring chamber and if the construction referred to above including two non-return valves is used it is preferred that the spring chamber communicates with the metering passage between the first and second non-return valves since the instantaneous increase in pressure which will occur in the metering passage when it comes into communication with the metering space at the end of each injection cycle will act on the valve member thereby accelerating its movement into its closed position and thus promoting an even more rapid termination ofthefuel injection.

The present invention also embraces a fuel injection system for an internal combustion engine including an injector of the type referred to above and compris ing a low pressure fuel pumptothe inletofwhich a fuel tank is connected and to the outlet ofwhich the metering passage of the injector is connected and a high pressurefluid pumptotheoutletofwhichthe supply port of the injector is connected. It will be appreciated that ifthe high pressure pump utilizes a fluid other than the fuel the first and second spill passages will communicate with the inlet side of the high and low pressure pump respectively.

In the preferred embodiment in which the high pressure pump utilizes fuel as the pressure medium, it is preferred that the outlet ofthe low pressure pump is connected tothe inletofthe high pressure pump and that the first and second spill passages communicate with the fuel tank. Thus in this embodiment the fuel vented from the pressure and meteringspacesatthe end of each injection cycle is returned immediately to the fuel tank which is generally at substantially atmospheric pressure.The invention embraces also a fuel injection system including a plurality of injectors andwhilstthese may be connectedto individual plunger pumps, the inlet of each of which is connected to a common low pressure pump, it is preferred that the injectors are connected to a respective outlet of a single high pressure pump of rotary distributor type fortheiractuation.

Furtherfeatures and details of the present invention will be apparentfrom the following description of one specific embodiment which is given by way of example only with reference to the accompanying drawings, in which: Figure lisa diagrammatic sectional elevation of a pump injector in accordance with the present invention; Figure 2 is a diagrammatic representation of a fuel system for a diesel engine incorporating such pump injectors; and Figures3to 5 are more detailed diagrammatic sectional elevations of a pump injector when fuel is entering the metering space, during fuel injection and atthe end of fuel injection, respectively.

The general principles of construction and operation ofthe present invention will be apparent from the following description with reference to Figures 1 and 2.The pumpinjectorof Figure 1 comprises an elongate body defining a space within which is a reciprocable plunger 5. Above the plunger 5 is a space 4, referred to as an actuating pressure space, communicating with a supply port 1. Below the plunger 5 is a further space 4a, referred to as a metering and injection space, which communicates with an outlet 6, to which the injection nozzle is connected, in use, as will be described below, and with a metering passage 3 via a spring loaded non-return valve 10. Communicating with the lowerface ofthe plunger 5 is a passage 37 which terminates in an annular port7 in the side surface ofthe plunger.Situated in the sidewall of the injector body are two spill ports 8 and 9 which communicate with a spill passage2 and which are spaced apart by a distance equal to that between the upper edge of the plunger 5 and the port 7.

In use,the supply port 1 is connected to the high pressure side of a fuel injection pump, the spill passage 2 to low pressure, such as the engine fuel tank and the metering passage 3 to a fuel supply at intermediate pressure, such astheoutletsideofa conventional engine fuel pump.

The mode of operation ofthe injector is as follows: high pressure is applied to the upper surface ofthe plunger5 by the normal fuel injection pump which results in the plunger moving rapidly downwards thereby raising the pressure of and injecting the fuel in the metering space 4athrough the outlet 6 and thence through the injection nozzle into an engine cylinder.

As the plunger moves down the pressure space is brought into communication with the spill passage 2 by virtue ofthe plunger uncovering the spill port 9 and simultaneously the metering space 4a is brought into communication with the spill passage also by virtue of the ports 7 and 8 coming into communication. This results in the pressure in the spaces 4 and 4a immediately dropping to substantially atmospheric pressurewherebythe plunger immediately stops moving and the injection offuel immediately ceases.

No significant pressure now exists above the plunger nor, momentarily, below the plunger. This causes the intermediate pressure prevailing inthe metering passage 3 to open the lightly loaded non-return valve 10 and the entering fuel starts to push the plunger 5 upward. This upward movement may be assisted, if required, by an upwardly acting return spring (not shown). The ports Band 9 are then closed again and the plunger rises more rapidly upwards. The fuel above the plunger is displaced back through the supply line which is now connected bythe fuel injection pump to a spill port at atmospheric pressure.The distance which the plunger moves upwards, so determining the volume of fresh fuel trapped below is readyforthe next injection, is determined by either a variable stop, not shown, above the plunger or by a variable area orifice within the metering passage 3. By limiting the rate offlowto the underside of the plunger, the variable area orifice determines howfarthe plunger rises beforethe imposition of high pressure via passage 1 recommences the cycle and starts to move the plunger 5 downwards again shutting the non-return valve 10 and injecting the trapped fuel below the plunger.

Figure 2 shows the layout ofthe whole injection and fuel supply system, though onlyonefuel injector, designated 48, is shown. In a multi-cylinder engine the high pressure, timed, fuel supply required to operate each pump-injector is provided buy a high pressure pump unit 40 which is preferably, though not necessarily, of rotary distributor type which produces a timed pressure pulse at each injector sequentially. The sequentially timed outputfrom the rotary pump is as is conventional, supplied via a respective pipe 47 to the supply port 1 ofthe individual injectors.

Fuel is taken from afuel supplytank53 byatransfer pump 42 which, in this case but not necessarily, is incorporated in the rotary high pressure fuel injection pump 40 driven, in the case of a four-cycle engine, at halfcrankshaftspeed by input shaft 41.The transfer pump raises the fuel to a pressure ofthe order of 4.5 bar at maximum operating speed,the pressure being controlled by a spring loaded relief valve 43, the excess fuel being returned buy a pipe 44to the pump suction pipe 45.The pressurised fuel passes from the transfer pump by pipes 51,46, which in practice are internal drillings within the high pressure pump unit, to the intake (suction) ports ofthe high pressure injection plunger or plungers which are part of the conventional construction offuel injection pumps of distributortype. The fuel injection pump is operated at a fixed fuel rackto supply a fixed quantity of high pressure fuel, suitable timed, to push down the pressurising plunger 5 ofthe pump injector shown in Figure 1. The spill passage 2 ofthe injector is connected via pipe 44to the low pressure suction pipe 45. Atapping 49 in the deliveryfrom the pressurised side of the transfer pump 42 communicates via a controllable cock 50 with the metering passages of the pump-injector unit. The degree to which cock 50 is open controls the rate at which fuel is supplied to the underside ofthe plunger Sin the pump-injector unit and hencethe quantity offuel availableforthe next injection. The cock 50 is controlled by a lever 52 itself either controlled by a governor or directly by the usual form of linkage to the driving accelerator pedal in the case of a vehicle engine.

Thus in a fuel supply system including an injector of the type illustrated diagrammatically in Figure 1, the duration of injection is controlled atthe injectoritself ratherthan at the high pressure pump which enables the injection parameters to be more closely defined and controlled. In particular, when the pressure space is brought into communication with the spill passage, the pressure in this space dissipates almostim- mediately. This effectively eliminates the risk of injection offuel continuing afterthe desired end of injection but any residual risk of this occurrence is eliminated by simultaneously placing the metering space also in communication with the spill passage.

Thusthefuel injection nozzle valve will reliably close and thus terminate injection atthe instant at which such termination is desired.

Figure 1 is of course a simplified diagrammatic representation of an injector but Figures 3to 5 are more detailed representations of a practical embodi ment, though for the sake of simplicity these are still in diagrammatic form.

In this embodiment, the body 17 of the injector is securely connected to the injection nozzle 59 by a cap nut 13which engages a shoulder 19On the nozzle body and is retained in position by mating screw threads 18. Within the body of the nozzle 9 is situated a central nozzle needle 58which is spring loaded downwards on to its conical seating 60 by means of a loading washer 14, a compressed helical spring 15 and an upperwasher 16whosethickness is chosen to compress the spring 15 to give the required nozzle opening pressure. When the fuel pressure is high enough to liftthe needle 58 fuel passes bythe conical seat 60 into a sac 61 and then discharged through a series of holes 62 to give the required spray directions and patterns within the engine combustion chamber.

Fuel is forced down to the injection nozzlethrough a drilled passage 20. The features described above are essentially known perse and the means for supplying the passage 20, which constitute the essence of the presentinventionwill now be described.

The major constructional difference, as compared with the construction of Figure 1, is that in order to overcome manufacturing problems of concentricity and clearances the plunger is of two piece construction comprising an upper portion 29 and a lower portion 27. In a manner similarto the embodiment of Figure 1,the diameter ofthe upper portion is greater than that ofthe lower portion. The passage 37 is formed in the lower portion 27 and communicates with an annular groove 33 on the peripheral surface of the portion 27. In addition, the metering passage 3 branches downstream ofthe non-return valve 10, constituted by a ball 22 and a return spring 23, into two passages.One ofthese, designated 28, communicates with the bore accommodating the lower portion 27 of the plunger at a point at which it will communicate with the annular space 33 atthe end of the injection strokeofthe plungerandtheother, designated 24, communicates with the metering space 4a via a non-return valve 25 similarto the valve 10.

The disposition of the va rious elements whilst fuel is entering the metering space is shown in Figure 3 and the direction offuel flow is indicated by the arrows.

During this metering phase, the pressure supplied by the high pressure rotary pump 48 is zero or at least a low nominal value only, thus as the portions 27 and 29 of the plunger move upwards, under the action of the pressure of the pump 42 which is applied to the metering passage 3, no resistance is met. Similarly anyfuel in the spill passage 2 is unpressurised. Thus the pressure provided by the primary pump 42 passes by the metering drilling 3 and non-return valves 10 and 25to the underside of the plunger member 27 thus causing plunger members 27 and 29 to move upwards.The amount of movement is determined either by the throttling valve 50 shown in Figure 2 in the transfer pressure linewhich determines the rate of fuel flow in the time available or by a mechanical movable stop (not shown) above thetop of the plunger member 29.The adjustable restrictorvalveor the adjustable mechanical stop, as the case may be, is arranged to beunderthecontrolofeitherthe accelerator pedal orthe engine's governor in the conventional manner. During this phase any fuel present between the two portions 29 and 27 ofthe plunger is freely vented to spill.

Having moved to the required height to contain the quantity offuel in the metering space 4a required for the next injection period, the plunger 27,29 is then acted on by the timed arrival of a high pressure pulse from the distributor pump 40 via the inlet drilling 1 which causes the plungerto move downwards.

Initially this causes the non-return valves 10 and 25 to seat, after which the pressure ofthe fuel trapped beneath the plunger rises until the spring-loaded injector needle 58 lifts and fuel injection into the engine cylinder starts. The disposition ofthevarious elements and the direction ofthefuel flows are shown in Figure 4. When the plunger has moved down to its final position, as shown in Figure 5, the top of the upper, larger diameter, portion ofthe plunger uncovers port 9thereby dropping the pressure above the plunger, the fuel passing to spill via drilling 2.

Approximately simultaneously, the annular groove 33 overruns port 8 releasing the pressure of the relatively small trapped volume of fuel 26 underthe plunger via the axial drilling 37 and the cross drilled hole 28 into a cavity 39 which houses the nozzle loading spring 15: the small increase in pressure in this cavity will assist the rapid closing ofthe nozzle. The injection cyclethen repeats itself.

It will be appreciated that a great many modifications may be made to the constructions described above whilst still retaining the advantages of those constructions. In one modified construction, which is not illustrated, the non-return valve 25 is omitted and the passage in which it is situated is connected directly to the valve 10 only. The passage 28 and the space 39 are connected to the spill passage 2. Thus the space is maintained constantly at spill pressure, i.e. substantially at atmospheric pressure. In a slight variant of this modification, the space 39 may be connected onlyto the passage 28 and notto the spill passage also. This may however lead to "lock-up" problems due to the build up of high pressures in the spring chamber 39 which will discourage the nozzle needlefrom lifting.

Alternatively the non-return valve 10 may be omitted from the construction of Figures 3 to 5 whilst making no other modifications.

In the constructions described above, the supply port 1 ofthe injector is supplied with pressurised fuel bythe pump 40. It will, however, be appreciated that the purpose ofthis fuel is merely to transmit powerto the fuel in the metering space and since there is no communication between the pressure and metering spaces the pump 40 may utilise some fluid other than fuel for hydraulic operation ofthe injector plunger.

If a mechanical variable stop instead of a variable throttle is used to control the fuel injection quantity, the metering and spill passages may be common, that isto say communicate with one another or be constituted by one and the same passage.

Whilst the non-return valves have been described as being of ball valve type they may be of any appropriate type. Similarly, whilst the high pressure pump has been described as being of rotary distributortype, an individual plunger pump may be associate with each injector of an engine.

The invention finds primary application with injectors having plain muiti-hole nozzles but may also be used with those having pintle type nozzles.

Whilst is may be desirable to provide the upper surface ofthe injector plungerwith a larger area than the lower surface since this produces a pressure amplificationthis isofcourse not essential.

Claims (15)

1. A pump injectorfor a fuel injection system for an internal combustion engine comprising an injector body connected to a nozzle which is controlled by a valve member, the injector body affording a bore accommodating a plunger which is reciprocable within the bore, the plunger and the bodytogether defining an actuating pressure space at one end ofthe plunger and a metering and injection space atthe other end of the plunger, the pressure space communicating with a supplyportwhich, in use, is connected to a timed source of high pressure fluid and the metering space communicating with the nozzle and with a metering passage which includes a first non-return valve and which, in use, communicates with a pressurised fuel supply, the injectorfurther including pressure release means which, when the plunger moves in the direction in which the volume of the pressure space is increasing and that ofthe metering space decreasing, vents the pressures in the pressure and metering spaces down to substantially reduced values substantially simultaneously.

2. A pump injector for a fuel injection system for an internal combustion engine comprising an injector body connected to a nozzle which is controlled by a valve member, the injector body affording a bore accommodating a plunger which is reciprocable within the bore, the plunger and the body together defining an actuating pressure space at one end ofthe plunger and a metering and injection space atthe other end of the plunger, the pressure space com municating with a supply port which, in use, is connected to a timed source of high pressure fluid and the metering space communicating with the nozzle and with a metering passagewhich includesafirst non-return valve and which, in use, communicates with a pressurised fuel supply, the plunger affording a pressure release passage which communicates with the metering space and terminates in a release port in the side surface ofthe plunger, the injector body affording a first spill passage which communicates with the bore and, in use, is connected to a low pressure supplyofthe said fluid and a second spill passage which communicates with the bore and, in use, is connectedto a fuel spill space, the first and second spill passages communicating with the bore at points which are so spaced apart in the direction of reciprocation ofthe plungerthat,when the plunger moves in thedirection inwhichthevolumeofthe pressure space is increasing andthat ofthe metering space decreasing, the pressure space is brought into communication with the first spill passage and the metering space is brought into communication with the second spill passage via the pressure release passage substantially simultaneously.

3. An injector as claimed in Claim 1 or Claim 2 including an adjustable mechanical stop which limits the maximum volume ofthe metering space.

4. An injector as claimed in Claims 2 and 3 in which the metering passage communicates with the second spill passage.

5. An injectorasclaimed in any one of the preceding claims in which the plungercomprises two separate members acting together.

6. An injector as claimed in any one of the preceding claims in which that portion of the plunger which partially defines the pressure space has a greater area than that portion which partially defines the metering space.

7. An injector as claimed in Claim 2 or any subsequent claim when dependent thereon in which thefirstspill passage communicates with the second spill passage.

8. An injector as claimed inClaim2and Claim 7 in which the pressure release passage communicates with a peripheral groove in the plungerso positioned that it communicates with the metering passage downstream ofthe first non-return valve at substantiallythe same time as the pressure and metering spaces come into communication withthefirst and second spill passages, and the metering passage includes a further non-retu rn valve adjacent the point atwhich it communicates with the metering space.

9. An injector as claimed in Claim 8 in which the valve member is biased into the closed position buy a spring accommodatedwithinaspring chamberwhich communicates with the metering passage between the first and second non-return valves.

10. A pump injector for a fuel injection system for an internal combustion engine substantially as specifically herein described with reference to Figure 1 or Figures 3 to 5 of the accompanying drawings.

11. Afuel injection system for an internal combustion engine including an injector as claimed in any one of the preceding claims and comprising a low pressurefuel pump to the inlet of which a fuettank is connected and to the outlet of which the metering passage ofthe injector is connected and a high pressure fluid pump to the outlet ofwhich the supply port ofthe injector is connected.

12. Asystem as claimed in Claim 11 including an injector as claimed in Claim 8 in which the outlet ofthe low pressure pump is connected to the inlet ofthe high pressure pump whereby, in use, the fluid pumped into the pressure space is fuel, the first and second spill passages communicating with the fuel tank.

13. A system as claimed in Claim 11 or Claim 12 including a plurality of injectors connected to a respective outlet of a single high pressure pump of rotary distributor type.

14. A system as claimed in Claim 11 or Claim 12 in which the connection between the metering passage and the outlet of the low pressure pump includes an adjustable throttle which limits the rate at which, in use, fuel flows through the metering passage into the metering space.

15. Afuel injection system for an internal combustion engine substantially as specifically herein described with reference to Figure 2 of the accompanying drawings.