A fuel injector for a diesel engine

Abstract

 A diesel engine fuel injector has a needle 10 controlled by two springs 24, 48 which act in series. The first spring 24 is overcome first to allow an initial volume of fuel to be injected. The second spring 48 is substantially overcome to allow a further volume to be injected. The allowable needle lift in each stage is set by stops 40, 46, and the positions of both these stops can be adjusted using a threaded and lockable adjustment.

Description

[0001] This invention relates to a fuel injector for a diesel engine.

[0002] In the quest for reduced emissions from diesel engines, it has been found that the behaviour of the diesel fuel injector can have a considerable influence on the composition of the exhaust gasses.

[0003] Diesel engines are also noisy. The noise is a by-product of the combustion process, and it is desirable to be able to reduce this noise.

[0004] The fuel injectors used in a diesel engine are conventionally driven by fuel pressure pulses from a fuel pump. When a pressure pulse arrives at an injector, it causes the injector needle to lift and fuel then passes through the injector nozzles into the combustion chamber. Because the pressure is high at the time that the injector opens, the fuel enters the combustion chamber as a steep fronted pressure pulse and the resulting combustion is less controlled than could be desired.

[0005] It is known to operate a diesel engine with so-called pilot injection where a small initial pressure pulse is fed by the fuel pump to the injector to inject an initial pilot quantity of fuel and then, after a short time interval, the main pressure pulse opens the injector and introduces the main charge. The advantage of this is that combustion can commence on a small scale with the pilot charge, and then the full combustion can take place in a more controlled manner than is possible when all the fuel is introduced as a single charge.

[0006] One example of a known two-stage fuel injection valve is shown in GB-A-2 071 760. In this patent, the first stage lift is set by a lift adjusting threaded member, but the second stage lift is set by abutment between two fixed parts of the casing. The spring load on the first stage lift is set by a shim. Also, the surface of the stop member which limits the first stage lift is a long way from the part of the stop member which actually sets the position of the stop member. This can lead to setting inaccuracies and to vulnerability to damage during operation.

[0007] According to the invention, there is provided a fuel injector for a diesel engine, the injector having a lower end with nozzles through which fuel can be injected, an upper end, a needle mounted inside a barrel and arranged to move back and forth in the barrel to open or close the injection nozzles, a pressurised fuel inlet between the ends of the needle, and a primary spring acting on the needle to urge the needle towards the lower end, wherein the travel of the needle towards the upper end is limited by a movable stop member, which stop member is movable towards the upper end against the force of an auxiliary spring and wherein the travel of the movable stop member towards the upper end is limited by a fixed stop member, the movable stop member, the auxiliary spring and the fixed stop member being contained in a limit stop unit which is screwed into a threaded bore in a head at the upper end of the injector with the fixed stop member being screwed into a threaded bore in the limit stop unit.

[0008] The pretension exerted by the auxiliary spring on the movable stop member can be set when the limit stop unit itself is assembled, and therefore the assembly stage where the limit stop unit is fitted in the head of the injector only needs to be controlled to set a clearance between the needle and the movable stop member.

[0009] A push rod may be provided between the top of the needle and the movable stop member such that when the push rod makes contact at one end with the needle and at the other end with the movable stop member, a first stage travel of the needle is set. A separate spring seat can be located between the push rod and the needle to support one end of the primary spring and to transmit compressive loads between the rod and the needle.

[0010] When the limit stop unit is threaded into the head of the injector, this clearance can be set by screwing in the limit stop unit until contact occurs between the needle and the movable stop, and then backing off the unit a predetermined amount corresponding to the desired axial clearance between the stop member and the needle. The setting can then be locked using the locking nut on the threads.

[0011] The fixed stop member can likewise be threaded into the limit stop unit, and the clearance between the fixed stop member and the movable stop member can be set by screwing in the fixed stop until contact occurs between the two stop members, and then backing off the fixed stop member a predetermined amount before locking it in position with a lock nut.

[0012] In some circumstances there is a requirement that the injector response should vary as the engine speed changes. In order to accomplish this, a piston may be mounted on the movable stop, and a working fluid space can be arranged between the piston and the limit stop unit in which the movable stop is supported. The working chamber can be fed with a suitable source of pressure fluid to add to the resistance offered by the movable stop member to movement of the injector needle.

[0013] The invention also provides a method of operating a fuel injector for a diesel engine in which the needle lift is controlled by two springs operating in series, wherein the fuel pressure which operates the injector reaches a minimum pressure of 180 bar before the biassing force of the first spring is overcome, and a minimum pressure of 260 bar before the biassing force of the second spring is overcome.

[0014] The invention will now be further described, by way of example, with reference to the accompanying drawings, in which :

Figure 1 is a section through the head of a fuel injector in accordance with the invention; and

Figure 2 is a section through the head of an alternative form of fuel injector in accordance with the invention.

[0015] Figure 1 shows a fuel injector with a needle 10 inside a barrel 12. At its lower end, the barrel 12 has a tip 13 with fuel injection nozzles 14 through which fuel passes into the combustion chamber of an engine. The injector has a head generally designated 16 at its upper end, and a pressurised fuel inlet 18. The inlet 18 leads into a chamber 20 which communicates with an annular passage 22 leading to the lower end of the injector and to the nozzles 14.

[0016] In use, the needle 10 is normally held in its closed position (where the tip of the needle closes communication between the passage 22 and the nozzles 14) by a compression spring 24. The spring 24 acts between a shoulder 26 on a spigot 27 and a bushing 28. The axial position of the bushing 28 can be adjusted by screwing it into or out of the barrel 12, to alter the compression in the spring 24, and then locking it with a lock nut 29. A push rod 38 passes through the bushing 28 and is in contact with the top of the spring seat 27. The bottom end of the rod 38 has an enlargement 39 which centres the rod within the turns of the spring 24. The spring retainer 27 has a corresponding diameter so that this is also centred.

[0017] The use of a push rod 38 which is separate from both the needle and the movable stop 56 ensures that any misalignment or tilting of the stop 40 does not affect the dimension of the first stage lift. The push rod is axially guided at its top end in the bushing 28 and at its lower end by the enlargement 39. The separation of the spring seat 27 from the needle and from the lower end of the push rod 38 prevents any slight off centre or non-axial application of the considerable operating forces from damaging the injector components.

[0018] The top end of the push rod 38 extends into the head 16 of the injector and takes up a position where there is a clearance 36, for example 25 µm, between the needle and a movable stop member 40. The movable stop member 40 is mounted in a limit stop unit which comprises two housing parts 42 and 44, a fixed stop member 46 and a compression spring 48. The fixed stop member 46 is screwed into a threaded bore in the top housing part 42 and is locked there by a lock nut 50. The top housing part 42 is screwed into a threaded bore in the bottom housing part and is locked there by a lock nut 52. The spring 48 acts between the top housing part 42 and a shoulder 54 on the movable stop member 40, and a second clearance 56 is formed between the bottom end of the fixed stop 46 and the top end of the movable stop. The magnitude of this clearance can be set by varying the position of the stop member 46 in the top housing part, before this is locked with the lock nut 50. A typical clearance will be 300 µm.

[0019] The limit stop unit which has been preassembled in this way is then screwed into the head 16 on a screw thread 51 until the movable stop 40 makes contact with the top of the push rod 38. In order to set the gap 36, the limit stop unit as a whole is then backed off by rotating it in its thread through a preset angle which will correspond to a known axial distance. The screw thread 51 will be accurately formed with a known number of threads per inch, eg 50 threads per inch (approx 0.5 mm pitch). The magnitude of the first stage lift can for example be 0.001˝ (25 µm). Once the position of the limit stop unit has been set, it will be locked in this position by a lock nut 49. It is of course of great importance that the position of the unit should not move once it has been correctly set. In view of this, other securing means such as a pinch bolt could be used in place of the lock nut 49.

[0020] There is a leak-off connection 60 to the head 16 which allows fuel leaking back into the head of the injector to be removed from the chamber 58. This leakage is an inevitable result of the injector configuration and is useful in providing lubrication for the movement of the needle and of the push rod.

[0021] In use, upward, opening movement of the needle 10 takes place in two stages. The first stage is limited by the size of the clearance 36 between the top of the push rod 38 and the movable stop member 40 and the second stage is limited by the size of the clearance 56. The first stage is a small needle lift which allows the injection of a throttled charge; the second stage is a full needle lift which allows the injection of the remainder of the charge.

[0022] Normally, in the absence of pressure in the line 18, the needle 10 closes the nozzles 14. When fuel is to be injected, a pressurised fuel pulse is sent along the fuel inlet 18 and enters the chamber 20. The needle 10 has a necked region 30 with an upper shoulder 32 and a lower shoulder 34 in the chamber 20. The shoulder 32 has a greater cross-sectional area facing in an axial direction than does the shoulder 34. The differential area between the shoulders results in the fuel pressure producing a net upwardly acting force on the shoulder 32 which overcomes the force of the spring 24 and lifts the tip of the needle off its seat. Fuel is then forced from the passage 22 out through the nozzles 14.

[0023] The pressure exerted by the fuel pulse will however continue to act in an upward direction on the needle, and once the needle starts to lift, a greater area is made available on which the fuel pressure can act. When the pressure reaches a sufficient level the movable stop 40 will be lifted against the pressure of the compression spring 48, until the clearance 56 is closed. Typically the first lift occurs when the pressure in the chamber 20 rises to 260 bar, and the second lift occurs when the pressure rises to 500 bar.

[0024] As a result of the two stage lift, the combustion chamber sees conditions very similar to that achieved by pilot injection, ie with a small volume of fuel injected initially, followed by a substantially larger volume. Ignition occurs amongst the pilot volume, and the fact that ignition has occurred before the main body of fuel enters ensures that combustion proceeds smoothly throughout the injected fuel volume. These improved combustion conditions produce a lessening of noise from the engine and also have a favourable effect on the emissions from the engine, in particular on the NO_x emissions and, to a lesser extent, on the particulate and hydrocarbon emissions.

[0025] It has also been found that it is desirable to vary the pressure at which the second lift occurs, in dependence on the engine speed. The second lift should occur at a lower pressure at low engine speed and at a higher pressure at high engine speed. Figure 2 shows a modified injector head which allows this speed control to occur.

[0026] In Figure 2, those parts which correspond directly to parts already described with reference to Figure 1 bear the same reference numerals.

[0027] In Figure 2, the movable stop member 40 is connected to a piston 70 which slides in the chamber 58 which takes the form of the cylinder for the piston. The walls of the chamber are lined with a sleeve 72 to provide an effective cylinder wall for the piston.

[0028] The space below the piston, ie chamber 58, is connected to a leak-off connection 60, and is therefore not pressurised.

[0029] The space above the piston 70 is connected via a pipe 62 with a source of pressure. The source of pressure is one which varies according to the engine speed, and it is preferred if the variation with engine speed is a linear variation. For example, the pressure applied through the pipe 62 to the space above the piston 70 could be the signal/transfer pressure from the fuel pump which will have typical values of 300 kpA at 1000 rpm rising to 740 kpA at 4000 rpm.

[0030] The application of this pressure above the piston 70 will be added to the resistance provided by the compression spring 48 in resisting the second stage lift of the needle. It will therefore require a higher pressure to lift the needle at high engine speeds than at low engine speeds.

[0031] The piston 70 is connected to the movable stop member 40 with an O-ring seal 74 to prevent fluid passing from one side of the piston to the other.

Claims

1. A fuel injector for a diesel engine, the injector having a lower end (13) with nozzles (14) through which fuel can be injected, an upper end (16), a needle (10) mounted inside a barrel (12) and arranged to move back and forth in the barrel to open or close the injection nozzles, a pressurised fuel inlet (18) between the ends of the needle, and a primary spring (24) acting on the needle to urge the needle towards the lower end (13), wherein the travel of the needle (10) towards the upper end is limited by a stop member (40), characterised in that the stop member (40) is movable towards the upper end (16) against the force of an auxiliary spring (48) and the travel of the movable stop member (40) towards the upper end is limited by a fixed stop member (46), the movable stop member (40), the auxiliary spring (48) and the fixed stop member (46) being contained in a limit stop unit which is screwed into a threaded bore in a head at the upper end of the injector with the fixed stop member (46) being screwed into a threaded bore in the limit stop unit.

2. A fuel injector as claimed in Claim 1, wherein a push rod (38) is provided between the top of the needle (10) and the movable stop member (40) such that when the push rod makes contact at one end with the needle and at the other end with the movable stop member, a first stage travel of the needle is set.

3. A fuel injector as claimed in Claim 2, wherein a separate spring seat (27) is located between the push rod (38) and the needle (10) and transmits compressive loads between the rod and the needle.

4. A fuel injector as claimed in any preceding claim, wherein the pretension exerted by the auxiliary spring (48) on the movable stop member (40) is set when the limit stop unit is assembled.

5. A fuel injector as claimed in any preceding claim, wherein the clearance (36) between the needle (10) and the movable stop member (40) is set when the limit stop unit is fitted in the head of the injector.

6. A fuel injector as claimed in any preceding claim, wherein a lock nut (49) is used to fix the position of the limit stop unit in the head of the injector.

7. A fuel injector as claimed in any one of Claims 1 to 5, wherein a pinch bolt is used to fix the position of the limit stop unit in the head of the injector.

8. A fuel injector as claimed in any preceding claim, wherein the fixed stop member (46) is threaded into the limit stop unit, and clamped once the correct position has been set.

9. A fuel injector as claimed in any preceding claim, wherein a piston (70) is mounted on the movable stop (40), and a working fluid space is arranged between the piston and the limit stop unit in which the movable stop is supported.

10. A fuel injector as claimed in Claim 9, wherein the working chamber can be fed with a suitable source of pressure fluid to add to the resistance offered by the movable stop member (40) to movement of the injector needle (10).

11. A method of operating a fuel injector for a diesel engine in which the needle lift is controlled by two springs (24,48) operating in series, wherein the fuel pressure which operates the injector reaches a minimum pressure of 180 bar before the biassing force of the first spring (24) is overcome, and a minimum pressure of 260 bar before the biassing force of the second spring (48) is overcome.

Drawing