Patent Grant
9140223
This application claims the benefit of priority, under 35 U.S.C. Section 119, to European Patent Application Serial No. 08425817.7 filed on Dec. 29, 2008, which is incorporated herein by reference in its entirety.
The present invention relates to a fuel injection system with high operation repeatability and stability for an internal combustion engine.
Normally, fuel injection systems comprise at least one fuel injector controlled by a metering servo valve, which comprises a control chamber supplied with pressurized fuel. An outlet passage of the control chamber is normally kept closed by an open/close element via elastic means. The open/close element is actuated for opening the servo valve, by an armature of an electric actuator acting in opposition to the elastic means, for controlling an injection of fuel. The fuel injection system also comprises a unit for controlling the electric actuator, which is designed to issue for each fuel injection a corresponding electrical command.
In order to improve the performance of the engine, from EP1795738, a fuel injection system is known in which, for each fuel injection in a cylinder of the engine, the control unit issues at least one first electrical command of a pre-set duration for generating a pilot fuel injection, and a subsequent electrical command of duration corresponding to the operating conditions of the engine for controlling a main fuel injection. In some examples, the two commands are separated by a time interval such that the main fuel injection starts without a solution of continuity with the pilot fuel injection, i.e., such that the diagram of the supply of fuel during the fuel injection phase or event will assume a humped profile. In some examples, the two commands are separated by a time interval such that the main fuel injection starts without any solution of continuity with the pilot fuel injection, i.e., such that the diagram of the supply of fuel during the fuel injection phase or event will assume a humped profile.
Given the same duration of the electrical commands for the actuation of the pilot fuel injection and of the main fuel injection, the total amount of fuel introduced into the combustion chamber via the pilot fuel injection and the main fuel injection varies as a function of the time interval between the two aforesaid commands issued by the control unit. In particular, it is possible to identify two different modes of behaviour of the injector as a function of the time interval that elapses between the command for the pilot fuel injection and the command for the main fuel injection. In fact, it is possible to identify a limit value for said interval, above which the amount of fuel injected during the main fuel injection depends, not only upon the duration of the electrical command, but also upon the oscillations of pressure that are set up in the intake duct from the rail to the injector, on account of the pilot fuel injection.
For durations of the interval between the two fuel injections shorter than this limit value, instead, the amount of fuel introduced during the main fuel injection is affected by numerous factors, among which the duration itself of said interval, the train of rebounds of the open/close element, the evolution of the fuel pressure in the control chamber, the position of the needle of the nebulizer at the instant of start of the command for the main fuel injection and again the fluid-dynamic conditions that are set up in the proximity of the sealing area. In addition, the state of ageing of the injector, insofar as the wear of the parts in fluid-tight contact or in mutual motion, with extremely small coupling play, significantly affects the mode of rebound of the open/close element.
This phenomenon is substantially due to the presence of the pilot fuel injection, which in effect alters the fluid-dynamic conditions of the injector at the moment of the command for the main fuel injection. In particular, the limit value of the duration of the interval that separates these two modes of behaviour is approximately 300 μs.
In addition, the robustness of operation of the injector is markedly jeopardized when the time interval between the commands of the two fuel injections occurs below the limit value defined previously, and in particular when said interval becomes very small so that the pilot fuel injection interferes to a greater extent with the subsequent main fuel injection.
Notwithstanding the fact that it is possible to program the control unit so as to vary this interval between the pilot fuel injection and the main fuel injection during the service life of the injector, it remains in any case impossible to predetermine the degree of the correction to be introduced to cause the profile of the two fuel injections to continue to be humped.
The drawback encountered in the known fuel injection systems of the type described is due to the fact that, in order to obtain an injection profile of the humped type, it is necessary to set a value of the interval between the pilot fuel injection and the main fuel injection that is very small. Consequently, the start of re-opening of the servo valve for the main fuel injection occurs when the injection dynamics of the injected fuel is markedly variable and dependent upon the parameters set forth previously, with deleterious effects on the efficiency of the engine and on the pollutant emissions of the exhaust gases. These drawbacks increase rapidly following upon wear of the parts of the servo valve.
The aim of the examples disclosed herein is to provide a fuel injection system with high operation repeatability and stability over time, eliminating the drawbacks of fuel injection systems of the known art.
According to several examples, the above purpose is achieved by a fuel injection system with high operation repeatability and stability for an internal combustion engine, as claimed in the attached Claims.
For a better understanding, some embodiments are described herein, purely by way of example with the aid of the annexed drawings, wherein:
With reference to
The casing 2 has an axial cavity 6, in which is housed a metering servo valve 5, which comprises a valve body 7 having an axial hole 9. A rod 10 is axially slidable in the hole 9, in a fluid-tight way for the pressurized fuel, for control of the injection. The casing 2 is provided with another cavity 14 housing an electric actuator 15, which comprises an electromagnet 16 designed to control an armature 17 in the form of a notched disk. The fuel injection system comprises an electronic unit 100 for controlling the electromagnet 16, which is designed to supply for each fuel injection a corresponding electrical command S. In particular, the electromagnet 16 comprises a magnetic core 19, which has a polar surface 20 perpendicular to the axis 3, and is held in position by a support 21.
The electric actuator 15 has an axial discharge cavity 22 of the servo valve 5, housed in which are elastic means defined by a helical compression spring 23. The spring 23 is pre-loaded so as to push the armature 17 in a direction opposite to the attraction exerted by the electromagnet 16. The spring 23 acts on the armature 17 through an intermediate body, designated as a whole by 12a, which comprises engagement means formed by a flange 24 made of a single piece with a pin 12 for guiding one end of the spring 23. A thin lamina 13 made of non-magnetic material is located between a top plane surface 17a of the armature 17 and the polar surface 20 of the core 19, in order to guarantee a certain gap between the armature 17 and the core 19.
The valve body 7 comprises a chamber 26 for controlling metering of the fuel to be injected, which is delimited radially by the side surface of the hole 9. Axially the control chamber 26 is delimited by an end surface 25 shaped like a truncated cone (i.e., frustoconical) of the rod 10 and by an end wall 27 of the hole 9 itself. The control chamber 26 communicates permanently with the inlet 4, through a duct 32 made in the body 2, and an inlet duct 28 made in the valve body 7. The duct 28 is provided with a calibrated length or stretch 29, which leads into the control chamber 26 in the vicinity of the end wall 27. On the outside of the valve body 7, the inlet duct 28 leads into an annular chamber 30, into which the duct 32 also leads.
The valve body 7 moreover comprises a flange 33 housed in a portion 34 of the cavity 6, having an oversized diameter. The flange 33 is axially in contact, in a fluid-tight way, with a shoulder 35 of the cavity 6 via a threaded ring nut 36 screwed on an internal thread 37 of the portion 34 of the cavity 6. The armature 17 is associated to a bushing 41 guided axially by a guide element, formed by an axial stem 38, which is made of a single piece with the flange 33 of the valve body 7. The stem 38 extends in cantilever fashion from the flange 33 itself towards the cavity 22. The stem 38 has a cylindrical side surface 39, coupled in a substantially fluid-tight way to a cylindrical inner surface 40 of the bushing 41.
The control chamber 26 also has an outlet passage 42a for the fuel, having a restriction or calibrated length or stretch 53, which in general has a diameter comprised between 150 and 300 micrometers (μm). The outlet passage 42a is in communication with a discharge duct 42, made inside the flange 33 and the stem 38. The duct 42 comprises a blind axial length or stretch 43, having a diameter greater than that of the calibrated length or stretch 53, and at least one substantially radial length or stretch 44, in communication with the axial length or stretch 43. Advantageously, there may be provided two or more radial lengths or stretches 44, set at a constant angular distance, which give out into an annular chamber 46, formed by a groove of the side surface 39 of the stem 38. In
The annular chamber 46 is made in an axial position adjacent to the flange 33 and is opened/closed by an end portion of the bushing 41, which forms an open/close element 47 for said annular chamber 46 and hence also for the radial lengths or stretches 44 of the duct 42. The open/close element 47 co-operates with a corresponding valve seat for closing the servo valve 5. In particular, the open/close element 47 terminates with a stretch having an inner surface shaped like a truncated cone 45 (
The armature 17 is made of a magnetic material, and is constituted by a distinct piece, i.e., separate from the bushing 41. It has a central portion 56 having a plane bottom surface 57, and a notched annular portion 58, having a cross section flared outwards. The central portion 56 has an axial hole 59, by means of which the armature 17 engages with a certain radial play along an axial portion of the bushing 41.
According to some examples, the axial portion of the bushing 41 has a projection designed to be engaged by the surface 57 of the armature 17 so as to enable the latter to perform an axial stroke greater than the stroke of the open/close element 47. In the embodiment of
In addition, the intermediate body 12a comprises an axial pin 63 for connection with the bushing 41, opposite to the pin 12, which is likewise made of a single piece with the flange 24 and is rigidly fixed to the bushing 41, in a corresponding seat 40a (
The distance, or space between the surface 65 of the flange 24 and the shoulder 62 of the bushing 41 constitutes the housing A of the armature 17 (see also
Assuming that the lamina 13 is fixed with respect to the polar surface 20 of the core 19, when the bushing 41, through the intermediate body 12a, is held by the spring 23 in the closing position of the servo valve 5, the distance of the plane surface 17a from the lamina 13 constitutes the stroke or lift C of the armature 17, which is always greater than the clearance G of said armature 17 in its housing A. The armature 17 is found hence resting against the shoulder 62, in the position indicated in
The stroke, or lift, I of opening of the open/close element 47 is equal to the difference between the lift C of the armature 17 and the clearance G. Consequently, the surface 65 of the flange 24 projects normally from the lamina 13 downwards by a distance equal to the lift I of the open/close element 47, along which the armature 17 draws the flange 24 upwards. The armature 17 can thus perform, along the neck 61, an over-stroke equal to said clearance G, in which the axial hole 59 of the armature 17 is guided axially by the neck 61.
Operation of a servo valve 5 of
When the electromagnet 16 is not energized, by means of the spring 23 acting on the body 12a, the open/close element 47 is kept resting with its surface shaped like a truncated cone 45 against the portion shaped like a truncated cone 49a of the connector 49 so that the servo valve 5 is closed. Assume that, on account of the force of gravity and/or of the previous closing stroke, which will be seen hereinafter, the armature 17 is detached from the lamina 13 and rests against the shoulder 62. This does not affect, however, the effectiveness of operation of the servo valve 5 described in various examples, which is irrespective of the axial position of the armature 17 at the instant of energization of the electromagnet 16.
In the annular chamber 46 there has hence been set up a pressure of the fuel, the value of which is equal to the pressure of supply of the fuel injector 1. When the electromagnet 16 is energized to perform a step of opening of the servo valve 5, the core 19 attracts the armature 17, which at the start performs a loadless stroke, equal to the clearance G (
When energization of the electromagnet 16 ceases, the spring 23, via the body 12a, causes the bushing 41 to perform the stroke I towards the position of
On account of the type of stresses, the small area of contact, and the hardness of the open/close element 47 and of the valve body 7, after impact the open/close element 47 rebounds, overcoming the action of the spring 23. The rebound is favoured also because the impact occurs in the presence of a considerable amount of vapour of the fuel that had formed at a point corresponding to the open/close element as a result of the flow rate of fuel leaving the chamber 46. The degree of the vapour phase present depends markedly in a proportional way upon the value of the pressure in the control chamber 26 at the instant of cessation of the energization of the electromagnet 16. Consequently, the degree of the rebound is greater the shorter the duration of the command of energization for pilot fuel injections of a small amount.
If the armature 17 were fixed with respect to the bushing 41 in its travel towards the valve body 7, at the instant in which the first impact occurs, the open/close element 47 would reverse its direction of motion together with the armature 17, performing the first rebound of considerable amplitude, consequently determining re-opening of the servo valve 5 and delaying the displacement of the rod 10 with consequent delay of closing of the needle of the nebulizer. The spring 23 then pushes the bushing 41 again towards the position of closing of the servo valve 5. There hence occurs a second impact with corresponding rebound, and so forth so that a train of rebounds of decreasing amplitude is generated, as indicated by the dashed line in
Instead, since the armature has the clearance G with respect to the flange 24, after a certain time from the first impact of the open/close element 47 against the connector 49, the armature 17 continues its travel towards the valve body 7, recovering the play existing in the housing A, until an impact of the plane surface 57 of the portion 56 occurs against the shoulder 62 of the bushing 41. As a result of this impact, and also on account of the greater momentum of the armature 17, due to its stroke C of greater length than the stroke I, the rebounds of the bushing 41 reduce sensibly or even vanish. In any case, the way with which the first rebound is modified, as compared to the case where the armature 17 is fixed with respect to the bushing of the open/close element, determines re-opening or otherwise of the servo valve 5 and consequently prolonging of the pilot injection. A lack of re-opening of the servo valve 5 in the instant immediately after the pilot fuel injection—and before the main fuel injection—decreases the likelihood of obtaining a humped injection profile.
By appropriately sizing the weights of the armature 17 and of the bushing 41, the stroke C of the armature 17, and the stroke I of the open/close element 47, it is possible to obtain impact of the armature 17 against the bushing 41, represented by point P in
As is highlighted in
In
The diagrams of
In general, given the same stroke I of the open/close element 47, the greater the clearance G between the armature 17 and the flange 24, the greater the delay of its travel with respect to that of the bushing 41 so that the dashed-and-dotted line of
The stroke of the armature 17 and of the open/close element 47 can be chosen so that the impact of the armature 17 with the shoulder 62 occurs exactly at the instant in which the open/close element 47 recloses the solenoid valve 5 after the first rebound, i.e., at the instant in which the point P coincides with the end of the first rebound, as indicated in the diagram of
From the
In this way, the degree of the first rebound of the open/close element is such as to enable a re-opening of the servo valve 5 with a fuel flow rate such as to stop the increase in pressure in the control space and hence such as to delay closing of the nebulizer. Consequently, by choosing an appropriate value for the time interval after which the command for the main fuel injection is to be issued, it is possible to obtain a humped fuel injection profile.
Since the degree of the rebound allowed is in any case smaller than in the case of the known art, and since the train of further rebounds is practically annulled, the wear of the parts that are in contact or that slide in relative motion manifests with much longer times, consequently increasing the robustness of operation and the service life of the fuel injector.
In fact, as has been said previously, in the case of the known art the wear of the surfaces 45 and 49, and 40 and 39 affects both the degree of the first rebound and the duration of the train itself. In particular, the wear causes increase in the sealing diameter between the surfaces 45 and 49. Hence, at the moment of impact, unbalancing forces tend to be introduced that favour re-opening (i.e., favour the first rebound), whilst the wear of the surfaces of mutual sliding 39 and 40 significantly reduces the friction between the bushing and the valve body, so favouring prolongation of the train of rebounds. Thanks to some examples described here, by reducing or eliminating the rebounds subsequent to the first rebound and reducing the degree of the first rebound itself, there is a smaller dependence of the behaviour of the servo valve 5 upon the wear of the components. Consequently, the servo valve 5 will present over time a high stability of operation, which, instead, is affected much less by the wear of the servo valve 5.
In the present description and in the claims, by the term “command” is understood a signal of electric current having a pre-set duration and a pre-set evolution.
In order to obtain a good efficiency of the engine and to reduce the emissions of pollutant exhaust gases, for each cycle of a cylinder of the engine, the control unit 100 controls the injector 1 for a fuel injection phase or event, comprising a pilot fuel injection and a subsequent main fuel injection. In certain instances, in order to optimize the fuel injection phase, it has been experimentally found that the main injection should start without a solution of continuity with the pilot fuel injection, i.e., that the fuel injection phase has a humped evolution. In certain instances, in order to optimize the fuel injection phase, it has been experimentally found that the main injection should start without any solution of continuity with the pilot fuel injection, i.e., that the fuel injection phase has a humped evolution.
For the above purpose, for each fuel injection phase, the control unit 100 issues at least one first electrical command S1 of a pre-set duration, for actuating the open/close element 47 thus determining the corresponding pilot fuel injection, and a second electrical command S2 of a duration corresponding to the operating conditions of the engine for actuating the open/close element 47 determining a corresponding main fuel injection. The two electrical commands S1 and S2 are separated by a dwell time DT, which will be seen more clearly in what follows. With reference to
In particular, the first electrical command S1 is generated starting from an instant T1, and has an evolution with a rising edge having a relatively fast growth up to a maximum value in order to energize the electromagnet 16. The duration of the maximum value of the electrical command S1 is constant and is followed by a stretch of maintenance of energization of the electromagnet 16 of an extremely short duration. The stretch of maintenance of the electrical command S1 is finally followed by a stretch of final decrease that terminates in the instant T2.
The second electrical command S2 is generated starting from an instant T3 such as to start the second lift, before the rod 10 has reached the end-of-travel position of closing of the nebulizer. Time T3-T2 constitutes the aforesaid dwell time DT between the two electrical commands S1 and S2.
The second electrical command S2 has likewise an evolution with a rising edge up to a maximum value, in order to energize the electromagnet 16, followed by a stretch of maintenance of energization of the electromagnet 16 of a duration greater than the stretch of maintenance of the first electrical command S1 and variable as a function of the operating conditions of the engine. Finally, the stretch of maintenance of the first electrical command S1 is followed by a stretch of final decrease that terminates at the instant T4.
As may be noted, the motion of the rod 10 occurs with a certain delay with respect to issuing of the corresponding electrical command, which depends upon the pre-loading of the spring 23 (see also
Advantageously, the bottom limit of the dwell time DT can be chosen in such a way that the lift of the rod 10 caused by the second electrical command S2 starts from the instant corresponding to the highest point of the lift of the rod caused by the first electrical command S1. Said limit is in the region of 100 μs. In turn, the upper limit of the dwell time DT can be chosen in such a way that the lift of the rod 10 due to the second electrical command S2 starts exactly at the instant in which the rod 10 returns in the closing position following upon the lift due to the first electrical command S1. In
For each injection phase, the unit 100 can issue more than one first electrical command S1. Said electrical commands can be separated by respective dwell times DT that can be equal to or different from one another, but comprised within the above limits indicated for said interval so that the evolution of the instantaneous fuel flow rate Qi does not present discontinuities.
As has been seen before, the displacement of the rod 10 is caused by a reduction of the fuel pressure in the control chamber 26. By bringing about displacement of the rod 10 by means of the electrical commands S1 and S2 spaced apart by the dwell time DT, the other conditions remaining the same, as said dwell time DT varies, the total amount of injected fuel Q for each fuel injection phase (pilot fuel injection+main fuel injection) varies. In
It has been found experimentally that, by damping the rebounds of the open/close element 47 by means of an impact with the armature 17 during the first rebound as indicated in the diagram of
On the other hand, if the first rebound of the open/close element 47 occurs freely, whilst the further rebounds are blocked as indicated in
In
In the embodiments of
According to the embodiment of
In order to obtain an operation in which the armature 17 impacts against the shoulder 62 at the end of the first rebound, as illustrated in
In the embodiment of
The central portion 56 of the armature 17 is here able to slide on an axial portion 82 of the bushing 41, adjacent to the rim 74. In addition, the rim 74 is adjacent to an end surface 80 of the bushing 41, which is in contact with the surface 65 of the flange 24. The annular depression 77 has a depth greater than the thickness of the rim 74 in order to enable the entire travel of the armature 17 towards the core 19 of the electromagnet 16. The shoulder 76 of the armature 17 is normally kept in contact with the plane surface 75 of the rim 74 by the compression spring 52, in a way similar to that has been seen for the embodiment of
In the embodiment of
The stem 85 moreover comprises a portion 92 of a reduced diameter on which the armature 17 is able to slide, said armature 17 normally resting by action of a compression spring 93 against a C-shaped ring 94 inserted in a groove 95 of the stem 85. The groove 95 separates the portion 92 of the stem 85 from the end portion 12a comprising the flange 24 on which the spring 23 acts and the pin 12 for guiding the end of the spring 23 itself. The spring 23 hence acts on the open/close element 84 through the engagement means comprising the flange 24 and the stem 85.
The projection means, designed to be engaged by the surface 57 of the central portion 56 of the armature 17, are constituted by an annular shoulder 97 set between the two portions 87 and 92 of the stem 85. The shoulder 97 is set in such a way as to define, with the bottom surface of the C-shaped ring 94, the housing A of the armature 17. In addition, the shoulder 97 forms, with the surface 57 of the portion 56 of the armature 17 the clearance G of the armature 17.
Instead, the top surface 17a of the armature 17 forms, with the lamina 13 on the polar surface 20 of the electromagnet 16, the stroke I of the stem 85, and hence also of the open/close element 84, whilst the stroke C of the armature 17 is formed by the sum of the clearance G and of the stroke I, in a way similar to that has been seen for the embodiment of
Operation of the servo valve 5 of
In the particular case of the fuel injector of
From what has been seen above, the advantages of the fuel injection system according to the some examples, as compared to those of the known art are evident. In the first place, the choice of the dwell time DT in such a way that the main fuel injection starts in the area Z of the diagram of
It emerges clearly that other modifications and improvements may be made to the fuel injection system described and to the corresponding fuel injector 1, without thereby departing from the scope of the present subject matter. In particular, the fuel injector 1 can be provided with a servo valve 5 of a balanced type, in which the armature 17 moves fixedly with the open/close element 47, for example causing the stroke C of the armature 17 to coincide with the stroke I of the open/close element 47 or making the open/close element of a single piece with the armature 17. In this case, the open/close element 47, when the servo valve 5 closes, performs freely the first rebound so that, with a dwell time DT substantially within the limits indicated above, there is generated, in the diagram of
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