This application claims priority to European Patent Application No. 08425458.0 filed on 27 Jun. 2008, the disclosure of which is incorporated herein, in its entirety, by this reference.
1. Technical Field
One or more embodiments of the present invention relate to a fuel injector with balanced metering servovalve for an internal-combustion engine, in which the servovalve governs a control rod for controlling injection.
2. The Relevant Technology
Normally, the metering servovalve of the injector comprises a control chamber having a calibrated hole for intake of the fuel under pressure. The control chamber is provided with an outlet or exhaust hole having a calibrated section, which is opened/closed by an open/close element that is axially mobile under the control of an electro-actuator. In particular, the exhaust hole is kept closed by the open/close element under the action of a spring, which acts upon an armature of an electromagnet. The exhaust hole is opened when the armature is actuated by the electromagnet, overcoming the action of the spring.
As long as the exhaust hole is closed, the pressure of the fuel in the control chamber, via the rod, keeps a needle of a nozzle or nebulizer for the fuel in a closed position. When the exhaust hole is open, the pressure of the fuel in the control chamber decreases, while the pressure in the usual injection chamber displaces the needle for opening the nebulizer to thereby displace the rod in the control chamber.
In known injectors, during closing of the needle of the nebulizer, upon arrest of the travel of the needle there occurs a rebound that causes a sort of re-opening of the nebulizer just after closing. This brings about a variation in the gradient of increase in the volume of the control chamber, and hence in the corresponding pressure, or even a temporary decrease in said volume. Furthermore, also the open/close element of the servovalve is subject to a rebound during closing of the hole for exhaust of the control chamber, this also causing a re-opening of said chamber and hence a temporary decrease in the pressure and consequently in the corresponding volume, thus increasing re-opening of the nebulizer.
The re-opening of the nebulizer and/or of the exhaust hole of the control chamber, due to the aforesaid rebounds, always causes injection of an amount of fuel greater than what is envisaged by the usual electronic control unit for controlling injection. On account of the large number of factors that affect the rebounds, the excess fuel thus introduced is not foreseeable so that it is not possible compensate for it via the electronic control unit, for example, by introducing a corrective factor for the time of excitation of the electromagnet. Consequently, especially during idling of the engine, the excess fuel causes a variation in the air/fuel ratio, which moves away from the optimal one, causing at the exhaust an excess of polluting emissions in the environment.
There have already been proposed injectors with a metering servovalve of a balanced type, in which the open/close element in a closed position is subject to substantially zero axial actions of pressure so that it is possible to reduce both preloading of the spring and the force of the electro-actuator. In a known balanced metering servovalve, the valve body comprises an axial stem, which is provided with an exhaust duct of the control chamber and is designed to guide the armature of the electromagnet axially. The open/close element is formed by a bushing engaging in a fluid-tight way with the stem, which is fixed with respect to the armature.
The exhaust duct of the control chamber comprises an axial stretch and at least one radial stretch, which gives out onto a lateral surface of the stem. Since the armature is in general in the form of a plate, or notched disk and is made of a single piece with the bushing, the moving element of the electro-actuator has a considerable mass, and is thus subject to considerable rebounds during closing, with a very low reactivity.
Furthermore, since the bushing must form a seal with the lateral surface of the stem and the open/close element must close the exhaust duct via engagement with an arrest element, the bushing must be machined with extreme precision and be made of a very hard material. The entire bushing-armature plate ensemble must hence be made of said hard material so that, on the one hand, there is a lot of swarf of said material and, on the other, machining thereof is very difficult and costly.
In this servovalve, even though the travel of the open/close element is just a few microns, the forces and the accelerations involved, to which it is subject, can lead to an inevitable rebound of the open/close element during closing. In turn, the marked hardnesses of the parts and the small surfaces, which are in contact along a ring of a width of 1-2 hundredths of a millimetre, favor said rebound, causing a re-opening and a corresponding emptying of the volume of the control chamber.
The aim of one or more embodiments of the invention is to provide a fuel injector with balanced servovalve for an internal-combustion engine, in which the servovalve enables a high reactivity of the servovalve to be obtained, eliminating the drawbacks referred to above.
The above aim may be achieved by a fuel injector with a balanced metering servovalve for an internal-combustion engine.
For a better understanding, some of the embodiments of the present invention are described herein, purely by way of non-limiting example, with the aid of the annexed drawings, wherein:
With reference to
The casing 2 has an axial cavity 6 in which a metering servovalve 5 comprising a valve body 7 having an axial hole 9 is disposed. A control rod 10 for controlling injection of the fuel under pressure is able to slide axially in the hole 9 in a fluid-tight way. The casing 2 is provided with another cavity 14, which is coaxial with the cavity 6 and houses an electro-actuator 15. The electro-actuator 15 comprises an electromagnet 16 designed to control an armature plate 17 in the form of a notched disk. In particular, the electromagnet 16 comprises a magnetic core 19, which has a polar surface 20 perpendicular to the axis 3, and is kept in position by a support 21.
The electro-actuator 15 has an axial cavity 22 in communication with the exhaust of the servovalve 5 towards the usual fuel tank. Housed in the cavity 22 are elastic means defined by a helical compression spring 23. The spring 23 is pre-loaded so as to exert an action of thrust on the armature plate 17, in a direction opposite to the attraction exerted by the electromagnet 16 when it is excited. The spring 23 acts upon the armature plate 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 guide pin 12 of one end of the spring 23. Set between a plane top surface 17a of the armature plate 17 and the polar surface 20 of the core 19 is a thin lamina 13 made of non-magnetic material in order to guarantee a certain gap between the armature plate 17 and the core 19.
The valve body 7 comprises a control chamber 26 for controlling metering of the fuel to be injected, which includes a volume delimited radially by the lateral surface of the hole 9. Axially, the volume of the control chamber 26 is delimited by a terminal surface 25 of the rod 10 and by a bottom wall 27 of the hole 9 itself To receive the fuel under pressure, 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 stretch 29, which gives out into the control chamber 26 in the vicinity of the bottom wall 27. In order to reduce the control volume 26 as much as possible, advantageously the terminal surface 25 of the rod 10 is shaped like a truncated cone. On the outside of the valve body 7, the inlet duct 28 gives out into an annular chamber 30, into which also the duct 32 gives out.
The valve body 7 moreover comprises a flange 33 housed in a portion 34 of the cavity 6, having an enlarged diameter. The flange 33 is set axially in contact with an internal shoulder 35 of the cavity 6, in a fluid-tight way, by a threaded ring nut 36 screwed on an internal thread 37 of the portion 34 of the cavity 6.
As it will seen more clearly hereinafter, the armature plate 17 is associated to a bushing 41 axially guided 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 has a diameter much smaller than that of the flange 33 and extends in cantilever fashion from the flange 33 itself along the axis 3 on the side opposite to the hole 9, i.e., towards the cavity 22.
The stem 38 is delimited externally by a cylindrical lateral surface 39, which guides the axial sliding of the bushing 41. In particular, the bushing 41 has a cylindrical internal surface 40, coupled to the lateral surface 39 of the stem 38 substantially in a fluid-tight way, i.e., by means of a coupling with appropriate diametral play, for example less than 4 μm, or else by interposition of specific seal elements.
The control chamber 26 also has a passage 42a for outlet of the fuel, having a restriction or calibrated stretch 53, which has in general a diameter comprised between 150 and 300 μm. The outlet passage 42a is in communication with an exhaust duct 42, made inside the flange 33 and the stem 38. The duct 42 comprises an axial blind stretch 43, made along the axis 3, in part in the flange 33 and in part in the stem 38. The axial stretch 43 has a diameter greater than that of the calibrated stretch 53.
The duct 42 also comprises at least one substantially radial stretch 44, in communication with the axial stretch 43. Advantageously, there can be provided two or more radial stretches 44, set at constant angular distances apart. Shown in
The annular chamber 46 is made in an axial position adjacent to the flange 33 and is opened/closed by a terminal portion of the bushing 41, which forms an open/close element 47 for said annular chamber 46 and hence also for the radial stretches 44 of the duct 42. The open/close element 47 terminates with a stretch having an internal surface shaped like a truncated cone 45 (
In particular, the truncated cone stretch 49 has two portions of truncated cone surface 49a and 49b, separated by an annular groove 50, which has a cross section substantially shaped like a right triangle. The truncated cone surface 45 of the open/close element 47 engages in a fluid-tight way the portion of truncated cone surface 49a, against which it stops in a closed position. On account of the wear between these surfaces 45 and 49a, the closed position of the open/close element 47 requires, after a certain time of use of the servovalve 5, a greater displacement of the bushing 41 towards the joining stretch 49.
The groove 50 has the function of enabling said greater displacement for closing of the open/close element 47, always defining a maximum diameter of the sealing surface equal to the diameter of the cylindrical stretch of the annular groove 50. Consequently, the groove 50 guarantees that the forces of unbalancing, due to the pressure acting on the surface 45 of the bushing 41, will always be contained within a certain value, in any case lower than the force exerted by the spring 23.
The armature plate 17, which is made of a magnetic material, 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 that has a cross section tapered toward the outside. The central portion 56 has an axial hole 59 through which the armature plate 17 is able to slide with a certain radial play along an axial portion of the bushing 41. Said axial portion is adjacent to a projection designed to be engaged by the surface 57 of the portion 56 of the armature plate 17.
In the embodiment of
Furthermore, the intermediate body 12a comprises an element for connection with the bushing 41, which is formed by another connection pin 63 made of a single piece with the flange 24. In the embodiment of
Advantageously, the seat 40a has a diameter slightly greater than that of the internal surface 40 of the bushing 41 that couples with the surface of the pin 39. In this way, the surface 40, which requires a more accurate grinding, i.e., the surface that is to form a dynamic seal with the surface 39 of the stem 38, has a smaller axial length, with evident economic advantages.
The connection pin 63 is coaxial with the guide pin 12 for the spring 23, and extends axially from a bottom surface 65 of the flange 24, in a direction opposite to that of said guide pin 12. Between the surface 39 of the stem 38 and the surface 40 of the bushing 41, there is in general a certain leakage of fuel, which gives out into a compartment 48 between the end of the stem 39 and the connection pin 63. To enable exhaust of the fuel that has leaked into the compartment 48 towards the cavity 22, advantageously the intermediate body 12a is provided with an axial hole 64.
For proper assembly of the intermediate body 12a, it is expedient for the surface 65 of the flange 24 to bear upon an end surface 66 of the collar 61 of the bushing 41. In fact, in this way, there is uniquely defined the distance, or space between the surface 65 of the flange 24 and the shoulder 62 of the bushing 41 that constitutes the housing A of the armature plate 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 is held by the spring 23 through the intermediate body 12a, in a closed position of the servovalve 5, the distance of the plane surface 17a from the lamina 13 defines the travel or lift C of the armature plate 17. The armature plate 17 is hence resting against the shoulder 62, in the position indicated in
The travel, or lift I of opening of the open/close element 47 is equal to the difference between the lift C of the armature plate 17 and the play G. Consequently, once again assuming that the lamina 13 is fixed with respect to the polar surface 20, the surface 65 of the flange 24 normally projects from the lamina 13 downwards by a distance equal to the lift I of the open/close element 47, along which the armature plate 17 draws the flange 24 upwards. The armature plate 17 can therefore perform, along the collar 61, an overtravel equal to said play G, which occurs along the housing A, in which the axial hole 59 of the armature plate 17 is guided axially by the collar 61.
Preferably, the lift I of the open/close element 47, and hence of the bushing 41, can be comprised between 12 and 30 μm. According to the embodiment of
Operation of the servovalve 5 of
When the electromagnet 16 is not excited, the open/close element 47 is kept, by the spring 23 through the body 12a rigidly connected to the bushing 41, resting with its truncated cone surface 45 against the truncated cone surface surface 49a of the joining stretch 49, so that the servovalve 5 is closed. It is assumed that, on account of the force of gravity and/or of the previous closing step, which will be seen hereinafter, the armature plate 17 comes to be detached from the lamina 13 and resting against the shoulder 62. This hypothesis does not affect the effectiveness of operation of the servovalve 5, which is irrespective of the axial position of the armature plate 17 at the instant of opening of the servovalve 5 itself
Hence in the annular chamber 46 there has been set up a pressure of the fuel, the value of which is equal to the pressure of supply of the injector 1. When the electromagnet 16 is excited to carry out a step of opening of the servovalve 5, the core 19 attracts the armature plate 17, which at the start effects an idle travel, equal to the play G illustrated in
Consequently in this step, the armature plate 17 and the bushing 41 move in a rigid way and thus traverse the stretch I by the entire travel C allowed for the armature plate 17. On account of the type of stresses to which the armature plate 17 is subjected and on account of the width of the surfaces that are in contact, i.e., the polar surface 20, lamina 13, and surface 17a, the impact of the armature plate 17 against the lamina 13/core 19 ensemble occurs with a practically negligible rebound.
When excitation of the electromagnet 16 ceases, the spring 23, via the body 12a, causes the bushing 41 to accomplish a travel of closing of the servovalve 5 towards the position of
After travelling this stretch I, the open/close element 47 collides with its conical surface 45 against the conical surface 49a of the joining stretch 49 of the valve body 7. On account of the small area of contact and of the hardness of the open/close element 47 and of the valve body 7, and also because the contact occurs in the presence of a considerable amount of vapour of the fuel, the open/close element 47 rebounds, overcoming the action of the spring 23, while the armature plate 17 continues its travel towards the valve body 7, recovering the play G existing in the housing A between the plane surface 57 of the portion 56 and the shoulder 62 of the flange 60.
It is evident that, at the instant in which rebound of the open/close element 47 occurs, this reverses its direction of motion and starts to move towards the armature plate 17. After a certain time, there then occurs a collision of the plane surface 57 of the portion 56 against the shoulder 62 of the bushing 41. As a result of this collision, and also on account the greater momentum of the armature plate 17, at the instant of this collision, the amount of the first rebound of the bushing 41 is sensibly reduced or even cancelled out, thus preventing the control chamber 26 from emptying suddenly. In this way, any alteration of the gradient of variation envisaged for the pressure in the control chamber 26 is eliminated and hence any delay of closing of the needle of the nebulizer.
In actual fact, after the first rebound thus reduced, there can be generated a train of rebounds of decreasing amplitude, the amount of which is much smaller than that of the first rebound already reduced so that not even these rebounds manage to determine a decrease in pressure in the control chamber 26. Consequently, there is no anomalous reconstitution in re-establishing the pressure of the fuel in the control chamber 26, and hence in the motion of the rod 10, which can close the nebulizer without any discontinuity in its motion of closing. The armature plate 17 finally remains in contact with the shoulder 62, also by the force of gravity.
In the embodiments of
In the embodiment of
In the embodiment of
Furthermore, the external diameter of the portion of the bushing underlying said annular flange 74 is smaller than the internal diameter of said annular flange 74. Consequently, during assembly, the armature plate 17 is inserted on the side of the open/close element 47 of the bushing 41. The central portion 56 of the armature plate 27 is 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 shoulder 76 of the armature plate 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 the embodiment of
In the embodiment of
Housing A is defined as the distance between the plane surface 75 and the surface of the projection means 78, 81 that is in contact with the surface 57 of the armature plate 17. The thickness S of the radial portion 56 that slides along the axial portion 82 of the bushing 41 is defined by the relation SA−G. Furthermore, the travel C of the armature plate 17 is C=I+G, as has been seen for the embodiment of
In this embodiment, the intermediate body 12a is connected to the bushing 41 by means of a unidirectional axial constraint. In particular, the flange 24 of the intermediate body 12a engages, with its surface 65, an end edge 80 of the bushing 41, but the connection pin 63 carried by the flange 24 is simply inserted in the axial seat 40a. Consequently, the pin 63 can have a certain radial play with respect to the seat 40a, and the intermediate body 12a can undergo an axial displacement with respect to the bushing 41 itself.
The retention ring 78 can have a modular thickness to enable an adjustment of the travel C of the armature plate 17. The retention ring 78 can be used as support for at least one spacer 81 having a modular thickness to enable an adjustment of the travel C of the armature plate 17 in addition to or instead of that of the ring 78. Also in this case, the play G can be between 10 and 30 μm, as in the embodiment of
In all the embodiments described above, the bushing 41 may be machined with extreme precision, for example, with a tolerance in the region of 1 μm, both to enable the fluid tightness of the fuel under pressure along the side wall 39 of the stem 38 and to enable the fluid tightness of the fuel of the annular chamber 46 by means of the truncated cone surface 45. For said purpose, the bushing 41 is made of very hard material, such as a steel for tooling. The internal surface 40 of the bushing 41 is grinded accurately, and the bushing 41 can possibly be subjected to one or more thermal treatments that will bestow thereon a greater resistance to wear and fatigue, such as hardening and/or nitridation.
For technological reasons, in one or more embodiments, the calibrated stretch 53 (
Both the armature plate 17 and the bushing 41 have been each made with a weight in the region of 2 g. The value “I”, indicated on the axis Y of the ordinates, represents the maximum travel I allowed for the open/close element 47. Represented by a dashed line is, instead, the lift of an open/close element according to the known art, in which the armature plate is made of a single piece with the bushing, the total weight of which is in the region of 4 g. The two plots are obtained by visualizing the effective displacement of the open/close element 47. From the two plots it is clear that the motion of opening of the open/close element 47 according to one or more embodiments of the invention occurs with a more prompt response with respect to the motion of opening of the open/close element according to the known art. This is due both to the fact that the armature plate 17 is made of a material with better characteristics of magnetization and to the fact that the armature plate 17 is separate from the bushing 41.
At the end of the motion of closing, the open/close element according to the known art makes a series of rebounds of decreasing amplitude, of which the amplitude of the first rebound is decidedly considerable. Instead, for the open/close element 47 according to one or more embodiments of the invention, having assumed for the ratio C/I a value between 0.7 and 5 and for the ratio I/G a value between 0.4 and 5, the amplitude of the first rebound is reduced to approximately 30% with respect to the one of the known art. Also the subsequent rebounds are damped more quickly.
In
Towards the end of the travel of closing of the armature plate 17, the latter at the instant designated by the point P hits against the projection means 62 of the bushing 41, which makes the first rebound. The bushing 41 is then pushed by the armature plate 17 towards the closed position. From the instant of this impact onwards, the armature plate 17 remains in contact with the retention means 62, oscillating imperceptibly together with the bushing 41.
Presented at a very enlarged scale in
In general, given the same travel I of the open/close element 47, the greater the play G between armature plate 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
Instead, with a smaller play between the armature plate 17 and the flange 24, at the first rebound at the end of the travel of closing of the open/close element 47 the retention means 62 or 78, 81 immediately encounter the armature plate 17. This is then drawn along, reversing its movement and exerting a reaction against the spring 23. In this case, the train of rebounds subsequent to the first could be temporally longer.
From what has been seen above, the advantages of the injector 1 according to one or more embodiments of the invention as compared to the injectors of the known art are evident. In the first place, the armature plate 17 is separate from the guide bushing 41 and displaceable independently of the latter to enable reduction or elimination of the rebounds of the open/close element 47 especially at the end of the travel of closing. In this way, there is prevented injection of a volume of fuel greater than the one envisaged, alteration of the air/fuel ratio, and reduction of environmental pollution by the engine exhaust gases.
Furthermore, since the armature plate 17 is separate from the guide bushing 41 the material for the armature plate 17 may be chosen so as to optimize the electromagnetic circuit and enable choosing a material with high resistance to wear for the bushing 41. In this way, there is prevented the drawback of also machining the armature plate 17 from said material, with considerable swarf of said material. The construction of the armature plate 17 from a softer material is thus considerably simplified. Finally, the mass of the moving element that the electromagnet 16 and the spring 23 must displace is reduced.
It is evident that further modifications and improvements can be made to the injector 1, without thereby departing from the scope of the embodiments of the invention. For example, in the embodiments of
To adjust the play G between the armature plate 17 in the housing A made between the surface 65 and the shoulder 62 of the bushing 41, there can be inserted at least one disk-shaped spacer having an appropriate modular thickness, for example in 5-μm steps, coaxial with the same armature plate 17. Said spacers contribute also to further damping of the collisions between the armature plate 17 and the bushing 41, with a further beneficial effect as regards elimination of the rebounds.
In the embodiment of
In turn, the lamina 13 can have an internal diameter smaller than the external diameter of the flange 24, and even the same as the internal diameter of the armature plate 17. In this case, the lamina 13 remains constrained in the housing A and consequently cannot undergo radial displacements. It is evident that in this case the axial length of the housing A must be increased by the thickness of the lamina 13 itself
In turn, the joining 49 between the stem 38 and the flange 33 of the valve body 7 can be without the groove 50, and the surface shaped like a truncated cone 45 of the open/close element 47 can be replaced by a sharp edge. The support 54 of the calibrated hole 53 can be eliminated, or else assumes a different shape from the one illustrated. Furthermore, the radial stretches 44 of the duct 42 can number more than two and be set at the same angular distance apart from one another and/or be perpendicular to the axis 3. The calibrated stretch 53 can also be set on the radial stretches 44 of the duct 42. The valve body 7 can be divided into two parts, one part containing the stem 38 and a portion of the flange 33, the other part containing the remaining portion of the flange 33 and the hole 9. Finally, the electromagnet 16 can be replaced by a piezoelectric actuation device.
Number | Date | Country | Kind |
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08425458.0 | Jun 2008 | EP | regional |