The present invention concerns a fuel injector equipped with a metering servovalve for an internal combustion engine.
Usually, injectors for internal combustion engines comprise a metering servovalve having a control chamber, which communicates with a fuel inlet and with a fuel discharge channel. The metering servovalve comprises a shutter, which is axially movable under the action of an electro-actuator to open/close an outlet opening of the discharge channel and vary the pressure in the control chamber. The pressure in the control chamber, in turn, controls the opening/closing of an end nozzle of the injector to supply the fuel in a associated cylinder.
The discharge channel has a calibrated segment, which is of particular importance for correct operation of the metering servovalve. In particular, in this calibrated segment, a fluid flow rate is associated with a predefined pressure differential.
In the injectors that are produced, the calibrated segment of the discharge channel is produced by making a perforation via electron discharge machining, followed by a finishing operation, necessary to eliminate any perforation defects that, even if small, would in any case result in large pressure drop errors in the flow of fuel and, consequently, in the flow rate of fuel leaving the control chamber.
In particular, the finishing operation is of an experimental nature and is carried out by making an abrasive liquid flow through the hole made via electron discharge machining, setting the pressure upstream and downstream of the hole and detecting the flow rate: the flow rate tends to increase progressively with the abrasion caused by the liquid on the lateral surface of the hole, until a preset design value is reached. At this point, the flow is interrupted: in usage, the section of the final passage obtained shall determine, with close approximation, a pressure drop equal to the difference in pressure established upstream and downstream of the hole during the finishing operation and a flow rate of fuel leaving the control chamber equal to the preset design value.
In the injector disclosed in patent EP1612403, the discharge channel has an outlet made in an axial stem guiding the shutter, which is defined by a sliding sleeve. The calibrated segment of the discharge channel is coaxial with the axial stem and is made in a perforated plate, which axially delimits the control chamber. Downstream of this calibrated segment, the discharge channel comprises an axial segment and then two opposed radial sections, which define, together, a relatively large passage section for the discharged fuel. Considering, for example, a fuel supply pressure of approximately 1600 bar to the injector, when the metering servovalve is open, or rather when the sleeve that defines the shutter is raised in the open position, the fuel inlet that runs into the control chamber determines a pressure drop down to approximately 700 bar in the control chamber; then, between the upstream and downstream ends of the calibrated segment of the discharge channel, the fuel pressure drops from approximately 700 bar to a few bar.
The curve shown with a line in
Experimentally, due to the large pressure drop, the onset of cavitation is encountered. In other words, the fuel pressure upstream of the discharge environment drops below the vapour pressure, indicated as PVAPOR, in correspondence to the outlet from the calibrated segment, where fuel flow velocity is maximum and the pressure is minimum (PMIN). In particular, the fraction or percentage of vapour is close to one.
As the passage sections from position XA to position XTEN are relatively narrow (even if larger than that of the calibrated segment), the fuel pressure slowly rises, and not all of the vapour that formed immediately downstream of position XA returns to the liquid state.
Thus, in correspondence to position XTEN the vapour fraction is still substantial. In correspondence to position XTEN, there is then the maximum increase in passage section. In this zone, it is possible to distinguish three undesired phenomena:
Summarizing: the wear deriving from the above-stated phenomena greatly reduces injector life, while the rebounds in the closure phase make the injector inaccurate.
Moreover, to generate a pressure drop of approximately 700 bar, the calibrated segment must have an extremely small diameter, which is extremely complex to make with precision and in a constant manner across the various injectors.
The same drawbacks are present in the embodiment disclosed in the US patent application having publication number US2003/0106533, as the discharge channel substantially has the same arrangement with two opposed radial outlet segments which define, together, a relatively large passage section. Unlike the embodiment disclosed in EP1612403, the discharge channel is made in the shutter, which is defined by a axially sliding pin.
The object of the present invention is that of embodying a fuel injector equipped with a metering servovalve for an internal combustion engine, which enables the above-stated problems to be resolved in a simple and economic manner, limiting as much as possible the risks of the presence of vapour around the sealing zone between the shutter and the axial stem.
According to the present invention, a fuel injector for an internal combustion engine is provided; the injector ending with a nozzle to inject fuel into an associated engine cylinder and comprising:
For a better understanding of the present invention, a preferred embodiment will now be described, purely by way of a non-limitative example, with reference to the attached drawings, in which:
With reference to
The casing 2 defines an axial cavity 6 in which a metering servovalve 5 is housed, comprising a valve body, made in a single piece and indicated with reference numeral 7.
The valve body 7 comprises a tubular portion 8 defining a blind axial hole 9 and a centring ridge 12, which radially projects with respect to a cylindrical outer surface of the portion 8 and couples with an inner surface 13 of the body 2.
A control rod 10 axially slides in a fluid-tight manner in the hole 9 to control, in a known and not shown manner, a shutter needle that opens and closes the injection nozzle.
The casing 2 defines another cavity 14 coaxial with the cavity 6 and housing an actuator 15, which comprises an electromagnet 16 and a notched-disc anchor 17 operated by the electromagnet 16. The anchor 16 is made in a single piece with a sleeve 18, which extends along the axis 3. Instead, the electromagnet 16 comprises a magnetic core 19, which has a surface 20 perpendicular to the axis 3 and defines an axial stop for the anchor 17, and is held in position by a support 21.
The actuator 15 has an axial cavity 22 housing a coil compression spring 23, which is preloaded to exert thrust on the anchor 17 in the opposite axial direction to the attraction exerted by the electromagnet 16. The spring 23 has one end resting against an internal shoulder of the support 21, and the other end acting on the anchor 17 through a washer 24 inserted axially between them.
The metering servovalve 5 comprises a control chamber 26 delimited radially by the lateral surface of the hole 9 of the tubular portion 8. The control chamber 26 is axially delimited on one side by an end surface 25 of the rod 10, usefully having a truncated-cone shape, and by a bottom surface 27 of the hole 9 on the other.
The control chamber 26 is in permanent communication with the inlet 4 through a channel 28 made in portion 8 to receive pressurized fuel. The channel 28 comprises a calibrated segment 29 running on one side to the control chamber 26 in proximity to the bottom surface 27 and on the other to an annular chamber 30, radially delimited by the surface 11 of portion 8 and by an annular groove 31 on the inner surface of the cavity 6. The annular chamber 30 is axially delimited on one side by the ridge 12 and on the other by a gasket 31a. A channel 32 is made in the body 2, is in communication with the inlet 4 and exits into the annular chamber 30.
The valve body 7 comprises an intermediate axial portion defining an external flange 33, which projects radially with respect to the ridge 12, and is housed in a portion 34 of the cavity 6 with enlarged diameter and arranged axially in contact with a shoulder 35 inside the cavity 6. The flange 33 is tightened against the shoulder 35 by a threaded ring nut 36, screwed into an internal thread 37 of portion 34, in order to guarantee fluid-tight sealing against the shoulder 35.
The valve body 7 also comprises a guide element for the anchor 17 and the sleeve 18. This element is defined by a substantially cylindrical stem 38 having a much smaller diameter than that of the flange 33. The stem 38 projects beyond the flange 33, along the axis 3 in the opposite direction to the tubular portion 8, namely towards the cavity 22. The stem 38 is externally delimited by a lateral surface 39, which comprises a cylindrical portion guiding the axial sliding of the sleeve 18. In particular, the sleeve 18 has an internal cylindrical surface 40, coupled to the lateral surface 39 of the stem 38 that is substantially fluid-tight, or rather via a coupling with opportune diameter play, 4 micron for example, or via the insertion of specific sealing elements.
The control chamber 26 is in permanent communication with a fuel discharge channel, indicated as a whole by reference numeral 42.
The channel 42 comprises a blind axial segment 43, made along the axis 3 in the valve body 7 (partly in the flange 33 and partly in the stem 38). The channel 42 also comprises at least one outlet segment 44, which is radial, begins from the segment 43 and defines, at the opposite end, an outlet opening onto lateral surface 39, at a chamber 46 defined by an annular groove made in the lateral surface 39 of the stem 38.
In particular, in the embodiment of
The chamber 46 is obtained in an axial position next to the flange 33 and is opened/closed by an end portion of the sleeve 18, which defines a shutter 47 for the channel 42. In particular, the shutter 47 ends with a truncated-cone inner surface 48, which is able to engage a truncated-cone connecting surface 49 between the flange 33 and the stem 38 to define a sealing zone.
The sleeve 18 slides on the stem 38, together with the anchor 17, between an advanced end stop position and a retracted end stop position. In the advanced end stop position, the shutter 47 closes the annular chamber 46 and thus the outlet of the sections 44 of the channel 42. In the retracted end stop position, the shutter 47 sufficiently opens the chamber 46 to allow the sections 44 to discharge fuel from the control chamber 26 through the channel 42 and the chamber 46. The passage section left open by the shutter 47 has a truncated-cone shape and is at least three times larger that the passage section of a single segment 44.
The advanced end stop position of the sleeve 18 is defined by the surface 48 of the shutter 47 hitting against the truncated-cone connection surface 49 between the flange 33 and the stem 38. Instead, the retracted end stop position of the sleeve 18 is defined by the anchor 17 axially hitting against the surface 20 of the core 19, with a nonmagnetic gap sheet 51 inserted in between. In the retracted end stop position, the chamber 46 is placed in communication with a discharge channel of the injector (not shown), via an annular passage between the ring nut 36 and the sleeve, the notches in the anchor 17, the cavity 22 and an opening 52 on the support 21.
When the electromagnet 16 is energized, the anchor 17 moves towards the core 19, together with the sleeve 18, and hence the shutter 47 opens the chamber 46. The fuel is then discharged from the control chamber 26: in this way, the fuel pressure in the control chamber 26 drops, causing an axial movement of the rod 10 towards the bottom surface 27 and thus the opening of the injection nozzle.
Conversely, on de-energizing the electromagnet 16, the spring 23 moves the anchor 17, together with the shutter 47, to the advanced end stop position in
In order to control the velocity of pressure variation in the control chamber 26 on the opening and closing the shutter 47, the channel 42 comprises calibrated restrictions. The term “restriction” is intended as a channel portion in which the passage section globally available for the fuel is smaller than the passage section that the fuel flow encounters upstream and downstream of this channel portion. In particular, if the fuel flows in a single hole, the restriction is defined by said single hole; on the other hand, if the fuel flows in a plurality of holes which are located in parallel and, therefore, are subjected to the same pressure drop between upstream and downstream, the restriction is defined by the entirety of said holes.
Instead, the term “calibrated” is intended as the fact that the passage section is made with precision in order to accurately define a predetermined fuel flow rate from the control chamber 26 and to cause a predetermined pressure drop from upstream to downstream.
In particular, for holes having relatively small, diameters, calibration is achieved in a precise manner via a finishing operation of an experimental nature, which is carried out by making an abrasive liquid run through the previously made hole (for example, by electron discharge or laser), setting a pressure upstream and downstream of this and reading the flow rate passing through: the flow rate tends to progressively increase with the abrasion caused by the liquid on the lateral surface of the hole (hydro-erosion or hydro-abrasion), until a pre-established design value is reached. At this point, the flow is interrupted: in use, having a pressure upstream of the hole equal to that established during the finishing operation, the final passage section that is obtained defines a pressure drop equal to the difference in pressure established upstream and downstream of the hole during the finishing operation and a fuel flow rate equal to the preset design flow rate.
For example, the restrictions of the channel 42 have a diameter between 150 and 300 micron, while segment 43 of the channel 42 is obtained in the valve body 7 via a normal drilling bit, without special precision, to achieve a diameter that is at least four times greater than the diameter of the calibrated restrictions.
There are at least two restrictions and they are arranged in series with each other along the channel 42 (in the attached figures, the diameter of the restrictions is only shown for completeness and is not in scale), so as to cause respective consequent pressure drops when the shutter is located in its retracted end stop position, as it will be better described later on. Obviously, between two consequent restrictions, the channel 42 comprises an enlarged intermediate segment, i.e. with a passage section larger that those of both the restrictions.
In the embodiment of
The calibrated restriction 53 axially extends for only part of the length of the bushing 54 and is in a position adjacent to segment 43, while the remainder of the bushing 54 has an axial segment 43a of larger diameter, for example, equal to that of segment 43 in the valve body 7. The volume of segment 43a is added to that defined by the bottom of the hole 9 to define the volume of the control chamber 26. Depending on the optimal volume required for the control chamber 26, the bushing 54 can be inverted so as to have the calibrated restriction 53 running directly into the bottom of the hole 9, as in the variants in
According to a variant that is not shown, the calibrated restriction 53 can also be arranged in an intermediate axial position along the bushing 54.
According to the variant in
According to the variant in
According to a variant that is not shown, the hole 9 comprises a bottom portion with a diameter corresponding to the external diameter of the plate 56: in this case, the plate 56 could be fixed in this bottom portion by interference fitting.
According to the variants in
By ensuring that the hole 58 is completely within the flange 33 of the valve body 7, the stem 38 proves to be more robust compared to the embodiment of
Furthermore, by reducing the axial length and enlarging the diameter of the hole 58 with respect to the segment 43, the making of the hole 58 and subsequent cleaning out of chips are facilitated. The hole 58 usefully has a diameter between 8 and 20 times that of the calibrated restriction 53. In this way, when making the holes 59, the intersection of the holes 59 with the bottom of the hole 58 is facilitated.
The calibrated restriction 53 is obtained in a cylindrical bushing 61 and extends for the entire length of the bushing 61. The bushing 61 is driven, or rather inserted by force, into an axial seat 60 after the hole 58 has been cleaned. The seat 60 has a larger diameter than that of the hole 58 and a shorter length than that of the hole 58, which facilitates press fitting; the bushing 61 could have a small, conical, external chamfer (not shown) on the side fitting into the flange 33 to facilitate its axial insertion into the seat 60.
According to the variant in
The hole 62 also comprises a blind segment 66 having a smaller diameter than that of segment 63, extending beyond the flange 33 into the stem 38 and defining a calibrated restriction. The diameter of segment 66 is greater than that of the calibrated restriction 53: for example, it is approximately two times that of the calibrated restriction 53. Notwithstanding the greater diameter, it is possible to obtain a pressure drop of the same order of magnitude of that caused by restriction 53, by calibrating in an appropriate way the length of the segment 66.
Since the diameter of segment 66 is still relatively small, the diameter of the stem 38 and thus the diameter of the seal with the sleeve 18 can be reduced with respect to the solution in
The channel 42 also comprises two diametrically opposed radial sections 67, which are made so as to define a larger passage section than that of segment 66 and without special machining precision. The sections 67 run directly to the calibrated segment 66 on one side and to the chamber 46 on the other.
According to variants of
The variants in
The remaining part of the bushing 61a and 64a has an axial hole 68 made with a larger diameter than the calibrated restriction 53 without special machining precision.
In the variant in
In the variants in
In the variant in
The plate 69 defines a through hole, the volume of which forms part of the control chamber 26, and is not interference fitted, but axially secured to the bottom of segment 63 by an insert defined by a sleeve 70, which is interference fitted to the inlet of segment 63 and is made of a relatively soft material to facilitate press fitting.
In the embodiment of
The stem 38 projects axially from the base 33c in the opposite direction to the disc 33b and comprises the calibrated restriction defined by the holes 44. The blind segment 43 is created partly in the base 33c and partly in the stem 38; the calibrated restriction 53 and the segment 43a are created in the disc 33b.
According to a variant of
According to a further variant of
In the embodiment of
The segment 43 is coaxial with the seat 55a and runs directly into the seat 55a. The seat 55a has a larger diameter than that of segment 43, and is engaged by an insert defined by a cylindrical bushing 54b, which is interference fitted in the seat 55b and arranged flush with the surface 77 of the base 33c.
La bushing 54b defines a calibrated restriction 79, arranged in series with the restrictions 44 and 53. The restriction 79 only extends for part of the axial length of the bushing 54b and is in a position adjacent to segment 43. The remainder of the bushing 54b has an axial segment 43b with a larger diameter than that of the restrictions and communicating directly with segment 43a.
According to variants of
In the embodiment of
The valve body 80 radially and axially delimits the control chamber 26 and comprises an end portion 82 provided with the ridge 12 and an external flange 33d axially secured between the flange 33c and the shoulder 35 (not shown).
The calibrated restriction 53 is made in portion 82 and runs into two coaxial sections 83 and 84 of the channel 42. The sections 83 and 84 have a larger diameter than that of the calibrated restriction 53 and substantially equal to that of segment 43. The segment 83 is defined by a hole in portion 82 and communicates directly with the control chamber 26; the segment 84 is defined by a sealing ring 85, which is housed in a seat 86 and arranged in contact against the surface 77 to define fluid-tight sealing of the channel 42 between the bodies 80 and 76. Alternatively, by opportunely reducing the diameter of segment 84, fluid sealing can still be achieved through metal-to-metal contact between the bodies 80 and 76 without any sealing ring.
According to variants of
According to further variants of
One of these variants is shown in
The disc 91 is kept in axial contact against the bottom of the seat 90 by a sealing ring 85a, provided in place of ring 85. The ring 85a has a rectangular or square cross-section, with an external diameter substantially equal to the diameter of the seats 90 and 86 and engages both of the seats 90 and 86 to define a centring member between the two bodies 80 and 76. In other words, the ring 85a provides three functions: axial centring between the bodies 80 and 76 when coupling, sealing between the bodies 80 and 76 around the fuel flow in the channel 42 and positioning of the disc 91 in the seat 90.
In the embodiment of
The axial end of valve body 7, opposite to portion 8, has an axial recess 139, which is defined by a surface 149 having substantially a frustum of cone shape and houses a shutter 147.
The shutter 147 is axially movable in response to the action of the actuator 15 in a manner known and not described in detail, to open/close an axial outlet of the channel 42. The shutter 147 has a external spherical surface 148, which engages the surface 149 when the shutter 147 is located in its advanced end stop position or closure position, so as to define a sealing zone.
In a manner similar to the embodiment of
The axial segment 43 is made in the flange 33 and exits in an axial segment 144 of the channel 42. The segment 144 defines a calibrated restriction located in series and coaxial with the restriction 53. At the opposite end, the segment 144 exit in a final axial segment 130, which has a passage section larger than that of the segment 144 and defines the outlet of the channel 42 onto the surface 149.
In all the above described embodiments, the pressure drop, which, in use, occurs in the control chamber 26 and in the discharge channel when the shutter 47 is in the open position, is divided into as many pressure drops as there are calibrated restrictions arranged in series along the channel 42.
Considering the two calibrated restrictions in series in
The linearized distance along the channel 42 with respect to the chamber 26 is indicated on the abscissas. In particular:
Thanks to the sequence of calibrated restrictions, the pressure drop shown in
As mentioned above, for a hole defining a calibrated restriction, a close correlation exists between the flow rate passing through and the difference in pressure upstream and downstream of this hole.
ρ=density of liquid,
ceffius=velocity coefficient of hole (experimentally obtainable),
Aforo=passage cross-section in hole,
Δp=difference in pressure between upstream and downstream of hole,
Q=flow rate.
Having a total number of n calibrated restrictions in series, which are crossed by the same flow rate Q, and assuming that the density of the fluid is constant and that cavitation is not present, gives:
Therefore, it is possible to write down a relation between the ratio of the pressure differences and the ratio of the passage sections. In fact, considering two restrictions indicated by subscripts 1 and 2, gives:
Assuming that the holes defining the restrictions are similar and consequently have the same velocity coefficient, gives:
It is understood that in the case of restrictions with velocity coefficients significantly different from each other, the above formulas are valid, but must be completed with the values of these coefficients, determined experimentally.
In injector 1, the total pressure drop of the fuel flow from control chamber 26 to the discharge environment is known. Indicating this pressure drop as Δp0 and wishing to divide this pressure drop into two differentials Δp1 and Δp2 (with Δp0=Δp1+Δp2), gives:
where A0 and D0 are respectively the passage cross-section and the diameter of the hole that one would have if a single calibrated restriction were used, instead of having two restrictions in series defined by the subscripts 1 and 2.
In a first approximation, having set how to subdivide the differential Δp0 between the two holes or restrictions in series and the flow rate that must be made to flow from the control chamber 26, it is possible to obtain the value of the diameters D1 and D2.
The more the calibrated restrictions are distanced from the sealing zone defined by the surfaces 48 and 49, the greater the probability of avoiding the presence of vapour and cavitation in correspondence to this seal.
To reduce the risks of the presence of vapour to a minimum in correspondence to position XTEN (
The calibrated restriction 53 is associated with a pressure drop of at least 60% of the total pressure drop and, conveniently, at least 80%.
For example, wishing to subdivide the pressure drop Δp0 in a way to associate 80% of this drop with the first restriction and 20% with the second restriction (Δp2=0.2 Δp0), and also assuming that the velocity coefficients are equal, a first approximation gives:
Generalizing the example shown above gives:
1<(D2/D1)<=2.088
or
1<(A2/A1)<=4.36
In particular, the condition D2/D1=1 corresponds to the case in which Δp1=Δp2=(0.5 Δp0).
Instead, the condition D2/D1=2.088 and A2/A1=4.36 corresponds to the case in which Δp1=(0.95 Δp0) and Δp2=(0.05 Δp0) (or Δp1/Δp2=19).
As explained above, the passage sections of the calibrated restrictions (A1 and A2) are easily calculated after having established the subdivision of the pressure drop Δp0 at design level and having set the flow rate Q with which it is wished to discharge the control chamber 26 in order to achieve certain performance levels from the injector (the desired flow rate Q determines the passage section A0 that one would have in the case of a single restriction to achieve the pressure drop Δp0).
According to the invention, considering the embodiment of
Considering the embodiment of
Noting that the radial sections are mutually parallel and thus associated with the same pressure drop, simply gives:
from which the diameter dfororad of each radial segment is obtained.
From what explained above, it emerges that the volumes of the channel 42, which are arranged in intermediate positions between the calibrated restrictions, have a pressure that is predetermined and a consequence of the pressure drops Δp1, Δp2, etc. set in the design and manufacturing phase.
Subdividing the total pressure drop into a number of parts reduces the risks of vapour being present, because the fuel's flow velocity in correspondence to the last pressure drop is relatively low. The risks of having local pressure values lower than the fuel's vapour pressure are thus limited: the vapour fraction in the sealing zone, if present, would in any case be much lower with respect to the situation with a single calibrated restriction.
By splitting the pressure drop in order to have the largest part—90% of the entire pressure drop for example—associated with the first restriction (calibrated restriction 53), the formation of vapour and possible cavitation, due to re-compression downstream of the restrictions, could possibly occur in proximity to this first calibrated restriction, but would not influence the life of the injector 1, as the phenomena would be relatively distant from the sealing zone between the shutter 47 and the stem 38.
Given that the second restriction is associated with a smaller pressure drop and therefore has larger diameters than the first restriction, the second restriction is easier to make. From the constructional viewpoint, only the first calibrated restriction requires special accuracy. In fact, as the second restriction is associated with a relatively small pressure drop, any dimensional manufacturing errors do not cause particularly adverse effects: in other words, the pressure drop of the second restriction is less sensitive to possible dimensional manufacturing errors.
Embodiments in which it is possible to reduce the diameter of the stem 38 and, in consequence, the sealing diameter of the shutter 47, with consequent reduction in leakage under dynamic conditions, and consequent reduction in the preloading required for the spring 23 and the force required of the actuator 15, are particularly useful.
In particular, the diameter of the stem 38 can be reduced to a value between 2.5 and 3.5 mm, according to the material chosen for the valve body, the heat treatment to which the valve body is subjected and, consequently, its toughness, and lastly, the manufacturing cycle adopted.
The reduction of the seal diameter on the shutter 47 also allows the axial length of the sleeve 18 to be reduced.
In fact, the flow rate of fluid leakage is directly proportional to the circumference of the coupling zone between the inner cylindrical surface of the sleeve 18 and the outer cylindrical surface 39 of the stem 38, but inversely proportional to the axial length of this coupling zone: as the circumference of the coupling zone has decreased, for the same fluid leakage flow rate it is possible to reduce the axial length of the coupling zone and, consequently, the axial length of the sleeve 18.
The reduction of the seal diameter and, in consequence, the external diameter of the shutter 47 and the reduction in length of the sleeve 18 have the effect of reducing the mass of the sleeve 18 and, consequently, the response times of the metering servovalve 5.
Furthermore, the reduction in the seal diameter allows the load of the spring 23 to be reduced: in fact, for the same coupling play between the stem 38 and the shutter 47, the circumference of the seal between the stem 38 and the shutter 47 decreases and, consequently, also the axial force that acts on the shutter 47 due to the fuel pressure, which although minimal, is still present even if the metering servovalve of the
The reduction in mass of the sleeve 18 and the reduction in load of the spring 23 have the effect of much smaller rebounds by the shutter 47 in the closure phase, and therefore better operating precision of the metering servovalve 5.
Finally, it is clear that modifications and variants can be made regarding the injector 1 described herein without leaving the scope of protection of the present invention, as defined in the attached claims.
In particular, the balanced-type metering servovalve 5 of the
The actuator 15 could be substituted by a piezoelectric actuator that, when subjected to an electric current, increases its axial dimension to operate the sleeve 18 in order to open the outlet of the channel 42.
Moreover, the chamber 46 could be at least partially excavated in the surface 40, but always with a shape such that the shutter 47 defined by the sleeve 18 is subject to a null pressure resultant along the axis 3 when it is positioned in the closure end stop position.
The axes of the sections 44 could lie on mutually different planes, and/or could not all be equally distanced around the axis 3, and/or the calibrated holes could be limited to just a part of the sections 44.
The channel 42 could be asymmetric with respect to the axis 3; for example, the sections 44 could have mutually different cross-sections and/or diameters, but always calibrated to generate an opportune pressure drop to cause a flow rate of discharged fuel that is balanced around the axis 3 and constant over time.
The present patent application is a continuation of U.S. patent application Ser. No. 12/491,938, filed Jun. 25, 2009, which in turn claims priority under 35 U.S.C. §119 to European Patent Application No. 08425460.6, filed Jun. 27, 2008, the entireties of which are hereby incorporated by reference.