Fuel injector for gaseous fuel

Abstract

To provide a fuel injector for a gaseous fuel which allows optimization of mixing of air and fuel and is easy and inexpensive to produce.SOLUTION: A fuel injector (1) has: an injection nozzle (3); a support body (4) with a tubular shape, which is provided with a feeding channel (5) therein, ending in the injection nozzle; an injection valve (7); a tubular sleeve (12), which is arranged around an end part of the support body, entirely contains the shutter (9) therein, and ends with the injection nozzle; and an actuator, which is configured to move the shutter. An injection duct (14) is created when the injection valve (7) is in an open position, and has a converging initial segment, in which the area of the injection duct decreases from a first maximum value at the beginning of the injection duct (14) to a minimum value, and a final segment, in which the area of the injection duct increases from the minimum value until reaching a second maximum value.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] (CROSS REFERENCE TO RELATED APPLICATIONS) This application claims priority to Italian Patent Application No. 102023000019236, filed September 19, 2023, the entire disclosure of which is incorporated herein by reference.

[0002] The present invention relates to a fuel injector for a gaseous fuel.

[0003] The present invention is advantageously applied to electromagnetic hydrogen injectors, and in the following discussion, explicit reference is made to electromagnetic hydrogen injectors without any loss of generality. [Background technology]

[0004] The electromagnetic fuel injector comprises a cylindrical tube-shaped support with a central supply channel, which serves as a fuel duct and ends in an injection nozzle adjusted by an injection valve controlled by an electromagnetic actuator, which comprises a needle, which is moved by the action of the electromagnetic actuator between a closed position and an open position of the injection nozzle against the action of a closing spring which pushes the needle towards the closed position, and which ends in a shutter adapted to seat sealingly against a valve seat of the injection valve.

[0005] The most suitable configuration of a fuel injector for injecting hydrogen provides an outward opening shutter such that the closed position of the shutter is not negatively affected by the increased pressure in the combustion chamber, i.e. when the shutter opens outward the increased pressure in the combustion chamber pushes the shutter towards the closed position, thus ensuring that the fuel injector is not caused to open undesirably by pressure peaks in the combustion chamber.

[0006] However, fuel injectors with an open head that flares outward can only produce a fuel spray that is concentric with the longitudinal axis of the injector, and this fuel spray configuration cannot optimize air-fuel mixing and therefore combustion.

[0007] U.S. Patent Application Publication No. 2001 / 025892 describes a supersonic fuel injector for gaseous fuels in which a nozzle for delivering the fuel is profiled with converging and diverging sections to accelerate the fuel flow to supersonic velocities.

[0008] DE 202023100690 A1 describes a fuel injector for gaseous fuels, in particular for hydrogen, in which a shutter is separated from a valve seat to create an injection duct through which the gaseous fuel flows and which has a converging initial segment and a diverging final segment. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] US Patent Application Publication No. 2001 / 025892 [Patent Document 2] German Utility Model No. 202023100690 Summary of the Invention

[0010] The object of the present invention is to provide a fuel injector for gaseous fuels that does not have the above-mentioned drawbacks (i.e. is capable of producing a fuel spray that allows optimal mixing of air and fuel), and which is particularly easy and cheap to produce.

[0011] According to the invention, a fuel injector for gaseous fuel is manufactured in accordance with the subject matter of the appended claims.

[0012] The appended claims, which form an integral part of this specification, illustrate preferred embodiments of the invention.

[0013] The invention will now be described with reference to the accompanying drawings, which show non-limiting exemplary embodiments of the invention. [Brief description of the drawings]

[0014] [Figure 1] FIG. 2 is a side view, partially in cross section, of a fuel injector. [Diagram 2] FIG. 2 is a longitudinal cross-sectional view of the final portion of the fuel injector of FIG. 1; [Diagram 3] FIG. 2 is a longitudinal section through the final portion of a variant of the fuel injector of FIG. 1; [Figure 4] FIG. 2 is a longitudinal section through the final portion of a variant of the fuel injector of FIG. 1; [Diagram 5] FIG. 2 is a longitudinal section through the final portion of a variant of the fuel injector of FIG. 1; [Figure 6] FIG. 2 is a longitudinal section through the final portion of a variant of the fuel injector of FIG. 1; [Figure 7] 2 illustrates a schematic of an internal combustion engine utilizing the fuel injector of FIG. 1; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] In Fig. 1, reference number 1 indicates as a whole an injector of a gaseous fuel, in particular of hydrogen, which is cylindrically symmetric about a longitudinal axis 2 and is adapted to be controlled to inject hydrogen through an injection nozzle 3 which leads directly into the combustion chamber of a cylinder of an internal combustion engine E (schematically shown in Fig. 7), that is to say that the internal combustion engine E is supplied with hydrogen and comprises an injection system which injects hydrogen directly into a number of cylinders by means of corresponding hydrogen injectors 1. The internal combustion engine E therefore comprises at least one cylinder and an injection system which injects hydrogen directly into said cylinder by means of a fuel injector 1.

[0016] As shown in FIG. 1, the fuel injector 1 comprises a support 4 having a cylindrical tubular shape with a cross-section that varies along a longitudinal axis 2 and having a supply channel 5 extending along the entire length of the support 4 for supplying pressurized hydrogen towards an injection nozzle 3.

[0017] The support 4 houses in its upper part an electromagnetic actuator 6 and in its lower part an injector 7 (shown in detail in FIG. 2). In use, the injector 7 is actuated by the electromagnetic actuator 6 in order to regulate the flow of hydrogen through the injection nozzle 3 provided by the injector 7. According to a different embodiment, not shown, the actuator 6 is of the piezoelectric type instead of being of the electromagnetic type.

[0018] The electromagnetic actuator 6 is configured to move axially (i.e. along the longitudinal axis 2) a movable unit comprising a needle 8 ending in a shutter 9 (shown in FIG. 2). The shutter 9 cooperates with a valve seat 10 (shown in FIG. 2) of the injector 7 to regulate the flow of hydrogen through the injection nozzle 3. In other words, the support 4 ends in a through-hole, in which the valve seat 10 is defined and which is engaged by the shutter 9. In particular, the electromagnetic actuator 6 is configured to move the shutter 9 between a closed position and an open position of the injector 7. Furthermore, the electromagnetic actuator 6 comprises a closing spring 11 which normally keeps the fuel injector 1 closed, i.e. the closing spring 11 pushes the shutter 9 towards the closed position of the injector 7. In other words, the injector 7 is normally closed by the effect of the closing spring 11 which pushes the needle 8 towards the closed position, in which the shutter 9 of the needle 8 is pressed against the valve seat 10 of the injector 7.

[0019] According to FIG. 2, the shutter 9 is arranged externally relative to the support 4 and is pressed against it by a closing spring 11 of the electromagnetic actuator 6. As a result, in order to move the closed position of the injector 7 to the open position, the shutter 9 moves along the longitudinal axis 2 towards the outside of the support 4, i.e. in a direction harmonic with the supply direction of hydrogen. In the open position of the injector 7 (shown in FIG. 2), the shutter 9 is separated from the valve seat 10, and a passage opening with a crown-shaped cross section is generated. As a result, the hydrogen injected through the injection nozzle 3 has an internally hollow conical shape at the outlet. In other words, the electromagnetic actuator 6 is configured to move the shutter 9 between the closed position of the injector 7, in which the shutter 9 is pressed against the valve seat 10, and the open position of the injector 7 (shown in FIG. 2), in which the shutter 9 is separated from the valve seat 10 to form an annular opening through which the hydrogen flows.

[0020] 2, around the end of the support 4 (i.e. at the injector 7 and the injection nozzle 3) is arranged a tubular sleeve 12 which contains the entire shutter 9 inside and which ends at the injection nozzle 3. The latter is thus bounded by the sleeve 12 and constitutes the final element of the fuel injector 1.

[0021] The shutter 9, in the embodiment shown in figures 2 to 5, has a bulb-like shape and has an outer surface, which comprises a first part facing the valve seat 10 and shaped to be sealingly connected to the inner surface of the valve seat 10 when the injector 7 is in the closed position, and a second part facing the inner surface of the sleeve 12 and always remaining a predetermined distance from the inner surface of the sleeve 12. Overall, between the outer surface of the shutter 9 and the inner surfaces of the valve seat 10 and the sleeve 12, an annular injection duct 14 is delimited, an initial part of which is created when the injector 7 is in the open position, and when the injector 7 is in the open position (shown in figure 2), gaseous fuel flows along the injection duct 14 towards the end zone 13 of the injection duct 14 (i.e. towards the injection nozzle 3). More generally, the injection duct 14 is externally delimited (defined) by the inner surface of the valve seat 10 which in turn is bounded (defined) by the inner surface of the sleeve 14 .

[0022] The injection duct 14 is created (initially between the shutter 9 and the valve seat 10 and sleeve 12) when the injection valve 7 is in the open position (as shown in FIG. 2) and has (wherever) a converging initial segment within which the area of ​​the injection duct 14 gradually decreases from a first maximum value at the start of the injection duct 14 to a minimum value, and (at least initially) a diverging final segment within which the area of ​​the injection duct 14 gradually increases from said minimum value until it reaches a second maximum value at the end of the injection duct 14 (which will generally be greater than the first maximum value).

[0023] The injection duct 14 has an intermediate segment, interposed between the convergent (wherever) initial segment and the divergent (at least initially) final segment, having a constant area equal to a minimum value and having a non-zero axial length.

[0024] In other words, the converging initial segment of the injection duct 14 extends from a zone where the area of ​​the injection duct 14 is equal to a first maximum value to a zone where the intermediate segment begins where the area of ​​the injection duct 14 is equal to a minimum value (i.e., the throat section), while the diverging final segment of the injection duct 14 extends from a zone where the intermediate segment ends where the area of ​​the injection duct 14 is equal to a minimum value (i.e., the throat section) to at least a zone where the area of ​​the injection duct 14 is equal to a second maximum value.

[0025] According to a preferred embodiment, the (at least initially) diverging final segment of the injection duct 14 is axially longer than the (anywhere) converging initial segment of the injection duct 14 (i.e., hydrogen must travel a longer distance to pass through the diverging final portion of the injection duct 14 than the converging initial segment of the injection duct 14).

[0026] Inside the sleeve 12 there is an end zone 13 of an injection duct 14 which is arranged downstream of the shutter 9 and which ends in the injection nozzle 3 .

[0027] The convergent-divergent alternation in the injection duct 14 constitutes a Laval nozzle, which is a supersonic injection nozzle made of a tube with a central bottleneck similar to an asymmetric hourglass. Its steady-state operation (where the hydrogen velocity is equal to the speed of sound in the throat section, i.e. in the smaller section) makes it possible to accelerate the hydrogen to supersonic speeds and to convey the injection flow so as to convert its thermal and pressure energy into kinetic energy.

[0028] 2-5, the shutter 9 has an initial part, which is connected to the valve seat 10 and has an increasing outer diameter along the longitudinal axis 2, an intermediate part, which has a constant outer diameter along the longitudinal axis 2, and an end part, which is closer to the injection nozzle 3 and has a decreasing outer diameter along the longitudinal axis 2. On the other hand, the sleeve 12 is arranged immediately downstream of the valve seat 10, has an initial part, which is arranged in the intermediate part of the shutter 9 and has a constant inner diameter along the longitudinal axis 2, an intermediate part, which has a decreasing inner diameter along the longitudinal axis 2, and an end part, which is closer to the injection nozzle 3, delimits an end zone 13 of the injection duct 14 and has a constant inner diameter along the longitudinal axis 2.

[0029] In the embodiment shown in Figures 2 to 5, in the diverging final segment, the area of ​​the injection duct 14 gradually increases from a minimum value until a second maximum value is reached, which is preferably reached at the end of the injection duct 14 (i.e. the second maximum value may be reached at the injection nozzle 3, at the end of the injection duct 14, or it may be reached upstream of the injection nozzle 3 and before the end of the injection duct 14).

[0030] In the alternative embodiment shown in Figure 6, the shutter 9 has substantially only an initial portion, which is connected to the valve seat 10 and has an outer diameter that gradually increases along the longitudinal axis 2, and an intermediate portion, which has a constant outer diameter along the longitudinal axis 2. That is to say, in the embodiment shown in Figure 6, the end portion, which is closer to the injection nozzle 3 and has an outer diameter that gradually decreases along the longitudinal axis 2, is substantially absent. Indeed, in the embodiment shown in Figure 6, the shutter 9 ends in a frustoconical shape, therefore closer to the injection nozzle 3 and has a flat lower wall.

[0031] In the alternative embodiment shown in FIG. 6, in the diverging final segment the area of ​​the injection duct 14 increases stepwise from a minimum to a second maximum at the flat lower wall of the shutter 9 .

[0032] In the embodiment shown in the accompanying drawings, the terminal zone 13 of the injection duct 14 is coaxial with the longitudinal axis 2 of the fuel (or other gaseous fuel) injector 1, thus imparting to the hydrogen spray leaving the injection nozzle 3 an orientation coaxial with the longitudinal axis 2 of the fuel injector 1. According to a different embodiment, not shown, the terminal zone 13 of the injection duct 14 is inclined (at an angle) with respect to the longitudinal axis 2 of the fuel injector 1 (i.e. the terminal zone 13 of the injection duct 14 forms an acute angle with the longitudinal axis 2 of the fuel injector 1), thus imparting to the fuel spray leaving the injection nozzle 3 an orientation inclined (at an angle) with respect to the longitudinal axis 2 of the fuel injector 1.

[0033] In the alternative embodiment shown in Fig. 3, the sleeve 12 is provided with at least one through-type lateral duct 15 penetrating it from side to side and leading to the end zone 13 of the injection duct 14, i.e. placing the combustion chamber of the corresponding cylinder (surrounding the final part of the fuel injector 1) in communication with the end zone 13 of the injection duct 14. According to a preferred embodiment, each lateral duct 15 is inclined with respect to the longitudinal axis 2 of the fuel injector 1 and forms an acute angle with said longitudinal axis 2 of the fuel injector 1, in particular the acute angle between each lateral duct 15 and the longitudinal axis 2 of the fuel injector 1 is in the range of 20° to 40°, preferably 30°.

[0034] In general, the sleeve 12 is provided with a number of lateral ducts 15 which may be arranged along the entire circular extent of the sleeve 12 (i.e. they form a closed ring and have a symmetrical distribution around the longitudinal axis 2) or they may be arranged only along a limited part of the circular extent of the sleeve 12 (i.e. they form an arc and have an asymmetrical distribution around the longitudinal axis 2).

[0035] The function of the lateral ducts 15 is to make it possible to first mix the already present hydrogen (fuel) and air (comburent) inside the end zone 13 of the injection duct 14, thus making it possible to suck air (i.e. air as a combustion improver) from the combustion chamber of the corresponding cylinder (surrounding the last part of the fuel injector 1) to the end zone 13 of the injection duct 14, thus making it possible to improve the overall mixing of hydrogen (fuel) and air (comburent) present in the combustion chamber of the corresponding cylinder (surrounding the last part of the fuel injector 1). In other words, the presence of the lateral ducts 15 makes it possible to improve the overall mixing of hydrogen (fuel) and air (comburent) present in the combustion chamber of the corresponding cylinder (surrounding the last part of the fuel injector 1).

[0036] It is important to note that, as mentioned above, the injection duct 14 has an initially converging and then diverging shape (to form a Laval nozzle), determining an increase in the fuel velocity and a decrease in its pressure, resulting in the creation, in the end zone 13 of the injection duct 14, of a (relative) low pressure that draws air from the combustion chamber of the corresponding cylinder (surrounding the final part of the fuel injector 1) through the lateral duct 15.

[0037] If the lateral ducts 15 are distributed symmetrically about the longitudinal axis 2 of the fuel injector 1, their presence does not affect the direction of the fuel spray leaving the injection nozzle 3. On the other hand, if the lateral ducts 15 are distributed asymmetrically about the longitudinal axis 2 of the fuel injector 1 (i.e. there are more or larger lateral ducts 15 on one side of the sleeve 12 than on the other side of the sleeve 12), their presence affects the direction of the fuel spray leaving the injection nozzle 3, which may have a direction that is inclined (has an angle) with respect to the longitudinal axis 2 of the fuel injector 1. In other words, the asymmetric distribution of the lateral ducts 15 with respect to the longitudinal axis 2 of the fuel injector 1 allows the fuel spray leaving the injection nozzle 3 to be given an inclination with respect to the longitudinal axis 2 of the fuel injector 1.

[0038] In order to obtain a fuel spray leaving the injection nozzle 3 with a given inclination relative to the longitudinal axis 2 of the fuel injector 1, it is possible to rely on an asymmetric distribution of the side ducts 15 relative to the longitudinal axis 2 of the fuel injector 1, or on an inclination of the end zone 13 of the injection duct 14 relative to the longitudinal axis 2 of the fuel injector 1 (as described above), or on a combination of both of these solutions.

[0039] In the embodiment shown in Figures 2 and 3, the outer edge 16 of the injection nozzle 3 is smooth. In the embodiment shown in Figure 4, the outer edge 16 of the injection nozzle 3 is sawtooth (i.e. has a series of convex and concave portions that are more or less irregular). In other words, the outer edge 16 of the injection nozzle 3 has a series of lobes (which may be more rounded or more pointed) that give the outer edge 16 a sawtooth shape. The (non-uniform and irregular) sawtooth shape of the outer edge 16 of the injection nozzle 3 can improve the mixing of the fuel and the air (combustion improver), since it forms greater disturbances in the fuel spray. Furthermore, the (non-uniform and irregular) notched shape of the outer edge 16 of the injection nozzle 3 makes it possible to reduce the noise (i.e. pressure waves in the audible range) generated during the fuel spray. The sawtooth shape improves the mixing process, which in turn weakens the formation of large-scale structures (associated with the generation of low-frequency noise present in the audible range) and excites smaller-scale structures (associated with the generation of high-frequency noise outside the audible range).

[0040] In the embodiment shown in Figures 2, 3 and 4, the end zone 13 of the injection duct 14 communicates towards the outside (i.e. towards the combustion chamber) and injects the fuel through a single opening. That is, in the embodiment shown in Figures 2, 3 and 4, the injection nozzle 3 has a single opening for the injection of the fuel. In an alternative embodiment shown in Figure 5, the end zone 13 of the injection nozzle 14 injects the fuel through a series of openings 17 that communicate towards the outside (i.e. towards the combustion chamber) and that may be on the left and right and have differentiated sizes (diameters). That is, in the embodiment shown in Figure 5, the injection nozzle 3 has a series of openings 17 that may be on the left and right and have differentiated sizes (diameters). In other words, in the embodiment shown in Figure 5, a "multi-hole" configuration is provided in which the fuel leaves the end zone 13 of the injection duct 14 through a series of openings 17 that constitute the injection nozzle 3 and that may be on the left and right and have differentiated sizes (diameters). In the embodiment shown in FIG. 5, the openings 17 are parallel to one another, but alternatively they may be angled (non-parallel) relative to one another.

[0041] Although in the above embodiment reference has been made to the injection of hydrogen, the fuel injector 1 may also be utilised to inject any other type of gaseous fuel, for example methane.

[0042] The embodiments described herein may be combined with each other without departing from the scope of protection of the present invention.

[0043] The fuel injector 1 described above has many advantages.

[0044] Firstly, the fuel injector 1 produces a fuel spray that allows for an optimized mixing of air (combustion aid) and hydrogen (fuel). In fact, the fuel injector 1 provides a high exit velocity to the hydrogen (fuel) spray, which increases the swirl and facilitates mixing with the air. Furthermore, the fuel injector 1 orients the hydrogen (fuel) spray at a certain angle relative to the longitudinal axis 2 of the fuel injector 1, allowing for an optimal direction for the fuel spray, regardless of the position and orientation of the fuel injector 1 in the crown of the cylinder (the tendency is for the fuel spray to surround the piston laterally rather than centrally, maximizing turbulence and thus obtaining a better mixture of fuel and air).

[0045] Finally, the electromagnetic fuel injector 1 described above is simple and cheap to produce as it has few easily manufacturable design differences compared to similar known hydrogen (fuel) injectors. [Explanation of symbols]

[0046] 1 Fuel Injector 2 Longitudinal axis 3. Injection nozzle 4 Support 5. Supply Channels 6 Electromagnetic Actuator 7 Injection valve 8 Needle 9. Shutter 10 Valve seat 11 Closing spring 12 Sleeve 13. Termination Zone 14 Injection duct 15 Side duct 16 Outer edge 17 Aperture E. Internal combustion engine

Claims

1. A fuel injector (1) for a gaseous fuel, said fuel injector (1) comprising: An injection nozzle (3); a support (4) having a tubular shape, said support (4) having a longitudinal axis (2) and having therein a feed channel (5) which terminates in said injection nozzle (3); an injection valve (7) configured to regulate the flow of the gaseous fuel through the injection nozzle (3) and comprising a movable shutter (9) arranged outside the support (4) and a valve seat (10) obtained in the support (4); a tubular sleeve (12) arranged around the end of the support (4), containing the entire shutter (9) therein and terminating in the injection nozzle (3); an actuator (6) configured to move the shutter (9) between a closed position of the injector (7), in which the shutter (9) is pressed against the valve seat (10), and an open position of the injector (7), in which the shutter (9) is separated from the valve seat (10); the shutter (9) comprises a first part facing the valve seat (10) and shaped to be sealingly coupled to an inner surface of the valve seat (10) when the injection valve (7) is in the closed position, and a second part facing the inner surface of the sleeve (12) and always remaining a predetermined distance from the inner surface of the sleeve (12); A fuel injector (1), wherein the inner surface of the valve seat (10) and the inner surface of the sleeve (12) define an annular injection duct (14), an initial portion of the injection duct (14) is created when the injection valve (7) is in the open position, and the gaseous fuel flows along the injection duct (14) towards the injection nozzle (3) when the injection valve (7) is in the open position, 1. The fuel injector (1), characterized in that the injection duct (14) is created when the injection valve (7) is in the open position, and has a converging initial segment within which the area of ​​the injection duct (14) gradually decreases from a first maximum value at the start of the injection duct (14) to a minimum value, and a diverging final segment, at least initially, within which the area of ​​the injection duct (14) increases from the minimum value until it reaches a second maximum value.

2. 2. The fuel injector of claim 1, wherein the injection duct has an intermediate segment interposed between the converging initial segment and the diverging final segment, the intermediate segment having a constant area equal to the minimum value and a non-zero axial length separating the initial segment from the final segment.

3. 2. The fuel injector (1) according to claim 1, wherein inside the sleeve (12) there is a terminal zone (13) of the injection duct (14), said terminal zone (13) being arranged downstream of the shutter (9) and terminating in the injection nozzle (3).

4. 4. The fuel injector (1) according to claim 3, wherein the end zone (13) of the injection duct (14) is arranged downstream of the shutter (9) and is coaxial with the longitudinal axis (2) of the fuel injector (1).

5. 4. The fuel injector (1) according to claim 3, wherein the terminal zone (13) of the injection duct (14) is arranged downstream of the shutter (9), is inclined with respect to the longitudinal axis (2) and forms an acute angle with the longitudinal axis (2).

6. 4. The fuel injector (1) according to claim 3, wherein the sleeve (12) has at least one through lateral duct (15) penetrating the sleeve (12) to the left and right and leading to the end zone (13) of the injection duct (14) located downstream of the shutter (9).

7. 7. The fuel injector (1) according to claim 6, wherein the lateral ducts (15) are inclined with respect to the longitudinal axis (2) and form an acute angle therewith.

8. A fuel injector (1) according to claim 7, wherein the acute angle between the lateral duct (15) and the longitudinal axis (2) is in the range of 20° to 40° and is preferably 30°.

9. 7. The fuel injector (1) according to claim 6, wherein the sleeve (12) is provided with a plurality of lateral ducts (15), said lateral ducts (15) being distributed symmetrically around the longitudinal axis (2).

10. 7. The fuel injector (1) according to claim 6, wherein the sleeve (12) is provided with a plurality of lateral ducts (15), said lateral ducts (15) being asymmetrically distributed around the longitudinal axis (2).

11. 11. The fuel injector (1) according to claim 10, wherein on one side of the sleeve (12) there are more or larger lateral ducts (15) than on the opposite side of the sleeve (12).

12. 2. The fuel injector (1) according to claim 1, wherein the outer edge (16) of the injection nozzle (3) is serrated and has a series of protrusions and recesses.

13. The fuel injector (1) of claim 1, wherein the diverging final segment of the injection duct (14) is axially longer than the converging initial segment of the injection duct (14).

14. The fuel injector (1) of claim 1, wherein the convergent-divergent alternation in the injection duct (14) constitutes a supersonic Laval nozzle.

15. The shutter (9) an initial portion coupled to said valve seat (10) and having an outer diameter that increases along said longitudinal axis (2); 15. The fuel injector (1) according to any one of the preceding claims, comprising exclusively an intermediate portion having a constant outer diameter along said longitudinal axis (2).

16. 15. A fuel injector (1) according to claim 1, wherein the shutter (9) terminates in a frustoconical shape and has a lower wall which is closer to the injection nozzle (3) and which is flat.

17. 15. The fuel injector (1) according to any one of the preceding claims, wherein in the final diverging segment, the area of ​​the injection duct (14) increases stepwise from the minimum value to the second maximum value.

18. The shutter (9) has a bulbous shape and an initial portion coupled to said valve seat (10) and having an outer diameter that increases along said longitudinal axis (2); an intermediate portion having a constant outer diameter along said longitudinal axis (2); 15. The fuel injector (1) according to claim 1, further comprising a terminal portion which is closer to the injection nozzle (3) and has an outer diameter which gradually decreases along the longitudinal axis (2).

19. 19. A fuel injector (1) as claimed in claim 18, wherein in the final diverging segment, the area of ​​the injection duct (14) gradually increases from the minimum value until it reaches the second maximum value, which is preferably reached at the terminal end of the injection duct (14).

20. The inner surface of the sleeve (12) is an initial portion having a constant inner diameter along said longitudinal axis (2); an intermediate portion having an inner diameter that decreases along said longitudinal axis (2); A fuel injector (1) according to any one of the preceding claims, comprising a terminal portion closer to the injection nozzle (3) and having a constant inner diameter along the longitudinal axis (2).

21. A fuel injector (1) according to any one of the preceding claims, wherein the injection nozzle (3) presents a series of openings (17) which are lateral and may have differentiated sizes.

22. A fuel injector (1) according to any one of the preceding claims, wherein the second maximum value is greater than the first maximum value.

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