Electromagnetic fuel injector with braking device

The present invention relates to a fuel injector. The fuel injector is provided with an injection nozzle, an injection valve, and an electromagnetic actuator. The injection valve has a movable needle to adjust the flow of fuel through the injection nozzle. The electromagnetic actuator is adapted to move the needle between a closing position and an opening position of the injection valve and is provided with a movable plunger which is mechanically connected to the needle and has at least one feeding through hole for the passage of fuel towards the injection nozzle. The plunger is provided with a hydraulic type braking device, which is coupled to the feeding hole and hydraulically dissipates kinetic energy to slow down the opening stroke of the needle when the needle moves towards the opening position of the injection valve.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims priority to Italian patent application BO2013A000169 filed on Apr. 17, 2013.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to fuel injectors and, more specifically, to an electromagnetic fuel injector with a braking device.

2. Description of Related Art

Electromagnetic fuel injectors known in the art (for example, the type described in European patent application EP1619384A2) generally include a cylindrical tubular body having a central feeding channel (which has a fuel conveying function) and ends with an injection nozzle adjusted by an injection valve controlled by an electromagnetic actuator. The injection valve includes a needle, which is moved by the action of the electromagnetic actuator between a closing position and an opening position of the injection nozzle against the bias of a closing spring, which tends to hold the needle in the closing position. The electromagnetic actuator is typically provided with a closing spring which pushes the needle towards the closing position, and with an electromagnet which pushes the needle towards the opening position against the elastic bias generated by the spring.

The electromagnet includes a coil externally arranged in a fixed position about the tubular body, a movable plunger firmly connected to the needle and movably mounted inside the tubular body, and a fixed magnetic pole made of ferromagnetic material; arranged within the tubular body at the coil; and adapted to magnetically attract the plunger. The magnetic pole is centrally perforated and has a central through hole which allows the fuel to flow towards the injection nozzle. The closing spring is arranged inside the central hole and is compressed between a perforated catch body driven into the central hole, and the plunger, so as to push the plunger (and, thus, the needle integral with the plunger) towards the closing position of the injection nozzle.

The manufacturers of Otto cycle heat engines (for example, spark-ignition internal combustion engines) often require increased fuel pressure (even in excess of 50 MPa) so as to improve the mixing of fuel, to support combustion and reduce the generation of black smoke (which indicates poor combustion), and to increase the dynamic performance of the electromagnetic injectors (for example, to increase the response speed of the electromagnetic injectors to commands) in order to inject small amounts of fuel with the goal of fractioning the fuel injection into multiple separate injections, whereby the generation of polluting substances during combustion can be reduced.

In an electromagnetic fuel injector, increasing the fuel feeding pressure causes a proportional increase of the hydraulic forces involved, and thus necessitates the use of stronger closing springs and more powerful electromagnets. In order to increase the power of an electromagnet (for example, to increase the magnetic attraction force generated by the electromagnet), either higher performance materials can be used (but, with a considerable increase in costs which is not normally acceptable by the modern automotive industry), or the size of the electromagnet can be increased. Regardless, an increase in the electromagnet size also causes an increase of the magnetic and mechanical inertia of the electromagnet, which then becomes slower. Specifically, increasing the size of the electromagnet inevitably degrades the dynamic performance of the electromagnet itself.

In order to obtain an increase in the force generated by the electromagnet without degrading the dynamic performance of the electromagnet itself, European patent EP1650428B1 suggests doubling the electromagnet; for example, two small-sized twin electromagnets are used instead of a single large-sized electromagnet.

When the injection valve is closed, there is a force of hydraulic nature which pushes on the shutter and maintains the shutter in the closing position (for example, the higher the fuel feeding pressure, the higher this force). Therefore, in order to open the injection valve, the electromagnetic actuator needs to generate a force on the needle which overcomes the force added to the elastic bias exerted by the closing spring. However, the force suddenly disappears as soon as the injection valve opens, thus the injection valve opens very quickly and violently with an extremely fast movement of the needle. Such a fast, violent opening of the injection valve causes a very steep and often irregular ramp in the initial part (referred to as the “ballistic zone”) of the injection law of the injector (for example, the law which relates the actuation time to the injected fuel amount; for example, the driving time).

Because the initial part of the injection law has a very steep and often irregular ramp, correctly controlling the fuel injection is very complex. Moreover, at such a steep ramp, tiny differences in the injection time (for example, in the control time) determine substantial differences in the injected fuel amount.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide an electromagnetic fuel injector which is free from the above-described drawbacks, for example, which allows to stabilize the initial part (for example, the ballistic zone) of the injection law, while being easily and cost-effectively manufactured.

According to the present invention, an electromagnetic fuel injector is provided. The injector includes an injection nozzle and an injection valve having a movable needle to adjust the flow of fuel through the injection nozzle. The injector further includes an electromagnetic actuator to move the needle between a closing position and an opening position of the injection valve. The actuator has at least one electromagnet including a coil, a fixed magnetic armature, and a movable plunger mechanically connected to the needle. The plunger and has at least one feeding through hole for the passage of fuel towards the injection nozzle. The injector also includes a closing spring which tends to hold the needle in the closing position. The injector still further has a tubular supporting body having a central channel which houses the fixed magnetic armature and the movable plunger. The plunger has a braking device of the hydraulic type, which is coupled to the feeding hole and hydraulically dissipates more kinetic energy when the needle moves towards the opening position of the injection valve than when the needle moves towards the closing position of the injection valve, so as to slow down the opening stroke of the needle when the needle moves towards the opening position of the injection valve.

Other objects, features and advantages of the present invention will be readily appreciated as the same becomes better understood after reading the subsequent description taken in connection with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION

InFIG. 1, reference numeral1generally indicates a fuel injector, which has a substantially cylindrical symmetry about a longitudinal axis2and is adapted to be controlled to inject fuel from an injection nozzle3which leads into a combustion chamber of an engine cylinder. The injector1includes a supporting body4, which has a variable-section cylindrical tubular shape along the longitudinal axis2, and has a feeding duct5extending along the length of the supporting body4to feed pressurized fuel towards the injection nozzle3.

The supporting body4houses an electromagnetic actuator6at an upper portion thereof, and an injection valve7(shown in greater detail inFIG. 2) at a lower portion thereof. In operation, the injection valve7is actuated by the electromagnetic actuator6to adjust the fuel flow through the injection nozzle3, obtained at the injection valve7.

As shown inFIG. 3, the electromagnetic actuator6includes a pair of twin electromagnets8(upper and lower electromagnets, respectively), which are activated together to work simultaneously. When energized, each electromagnet8is adapted to move a respective plunger9made of ferromagnetic material along the axis2, from a closing position to an opening position of the injection valve7against the bias of a single, common closing spring10which tends to hold plunger9in the closing position of the injection valve. Each electromagnet8includes a coil11(which is electrically controlled by a control unit and is housed outside the supporting body4) and a magnetic armature12or magnetic pole12(which is housed within the supporting body4and has a central hole13to allow the fuel to flow towards the injection nozzle3). A catch body14(seeFIG. 1), which has a tubular cylindrical shape (possibly open along a generating line) to allow the fuel to flow towards the injection nozzle3. The catch body14is adapted to maintain the common spring10compressed against the plunger9of the upper electromagnet8, and driven in fixed position into the central hole13of the magnetic armature12of the upper electromagnet8.

Each coil11is wound directly inside an annular slot15formed in the outer surface of the supporting body4. Each coil11includes an enameled conductor wire having a self-bonding paint. He conducting wire has a small axial dimension (for example, a dimension measured along the longitudinal axis2) so as to minimize the dispersed magnetic fluxes. At the coils11, a protection body16is coupled about the supporting body4. The protection body16has a tubular shape and is used to ensure suitable mechanical protection to the coils11so as to allow the magnetic flux lines generated by the coils11to be closed, and so as to increase the mechanical strength of the supporting body4at structural weak points introduced by the presence of the slots15.

The plungers9form part of a movable equipment, which includes a shutter or needle17having an upper portion integral with each plunger9, and a lower portion cooperating with a valve seat18(shown inFIG. 2) of the injection valve7to adjust the fuel flow through the injection nozzle3.

In operation, when the electromagnets8are de-energized, each plunger9is not attracted by its magnetic armature12and the elastic bias of spring10pushes the plungers9, together with the needle17, downwards. In this situation, the injection valve7is closed. When the electromagnets8are energized, however, each plunger9is magnetically attracted by its magnetic armature12against the elastic bias of spring10, whereby and the plungers9, together with the needle17, move upwards to cause the injection valve7to open.

In order to accurately determine the upward stroke of needle17, the plunger9of the upper electromagnet8has a shorter effective stroke than that of the plunger9of the lower electromagnet8. In this way, when the electromagnets8are energized, only the plunger9of the upper electromagnet8comes into contact with its magnetic armature12, regardless of manufacturing tolerances. In order to limit the effective stroke of the plunger9of the upper electromagnet8, either the lower surface of armature12or the upper surface of plunger9is coated with a layer of hard, non-ferromagnetic metal material, such as chromium. Thus, the thickness of the chromium layer determines the reduction of the effective stroke of the plunger9of the upper electromagnet8. Moreover, the chromium layer increases the shock resistance of the zone and helps avoid magnetic sticking phenomena (due to a direct contact between the ferromagnetic material of plunger9and the ferromagnetic material of armature12). Specifically, the chromium layer defines a gap that prevents the magnetic attraction forces (caused by the residual magnetism between plunger9and armature12) from reaching specific high values (for example, higher than the elastic bias generated by spring10).

Furthermore, only the plunger9of the upper electromagnet8is subjected to mechanical precision machining to ensure a calibrated outer diameter substantially equal (obviously by defect) to the inner diameter of the feeding channel5. The plunger9of the lower electromagnet8, however, has a non-calibrated outer diameter that is smaller than the inner diameter of the feeding channel5. In this way, only the plunger9of the upper electromagnet8serves the function of guiding the needle17so as to control the axial sliding of the needle17along the longitudinal axis2. This arrangement reduces manufacturing costs because only the plunger9of the upper electromagnet8needs to be subjected to costly precision machining operations.

A shown inFIG. 2, the valve seat18is defined in a sealing element19, which is monolithic, seals the bottom of the feeding channel5of the supporting body4, and is crossed by the injection nozzle3. In particular, the sealing element19includes a disc-shaped capping element20which seals the bottom of the feeding channel5of the supporting body4and is crossed by the injection nozzle3. A guiding element21rises from the capping element20, has a tubular shape, houses a needle17therein for defining a lower guide of the needle17itself, and has an outer diameter which is smaller than the inner diameter of the feeding channel5of the supporting body4, so as to define an outer annular channel22through which the pressurized fuel can flow.

A set of four through feeding holes23(two of which are shown inFIG. 2), which lead towards the valve seat18to allow the flow of pressurized fuel towards the valve seat18itself, are obtained in the lower part of the guiding element21. The feeding holes23may be either offset with respect to the longitudinal axis2so as not to converge towards the longitudinal axis2and to create a vortex pattern to the respective fuel flows, or the feeding holes23may converge towards the longitudinal axis2. As shown inFIG. 4, the feeding holes23are arranged at an 80° angle (generally from 70° to 90°) with the longitudinal axis2. In one embodiment, the feeding holes23form a 90° angle with the longitudinal axis2.

The needle17ends with a substantially spherical shutter head24adapted to rest against the valve seat18fluid-tight. In one embodiment, the shutter head24has a substantially cylindrical shape and the abutment zone has a spherical shape. Furthermore, the shutter head24rests slidingly on an inner surface21of the guiding element29so as to be guided in its movement along the longitudinal axis2. The injection nozzle3is defined by a plurality of injection through holes25obtained from an injection chamber26arranged downstream of the valve seat18.

As shown inFIG. 4, each plunger9includes an annular (tubular) element27and a discoid element28, which closes the bottom of the annular element27and has a central through hole29adapted to receive a portion of the needle17and a plurality of peripheral feeding through holes30(two of which are shown inFIG. 4) adapted to allow the fuel to flow towards the injection nozzle3. The needle17is integral with the discoid element28of each plunger9with an annular weld that surrounds the central hole29. A central portion of the discoid element28of the plunger9of the upper electromagnet8abuts against a lower end of spring10.

As mentioned above, the outer diameter of the annular element27of the plunger9of the upper electromagnet8is substantially the same as the inner diameter of the corresponding portion of the feeding channel5of the supporting body4. In this way, the plunger9can slide with respect to the supporting body4along the longitudinal axis2, but cannot perform any movement transverse to the longitudinal axis2with respect to the supporting body4. Because the needle17is firmly connected to the plunger9of the upper electromagnet8, the plunger9also serves the function of upper guide of the needle17. As such, the needle17is guided by the plunger9of the upper electromagnet8on the top and by the guiding element21at the bottom.

A hydraulic type anti-rebound device31, adapted to attenuate the bouncing of the shutter head20of the needle17against the valve seat18when the needle17moves from the opening position to the closing position of the injection valve7, is connected to the lower face of the discoid element28of the plunger9. The anti-rebound device31includes a plurality of valve elements32, each of which is coupled to a respective peripheral feeding hole30of the plunger9and has a different permeability to the passage of fuel as a function of the passage direction of the fuel through the feeding hole30. In particular, each valve element32includes an elastic blade33, which is partially fixed to a lower surface34of the plunger9on one side of the respective feeding hole30and has a small-sized calibrated hole35aligned with the feeding hole30itself. When the fuel flows downwards (for example, towards the injection nozzle3), the blade33is deformed under the bias of the fuel and leaves the fuel passage through the feeding hole30substantially free. When the fuel flows upwards, the blade33adheres to the lower surface34of the plunger9under the bias of the fuel, thus closing the feeding hole30and allowing the fuel to pass through its small-sized calibrated hole35only. In other words, the anti-rebound device31forms a system for asymmetrically damping the kinetic energy of the plunger9of the upper electromagnet8.

The blade33of the anti-rebound device31is calibrated so as to adhere to the lower surface34of the plunger9only when the needle17is close to the closing position. In this way, the anti-rebound device31slows down the closing movement of the needle17only just before the impact of the needle17against the valve seat18and not along the whole closing stroke. By virtue of the slowing action exerted by the anti-rebound device31, the needle17is slowed down just before impacting against the valve seat18, and thus the elastic bouncing of the needle17against the valve seat18is greatly reduced. In order to achieve this result, the blade33of the anti-rebound device31is dimensioned to be relatively rigid and heavy, and thus have a higher mechanical inertia. Thus, the blade33of the anti-rebound device31is relatively thick. By virtue of the relatively high mechanical inertia of the blade33of the anti-rebound device31, the intervention of the anti-rebound device31is delayed with respect to the beginning of the step of closing the injection valve7, and thus the anti-rebound device31only intervenes when the needle17is about to impact against the valve seat18.

Specifically, the anti-rebound device31hydraulically dissipates more kinetic energy when the needle17moves towards the closing position of the injection valve7than when the needle17moves towards the opening position of the injection valve7. Such an effect is achieved because each valve element34of the anti-rebound device31has a different permeability to the passage of fuel as a function of the passage direction of the fuel through the feeding hole30, so as to have a lower permeability to the passage of fuel when the needle17moves towards the closing position of the injection valve7, and a higher permeability to the passage of fuel when the needle17moves towards the opening position of the injection valve7.

A hydraulic type braking device36adapted to slow down the opening movement of needle17(for example, the movement with which the needle17moves from the closing position to the opening position of the injection valve7) is connected to the upper face of the discoid element28of the plunger9of the upper electromagnet8(for example, on the opposite side with respect to the anti-rebound device31). The braking device36includes respective valve elements37, each of which is coupled to a respective peripheral feeding hole30of the plunger9, and has a different permeability to the passage of fuel as a function of the passage direction of the fuel through the feeding hole30. In particular, each valve element37includes an elastic blade38, which is partially fixed to an upper surface39of the plunger9only on one side of the respective feeding hole30, and has a small-sized calibrated hole40aligned with the feeding hole30. When needle17moves downwards (for example, when needle17moves from the opening position to the closing position), the blade38is deformed under the bias of the fuel and leaves the passage of fuel through the feeding hole30substantially free, while when the needle17moves upwards (for example, when needle17moves from the closing position to the opening position), the blade33adheres to the upper surface39of the plunger9under the bias of the fuel, thus closing the feeding hole30and allowing the fuel to pass only through its small-sized calibrated hole40. Thus, the braking device36forms a device for asymmetrically damping the kinetic energy possessed by the plunger9of the upper electromagnet8.

Specifically, the braking device36hydraulically dissipates more kinetic energy when the needle17moves towards the opening position of the injection valve7than when the needle17moves towards the closing position of the injection valve7, so as to slow down the opening stroke of the needle17when the needle17moves towards the opening position of the injection valve7. Such an effect is achieved because each valve element37of the braking device36has a lower permeability to the passage of the fuel when the needle17moves towards the opening position of the injection valve7, and a higher permeability to the passage of the fuel when the needle17moves towards the closing position of the injection valve7.

When the injection valve7is closed, there is a force of hydraulic nature which pushes on the shutter head24and maintains the shutter head24in the closing position. Therefore, in order to open the injection valve7, the electromagnetic actuator6needs to generate a force on the needle17to overcome the force added to the elastic bias exerted by the closing spring10. However, the force suddenly disappears as soon as the injection valve7opens, and thus the injection valve7tends to open very quickly and violently with an extremely fast upward movement of the needle17. When the injection valve7opens and the force suddenly disappears, the action of the braking device36slows down the opening movement (for example, the upward movement) of the needle17, because it determines a hydraulic dissipation of part of the kinetic energy possessed by the needle17. Such a slowing action determined by the braking device36is particularly valuable, because it prevents the injection valve7from opening very quickly and violently with an extremely fast upward movement of needle17. Essentially, because of the presence of the braking device36, the opening of the injection valve7is slowed down to the benefit of greater controllability (for example, better accuracy and repeatability) of the fuel injection in the ballistic zone of the injection law (for example, the law which relates the actuation time, for example, the control time, to the injected fuel amount). Specifically, the action of the braking device36stabilizes the initial part (the ballistic zone) of the injection law.

The blade38of the braking device36is calibrated so as to have a low mechanical inertia to allow a nearly instantaneous intervention of the braking device36as soon as the injection valve7starts opening. Indeed, the braking device36must intervene more quickly as soon as the injection valve7starts opening. To this end, the blade38of the braking device36is dimensioned to be very flexible and light in weight, and thus to have a low mechanical inertia. Thus, the blade38of the braking device36is relatively thin. Because of the low mechanical inertia of the blade38of the braking device36, the intervention of the braking device36is nearly simultaneous to the beginning of the step of opening of the injection valve7.

It will be appreciated hat the mechanical inertia of the braking device36(for example, of the blade38of the braking device36) is lower than the mechanical inertia of the anti-rebound device31(for example, of the blade33of the anti-rebound device31), because the braking device36must intervene instantaneously, while the anti-rebound device31must intervene with a given delay.

As shown inFIG. 5, the elastic blade33of the anti-rebound device31is fixed to the discoid element28at a peripheral edge thereof and is provided with a series of petals, each of which is coupled to a respective feeding hole30and has a calibrated hole35in the center. Each petal of the elastic blade33is normally arranged in a closing position of the feeding hole30and is movable, during the opening stroke of the piston21, from the closing position to an opening position of the feeding hole30itself. The elastic blade33includes an outer crown, which is fixed to the lower surface34of the discoid element28by a weld (for example, a spot laser weld). The petals extend inwards from the crown, with each petal having a circular sealing element (in the center of which a calibrated hole35is obtained) connected to the crown by a thin shaft (for example, having a length much longer than the width) in order to be elastically deformed.

As shown inFIG. 6, the elastic blade38of the braking device36is fixed to the discoid element28at a peripheral edge thereof and is provided with a series of petals, each of which is coupled to a respective feeding hole30and has a calibrated hole40in the center. Each petal of the elastic blade38is normally arranged in a closing position of the feeding hole30and is movable, during the closing stroke of piston21, from the closing position to an opening position of the feeding hole30itself. The elastic blade38includes an outer crown, which is fixed to the upper surface39of the discoid element28by a weld (for example, a spot laser weld). The petals extend inwards from the crown, with each petal having a circular sealing element (in the center of which a calibrated hole40is obtained) connected to the crown by a thin shaft (for example, having a length much longer than the width) in order to be elastically deformed.

In the embodiment shown throughout the figures, the plunger9of the upper electromagnet8is the only upper guide of the needle17and supports both the anti-rebound device31and the braking device36. The devices31and36may be coupled to the plunger9of the upper electromagnet8, as the plunger9provides a better lateral hydraulic sealing with respect to the inner surface of the feeding channel5(for example, lesser lateral leakages of fuel) and thus provides better operation of the devices31and36themselves. In one embodiment, the plunger9of the lower electromagnet8could form the only upper guide of the needle17and thus, the devices31and36would be coupled to the plunger9of the lower electromagnet8. Moreover, in one embodiment, both plungers9could form the two upper guides of the needle17and thus, the devices31and36could be either coupled to the plunger9of the lower electromagnet8or to the plunger9of the upper electromagnet8.

The needle17has a cylindrical symmetry shaft, to which the substantially spherical shutter head24is connected by an annular weld. Similarly, the shaft is connected to the discoid element28of each plunger9by an annular weld.

In this way, the injector1of the present invention provides many advantages. Firstly, the injector1has extremely high dynamic performance (for example, is capable of opening and closing the injection valve7very quickly) even when the fuel feeding pressure is high (even higher than 50 MPa) because of the use of two twin electromagnets8of relatively small size and having low mechanical and magnetic inertia. Furthermore, the injector1has a linear, uniform (for example, without irregularities) injection law (for example, the law which relates the driving time to the injected fuel amount), even for short driving times (for example, in the ballistic zone) and thus for small injected fuel amounts. Thus, the injector1allows injection of small fuel amounts in an accurate and repeatable manner. Moreover, the injector1is simple and cost-effective to manufacture, because no machining and/or assembly operations substantially different from those of a traditional electromagnetic fuel injector are required.