INJECTOR FOR GASEOUS FUEL

A fuel injector suitable for gaseous fuels comprises an injection nozzle having a tip region that is shaped to define a valve seat that extends about a central outlet opening and an outward opening injection needle slidably received in the injection nozzle. A nozzle cap is received over the tip region of the injection nozzle, wherein the tip region and the nozzle cap are configured to define an thermal barrier layer between them to reduce thermal transfer. The nozzle cap is releasably clipped onto the tip region. It comprises a slot and is configured to be compressible in a radial direction so as to reduce its outer diameter.

TECHNICAL FIELD

The present invention relates generally to a configuration of a fuel injector suitable for injecting a gaseous fuel such as hydrogen into a combustion chamber of an internal combustion engine. The injector may be suitable for injection of other fuels.

BACKGROUND OF INVENTION

Different types of fuel injectors for gaseous fuels are known. One approach is a so-called ‘inward opening’ fuel injector in which an injector valve needle is configured to lift away from a valve seat against the flow direction of fuel so as to open a set of injector ports at the tip of the injector. Another approach is a so-called ‘outward opening’ fuel injector in which an injector valve needle, which is also known as a ‘pintle’, is configured to open in a direction that is the same as the fuel flow direction.

One challenge particularly associated with outward opening fuel injectors is the thermal effects of the combustion processes. The combustion properties of hydrogen tend to cause excessive heating of fuel injectors due to the tendency toward high flame temperatures, low ignition energy and small quenching distances. Uncontrolled heating of the injector tip can cause undesirable auto-ignition or detonation of the fuel-air mixture and can lead to excessive wear of the injector. It is therefore desirable to control and improve the flow of thermal energy into and out of the injector in order to address these issues.

US 2017/328310 A1 discloses a gas injector with a cooling ring and a shielding element, wherein the cooling ring is fixed in place on the gas injector with the aid of the two welded joints. More precisely, the cooling ring has a first contact area with the shielding element and a second contact area with the cylinder head. The cooling ring has an axial slot and is connected to the shielding element with the aid of the first welded joint. The cooling ring is also connected to the valve body by the second welded joint.

SUMMARY OF THE INVENTION

Against this background, the invention provides a fuel injector suitable for gaseous fuels as claimed in claim1. The fuel injector comprises an injection nozzle having a tip region that is shaped to define a valve seat that extends about a central outlet opening; an outward opening injection needle slidably received in the injection nozzle, wherein the injection nozzle further includes a nozzle cap that is received over the tip region of the injection nozzle, wherein the tip region and the injection cap are configured to define a thermal barrier layer between them to reduce thermal transfer.

Beneficially, the thermal barrier layer that is defined between the tip region and the injector cap reduces the thermal energy that is transferred from the nozzle cap to the nozzle and, in doing do, promotes thermal transfer into a bore of a cylinder head into which the injector is received in use. The barrier layer has an insulation effect.

The invention therefore extends to an injector arrangement including a fuel injector as defined above, which is received in a bore defined in a wall at least partly bounding a combustion chamber. The injection nozzle and the nozzle cap are received in the bore such that the nozzle cap is in direct thermal contact with the bore. Direct thermal contact may mean that the bore and the nozzle cap have comparable diameters so that a tight fit is achieved between the nozzle cap and the bore thereby promoting thermal conductivity between those components.

Notably, the nozzle may have a length that is substantially the same diameter as the nozzle cap so that the nozzle and the cap can be fitted into the bore by way of a tight fit. Preferably some radial expansion of the nozzle cap is beneficial to ensure good thermal contact.

As the skilled person would understand, the term ‘outward opening’ is used to refer to an injector in which the valve needle is configured to be movable between a first position in which a head of the needle seals against the valve seat to prevent fuel from being delivered through the central outlet opening, and a second position in which the head of the needle moves away from the valve seat to permit fuel to be delivered through the outlet opening, wherein when moving from the first position to the second position the injection control head moves in a fuel delivery direction.

The injector may be configured such that the nozzle cap is received over a reduced-diameter portion of the tip region of the injection nozzle. As a result of this configuration, a radially outer surface of the nozzle cap may be provided with substantially the same cross section profile as the tip region, which promotes thermal conductivity between the nozzle cap and the bore within which it is installed.

The thermal barrier layer may be defined by a radial clearance between an outer surface of the tip region of the nozzle and an inner surface of the nozzle cap. The clearance may preferably be greater than 50 micrometres in order to ensure a sufficient thermal barrier. Other options may be for the radial clearance to be filled with an insulating material. Optionally, an insulating coating may be applied to the outer surface of the tip region and/or to the inner surface of the nozzle cap, whereupon the radial clearance may be reduced or eliminated.

For ease of assembly, the nozzle cap and the tip region of the nozzle are configured so that the end cap is releasably clipped onto the tip region of the nozzle. This means that the nozzle cap can readily be assembled onto the nozzle without the use of welding or other bonding techniques which would inhibit the ability of the nozzle cap to expand so as to press against the cylinder head bore to make good thermal contact therewith.

The nozzle cap is configured to be compressible in a radial direction so as to reduce its outer diameter. The compressibility is achieved by the provision of a slot that extends from a first end to a second end of the nozzle cap. The slot may extend in a straight path, which is simple to manufacture. Alternatively, the slot may extend along a path that is not straight, for example a meandering or zigzag path.

In some embodiments, the nozzle cap may define an open area that leaves the outlet opening in the nozzle substantially unrestricted when projecting the profile of the nozzle below it. Expressed in another way, the nozzle cap may be configured so as not to overlap the outlet opening of the nozzle. However, in other embodiments, the nozzle cap may further define a hood portion that extends beyond the tip region of the nozzle and provides a fuel delivery guide. The hood portion may be shaped to define a bulbous cavity or chamber which at least partially encloses the outlet opening of the nozzle. In this way, the hood portion defines a sac volume of the nozzle which promotes mixing of air and fuel. Moreover, the hood portion may be provided with one or more exit openings to guide the flow of fuel from the nozzle. The guiding function may be different from what could be achieved simply by the nozzle itself without the nozzle cap. The hood portion may be provided with a plurality of outlet openings, the positions of which are selected to achieve predetermined spray shape objectives.

Further optional and advantageous features are referenced in the detailed description and the appended claims.

DETAILED DESCRIPTION OF THE EMBODIMENTS

With reference toFIG.1, a direct injection fuel injector2comprises an injector body4that houses an actuator arrangement6that is configured to act on an injection valve needle or ‘pintle’8which is contained in an elongated valve housing or ‘nozzle’10.

The fuel injector2is an outward opening type such that the valve needle8is arranged to move downwards, in the orientation of the figures, in order to initiate a delivery of fuel, and to move upwards to terminate fuel delivery. The nozzle10houses a supply of fuel which extends down into an annular chamber12that surrounds the valve needle8. When the valve needle8opens, fuel is thus delivered from the nozzle10into the combustion chamber. Therefore, the valve needle8moves in the same direction as the flow of fuel through the nozzle10during an injection event.

The valve needle8is biased into a closed position by way of a closure spring11. Actuation of the valve needle8is achieved by way of an electromagnetic actuator13. Although in this example an electromagnetic actuator is used, other forms of actuation are acceptable, such as piezoelectric actuators.

As will be appreciated inFIG.1, a significant portion of the injector2, and particularly the nozzle10thereof, is fitted within a cylinder head14of an engine. The nozzle10is held in a passage or bore16that is shaped to receive the nozzle10in a relatively loose fit. The nozzle10and the passage16are configured to that a tip region18of the nozzle10is positioned at a lower opening20of the passage which opens into a combustion chamber22of the engine.

The general form of the fuel injector2is not central to the injection so further detailed discussion will be avoided so as not to obscure the invention. However, discussion will focus on the details of the nozzle10which is shown in an enlarged form inFIGS.2and3.

The nozzle10carries a seal24. The seal24is in the form of a ring-shaped element which is received in a correspondingly-shaped annular slot28formed in the outer surface of the nozzle9. The seal24forms a tight seal with the bore16of the cylinder head14as a measure to guard against combustion gases blowing by the nozzle10. The seal24has a rectangular cross section as shown here, but other profiles would be acceptable, the key functionality being to create a gas-tight seal between the nozzle10and the bore16.

The tip region18is shaped to define a seal with the valve needle8, as is generally known. As such, the valve needle8includes a head30which defines a frustoconical sealing surface32on its upper side, as shown in the drawings. The sealing surface32of the head30is engageable with a correspondingly-shaped valve seat34provided on a lower surface of the tip region18. That valve seat34defines a leading edge35of the nozzle.

As is typical with outward opening fuel injectors, that valve seat34is circular in form and so defines a circular opening at the end of the nozzle. The head30of the valve needle8fills the opening and so defines a narrow annular outlet opening36, when the valve needle8is in an open position, which shapes the flow of fuel into a conical jet. The respective geometries of the valve seat34and the head30can influence the shape and direction of the jet of fuel during delivery, but the precise details of this are not central to the invention and so will not be the focus of further discussion.

The valve needle8is movable between a first position in which the head30seals against the valve seat34to prevent fuel being delivered through the outlet opening36and a second position in which the head30moves outwardly from the nozzle10thereby spacing the head30from the valve seat34which therefore allows fuel to be delivered in a fuel delivery direction. It will therefore be appreciated that the fuel flow direction is the same direction as the opening movement of the valve needle

In use, it is important to control the temperature of the tip region18of the nozzle10. If the fuel within the nozzle and/or the sealing ring24is heated excessively, then this risks auto-ignition in the combustion chamber upon delivery through the outlet opening and can also degrade the mechanical properties of these components.

To guard against this, the nozzle10includes an end cap or “nozzle cap”40. The nozzle cap40is fitted onto, or received on to, the tip region18of the nozzle10and functions to control the thermal energy that is transferred to the nozzle10in use during combustion. Advantageously, the nozzle cap40provides a thermally conductive pathway which encourages thermal energy to transfer from the nozzle cap40and into the surrounding material of the cylinder head14in preference to being transferred into the tip region18of the nozzle10. As a result, the nozzle cap40can guard against excessive hot spots within the nozzle10.

The nozzle cap40is collar-shaped, and as such has a short cylindrical profile defining an upper edge41, a lower edge42and an open central area44. Due to its shape, the end cap40defines a radial outer surface45and a radial inner surface46which together define a thickness dimension T. Note that in this embodiment, the thickness dimension of the nozzle cap40is substantially uniform along its length.

As will be apparent from the figures, the outer diameter of the nozzle cap40and the outer diameter of the adjacent part of the nozzle10are substantially equal. Therefore, the outer surface45of the nozzle cap40is substantially flush with the outer surface of the nozzle10. To allow this, the tip end18of the nozzle10defines a reduced diameter region48. The reduced diameter region48is defined between the leading edge35of the tip region18and a step or shoulder49of the nozzle10. The shoulder49is defined at a lower position on the nozzle10than the position of the seal16.

From the figures, it will be appreciated that the radius of the tip region18is less than the radius of the nozzle10by an amount that is comparable to the thickness dimension T of the nozzle cap40. This configuration is useful to allow the nozzle10and the nozzle cap40to be fitted within the uniform bore16in the cylinder head14.

The nozzle cap40is releasably retained on the tip region18nozzle10. Notably, it is not fixed by welding or other bonding process. The geometry of the nozzle cap40is such that it can be slipped over the tip region18loosely so as to define a radial clearance or gap G between the inner surface46of the nozzle cap40and the outer surface47of the end tip region18. Retention is achieved by a retention feature50defined in part by the tip region18and in part by the nozzle cap40. In the illustrated embodiment, the retention feature50takes the form of an annular rib that extends about the outer surface of the tip region18that engages with an annular groove54which extends about the inner surface of the nozzle cap40.

As is shown clearly inFIG.2, the annular rib50sits inside the annular groove54and so functions to retain the nozzle cap40on the tip region18.

Due to the way in which the nozzle cap40is clipped over the tip region18thereby defining a radial clearance G, a thermal barrier58is created between those components thereby providing some thermal insulation. The effect of this insulation layer is to establish thermal break or barrier between the nozzle cap40and the tip region18which reduces the transmission of thermal energy. It is currently envisaged that a radial gap having a dimension of between 100 and 200 micrometres would be sufficient, although this is exemplary and some variants around these values is acceptable. Generally, the gap dimension is driven my manufacturing tolerances, as it is preferred for the gap to be as small as possible. In known configuration of injectors, typically injector caps are welded onto the tip of the inject which, contrary to the invention, promotes thermal transfer and increases the risk of hotspots.

In the illustrated embodiment, the insulation layer58is formed by the air gap G as this provides sufficient insulative properties to reduce the thermal energy transfer between the nozzle cap40and the tip region18of the nozzle10.

Notably, the air gap G is substantially continuous in this embodiment, and although this is preferred, it is not crucial. Therefore, some areas of contact may be acceptable between these components yet still retaining the benefits of the invention.

The material of the nozzle cap40is preferably a high temperature resistant metal such as a suitable stainless steel or similar alloy. An example of a suitable material is Inconel®. The material should be capable of withstanding the combustion temperatures of hydrogen (approx. 530 degrees centigrade) without glowing, as this would provide an undesirable source of ignition.

In order for the nozzle cap40in order to be slipped over the tip region18and clipped in place, in the illustrated embodiment the nozzle cap40is provided with a degree of flexibility in the radial direction so that its outer diameter is able to increase and decrease slightly. To achieve this effect, the nozzle cap40comprises an axial slot60.

As can be seen inFIG.2, the axial slot60in this embodiment is linear and extends between either end of the nozzle cap40along a straight path. The axial slot60therefore makes the nozzle cap40resemble a circlip that has some radial resilience.

Notably, the width of the axial slot60, that is to say the dimension of the slot in the circumferential direction of the nozzle cap40should be kept small so as to limit the travel of combustion gases up the slot60from the leading edge of the nozzle cap40. Currently preferred is for the slot to be at least 50 micrometres in width, and preferable between 50 and 100 micrometres. This is to ensure that the width of the gap is sufficient to provide the nozzle cap40with enough radial flexibility, but also small enough to limit the travel of combustion gases up the slot60.

Note that the above dimension of the slot60is given as the dimension when the nozzle10together with the nozzle cap40are installed in the bore16of the cylinder head14. Before installation, it is envisaged that the geometry of the nozzle cap40such that its outer diameter is slightly larger than the internal diameter of the bore14. During installation, therefore, the nozzle cap40is arranged to be pushed into the bore14by way of a press fit. This urges the nozzle cap40to be compressed inwards in a radial direction so that the outer surface45of the nozzle cap40is urged into engagement with the surface of the bore16. This ensures a tight fit that promotes efficient transfer of thermal energy.

As will be noted fromFIGS.2and3, in the illustrated embodiment the central area44of the nozzle cap40exposes the entirety of the head30of the valve needle8and so provides a substantially unobstructed flow of fuel. In fact, the nozzle cap can be shaped to control the shape of the fuel flow that is injected into the combustion chamber. Here, the lower edge42of the nozzle cap40has a small radial lip62that extends a small distance in the radially inwards direction. The configuration of the lip62means that the nozzle cap40at least partly overlaps or covers the end of the nozzle10which limits direct heat exposure to combustion gases. The radial lip62defines a circumferential inner surface64that is inclined with respect to the longitudinal axis L of the nozzle10(by about 30 degrees in this embodiment) which contributes to shaping the injected fuel into a conical jet.

Furthermore, it will be noted that the radial lip62extends slightly over the leading edge35of the nozzle10. Beneficial aspects of this is that 1) it positions the inner surface64of the radial lip62close to the edge of the outlet opening36to improve the spray shaping; 2) it provides at least a partial cover to the leading edge35of the nozzle10, thereby limiting heat exposure of the nozzle at this point; 3) it acts as an end stop to define an end point of travel for the nozzle cap40onto the reduced diameter tip region18.

The illustrated embodiment shown inFIGS.2and3shows one option for providing spray shaping of the fuel delivery from the nozzle10. However, the nozzle cap40can be modified to provide further spray shaping options.

An example of this is shown in the embodiment ofFIGS.4and5. Note that the embodiment shares many features of configuration with the previous embodiment so only the differences will be described. The same reference numerals will be used to define common parts or features.

The principal difference in the embodiment ofFIGS.4and5is that the nozzle cap40includes a cowl or hood portion70. As can be seen the hood portion70extends beyond the tip region18to partially enclose a bulbous chamber72beneath the head30of the valve needle8. The hood portion70is penetrated by a hood opening74. Although in theory the hood opening74may be defined by the hood portion70with any size and shape, in the illustrated embodiment it will be noted that the hood opening74is offset to one side. That is to say the hood opening74is offset from the longitudinal axis L of the nozzle10.

The offset position of the hood opening74has the effect of controlling the spray direction from the nozzle10since the bulbous interior form of the hood opening74captures and redirected the flow of fuel exiting the output opening36of the tip region18, as seen by the arrows inFIG.4. The chamber72therefore constitutes a type of sac volume for the nozzle, which promotes mixing of the air and fuel during an injection event by controlling the spray direction to optimum parts of the combustion chamber.

Although the illustrated embodiment includes a single large hood opening74, it should be appreciated that more than one opening may be provided. Those plurality of openings may be the same size, or different sizes and may be spaced regularly or irregularly about the hood portion70.

Configuring the hood portion70with one or more angularly offset openings as described may have the effect of applying an angularly directed force to the nozzle cap40. As can be seen particularly well, the opening74in the illustrated embodiment has a geometry that resembles a stadium shape or obround, which contributes to the spray shaping function of the nozzle cap40. Such shape is exemplary only and other shapes would be acceptable.

To guard against any unwanted angular movement of the nozzle cap40, it may be configured with rotational locking means. In the illustrated embodiment, the rotational locking means is embodied as a locking protrusion or tab76. The locking tab76extends axially from the upper edge41of the nozzle cap10and is accommodated in a complementary notch78defined in the tip region18. The mating between the tab76and the notch78therefore acts as a location feature during installation of the nozzle cap10on the tip region18and, once installed, prevent the nozzle cap10from rotating out of its selected position.

A single locking feature is likely to be sufficient for the purpose of rotationally locking the nozzle cap10to the tip region18, although it should be appreciated that the configuration of the locking feature may take different forms as would be apparent to the skilled person.

Some variants on the specific embodiments have already been described. However, the skilled person would appreciate that further modifications could be made to the specific embodiments that do not depart from the scope of the invention as defined by the claims.

One exemplary variant is shown inFIGS.6and7, each of which shows a nozzle cap10with a different form of axially extending slot60. In the previous illustrated embodiment, the axially extending slot60extended along a straight path between the upper edge41of the nozzle cap10to the lower edge42.

InFIG.6, however, the axially extending slot60extends along an undulating path80. More specifically the path is zigzag shaped, as shown here, as defined by a plurality of equal length sections that are disposed at equal angular intervals to one another.

InFIG.7, the axial slot60again takes an undulating path80but instead of being zigzag, as inFIG.6, the path80is shaped to define a plurality linked horizontal and vertical path segments.

Both of the undulating slot shapes shown inFIGS.6and7retain the ability of the nozzle cap10to contract and dilate. However, it is envisaged that the elongated paths80provide a more circuitous route for combustion gases and may be more effective at insulating the nozzle10from high combustion temperatures.