INJECTOR FOR GASEOUS FUEL

A fuel injector for injecting gaseous fuels into a combustion chamber, comprising an injection nozzle including a nozzle cap, and an outward opening injection needle. The injection nozzle has a longitudinal injector axis and a tip region that is shaped to define a valve seat that extends about a central outlet opening for gaseous fuel. The outward opening injection needle is slidably received in the injection nozzle and is engageable with the valve seat to control a flow of gaseous fuel into a sac volume of the fuel injector. The nozzle cap is received over the tip region to define the sac volume, and is provided with a plurality of openings to enable the flow of gaseous fuel to pass from the sac volume into the combustion chamber when the injection needle is moved away from the valve seat. The plurality of openings comprise at least a first opening and a second set of openings.

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

Fuel injectors are used in combustion engines to inject fuel into a runner of an air intake manifold ahead of a cylinder intake valve or directly into the combustion chamber of an engine cylinder. Different types of fuel injectors for gaseous fuels are known. One approach is a so-called ‘outward opening’ fuel injector in which an injector valve needle (or pintle) is configured to lift away from a valve seat towards the combustion chamber in order to allow fuel to flow into the combustion chamber.

One challenge associated with outward opening injectors is that of achieving efficient air-fuel mixing required for homogeneous combustion. It is particularly challenging to achieve such an efficient combustion at increasing engine loads and speeds. Typically, outward opening injectors create a spray or jet plume of fuel with a hollow cone shape which is susceptible to either inward or outward collapse. As a result, it can be difficult to target the delivery of fuel to optimal regions within the combustion chamber especially at high cylinder pressures which result from high engine loads and speeds. This is especially true for gas fuel injectors due to the low fuel density. Therefore, internal combustion systems that employ such fuel injectors experience poor engine efficiency at high cylinder pressures, the efficiency varying, and typically being improved with lowering engine loads and speeds. However, without optimal jet targeting, homogeneous combustion cannot be achieved, and engine efficiency suffers as a result.

It is against this background that the invention has been devised.

SUMMARY OF THE INVENTION

Against this background, a first aspect of the invention provides a fuel injector suitable for injecting gaseous fuels into a combustion chamber, the fuel injector comprising an injection nozzle having a longitudinal injector axis and a tip region that is shaped to define a valve seat that extends about a central outlet opening for gaseous fuel; and an outward opening injection needle slidably received in the injection nozzle and engageable with the valve seat to control a flow of gaseous fuel into a sac volume of the fuel injector. The injection nozzle further includes a nozzle cap that is received over the tip region to define the sac volume, the nozzle cap being provided with a plurality of openings to enable the flow of gaseous fuel to pass from the sac volume into the combustion chamber when the injection needle is moved away from the valve seat. The plurality of openings is comprised of at least(i) at least one first opening configured to direct a first flow portion of the flow of gaseous fuel into a first zone of the combustion chamber; and(ii) a second set of openings, configured to direct a second flow portion of the flow of gaseous fuel into a second zone of the combustion chamber.

The second zone of the combustion chamber may be substantially different from the first zone of the combustion chamber and comprise regions within the combustion chamber which cannot be reached by the first flow portion of the flow of gaseous fuel. For example, the first zone may be defined as a lower portion of the combustion chamber and the second zone may be defined as an upper portion of the combustion chamber. In this way, the first and second flow portions of gaseous fuel achieve greater coverage within the combustion chamber thereby promoting greater air-fuel mixing and more efficient combustion.

The first opening may form one of a set of first openings.

The or each opening of the first set may have a diameter that is different to the diameter of the or each opening of the second set. In some embodiments, the first opening may form one opening of a first set of openings. If there is only one opening in the first set, the opening may have a first flow axis which aligns with the longitudinal injector axis.

Additionally or alternatively, each opening of the second set may have a second central flow axis which defines a first angle to the longitudinal injector axis which is the same as for the other openings of the set. Further, each opening of the second set may have an inlet which opens into the sac volume which is located on a first plane perpendicular to the longitudinal injector axis. The inlets of each opening of the second set may be equiangularly spaced around the longitudinal injector axis in the first plane.

The plurality of openings may further include a third set of openings configured to direct a third flow portion of the flow of gaseous fuel into a third zone of the combustion chamber. The third zone may be substantially different to the first and second zones and comprises regions of the combustion chamber which cannot be reached by the first and second flow portions of the flow.

Each opening of the third set may have a third central flow axis which defines a second angle to the longitudinal injector axis which is different to the first angle defined by the second central flow axis to the longitudinal injector axis. Additionally or alternatively, each opening of the third set may have an inlet which opens into the sac volume which is located on a second plane perpendicular to the longitudinal axis. The second plane adopts a lower position along the longitudinal injector axis than the first plane—i.e. the second plane is lower than the first plane according to the orientation of the Figures which follow.

In some embodiments, each opening of the second set may have a diameter that is different to the diameter of each opening of the third set. Alternatively, each opening of the second set may have a diameter that is the same as the diameter of each opening of the third set.

In some embodiments, the second set of openings may include between two and five openings.

In some embodiments, the inlets of the second set of openings are angularly spaced apart from the inlets of the third set of openings around the nozzle cap in their respective planes perpendicular to the longitudinal injector axis/nozzle cap axis; and wherein the outlets of the second set of openings are angularly spaced apart from the outlets of the third set of openings around the nozzle cap in their respective planes perpendicular to the longitudinal injector axis/nozzle cap axis, such that the second central flow axis of the second set of openings is not axially aligned with the third central flow axis of the third set of openings.

In another aspect of the invention, there is provided a fuel injection system for gaseous fuel comprising a fuel injector and a combustion chamber into which the gaseous fuel is injected.

It will be appreciated that preferred or optional features of the first aspect of the invention may be incorporated in other aspects of the invention also, alone or in appropriate combination.

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

DETAILED DESCRIPTION OF THE EMBODIMENTS

To provide context for the invention,FIG.1schematically shows a cross-section view of part of an engine2. The engine2comprises a cylinder or combustion chamber4with a cylinder axis6which, in this context, defines a vertical direction although this vertical direction may not coincide with the direction of gravity depending on the design of the engine2. An air intake duct8and an air outlet duct10are connected to a head12of the cylinder4with an intake valve14and an outlet valve16disposed to alternatingly open and close the respective ducts8,10.

A fuel injector18(shown only in simplified form) is installed on the head12of the cylinder4in a central mounting configuration, meaning that the injector18is mounted in a centre region of the engine cylinder head12among the intake and outlet valves14,16. The injector18has a longitudinal injector axis20which is aligned with the cylinder axis6. Alternatively, as shown inFIG.2, the fuel injector18may be mounted to the side of the cylinder4where the intake valve14is installed. In this case, the injector axis20is titled at an angle (e.g., of about) 70° with respect to the cylinder axis6.

FIG.3shows an embodiment of the fuel injector18which is an outward opening type. As shown, the injector18comprises an injector body22and an elongated valve housing24(also known as a ‘nozzle’) having the longitudinal injector axis20. The injector body22houses an actuator arrangement26that is configured to act on an injection valve needle28(or ‘pintle’) contained within the nozzle24. The injection nozzle24has a tip region that is shaped to define a valve seat32that extends about a central outlet opening for gaseous fuel. The valve needle28is biased against the valve seat32by means of a closure spring34.

The nozzle24houses a supply of fuel in an annular chamber30that surrounds the needle28. The valve needle28is configured to be slidingly actuated along the injector axis20in order to open and close the valve needle28against the valve seat32as required. When the valve needle28is in a closed position, an end of the needle28engages with the valve seat32defined at a tip region of the nozzle24so as to block the flow of fuel out of the nozzle24. When the valve needle28is actuated to move downwards (in the orientation of the Figures), the end of the valve needle28moves away from the valve seat32thereby allowing the fuel to flow out of the tip region of the nozzle24and into the combustion chamber4. In other words, the valve needle28is moved into an open position under the force of the actuator26.

In the present embodiment, actuation of the valve needle28is achieved by way of an electromagnetic actuator. Other forms of actuation are acceptable, such as piezoelectric actuators.

The nozzle24includes a nozzle cap36, which defines a sac volume38and is positioned over the tip region of the nozzle24. The nozzle cap36is hemispherical in shape, however, other embodiments in which the nozzle cap36is cylindrical in shape with a hemispherical end or taking other axisymmetric form are possible. As will be described in more detail below, the nozzle cap36comprises a plurality of holes or openings40, each opening40defining a flow passage through the wall of the nozzle cap36so as to enable the fuel to flow from the tip region of the nozzle24into the combustion chamber4via the sac volume38when the valve needle28is in the open position. That is to say, the injection needle28is configured to be slidably received in the injection nozzle24and engageable with the valve seat32so as to control the flow of fuel into the sac volume38of the fuel injector18. Accordingly, when the needle28is moved away from the valve seat32, gaseous fuel passes from the sac volume38and through the plurality of openings40into the combustion chamber4. The nozzle cap36has a central longitudinal axis42which aligns with the longitudinal axis of the injector20.

Each opening40extends from an inlet44at an inner surface46of the nozzle cap36to an outlet48at an outer surface50of the nozzle cap36along an opening axis or central flow axis52which is a symmetrical axis of the respective opening. Each inlet44opens into the sac volume38while each outlet48opens into the combustion chamber4. Accordingly, the opening axis52corresponds to a general direction of fuel flow from the inside of the nozzle cap36(i.e. the sac volume38) to the outside of the nozzle cap36(i.e. the combustion chamber4), although the precise motion of the fuel is more complicated.

The inlet44of each opening40is similarly shaped to the outlet48of the respective opening40. In this way, the walls of the opening40through the nozzle cap36are parallel to each other, thereby providing a cylindrical passage through which fuel can flow. In other words, the diameter of each opening40is constant along its length. Flow of fuel from cylindrical passages is advantageously robust and less susceptible to collapse. In instances where the opening axis52of an opening40is perpendicular to the inner and outer surfaces46,50of the nozzle cap36, the inlet and outlet44,48of the opening40have a circular cross section in a plane tangential to the outer surface50of the nozzle cap36. Conversely, embodiments are contemplated where the opening axis52is at a non-perpendicular angle to the inner and outer surfaces of46,50the nozzle cap36. In such instances, the inlet and outlet44,48are of elliptical cross section in a plane tangential to the outer surface50of the nozzle cap36.

Generally, the plurality of openings40is configured to provide a controlled spray plume and may be optimised according to the requirements of the engine arrangement2. Specifically, the openings40are configured to direct fuel jets, or portions of the fuel flow, to various zones within the combustion chamber4in order to achieve maximum flow separation and promote efficient air-fuel mixing. As will be explained in more detail below, the openings40are grouped in sets of openings according to the zones of the combustion chamber to which they direct fuel jets.

The optimal spray plume, and thus the optimal configuration of openings40, for a particular engine2depends on the injector18mounting position as well as the type of air motion within the cylinder4. There are two general air motions which may be achieved depending on the arrangement of the air inlet and outlet ducts8,10. Swirl motion is defined by a rotational motion of the incoming air about the cylinder axis6, while tumble motion is defined by a rotational motion of the incoming air about an axis which is normal to the cylinder axis6. Therefore, several arrangements for the plurality of openings40in the nozzle cap36are contemplated and described with reference to the embodiments shown inFIGS.4to9.

In each arrangement, the plurality of openings40is comprised of at least a first set of openings and a second set of openings. Generally, one or more sets of openings are configured to direct fuel so that it interacts with the tumbling or swirling air motion within the cylinder, thus actively mixing with the air as it circulates the cylinder. Inevitably, however, the circulating air and fuel does not effectively reach all regions within the cylinder when only one set of openings is used. Therefore, one or more further sets of openings are configured to direct fuel so that it flows to regions of the cylinder that is largely inaccessible by the fuel directed through the first set of openings, thereby improving the air-fuel mixing achieved. Accordingly, the first set of openings is configured to direct fuel into a first zone of the cylinder, which may be a defined in a lower portion of the cylinder, while the second set of openings is configured to direct fuel into a second zone of the cylinder, which may be defined in an upper portion of the cylinder. The two zones preferably comprise different portions of the cylinder, but it is contemplated that they may overlap.

In the arrangements ofFIGS.4to6, the first set of openings consists of only one opening154′ which is arranged in the nozzle cap36to have an opening axis52which aligns with the longitudinal injector axis20. That is to say, the one opening is arranged to extend from a circular inlet44at the centre of the inner surface46of the nozzle cap36to a circular outlet48at the centre of the outer surface50of the nozzle cap36in a cylindrical manner. The diameter of the inlet and outlet44,48are substantially the same and the internal diameter of the one opening is constant along its length.

Additionally, each aperture or opening of the second set of openings is offset from the centre of the nozzle cap36and arranged to extend from the inner surface46of the nozzle cap36to the outer surface50of the nozzle cap36in a cylindrical manner. The internal diameter of each second set opening is different to, and preferably smaller than, the internal diameter of the first set opening so that the fuel flow rate through the first set opening is different to, and preferably greater than, the fuel flow rate through each second set opening.

These arrangements further comprise a third set of openings, similar to the second set of openings in that each aperture or opening of the third set is offset from the centre of the nozzle cap36and arranged to extend from the inner surface46of the nozzle cap36to the outer surface50of the nozzle cap36in a cylindrical manner. In the embodiments shown, the internal diameter of each third set opening is the same as the internal diameter of the second set openings, but it is contemplated that they could be different.

More specifically,FIGS.4ato4fshow embodiments of a centre-mounted injector18with a nozzle cap136optimised for a tumble motion cylinder4.FIGS.4aand4bdepict the nozzle cap as seen from the side, whileFIGS.4cand4dprovide cross-sectional plan views of one embodiment andFIGS.4eand4fprovide cross-sectional plan views of another embodiment. In both embodiments, the nozzle cap136comprises a first opening154′ and second and third sets of openings,156a,156brespectively, as described above.

The second set of openings156aincludes four apertures or openings156′, each of which has a respective inlet144aand a respective outlet148a. The inlets144aof the openings of the second set156aare equiangularly spaced on the inner surface of the nozzle cap136around an axis142of the nozzle cap (the longitudinal nozzle cap axis) which is aligned with the injector axis20. The inlets144aare defined in a plane perpendicular to the longitudinal injector axis20/nozzle cap axis142. The third set of openings156balso includes four apertures or openings156″, each of which has a respective inlet144band a respective outlet148b. The inlets144bof the openings of the third set156bare equiangularly spaced on the inner surface146of the nozzle cap136around the longitudinal nozzle cap axis142and are defined in a plane perpendicular to the longitudinal injector axis20/nozzle cap axis142.

Likewise, the outlets148aof the openings of the second set156aare equiangularly spaced on the outer surface of the nozzle cap136(i.e. around the longitudinal nozzle cap axis142) and are defined in a plane perpendicular to the longitudinal injector axis20, and the outlets148bof the openings of the third set156bare equiangularly spaced on the outer surface of the nozzle cap136around the longitudinal nozzle cap axis142and are defined in a plane perpendicular to the longitudinal injector axis20.

The opening axis152of each opening156′ of the second set156adefines an angle to the longitudinal nozzle cap axis142which is the same as for the other openings in the second set156a. Likewise, the opening axis152of each opening156″ of the third set156bdefines an angle to the longitudinal nozzle cap axis142which is the same as for the other openings in the third set156b. Further, the centre of the inlet144aof each of the openings156′ of the second set156alies on the same plane perpendicular to the longitudinal injector axis20as for the inlets144aof the other openings in the same set, and the centre of the inlet144bof each of the openings156″ of the third set156blies on the same plane (i.e. the inlet plane) perpendicular to the longitudinal injector axis20as for the inlets144bof the other openings in the third set156b. Likewise, the centre of the outlet148aof each opening of the second set156alies on the same plane (i.e. the outlet plane) perpendicular to the longitudinal injector axis20as for the outlets148aof the other openings156′ of the second set, and the centre of the outlet148bof each opening of the third set156blies on the same plane (i.e. the outlet plane) perpendicular to the longitudinal injector axis20as for the outlets148bof the other openings156″ of the third set156b. That is to say, the fluid path through each opening156′156″ in a set156a,156bis of the same length.

The second and third sets of openings156a,156bare arranged relative to each other about the longitudinal injector axis20such that the inlet and outlet planes of the second set156aare further away from the cylinder4(i.e. axially higher in the orientation of the Figures) than the inlet and outlet planes of the third set of openings156b. In other words, the inlet and outlet planes of the third set156bof openings156″ adopt a lower position along the longitudinal injector axis20than the inlet and outlet planes of the second set156aof openings156′. Further, as shown inFIG.4b, the angle α1of the opening axis152relative to the longitudinal injector axis20defined for the second set156ais different to, and preferably larger than, the angle α2of the opening axis152relative to the longitudinal nozzle cap axis142defined for the third set156b.

Furthermore, the respective inlets144a,144bof the second and third set openings156′,156″ are angularly spaced apart around the inner surface of the nozzle cap136(in their planes perpendicular to the nozzle cap axis142) so that they are not axially aligned with one another in their respective planes. Similarly, the respective outlets148aof the second set of openings156aare equiangularly spaced around the outer surface of the nozzle cap136are alternately and equiangularly spaced around the inner surface of the nozzle cap136(in their plane perpendicular to the nozzle cap axis142) so that they are not axially aligned with the respective outlets148bof the third set of openings156b. This provides an alternating configuration of the inlets144a,144b, equiangularly spaced around the longitudinal nozzle cap axis142whereby the inlets144aof the second set of openings156aare interspersed between the inlets144bof the third set of openings156band a similar configuration of the outlets148a,148bwhich are arranged in the same alternating manner. This is especially clear when looking at the plan views of the openings. The opening axes152of the second and third sets of openings156a,156bare therefore not axially aligned with one another.

FIGS.4(e) and4(f)show a similar embodiment to that described with reference toFIGS.4(c) and4(d), except that there are only three openings156′,156″ in each of the second and third sets. As before, the inlets144aand outlets148aof the openings of the second set156aand the inlets144band outlets148bof the openings of the third set156bare arranged such that the opening axes of the second and third sets of openings are not axially aligned. The inlets144aof the second set of openings156aand the inlets144bof the third set of openings156bare alternately and equiangularly spaced around the inner surface of the nozzle cap136(in their respective planes perpendicular to the nozzle cap axis142) so that the inlets144aof the second set of openings156aare interposed between the inlets144bof the third set of openings156b(this is especially clear when looking at the plan views of the openings).

Likewise, the outlets148aof the second set of openings156aand the outlets148bof the third set of openings156bare alternately and equiangularly spaced relative to one another around the outer surface of the nozzle cap136so that the outlets148aof the second set of openings156aare interposed between the outlets148bof the third set of openings156b(this is especially clear when looking at the plan views of the openings).

In the embodiment ofFIGS.4(c) and4(d), the second and third sets of openings156a,156beach comprise four openings,156′,156″, respectively, while in the embodiment ofFIGS.4eand4f, the second and third set of openings156a,156beach comprise only three openings156′,156″, respectively. It is further contemplated that each set of openings156a,156bmay comprise up to eight openings, for example.

In each case, the third set156bof openings is arranged relative to the second set156aof openings such that the nozzle cap136has at least one plane of symmetry in the vertical direction. It should be noted, however, that any arrangement of openings which gives the nozzle cap136at least one plane of symmetry in the vertical direction is particularly suitable for a centre mounted injector18of a tumble motion cylinder4. Such an arrangement of second and third sets of openings156′,156″, in combination with the single opening154′ as described above, will produce an unbiased spray pattern which evenly distributes the fuel within the compression chamber4. Specifically, the first opening154′, being aligned with the longitudinal injector axis20and thus being parallel with the cylinder axis6, targets fuel into a lower portion of the cylinder so that it interacts with the tumble motion of air in the cylinder4. The second and third set openings156′,156″ being angled away from the longitudinal injector axis, target fuel to an upper portion of the cylinder where fuel targeted through the first set opening cannot effectively reach. By virtue of this opening arrangement, the injection of fuel is less likely to disrupt the tumble motion initiated by the air intake8. Thus, the tumble motion of air can be exploited to promote better air-fuel mixing.

FIGS.5ato5fshow embodiments of a centre mounted injector18with a nozzle cap236optimised for a swirl motion cylinder4. The injection nozzle cap236includes a first opening254′ and second and third sets of openings,256a,256brespectively, having second and third set openings,256′,256″ respectively. More specifically,FIGS.5aand5bdepict the nozzle cap236as seen from the side, whileFIGS.5cand5dprovide cross-sectional plan views of one embodiment andFIGS.5eand5fprovide cross-sectional plan views of another embodiment. These embodiments differ from those described above with reference toFIGS.4ato4fin that the inlet244a,244bof each second and third set of openings256′,256″ is angularly offset from each respective outlet248a,248bsuch that each opening axis252defines an angle θ1between the inner and outer surfaces246,250of the nozzle cap236. Accordingly, the inlets244a,244band outlets248a,248bof each of the second and third set openings256′,256″ are elliptical in shape. The angle θ1defined for each second and third sets of openings256′,256″ is the same as for all other second and third sets of openings256′,256″ in the arrangement.

Although the nozzle cap according to these embodiments does not have a plane of symmetry in the vertical direction, the nozzle cap does have rotational symmetry. The embodiment ofFIGS.5cand5dcomprise four openings256′,256″, respectively, in each of the second and third sets and so the openings are arranged to provide rotational symmetry to the order of four. Similarly, in the embodiment ofFIGS.5eand5feach of the second and third sets comprise three openings256′,256″, and so the openings are arranged to provide rotational symmetry to the order of three. Other embodiments are contemplated in which each of the second and third sets comprise up to eight openings being arranged such the rotational symmetry order increases correspondingly. By virtue of this rotational symmetry, the arrangement of openings will produce an unbiased spray pattern which evenly distributes the fuel within the compression chamber4.

The second and third set openings256′,256″, being angled between the inner and outer surfaces of the nozzle cap, direct fuel such that it enters the cylinder with rotational momentum in the direction of the swirl motion induced by the air intake. Accordingly, the fuel flow from the second and third set openings256′,256″ is encouraged to interact with the swirl motion in an upper portion of the cylinder. Meanwhile, the first set opening254′, being aligned with the longitudinal injector axis20and thus being parallel with the cylinder axis6, targets fuel into a lower portion of the cylinder so that it induces a tumble motion of air in the cylinder which compliments the swirl motion induced by the air intake. This leads to greater circulation within the cylinder and thus greater air-fuel mixing.

Referring toFIG.5b, the angle α1of the opening axis252relative to the longitudinal nozzle cap axis242defined for the second set of openings256ais different to, and preferably larger than, the angle α2of the opening axis252relative to the longitudinal nozzle cap axis242defined for the third set of openings256b. For example, the angle α1may range between 60 to 75 degrees relative to the longitudinal nozzle cap axis242, and preferably between 65 to 70 degrees relative to the longitudinal nozzle cap axis242. Additionally, the angle α2may range between 40 to 60 degrees relative to the longitudinal nozzle cap axis242, more preferably between 45 to 50 degrees relative to the longitudinal nozzle cap axis242.

The configurations of the openings inFIGS.5(c) and5(d)andFIGS.5(e) and (f)are similar to those shown inFIGS.4(c) and4(d)andFIGS.4(e) and4(f), respectively, in that the inlets244a,248ato the openings are equiangularly spaced around the inner surface of the nozzle cap236, relative to one another and in their respective axial planes, so that they form an alternating arrangement of inlets between the sets (this is especially clear when looking at the plan views of the openings). Likewise, the outlets244b,248bfrom the openings are angularly spaced around the outer surface of the nozzle cap236, relative to one another and in their respective axial planes, so that they form an alternating arrangement of outlets between the sets (this is especially clear when looking at the plan views of the openings).

FIGS.6aand6bshow an embodiment of a side mounted injector18with a nozzle cap336optimised for a tumble motion cylinder4. More specifically,FIG.6adepicts the nozzle cap336as seen from the side, whileFIG.6bprovides a cross-sectional plan view of the nozzle cap336. As can be seen, the nozzle cap336comprises a first set opening354′ as described above and further second and third sets of openings356a,356b, with each of the second and third sets of openings356a,356bcomprising only two openings356′,356″ respectively. As in the previously described embodiments, and as shown inFIG.6a, the second and third sets of openings356a,356bare arranged relative to each other about the longitudinal injection axis20such that the inlet and outlet planes of the first set356aare further away from the cylinder4(i.e. higher in the orientation of the Figures) than the inlet and outlet planes of the second set356b.

Also as above, the opening axis352of each opening356′,356″ in the second and third sets356a,356bdefines an angle to the longitudinal injector axis20which is the same as for the other openings356′,356″ in the associated set356a,356b. The angle d1to the longitudinal axis20defined for the first set356ais different to, and preferably larger than, the angle α2to the longitudinal axis20defined for the second set356b.

However, unlike the previously described embodiments, the second and third set openings356′,356″ are not necessarily arranged equiangularly about the longitudinal nozzle cap axis342, but they are arranged symmetrically about an axis358perpendicular to the longitudinal nozzle cap axis342(i.e. a lateral axis).

More specifically, and as shown inFIG.6b, the outlet348of each second set opening356′ is offset from each respective inlet344such that each respective opening axis352defines an angle1to the lateral axis358which is the same as for the other minor opening356′ in the set356b. That is to say, the second set openings356′ are arranged either side of the lateral axis358such that the inlet344and outlet348of one opening356′ are arranged symmetrically to the inlet344and outlet348of the other opening356′ about the lateral axis358. The same is true for the third set openings356″—i.e. the opening axis352of each third set opening356″ defines an angle β2to the lateral axis358which is the same as for the other opening356″ in the set356b. The angle β2defined by the opening axes352of the third set356bopenings356′ is different to and preferably smaller than the angle β1defined by the opening axes352of the second set356aopenings356′. Accordingly, the arrangement of all the second and third set openings356′,356″ is symmetric about the lateral axis358and biased such that all the opening axes352are angled towards one side of the nozzle cap336, with the opening axes352of the third set356bbeing angled more towards that side than the opening axes352of the second set356a.

Since the opening axes352of the second and third set openings356′,356″ are not normal to the inner and outer surfaces342,350of the nozzle cap336, the inlet and outlet344,348of each opening356′,356″ is elliptical in shape so as to maintain parallelism of the opening walls through the nozzle cap336. The embodiment ofFIG.6is particularly advantageous for use with an injector side mounted to a tumble motion engine cylinder4because the first set opening targets fuel to interact with the tumbling air motion, while the second and third set openings356′,356″ target fuel to the regions either side of the main tumble motion. In this way, the second and third set openings356′356″ direct fuel to a second zone of the cylinder, that is, one which the fuel from the first set opening does not reach effectively.

FIG.7shows an embodiment of a side mounted injector18with a nozzle cap436optimised for a swirl motion cylinder4. As can be seen inFIG.7a(which shows the nozzle cap436as seen from the side), the first opening454′ is similar to that of the previously described embodiments in that it is arranged to extend from an inlet444at the centre of the inner surface446of the nozzle cap436to an outlet448at the outer surface450of the nozzle cap436in a cylindrical manner such that the walls of the opening454′ are parallel to each other. While the inlet444is aligned with the centre of the inner surface446of the nozzle cap436, the outlet448is offset from the centre of the outer surface450of the nozzle cap436such that the opening axis452defines an angle γ to the longitudinal axis20of the injector18. Specifically, the outlet448is offset from the longitudinal injector axis20so that the opening axis452is angled towards a lining60of the cylinder4associated with the injector18. In this way, the first opening454′ directs fuel downwards and towards the cylinder lining60, thereby inducing a swirl motion of fuel within the lower portion (or a first zone) of the cylinder.

Also shown is a second set456′ of openings comprising five openings456′ arranged around the longitudinal nozzle cap axis442and with each opening456′ extending cylindrically from respective inlets444on the inner surface446of the nozzle cap436to respective outlets448on the outer surface450of the nozzle cap436. The five second set openings456′ are equiangularly spaced around one half of the nozzle cap436, and specifically around the half of the nozzle cap436furthest from the cylinder lining60. Accordingly, the second set openings456′ direct fuel into the upper portion (or a second zone) of the combustion chamber4where the fuel from the first opening454′ does not reach effectively.

It is contemplated that the nozzle cap436may comprise additional sets of openings, each similarly arranged about the longitudinal nozzle cap axis442as the second set openings456′, but with differing inlet and outlet planes perpendicular to the injector axis20. AlthoughFIG.7bindicates five minor openings456′ for the second set456, any number between two and five per set456is considered.

Generally, for a swirl motion side mounting injector as shown inFIG.7, it is recommended to include between 1 and 3 openings in the nozzle cap136. One major opening (for example, opening454′) should have at least 60% of the fuel mass or higher. As mentioned, the major jet should be targeted in a way of impinging on the cylinder lining60of the wall opposite the injector location.

In any embodiment of the invention for a swirl engine, the first set openings254′,454′ may comprise one opening254′,454′, whilst the second set openings256′,456′ may comprise between one to three openings (for example, for injection of fuel up to 60 bar, in an outward opening injector). There is also a minimum spray momentum requirement for the spray jet to propagate into the combustion chamber, and it has been found that high spray momentum gives better mixing.

In embodiments, the first opening454′ may be at an angle γ relative to the longitudinal axis20such that the fuel flow through the first opening454′ impinges the cylinder lining60. Advantageously, the fuel to flow through the opening454′ should be directed so as not to impinge on the spark plug or the air intake port. In this way, the fuel may flow directly into the combustion chamber4such that no fuel flow is sucked back into the intake port of the engine2.

FIGS.8to10depict other possible nozzle cap embodiments 536, 636, 736 as seen from the bottom when viewed along the longitudinal nozzle cap axis. As can be seen, the plurality of openings40is comprised of at least a first set of openings and a second set of openings. Unlike the previously described embodiments, the first set of openings comprise more than one opening—there are three first set openings554′,654′ in the embodiments ofFIGS.8and9, and four first set openings754′ in the embodiment ofFIG.10. The second set of openings also comprise more than one opening556′,656′756′—three in the embodiments ofFIGS.8and9, and four in the embodiment ofFIG.10. In each of these embodiments all of the openings554′,556′,654′,656′,754′,756′ are arranged such that the opening axis is normal to the inner and outer surfaces of the nozzle cap536,636,736. The first and second sets of openings are arranged relative to reach other about the longitudinal nozzle cap axis such that the inlet and outlet planes of the first set are closer to the bottom of the nozzle cap536,636,736than the inlet and outlet planes of the second set of openings. In the embodiments ofFIGS.8and10, the first set openings554′,654′ are larger (in diameter) than the second set openings556′,656′, whereas, in the embodiment ofFIG.9, all the openings754′,756′ have the same diameter.

Also, the first set openings554′,654′,754′ are equiangularly arranged around the longitudinal nozzle cap axis (which aligns with the injector axis20). Similarly, the second set openings556′,656′,756′ are equiangularly arranged around the longitudinal nozzle cap axis. The first set of openings and the second set of openings are rotated about the longitudinal nozzle cap axis relative to each other such that symmetry in the overall arrangement of both first and second set openings554′,556′,654′,656′,754′,756′ is maintained. More specifically, in the embodiments ofFIGS.8and9, the second set of openings is rotationally offset (about the longitudinal nozzle cap axis) from the first set of openings by 120° so that the nozzle cap536,636,736has three planes of symmetry and a rotational order of symmetry of three about the longitudinal nozzle cap axis. Similarly, in the embodiment ofFIG.10, the second set of openings is rotationally offset (about the longitudinal nozzle cap axis) from the first set of openings by 90° so that the nozzle cap536,636,736has four planes of symmetry and a rotational order of symmetry of four about the longitudinal nozzle cap axis.

FIG.11shows yet another nozzle cap embodiment 836 as seen from the bottom when viewed along the longitudinal nozzle cap axis. As can be seen, the plurality of openings40comprises a first set of openings and a second set of openings, each set comprising two openings854′856′ arranged opposite each other. The second set of openings is arranged rotationally offset from the first set of openings by 90° about the longitudinal nozzle cap axis so that the nozzle cap836has two planes of symmetry and a rotational order of symmetry of two.

The first and second sets are arranged relative to each other about the longitudinal injector axis such that the inlet and outlet planes of the first set are closer to the bottom of the nozzle cap836than the inlet and outlet planes of the second set of openings. Accordingly, first set of openings direct fuel to a lower portion (or a first zone) of the cylinder, while the second set of openings direct fuel to an upper (or second) zone of the cylinder4. Finally,FIG.12shows an example of a nozzle cap936which is identical toFIG.11but in which the second set of openings is removed. This may be useful for some cylinder configurations, where effectively the nozzle cap openings are grouped into first and second sets of openings which each direct gaseous fuel into a different region or zone of the combustion space.

Further investigations into the performance of the fuel injector18have concluded that high chamber pressures as a result of high load and high engine speed operating conditions result in poorer air-fuel mixing and, therefore, lower engine combustion efficiency overall. Generally, the higher chamber pressures require the fuel injector18to operate at higher injection pressures to provide the jet momentum required to achieve performance requirements. Therefore, generally, gaseous fuel injectors with high fuel flow rates provide better flow separation and better performance in high chamber pressures than gas fuel injectors with lower fuel flow rates.

Therefore, the nozzle cap has been designed with an appropriate number of holes with respective diameters to provide optimum jet targeting such that the minimum performance requirements are met for the various engine operating conditions.

It will be appreciated that various modifications may be made to the aforementioned embodiments without departing from the scope of the appended claims.