"A Gaseous Fuel Injection System"

Field of the Invention

The present invention relates to the injection of gaseous fuels for use in internal combustion engines. More particularly, the invention relates to an improved two stage gaseous fuel direct injection system for an internal combustion engine.

The term "direct injection" refers to the use of delivery injectors for injecting a fluid directly into the combustion chambers of internal combustion engines. The term "gaseous fuel" is defined herein as referring to both compressed gas fuels such as hydrogen (H2) or compressed natural gas (CNG) and to liquefied gas fuels such as liquefied petroleum gas (LPG) and liquefied natural gas (LNG).

Background Art

There exist many potential advantages in using gaseous fuels in engines in place of, or together with, the more commonly used liquid fuels. For example, it is weli appreciated that the undesirable exhaust emissions from an engine using a gaseous fuel can be lower than for a comparable engine using liquid fuel. Further, the use of gaseous fuels can in certain cases translate to a significant cost saving for the user due to the per litre cost thereof as compared to the per litre cost of the more commonly used liquid fuels.

The applicant has developed various proprietary direct fuel injection systems for use with two stroke and four stroke cycle internal combustion engines which primarily use liquid fuel. However, developments and trends in the automotive industry over recent years have highlighted certain potential advantages of also having a system which can inject gaseous fuel directly into the combustion chambers of the engine. As well as potentially leading to simpler systems requiring less mechanical components and facilitating improved metering control of the gaseous fuel to the engine as compared with prior art systems, such systems are likely to enable certain costs benefits to be realised by users,

United States Patent No. 5,941,210, the contents of which are included herein by way of reference, discloses a prior art gaseous fuel injection arrangement developed by the applicant. A system is described wherein gaseous fuel is directly metered and injected into a combustion chamber of the engine by way of a 'single stage' process. A pressure regulator is provided to regulate the supply pressure of the gaseous fuel to a delivery injector. With this arrangement, the gaseous fuel, for example hydrogen, is supplied to the delivery injector at a substantially constant pressure. However, the pressure within the combustion chamber varies according to the relative position of the piston within the coσesponding engine cylinder and whether the inlet and outlet ports of the engine ane open or closed. Accordingly, operation of the delivery injector is controlled by a control means in order to control the differential pressure across the delivery injector, the differential pressure being the difference between the supply pressure of the gaseous fuef being metered and injected by the delivery injector and the pressure within the combustion chamber. The differential pressure is a determining factor in the quantity of fuel injected into the combustion chamber.

The control means controls the duration of the opening of the delivery injector, as well as the point at which the delivery injector is opened and closed during an engine cycle. The quantity of fuel injected by the delivery, injector can therefore be controlled by controlling the timing of the injection process. Alternatively, the quantity of fuel injected by the delivery injector can be controlled by maintaining the duration of the opening of the delivery injector constant while the start or end of the injection process is set at a predetermined point in the engine cycle. In either scenario, the single delivery injector which essentially functions as a pressure-time device performs both the metering and direct injection function.

One disadvantage of this arrangement is that there may be conditions at which the vapour pressure of the gaseous fuel is less than the regulated pressure needed for satisfactory operation of the engine. Such a situation can arise, for example, at low ambient temperatures where the engine is cold and the vapour pressure of the gaseous fuel is consequently at a level below the regulated

prcssure. In such circumstances, vapour pressure of the gaseous fuel at a level below the regulated pressure may affect the metering accuracy of the gaseous injection system. This may then have a detrimental effect on the performance of the engine in terms of factors such as efficiency of operation, smoothness of running, and emission levels.

In the above-mentioned US Patent, there is also disclosed a second arrangement in which the pressure of the gaseous fuel supplied to the delivery injector is unregulated. With such an arrangement, the control means can respond to changes in the pressure of the gaseous fuel supplied to the delivery injector and vary operation of the delivery injector accordingly. However, with this arrangement, a pressure measurement device is required for measurement of the gaseous fuel pressure which may be disadvantageous in certain applications due to the additional cost associated with utilisation of such a pressure measurement device.

Further, whilst such an arrangement may indeed be possible, being unregulated, it would also require a delivery injector with a "crack" pressure that exceeds the maximum vapour pressure of the gaseous fuel. As a result the delivery injector may typically require to be a much higher power consumption device. Such an injector would also need to have better sealing properties and hence be manufactured to much higher tolerances, factors which would typically add significant expense and complexity.

The applicant has also previously developed systems whereby a gaseous substance may be delivered to the combustion chambers of an internal combustion engine together with the fuel and/or air delivered thereto. United States Patent No. 5,546,902, the contents of which are included herein by way of reference, discloses a two fluid fuel injection arrangement developed by the applicant wherein fuel is delivered to a combustion chamber by way of a compressed gas such as air. The specific arrangement of a fuel metering means and a delivery injector serve to separate the fuel metering and fuel delivery

functions such that the compressed gas can entrain and deliver a metered quantity of fuel for delivery directly into the combustion chambers of the engine.

More particularly, this US Patent discusses the introduction of a combustion enhancing substance, typically hydrogen, to promote the initial ignition of the delivered fuel. The specification discloses a number of arrangements whereby, over a selected portion of the engine operating load range, a specific quantity of a combustion control substance can be delivered to the combustion chamber in timed relation to the delivery of a liquid fuel thereto, For example, in one arrangement, the combustion control substance may be delivered into the combustion chamber together with the fuel. In an alternative arrangement, the combustion control substance may be delivered to combustion chamber together with the compressed gas so as to provide a substantially uniform air / control substance mixture.

This arrangement was however not developed with the delivery of only gaseous fuel in mind and rather was aimed at providing a desired combustion control substance within the engine cylinder at the point of ignition to promote the combustion process. Furthermore, as this arrangement was based, around the applicant's proprietary dual fluid fuel injection technology, it requires the addition of a specifically configured air system to properly function. Hence, such an arrangement would typically also include a source of compressed gas such as a compressor, an air regulator, an øir rail and various other components necessitated by the addition of the air system. As well as introducing further cost to the overall injection system, such components also introduce a degree of additional complexity to the system, particularly in relation to the control thereof.

Attempts have also been made to develop gaseous fuel systems where a gaseous fuel, typically hydrogen, is injected into the manifold of a port injected or MPI engine. Such endeavors have typically resulted in significant power loss (due to air displacement by the gaseous fuel) suggesting that direct injection of the gaseous fuel may provide a more advantageous alternative with minimal to no loss in overall engine power.

The above discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.

With the above background in mind, it is one object of the present invention to provide a system and method for the direct injection of a gaseous fuel into an internal combustion engine which provides an advantageous alternative to the above-mentioned arrangements.

Disclosure of the Invention

Therefore, according to a first aspect of the present invention, there is provided an injection system for the direct injection of a gaseous fuel into a combustion chamber of an internal combustion engine, the injection system comprising a metering means for delivering a metered amount of gaseous fuel during a metering event for subsequent delivery to the engine, and a delivery injector arranged to receive the gaseous fuel metered by the metering means and to effect the direct injection thereof into the combustion chamber of the engine, wherein the injection system is configured to promote operation of the metering means with a sonic flow during the gaseous fuel metering event.

Preferably, a region downstream of the metering means and within the injection system is configured and sized to promote operation of the metering means with a sonic flow during the gaseous fuel metering event. The metering means and the delivery injector may hence be specifically arranged and/or configured to ensure sonic flow through the metering means when it is opened and is metering gaseous fuel.

Preferably, the delivery injector is also arranged and/or configured so as to promote sonic flow throughout a gaseous fuel delivery event. That is, the delivery injector also functions sonically whilst the metered amount of gaseous fuel is injected thereby directly into the engine combustion chamber. In this way,

delivery of the gaseous fuel directly in to the combustion chamber is able to occur irrespective of the pressures of the prevailing gases within the combustion chamber.

The injection system is essentially a two stage ¹ system wherein the separate metering means and delivery injector enable the gaseous fuel metering event and the gaseous fuel delrvery event to be separated in time. In this way, the metering means, which is conveniently provided as a calibrated metering injector, is able to meter a predetermined amount of gaseous fuel throughout any portion of the engine cycle.

Conveniently, the injection system also comprises a holding chamber arranged to receive the metered quantity of gaseous fuel delivered by the metering means. Preferably, the holding chamber is in communication with the delivery injector such that the metered quantity of gaseous fuel may be directly injected into the combustion chamber of the engine. The holding chamber is conveniently arranged downstream of the metering means and upstream of the delivery injector. Preferably, the holding chamber is an intermediate volume located between the metering means and the delivery injector which stores the metered gaseous fuel until the direct injection event is timed to occur.

In certain arrangements, the holding chamber may preferably be formed as part of the delrvery injector such that the metered gaseous fuel may be delivered into the delivery injector and held within the holding chamber therein prior to delivery to the combustion chamber. Conveniently, the holding chamber may be provided as a suitably configured volume within ihe delivery injector.

Preferably, the holding chamber is configured to be of a sufficient volume to ensure sonic flow throughout the metering event. If the holding chamber is configured with insufficient volume, the pressure in the holding chamber may fluctuate considerably during the metering event resulting in sonic conditions not being maintained. This is of course undesirable as fuel metering would become non-linear as a function of time leading to variations in the metered amount of gaseous fuel expected during the metering event. Hence, the metering means

effectively operates in the choked condition during the metering event such that the metering of fuel thereby is not affected by the downstream pressure conditions.

Conveniently, the volume of the holding chamber is also sized such that it is small enough to ensure satisfactory transient response to changes in the metered quantity of gaseous fuel required by the engine.

Preferably, the delivery injector is of the outwardly opening poppet-type and comprises a valve means arranged within a delivery port, the delivery port being configured with a large flow area when opened. Conveniently, the delivery injector is provided as high flow rate direct injector. In this way, rapid delivery of the gaseous fuel through the delivery port and into the combustion chamber during a compression stroke can be promoted. Furthermore, the delivery injector is able to function as an effectively self-sealing injector by virtue of the combustion chamber pressures acting against the underside of the injector vatve means when the valve means is closing the delivery port.

Preferably, the metering means is connected to a source of compressed gas at a constant high pressure. In this way, when the metering means is opened, the compressed gas is self-propelled into the holding chamber by virtue of the high storage pressure thereof. Conveniently, the compressed gas may be hydrogen.

In alternative arrangements, the source of compressed gas may be regulated to maintain a substantially constant high pressure so as to maintain accurate fuel metering function by the fuel metering means.

Preferably, the gaseous fuel is delivered to the combustion chamber of the engine by the delivery injector following closure of an air inlet of the engine. The fuel delivery event typically occurs during a compression stroke of the engine operating cycle such that no air is displaced from the combustion chamber by the injection of the gaseous fuel.

The metering means may be designed such that it farms a converging/diverging delivery nozzle.

The delivery injector may be designed such that it forms a converging/diverging delivery nozzle.

Preferably, the metering means and the delivery injector are designed such that they each form a converging/diverging delivery no2.de, with minimal friction for the fluid flow therethrough. This aids in promoting the establishment and maintenance of sonic conditions at the region of minimum cross-section within the nozzle and results in the flow therethrough being independent of downstream pressures.

According to a second aspect of the present invention, there is provided a method for injecting gaseous fuel directly into a combustion chamber of an internal combustion engine by way of a gaseous fuel injection system, the injection system comprising a metering means for delivering a metered amount of gaseous fuel during a metering event for subsequent delivery to the engine, and a delivery injector arranged to receive the gaseous fuel metered by the metering means and to effect the direct injection thereof into the combustion chamber of the engine, each of the metering means and delivery injector arranged to be controlled by an engine controller, wherein operation of the injection system by the controller is effected so as to promote a sonic flow through the metering means during the gaseous fuel metering event.

Conveniently, the engine controller manages the timing of and period of opening for the metering means so as to promote sonic operation thereby. Conveniently, the engine controller also manages the timing of and period of opening for the delivery injector so as to further promote sonic operation thereby.

Preferably, the engine controller controls the operation of the delivery injector such that the metered quantity of gaseous fuel is delivered thereby to the combustion chamber of the engine following closure of an air inlet of the engine. Conveniently, the fuel delivery event is controlled to occur during a compression

stroke of the engine operating cycle such that no air is displaced from the combustion chamber by the injection of the gaseous fuel.

Brief Description of the Drawings

It will be convenient to further describe the present invention with respect to the accompanying drawings which illustrate several possible embodiments of the invention. Other embodiments of the invention are possible and, consequently, the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention. In the drawings:

Figure 1 is a sectional side view of a gaseous fuel injection system according to a first embodiment of the present invention;

Figure 2 is a schematic representation of the gaseous fuel injection system of Figure 1 when arranged on a four stroke internal combustion engine;

Figure 3 is a sectional side view of a delivery injector of a gaseous fuel injection system according to a second embodiment; and

Figure 4 is a schematic representation of a prior art gaseous fuel injection arrangement arranged for use on a two stroke internal combustion engine.

Best WlOdIe(S) for Carrying Out the Invention

In Figure 1 there is shown a gaseous fuel injection system 10 according to a first embodiment for delivering metered amounts of gaseous fuel directly into the combustion chamber of an engine. The injection system 10 is configured as a 'two stage ¹ system and comprises a gaseous fuel metering means 12 and a delivery injector 14. The metering means 12 is arranged coaxially with the delivery injector 14 such that gaseous fuel metered thereby can be received by the delivery injector 14 for subsequent delivery to the engine.

A holding chamber 16 is arranged intermediate of the metering means 12 and delivery injector 14. In the arrangement shown, the holding chamber 16 is provided as a volume between the metering means 12 and the downstream delivery injector 14, though it is to be appreciated that for certain applications the holding chamber 16 may alternatively be provided as a volume within the delivery injector 14, The metering means 12 is aligned to the delivery injector 14 by way of a housing 18 which also serves to define part of the holding chamber volume.

A nozzle end 19 of the delivery injector 14 is configured to be snugly received in a corresponding aperture 20 in a cylinder head 22 of an engine. In this way, the delivery injector 14 is able to effect direct injection of the gaseous fuel into a combustion chamber 24 of the engine. The metering means 12 is configured at an upper end 26 thereof to receive an interface 28 providing communication with a source of compressed gas, typically hydrogen. The hydrogen is typically stored and supplied at a constant high pressure such that the metering means 12 is able to reliably and repeatably function as a pressure-time gaseous metering injector.

Figure 2 shows the injection system 10 when arranged for use on a multi-valve four stroke internal combustion engine 30. The engine 30 is of otherwise standard configuration and includes a piston 32 arranged within a cylinder 34 which together with the cylinder head 22 define the combustion chamber 24. The engine 30 further comprises an air inlet 36 and exhaust outlet 38 which are opened and closed by corresponding inlet valve means 40 and exhaust valve means 42. A spark plug 43 is also arranged with the cylinder head 22 so as to enable ignition of the gaseous fuel delivered into the combustion chamber 24 at a desired time.

Operation of the engine 30 and the injection system 10 is controlled by an engine control means comprising a suitably programmed electronic control unit (ECU)

(not shown) which receives signals from a number of sensors (not shown) and determines the period and/or timing of various events including fuel metering

events, fuel delivery events and ignition events. In one arrangement, the control means may control the operation of the metering means 12 and the delivery injector 14 as a function of operator load demand, this being the engine load demanded by an operator of the engine 30. For example, in the case of an engine within a vehicle, the load demand may be controlled by the vehicle driver by displacement of an accelerator pedal and may be in response to the operator's desire to, for example, overtake a slow moving vehicle or ascend a steep incline. The control means may determine the operator load demand as a basis for determining the required operating parameters for the metering means 12 and delivery injector 14, and may control the operation thereof accordingly. To this end, the control means may include one or more "look-up" maps to determine the required operating parameters for the metering means 12 and/or the delivery injector 14.

In use, the separate metering means 12 and delivery injector 14 enable the gaseous fuel metering event and the gaseous fuel delivery event to be separated in time. The metering means 12, typically a calibrated metering injector, under the control of the engine control means, is able to meter a predetermined amount of gaseous fuel into the holding chamber 16 throughout any portion of the engine cycle. That is, the gaseous fuel can be metered at almost any time rather than being limited to the short time of the compression stroke. This serves to greatly improve the turndown ratio of the delivery injector 14.

The delivery injector 14, arranged as a direct injection means which communicates with the upstream holding chamber 16 when opened, is able to then deliver the metered quantity of gaseous fuel to the combustion chamber 24. This delivery event typically occurs during the compression stroke of the engine cycle after closure of the air inlet 36 by the inlet valve means 40. The delivery event is also managed by the engine control means such that rt is typically completed before the cylinder pressure rises above the pressure in the holding chamber 16 which is communicated with the delivery injector 14 during the injection event. This 'two stage' injection process enables great flexibility with

the timing of metering and delivery events and facilitates high injector turndown ratios.

Direct injection of the gaseous fuel, typically hydrogen, after inlet valve closure is important to the satisfactory operation of the engine 30. Due to the low density of gaseous hydrogen, it is able to displace a considerable volume of air from the cylinder 34 if mixed before inlet valve closure. In fact, a stoichiometric mixture of hydrogen and air typically contains 30% less oxygen per volume than air alone. By direct injecting the hydrogen after intake valve closure, no air is displaced from the cylinder 34 such that the maximum mass of oxygen can be retained therein. Ideally, this may equate to a power increase exceeding 40%, Furthermore, since the hydrogen is typically stored at very high pressure in storage tanks, a direct injection arrangement such as the injection system 10 can be realised with a minimum of additional components.

Unlike the metering means 12, the delivery injector 14 is not required to be a calibrated metering injector. As best seen in Figure 1 , the delivery injector 14 is an outwardly opening poppet-type injector which is effectively self-sealing by virtue of the combustion chamber gases acting thereon when the injector 14 is closed. The delivery injector 14 comprises a delivery port 44 which is open and closed by a valve means 46. The delivery port 44 is configured with a large open area (being the annular flow area between the valve means 46 and the delivery port 44) to enable rapid delivery of the gaseous fuel or hydrogen into the combustion chamber 24 during the compression stroke. Hence, the delivery injector 14 is able to function as a high flow rate direct injector. This can be contrasted to the use of a pintle-type inwardly opening injector wherein the flow area is defined by surfaces which are at a smaller radius than for an outwardly opening valve, and the combustion pressures tend to act so as to open the pintle-type valve)

During the metering events, the metering means 12 operates in the choked condition such that delivery of the gaseous fuel therefrom is not affected by the downstream pressure conditions in the holding chamber 16. This may ideally be

achievecf by suitable sizing of the volume of the downstream holding chamber 16. In the arrangement described, two conflicting requirements were used to determine the volume of the holding chamber 16.

Firstly, the volume of the holding chamber 16 required to be large enough to ensure sonic flow throughout the metering event. An insufficient holding chamber volume may cause the pressure within the holding chamber 16 to fluctuate considerably during metering resulting in sonic conditions not being maintained. That is, if the holding chamber volume was too small the pressure in the holding chamber would rise during the fuel metering process to a point wherein the backpressure created in the holding chamber would begin to influence the fuel flow into the holding chamber. This typically leads to the metering of fuel becoming non-linear as a function of time, leading to variations in the expected metered amount of gaseous fuel. Secondly, the holding chamber volume is also required to be sized small enough to ensure rapid transient response to changes in the metered gaseous fuel quantity delivered by the metering means 12.

The delivery injector 14 also operates in the choked condition when open and is configured so as to promote sonic flow throughout a gaseous fuel delivery event. Sonic operation of the delivery injector 14 ensures that the delivery of the gaseous fuel directly into the combustion chamber 24 is able to occur irrespective of the pressures of the prevailing gases therein. If the delivery injector 14 were to not operate with a sonic flow, the downstream combustion cylinder pressures may compromise the delivery event and result in a certain amount of hydrogen being retained in the holding chamber 16, such amount being different at different operating conditions and thereby affecting the net amount of fuel being delivered to the combustion chamber.

In this regard, both the metering means 12 and delivery injector 14 of the injection system 10 are designed such that they each form a converging/diverging delivery nozzie with minimal friction for the fluid flow therethrough. This aids in promoting the establishment and maintenance of

sonic conditions at the regions of minimum cross-section within the nozzles and results in the respective flows therethrough being independent of. downstream pressures.

Whilst the injection system 10 has been shown in Figures 1 and 2 as an axial metering means 12 and delivery injector 14 arrangement, it is to be noted that such an axial or coaxial arrangement is not necessarily required by the present invention, in fact, as the fuel being injected is gaseous in nature, there should not exist any "fuel hang-up" issues to contend with suggesting that other arrangements may be equally workable. For example, the injection arrangement may comprise a side feed axial delivery injector arranged to receive metered amounts of gaseous fuel from a laterally arranged metering means. This of course may lead to certain packaging advantages wherein different injection arrangements may be adapted to suit specific engine applications.

Referring now to Figure 3, there is shown the delivery injector 14 of an injection system according to a second embodiment. The injection system according to the second embodiment is similar in many respects to that of the first embodiment and so corresponding reference numerals are used to identify similar parts.

As is the case with the first embodiment, the delivery injector 14 of the injection system 10 according to the second embodiment is designed to form a converging/diverging delivery nozzle with minimum friction for the fluid flow therethrough. The delivery injector 14 comprises a valve needle 50 of the outwardly opening poppet valve type having a valve stem 52 and a valve head 54 at one end of the valve stem. The valve head 54 cooperates with the delivery port 44 to cause opening and closing thereof. The valve stem 52 is tubular to provide a central bore 56 for fluid flow therealong. Openings 58 provided in the wall of the valve stem 52 adjacent the valve head 54 to permit the fluid flow to pass from the central bore 56 to an outer zone 60 from where it can be delivered into the combustion chamber upon opening of the delivery port 44.

The delivery port 44 is defined by a valve seat 62 having an annular face 64 of arcuate profile, The valve head 54 has an arcuate face 66 confronting the valve seat face 64. With this arrangement, the flow path established between the arcuate faces 64, 66 upon opening of the delivery port 44 comprises an upstream convergent section converging to a minimum choke area and a downstream divergent section diverging from the minimum choke area. The convergent and divergent sections thus define the converging/diverging delivery nozzle.

Operation of the injection systems 10 according to the two embodiments during initial testing by the applicant revealed good results over a range of engine operating conditions, including the extremes of high-speed high-load conditions and low-speed low-load conditions. Sonic conditions for both the metering means 12 and delivery injector 14 were able to be maintained, resulting in the metering of gaseous fuel and the subsequent delivery thereof being independent of the respective downstream pressure conditions.

Accordingly, the two stage" or two injector injection system 10 can enable the realisation of certain advantages over the prior art gaseous fuel injection arrangements discussed hereinbefore. For example, the injection system 10 is not limited by requiring the metering and delivery events having to occur at the same time. This is a specific feature of the 'single stage' gaseous fuel injection system as show in Figure 4 and as alluded to hereinbefore. In the injection arrangement 100 as shown, gaseous fuel is directly injected into a combustion chamber 101 of the engine 102 with a pressure regulator 104 being provided to regulate trie supply pressure of the gaseous fuel to a single injector 106.

In this arrangement, the gaseous fuel, for example hydrogen, is supplied to the delivery injector 106 at a substantially constant pressure from a pressurized gas source 110. The operation of the injection arrangement 100 is controlled by a control means 108 in order to control the differential pressure across the delivery injector 106 when it is open. The differential pressure is a determining factor in the quantity of fuel simultaneously metered and injected into the combustion chamber 101.

The control means 108 controls the duration of the opening of the delivery injector 106, as well as the point at which the delivery injector 106 is opened and closed during an engine cycle. The quantity of fuel injected by the delivery injector 106 is as a result dependant on the timing of the injection process with the single delivery injector 106 performing both the metering and direct injection functions.

Due to the nature of operation of the injection arrangement 100, having a vapour pressure of the gaseous fuel at a level below the regulated pressure may affect the metering accuracy of the fuel. This may then have a detrimental effect on the performance of the engine 102 in terms of factors such as efficiency of operation, smoothness of running, and emission levels. The injection system 10 according to the present invention is not compromised by issues of this nature as the two stage' functioning thereof enables the metering and delivery events to be separated without complicating the nature of the system 10.

To operate satisfactorily, a 'single stage' injection arrangement such as that shown in Figure 4 is also likely to require a source of compressed gas having dual supply pressures in order to achieve a satisfactory injector turndown ratio. That is, high gas pressures are required to facilitate the delivery of high fuelling rates at high-speed high-load conditions whilst low gas pressures are required for lower fuelling rates. Alternatively, such an arrangement could be made to function with a, delivery injector having a faster response than that of a typical direct acting solenoid injector valve, although this is likely to incur a significant cost and complexity penalty. In contrast, the injection system 10, by virtue of its 'two stage" functionality, is well suited to be able to meet the injector turndown requirements for engine operation over a wide range of operating conditions.

Furthermore, whilst the injection system 10 is provided as a 'two stage' system, it does not necessitate the provision of an additional propellant such as air or the air system required with certain prior art two stage two fluid' fuel injection systems. In particular, as there is no requirement for air to assist with the entrainrneπt and delivery of fuel to the engine 30, in its simplest form, the

injection system 10 does not require an air compressor, air regulators or an air rail. Equally, other additional components and corresponding control strategies that would typically be required due to the presence of an air system in addition to the fuel system are not required.

The present invention offers a direct injection gaseous fuel solution which may have applicability in a variety of applications and which avoids the undesirable loss in engine power that has been found to exist with manifold injection alternatives. For example, application of a direct injection gaseous fuel injection system such as that described may be particularly suited to a hydrogen combustion engine hybrid electric vehicle seeking to deliver improved fuel economy and emissions.

Whilst the above embodiments have predominantly be discussed in respect of the delivery of hydrogen to an internal combustion engine, it is to be appreciated that the injection systems 10 are equally applicable for use with other gaseous fuels that may require to be delivered to an engine. Still further, whilst the injection systems 10 have primarily been discussed with respect to the direct injection of gaseous fuel to the combustion chambers) of an internal combustion engine, it is to be appreciated that the injection systems may also have applicability to other applications where the direct injection of a gas into a downstream volume is required.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Modifications and variations as would be deemed obvious to the person skilled in the art are included within the ambit of the present invention.