Compressor system

Abstract

A compressor system 10 for compressing gaseous fuel, comprising a first housing portion 18 with a first housing bore 20, a first piston head 12 for reciprocating movement along the first housing bore

Description

FIELD OF THE INVENTION This invention relates to a compressor system for gaseous fuel. In particular, but not exclusively, the invention relates to a compressor system for use in compressing hydrogen gas or natural gas in a fuel system of an internal combustion engine. BACKGROUND Gaseous fuels such as hydrogen are promising alternative fuels to gasoline and diesel due to their potential for low or zero emissions and there has been considerable interest in developing traditional internal combustion engines to run on ecologically produced hydrogen. Research into the feasibility of this solution suggests that injecting hydrogen at a pressure considerably higher than atmospheric pressure beneficially affects engine efficiency. As a consequence, a compressor system is required to locate between the hydrogen tank of the vehicle and the fuel injection system itself. The compressor system acts on the fuel within the tank and elevates the pressure to a suitable pressure for injection to optimize efficiency. The compressor system is designed to act on the fuel in the event that the fuel in the fuel tank is at a pressure below 300 bar. In the event that the fuel in the fuel tank is above 300 bar, for example - a full fuel tank may contain fuel stored at up to 700 bar - the compressor system is inactive. The compressor system typically takes the form of a positive displacement compressor to cope with the requirement for a high pressure ratio between outlet pressure and inlet pressure. The compressor has a number of demanding running conditions which have to be satisfied, including that the outlet pressure must be substantially fixed while the inlet pressure depends on the amount of fuel left inside the tank. Sealing within the system is therefore of paramount importance. It is an object of the invention to provide a compressor system which is effective for use in a gaseous fuel delivery and which has effective sealing. SUMMARY OF THE INVENTION According to the invention, there is provided a compressor system for compressing gaseous fuel, the compressor system comprising a first housing portion provided with a first housing bore, a first piston head arranged for reciprocating movement along the first housing bore through a range of movement between a first position and a second position to apply a compressive force to fuel in a first compression chamber arranged on a first side of the first piston head, and a first oil chamber arranged on a second side of the first piston head, opposed to the first side, in which drive fluid is received to drive reciprocating motion of the first piston head within the first housing bore. The first piston head comprises an outer surface carrying a first seal assembly including a first annular seal arranged to limit or substantially prevent fuel leakage from the first compression chamber past the first piston head; a second annular seal arranged to limit or substantially prevent fluid leakage from the first oil chamber past the first piston head, the first and second annular seals being axially spaced along the outer surface; and a first leak collection chamber having an axial length along the outer surface of the first piston head, the first leak collection chamber being disposed between the first and second annular seals to collect fuel leakage and drive fluid leakage. The first leak collection chamber communicates with a first collection port in the first housing portion and the distance between the first position and the second position is less than the axial length of the first leak collection chamber such that the first collection port is maintained in fluid communication with the first leak collection chamber throughout the full range of reciprocal movement of the first piston head. The compressor system provides a convenient means for allowing leakage fluid that has collected in the first leak collection chamber to exit the system without risk of contaminating the fluid found in either of the first compression chamber or the first oil chamber. It is a particular advantage that as soon as any leakage fluid, whether drive fluid or gas, enters the first collection chamber it loses pressure, and so there is a much reduced likelihood of any leakage fluid passing across the first or second annular seals, and rather it exits the system through the first collection port. Moreover, the first collection port provides a convenient means for detecting whether leakage has occurred within the system. In embodiments, the first collection port is disposed mid way along the first leak collection chamber when the first piston head is positioned mid way between the first position and the second position. Arranging the first collection port in this way allows the full range of movement of the first piston head to be maximised whilst ensuring that the first collection port is in communication with the first leak collection chamber at all times. A second collection port is arranged diametrically opposed to the first collection port, and the first leak collection chamber is configured to communicate with the second collection port over the full range of movement of the first piston head. The first piston head is formed from a piston material and the first piston head includes an internal chamber absent any piston material. Forming the first piston head as described above allows for a first piston head of reduced mass. Reducing the mass of the first piston head reduces the wear and tear experienced by the system during reciprocating movement of the first piston head. The first piston head further comprises a flow path for drive fluid which communicates with the outer surface of the first piston head to deliver drive fluid to the outer surface. The flow path communicates, at one end, with the outer surface and, at the other end, with the first oil chamber. The above flow path, communicating the first oil chamber with the outer surface of the first piston head, will provide oil to the outer surface of the first piston head, thus lubricating the outer surface of the first piston head. The flow path comprises an axial drilling which communicates with the first oil chamber and at least one radial drilling which communicates with the axial drilling at one end thereof and the outer surface at the other end thereof. The flow path includes a restriction to restrict the flow of drive fluid to the outer surface. By providing the flow path with a restriction, the pressure of the fluid found in the flow path can be controlled such that it is always less than the pressure found in either of the first compression chamber or the first oil chamber. If the pressure in the flow path were to exceed the pressure of the fluid found in either of the first compression or oil chambers the first or second annular seal may experience undesirable additional loading. The compressor system may further comprise a second housing portion, provided with a second housing bore, and a second piston head, arranged for reciprocating movement along the second housing bore to compress fuel within a second compression chamber on a first side of the second piston head. The second housing portion, the second piston head and the second compression chamber are configured identically to the first housing portion, the first piston head and the first compression chamber, respectively. The compressor system further comprises a piston rod carrying the first piston head at one end of the piston rod and the second piston head at the other end of the piston rod. The compressor system is particularly suitable for use in compressing hydrogen gas for a fuel injection system of an engine. BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention may be more readily understood, preferred nonlimiting embodiments thereof will now be described, byway of example only, with reference to the accompanying drawings, in which: Figure 1a is a schematic drawing of a part of a compressor system of a first embodiment of the invention; Figure 1b is an enlarged view of a leak collection chamber and collection port of the compressor system in Figure 1a; Figure 2 shows a schematic drawing of a double-ended compressor system based on Figure 1a; Figure 3 is a schematic drawing of a part of a compressor system of a second embodiment of the invention; Figure 4 shows a schematic drawing of a doubled-ended compressor system based on Figure 3; Figure 5a shows a schematic drawing of a part of a compressor system of a third embodiment of the invention; Figure 5b shows an enlarged view of a portion of the compressor system in Figure 5a; Figure 5c shows an enlarged view of another portion of the compressor system in Figure 5a; Figure 6 shows a schematic drawing of a doubled-ended compressor system based on Figure 5a; Figure 7 shows a schematic drawing of a part of a compressor system of a fourth embodiment of the invention; and Figure 8 shows a double-ended compressor system of the embodiment in Figure 7. Throughout this description, terms such as ‘left and Tight, and other directional references, are used with reference to the orientation of the compressor system as shown in the accompanying drawings. However, it will be appreciated that such references are not limiting and that compressor systems according to the invention can be used in any orientation. DETAILED DESCRIPTION OF THE INVENTION As used herein, the terms "above" and "below" refer to the relative vertical positions of elements when the device is oriented as shown in the Figures, with "above" indicating a higher position and "below" indicating a lower position. Similarly, the terms "left" and "right" refer to relative horizontal positions, with "left" indicating a position to the “left” and "right" indicating a position to the right when the device is oriented as shown in the Figures. These terms do not imply direct vertical or horizontal alignment unless explicitly stated. Figures 1a, 1 b and 2 show a gaseous fuel compressor system 10 according to an embodiment of the invention in the context of a vehicle (not shown) that comprises an internal combustion engine (not shown). The compressor system 10 forms part of a fuel delivery system that is configured to deliver gaseous fuel from a fuel tank (not shown) to a fuel injection system (not shown) and, from there, to the internal combustion engine. It is noted, however, that compressor systems according to the invention may be used in a wide range of applications and so the system shown in Figures 1a, 1b and 2 is purely an example. Indeed, in some embodiments compressors may find application outside the context of a vehicle, for example in fuelling stations. The compressor system 10 may be used in a fuel system for delivering a gaseous fuel such as hydrogen. The compressor system 10 comprises a central housing portion 16 for receiving a piston assembly including a piston rod 14 which is slidable through a central housing bore 22 formed in the central housing portion 16. For the sake of clarity, the left-hand side of the compressor system 10 (to the left of the central housing portion 16) will be described first. The compressor system 10 further comprises a first housing portion 18 which extends to the left-hand side of the central housing portion 16. The first housing portion 18 takes the form of a first cylinder, wherein the inside of the cylinder is hollow - thus defining a first cylinder bore 20. The first cylinder bore 20 is coaxially aligned with the central housing bore 22 so that the piston rod 14 extends through and is slidable within the central housing bore 22 and a first piston head 12 extends through and is slidable within the first cylinder bore 20. A first compression chamber 24 is defined within the first housing portion 18 for receiving hydrogen gas, the first compression chamber 24 being closed by an end plate 28 which is mounted to the first housing portion 18. When gas fills the first compression chamber 24, a first side 122 of the first piston head 12 is exposed to gas within the first compression chamber 24. Moreover, the end plate 28 comprises two one-way valves (not shown) configured to control the flow of gaseous fuel to and from other parts of the fuel system (not shown). The first of these two one-way valves is an inlet valve (not shown) configured to allow gaseous fuel to enter the first compression chamber 24 when the first side 122 of the first piston head 12 is moving away from the end plate 28 during actuation of the piston assembly. The second of these two one-way valves is an outlet valve (not shown) configured to provide compressed gaseous fuel to downstream components of the fuel system once the gaseous fuel has been compressed through further actuation of the piston assembly such that the first side 122 of the first piston head 12 is moving towards the end plate 28. On the other side of the first piston head 12 to the first compression chamber 24, a further chamber is defined within the first housing portion 18. Whereas the first compression chamber 24 receives hydrogen gas, this further chamber receives a drive fluid such as oil, which may have lubricating properties. Therefore, in this embodiment, the further chamber is referred to as the first oil chamber 26 but may also be considered as a drive chamber in which a driving fluid is introduced to drive reciprocating movement of the first piston head 12. The first oil chamber 26 is defined between a second side of the first piston head 124 opposed to the first side 122 and a first side 22 of the central housing portion 16. In normal operating conditions the pressure of gas within the first compression chamber 24 can be higher or lower than the pressure of oil within the first oil chamber 26. In other embodiments the fluid within the further chamber may be provided to have lubrication properties, rather than being selected simply for the purpose of providing the fluid pressure necessary to move the piston arrangement. A channel (not shown) is provided in the central housing portion 16 to allow fluid to flow into the first oil chamber 26 allowing pressurised oil to drive reciprocating movement of the first piston head 12 along a longitudinal piston axis defined by the piston rod 14. The first piston head 12 is arranged for reciprocating motion, between a first position and a second position. In order to drive reciprocating motion of the first piston head 12, a drive fluid pump (not shown) is configured to provide high pressure drive fluid to the first oil chamber 26 through an inlet valve in the central housing portion 16. The inlet and outlet valves for the hydrogen gas are pressure operable. The first side 122 of the first piston head 12 may be substantially flat and may be arranged perpendicular to the internal bore 20 of the first housing portion 18. The second side 124 of the first piston head 12, which is exposed to oil within the first oil chamber 26, may be profiled. The profiled form of the second side 124 of the first piston head 12 may define a radially outer flange 126 on the first piston head 12 adjacent to an annular recessed portion 128 of the first piston head 12. It may be advantageous to provide the second side 124 of the first piston head 12 in a profiled form to minimise the stress in the system and improve the flow of oil entering and exiting the first oil chamber 26 through the one-way valves in the end plate 28. The first piston head 12 comprises an outer surface 32 in the form of an annular cylindrical surface with a first continuous annular groove disposed therein. The annular groove extends axially along the outer surface 32 to define a groove axial length. The outer surface 32 is configured to be in a close fit configuration with the internal bore 20 of the first housing portion 18. The annular groove defines a first leak collection chamber 34. The first piston head 12 is also provided with a first seal assembly including first and second annular seals 36, 38 disposed on the outer surface 32 thereof, one being located on each side of the annular groove. The first annular seal 36 is located towards the first side 122 of the first piston head 12 and the second annular seal 38 is provided on the opposite side of the first leak collection chamber 34, towards the second side 124 of the first piston head 12 along the outer surface 32, such that the first and second annular seals 36, 38 are axially spaced along the outer surface 32 of the first piston head 12 either side of the first leak collection chamber 34. The provision of the two annular seals 36, 38 serves to prevent contamination of the fluids between the first compression chamber 24 and the first oil chamber 26. The use of a dual seal system in this way is preferable to a single seal arrangement where the seal can be prone to movement both left and right, depending on which of the first compression and first oil chamber 24, 26 has the greater pressure. Movement of a seal in this way, in a single seal system, can lead to seal wear and fatigue. Also, extended periods of inactivity with pressurized hydrogen in the first compression chamber 24, and environmental pressure in the first oil chamber 26, may result in hydrogen accumulation in the lubrication system, which is not desirable. The use of the double seal system reduces this risk of contamination. A further benefit of a dual seal system is that the first annular seal 36 can be optimised for gas leakage whereas the second annular seal 38 can be optimised for oil leakage. The first annular seal 36 is arranged to limit or substantially prevent fuel leakage from the first compression chamber 24 past the first piston head 12 and the second annular seal 38 is arranged to limit or substantially prevent fluid leakage from the first oil chamber 26 past the first piston head 12. The first leak collection chamber 34 serves to collect any leakage fluid that inadvertently flows from the first compression chamber 24 along the first piston head 12 and past the first annular seal 36 and / or from the first oil chamber 26 along the first piston head 12 past the second annular seal 38. In the case of fluid flow from the first compression chamber 24, the leakage fluid is hydrogen and, in the case of fluid flow from the first oil chamber 26, the leakage fluid is oil. The first leak collection chamber 34 communicates with a first outlet or collection port 40 formed in the first housing portion 18 such that any leakage fluid which collects in the first leak collection chamber 34 is able to flow through the first collection port 40 and into a volume (not shown) external to both the first housing portion 18 and the central housing portion 16. The first collection port 40 is formed in the first housing portion 18 such that, when the first piston head 12 is mid way between the first position and the second position (i.e. halfway between the extremes of its range of travel), the first collection port 40 is axially mid way along the first leak collection chamber 34. This is beneficial in that it maximises the permitted range of travel of the piston, whilst maintaining communication between the first fluid collection port 40 and the first leak collection chamber 34 over the full range of movement of the first piston head 12. It will be appreciated that the first collection port 40 must be positioned so that neither the first compression chamber 24 nor the first oil chamber 26 communicate with the first collection port 40 at any position of the piston. Figure 1b shows an enlarged depiction of the first collection port 40 seen in Figure 1a. The first collection port 40 is shown to comprise a narrow passage 402 between the first leak collection chamber 34 and a volume external to all housing portions (an external volume). This may be advantageous because the external volume may not be confined to the same size restraints as the rest of the system and may therefore contain a sensing arrangement (not shown) configured to indicate when leakage is detected and / or measure a rate of flow of leakage fluid into the external volume. Moreover, by providing a flow path by which leakage fluid may leave the first leak collection chamber 34, the fluid within the first leak collection chamber 34 is maintained at a lower pressure than the fluid in both the first compression chamber 24 and the first oil chamber 26. Pressure in the first leak collection chamber 34 should always be as close as possible to atmospheric pressure. This is advantageous because if the pressure in the first leak collection chamber 34 were to become close, or equal, to the pressure in either of the first compression chamber 24 or the first oil chamber 26 then leakage fluid may inadvertently flow from the first leak collection chamber 34 into the first compression chamber 24 or the first oil chamber 26 and contaminate the fluid therein. Moreover, if the pressure in the first leak collection chamber 34 were to become close, or equal, to the pressure in either of the first compression chamber 24 or the first oil chamber 26 then the first or second annular seal 36, 38, respectively, may be forced to move left and right during movement of the first piston head 12; as previously described, this is known to increase seal wear. Maintaining that the pressure of the fluid within the first leak collection chamber 34 is lower than that of the fluid in the first compression chamber 24 and the first oil chamber 26 ensures that the first and second annular seals 36, 38, respectively, are always biased toward the first leak collection chamber 34. Further to the above, the collection port 40 also comprises an annual collection port groove 404. The annular collection port groove 404 is provided to reduce the wear on the first and second annular seals 36, 38 in the event that they pass over the narrow passage 402 during assembly of the compression system 10. This improves the longevity of the first and second annular seals 36, 38 because otherwise an increased seal wear leads to a reduced ability to seal. In addition to the above, further components are mounted on the first piston head 12 in the form of piston guides 44, 46. The first piston guide 44 is an annular component located proximal to the first annular seal 36, but spaced apart axially from the first annular seal 36 in the direction of the second side 124 of the first piston head 12. The second piston guide 46 is an annular component located proximal to the second annular seal 38, but spaced apart axially from the second annular seal 38 in the direction of the first side 122 of the first piston head 12. The piston guides 44, 46 ensure concentricity between the first piston head 12 and the first housing portion 18 during actuation of the first piston head 12 within the first cylinder bore 20. Figure 1a shows an internal chamber 48 disposed inside the first piston head 12 which is absent any piston material. This internal chamber 48 may be a vacant annular chamber with an elongate cross-section. Alternatively, the internal chamber 48 may be filled with a material less dense than the piston head material and may have many different shapes and sizes of cross-section to that shown. For example, the annular chamber 48 may be in communication with the oil chamber 26 so as to be filled with oil. By including the internal chamber 48, absent the piston material, the total mass of the first piston head 12 is reduced. Reducing the mass of the first piston head 12 reduces the force required to drive the first piston head 12 and the momentum carried by the first piston head 12 during reciprocating movement. By reducing the above, the wear and tear experienced by the compressor system 10 over time can be reduced. In an alternative embodiment to that shown, the internal chamber 48 may be in fluid communication with the first oil chamber 26 such that fluid may flow between the internal chamber 48 and the first oil chamber 26. Figure 2 shows the full compressor system 10 based on the assembly shown in Figure 1, wherein the compressor is double-ended. Each end of the compressor system 10 embodies the compressor system 10 shown in Figure 1, therefore illustrating that the piston rod 14 may carry a piston head at each end; a first piston head 12 at the left-hand end and a second piston head 12b at the right-hand end. In this embodiment, a second housing portion 18b in the form of a second cylinder extends to the right-hand side of the central housing portion 16. The inside of the second cylinder is hollow and defines a second cylinder bore 20b in which the second piston head 12b is configured for reciprocating movement. An equivalent arrangement is provided for the second piston head 12b as is described for the first piston head 12, wherein the second piston head 12b comprises corresponding first and second annular seals 36b, 38b, a leak collection chamber 34b, an internal chamber 48b absent any piston material and piston guides 44b, 46b, as seen for the first piston head 12, as well as a collection port 40b. It will be appreciated by comparing the piston heads 12, 12b on each side of the piston rod 14 that both sides of the arrangement are substantially identical. As described above for the first compression chamber 24 of Figure 1a, a second compression chamber 24b (shown in Figure 2) is defined in the second housing portion 18b and is closed by a second end plate 28b mounted to the second housing portion 18b. When gas fills the second compression chamber 24b, a first side 122b of the second piston head 12b is exposed to gas within the second compression chamber 24b. The first and second compression chambers 24, 24b are located at opposed ends of the piston rod 14. When the piston assembly is activated - driven by the introduction of oil to the first oil chamber 26 - and the first piston head 12 is moved to the left (left, in the illustration shown), gas within the first compression chamber 24 is compressed. Likewise, when the second piston head 12b is moved to the right - driven by an increase in pressure inside the second oil chamber 26b- (right, in the illustration shown) gas within the second compression chamber 24b is compressed. The first and second oil chambers 26, 26b are located on the left and right sides of the central housing portion 16, respectively. When the drive arrangement acts on the piston rod 14, and the first piston head 12 is moved to the right (right, in the illustration shown), fluid within the first oil chamber 26 is compressed. Likewise, when the second piston head 12b is moved to the left (left, in the illustration shown) fluid within the second oil chamber 26b is compressed. In addition to the above, Figure 2 shows a first annular piston rod seal 50 (i.e. a left-hand annular piston rod seal) located towards one end of the piston rod 14 to prevent oil within the first oil chamber 26 from flowing past the piston rod 14 into the central housing bore 22. A second annular piston rod seal 50b (i.e. a right-hand annular piston rod seal) is located towards the other end of the piston rod 14 to prevent oil within the second oil chamber 26b from flowing past the piston rod 14 into the central housing bore 22. As the piston assembly is driven back and forth through the introduction of oil into the first and second oil chambers 26, 26b, causing the piston assembly to reciprocate within the central housing bore 22, the first and second piston heads 12, 12b either side of the central housing portion 16 are drawn into and pushed outwards from the central housing portion 16 (reciprocating within first and second housing bores 20, 20b, respectively)!© reduce and increase the volumes of the first and second compression chambers 24, 24b, respectively. This reciprocation occurs through a range of movement defined between the first position and the second position of the piston rod 14. As a result, the hydrogen gas in the first and second compression chambers 24, 24b is compressed, in a cyclical manner, for delivery to the downstream parts of the fuel system. The range of movement is limited such that the collection ports 40, 40b are maintained in fluid communication with their respective leak collection chamber 34, 34b throughout the full range of movement of the piston assembly. As both sides of the compressor system 10 are identical, the right-hand side of the system will not be described in further detail. It will be appreciated that the right-hand side of the system, comprising the second piston head 12b, comprises equivalent components to those described above for the first piston head 12, as can be seen in Figure 2 where equivalent components of the second piston head 12b (right-hand side) are indicated by equivalent reference numerals to those used in Figure 1. One advantage of the invention is that as soon as any leakage fluid (whether gas or oil) escapes from its respective chamber 24, 24b, 26, 26b and passes the respective annular seal 36, 36b, 38, 38b, it enters the first / second leak collection chamber 34, 34b and loses pressure (as each collection chamber 34, 34b is in communication with a collection port 40, 40b). As a result of this immediate pressure loss, the risk of the oil or gas leakage, inside the leak collection chambers 34, 34b traversing the annular seals 36, 36b, 38, 38b, respectively, is considerably reduced. A further benefit of the invention is that if fluid is detected exiting the collection port 40, 40b it provides an immediate indication that there is a leak within the compressor system 10. In the embodiment of the invention shown in Figure 2, the reciprocating movement of the first piston head 12, arranged for reciprocating movement along the first cylinder bore 20, is driven by increasing the pressure of the oil fluid in the first oil chamber 26 such that during the reciprocating movement of the first piston assembly along the first cylinder bore 20, the movement of the first piston head 12 towards the end plate 28 is driven by providing oil to the first oil chamber 26. Moreover, the reciprocating movement of the second piston head 12b arranged for reciprocating movement along the second cylinder bore 20b is driven by increasing the pressure of the oil in the second oil chamber 26b such that during the reciprocating movement of the second piston assembly along the second cylinder bore 20b, the movement of the second piston head 12b towards the end plate 28b is driven by providing oil to the second oil chamber 26b. Figure 3 shows an alternative embodiment of the compressor system 10 shown in Figure 1. The system of Figure 3 comprises an additional collection port (a second collection port 42) arranged diametrically opposite the first collection port 40. In addition to the communication with the first collection port 40, the first leak collection chamber 34 also communicates with the second collection port 42 such that any leakage fluid which collects in the first leak collection chamber 34 is able to flow through both the first and second collection ports 40, 42 and into a volume (or volumes - not shown) external to both the first housing portion 18 and the central housing portion 16. The embodiment of the invention shown in Figure 3 is therefore configured such that leakage fluid that is collected in the first leak collection chamber 34 is able to flow into an external volume via both the first and second collection ports 40, 42. As seen for the first collection port 40, the second collection port 42 is also shown to comprise a narrow passage 422 between the first leak collection chamber 34 and a volume external to all housing portions (an external volume). It will be appreciated that the external volume may be such that one volume communicates with both the first and second collection ports 40, 42. It will also be appreciated that each of the first and second collection ports 40, 42 may communicate the first leak collection chamber 34 with separate first and second external volumes, respectively. In one preferred embodiment, the compressor system is oriented so that the first collection port 40 is above the second collection port 42. By way of example, the first collection port 40 may be directly above the second collection port 42. In this instance, directly above is intended to describe a first component (the first collection port 40) being on a same vertical axis as a second component (the second collection port 42), wherein the vertical axis is normal to a surface (not shown) on which the fuel system is disposed, and that the first component is further from the surface along the vertical axis than the second component. This arrangement may be advantageous in that, in use, the less dense fluid, (the gaseous fuel) will exit the first leak collection chamber 34 through the vertically higher first collection port 40, and the more dense fluid (the drive fluid), will exit the first leak collection chamber 34 through the vertically lower second collection port 42. In a specific embodiment where the external volume comprises separate first and second external volumes - a first external volume communicating with the first collection port 40 and a second external volume communicating with the second collection port 42 - gaseous fuel in the first leak collection chamber 34 will exit into the first external volume and drive fluid found in the first leak collection chamber 34 will exit into the second external volume. This may be advantageous in that the first external volume may be optimised for capturing and containing gaseous fuel and the second external volume may be optimised for capturing and containing drive fluid. The range of movement of the first piston head 12 is configured such that both the first and second collection ports 40, 42 are maintained in communication with the first leak collection chamber 34 at all times, for all positions of the piston assembly. Moreover, the second collection port 42 is positioned in the first housing portion 18 such that, when the first piston head 12 is mid way between the first position and the second position, the second collection port 42 is axially mid way along the first leak collection chamber 34. This is beneficial in that it maximises the distance between the first and second positions that can be permitted whilst maintaining communication between the first and second collection ports 40, 42 and the first leak collection chamber 34. Figure 4 shows a full compressor system 10, where the compressor is double-ended. Each end of the compressor system 10 embodies the compressor system 10 shown in Figure 3, illustrating that the piston rod 14 carries an identical piston head at each end; a first piston head 12 at the left-hand end and a second piston head 12b at the right-hand end. In this embodiment, a second housing portion 18b in the form of a second cylinder extends to the right-hand side of the central housing portion 16. The second piston head 12b is slidable within the second cylinder bore 20b defined in the second housing portion 18b. An equivalent arrangement is provided for the second piston head 12b as is shown for the first piston head 12 in Figure 3, wherein the second piston head 12b comprises corresponding first and second annular seals 36b, 38b, a leak collection chamber 34b and piston guides 44b, 46b, as seen for the first piston head 12, as well as two collection ports 40b, 42b diametrically opposed to one another around the annular leak collection chamber 34b. The first annular piston rod seal 50 (i.e. the left-hand annular piston rod seal) is located towards one end of the piston rod 14 to prevent oil within the first oil chamber 26 from flowing past the piston rod 14 into the central housing bore 22. The second annular piston rod seal 50b (i.e. a right-hand annular piston rod seal) is located towards the other end of the piston rod 14 to prevent oil within the second oil chamber 26b from flowing past the piston rod 14 into the central housing bore 22. It will be appreciated by comparing the piston heads 12, 12b on each side of the piston rod 14, as shown in Figure 4, that both sides of the arrangement are substantially identical. As the piston assembly is driven back and forth by selectively introducing oil into the first and second oil chambers 26, 26b, the first and second piston heads 12, 12b either side of the central housing portion 16 are drawn into, and pushed outwards from, the central housing portion 16 to reduce and increase the volumes of the first and second compression chambers 24, 24b, respectively. As a result, the hydrogen gas in the first and second compression chambers 24, 24b is compressed, in a cyclical manner, for delivery to the downstream parts of the fuel system. As both sides of the compressor system 10 shown in Figure 4 are identical, the right-hand side of the system will not be described in further detail. It will be appreciated that the right-hand side of the system, comprising the second piston head 12b, comprises equivalent components to those described above for the first piston head 12, as can be seen in Figure 4 where equivalent components of the second piston head 12b (right-hand side) are indicated by equivalent reference numerals with the addition of the letter ‘b’ appearing at the end of the reference numerals. Figure 5 shows an alternative embodiment of the invention in which the first piston head 12 is provided with an additional flow path comprising a network of drillings. The network of drillings comprises first and second radially extending drillings 54, 56 and an axially extending drilling 58 formed in the first piston head 12. The additional flow path defined by the network of drillings provides a means to communicate the first oil chamber 26 with the outer surface 32 of the first piston head 12 such that oil is permitted to flow from the first oil chamber 26 to the outer surface 32 of the first piston head 12. Figure 5b shows how the first radial drilling 54 is configured to communicate a second end of the axial drilling 58 with a portion of the outer surface 32 of the first piston head 12 proximal to the first annular seal 36. Figure 5c shows how the second radial drilling 56 is configured to communicate the first end of the axial drilling 58 with a portion of the outer surface of the first piston head 12 proximal to the second annular seal 38. The additional flow path further comprises first and second annular lubrication grooves 60, 62, each being provided at an end of a respective one of the first and second radial drillings 54, 56. The first annular lubrication groove 60 is therefore disposed in the outer surface 32 of the first piston head 12 between the first piston guide 44 and the first annular seal 36 such that it communicates with the first radial drilling 54. The second annular lubrication groove 62 is disposed in the outer surface 32 of the first piston head 12 between the second piston guide 46 and the second annular seal 38 such that it communicates with the second radial drilling 56. The first and second annular lubrication grooves 60, 62 extend around the circumference of the first piston head 12 and ensure that the oil is delivered uniformly to the entire circumference of the outer surface 32 through the network of drillings. The pressure in the first oil chamber 26 may reach pressures of the order of 400 bar. Therefore, a restriction 66 is disposed at a first end of the axial drilling 58, the first end of the axial drilling 58 opening into the first oil chamber 26. The restriction 66 is configured to restrict the flow of oil from the first oil chamber 26 into the first axial drilling 58. By restricting the flow of oil from the first oil chamber 26 to the first axial drilling 58 the pressure of the fluid within the first axial drilling 58 is maintained to be less than the pressure of the oil within the first oil chamber 26. The restriction 66 therefore has the effect to calibrate the oil flow rate. Thus, given that the additional flow path communicates with the outer surface 32 of the first piston head 12 - via the first and second radial drillings 54, 56, at two locations proximal to the first and second annular seals 36, 38 - and the pressure of the fluid within the first axial drilling 58 is maintained lower than the pressure of the oil within the first oil chamber 26, axial movement of each annular seal 36, 38 (in both left and right directions) can be reduced. As described previously, axial movement of a seal to the left or right, as often found in a single seal system, can lead to seal wear and fatigue. Also, the resulting pressure differential across the second annular seal 38 prevents the fluid which fills the first axial drilling 58, which may be a mixture of hydrogen fuel and oil, from passing through to the first oil chamber 26 and contaminating the oil therein. In order to operate correctly, the pressure of fluid within the first leak collection chamber 34 must always be lower than the pressure of oil in the first oil chamber 26. To some extent the restriction 66 may help to prevent the unwanted reverse flow to the leak collection chamber 34, but in other embodiments a one way valve may be included within the axial drilling 58 to maintain this pressure differential. In the embodiment of the invention shown in Figures 5a to 5c any fluid in the additional flow path will largely be oil. Hydrogen fuel will only be found in the additional flow path in the event that some hydrogen fuel inadvertently flows past the first annular seal 36. By providing the radial drillings with oil, the outer surface 32 of the first piston head 12 is provided with oil, thus reducing the friction experienced by the first and second annular seals 36, 38 and the first and second piston guides 44, 46 during the reciprocating motion of the piston assembly. Maintaining the pressure of the fluid within the radial drilling 58 lower than fluid in the first oil chamber 26 through the use of the restriction 66, as described above, also reduces the movement both left and right of the first annular seal 36 by ensuring that the pressure in the first radial drilling 54 is less than the pressure in the first compression chamber 24, such that the first annular seal 36 is always biased away from the first side 122 of the first piston head 12. Figure 6 shows a full compressor system based on the compressor in Figure 5a, where the compressor is double-ended. Each end of the compressor system 10 embodies the compressor system 10 shown in Figure 5a, illustrating that the piston rod 14 may carry such a piston head at each end; a first piston head 12 at the left-hand end and a second piston head 12b at the right-hand end. In this embodiment, a second housing portion 18b in the form of a second cylinder extends to the right-hand side of the central housing portion 16. The second piston head 12b is slidable within the second housing portion 18b. An equivalent arrangement is provided for the second piston head 12b as is described for the first piston head 12 in Figure 5, wherein the second piston head 12b comprises corresponding first and second annular seals 36b, 38b, a leak collection chamber 34b, piston guides 44b, 46b and a collection port 40b, as seen for the first piston head 12, as well as a flow path in the form of a network of drillings (formed of an axial drilling 58b and first and second radial drillings 54b, 56b) and annular lubrication grooves 60b, 62b at the end of each radial drilling 54b, 56b. As both sides of the compressor system 10 shown in Figure 6 are identical, the right-hand side of the system will not be described in further detail. It will be appreciated that the right-hand side of the system, comprising the second piston head 12b, comprises equivalent components to those described above for the first piston head 12 as can be seen in Figure 5, indicated by equivalent reference numerals to those used in Figure 5 to indicate components of the first piston head 12. Figure 7 shows an alternative embodiment of the compressor system 10 shown in Figures 5 and 6. The system of Figure 7 comprises an additional collection port (a second collection port 42) arranged diametrically opposite the first collection port 40 in the housing portion 18. In addition to the communication with the first collection port 40, the first leak collection chamber 34 also communicates with the second collection port 42 such that any leakage fluid which collects in the first leak collection chamber 34 is able to flow through both the first and second collection ports 40, 42 and into a respective volume (or volumes - not shown) external to both the first housing portion 18 and the central housing portion 16. The embodiment of the invention shown in Figure 7 is therefore configured such that leakage that is collected in the first leak collection chamber 34 is able to flow into an external volume via both the first and second collection ports 40, 42. It will be appreciated that the external volume may be such that one volume communicates with both the first and second collection ports 40, 42. Alternatively, each of the first and second collection ports 40, 42 may allow the first leak collection chamber 34 to communicate with separate first and second external volumes, respectively. In one preferred embodiment, the compressor system 10 is oriented so that the first collection port 40 is above the second collection port 42. By way of example, the first collection port 40 may be directly above the second collection port 42. In this instance, directly above is intended to describe a first component (the first collection port 40) being on a same vertical axis as a second component (the second collection port 42), wherein the vertical axis is normal to a surface (not shown) on which the fuel system is disposed, and that the first component is further from the surface along the vertical axis than the second component. This arrangement may be advantageous in that, in use, the less dense fluid (the gaseous fuel) will exit the first leak collection chamber 34 through the vertically higher first collection port 40 and the more dense fluid (the drive fluid) will exit the first leak collection chamber 34 through the vertically lower second collection port 42. In a specific embodiment where the external volume comprises separate first and second external volumes - a first external volume communicating with the first collection port 40 and a second external volume communicating with the second collection port 42 - gaseous fuel in the first leak collection chamber 34 will exit into the first external volume and drive fluid in the first leak collection chamber 34 will exit into the second external volume. This may be advantageous in that the first external volume may be optimised for capturing and containing gaseous fuel and the second external volume may be optimised for capturing and containing drive fluid. The range of movement of the first piston head 12 is configured such that both the first and second collection ports 40, 42 are maintained in communication with the first leak collection chamber 34 at all times, for all positions of the piston assembly. Moreover, the second collection port 42 is positioned in the first housing portion 18 such that, when the first piston head 12 is mid way between the first position and the second position, the second collection port 42 is axially mid way along the first leak collection chamber 34. This is beneficial in that it maximises the distance between the first and second positions that can be permitted whilst maintaining communication between the first and second collection ports 40, 42 and the first leak collection chamber 34. Figure 8 shows a full compressor system 10 based on the compressor system in Figure 7, where the compressor is double-ended. Each end of the compressor system 10 embodies the compressor system 10 shown in Figure 7, illustrating that the piston rod 14 may carry such a piston head at each end; a first piston head 12 at the left-hand end and a second piston head 12b at the right-hand end. In this embodiment, a second housing portion 18b in the form of a second cylinder extends to the right-hand side of the central housing portion 16. The second piston head 12b is slidable within the second housing portion 18b. An equivalent arrangement is provided for the second piston head 12b as is shown for the first piston head 12 in Figure 7, wherein the second piston head 12b comprises corresponding first and second annular seals 36b, 38b, a leak collection chamber 34b, piston guides 44b, 46b, a network of drillings (formed of an axial drilling 58b and first and second radial drillings 54b, 56b), and annular lubrication grooves 60b, 62b at the end of each radial drilling 54b, 56b as seen for the first piston head 12, as well as two collection ports 40b, 42b diametrically opposed to one another around the annular leak collection chamber 34b. It will be appreciated by comparing the piston heads 12, 12b on each side of the piston rod 14, as shown in Figure 8, that both sides of the arrangement are substantially identical. As both sides of the compressor system 10 shown in Figure 8 are identical, the right-hand side of the system will not be described in further detail.

Claims

1. A compressor system (10) for compressing gaseous fuel, the compressor system (10) comprising:a first housing portion (18) provided with a first housing bore (20),a first piston head (12) arranged for reciprocating movement along the first housing bore (20) through a range of movement between a first position and a second position to apply a compressive force to fuel in a first compression chamber (24) arranged on a first side (122) of the first piston head (12), anda first oil chamber (26) arranged on a second side (124) of the first piston head (12), opposed to the first side (122), in which drive fluid is received to drive reciprocating motion of the first piston head (12) within the first housing bore (20),wherein:the first piston head (12) comprises an outer surface (32) carrying a first seal assembly including:a first annular seal (36) arranged to limit or substantially prevent fuel leakage from the first compression chamber (24) past the first piston head (12);a second annular seal (38) arranged to limit or substantially prevent fluid leakage from the first oil chamber (26) past the first piston head (12), the first and second annular seals (36, 38) being axially spaced along the outer surface (32); anda first leak collection chamber (34) having an axial length along the outer surface (32) of the first piston head (12), the first leak collection chamber (34) being disposed between the first and second annular seals (36, 38) to collect fuel leakage and drive fluid leakage, wherein the first leak collection chamber (34) communicates with a first collection port (40) in the first housing portion (18);and wherein the distance between the first position and the second position is less than the axial length of the first leak collection chamber (34) such that the first collection port (40) is maintained in fluid communicationwith the first leak collection chamber (34) throughout the full range of movement of the first piston head (12).

2. The compressor system (10) of claim 1, wherein the first collection port (40) is disposed mid way along the first leak collection (34) chamber when the first piston head (12) is positioned mid way between the first position and the second position.

3. The compressor system (10) as claimed in claim 1 or claim 2, further comprising a second collection port (42) arranged diametrically opposed to the first collection port (40), wherein the first leak collection chamber (34) is configured to communicate with the second collection port (42) over the full range of movement of the first piston head (12).

4. The compressor system (10) as claimed in any of claims 1 to 3, wherein the first piston head (12) is formed from a piston material and wherein the first piston head (12) includes an internal chamber (48) absent any piston material.

5. The compressor system (10) as claimed in any of claims 1 to 4, the first piston head (12) further comprising a flow path for drive fluid which communicates with the outer surface (32) of the first piston head (12) to deliver drive fluid to the outer surface (32).

6. The compressor system (10) as claimed in claim 5, wherein the flow path communicates, at one end, with the outer surface (32) and, at the other end, with the first oil chamber (26).

7. The compressor system (10) as claimed in claim 6, wherein the flow path comprises an axial drilling (58) which communicates with the first oil chamber (26) and at least one radial drilling (54, 56) which communicates with the axial drilling (58) at one end thereof and the outer surface (32) at the other end thereof.

8. The compressor system (10) as claimed any of claims 5 to 7, wherein the flow path includes a restriction (66) to restrict the flow of drive fluid to the outer surface (32).

59. The compressor system (10) as claimed in any of claims 1 to 8, the compressor system (10) further comprising a second housing portion (18b), provided with a second housing bore (20b), and a second piston head (12b), arranged for reciprocating movement along the second housing bore (20b)10 to compress fuel within a second compression chamber (24b) on a first side (122b) of the second piston head (12b), wherein the second housing portion (18b), the second piston head (12b) and the second compression chamber (24b) are configured identically to the first housing portion (18), the first piston head (12) and the first compression chamber (24), respectively, the com-15 pressor system (10) further comprising a piston rod (14) carrying the first piston head (12) at one end of the piston rod (14) and the second piston head (12b) at the other end of the piston rod (14).

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