Compressor System

Abstract

A compressor system (10, Fig. 1) for compressing gaseous fuel such as hydrogen, comprising a housing 12 provided with a bore 16, a piston assembly arranged for reciprocating movement along the bore 16 and comprising a piston rod 14 defining a piston axis (A-A, Fig. 1) and a first piston head 20a arranged at a first end of the piston rod 14, a first compression chamber 26a arranged on a first side of the first piston head 20a, a first fluid chamber 30a on a second, opposed side of the first piston head 20a in which lubricating fluid is received in use, a seal assembly is carried by the piston assembly including a first annular seal 60a to substantially prevent fuel leakage past the first piston head 20a and a second annular seal 62a to substantially prevent fuel leakage from the first fluid chamber 30a, the annular seals being axially spaced along the piston head 20a; A collection chamber 64a is located axially between the first and second annular seals 60a, 62a and a primary flow path is defined, at least in part, through the piston assembly to allow fluid which has collected in the collection chamber 64a to flow through the piston assembly; The primary flow path comprises an outlet 70 defined in the housing 12 to allow fluid from the collection chamber 64a to exit the compressor system via the housing 12.

Description

FIELD OF THE INVENTION This invention relates to a compressor system for natural gas. In particular, but not exclusively, the invention relates to a compressor system for use in compressing hydrogen 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 on 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 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 achieve a compressor system which is effective for use in a natural gas 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 fuel compressor comprising a housing and a piston assembly arranged for reciprocating movement along a bore provided in the housing, wherein the piston assembly comprises a piston rod defining a piston axis and a first piston head arranged at a first end of the piston rod; the system comprising a first compression chamber in which gaseous fuel is compressible, in use, by movement of the piston in a first direction along the piston axis, the first compression chamber being arranged on a first side of the first piston head; and a first fluid chamber being arranged on a second, opposed side of the first piston head to the first compression chamber in which lubricating fluid is received, in use. A seal assembly is carried by the piston assembly comprising a first annular seal arranged to limit or substantially prevent fuel leakage from the first compression chamber past the first piston head and further comprising a second annular seal arranged to limit or substantially prevent fuel leakage from the first fluid chamber past the first piston head, the first and second annular seals being axially spaced along the piston head. A collection chamber is located axially between the first and second annular seals; and a primary flow path is defined, at least in part, through the piston assembly to allow fluid which has collected in the collection chamber to flow through the piston assembly. The primary flow path comprises an outlet defined in the housing to allow fluid from the collection chamber to exit the compressor system via the housing. The compressor system provides a convenient means for allowing leakage fluid that has collected in the collection chamber to exit the system altogether, passing through the piston of the piston assembly itself, without risk of contamination to other fluid. It is a particular advantage that as soon as one of the fluids, whether lubricating fluid or gas, enters the collection chamber (which communicates with the outlet) it loses pressures, and so there is a much reduced likelihood of the leakage fluid or the gas passing across the other seal, but rather it exits the system through the primary flow path and hence the outlet. The outlet from the system, which provides the exit point for collected leakage fuel in the collection chamber, provides a convenient means of detecting whether leakage has occurred within the system. In embodiments, the primary flow path comprises an axially extending drilling which extends through the piston rod along the piston axis. The bore may comprise a first annular recess which forms a part of the primary flow path. The first annular recess may communicate with a drilling in the housing, and wherein the outlet may be defined at the end of the drilling. The compressor system may further comprise a secondary flow path defined, at least in part, through the piston assembly, wherein the secondary flow path communicates with a source of flushing fluid to allow leakage fluid within the collection chamber to be flushed out of the compressor system through the primary flow path. By way of example, the secondary flow path may comprise a further axially extending drilling provided in the piston rod. The secondary flow path may be configured to extend parallel with the first axially extending drilling . In embodiments, the secondary flow path may comprise a second annular recess provided in the bore. The first and second annular recesses may be axially spaced by a first annular piston seal carried by the piston rod. For example, the compressor system may comprise first and second annular piston seals, the first piston seal being arranged at an end of the first annular recess remote from the central annular piston seal and the second piston seal being arranged at an end of the second annular recess remote from the central piston seal. In embodiments, the piston assembly may comprise a second piston head arranged at a second end of the piston rod opposite to the first end. The compressor system may further comprise a second compression chamber in which gaseous fuel is compressed by movement of the piston rod in a second direction along the piston axis, in use, the first and second compression chambers being are arranged at opposed ends of the piston assembly. The compressor system may further comprise a drive arrangement arranged to drive reciprocating movement of the piston assembly along the piston axis. BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention may be more readily understood, preferred non-limiting embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a cross section of a part of a compressor system of a first embodiment of the invention; Figure 2 is a cross section of a compressor system of a second embodiment of the invention; and Figure 3 shows the compressor system in Figure 2, with the direction of flow through the system indicated by arrows. Throughout this description, terms such as ‘left and ‘right, 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 Figure 1 shows a gaseous fuel compressor system according to an embodiment of the invention in the context of a vehicle (not shown) that also comprises an internal combustion engine. The compressor system, referred to generally as 10, forms part of a fuel delivery system that is configured to deliver gaseous fuel from a fuel tank (not shown) to fuel injection system and, from there, to the 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 Figure 1 is purely an example. Indeed, in some embodiments compressors may find application outside the context of a vehicle, for example in fuelling stations. Figure 1 shows a part of a compressor system 10 for use in a fuel system such as that used for delivering a natural gas such as hydrogen. The compressor system 10 comprises a main (central) housing 12 for receiving a piston assembly including a slidable piston rod 14 which is slidable through a central housing bore 16 of the main housing 12. The piston rod 14 carries a piston head at each end; a left-hand piston head 20a at the left-hand end and a right-hand piston head at the right-hand end. Only one of the piston heads is shown in Figure 1, being the left-hand piston head 20a. In other embodiments, if manufacturing allows, the piston heads 20a and the piston rod 14 may be integrally formed. A first housing in the form of a first cylinder 22a extends to the left-hand side of the main housing 12 and a second housing in the form of a second cylinder 22b extends to a first, right-hand side of the main housing 12. Only a part of the second cylinder 22b is visible in the right-hand side of Figure 1. The left-hand piston head 20a is slidable within the first cylinder 22a (as shown in Figure 1) and the righthand piston head is slidable in the second cylinder22b. In use, a drive arrangement (not shown) is arranged to drive reciprocating movement of the piston rod 14 along a longitudinal piston axis, indicated by A-A. A first gas chamber 26a is defined within the first cylinder 22a for receiving hydrogen gas, the first gas chamber 26a being closed by an end plate (not shown in Figure 1) which is mounted to the first cylinder 22a. When gas fills the first gas chamber 26a, a first surface 28a of the piston head 20a is exposed to gas within the first gas chamber 26a. In a similar manner, a second gas chamber (not shown in Figure 1) is defined in the second cylinder 22b and is closed by a second end plate (not shown) mounted to the second cylinder 22b. When gas fills the second gas chamber, a first surface of the right-hand piston head is exposed to gas within the second gas chamber. The first and second gas chambers are located at opposed ends of the piston rod 14. When the drive arrangement acts on the piston rod 14, and the left-hand piston head 20a is moved to the left (left, in the illustration shown), gas within the first chamber 26a is compressed. Likewise, when the right-hand piston head is moved to the right (right, in the illustration shown) gas within the second chamber is compressed. On the other side of the piston head 20a to the first gas chamber 26a, a further chamber 30a is defined within the first cylinder 22a. Whereas the first gas chamber 26a receives hydrogen gas, the further chamber receives oil and is therefore referred to as the first oil chamber 30a. In normal operating conditions the pressure of gas within the first gas chamber 26a can be higher or lower than the pressure of oil within the first oil chamber 30a. A first annular piston rod seal 27 (i.e. a left-hand annular piston rod seal) is located towards one end of the piston rod 14 to prevent oil within the oil chamber 30 from flowing past the piston rod 14. A second annular piston rod seal 29 (i.e. a righthand annular piston rod seal) is located towards the other end of the piston rod 14 to prevent oil within the oil chamber of the second piston head from flowing past the piston rod 14. As the piston rod 14 is driven back and forth by the drive arrangement to reciprocate within the main housing bore 16, the left and right-hand piston heads on either side of the main housing 12 are drawn into, and pushed outwards from, the main housing bore 16 to reduce and increase the volumes of the first 20a and second gas chambers respectively. As a result, hydrogen gas in the first 20a and second chambers is compressed, in a cyclical manner, for delivery to the downstream parts of the fuel system. As both sides of the compressor system 10 are identical, the right-hand side of the system will not be described in further detail with regards to Figure 1. The first surface 28a of the piston head 20a is substantially flat and is arranged perpendicular to the internal wall of the first cylinder 22a. The opposite surface 32a of the piston head 20a, which is exposed to oil within the oil chamber 30a, is profiled. The profiled form of the piston head 20a defines a radially outer flange 34a on the piston head 20a which is adjacent to an annular recessed portion 36a of the piston head 20a. A frusto-conical central portion 38a of the piston head 20a projects centrally and symmetrically along the piston axis A-A from the annular recessed portion 36a. The central portion 38a of the piston head 20a is provided with a stepped bore within which the piston rod 14 is received fixedly. The piston rod 14 comprises a relatively smaller diameter end portion 14-1 and a relatively larger diameter main body portion 14-2. The end portion 14-1 defines an end surface 40a which, together with the stepped bore in the piston head 20a, defines a leakage chamber 42a within the piston head 20a. The leakage chamber 42a communicates with a primary flow path defined, in part, by a first axially extending drilling 44 provided along the axis A-A of the piston rod 14. A first radially extending drilling 46 extends from the first axially extending drilling 44 to complete a communication path through the piston rod 14 between the leakage chamber 42a and a first annular recess 48 provided in the main housing bore 16. The first radially extending drilling 44 and the first annular recess 48 also form a part of the primary flow path. The first annular recess 48 extends around the piston rod 14 and communicates with a second radially extending drilling 50 provided in the main housing 12, the second radially extending drilling 50 extending perpendicularly to the piston rod axis A-A. The second radially extending drilling 50 also forms a part of the primary flow path. The piston head 20a is also provided with a primary piston head drilling 54a which forms a part of the primary flow path, the primary piston head drilling 54a extending generally radially outward from the leakage chamber 42a through the piston head 20a towards the collection chamber 64a. The piston head 20a is also provided with two annular seals 60a, 62a, one being located on each side of an annular groove 64a provided on the outer surface of the piston head 20a. A first annular seal 60a is located towards the first surface 28a of the piston head 20a and a second annular seal 62a is provided on the other side of the annular groove 64a, towards the second surface 32a of the piston head 20a. The provision of the seals 60a, 62a serves to prevent contamination of the fluids between the gas chamber 26a and the oil chamber 30a. 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 left and right, depending on which of the gas and oil chambers 26a, 30a has the greater pressure. Movement of the 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 gas chamber 26a, and environmental pressure in the oil chamber 30a, may result in hydrogen accumulation in the oil system, which is not desirable. The use of the double seal system reduces this risk of contamination. A further component is mounted on the piston in the form of a piston guide 66a. The piston guide 66a is an annular component, located between the second seal 62a and the second surface 32a of the piston had 20a. The piston guide 66a ensures concentricity between the piston head 20a sliding in the first cylinder 22a. The annular groove 64a in the left-hand piston head 20a defines a collection chamber which serves to collect any leakage fluid that inadvertently flows from the gas chamber 26a along the piston head 20a and past the first annular seal 60a and / or from the oil chamber 30a along the piston head 20a past the second seal 62a. In the case of flow from the gas chamber 26a, the leakage fluid is hydrogen and, in the case of flow from the oil chamber 30a, the leakage fluid is oil. The collection chamber 64a communicates with the primary piston head drilling 54a so that any leakage fluid which collects in the collection chamber 64a is able to flow through the primary piston head drilling 54a and into the leakage chamber 42a at the end of the piston rod 14. From the leakage chamber 42a, leakage fluid is able to flow through the axially extending drilling 44 in the piston rod 14, out through the first radially extending drilling 46, through the first annular recess 48 and out through the second radially extending drilling 50 in the main housing 12. The second radially extending drilling 50 delivers the leakage fluid through an outlet port 70 to a collection volume (not shown) external to the compressor system 10 (i.e. a volume which is separate and distinct from the gas chamber 26a and the oil chamber 30a). Although not shown, a similar arrangement is provided for the right-hand piston head, on the right-hand side of the compressor system 10, where the right-hand piston head comprises first and second annular seals, a collection chamber and a piston guide, as for the left-hand piston head 20a, as well as a leakage chamber and a radially extending drilling to an associated collection chamber. One advantage of the invention is that as soon as any leakage fluid (whether gas or oil) escapes from its respective chamber 26a, 30a and past the respective seal 60a, 62a, it enters the collection chamber 64a and loses pressure (as the collection chamber is in communication with the outlet port 70 from the compressor system). As a result of this immediate pressure loss the risk of the oil or the gas traversing the first annular seal 60a or the second annular seal 62a, respectively, is considerably reduced. A further benefit of routing the leakage fluid from the collection chamber 64a through the piston head 20a and the piston rod 14, and through the main housing 12 to the outlet port 70, is that if fluid is detected at the outlet port 70 it provides an immediate indication that there is a leak within the compressor system and the system should be stopped for maintenance. A further benefit of a dual seal system is that the first annular seal 60a can be optimised for gas leakage whereas the second annular seal 62a can be optimised for oil leakage. Figure 2 shows an alternative embodiment of the invention in which like parts to those shown in Figure 1 are denoted with like reference numerals. In this case both sides of the compressor system are shown, including the left-hand piston head 20a on the left-hand side and the right-hand piston head 20b on the right-hand side. It will be appreciated by comparing the piston heads 20a, 20b on each side of the piston rod 14 that both sides of the arrangement are substantially identical. The left- and right-hand end plates 72a, 72b respectively are shown in Figure 2. In this embodiment the compressor system is provided with a primary flow path in the form of a primary network of drillings, as for Figure 1, but also a secondary flow path in the form of a secondary network of drillings. As before, and referring initially to the left-hand piston head 20a, the primary network of drillings comprises the primary piston head drilling 54a and the leakage chamber 42a in the left-hand piston head 20a, the first axially extending drilling 44 and the first radially extending drilling 46 in the piston rod 14, the first annular recess 48, the first radially extending drilling 50 through the main housing 12 and the outlet port 70. The primary network of drillings also comprises the equivalent drillings in the right-hand piston head 20b, which are labelled with the same reference numbers as for the features of the lefthand piston 20a, followed by the letter b, namely a primary piston head drilling 54b and a leakage chamber 42b in the right-hand piston head 20b. The leakage chamber 42b in the right-hand piston head 20b communicates with the first axially extending drilling 44 in the piston rod 14. In addition, and again referring initially to the left-hand piston head 20a, the secondary network of drillings comprises a secondary piston head drilling 80a provided in the left-hand piston head 20a which communicates, at one end, with the collection chamber 64a. The other end of the secondary piston head drilling 80a communicates with an intermediate piston head drilling 82a extending along an axis parallel with the piston rod axis A-A. In addition, the piston rod 14 is provided with a secondary axially extending drilling 90 forming part of the secondary network of drillings. The secondary axially extending drilling 90 is arranged in parallel with the primary axially extending drilling 44. The secondary axially extending drilling 90 communicates with a second annular recess 92 provided in the bore 16 of the main housing 12 through a link passage 94. The first annular recess 48 and the second annular recess 92 are axially spaced from one another along the piston rod axis A-A and are separated by a central annular piston rod seal 96. The right- and left-hand annular seal members, 29, 27, respectively, are located one at each end of the first and second recesses 48, 92, respectively. In addition to the first radially extending drilling 50 provided in the main housing 12, a second radially extending drilling 102 is also provided in the main housing 12 to communicate with the second annular recess 92. The second radially extending drilling 102 forms a further part of the secondary network of drillings. As described previously for Figure 1, leakage fluid which collects in the collection chamber 64a, 64b in each of the piston heads 20a, 20b is able to flow through the primary network of drillings to the outlet port 70. In addition, a flushing flow of fluid is introduced to the secondary network of drillings via an inlet port 104 at the end of the second radially extending drilling 102 in the main housing 12. This flushing flow of fluid flows into the second annular recess 92 and flows through the link channel 94 into the second axially extending drilling 90 in the piston rod 14 before passing into the piston head drillings 80a, 80b in both the left-hand and right-hand piston heads, 20a, 20b respectively. The fluid that is introduced through this secondary network of drillings therefore generates a continuous circulating flow of fluid to clean or flush the collection chambers 64a, 64b, encouraging the exit of fluid out of the collection chambers 64a, 64b through the primary network of drillings. This circulating flow of fluid prevents the formation of oil deposits on the piston heads 20a, 20b. The circulating fluid may be any suitable liquid such as gas (e.g. nitrogen). It will be appreciated that the right-hand piston head 20b has an identical arrangement to the left-hand piston head 20a, with the collection chamber 64b of the right-hand piston head 20b communicating with the axially-extending drilling through the piston rod 14 in the same manner as for the left-hand piston head 20a. Figure 3 is identical to Figure 2 except that the flow paths through the compressor system are shown by the arrows which indicate how the circulating fluid flows through the inlet port 104, through the secondary network of drillings and through the collection chambers 64a, 64b, out through the primary network of drillings and eventually the outlet port 70. The reference numbers are mostly removed from Figure 3 (which is identical to Figure 2), for the sake of clarity. 5 It will be appreciated that further embodiments of the invention are envisaged without deviating from the scope of the appended claims.

Claims

1. A compressor system (10) for compressing gaseous fuel, the compressor system comprising;a housing (12) provided with a bore (16);a piston assembly arranged for reciprocating movement along the bore (16), wherein the piston assembly comprises a piston rod (14) defining a piston axis (A-A) and a first piston head (20a) arranged at a first end of the piston rod (14);a first compression chamber (26a) in which gaseous fuel is compressible, in use, by movement of the piston assembly in a first direction along the piston axis (A-A), the first compression chamber (26a) being arranged on a first side of the first piston head (20a);a first fluid chamber (30a) on a second, opposed side of the first piston head (20a) to the first compression chamber (26a) in which lubricating fluid is received, in use,a seal assembly (60a, 62a) carried by the piston assembly including a first annular seal (60a) arranged to limit or substantially prevent fuel leakage from the first compression chamber (26a) past the first piston head (20a) and a second annular seal (62a) arranged to limit or substantially prevent fuel leakage from the first fluid chamber (30a) past the first piston head (20a), the first and second annular seals being axially spaced along the piston head (20a);a collection chamber (64a) being located axially between the first and second annular seals (60a, 62);a primary flow path defined, at least in part, through the piston assembly to allow fluid which has collected in the collection chamber (64a) to flow through the piston assembly; wherein the primary flow path comprises anoutlet (70) defined in the housing (12) to allow fluid from the collection chamber (64a) to exit the compressor system via the housing (12).

2. The compressor system (10) as claimed in claim 1, wherein the primary flow path comprises an axially extending drilling (44) which extends through the piston rod (14) along the piston axis (A-A).

3. The compressor system (10) as claimed in claim 2, wherein the bore (16) comprises a first annular recess (48) which forms a part of the primary flow path.

4. The compressor system (10) as claimed in claim 3, wherein the first annular recess (48) communicates with a drilling (50) in the housing (12), and wherein the outlet (70) is defined at the end of the drilling (50).

5. The compressor system (10) as claimed in any of claims 1 to 4, further comprising a secondary flow path defined, at least in part, through the piston assembly, wherein the secondary flow path communicates with a source of flushing fluid to allow leakage fluid within the collection chamber (64a) to be flushed out of the compressor system through the primary flow path (44, 50, 70).

6. The compressor system (10) as claimed in claim 5 when dependent on claim 3, wherein the secondary flow path comprises a further axially extending drilling (90) provided in the piston rod (14).

7. The compressor system (10) as claimed in claim 6, wherein the secondary flow path extends substantially parallel with the first axially extending drilling (44).

8. The compressor system (10) as claimed in claim 6 or claim 7, wherein the secondary flow path comprises a second annular recess (92) provided in the bore (16).

9. The compressor system (10) as claimed in claim 8 when dependent on claim 3, comprising a first annular piston seal (96) carried by the piston rod(14) which is located between the first annular recess (48) and the second annular recess (92).

10. The compressor system (10) as claimed in claim 9, further comprising first and second annular piston seals (27, 29), the first piston seal (29) being arranged at an end of the first annular recess (48) remote from the central annular piston seal (96) and the second piston seal (27) being arranged at an end of the second annular recess (92) remote from the central piston seal (96).11 .The compressor system (10) as claimed in any of claims 1 to 10, wherein the piston assembly comprises a second piston head (20b) arranged at a second end of the piston rod (14) opposite to the first end; and further comprising a second compression chamber (26b) in which gaseous fuel is compressed by movement of the piston rod (14) in a second direction along the piston axis (A-A), in use, the first and second compression chambers (26a, 26b) being arranged at opposed ends of the piston assembly.

12. The compressor system (10) as claimed in claim 11, further comprising a drive arrangement arranged to drive reciprocating movement of the piston assembly along the piston axis (A-A).15