Patent Publication Number: US-2013239569-A1

Title: Gas expander system

Description:
The present invention relates to a gas expander system, and in particular to a gas expander system suitable for use in a turbomachine (e.g. a machine comprising a turbocharger, such as a motor vehicle). 
     Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric pressure (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing connected downstream of an engine outlet manifold. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to an engine intake manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housings. 
     In known turbochargers, the turbine stage comprises a turbine chamber within which the turbine wheel is mounted; an annular inlet passageway defined between facing radial walls arranged around the turbine chamber; an inlet volute arranged around the inlet passageway; and an outlet passageway extending from the turbine chamber. The passageways and chambers communicate such that pressurised exhaust gas admitted to the inlet chamber flows through the inlet passageway to the outlet passageway via the turbine and rotates the turbine wheel. It is also known to improve turbine performance by providing vanes, referred to as nozzle vanes, in the inlet passageway so as to deflect gas flowing through the inlet passageway towards the direction of rotation of the turbine wheel. 
     Turbines may be of a fixed or variable geometry type. Variable geometry turbines differ from fixed geometry turbines in that the size of the inlet passageway can be varied to optimise gas flow velocities over a range of mass flow rates so that the power output of the turbine can be varied to suit varying engine demands. For instance, when the volume of exhaust gas being delivered to the turbine is relatively low, the velocity of the gas reaching the turbine wheel is maintained at a level which ensures efficient turbine operation by reducing the size of the annular inlet passageway. Turbochargers provided with a variable geometry turbine are referred to as variable geometry turbochargers. 
     Some turbomachines are provided with a power turbine. A power turbine may be used, for example, to transmit power to an engine crankshaft. The power turbine may be powered by exhaust gases leaving a turbine housing of a turbocharger. Since exhaust gases are used to provide power to the engine crankshaft, the overall efficiency of the turbomachine may be improved by the incorporation of the power turbine. 
     Whereas the turbine of a turbocharger drives a compressor, in a power turbine the end of the turbine shaft remote from the turbine wheel transmits power via a mechanical coupling. In a turbocompound engine comprising a power turbine connected in series with the turbine of a turbocharger, a gear wheel may be fixed to the end of the power turbine shaft to transmit power to the crankshaft of the engine via an appropriate coupling (for example, a mechanical gear). As with a turbocharger, the shaft of a power turbine is supported on bearing assemblies, including appropriate lubricating systems, located within a bearing housing connected to the turbine housing. The bearing arrangement at the turbine end of the shaft may be substantially the same as that found in a turbocharger, although the bearing arrangement at the drive end of the shaft may be a ball bearing assembly. 
     A power turbine may alternatively or additionally be provided in machines that do not have a turbocharger. Furthermore, a power turbine may be used to transmit power to something other than an engine crankshaft. For instance, a power turbine may be used to transmit power to, or generate power in, any suitable load, such as an electrical, hydraulic or mechanical arrangement. Although the power turbine has been described as comprising a turbine wheel, other gas expanders may be used instead of a turbine wheel (a gas expander being a device in which expansion of a gas may be used to derive mechanical work). For instance, the gas expander could be of a piston-type, a swash-plate type, or a screw-type expander. Generically speaking, then, a gas expander may be connected to a load. In known arrangements, the gas expander may be connected to the load via a moveable (for example, rotatable) shaft and a mechanical coupling such as a mechanical gear. 
     Gas expanders that are connected to, or are connectable to a load via a moveable shaft and a mechanical coupling (such as a mechanical gear) are well known, and have been in use for many decades. A gas expander, one or more moveable shafts and a coupling (e.g. a mechanical gear) may together be referred to as a gas expander system. Despite the existence of such gas expander systems over long period of time and in a wide variety of applications, there are problems associated with the use of such gas expander systems. The problems lie in the use of a mechanical gear in the gas expander system. The use of a mechanical gear requires continued maintenance, for example by way of the provision of a lubricant or the like. The efficiency of the mechanical gear could also be improved. A mechanical gear may also become damaged if excessive force is applied to the gear, for example via a torque in an input or output shaft to which the gear is connected. Another problem is the need to ensure consistently accurate alignment between members of a mechanical gear (e.g. cogs, or the like) over the lifetime of the mechanical gear. A further problem is the inherent high level of acoustic noise and vibration that is associated with the use of mechanical gears 
     It is an object of the present invention to provide a gas expander system that obviates or mitigates at least one problem of the prior art, whether identified herein or elsewhere. 
     According to an first aspect of the present invention, there is provided a gas expander system suitable for use in a turbomachine, the gas expander system comprising: a gas expander provided with a moveable part; a magnetic gear arrangement; and a shaft; the moveable part of the gas expander being connectable to a load via the magnetic gear arrangement and the shaft, and movement of the moveable part of the gas expander being arranged to cause movement of the shaft. 
     By connecting the moveable part of the gas expander to a load via the magnetic gear arrangement and the shaft, one or more problems associated with the use of mechanical gears as used in existing gas expander systems can be obviated or mitigated. In comparison with mechanical gears, magnetic gears require reduced maintenance and offer improved reliability. Magnetic gears are lubrication free in some embodiments, and other embodiments require less lubrication than mechanical gears. Magnetic gears are more efficient than conventional mechanical gears. Magnetic gears offer precise peak torque transmission and inherent overload protection (e.g. the magnetic gears are not damaged when an excessive force is applied to the magnetic gear which would, in mechanical gears, cause damage to teeth of the gears). The use of the magnetic gear may allow the physical isolation between an input and an output shaft and this can be taken advantage of as described in more detail below. Magnetic gears offer inherent anti-jamming transmission and significantly reduce potentially harmful drive-train pulsations. Furthermore, magnetic gears allow for misalignment of for example, input and output shafts of the gas expander system. Finally, when operating, magnetic gears have very low inherent acoustic noise and vibration in comparison with mechanical gears. 
     According to a second aspect of the present invention, there is provided a turbocharger system comprising: a turbocharger, comprising a turbine and a compressor; the gas expander system according to the first aspect of the invention. 
     The system (i.e. the gas expander system, and/or the turbocharger system) may be arranged such that a gas flowing into or out of the turbocharger or the gas expander is arranged to cause movement of the moveable part of the gas expander. 
     The system may comprise, in use, a source of heat. The source of heat may be provided by, in use, a part of the turbocharger system, an engine to which the turbocharger is connected, or a fluid flowing into or out of the turbocharger or the engine. The source of heat may be arranged to heat a working fluid provided in a closed-loop system and cause expansion of that working fluid. Expansion of the working fluid in a gaseous form in the closed loop system may be arranged to move the moveable part of the gas expander. The moveable part of the gas expander may be located within the closed loop system. The magnetic gear may comprise a first rotor and a second rotor, the first rotor and second rotor being separated from one another by at least a part of a wall forming part of the closed loop system. The at least a part of a wall forming part of the closed loop system that separates the first rotor from the second rotor may comprises (e.g. may be attached to or form part of) a stator of the magnetic gear. 
     The system may further comprise an impeller arranged to induce a fluid flow through the magnetic gear arrangement. The impeller may be attached to or form part of a rotor of the magnetic gear arrangement. One or more members of the magnetic gear arrangement may be provided with surface profiling or apertures configured to induce a fluid flow through the magnetic gear arrangement. The surface profile may comprise rifling. The fluid flow through the magnetic gear arrangement may, in general, be arranged to be directed radially away from an axis about which one or more members of the magnetic gear arrangement is configured to rotate. The fluid flow through the magnetic gear arrangement may be configured to have a serpentine like path through the magnetic gear arrangement, such that the fluid passes along one or more surfaces of two or three members of the magnetic gear arrangement (e.g. to improve cooling of those surfaces). A housing or casing of one or more members of the magnetic gear arrangement is configured (e.g. shaped) to direct, or assist in the direction of, the flow of air in the serpentine like path. 
     The magnetic gear arrangement may comprise a first rotor, a second rotor, and a magnetic gear member, and wherein the magnetic gear member is rotatable. The magnetic gear member may be located in-between the first rotor and the second rotor. The magnetic gear member may be selectively allowed to freewheel. The system further comprises an electricity generation arrangement for generating electricity from rotation of the magnetic gear member. 
     The system may further comprise a driving arrangement configured to drive rotation of the magnetic gear member. The magnetic gear arrangement may be configured such that, when the magnetic gear member is not rotated by the driving arrangement, the magnetic gear arrangement has a first, inherent, gear ratio. The magnetic gear arrangement may be configured such that, when the magnetic gear member is arranged to be rotated by the driving arrangement, the magnetic gear arrangement has a second gear ratio that is related to the first, inherent, gear ratio as defined by: 
       ( S   mm   −S   fr )/ R=S   sr    
     where S mm  is the speed of the magnetic gear member, S fr  is the speed of the first rotor, S sr  is the speed of the second rotor and R is the first, inherent, gear ratio. The driving arrangement may comprise a motor having a motor shaft. The driving arrangement may further comprise an abutment member (e.g. a cog or gear or disc like member) attached to or forming a part of the motor shaft and which abuts against the magnetic gear member. Rotation of the shaft may thus cause rotation of the abutment member, which in turn causes rotation of the magnetic gear member. 
     The magnetic gear arrangement may comprise a first rotor, a second rotor, and a magnetic gear member located, in use, in-between the first rotor and the second rotor, and wherein the magnetic gear member is moveable into and out of a position substantially in-between the first rotor and the second rotor. The magnetic gear member may be moveable in a radial direction (e.g. if the first rotor, second rotor, and magnetic gear member are disc-like in shape) or in an axial direction (e.g. if the first rotor, second rotor, and magnetic gear member are cylinder-like in shape). The magnetic gear member may be moveable in any suitable way, for example in a circumferential direction (e.g. in a direction parallel, perpendicular or tangential to a circumference of the magnetic gear member). 
     The magnetic gear arrangement may comprise a first rotor, a second rotor, and a magnetic gear member located in-between the first rotor and the second rotor, and wherein the magnetic gear member comprises a first part of the magnetic gear member and a second part of the magnetic gear member, relative movement being possible between the first part of the magnetic gear member and the second part of the magnetic gear member. One or both of the first part of the magnetic gear member and second part of the magnetic gear member are moveable from a first position to a second position, the first position being such that magnetic flux is prevented from passing from the first rotor to the second rotor, or from the second rotor to the first rotor, and the second position being such that magnetic flux is allowed to pass from the first rotor to the second rotor, or from the second rotor to the first rotor. The first configuration may be when pole-pieces of the first and second parts of the magnetic gear member are not aligned with one another, and the second configuration may be when pole-pieces of the first and second parts of the magnetic gear member are in alignment with one another 
     The magnetic gear arrangement may comprise a first rotor, a second rotor and a magnetic gear member located in-between the first rotor and the second rotor, and wherein the first rotor, second rotor and magnetic gear member each have a substantially cylindrical shape. Alternatively, the magnetic gear arrangement may comprise a first rotor, a second rotor and a magnetic gear member located in-between the first rotor and the second rotor, and wherein the first rotor, second rotor and magnetic gear member each have a substantially disc-like shape. The magnetic gear member may be a stator of the magnetic gear arrangement. The first rotor and second rotor may each comprises a plurality of permanent magnets. The magnetic gear member (e.g. the stator) may comprise a plurality of pole pieces. 
     The moveable part of the gas expander system may be rotatable. The shaft of the gas expander system may be rotatable. 
     The gas expander may be one of a group comprising: a turbine, a piston-type expander, a swash-plate type expander, or a screw-type expander. The moveable part may be a turbine wheel, a piston, a swash-plate or a screw-type arrangement (e.g. having a screw-like thread). 
     The moveable part of the gas expander may be connected to a first rotor of the magnetic gear by a first shaft, and a second rotor of the magnetic gear is connectable to the load via a second shaft. 
     The load may comprise one of a group comprising: an electricity generator (e.g. an alternator or a dynamo); a hydraulic system; a mechanical transmission; a mechanical gear; an engine crankshaft. 
     The gas expander may be a turbine, the moveable part of the gas expander being a turbine wheel, the gas expander forming part of a turbocharger, and wherein the system may further comprise a the load, the load being a compressor wheel of the turbocharger. 
     A turbocharger may be provided that incorporates the gas expander system as described herein. 
    
    
     
       Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying Figures, in which: 
         FIG. 1  is a section-view schematically depicting an embodiment of a turbocharger; 
         FIG. 2  schematically depicts an existing turbocompound engine system; 
         FIG. 3  schematically depicts in perspective view a part of an existing turbocompound engine system; 
         FIG. 4  schematically depicts another turbocompound engine system, together with potential sources of heat in that system; 
         FIG. 5  schematically depicts an embodiment of a waste heat recovery system that may be used in conjunction with the turbocompound engine system shown in and described with reference to  FIG. 4 ; 
         FIG. 6  schematically depicts an end-on-view of a magnetic gear that may be used in a gas expander of the turbocompound engine systems shown in and described with reference to  FIGS. 2 to 5 ; 
         FIG. 7  schematically depicts in perspective-view the magnetic gear shown in and described with reference to  FIG. 6 ; 
         FIG. 8  schematically depicts a section-view of a gas expander system in accordance with an embodiment of the present invention; 
         FIG. 9  schematically depicts a turbocompound engine system incorporating the gas expander shown in and described with reference to  FIG. 8 ; 
         FIG. 10  schematically depicts a waste heat recovery system for use in a turobcompound engine system, incorporating the gas expander shown in and described with reference to  FIG. 8 ; 
         FIG. 11  schematically depicts a modification to the waste heat recovery system of  FIG. 10 ; 
         FIG. 12  schematically depicts an operating principle associated with the waste heat recovery system shown in and described with reference to  FIG. 11 ; 
         FIG. 13  schematically depicts an end-on-view of a magnetic gear; 
         FIG. 14  schematically depicts an end-on-view of an impeller suitable for use in connection with the magnetic gear shown in and described with reference to  FIG. 14 ; 
         FIG. 15  schematically depicts an end-on-view of the magnetic gear of  FIG. 13  provided with the impeller of  FIG. 14 ; 
         FIG. 16  schematically depicts a perspective-view of a member of the magnetic gear shown in and described with reference to  FIG. 13 , the member be provided with surface features in the form of rifling; 
         FIG. 17  schematically depicts a section-view of a magnetic gear, and the flow of air in an around that gear, in accordance with an embodiment of the present invention; 
         FIG. 18  schematically depicts an end-on-view of a magnetic gear together with relative movement of members of that magnetic gear; 
         FIG. 19  schematically depicts the effects on the relative movement of members of the magnetic gear when the stator of the magnetic gear is free to rotate, and an outer member of the magnetic gear is rotated; 
         FIG. 20  schematically depicts the effects on the relative movement of members of the magnetic gear when the stator of the magnetic gear is free to rotate, and an inner member of the magnetic gear is rotated; 
         FIG. 21  schematically depicts an end-on-view of a magnetic gear in accordance with an embodiment of the present invention in which the stator of the magnetic gear comprises two concentric parts, one part being moveable relative to the other; 
         FIG. 22  schematically depicts relative movement between the first and second moveable parts of the stator of the magnetic gear shown in and described with reference to  FIG. 21 ; 
         FIG. 23  schematically depicts a perspective-view of a magnetic gear in accordance with another embodiment of the present invention, in which a stator of the magnetic gear is axially moveable; 
         FIG. 24  schematically depicts a first magnetic gear arrangement; 
         FIG. 25  schematically depicts a second magnetic gear arrangement; 
         FIG. 26  schematically depicts a magnetic gear arrangement according to another embodiment of the present invention, in which a magnetic gear member may be rotated by a driving arrangement; 
         FIG. 27  schematically depicts a magnetic gear arrangement according to a further embodiment of the present invention, in which a magnetic gear member may be rotated by a driving arrangement; 
         FIG. 28  schematically depicts gas expander system provided with the magnetic gear arrangement of  FIG. 26 ; 
         FIG. 29  schematically depicts gas expander system provided with the magnetic gear arrangement of  FIG. 27 ; and 
         FIG. 30  schematically depicts a turbocharger provided with a gas expander system according to an embodiment of the present invention. 
     
    
    
       FIG. 1  is an axial cross-section through a typical turbocharger  2  with a fixed geometry turbine which illustrates the basic components of a turbocharger. The turbocharger  2  comprises a turbine  4  joined to a compressor  6  via a central bearing housing  8 . The turbine  4  comprises a turbine housing  10  which houses a turbine wheel  12 . Similarly, the compressor  6  comprises a compressor housing  14  which houses a compressor wheel  16 . The turbine wheel  12  and compressor wheel  16  are mounted on opposite ends of a common turbo shaft  18  which is supported on bearing assemblies  20  within the bearing housing  8 . 
     The turbine housing  10  is provided with an exhaust gas inlet  22  and an exhaust gas outlet  24 . The inlet  22  directs incoming exhaust gas (e.g. from an engine outlet manifold) to an annular inlet chamber, i.e. a volute  26  surrounding the turbine wheel  12  and communicating therewith via a radially extending annular inlet passageway  28 . Rotation of the turbine wheel  12  rotates the compressor wheel  16  (via the shaft  18 ) which draws in air through an axial inlet  30  and delivers compressed air to an engine intake (not shown) via an annular outlet volute  32 . 
     The turbocharger shown in and described with reference to  FIG. 1  may be used, for example, in a turbocompound engine system.  FIG. 2  schematically depicts an example of a turbocompound engine system. The turbocompound engine system comprises the turbocharger  2  shown in and described with reference to  FIG. 1 . Referring back to  FIG. 2  the turbocharger  2  is connected to an engine  34 . The connection is such that exhaust gas exiting an outlet manifold  38  is used to drive the turbine wheel of the turbine  4  of the turbocharger  2 . Such rotation causes the compressor wheel of the compressor  6  to draw in air  40  and compress it, such that compressed air  41  may be delivered to an engine intake manifold  42 . 
     The turbine  4  is a gas expander. As known in the art, this means that gas expansion within the turbine  4  allows mechanical work to be derived from rotation of the turbine wheel within the turbine. In the case of a turbine  4  provided with a turbine wheel, expansion of the exhaust gas  36  in the turbine  4  causes rotation of the turbine wheel and therefore rotation of the shaft  18 . 
     After causing rotation of the turbine wheel, the exhaust gas  43  leaves the turbine  4 . In some applications, this exhaust gas  43  may then be vented to atmosphere, for example via the exhaust pipe of an automobile. However, in order to increase the efficiency of such an automobile (or other machine incorporating such a turbocharger) the exhaust gas  43  may be passed through another gas expander in order to derive further mechanical work from the exhaust gas  43 . In this example, the exhaust gas  43  is passed into a gas expander in the form of an additional turbine  44 . Since, in this example, the gas expander is a turbine  44 , and power is derived from that turbine  44 , it is common to refer to the turbine  44  as a power turbine. As is the case in the turbocharger described previously, when the exhaust gas  43  enters into the additional turbine  44  the gas  43  expands and causes rotation of a turbine wheel. Rotation of the turbine wheel causes rotation of an additional shaft  46  to which the turbine wheel is connected. Exhaust gas  48  leaving the additional turbine  44  is then vented to atmosphere, for example by way of an exhaust pipe or the like. 
     Rotation of the additional shaft  46  may be used, for example, to transmit power to a hydraulic, electrical or mechanical system (or, generically speaking, a load  50 ). For example, the load may be a crank shaft of the engine  34 . Although it may be possible to transmit power directly from the additional shaft  46  to the load  50 , it is more common to transmit the power via a mechanical gear  52  and an output shaft  54 . The mechanical gear  52  may be used, for example, to ensure that the output shaft  54  is rotated at a desired speed or rate, and/or may be used to ensure that the rotation of the output shaft  54  is in a certain direction. 
       FIG. 2  is a relatively simplified representation of an example of a turbocompound engine system.  FIG. 3  is a perspective-view schematically depicting a more detailed representation of the turbocompound engine system shown in and described with reference to  FIG. 2 . The description of  FIG. 2  and the reference numerals used in  FIG. 2  are equally applicable to the description of  FIG. 3 , and therefore no further description of  FIG. 3  will be undertaken here. 
     As discussed above, the use of mechanical gears has various associated problems and disadvantages. It is desirable to obviate or mitigate such problems and disadvantages. Such problems and disadvantages are not only applicable to the use of mechanical gears in gas expander systems in a turbocompound engine system, but are applicable to gas expander systems in general, for example in a waste heat recovery system comprising such a gas expander system. 
       FIG. 4  schematically depicts another example of a turbocompound engine system. The turbocompound engine system is substantially the same as the turbocompound engine system shown in and described with reference to  FIG. 2 , and therefore like features are given the same reference numerals. The turbocompound engine system of  FIG. 4  does, however, include additional gas flow paths and some additional components. For instance, a portion  56  of the gas  36  leaving the engine outlet manifold  38  of the engine  34  may be directed into the engine inlet manifold  40 . This is known as exhaust gas recirculation. Alternatively or additionally, a portion  58  of exhaust gas  48  leaving the additional turbine  40  may be directed such that it combines with the intake gas  40  (e.g. air) for the compressor  6  of the turbocharger  2 . Again, this is known as exhaust gas recirculation. Exhaust gas recirculation may, for example, improve efficiency of an engine system and/or reduce the level of harmful emissions present in exhaust gases vented to atmosphere turbocompound engine system.  FIG. 4  also shows that an engine cooling circuit  60  may be provided for cooling the engine  34 . Alternatively or additionally, an engine cooling jacket  62  may be provided for cooling the engine  34 . 
     It has already been described that use of a power turbine may be used to improve the efficiency of an engine by using energy present in an exhaust gas to, for example, power a turbine which in turn transmits power to a crank shaft of the engine. Further gains in efficiency can be achieved by taking advantage of exhaust gas recirculation principles. Still further efficiency gains can be obtained by recovering energy from one or more sources of waste heat within or forming part of the engine system, and using energy extracted from the heat source to power a hydraulic, electrical or mechanical system (e.g. the crank shaft of the engine  34 ). A system that recovers energy from a source of wasted heat is known as a waste heat recovery system. A suitable source of heat may be found at one of a number of locations in or around the turbocompound engine system. For example, the source of heat may be one or more of the following: the engine  34 , the cooling jacket for the engine  62 , the cooling circuit for the engine  60 , exhaust gas  36  leaving the engine  34 , exhaust gas  43  leaving the turbine  4 , exhaust gas  48  leaving the additional turbine  44 , compressed gas  41  leaving the compressor  6  and being directed towards the engine  34 , or the heat present in one or more gas recirculation paths  56 ,  58 . 
       FIG. 5  schematically depicts a waste heat recovery system. The waste heat recovery system is configured to recover (e.g. extract) energy from wasted heat generated by one or more of the heat sources described above and to convert this heat (or thermal energy) into mechanical work. 
     Referring to  FIG. 5 , the waste heat recovery system is a closed loop system  64 . The closed loop system  64  is provided with a working fluid (e.g. water, refrigerant, or any other fluid capable of carrying heat). The fluid in the closed loop system  64  is heated by one or more of the heat sources described above, generically denoted by reference numeral  66  in the Figure. The heating may be achieved by, for example, one or more conduits of a closed loop system  64  being in contact with one or more of the heat sources  66 , or any other suitable arrangement. Heating of the working fluid causes the working fluid to expand into a gaseous form (e.g. by boiling, evaporation, vaporisation or the like). The heated fluid  68  in gaseous form is allowed to expand in a gas expander  70 , for example a turbine comprising a rotatable turbine wheel. Expansion of the heated fluid  68  in the gas expander  70  causes movement of a moveable part of the gas expander (for example a turbine wheel, piston, swash-plate, screw-thread arrangement, or the like). Movement of the moveable part of the gas expander  70  in turn causes movement (e.g. rotation) of a shaft  72  to which the moveable part is connected. The shaft  72  may be connected to an output shaft  74  by way of a mechanical gear  76 . The mechanical gear  76  may be used to, for example, ensure that the output shaft  74  moves at the required speed, rate and/or in a desired direction. The output shaft  74  is connected to a load  77 , for example an electrical, hydraulic or mechanical load such as a crank shaft of an engine. 
     A pump  78  forming part of the closed loop system ensures that after expanding in the expander, the working fluid  80  is forced around the closed loop system  64 . The fluid is then condensed by a condenser  82  before it is pumped into thermal contact with the heat source  66 , completing the closed loop system. 
     The closed loop system may work in any one of a number of ways and may operate on the basis of the Carnot, Rankine, or Brayton thermodynamic cycles, or any other suitable thermodynamic cycle. 
     The problems and disadvantages associated with the use of a mechanical gear in a gas expander system (discussed above) are also applicable to the system&#39;s shown in  FIGS. 4 and 5 , since these systems comprise a gas expander provided with a mechanical gear. It is desirable to overcome the problems and disadvantages associated with the use of a mechanical gear in a gas expander system. In particular, it is desirable to overcome the problems associated with the use of a mechanical gear which connects a moveable part of a gas expander to a load. 
     According to embodiments of the present invention, one or more problems or disadvantages associated with the use of a mechanical gear in a gas expander system can be overcome by replacing the mechanical gear with a magnetic gear. 
     By connecting a moveable part (e.g. a turbine wheel, a piston, a swash-plate, or a screw-thread arrangement) of a gas expander (e.g. a turbine, a piston-type, a swash-plate type, or a screw-type expander) to a load via a magnetic gear arrangement and a shaft, one or more problems associated with the use of mechanical gears as used in existing gas expander systems can be obviated or mitigated. For instance, in comparison with mechanical gears, magnetic gears require reduced maintenance and offer improved reliability. Magnetic gears are lubrication free in some embodiments, and in other embodiments require less lubrication than mechanical gears. Magnetic gears are more efficient than conventional mechanical gears. Magnetic gears offer precise peak torque transmission and inherent overload protection (e.g. the magnetic gears are not damaged when an excessive force is applied to the magnetic gear which would, in mechanical gears, cause damage to teeth of the gears). The use of the magnetic gear may allow the physical isolation between an input and an output shaft, and this can be taken advantage of as described in more detail below. Magnetic gears offer inherent anti-jamming transmission and significantly reduce harmful drive-train pulsations. Furthermore, magnetic gears allow for misalignment of for example, input and output shafts of a gas expander system. Finally, magnetic gears have very low acoustic noise and vibration associated when in use, at least in comparison with mechanical gears. 
     Specific embodiments of the present invention will now be described, by way of example only, with reference to  FIGS. 6 to 25 . 
       FIGS. 6 and 7  schematically depict one example of a magnetic gear.  FIG. 6  schematically depicts an end-on view of the magnetic gear, and  FIG. 7  schematically depicts a perspective view of the magnetic gear.  FIGS. 6 and 7  will be referred to in combination. 
     The magnetic gear comprises a first member in the form of an inner rotor  84  and a second member in the form of an outer rotor  86 . Located in-between the inner rotor  84  and outer rotor  86  is a stator  88 . The inner rotor  84 , stator  88  and outer rotor  86  are all shaped like a hollow cylinder (that is, a cylinder with a bore extending through the entire length of the cylinder), and are concentrically arranged with respect to one another. In another example, the inner rotor  84  may not be shaped like a hollow cylinder, but may instead be shaped like a cylinder. 
     In this embodiment, the inner rotor  84  and outer rotor  88  are rotatable, whereas the stator  88  remains fixed in position. One or more bearings or the like (not shown in  FIG. 6  or  7 ) may be located in-between the inner rotor  84  and the stator  88  and/or between the stator  88  and the outer rotor  86 . 
     The inner rotor  84  and the outer rotor  86  are both provided with a plurality of permanent magnets  90  located in, or mounted in, spacing material  91 . The permanent magnets  90  are spaced apart and extend around the inner rotor  84  and the outer rotor  86 , respectively. The permanent magnets  90  also extend along the length of the inner rotor  84  and the outer rotor  86 , respectively. The permanent magnets  90  may be, for example, rare-earth permanent magnets. The stator  88  is provided with a plurality of ferromagnetic pole-pieces  92  located in, or mounted in, spacing material  93 . The pole-pieces  92  are spaced apart and extend around the stator  88 . The pole-pieces  92  also extend along the length of the stator  88 . 
     By providing the magnets and  90  and pole-pieces  92  in one or more particular configurations (e.g. by choosing appropriate numbers, spacings, distributions, and the like, of the magnets  90  and/or pole pieces  92 ) rotation of the inner rotor  84  can be made to cause rotation of the outer rotor  86 . Similarly, rotation of the outer rotor  86  can be made to cause rotation of the inner rotor  84 . The configuration of the permanent magnets  90  and/or pole pieces  92  can be chosen to ensure that there is a specific ratio between the rate of rotation of the inner rotor  84  and outer rotor  86 —i.e. to form a gear. The gear ratio may be any suitable ratio. For a given arrangement or configuration of the permanent magnets  90  and/or pole pieces  92 , the magnetic gear arrangement will have an inherent and fixed gear ratio. 
     Operation of the magnetic gear depends on the modulation of the magnetic fields provided by each of the permanent magnets  90  of the inner and outer rotors  84 ,  86  by the ferromagnetic pole-pieces  92  of the stator  88 . The modulation should result in appropriate harmonics (in the magnetic flux density distribution) with the requisite number of poles as the associated permanent magnetic gear rotor in question. More details of the functionality, operation and design of magnetic gears can be found in, for example, “ Design, analysis and realisation of a high - performance magnetic gear , by K. Atallah, S. D. Calverley and D. Howe, IEE Proc.—Electr. Power Appl., Vol. 151, No. 2, March 2004”, and also in “High-performance magnetic gears, by Kais Atallah, Stuart D. Calverley, David Howe, Journal of Magnetism and Magnetic Materials 272-276 (2004) e1727-e1729” and publications by Magnomatics™. 
       FIG. 8  is a section-view of a gas expander system in accordance with an embodiment of the present invention. The gas expander system comprises a gas expander in the form of a turbine  94  which is connected, via an input shaft  96 , a magnetic year  98 , and an output shaft  100 , to a load  102  in the form of an output gear  102 . 
     The turbine  94  is similar to the turbine discussed above. The turbine  94  comprises an annular fluid inlet  104 . Fluid flowing through the inlet  104  rotates a turbine wheel  106  which is connected to the input shaft  96 . Fluid which has caused rotation of the turbine wheel  106  leaves the turbine via an outlet  108 . 
     The fluid used to rotate the turbine wheel  106  will depend on the application of the gas expander system. In one example, the fluid may be exhaust gas from an engine, or exhaust gas leaving a turbine of a turbocharger or the like. In another example, the fluid may be the working fluid of a closed-loop waste heat recovery system. Such different applications will be discussed in more detail below. 
     The input shaft  96  extends from the turbine wheel  106  and into connection with an inner rotor  84  of the magnetic gear  98 . Rotation of the turbine wheel  106  thus causes rotation of the input shaft  96 , which in turn causes rotation of the inner rotor  84  of the magnetic gear  98 . 
     As discussed above, rotation of the inner rotor  84  of the magnetic gear  98  is will cause rotation of an outer rotor  86  of the magnetic gear  98  (i.e. torque can be transmitted from the inner rotor  84  to the outer rotor  86 ). The outer rotor  86  of the magnetic gear is attached to the output shaft  100 . Thus, rotation of the outer rotor  86  causes rotation of the output shaft  100 , and this allows power to be transmitted to the load in the form of output gear  102 . 
     The stator  88  of the magnetic gear  98  may be located in, or form part of a wall  108 . The wall  108  separates a housing  110  of the turbine  94  from a housing  112  of the magnetic gear  98  and (at least a part of) the output shaft  100 . Such separation may be taken advantage of as will be discussed in more detail below. 
     In addition to the components already described, the gas expander system may also be provided with additional components. Referring back to  FIG. 8 , the gas expander system may be provided with one or more bearings  114  for supporting the output shaft  100 . The gas expander system may also be provided with one or more bearings  116  for supporting the input shaft and/or inner rotor  84  of the magnetic gear  98  to which the input shaft  96  is connected. One or more lubrication inlets  118  and lubrication channels  120  may be provided for lubrication of, for example, the bearings  116  supporting the input shaft  96 . A lubricant drain  122  may also be provided to allow lubricant to leave the gas expander system. 
     The gas expander system of  FIG. 8  may be used in a wide variety of applications, and in particular in a-turbomachine (i.e. a machine provided with a turbocharger). A turbomachine incorporating the gas expander system of  FIG. 8  may be made to operate more efficiently. 
       FIG. 9  schematically depicts one application of the gas expander system shown in and described with reference to  FIG. 8 , in accordance with an embodiment of the present invention. In general,  FIG. 9  schematically depicts substantially the same turbocompound engine system shown in and described with reference to  FIG. 2 . As a consequence of this, like features appearing in  FIG. 9  have been given the same reference numerals as they were given in the description of  FIG. 2 . In stark contrast to the turbocompound engine system shown in and described with reference to  FIG. 2 , however, the turbocompound engine system described with reference to  FIG. 9  is provided with the gas expander system  124  as shown in and described with reference to  FIG. 8 . 
     The gas expander system  124  forms the power turbine of the turbocompound engine system. Exhaust gas  43  leaving the turbine  4  of the turbocharger  2  is used to rotate the turbine wheel of the turbine  94  forming part of the gas expander system  124 . Rotation of the turbine wheel of the turbine  94  causes power to be transmitted to the load  102  via appropriate rotation of the input shaft  96 , rotation of the inner and outer rotors of the magnetic gear  98  and rotation of the output shaft  100 . A significant difference between the turbocompound engine system shown in  FIG. 9  and that shown in and described with reference to  FIG. 2  is that, in  FIG. 9 , the gear  98  connecting the turbine wheel of the turbine  94  to the load  102  is magnetic, and not mechanical, in nature. The turbocompound engine system as shown in and described with reference to  FIG. 9  thus takes advantage of all the benefits associated with the use of a magnetic gear, as opposed to the use of a mechanical gear, as discussed above. 
       FIG. 10  schematically depicts another application of the gas expander system of  FIG. 8 , according to an embodiment of the present invention.  FIG. 10  schematically depicts a waste heat recovery system that is, in general, substantially the same as the waste heat recovery system shown in and described with reference to  FIGS. 5 and 4 . Thus, the description of the waste heat recovery system of  FIG. 5 , described in relation to the turbocompound engine system of  FIG. 4 , is equally applicable to the description of the waste heat recovery system of  FIG. 10 . However, in contrast with the waste heat recovery system shown in  FIG. 5 , the waste heat recovery system of  FIG. 10  is provided with the gas expander system  124  shown in and described with reference to  FIG. 8 . That is, fluid flowing around and expanding within the closed loop system  64  is used to cause rotation of the turbine wheel within the turbine  94  of the gas expander system  124 . As described above, the gas expander system  124  transmits power to the load  102  via an input shaft  96 , a magnetic gear  98  and an output shaft  100 . By using the magnetic gear  98  in the waste heat recovery system, the waste heat recovery system does not have associated with it the disadvantages usually associated with the use of one or more mechanical gears, as discussed above. 
     Referring back to  FIG. 8 , it was described that the stator  88  may be, form part of, or be located within or on a wall  108  which separates the turbine housing  110  from the housing  112  of the magnetic gear  98  and (at least a part of) the output shaft  100 . In some embodiments, this arrangement may not be required. In other embodiments, however, this arrangement may be advantageous.  FIG. 11  schematically depicts, in general, the same waste heat recovery system that is shown in and described with reference to  FIG. 10 . In the embodiments shown in  FIG. 11 , however, the stator  88  of the magnetic gear  94 , which is located in the wall  108 , is shown as forming part of a wall or enclosure that encloses the closed loop system  64 . By locating the stator  88  in a wall  108  which is or forms part of a wall  126  of a closed loop system  64  of the waste heat recovery system, the working fluid of the waste heat recovery system, cannot escape—i.e. there is no opening in the wall  108  through which the working fluid map pass. Despite there being no opening in the wall  108 , due to the fact that magnetic flux can pass through solid objects, power derived from rotation of the turbine wheel of the turbine  94  can be transmitted across the wall  126  of the closed loop system  64  via the use of the magnetic gear  98 . 
       FIG. 12  schematically depicts an expanded view of a part of the system of  FIG. 11 .  FIG. 12  shows the magnetic gear  98  in relation to the wall  126  of the closed loop system of the waste heat recovery system. The location of the stator  88  can be more clearly seen in this expanded view. 
     Although the use of magnetic gears is advantageous, for example in comparison with mechanical gears, the use of magnetic gears may still have associated problems. It is desirable to eliminate or reduce these problems or the effects of these problems. In one example, rotation of the inner and/or outer rotors of the magnetic gear may cause heat to be generated within the magnetic gear (e.g. 500 W). It is desirable to remove this heat from the magnetic gear to, for example, prevent overheating of the magnetic gear and/or undesirable expansion of component parts of the magnetic gear which could cause the magnetic gear to malfunction or cease to operate. 
     According to an embodiment of the present invention, the magnetic gear may be cooled by passing air through/or around component parts (i.e. members) of the magnetic gear. In principle, air could be directed in and/or around the magnetic gear using an air flow that is already present in or around the magnetic gear, for example due to motion of the machine in which the magnetic gear is present. Preferably, however, cooling of the magnetic gear should not be linked to such external criteria (e.g. movement of the machine in which the gear is located). This independence can be achieved by providing one or more rotors of the magnetic gear with an impeller configured to push air in and/or around the members of the magnetic gear. The impeller may be, for example, attached to, or form part of an inner or outer rotor of the magnetic gear. Preferably the magnetic gear and impeller arrangement is configured to ensure that, in general, air is directed away from an axis of rotation of the inner or outer rotor such that heat is not pushed towards the centre of the magnetic gear, but is instead pushed out and away from the magnetic gear. Specific embodiments of the application of the impeller for cooling the magnetic gear will now be described. The same or similar effect can be achieved by providing one or more members of the magnetic gear with apertures or a surface profile which encourages such air flow upon rotation of the respective member. 
       FIG. 13  schematically depicts an end-on view of a magnetic gear  128 . As with the magnetic gears shown in and described with reference to previous Figures, the magnetic gear  128  of  FIG. 13  comprises an inner rotor  130 , a stator  132  and an outer rotor  134 . The inner rotor  130 , stator  132  and outer rotor  134  are arranged concentrically about a common longitudinal axis  136 . The inner rotor  130 , stator  132  and outer rotor  134  all have a hollow-cylinder shape, the cylinders extending along the longitudinal axis  126 . Spaces  138  are formed between the inner rotor  130  and the stator  132 , and also between the stator  132  and the outer rotor  134 . One or more bearings may be located in the spaces  138 . 
       FIG. 14  shows an example of an impeller  140  that may be attached to, or formed as part of one of the rotors of the magnetic gear. It can be seen that the impeller  140  has a substantially annular shape. The impeller may be formed from any suitable material, and may be formed from the same material that forms part of the rotor of the magnetic gear. 
       FIG. 15  shows how the impeller  140  may be located relative to the magnetic gear  128 . The impeller  140  is attached to an end of the inner rotor  130 , and such that the impeller  140  extends radially across a space formed between the inner rotor  130  and the stator  132 . Upon rotation of the inner rotor  130 , the impeller  140  is configured to draw air into the magnetic gear and along and through the space between the inner rotor  130  and the stator  132 . Depending on the arrangement of the magnetic gear  128  and, for example, a housing of the magnetic gear  128 , air may not simply flow along the magnetic gear in the space between the inner rotor  130  and the stator  132  and then leave the gear  128 . Instead, the air may then be redirected back through the magnetic gear in an opposite direction, along and through the space  138  located in-between the stator  132  and the outer rotor  134 . Furthermore, the air may then be redirected again along an outer surface of the outer rotor  134 . If the air is made to flow in this serpentine-like manner, the air will pass along the inner rotor  130 , stator  132  and outer rotor  134 , cooling these members. Furthermore, the air is, in general, flowing radially away from the longitudinal axis  136  of the magnetic gear, thus ensuring that heated air is not pushed towards the centre of the magnetic gear, but is instead pushed out and away from the magnetic gear 
       FIGS. 14 and 15  show an impeller may be used to cause a flow of air in and around the members of the magnetic gear. In other embodiments, one or more members of the magnetic gear may be provided with apertures or a surface profile which encourages such air flow. For example,  FIG. 16  shows how a rotor  142  of the magnetic gear (for example an inner or outer rotor) can be provided with surface profiling in the form of rifling  144 . As with the impeller embodiment described above, rotation of the rotor  142  will cause rotation of the rifling  144  which will, in turn, induce an air flow along the rotor  142  in a direction dependent on the direction of rotation of the rotor  142 . The rifling  144  (or other surface profile) may be provided on an inner or outer surface of the rotor, or on both surfaces. 
     As described above, an advantage of providing one or more rotors of the magnetic gear with, apertures, surface profiling and/or an impeller, is that the magnetic gear provides its own flow of air. A further advantage is that the flow rate of the air increases as the rotation of, for example, the rotor increases. This means that as the rotor gets hotter due to increased rotation, the rate of flow of cooling air also increases to counteract the heating effect. Therefore, not only is the embodiment useful for providing self-sufficient cooling, but the self-sufficient cooling is, at least to some extent, tailored to the speed rotation of members of the magnetic gear and thus the heating of the magnetic gear. 
       FIG. 17  schematically depicts a side-on section view of the magnetic gear and impeller arrangement of  FIG. 14  in-situ relative to an input shaft and output shaft (for example of a gas expander system). The inner rotor  130  of the magnetic gear is shown as being connected to an input shaft  146  via a hub  148 . The impeller  140  is shown as being attached to an end of the inner rotor  130 . The stator  132  and the outer rotor  134  are shown as surrounding, in a concentric manner, the inner rotor  130 , as described above. The outer rotor  134  is attached to an output shaft  150 . As discussed above, in relation to previous embodiments, rotation of the input shaft  146  (for example, by a turbine wheel connect to the input shaft  146 ) causes rotation of the inner rotor  130 . Rotation of the inner rotor  130  causes, in turn, rotation of the outer rotor  134  and the output shaft  150  to which the outer rotor  134  is attached. The input shaft  146 , inner rotor  130 , outer rotor  134  and output shaft  150  all have a common axis of rotation  136 . 
     Rotation of the input shaft  146  causes rotation of the inner rotor  130  and the impeller  140 , which is attached to the inner rotor  130 . Rotation of the impeller  140  causes air  152  to be drawn into the space  138  between the inner rotor  130  and the stator  132 . Air flows along this space  138 . When the air  152  reaches the opposite end of the inner rotor  130 , it impinges against and is directed by a housing  154  of the outer rotor  134 . The air  152  is directed away from the longitudinal axis  136  and, due to the shape of the housing  154 , is then directed between a space  138  in-between the stator  132  and the outer rotor  134 . When the air  152  reaches the opposite end of the outer rotor  134 , it is redirected by at least one of a stator housing  156 , an input shaft housing  158  and/or a magnetic gear housing  160 , such that the air  152  is then directed along an outer surface of the outer rotor  134  and/or outer rotor casing  154  and out through one or more vents  162  provided in the magnetic gear housing  160 . 
     It can be seen from the Figure that the air flow path is serpentine in nature. The path that the air flow  152  takes after being drawn into the magnetic gear by the impeller  140  leads around the members of the magnetic gear before leaving the magnetic gear housing  160 . Furthermore, and in an advantageous manner as described above, the direction of the flow of air  152  is, in general, radially away from the axis of rotation  136  of the members of the magnetic gear. 
       FIG. 18  schematically depicts an end on view of a magnetic gear. The magnetic gear is the same as the magnetic gear shown in and described with reference to  FIG. 6 . Therefore, like features appearing in both Figures have been given like reference numerals.  FIG. 18  shows that if the inner rotor  84  is rotated in a first direction (e.g. clockwise in the Figure) then the outer rotor  86  will rotate in the opposite direction (e.g. anti-clockwise in the Figure). It will be appreciated that if the outer rotor  86  is rotated in one direction (e.g. anti-clockwise in the Figure) the inner rotor  84  will rotate in the opposite direction (e.g. clockwise in the Figure). It will thus be appreciated that rotation of the inner rotor can be transmitted to rotation of the outer rotor (i.e. torque can be transmitted in a first direction), and rotation of the outer rotor can be transmitted to rotation of the inner rotor (i.e. torque can be transmitted in a first direction). In some applications, this is not desirable. 
     In previous embodiments, the inner rotor  84  of the magnetic gear has been described as being attached to or forming part of an input shaft that may be attached to, for example a moveable part of a gas expander. The moveable part may be, for example a turbine wheel of a turbine. Rotation of the turbine wheel will cause rotation of the input shaft, and corresponding rotation of the inner rotor  84 . Rotation of the inner rotor  84  will cause rotation of the outer rotor  86 . As discussed in previous embodiments, the outer rotor  86  may be attached to a load, for example an engine crank shaft, or an output shaft or one more gears or the like. Thus, rotation of the outer rotor  86  can be used to transmit power to the load. This situation is desirable, since exhaust gases which would otherwise be vented to atmosphere can be used to drive the turbine, cause rotation of the members of the magnetic gear, which in turn transmits power to the engine crank shaft. Thus, the efficiency of the machine in incorporating the gas expander is increased, since exhaust gases are no longer simply being vented to atmosphere, but instead being used to provide power to the engine crank shaft. 
     As discussed above, the magnetic gear will be connected to a load. In some circumstances, for example when the load is an engine crank shaft, the load itself may cause rotation of the outer rotor  86  of the magnetic gear. This will cause rotation of the inner rotor  84  of the magnetic gear which will in turn cause rotation of the turbine wheel. This may be described as negative torque. If the flow of exhaust gas about the turbine wheel is not great enough to provide sufficient ‘positive’ torque to overcome the negative torque, the efficiency of the system is decreased. This is because the engine crank shaft will be driving the turbine wheel in some circumstances, rather than the rotation of the turbine wheel driving the crankshaft. It is therefore desirable to provide a de-clutching mechanism which, in some circumstances, prevents rotation of the outer rotor of the magnetic gear causing rotation of the inner rotor of the magnetic gear. In other words, it is therefore desirable to provide a de-clutching mechanism which prevents negative torque transmission. 
     The operation of the magnetic gear shown in  FIG. 18  is similar to that of an epicyclic gear box. In such an epicyclic gear box, by allowing the stator of the gear to freely rotate (or in other words free wheel) no torque can be transmitted from the inner rotor to the outer rotor, or from the outer rotor to the inner rotor. According to an embodiment of the present invention, by allowing the stator  88  of the magnetic gear to freely rotate (or in other words free wheel) no torque can be transmitted from the inner rotor  84  to the outer rotor  86 , or from the outer rotor  86  to the inner rotor  84 . 
     It will be understood that, in normal terminology, a stator is a component that does not move. The stator does not move in many of the embodiments described herein. However, in order to keep the description of the Figures consistent, the stator is sometimes described herein as being moveable. When the stator is prevented from moving, it once again serves as a stator in the true sense of the word. In this context, the stator may thus be generically described as a magnetic gear member located in-between a first (e.g. inner) rotor of a magnetic gear and a second (e.g. outer) rotor of a magnetic gear. This magnetic gear member may be moveable or be fixed in position. However, for the remainder of this description, the term ‘stator’ will be used for consistency and to avoid confusion, since a magnetic gear member located in-between a first (e.g. inner) rotor of a magnetic gear and a second (e.g. outer) rotor of a magnetic gear is commonly referred to as a stator. 
       FIG. 19  shows the same magnetic gear that was shown in and described with  FIGS. 18 and 6 . However, in  FIG. 19 , the stator  88  is no longer being maintained in a fixed position, but is allowed to rotate freely.  FIG. 19  shows that when the stator  88  is allowed to freely rotate, rotation of the outer rotor  86  causes rotation of the stator  88 , but does not cause rotation of the inner rotor  84 .  FIG. 20  shows the same magnetic gear. However, in  FIG. 20 , the inner rotor  84  is shown as rotating, and causing rotation of the freely rotatable stator  88 . Again, no torque is transmitted to the outer rotor  86 . 
     The stator may be allowed to rotate around or about one or more bearings. When it is desirable to prevent torque transmission between the inner rotor  84  and the outer rotor  86 , the stator  88  can be allowed to freely rotate by, for example, appropriate engagement or disengagement of an actuator or the like. The actuator may lock into position in the stator  88  to prevent rotation of the stator  88  when such rotation is not required. 
     An engine management system or the like may detect when torque transmission is ‘positive’ (i.e. from the turbine to the engine crank shaft) or ‘negative’ (from the engine crank shaft to the turbine wheel). When negative transmission of torque is detected, the stator  88  may be put into a free-wheeling state. 
     In some embodiments, detection and active actuation of the free-wheeling state of the stator may not be required. For instance, any commonly known freewheeling mechanism may be applied to the magnetic gear arrangement to prevent negative torque transmission. One freewheel mechanism consists of two saw-toothed, spring-loaded discs pressing against each other with the toothed sides together, somewhat like a ratchet. Rotating in one direction, the saw teeth of the drive disc lock with the teeth of the driven disc, making it rotate at the same speed. If the drive disc slows down or stops rotating, the teeth of the driven disc slip over the drive disc teeth and continue rotating. The discs may form part of, or be attached to, appropriate parts of the magnetic gear such as the stator and outer rotor. Alternatively, the teeth may form part of the stator and outer rotor. The stator may be allowed to freewheel only in one direction, for example when the torque transmission is negative and the teeth engage with one another. 
     Other arrangements for preventing torque transmission between the inner and outer rotor of a magnetic gear are also possible. Such arrangements will now be discussed in relation to  FIGS. 21 to 23 . 
       FIG. 21  schematically depicts, in an end-on view, a magnetic gear in accordance with an embodiment of the present invention. As described above in previous embodiments, the magnetic gear comprises an inner rotor  164 . Surrounding the inner rotor  164  is a stator  166 . Surrounding the stator  166  and the inner rotor  164  is an outer rotor  168 . The inner rotor  164 , stator  166  and outer rotor  168  all have a hollow cylinder shape and are arranged concentrically with respect to one another. The inner rotor  164  and outer rotor  168  comprise, as discussed above, permanent magnets  170 . In a similar manner to the stator as described above, the stator  166  of the magnetic gear of  FIG. 21  is provided with ferromagnetic pole-pieces  172 . 
     In contrast with the magnetic gears discussed above, where, in terms of its overall shape, the stator forms a single-piece hollow cylinder, the stator  166  of the magnetic gear of  FIG. 21  comprises two concentrically arranged stator members: an inner stator member  174  and an outer stator member  176 . At least one of the inner stator member  174  and outer stator member  176  is selectively moveable relative to the other stator member. 
     The inner stator member  174  and outer stator member  176  are each provided with a plurality of pole-pieces  172  which are equally spaced apart and extend around each respective stator member  174 ,  176 . The pole-pieces  172  of the inner stator member  174  and outer stator member  176  are arranged such that the pole-pieces  172  of the inner stator member  174  can be brought into or moved out of alignment with the pole-pieces  172  of the outer stator member  176 . 
       FIG. 21  shows the situation wherein the pole-pieces  172  of the inner stator member  174  are in direct alignment with the pole-pieces  172  of the outer stator  176 . In this situation, the inner stator member  174  and outer stator member  176  form and function as a single stator member, for example the single stator discussed above in relation to previous embodiments. When the inner stator member  174  and outer stator member  176  are in the configuration shown in  FIG. 21 , and when both the inner stator member  174  and outer stator member  176  are prevented from rotating, the magnetic gear shown in  FIG. 21  functions in the same way as shown in and described above in relation to previous embodiments. For instance, torque may be transmitted from the outer rotor  168  to the inner rotor  164 , or from the inner rotor  164  to the outer rotor  168 . 
       FIG. 22  shows a situation wherein the inner stator member  174  has been rotated by an angle θ relative to its position in  FIG. 21  and relative to the outer stator  176 . The angle θ is chosen such that the pole-pieces  172  of the inner stator member  174  are no longer in direct alignment with the pole-pieces  172  of the outer stator member  176 . The angle θ is chosen such that the pole-pieces  172  of the inner stator member  174  align with spaces in-between the pole-pieces  172  of the outer stator member  176 . 
     As discussed above, rotation of an inner rotor of a magnetic gear can only cause rotation of an outer rotor of a magnetic gear if the magnetic flux density or densities have a specific configuration which enables such rotation to be transmitted between the rotors. When the stator  166  of the magnetic gear has the configuration shown in  FIG. 22 , magnetic flux from the inner rotor  164  cannot be transmitted to the outer rotor  168 , and magnetic flux from the outer rotor  168  cannot be transmitted to the inner rotor  164 . Thus, when the stator  166  is in the configuration shown in  FIG. 22 , the rotors  164  and  168  are de-clutched from one another. 
     Movement of one or both of the inner and outer stator members  174 ,  176  may be achieved in any one of a number of ways. Preferably, only one of the stator members  174 ,  176  is moveable to limit the complexity and costs of any arrangement used to move one of the stator members and to reduce the chances of malfunction in the arrangement, and also to reduce the complexity, costs and chances of malfunction of the stator  166  itself. One or both of the stator members  174 ,  176  may be moved by, for example, a motor. Preferably this may be accomplished using a non-backdrive mechanism such as a lead-screw, so that energy only needs to be used to move one of the stator members  174 ,  176  and not to keep one or both of the stator members  174 ,  176  in a desired position. 
     Another approach to controlling the arrangement of the inner and/or outer stator members  174 ,  176  would be to allow only one of the stator members  174 ,  176  to be freely moveable. The freely moveable stator member may be arranged such that it is selectively rotated in a given direction by the magnetic forces present within the magnetic gear, and in a direction in which torque was being transmitted through the magnetic gear. For example, the inner rotor  174  could be rotatably mounted. The rotatable mounting might allow rotation of the inner stator member  174  in only a certain direction, for example by providing a stop or the like which prevents the inner stator member  174  from rotating in the opposite direction. The direction in which rotation of the inner stator member  174  is allowable may correspond to the direction in which the stator member would move if the torque transmission through the gear was negative, i.e. from the outer rotor  168  to the inner rotor  164 . At the point at which negative torque transmission occurs, the inner stator  174  member would (in a passive manner, i.e. without any active input) be dragged into a position shown in and described with reference- to  FIG. 22 , thereby preventing torque being transmitted from the outer rotor  168  to the inner rotor  164 , automatically de-clutching the magnetic gear. 
       FIG. 23  depicts another embodiment of a magnetic gear in accordance with an embodiment of the present invention. The magnetic gear comprises the same constituent parts as shown in and described with reference to  FIGS. 6 and 7 , and therefore the like features appearing in  FIGS. 6 and 7  have been given the same reference numerals in  FIG. 23 . In summary, the magnetic gear of  FIG. 23  comprises of an inner rotor  84 , a stator  88  which surrounds the inner rotor  84  and an outer rotor  86  which surrounds both the stator  88  and the inner rotor  84 . 
     Referring back to  FIGS. 6 and 7 , the stator  88  was described as being fixed in position. Referring back to  FIG. 23 , in contrast the stator  88  is now actively moveable in an axial direction as indicated by the arrows in the Figure. When the stator  88  is located substantially between the inner rotor  84  and the outer rotor  86 , torque transmission is possible between the inner rotor  84  and the outer rotor  86 . This is due to the appropriate modulation of the magnetic flux density by the pole-pieces of the stator. However, when the stator  88  is substantially and axially withdrawn from its location in-between the inner rotor  84  and outer rotor  86 , the magnetic flux density is no longer modulated to allow torque to be transmitted between the inner rotor  84  and the outer rotor  86 . Thus, by substantially and axially withdrawing the stator  88 , a de-clutching mechanism is achieved. 
     The stator  88  may be axially moved in any one of a number of ways. For instance, one or more actuators may be used to selectively push and/or pull the stator into or out of a location in-between the inner rotor  84  and the outer rotor  86 . 
     The embodiments discussed above in relation to  FIGS. 18 to 23  have shown how torque transmission can be prevented between the inner and outer rotors of a magnetic gear. All of the embodiments have advantages, as described in relation to each respective embodiment. However, it is envisaged that the embodiment shown in and described with reference to  FIGS. 18-20  may be preferable. This is because this embodiment, which relies on allowing the stator to be freely rotatable, does not require axial movement of the stator (which might require more space within a machine in which the magnetic gear is used), and does not require the stator to be formed from two separate parts (which can be more expensive and lead to complications in terms of construction, maintenance and operation of the magnetic gear). A further possible advantage associated with the free-wheel stator embodiment shown in and described with reference to  FIGS. 18 to 20  is that the free-wheeling stator can be used to drive a generator or dynamo (or any other electricity generation arrangement), such that the electrical energy may be extracted from rotation of the free-wheeling stator. 
     All of the embodiments described above have shown a magnetic gear in which the outer rotor, stator and inner rotor are concentrically arranged about a common rotational axis. In those embodiments, the outer rotor at least partially surrounds the stator, which at least partially surrounds the inner rotor.  FIG. 24  schematically depicts this arrangement of the magnetic gear.  FIG. 24  schematically depicts a magnetic gear system comprising an input shaft  180  attached to an inner rotor  182 . At least partially surrounding the inner rotor  182  is a stator  184 . At least partially surrounding the stator  184  is an outer rotor  186 . The outer rotor  186  is attached to an output shaft  188 . Although the magnetic gear arrangement shown in  FIG. 24  has been the magnetic gear arrangement used to describe the embodiments of the invention included above, it is not the only magnetic gear arrangement. 
       FIG. 25  depicts another magnetic gear arrangement. The magnetic gear arrangement comprises an input shaft  190  attached to a first rotor  192 . Disposed between the first rotor  192  and a second rotor  194  is a stator  196 . The second rotor  194  is attached to an output shaft  198 . The first rotor  192 , stator  196  and second rotor  194  are all substantially planar, and in this example are substantially disc-shaped. The embodiments discussed above in relation to the magnetic gear arrangement having the configuration shown in  FIG. 24  are applicable to the magnetic gear arrangement shown in and described with reference to  FIG. 25 . However, it will be appreciated that, in some embodiments, slight modifications may need to be made to the arrangement shown in  FIG. 25  to implement embodiments of the present invention discussed above in relation to the arrangement shown in and described with reference to  FIG. 24 . 
     With regard to the embodiments shown in and described with reference to  FIGS. 9 and 10 , the magnetic gear arrangement shown in  FIG. 25  could simply be replace the arrangement akin to that shown in  FIG. 24 . 
     With regard to the embodiment shown in  FIG. 11 , the arrangement shown in  FIG. 25  could be implement therein by ensuring that the first rotor  192  and second rotor  194  are separated from each other by a wall of a the closed loop system, the stator  196  being part of or being attached to that wall. 
       FIGS. 13 to 16  described how an impeller could be attached to a rotor of a magnetic gear arrangement as shown in and described with reference to  FIG. 24  to cool the magnetic gear arrangement. Such cooling could be implemented with the magnetic gear arrangement shown in and described with reference  FIG. 25  by, for example, attaching an impeller to the periphery of the first rotor  192  which is arranged to direct air in-between the first rotor  192  and the stator  196 . An additional impeller could be added to the second rotor  194  to, similarly, direct air in-between a second rotor  194  and the stator  196 . Rifling or the like could be provided on one or more surfaces of the magnetic gear arrangement shown in  FIG. 25  to either draw air toward or away from parts of the arrangement. 
     The free-wheeling stator embodiments shown in and described with reference to  FIGS. 18-20  can be implemented using the magnetic gear arrangement shown in  FIG. 25  by the selectively allowing or preventing the stator  196  from rotating. 
     With regard to the two stator member stator member arrangements shown in and described with reference to  FIGS. 21 and 22 , this may be implemented using the magnetic gear arrangement shown in the  FIG. 25  by forming the disc-like stator from two concentric disk-like stator parts or members. Rotation of one or both of the disc stator members can be used to move pole-pieces of the disc-like members into or out of alignment with one another to selectively allow or prevent magnetic flux being transmitted from the first rotor  192  to the second rotor  194 , and/or from the second rotor  194  to the first rotor  192 . 
       FIG. 23  describes axial movement of the stator to prevent flux transmission between the inner rotor and outer rotor of the magnetic gear arrangement. This same effect may be implemented using the magnetic gear arrangement shown in  FIG. 25  by radial movement of the stator  196  such that the stator  196  is substantially moved from its location between the first rotor  192  and second rotor  194 . 
     In the above-mentioned embodiments, the gear ratio of the magnetic gear arrangement has a fixed value that is related to the arrangement of pole-pieces and permanent magnets in the magnetic gear arrangement. In some embodiments, however, it can be desirable to be able to vary the gear ratio, or to provide what is known in the art as a continuously variable transmission (CVT). 
     According to an embodiment of the present invention, an arrangement for varying the gear ratio of a magnetic gear arrangement is provided. The arrangement comprises a first rotor connectable to an input shaft, and a second rotor connected to an output shaft. A magnetic gear member (which may be intermediate to the first and second rotors, or surround the first and second rotors), is drivable at a certain speed by a driving arrangement. The speed of rotation of the magnetic gear member affects the magnetic gear ratio of the magnetic gear arrangement as a whole. The speed and/or direction of rotation of the magnetic gear member (for example, ‘the stator’ as described above) can be used to vary the gear ratio of the magnetic gear arrangement about a nominal gear ratio which would be established if the magnetic gear member was not rotated. Rotation of the magnetic gear member may be achieved by, for example, using a motor to drive the rotation of the magnetic gear member. The motor may be connected to the magnetic gear member by an abutment member that abuts against the magnetic gear member. For example, the abutment member could be a cog or gear or the like. 
     When the magnetic gear member is arranged to be rotated by the driving arrangement, the magnetic gear arrangement will have a second (e.g. varied) gear ratio that is related to an initial (e.g. first or inherent) gear ratio by the following equation: 
       [ S   mm   −S   fr   ]/R=S   sr    
     where S mm  is the speed of rotation of the magnetic gear member, S fr  is the speed of the rotation of the first rotor, S sr  is the speed of the rotation of the second rotor, and R is the first, inherent gear ratio of the magnetic gear arrangement. 
     It will be appreciated from the above-mentioned equation that the gear ratio can be varied about the first, inherent gear ratio of the magnetic gear arrangement by control of the speed of rotation of the magnetic gear member. 
       FIG. 26  schematically depicts a magnetic gear arrangement. The magnetic gear arrangement comprises a first shaft  200 . The first shaft  200  is attached to a first rotor  202  of the magnetic gear arrangement. The first rotor  202  has a cylinder like shape, as discussed above in relation to previous embodiments. Surrounding the first rotor  202  is a magnetic gear member  204 . The magnetic gear member  204  has a cylinder like shape, as discussed above in relation to previous embodiments. Surrounding both the first rotor  202  and the magnetic gear member  204  is a second rotor  206  which is connected to a second shaft  208 . The second rotor  206  has a cylinder like shape, as discussed above in relation to previous embodiments. A driving arrangement  210  is also provided. The driving arrangement  210  comprises a motor  212  and a gear  214  which abuts against the magnetic gear member  204  and is arranged to rotate the magnetic gear member  204  upon activation of the motor  212 . 
     The inner rotor  202  and outer rotor  206  are provided with a plurality of permanent magnets. The magnetic gear member  204  is provided with a plurality of pole-pieces. The configuration of the permanent magnets and pole-pieces is such that the magnetic gear arrangement has an inherent gear ratio when the magnetic gear member  204  is not rotated. This means that there is an inherent ratio between the speed of rotation of the first shaft  200  and first rotor  202 , and the resulting speed of rotation of the second rotor  206  and second shaft  208 . If the magnetic gear member  204  is fixed in position and does not rotate, this gear ratio is fixed. If, however, the driving arrangement  210  is used to drive rotation of the magnetic gear member  204  the gear ratio is changed, and this change depends on the speed of rotation of the magnetic gear member as described above. 
       FIG. 27  shows another embodiment of the magnetic gear arrangement. The magnetic gear arrangement comprises a first shaft  220 . Attached to the first shaft is a first rotor  222 . The first rotor  222  is substantially disc-shaped. A second rotor  224  is provided that is also substantially disc-shaped. The second rotor  224  is attached to a second shaft  226 . Disposed between the first and second rotors  222 ,  224  is a disc-shaped magnetic gear member  228 . The magnetic gear arrangement is provided with a driving arrangement  230 . 
     The driving arrangement  230  comprises a motor  232  and a gear  234  which abuts against a peripheral surface of the magnetic gear member  228  and is arranged to rotate the magnetic gear member  228  upon activation of the motor  232 . 
     The first rotor  222  and second rotor  224  are provided with a plurality of permanent magnets. The magnetic gear member  228  is provided with a plurality of pole-pieces. The configuration of the permanent magnets and pole-pieces is such that the magnetic gear arrangement has an inherent gear ratio. This means that there is a specific ratio between the speed of rotation of the first rotor  222  and second rotor  224 . If the magnetic gear member  228  is fixed in position and does not rotate, this gear ratio is fixed. If, however, the driving arrangement  230  is used to drive rotation of the magnetic gear member  228 , the gear ratio can be varied. The variation in the gear ratio depends on the speed of rotation of the magnetic gear member  228 , as discussed above. 
       FIG. 28  schematically depicts a perspective section-view of a gas expander system according to an embodiment of the present invention. The gas expander system of  FIG. 28  incorporates the magnetic gear arrangement  240  shown in and described with reference to  FIG. 26 . 
     The gas expander system comprises a turbine  242 . The turbine is provided with an inlet  244  and an outlet  246 . Fluid flowing through the inlet  244  passes and causes rotation of a turbine wheel  248 . The turbine wheel  248  is attached to the first shaft  200 . The first shaft  200  is supported by bearings  250 . 
     The first shaft  200  extends towards and is attached to the first rotor  202 . Surrounding the first rotor  202  is the magnetic gear member  204 . A lip  252  of the magnetic gear member  204  (or a housing of the magnetic gear member  204 ) engages with the gear  214  of the driving arrangement  210 . Surrounding the magnetic gear member  204  is the second rotor  206 , which is attached to the second shaft  208 . The second shaft  208  is supported by bearings  254 . 
     As discussed above, rotation of the turbine wheel  248  can effect rotation of the second shaft  208  and allow power to be transmitted to a load  256  to which the second shaft  208  is connected. 
     Rather than taking advantage of the inherent gear ratio of the magnetic gear arrangement, it may be desirable to vary the gear ratio as described above. This may be achieved by causing rotation of the magnetic gear member  204  using the driving arrangement  210 . The driving arrangement  210  comprises a motor  212  and a gear  214  which abuts against a peripheral lip  252  or surface of the magnetic gear member  204  (or a housing of that member) and is arranged to rotate the magnetic gear member  204  upon activation of the motor  212 . 
       FIG. 29  schematically depicts a perspective section-view of a gas expander system according to another embodiment of the present invention. The gas expander system of  FIG. 29  incorporates the magnetic gear arrangement  260  shown in and described with reference to  FIG. 27 . 
     The gas expander system comprises a turbine  262 . The turbine is provided with an inlet  264  and an outlet  266 . Fluid flowing through the inlet  264  passes and causes rotation of a turbine wheel  268 . The turbine wheel  268  is attached to the first shaft  222 . The first shaft  222  is supported by bearings  270 . 
     The first shaft  222  extends towards and is attached to the first rotor  222 . The second rotor  224  is attached to the second shaft  226 . The second shaft  208  is supported by bearings  272 . Disposed in-between the first rotor  222  and the second rotor  224  is the magnetic gear member  228 . As discussed above, rotation of the turbine wheel  268  can effect rotation of the second shaft  226  and allow power to be transmitted to a load  274  to which the second shaft  226  is connected. 
     Rather than taking advantage of the inherent gear ratio of the magnetic gear arrangement, it may be desirable to vary the gear ratio as described above. 
     This may be achieved by causing rotation of the magnetic gear member  228  using the driving arrangement  230 . The driving arrangement  230  comprises a motor  232  and a gear  234  which abuts against a peripheral surface or lip of the magnetic gear member  228  (or a housing of that member) and is arranged to rotate the magnetic gear member  228  upon activation of the motor  232 . 
     In the above described embodiments, the gas expander system has been described in relation to a waste heat recovery system and a power turbine. In the case of the waste heat recovery system, the load to which power was transmitted was described as being, for example, a hydraulic or mechanical or electrical system. In the case of the power turbine, the load to which power was transmitted was described as being, for example, an engine crankshaft or the like. These are only one of many examples of the use of the gas expander system of the present invention. In another example, the gas expander system may form part of a turbocharger, the load to which power is transmitted being a compressor wheel of that turbocharger. The magnetic gear arrangement, when used in the turbocharger, may allow physical isolation between the shafts that are respectively attached to a compressor wheel and a turbine wheel is attached. This, as described above, may allow for the gear ratio to be varied, such that the ratio on the speeds of rotation of the turbine wheel and compressor wheel can be set to be at a particular (e.g. desired) value. This may be advantageous when, for example, the compressor wheel and turbine wheel have optimum speeds of rotation which are not achievable using a fixed gear ratio. 
       FIG. 30  schematically depicts a side-on section view of a turbocharger provided with a magnetic gear arrangement. The turbocharger is provided with a turbine wheel  280 . The turbine wheel  280  is attached to a first (e.g. input) shaft  282 . The first shaft  282  extends into a bearing housing  284 , where the first shaft is supported by bearings  286 . The first shaft  282  is attached to a first (inner) rotor  288  of the magnetic gear arrangement. Surrounding the first (inner) rotor  288  is a stator  290  (or in other words, a magnetic gear member) of the magnetic gear arrangement. Surrounding the stator  290  is a second (outer) rotor  292  of the magnetic gear arrangement. The second (outer) rotor  292  is attached to a second (e.g. output) shaft  294 . The second shaft  294  is supported by bearings  296  in the bearing housing  284 . An end of the second shaft  294  remote from the second rotor  292  is attached to a compressor wheel  298 . 
     Rotation of the turbine wheel  280  causes rotation of the compressor wheel,  298 , via appropriate interaction between members of the magnetic gear arrangement as described above. 
     The turbocharger of  FIG. 30  may be modified to include one or more of the gas expander embodiments described above. 
     The embodiments described above have been described in relation to the use of a magnetic gear and a gas expander in a gas expander system. The gas expander system may be used in any suitable environment, for example, in a waste heat recovery system, or a system in which it is desired to extract energy from a flowing and expanding fluid. The embodiments described above, however, are particularly applicable to turbocharger systems (i.e. a system comprising turbocharger) comprising such a gas expander, and in particular those systems that comprise or form part of internal combustion engine systems or motor vehicles. This is because, as discussed above, the use of a magnetic gear in a gas expander system offers improvements in reliability, maintenance, performance and efficiency. These improvements are particularly advantageous in the field of turbocharger systems, and in particular turbocharged motor vehicles and internal combustion engine systems, where there is a constant drive to improve reliability, performance and efficiency and reduce maintenance requirements. 
     It will be appreciated that a wide range of modifications and alterations may be made to the embodiments of the invention described hereinbefore without departing from the scope of the invention as defined by the claims that follow.