Stirling cycle machine

Abstract

In one aspect the invention provides a Stirling cycle machine including a chamber containing a working fluid and defining a terminal end displaced from a thermal transfer zone, and a displacer shuttle free to reciprocate within the chamber towards and away from the terminal end of the chamber. Also provided is a drive piston with an end linked to a mechanical or electrical drive configured to reciprocate the drive piston towards and away from the terminal end of the chamber, the displacer shuttle being at least partly located inside the drive piston with the drive piston being free to move axially around and relative to the displacer shuttle. At least one switch magnet is engaged with the drive piston whereby motion of a magnet mounted to the displacer shuttle through the switch magnet forces the displacer shuttle to move in the opposite direction of travel to the drive piston.

Description

[0001] STIRLING CYCLE MACHINE

[0002] Technical Field

[0003] This invention relates to a Stirling cycle machine. In various implementations this machine may be configured to implement a mechanical drive system when provided access to a source of thermal energy, or may implement a heating or cooling system when connected to a mechanical or electrical drive system.

[0004] Background art

[0005] Electric motors and combustion engines are commonly used to generate kinetic energy as part of mechanical drive systems. These mechanical drive systems are used in a wide range of applications including the operation of pumps in thermal transfer applications such as refrigerators, freezers or heat pumps. It is very common for electrical motors to drive gas compressors used in heat pump or refrigeration systems where the gas used is a known greenhouse gas.

[0006] Stirling cycle machines, engines or systems are well known alternatives to electrical or combustion engines and can be operated without the use of a greenhouse gas. A Stirling cycle machine can be implemented using a variety of different fluids or gases transported between a hot region and a cold region under the action of a driving piston reciprocating within a closed chamber. A Stirling cycle machine can function as a heat pump to cool or heat a region when connected to a mechanical or electrical drive system used to reciprocate its driving piston. A Stirling cycle machine may alternatively form a mechanical drive system when a temperature differential between its hot and cold regions is used to generate reciprocal motion in its drive piston.

[0007] Commonly Stirling engines and heat pumps rely on the use of a return spring in the operation of the reciprocating drive piston which compresses and expands the working fluid contained in its chamber. A return spring is commonly used to conserve and recycle kinetic energy as the direction of motion of the drive piston changes during an entire cycle of reciprocal movement.

[0008] These systems will operate at maximum efficiency when the speed or reciprocation frequency of the piston is at resonance with the resonant frequency of the return spring and respective assembly. This design constraint in practice limits existing Stirling engines and heat pumps to fixed and limited bands of operating speeds dictated by the geometry of the components used and in particular the construction of any return spring employed.

[0009] Such prior art systems cannot be reconfigured with different operating parameters as they consist of tuned systems of masses and springs designed for a particular geometry of reciprocation chamber and associated hot and cold regions. In particular the use of a mechanical piston return spring dictates the effective geometry of the hot and cold regions which may be utilised or serviced through its operation. A miniaturised Stirling cycle machine which needs to engage with a very small heating or cooling region may not be practical when considering the most efficient drive piston speed required by its return spring. Alternatively a Stirling cycle machine using or servicing relatively large hot and cold regions may require a return spring which dictates an impractically slow drive piston reciprocation rate.

[0010] It would be of advantage to have improvements to a Stirling cycle machine which addressed any or all of the above referenced issues, or at least provided the public with an alternative choice to the prior art.

[0011] In particular, it would be of advantage to have a Stirling cycle machine which could operate without significantly reduced efficiency over a wide range of operating speeds or frequencies.

[0012] An improved machine which could have its dimensions scaled up and down as desired without impacting operational efficiency or limiting operational speeds would also be of advantage.

[0013] An improved lightweight machine which avoided the need to use greenhouse gas would be of advantage, as would improvements which avoided the need for complex control systems or large numbers of moving parts. Disclosure of Invention

[0014] According to one aspect of the present invention there is provided a Stirling cycle machine which includes a chamber arranged to contain a working fluid and defining a terminal end displaced from a thermal transfer zone, and a displacer shuttle free to reciprocate within the chamber towards and away from the terminal end of the chamber, and a drive piston with an end linked to a mechanical or electrical drive configured to reciprocate the drive piston towards and away from the terminal end of the chamber, the displacer shuttle being at least partly located inside the drive piston with the drive piston being free to move axially around and relative to the displacer shuttle, and at least one switch magnet engaged with the drive piston whereby motion of a magnet mounted to the displacer shuttle through the switch magnet forces the displacer shuttle to move in the opposite direction to the direction of travel of the drive piston.

[0015] According to another aspect of the present invention there is provided a Stirling cycle machine which includes a chamber arranged to contain a working fluid and defining a terminal end displaced from a thermal transfer zone, and a displacer shuttle free to reciprocate within the chamber towards and away from the terminal end of the chamber, and a drive piston at least partly located inside the displacer shuttle with an end of the drive piston linked to a mechanical or electrical drive configured to reciprocate the drive piston towards and away from the terminal end of the chamber, the drive piston being free to move axially within and relative to the displacer shuttle, and at least one switch magnet engaged with the displacer shuttle whereby motion of a magnet mounted to the drive piston through the switch magnet forces the displacer shuttle to move in the opposite direction to the direction of travel of the drive piston.

[0016] According to a further aspect of the present invention there is provided a Stirling cycle machine substantially as described above which includes at least one braking magnet engaged with the chamber and at least one braking magnet engaged with the displacer shuttle, whereby motion of braking magnets towards and in proximity to each other applies a force which slows the motion of the displacer shuttle relative to the chamber. According to another aspect of the present invention there is provided a Stirling cycle machine substantially as described above which includes at least one stop magnet engaged with the drive piston and at least one control magnet engaged with the displacer shuttle, whereby motion of said at least one stop magnet towards and in proximity to said at least one control magnet applies a force which slows the motion of the displacer shuttle relative to the drive piston.

[0017] According to an alternative aspect of the present invention there is provided a Stirling cycle machine substantially as described above which includes at least one stop magnet engaged with the displacer shuttle and at least one control magnet engaged with the drive piston, whereby motion of said at least one stop magnet towards and in proximity to said at least one control magnet applies a force which slows the motion of the displacer shuttle relative to the drive piston.

[0018] In some embodiments a control magnet engaged with the drive piston moves through the switch magnet to apply a force which moves the displacer shuttle in the opposite direction to the direction of travel of the drive piston. In other preferred embodiments a control magnet engaged with the displacer shuttle moves through the switch magnet to apply a force which moves the displacer shuttle in the opposite direction to the direction of travel of the drive piston.

[0019] Preferably the Stirling machine includes a regenerator passage connecting the thermal transfer zone and the terminal end of the chamber.

[0020] Preferably the motion of the displacer shuttle towards the terminal end of the chamber acts to move working fluid away from the terminal end, through the regenerator passage, towards the thermal transfer zone and compresses the fluid.

[0021] Preferably the motion of the displacer shuttle away from the terminal end of the chamber acts to move working fluid away from the thermal transfer zone, through the regenerator passage, towards the terminal end and expands the fluid.

[0022] The present invention is arranged to provide a Stirling cycle machine. Those skilled in the art will appreciate that this machine may be configured or designed to perform a variety of roles. Any available heat source can be used to generate mechanical work and provide a drive apparatus of various forms. Alternatively, the machine may be configured as a heat pump when connected to a powered mechanical or electrical drive system.

[0023] Reference will be made throughout the specification to the invention providing a Stirling cycle machine. Those skilled appreciate that this reference encompasses the variety of applications and roles which the machine may be configured for use in.

[0024] The machine provided by the invention has a chamber used to contain a fixed amount of working fluid or gas. The interior of the chamber is sealed against loss of the working fluid which travels between a terminal end of the chamber and a thermal transfer zone defined by the chamber and the piston. In this respect the machine employs well known characteristics of existing Stirling cycle machines.

[0025] The machine provided by the invention includes a displacer shuttle located inside the chamber. The displacer shuttle is free to move within the hollow interior of the chamber and in particular to execute a reciprocal motion towards and away from the chamber's terminal end.

[0026] Preferably at least a portion or section of the displacer shuttle has a complimentary bore, profile or shape to the interior of the volume defined by the chamber. These complimentary shapes facilitate the motion of the shuttle. In further preferred embodiments a portion of the displacer shuttle may have a restricted diameter at its end adjacent to the drive piston, which this restricted diameter end extends into the drive piston as discussed further below.

[0027] Preferably the displacer shuttle is located centrally and concentrically within the chamber and reciprocates along the same longitudinal axis as the chamber.

[0028] This reciprocal motion executed by the displacer shuttle functions to operate the Stirling cycle machine. The movement of the displacer shuttle towards the terminal end of the chamber moves the working fluid, through the regenerator passage, towards the thermal transfer zone and compresses the fluid. The movement of the displacer shuttle away from the terminal end of the chamber draws working fluid away from the thermal transfer zone, through the regenerator passage and expands the fluid at the terminal end. The Stirling cycle machine provided by the invention includes a drive piston. In some embodiments the drive piston is at least partly located inside the displacer shuttle. In other preferred embodiments at least a portion of the displacer shuttle is located inside the drive piston. In these embodiments the displacer shuttle may be configured with a restricted diameter at its end adjacent to the drive piston. This restricted diameter portion of the displacer shuttle may extend into the drive piston and can include a magnet which moves through a switch magnet mounted to the drive piston.

[0029] Preferably the drive piston is located centrally and concentrically or axially aligned with the longitudinal axis of the displacer shuttle, and also preferably the longitudinal axis of the chamber.

[0030] Reference throughout this specification will also be made to a portion of the displacer shuttle piston being located within the drive piston. However those skilled in the art should also appreciate that an alternative embodiment is also within the scope of the invention where the displacer shuttle is formed so as to surround at least part of the drive piston. These two potential implementations both exhibit the same mode of operation, irrespective of whether a portion of the shuttle is located inside the piston, or if a portion of the piston is located inside the shuttle. In either instance the magnets used in the operation of the invention can be mounted to appropriate parts of the invention so that when a magnet moves through the switch magnet it forces either

[0031] • the displacer shuttle to move in the opposite direction to the direction of travel of the drive piston, or

[0032] • the piston to move in the opposite direction to the direction of travel of the displacer shuttle.

[0033] In various embodiments the end of the drive piston opposite to the terminal end of the chamber is linked to a mechanical or electrical drive system. In some cases this drive system may be formed by a separate engine or motor used to reciprocate the drive piston within the chamber when it operates in a heat pump role. In other cases some form of power take-off or transmission linkage can be used to form the mechanical or electrical drive system or generate electricity. Those skilled in the art will appreciate that the drive system being engaged with may be formed by a mechanical drive - such as by an internal combustion engine - or an electrical drive - such as an electric motor. In a number of embodiments the drive piston may define a free end located closest to the terminal end of the chamber, and a base located at the opposite end of the chamber and linked to the drive system. In such embodiments the base of the piston may close the chamber and face the end of the displacer shuttle furthest away from the chamber's terminal end.

[0034] The drive piston is free to move axially relative to the displacer shuttle and preferably over and around a portion of the shuttle. During use of the machine the drive piston reciprocates towards and away from the terminal end of the chamber. The drive piston is free to move axially over and relative to a portion of the displacer shuttle which also reciprocates towards and away from the terminal end of the chamber. This arrangement of chamber, shuttle and piston deploys two moving parts having cyclic reciprocal motion which can be out of phase with one another.

[0035] The chamber defines a terminal end displaced from a thermal transfer zone. Preferably the chamber also defines a regenerator passage connecting the terminal end and thermal transfer zones. The regenerator passage therefore displaces the terminal end and thermal transfer zone from one another.

[0036] In various preferred embodiments the regenerator passage may site, locate or otherwise contain heat sink material promoting transfer of thermal energy with the working fluid. For example, in one particular implementation the regenerator passage may also be used to house a perforated film or mesh type structure which maximises surface area contact with the working fluid.

[0037] In a preferred embodiment the chamber's thermal transfer zone may be defined by the volume of space bounded by a surface of the displacer shuttle extending across the width of the chamber and displaced from the terminal end, and by an adjacent facing surface of the drive piston. For example in one preferred embodiment the displacer shuttle may have a restricted diameter at its end adjacent to the drive piston and which extends into the drive piston. In this arrangement the chamber's thermal transfer zone may be defined by the surface of the displacer shuttle which spans the entire bore of the chamber and the facing surface of the piston which receives the restricted diameter portion of the shuttle. In other embodiments where a portion of the piston moves inside the displacer shuttle the thermal transfer zone may be defined by the face of the shuttle furthest from the terminal end and by an abutting face of the piston base. In such embodiments the regenerator passage connects to this zone of the chamber as well as to the chamber's terminal end.

[0038] In various embodiments the thermal transfer zone may include heat sink material which promote the transfer of thermal energy with the working fluid. For example in a range of embodiments the interior surface of the chamber at the thermal transfer zone may define an array of fins or sheets of metal forming a heat sink which improves the efficiency of the resulting Stirling machine.

[0039] In various embodiments the Stirling machine may also define a terminal end zone defined by the volume of space bounded by the closest end of the displacer shuttle and by the chamber's facing terminal end. In further preferred embodiments this terminal end zone may include heat sink material which promote the transfer of thermal energy with the working fluid. For example, in a range of embodiments the interior surface of the chamber, and / or the terminal end itself may define an array of fins or sheets of metal arranged to define a heat sink which improves the efficiency of the resulting Stirling machine.

[0040] In one embodiment the Stirling machine provided includes at least one switch magnet engaged, connected or otherwise mounted in association with the displacer shuttle. The switch magnet therefore also follows the same reciprocal motion of the shuttle within the chamber.

[0041] In one preferred embodiment the Stirling machine includes at least one switch magnet engaged, connected or otherwise mounted in association with the drive piston. This arrangement may be used when the piston is formed so as to surround at least part of the displacer shuttle. In this configuration the switch magnet follows the same reciprocal motion of the drive piston within the chamber.

[0042] Reference will be made throughout this specification to the Stirling machine provided having a switch magnet mounted to the drive piston. Those skilled in the art will also appreciate that an alternative arrangement is within the scope of the invention where a switch magnet is mounted to the displacer shuttle. In preferred implementations of the invention a switch magnet may be formed by a single ring magnet with a circular profile having a centralised aperture. Reference throughout this specification will also be made to a switch magnet used with the invention being a ring magnet. However those skilled in the art will appreciate that in other instances arrays of several separate magnets may be grouped together to form a composite switch magnet if desirable or applicable.

[0043] Reference throughout this specification will also be made to the present invention incorporating or deploying a single switch magnet only in association with the shuttle or potentially with the drive piston as an preferred alternative. However those skilled in the art will appreciate that there is an element of design choice available and multiple instances of switch magnets may be provided in other implementations if desired.

[0044] In a preferred embodiment a switch magnet mounted to the drive piston cooperates with a further magnet mounted or otherwise engaged with the shuttle. The interfering magnetic fields exhibited by the switch magnet and the displacer shuttle magnet are selected so as to initially resist the motion of the shuttle as its magnet approaches the switch magnet. The approaching shuttle magnet will therefore apply a repulsive force to the drive piston through the switch magnet in the direction of motion orientated in the direction of motion of the shuttle magnet.

[0045] As it follows through a full cycle of reciprocal motion the shuttle magnet will move through the central aperture of the switch magnet. After it has traversed the switch magnet the repulsive force applied by the interaction of the magnetic fields will urge the drive piston to move in a direction opposite to that of the direction of travel of the shuttle.

[0046] This arrangement of magnetic fields can be used to drive the reciprocal motion required of the displacer shuttle. For example one position in the stroke of the drive piston will see it moving towards the terminal end of the chamber. There repulsive magnetic force applied through or to the shuttle's magnet drives the shuttle in front of and in the same direction as the driveshaft.

[0047] Subsequently the driven motion of the piston will push the shuttle's magnet through the piston's switch magnet reversing their relative positions. At this time the shuttle is repulsed from the piston and reverses its direction of travel. In this way both moving parts can exhibit reciprocal motion where the use of a magnet mounted to the shuttle and the drive piston switch magnet allows these motions to be out of phase relative to each. Those skilled in the art will also appreciate that the same mode of operation can also be executed when the invention is configured with a switch magnet mounted to the displacer shuttle and with an end of the drive piston mounting a magnet extending into the displacer shuttle

[0048] These arrangements allows the switch magnet to operate as an alternative to the prior art's use of a mechanical spring. As with a mechanical spring, the shuttle experiences increasing resistance to its motion toward the terminal end of the chamber until it reaches a point in its travel where this resistance effectively reverses direction and accelerates the shuttle away from the terminal end, towards the drive piston.

[0049] Similarly, the shuttle experiences increasing resistance to its motion away from the terminal end of the chamber until it reaches a point in its travel where this resistance effectively reverses direction and accelerates the shuttle, away from the drive piston, towards the terminal end.

[0050] In various preferred embodiments the invention includes at least one braking magnet engaged with the chamber and at least one braking magnet engaged with the displacer shuttle. These braking magnets are positioned in the chamber and on the shuttle so that the motion of braking magnets towards and in proximity to each other applies a force which slows the motion of the displacer shuttle relative to the chamber. The braking magnets can therefore be used to control the reciprocal motion of the shuttle inside the chamber, with their relative positions in the chamber and on the shuttle stopping the motion of the shuttle when it reaches the end of its trajectory in one direction or the other.

[0051] In some alternative embodiments the invention includes at least one stop magnet engaged with the displacer shuttle and at least one control magnet engaged with the drive piston. A stop magnet is positioned on the shuttle so that the motion of the stop and control magnets towards and in proximity to each other applies a force which slows the motion of the displacer shuttle relative to the piston.

[0052] However in a preferred embodiment the invention may include at least one stop magnet engaged with the drive piston and at least one control magnet engaged with the shuttle. This arrangement may be used when the invention is arranged with a portion of the displacer shuffle extends into the piston. Similarly a stop magnet may be positioned on the piston with a control magnet on the end of the shuttle so that the motion of the magnets towards each other applies a force which slows the motion of the displacer shuttle relative to the piston.

[0053] A stop magnet and a control magnet can therefore be used to control the reciprocal motion of the shuttle relative to the piston, with their relative positions in the piston and on the shuttle stopping the motion of the shuttle when it reaches the end of its trajectory in one direction or the other.

[0054] In a further preferred embodiment where the invention includes both control and switch magnets, a control magnet engaged with the displacer shuttle may move through the switch magnet to apply a force which moves the displacer shuttle in the opposite direction to the direction of travel of the drive piston. In such embodiments the control magnet engaged with the shuttle can interact with stop magnets and the switch magnet mounted within the piston. This single control magnet can therefore be used to apply driving forces to the shuttle, as well as an acceleration force which reverses the direction of travel of the shuttle.

[0055] Reference in general has been made throughout this specification to the Stirling machine including a number of arrangements of braking magnets and stop magnets. Those skilled in the art will however appreciate that various embodiments of the invention may not necessarily incorporate or require these magnets and may instead rely on other known prior art systems for braking or stopping a reciprocating moving part.

[0056] In various additional embodiments multiple instances of the Stirling machines described may be linked together to form a Stirling assembly. In particular in various embodiments the invention may provide a Stirling machine assembly composed from two or more Stirling machines as described above, the Stirling machines being connected together to position the mechanical or electrical drives of each Stirling machine adjacent to each other. These assemblies may be deployed in various applications to balance forces and pressures experienced during the normal operation of a single Stirling machine.

[0057] For example, in one preferred embodiment two Stirling machines may be connected adjacent to the mechanical or electrical drive used by either or potentially both machine with their terminal ends projecting away from each other. This arrangement may therefore connect the two Stirling machines together to position the drives of each machine adjacent to each other. In such embodiments the timing or phase of the reciprocal motions executed by each machine may be controlled so as to balance the operation of the resulting Stirling assembly. In particular the sequence of motion executed by each machine's drive piston may be 180° out of phase with the opposite of the pair.

[0058] Those skilled in the art will also appreciate that these forms of Stirling assemblies may be implemented by a two member set, or in alternative embodiments various multiples of two machines may potentially be used form such assemblies. For example, in one possible embodiment a Stirling assembly may be formed by four Stirling machines preferably linked or connected as described above.

[0059] The present invention may provide many potential advantages over prior art.

[0060] In various embodiments the invention can provide a Stirling cycle machine which capable of operating without significantly reduced efficiency over a wide range of operating speeds or frequencies.

[0061] A Stirling machine provided by the invention could have its dimensions scaled up and down as desired without impacting operational efficiency or limiting operational speeds.

[0062] In various implementations the invention may provide an improved lightweight machine and operate as a heat pump function without the use of a greenhouse gas. In a range of embodiments the invention avoids the need for complex control systems or large numbers of moving parts.

[0063] Brief description of the drawings

[0064] Additional and further aspects of the present invention will be apparent to the reader from the following description of embodiments, given in by way of example only, with reference to the accompanying drawings in which:

[0065] • Figure la provides a side cross section view of a Stirling machine as provided by one embodiment, • Figure lb provides a perspective cross section view of the Stirling machine of figure la.

[0066] • Figure 2a through 2d shows perspective and side cross section views of the piston, shuttle and chamber of figures la, lb when separated from each other,

[0067] • Figures 3A through 3F show a complete cycle of reciprocal movements executed by the shuttle and piston of figures 1 and 2,

[0068] • Figure 4 shows the use of two instances of the Stirling machine of figures 1-3 connected to form a Stirling assembly,

[0069] • Figure 5a through 5d provides a side cross section, perspective cross section and isolated piston and displacer shuttle views of a Stirling machine as provided in an alternative embodiment to that shown by figures 1-4, and

[0070] • Figure 6 provides a plot of degrees through a full cycle of reciprocal motion against the distance between shuttle and piston for the Stirling machine shown in figures 1 to 4.

[0071] Further aspects of the invention will become apparent from the following description of the invention which is given by way of example only of particular embodiments.

[0072] Best modes for carrying out the Invention

[0073] Figure la provides a side cross section view of a Stirling machine 1 as provided by one embodiment, and Figure lb provides a perspective cross section view of the same machine. Figures 2a-d shows separate views of the chamber 2, shuttle 6 and piston 7 shown in figure 1.

[0074] As can be seen from these figures the Stirling machine 1 includes a chamber 2 used to locate and define other parts of the machine. The chamber has a terminal end 3 which is linked to a regenerator passage 5 and thermal transfer zone 4 bounded by the interior walls of the chamber and piston base 7c and shuttle 6. Working fluid or gas in the terminal end 3 can escape through the regenerator passage 5, and move to the thermal transfer zone 4 and the same working gas is also free to flow back to the terminal end 3. These features are shown with particular clarity by figure 2.

[0075] The chamber houses the displacer shuttle 6 which is free to move in a reciprocal pattern towards and away from the terminal 3 and opposite end of the chamber 2 defining the thermal transfer zone 4. The machine includes a drive piston 7 where the free end 7a and majority of the piston shaft 7b are located inside the displacer shuttle 6. The complimentary and concentric nature of these parts is shown with particular clarity by figure 2a and 2b. A piston base 7c is linked to a separate mechanical drive 8 (not shown) acting as either a power transmission system or a mechanical drive to reciprocate the piston.

[0076] As can also be seen by figures 2 a, b the shuttle defines an interior cavity with a complimentary shape to that of the shaft of the piston. The end of the shuttle closest of the chamber's terminal end also defines a further cavity which receives the moving free end 7a of the piston.

[0077] Figures 2c and 2d show the chamber used in this implementation, and in particular the terminal end 3, thermal transfer zone 4 and regenerator passage 5. In various embodiments the regenerator passage 5 can be packed with matrix materials which allow the passage of the working fluid while increasing surface area contact with the working fluid. These materials promote the transfer of thermal energy between the working fluid and the regenerator passage, and may include - for example - various metal meshes, perforated films or porous packing materials as currently used in the prior art.

[0078] The thermal transfer zone 4 is defined by the volume of space bounded by the interior wall of the chamber, the face of the shuttle 6 furthest from the terminal end and by an abutting face of the piston base 7c. Those skilled in the art will appreciate that these piston and shuttle boundaries of this zone will change over time with their respective motion during operation of the Stirling machine, but will always encompass the entry area to the regenerator passage.

[0079] The thermal transfer zone also includes a heat sink 4a defined by the interior surface of the chamber. In the embodiment shown this heat sink is formed by an array of metal fins which increase surface area contact with working fluid present in the zone.

[0080] The drive piston shaft and free end 7a, 7b are predominantly located inside the displacer shuttle 6 and are free to move axially inside the shuttle with an equivalent reciprocal motion to that executed by the shuttle. This arrangement allows both piston and shuttle to reciprocate towards and away from the chamber terminal end freely inside and outside of each other. The reciprocal motion of the displacer shuttle implements the Stirling cycle of the machine, with its motion towards the terminal end moving the working gas through regenerator passage 5, into the thermal transfer zone 4 where it is compressed. The withdrawal of the of the displacer shuttle from the terminal end 3 acts to draw the working gas back towards the terminal end where it is expanded.

[0081] In the embodiment shown the machine relies on a series of magnets to control the motion of the shuttle.

[0082] Braking magnets 9a, 9b are mounted both to the interior of the chamber and also to the exterior 9c of the shuttle. The magnets mounted by the chamber 9a, 9b set boundaries for the span of the reciprocal motion which can be completed by the shuttle. Once the braking magnet 9c mounted to the shuttle has moved far enough away from the terminal end to encounter the leftmost braking magnet 9a its further motion in this direction will be resisted. Conversely as the shuttle travels towards the terminal end its continued motion will be resisted by the chamber's right hand braking magnet 9b.

[0083] A control magnet 10 is also mounted to the free end 7a of the drive piston. During the reciprocal cycle executed by the shuttle the control magnet 10 will come into proximity to one of a pair of stop magnets 11a, lib mounted on the shuttle. The poles of these magnets are arranged so that a repulsion force will be experienced by the shuttle as the control magnet 10 approaches.

[0084] The shuttle 6 also mounts a switch magnet 12 between the two stop magnets 11a, lib. The switch magnet 12 is formed from a ring magnet which the control magnet must pass through twice to complete a single cycle of reciprocal motion. The poles of the control 10 and switch magnets 12 are arranged so that motion of the control magnet through the switch magnet forces the displacer shuttle 6 to move in the opposite direction to the direction of travel of the drive piston 7. As the switch and control magnet approach each other they experience a repulsion force. However when the piston continues to travel towards and subsequently through the ring magnet the same repulsing forces are present but now switches directions to repel the displacer shuttle in the opposite direction. Figures 3A through 3F show a complete cycle of reciprocal movements executed by the shuttle and piston of figures 1 and 2.

[0085] Figure 3A shows the arrangement of the machine when the distance between the piston and shuttle is at its maximum. At this position the piston has reached the leftmost extent of its travel and the shuttle has moved asynchronously to the right due a prior interaction of the control and switch magnets.

[0086] Figure 3B shows the machine when the shuttle has stopped at its rightmost position while the piston is moving to the right. In this position the shuttle has displaced most of the working gas from the terminal end into the thermal transfer zone.

[0087] Figure 3C shows the shuttle still at the rightmost side of its range while the piston is continuing to move to the right. At this position the piston control magnet passes through the switch magnet mounted to the shuttle. This reverses the direction of travel of the shuttle to be towards the opposite end of the chamber.

[0088] Figure 3D shows the piston at the rightmost extent of its range of motion, while the shuttle has completed its accelerated movement towards the left.

[0089] At this point in the Stirling cycle the required compression of the working gas has been completed and the shuttle next starts to move towards the opposite end.

[0090] Figure 3E shows where the shuttle has stopped at the leftmost extent of its motion, while the piston moves to the left. At this stage of the cycle the shuttle is expanding the working gas at the terminal end.

[0091] Figure 3F shows the movement of the piston leftward with the piston's control magnet passing through the shuttle's switch magnet. This applies a repulsive force to the shuttle urging it towards the right and the terminal end of the chamber. At this point the shuttle's expansion of gases has been completed and the shuttle moves into the position in the cycle shown by figure 3A.

[0092] Figure 4 shows the use of two instances of the Stirling machine 1 of figures 1-3 connected to form a Stirling assembly 101. The pair of Stirling machines can have the ends of each chamber aligned with the other of the pair. This allows the mechanical drive to share a common volume of space with that of the other paired machine.

[0093] With this arrangement of a Stirling assembly the drive pistons may be operated at the same speed but 180° out of phase with each other. This arrangement can balance the weight of the moving parts of the assembly as it operates and can balance the gas pressures experienced by the mechanical drives over a greater volume.

[0094] Figures 5a through 5d provide a number of views of a Stirling machine as provided in an alternative embodiment to that shown by figures 1-4. In particular figure 5a shows a side cross section view, figure 5b shows a perspective cross section view and figures 5c and 5d show isolated views of a drive piston and displacer shuttle.

[0095] In the embodiment shown with respect to these figures the piston 107 defines a hollow interior region which locates a portion of the displacer shuttle 106 while the chamber 102 is substantially similar to that shown in the embodiments of figures for 1-4.

[0096] The end of the piston 107 shown on the left-hand side of figure 5 is linked to a drive shaft of a mechanical drive assemblyl08. The end of the shuttle furthest from the terminal end 103 of the chamber interacts with a switch magnet 112 and stop magnets 111 located within the hollow interior region of the piston. The mechanical drive assembly can - for example - be used to induce a reciprocal motion in the piston where the switch 112 and stop magnets 111 induce a corresponding reciprocal motion in the shuttle as a restricted diameter end of the shuttle has a control magnet 110 mounted to it. The restricted diameter end of the shuttle located within the hollow cavity of the piston is engaged with a further wide bore displacer body extending towards a terminal end 103 of the chamber, which when reciprocated drives the Stirling cycle of the resulting machine.

[0097] Similar to the embodiment of figures 1-4 braking magnets 109a, 109b are mounted within the interior wall of the chamber to interact with a further braking magnet 109c mounted to the displacer shuttle.

[0098] The embodiment shown by figures 5a-5d also illustrates the formation of a regenerator passage 105 linking the terminal end 103 of the chamber 102 with a thermal transfer zone 104. This thermal transfer zone is defined by the surface 106a of the displacer shuttle which spans the entire bore of the chamber and the facing surface 107a of the piston which receives the restricted diameter portion 106b of the shuttle. A heat sink element 104a is also embedded in the surface of the chamber at this location.

[0099] Figure 6 provides a plot of degrees through a full cycle of reciprocal motion against the distance between shuttle and piston for the Stirling machine shown in figures 1 to 4.

[0100] As can be seen from this figure both the drive piston and displacer shuttle have the same frequency of reciprocation within the chamber but are usually offset in terms of their relative positions at any instant time. As can also be seen from figure 6 the displacer shuttle experiences a period without motion when located at its closest point to the chamber's terminal and opposite ends. During these periods the distance between the displacer shuttle and piston shrinks or extends, depending on direction of travel, until the piston passes through the shuttle's switch magnet and the shuttle is respectively drawn to or repelled from the piston.

[0101] This characteristic of the invention has the effect of increasing the dwell time of the displacer piston at the maximum extent of its travel, in either direction. This ensures that the majority of fluid compression occurs in the thermal transfer zone and the majority of fluid expansion occurs at the terminal end.

[0102] In the preceding description and the following claims the word "comprise" or equivalent variations thereof is used in an inclusive sense to specify the presence of the stated feature or features. This term does not preclude the presence or addition of further features in various embodiments.

[0103] It is to be understood that the present invention is not limited to the embodiments described herein and further and additional embodiments within the spirit and scope of the invention will be apparent to the skilled reader from the examples illustrated with reference to the drawings. In particular, the invention may reside in any combination of features described herein, or may reside in alternative embodiments or combinations of these features with known equivalents to given features. Modifications and variations of the example embodiments of the invention discussed above will be apparent to those skilled in the art and may be made without departure of the scope of the invention as defined in the appended claims.

Claims

WHAT I CLAIM IS:

1. A Stirling cycle machine which includes a chamber arranged to contain a working fluid and defining a terminal end displaced from a thermal transfer zone, and a displacer shuttle free to reciprocate within the chamber towards and away from the terminal end of the chamber, and a drive piston with an end linked to a mechanical or electrical drive configured to reciprocate the drive piston towards and away from the terminal end of the chamber, the displacer shuttle being at least partly located inside the drive piston with the drive piston being free to move axially around and relative to the displacer shuttle, and at least one switch magnet engaged with the drive piston whereby motion of a magnet mounted to the displacer shuttle through the switch magnet forces the displacer shuttle to move in the opposite direction to the direction of travel of the drive piston.

2. A Stirling cycle machine as claimed in claim 1 wherein the terminal end zone includes heat sink material.

3. A Stirling cycle machine as claimed in claim 1 wherein the thermal transfer zone includes heat sink material.

4. A Stirling cycle machine as claimed in claim 1 wherein the chamber defines a regenerator passage connecting the terminal end and thermal transfer zones.

5. A Stirling cycle machine as claimed in claim 4 wherein the regenerator passage contains heat sink material.

6. A Stirling cycle machine as claimed in claim 1 wherein the movement of the displacer shuttle towards the terminal end of the chamber moves the working fluid through the regenerator passage, towards the thermal transfer zone and compresses the fluid.

7. A Stirling cycle machine as claimed in claim 1 wherein the movement of the displacer shuttle away from the terminal end of the chamber draws working fluid away from the thermal transfer zone, through the regenerator passage and expands the fluid at the terminal end.

8. A Stirling cycle machine as claimed in claim 1 wherein at least a portion of the displacer shuttle has a complimentary shape to the interior of the volume defined by the chamber.

9. A Stirling cycle machine as claimed in claim 1 wherein the shuttle is located centrally and concentrically within the chamber and reciprocates along the same longitudinal axis as the chamber.

10. A Stirling cycle machine as claimed in claim 1 wherein the displacer shuttle has a restricted diameter at its end adjacent to the drive piston which extends into the drive piston.

11. A Stirling cycle machine as claimed in claim 1 wherein the drive piston is located centrally and axially aligned with the longitudinal axis of the displacer shuttle.

12. A Stirling cycle machine as claimed in claim 1 wherein the end of the drive piston opposite to the terminal end of the chamber is linked to a mechanical or electrical drive system formed by a power take-off transmission linkage.

13. A Stirling cycle machine as claimed in claim 1 wherein the end of the drive piston opposite to the terminal end of the chamber is linked to a mechanical or electrical drive system formed by an engine used to reciprocate the drive piston within the chamber.

14. A Stirling cycle machine as claimed in claim 1 wherein a switch magnet is formed by a single ring magnet with a circular profile and a centralised aperture.

15. A Stirling cycle machine as claimed in claim 1 wherein an array of several separate magnets are grouped together to form a composite switch magnet.

16. A Stirling cycle machine as claimed in claim 1 which includes at least one braking magnet engaged with the chamber and at least one braking magnet engaged with the displacer shuttle, said at least one braking magnet or magnets being positioned so that the motion of braking magnets towards and in proximity to each other applies a force which slows the motion of the displacer shuttle relative to the chamber.

17. A Stirling cycle machine as claimed in claim 1 which includes at least one stop magnet engaged with the drive piston and at least one control magnet engaged with the displacer shuttle, whereby motion of said at least one stop magnet towards and in proximity to said at least one control magnet applies a force which slows the motion of the displacer shuttle relative to the drive piston.

18. A Stirling cycle machine as claimed in claim 17 wherein the control magnet engaged with the displacer shuttle moves through the switch magnet to apply a force which moves the displacer shuttle in the opposite direction to the direction of travel of the drive piston.

19. A Stirling cycle machine which includes a chamber arranged to contain a working fluid and defining a terminal end displaced from a thermal transfer zone, and a displacer shuttle free to reciprocate within the chamber towards and away from the terminal end of the chamber, and a drive piston at least partly located inside the displacer shuttle with an end of the drive piston linked to a mechanical or electrical drive configured to reciprocate the drive piston towards and away from the terminal end of the chamber, the drive piston being free to move axially within and relative to the displacer shuttle, and at least one switch magnet engaged with the displacer shuttle whereby motion of a magnet mounted to the drive piston through the switch magnet forces the displacer shuttle to move in the opposite direction to the direction of travel of the drive piston.

20. A Stirling machine assembly composed from two or more Stirling machines as claimed in any one of claims 1 to 19, the Stirling machines being connected together to position the mechanical or electrical drives of each Stirling machine adjacent to each other.

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