Patent Publication Number: US-6701708-B2

Title: Moveable regenerator for stirling engines

Description:
PRIORITY INFORMATION 
     This application claims the priority benefit under 35 U.S.C. §119(e) of Provisional Application No. 60/288,405 filed May 3, 2001 and Provisional Application No. 60/291,718 filed May 17, 2001, the entire contents of which are expressly incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to engines and, in particular, to Stirling cycle engines. 
     2. Description of the Related Art 
     Stirling cycle engines have a theoretical thermodynamic efficiency that is much higher than internal combustion engines. However, Stirling cycle engines are not as widely used as internal combustion engines because Stirling cycle engines typically require complicated hardware, which results in very low power-to-weight and power-to-volume ratios. 
     For example, a typical Stirling cycle engine includes an enclosed chamber, a displacer piston, a power piston and a crankshaft. The displacer piston is positioned within the enclosed chamber and is connected to the crankshaft by a shaft, which extends through the walls of the chamber. The power piston is also connected to the crankshaft and has one end that is in communication with the interior of the chamber. With respect to the crankshaft, the displacer piston and the power piston are typically 90 degrees out of phase with each other. 
     In operation, the displacer piston moves working fluid from a cold side of the chamber to a hot side of the chamber. This causes the working fluid to expand. This expansion pushes the power piston, thereby rotating the crankshaft. As the crankshaft rotates, the displacer piston moves the working fluid to the cold side of the chamber. This causes the working fluid to contract, pulling the piston down. As the piston moves back down, the crankshaft rotates and the displacer piston moves the working fluid to the hot side of the chamber, thereby completing the cycle. 
     There is, therefore, a need for an improved design for a Stirling cycle engine that minimizes at least some of the disadvantages described above. 
     SUMMARY OF THE INVENTION 
     The present invention provides for several novel Stirling cycle engine designs, which provide for increased efficiency and better power to volume ratios than conventional designs. In one preferred embodiment, the engine comprises a sealed engine block that defines a cylindrical chamber. A rotary displacer is suitably journalled for rotation within the engine block. A displacer drive motor rotates the rotary displacer and is controlled by a microprocessor. Working fluid in the chamber is in communication with a rolling sock seal piston, which, in turn, is coupled to a generator. For alternately heating and cooling the working fluid, a heat source is located on one side of the sealed chamber and a heat sink is located on another side of the sealed chamber. In modified embodiments, the rotary displacer is counter balanced and/or shaped to reduce aerodynamic drag. 
     In another embodiment, a Stirling engine comprises a sealed engine block that defines a cylindrical chamber, which encloses a working fluid. The engine block including a first quadrant, a second quadrant, a third quadrant and a fourth quadrant. A rotary displacer is suitably journalled for rotation within the engine block. A displacer drive motor rotates the rotary displacer and is controlled by a microprocessor. Working fluid in the chamber is in communication with a piston. A heat source is configured to heat the first and third quadrants, which oppose each other. A heat sink is configured to cool the second and fourth quadrants, which oppose each other. The rotary displacer moves between a first position wherein most of the working fluid is the second and forth quadrants and a second position wherein most of the working fluid is in the first and third quadrants. 
     In yet another embodiment, a Stirling engine comprises a sealed engine block that defines a generally triangular chamber, which encloses a working fluid. The engine block comprises a hot side, a cold side and a base. A displacer is suitably journalled for pivotal movement within the engine block. A displacer drive motor moves the displacer in an oscillating arc shaped motion and is controlled by a microprocessor. A heat source is configured to heat the hot side of the engine block and a heat sink is configured to cool the cold side of the engine block. The displacer is moveable between a first position wherein most of the working fluid is near the hot side of the engine block and a second position wherein most of the working fluid is near the cold side of the engine block. 
     In still yet another embodiment, a Stirling engine comprises a sealed engine block, which encloses a working fluid. The engine block comprises a cylindrical inner member and a coaxial cylindrical outer member. A heat source and a heat sink are configured to keep the inner member and the outer member at different temperatures. A displacer is positioned within the chamber and is configured to move between a first position wherein most of the working fluid is near the outer member and a second position wherein most of the working fluid is near the inner member. 
     In another embodiment, a Stirling engine comprises a sealed engine block, which encloses a working fluid. The engine block defines a working fluid space, a hot path and a cold path. The hot path is connected to the working fluid space at a hot inlet and a hot outlet. The hot path includes a hot inlet valve and a hot outlet valve. The cold path is connected to the working fluid space at a cold inlet and a cold outlet. The cold path includes cold inlet valve and a cold outlet valve. The engine further including a working fluid circulator for circulating the working fluid within the engine. A heat source and a heat sink are configured to keep the hot path and the cold path at different temperatures. A control system is configured to alternately open and close the hot path and the cold path such that the working fluid is alternately circulated through a first past that is defined, at least in part, by the hot path and the working fluid space and a second path that is defined, at least in part, by the cold path and the working fluid space. 
     In another embodiment, a Stirling cycle engine comprises a substantially sealed engine block that defines a working fluid space, a hot path and a cold path. A heat source and a heat sink are configured to keep the hot path and the cold path at different temperatures. The engine includes a valve chamber that is communication with the working fluid space, the hot path and the cold path. A valve is moveably positioned within the valve chamber between at least a first position and a second position. The valve defines a passage that, in the first position, places the working fluid space in communication with the hot path and, in the second position, places the working fluid space in communication with the cold path. A regenerator positioned within the passage. 
     In another embodiment, a method of operating a Stirling cycle engine having a substantially sealed engine block that defines a working fluid space, a hot path and a cold path, the method comprises passing a working fluid through the hot path, passing the working fluid into the working space, passing the fluid through a regenerator and into the cold path, passing the fluid through the cold path, moving the regenerator such that it is in communication with the hot path and the working space, passing the fluid into the working space; and passing the fluid through the regenerator into the hot path. 
     In another embodiment, a Stirling cycle engine comprises a substantially sealed engine block that defines a working fluid space, a hot path and a cold path. A heat source and a heat sink are configured to keep the hot path and the cold path at different temperatures. A valve chamber is in communication with the working fluid space, the hot path and the cold path. The engine further comprises a regenerator and means for moving the regenerator so as to alternately direct working fluid from the working fluid space to the hot path and the cold path. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a first embodiment of a Stirling engine. 
     FIG. 2 is a side perspective view with a portion cut away of the engine of FIG.  1 . 
     FIGS. 3A-C are top plan views of fin plates and chamber plates that are used to form an engine block of the engine of FIG.  1 . 
     FIG. 4 is a perspective view of hot passages and cold passages in the engine of FIG.  1 . 
     FIG. 5 is a cross-sectional view of a modified embodiment of the engine of FIG.  1 . 
     FIG. 6 is a top plan view of a modified embodiment of the fin plates of FIG.  3 A. 
     FIGS. 7A-D illustrate several modified embodiments of the displacer. 
     FIGS. 8A-C illustrate several more modified embodiments of the displacer. 
     FIG. 9 is a graph that illustrates the theoretical movement of working fluid in the engine of FIG.  1 . 
     FIG. 10 is a cross-sectional view of another modified embodiment of the engine of FIG.  1 . 
     FIG. 11 is a top view of a second embodiment of a Stirling engine. 
     FIGS. 12A-B illustrate the fin plates, chamber plates, cold passages and hot passages of the engine of FIG.  11 . 
     FIG. 13 is a modified embodiment of the fin plate of FIG.  12 A. 
     FIG. 14A is a side elevational view of a third embodiment of a Stirling engine. 
     FIG. 14B is a modified embodiment of the engine of FIG.  14 A. 
     FIGS. 15A-B are top plan views of a fourth embodiment of a Stirling engine. 
     FIG. 16 illustrates a fifth embodiment of a Stirling engine. 
     FIG. 17 illustrates a modified embodiment of the engine of FIG.  16 . 
     FIG. 18 illustrates a modified embodiment of an air circulator for the engine of FIG.  17 . 
     FIG. 19 illustrates a rotor valve for the engine of FIG.  17 . 
     FIG. 20 illustrates a modified embodiment of a portion of the engine of FIG. 16 or  17 . 
     FIGS. 21A-C illustrate a regenerator having certain features and advantages according to the present invention positioned within the Stirling engine of FIG.  16 . 
     FIGS. 22A-B are perspective views of a modified embodiment of a regenerator. 
     FIGS. 23A-B are cross-sections views of the regenerator of FIGS. 22A-B. 
     FIG. 24 is an exploded view of another modified embodiment of a regenerator. 
     FIG. 25A is a cross-sectional view of the regenerator of FIG. 24 in a first position. 
     FIG. 25B is a cross-sectional view of the regenerator of FIG. 24 in a second position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is directed to several novel arrangements of a Stirling cycle engine. In a first embodiment, which will be explained in greater detail below, the engine includes a sealed engine block that defines a chamber that may be generally cylindrical in shape. A rotary displacer is suitably journalled for rotation within the engine block. Preferably, the displacer includes a plurality of blades and the engine block includes a plurality of internal fins that are located between each blade of the displacer. A displacer drive motor rotates the rotary displacer and is controlled by a microprocessor. A sealed piston, such as, a rolling sock seal piston is in communication with working fluid in the chamber. Preferably, the piston is coupled to a generator so as to convert the movement of the piston to electrical energy. For alternately heating and cooling the working fluid, a heat source is located on one side of the sealed chamber and a heat sink is located on another side of the sealed chamber. Optionally, the rotary displacer is counter balanced and/or shaped to increase heat transfer between the internal fins and the working fluid. 
     FIGS. 1-4 illustrate a first embodiment of a rotary Stirling engine  10 . With initial reference to FIG. 1, the engine  10  includes an engine block  12 , which defines a substantially sealed, generally cylindrical chamber  14 . A rotary displacer  16  is positioned within the chamber and comprises a plurality of blades  18 , which are coupled to a shaft  20 . The shaft  20 , in turn, is suitably journalled for rotation within the engine block  12 . Specifically, in the illustrated example arrangement, a first end  21  a of the shaft is journalled with bearings  23 , which are supported in the engine block  12 . A second end  21   b  of the shaft  20  is supported by a drive motor  25 , which will be described in more detail below. Of course, alternative methods of journalling the shaft  20  for rotation may be used. 
     A plurality of internal fins  22  extend between adjacent blades  18 . The internal fins  22  divide the chamber  14  into a plurality of sub-chambers  24 . Preferably, one blade  18  is positioned within each sub-chamber  24 . As shown in FIG. 1, the illustrated engine  10  includes seven sub-chambers  24  and seven blades  18 . However, it should be appreciated that the illustrated number of sub-chambers  24  and blades  18  is merely exemplary and modified arrangements may include more or less sub-chambers  24  and/or blades  18 . 
     With particular reference to FIGS. 3A and 3B, the engine block  12 , the fins  22  and sub-chambers  24  preferably are formed by alternately stacking and rotating a plurality of fin plates  30  and chamber plates  32 . With particular reference to FIG. 3B, the chamber plates  32  include a housing portion  34  and an inner surface  36 , which defines a generally cylindrical cavity  37 . As shown in FIG. 1, the inner surfaces  36  of a series of chamber plates  32  define an outer boundary of the chamber  14  and sub-chambers  22 . The chamber plate  32  preferably is made from a material that seals smoothly against the other plates that make up the engine  10 , has a coefficient of expansion compatible with plates that contact the chamber plate  32 , has high strength at elevated temperatures and has good thermal conductivity, such as, for example, stainless steel. 
     As shown in FIG. 3A, the fin plates  30  include a housing portion  40  and a fin portion  42 . When stacked between the chamber plates  32 , the fin portions  42  form the fins  22  that extend between the blades  18 . In a similar manner, the housing portions  34 ,  40  of the chamber plate  32  and the fin plates  30  define the walls of the engine block  12 . Preferably, the fin plates  30  are made from a material that has a high thermal conductivity and retains adequate strength at elevated temperatures, such as, for example, copper or aluminum. 
     With reference back to FIG. 1, two end assemblies  50  are provided for closing the ends the chamber  14 . In the illustrated embodiment, the end assemblies  50  include a fin plate  30  and a end plate  54 . Of course, the end assemblies  50  may be formed without the fin plate  30 . 
     The fin plates  30 , chamber plates  32  and end assemblies  50  preferably are coupled together by a plurality of bolts  58 . Preferably, to seal the engine block  12 , gaskets (not shown) are provided between the fin plates  30 , chamber plates  32  and end assemblies  50 . In a modified embodiment, small grooves may be provided in the fin plates  30 , chamber plates  32  and/or end assemblies  50 . A compressible material, such as, a copper wire, for example, is then positioned within the small grooves. When the engine block  12  is assembled, sufficient pressure is applied to compress the wire and form a tight seal between the parts of the engine block  12 . 
     In the illustrated embodiment, the heat source and heat sink comprise a plurality of hot passages  62  and cold passages  64 , which are formed in the walls of the engine block  12 . With particular reference to FIGS. 3A-C and FIG. 4, the hot and cold fluid passages are defined by the hot channels  63  and cold channels  64  formed in the housing portions  40 ,  34  of the fin and chamber plates  30 ,  32 . By alternately stacking and rotating 180 degrees the fin plates  30  and chamber plates  32  as shown in FIG. 3C, the hot and cold fluid passages  62 ,  64  shown in FIG. 4 may be formed. As such, the engine block has a cold side  66  and a hot side  68  (see FIG.  1 ). 
     Preferably, the heating fluid (i.e., the fluid in the hot passages) remains liquid at the resting and operating temperatures of the engine (i.e., the boiling point is above the operating temperature of the engine and the melting point is below the resting temperature of the engine), has high thermal conductivity, a low viscosity and is non-corrosive and chemically stable, such as, for example, water (for operating temperatures below 100 degrees Celsius) and silicone oils, perfluorinate polyethers, and liquid sodium (for extremely high operating temperatures). A wide variety of methods may be used to heat the heating fluid. For example, the heating fluid/gas may be heated in a furnace that burns fossil and/or waste fuels. In other embodiments, the heating fluid may be heated by sunlight or geothermal heat. 
     The cooling fluid (i.e., the fluid in the cooling passages) preferably has good thermal conductivity, a low viscosity and remains a liquid at the resting and operating temperatures of the engine (i.e., the boiling point is above the operating temperature of the engine and the melting point is below the resting temperature of the engine), such as, for example, water at low to intermediate temperatures (i.e., below 100 degrees Celsius), silicone oils, perfluorinate polyethers and commercially available refrigerant liquids that are appropriate for the operating temperatures of the engine. In modified arrangements, it is anticipated that the cooling fluid/gas may be a low-melting-temperature metal alloy, such as, for example, Wood&#39;s metal, Bismuth, Lead-Tin solder, Bismuth-Tin allays and Mercury and/or Cadmium. Such metal allows are useful because they have high thermal conductivity and high boiling points, which allows the engine to be operated extremely high temperatures. Large temperature differentials between the hot and cold side of the engine increase the thermodynamic efficiency of the engine. 
     As with the heating fluid, a wide variety of methods may be used to cool the cooling fluid. For example, the cooling fluid may be cooled by passing the cooling fluid through a cooler, which uses ambient air or water. 
     FIG. 5 illustrates a modified embodiment of a Stirling engine  80  having certain features and advantages according to the present invention. In this embodiment, the engine  80  does not include hot passages and/or cold fluid passages. Instead, the hot side  68  of the engine block is exposed directly a heating source  82 , such as, for example, a flame or reflected sunlight. In a similar manner, the cold side  66  may be exposed to a heat sink  84 , such as, for example, ambient air or a cooling fluid. Preferably, external fins  86  are provided for increasing the heat transfer between the heat source  82  and/or heat sink  84 . More preferably, the external fins  86  form part of the fin plate  22 . 
     With reference back to FIG. 3A, it is readily apparent that one side of the fin plate  30  will be hot while the other side is cold. To prevent excessive heat transfer between the hot and cold sides, the fin plate  30  preferably includes an insulating slot  70 . In the illustrated arrangement, the slot  70  has a length that is approximately equal to the diameter of the chamber  14 . In a modified embodiment, the insulating slot  70  can be filled with an insulating material that is durable at high temperatures and has low thermal-conductivity, such as, for example, glass, solid ceramics or closed-cell materials that seal well, high temperature polymers, such as various phenolics or teflons. The slot  70  tends to reduce conductive heat transfer by reducing the effective cross-sectional area available for conductive heat transfer. In a modified embodiment, the fin plate  30  may be formed in two separate pieces with an insulating material, such as, for example, the insulating materials described above, separating the two pieces. In a similar manner, the chamber plate  32  may be formed in two separate pieces with an insulating material separating the two pieces. 
     With reference to FIG. 6, the internal fins  22  may be modified in several ways so as to increase the heat transfer to/from the working fluid. In FIG. 6, the fin portion  42  of the fin plate  30  includes a plurality of thin slots  88 . The slots  88  are designed to promote fluid flow between sub-chambers  24  and to increase turbulence within the chamber  14 . The slots  88  also increase the surface area of the internal fins  22 . As such, the slots  88  may increase heat transfer between the fins  22  and the working fluid. For corresponding applications, it is anticipated that the dimensions, shape, orientation and number of slots  22  may be further optimized through experimentation and/or modeling. 
     In the preferred embodiment described above, the displacer  16  is formed from an assembly of interchangeable flat plates configured to fit within the sub-chambers  24  between the fins  22 . Such an arrangement is useful because it provides a modular engine block  12 . That is, standard sizes of the fin plates  32  and chamber plates  30  may be mass produced and the engine size may be easily modified by varying the number of fin plates/chamber plates  30 ,  32  combinations. However, it should be appreciated that in modified embodiments, the engine block may be formed from a single or plurality of cast, extruded and/or milled blocks, which combine one or more features of the fin plates  30  and chamber plates  32  described above. 
     Each blade  18  of the displacer  16  has a generally half cylindrical shape and is configured to fit within the sub-chambers  24 . In the preferred embodiment, the displacer is configured such that a {fraction (1/16)}th-{fraction (1/32)}nd inch gap exists between the displacer  16 , the fins  22  and the inner surface of the chamber plates  32  though gaps of other sizes can be used. The rotary displacer  16  also includes a hub  88 , which is attached to the shaft  20 . The material that forms the displacer  16  preferably has a low thermal conductivity, a low mass density, a low coefficient of aerodynamic friction and retains adequate strength at high temperatures, such as, for example, Flourocarbon polymers, Fluorosilicate polymers, Glass, Glass-Epoxy composites, High-temperature thermosetting plastics, Magnesium alloys, Aluminum alloys, and/or ceramic foams or aluminum honeycomb. 
     FIGS. 7A-7D illustrate several modified embodiments of a rotary displacer. These modified embodiments provide for a displacer that is substantially counter-balanced. This can increase the efficiency of the engine  10  by reducing the energy required to rotate and stop the rotary displace  16   r . With initial reference to FIG. 7A, a rotary displacer  90  is formed from a first portion  92  made of a first material  92  (e.g., aluminum) and a second portion  94  made of a less dense second material (e.g., a closed cell foam). The first portion forms a frame with a first thickness T 1  on an open side  98  of the displacer  90  and a second thickness T 2  on a closed side  100  of the displacer  90 . On the closed side  100 , the second portion  94  fills the area between the frame  94  and a hub  101 . Given the relative densities of the first and second materials  92 ,  94 , the first and second thicknesses T 1 , T 2  may be selected to produce a rotary displacer  90  that is balanced about a central axis  102 . 
     In FIG. 7B, a displacer  103  includes a frame  104  with a generally uniform thickness. To balance the displacer  90 , a thick portion  106  is added to the frame  104  generally opposite the closed side  100 . As with the previous embodiment, given the relative densities of the materials of the first and second portion  92 ,  94 , the area of the thick portion  106  can be adjusted to balance the rotary displacer  90  about the central axis  102 . 
     FIG. 7C illustrates another embodiment of a displacer  110 . In this embodiment, the rotary displacer  110  is crescent shaped. End portions  112  of the crescent shaped displacer  110  lie on one side of the central axis  102  while a main portion  114  of the crescent lies on the other side of the central axis  102 . Weight plugs  116  (i.e., a material that is denser than the main portion  114  and end portions  112 ) are provided on the end portions  112  to balance the rotary displacer  110 . 
     FIG. 7D illustrates yet another embodiment of a rotary displacer  120 . In this embodiment, the rotary displacer  120  has a generally half-circular shape, which includes a main portion  122  located on one side of the central axis  102  and a weight portion  124  located on the other side of the central axis  102 . The weight portion  124  is wide enough to support a weight plug  126 , which is used to balance the rotary displacer  120  about the central axis  102 . 
     In other modified embodiments, the rotary displacer may be counter-balanced outside of the engine block  12 . For example, in such an arrangement, the shaft  20  (see FIG. 1) may extend outside the engine block and weights may be attached to the shaft  20 , generally opposite the displacer  16 , to counter-balance the rotary displacer  16 . 
     FIGS. 8A-C illustrate additional embodiments of a rotary displacer. These modified embodiments are designed to increase the heat transfer to/from the working fluid and the internal fins  22  and/or to promote the flow of working fluid between sub-chambers  24 . It should also be appreciated that these embodiments can also be used in combination with the embodiments described above with reference to FIGS. 7A-D. 
     With initial reference to FIG. 8A, a rotary displacer  130  has generally circular shape. One half  132  of the displacer includes a plurality of wide slots  134 . These slots  134  are designed to increase turbulence in the working fluid and thereby increase heat transfer between the working fluid and the internal fins  22 . A rotary displacer  136  in FIG. 8B includes a plurality of blades  138 , which are designed to perform the same function as the slots  134  of FIG.  8 A. As shown in FIG. 8C, the blades  138   a,b,c  may be shaped and orientated in a variety of ways. For corresponding applications, it is anticipated that the dimensions, shape, orientation and number of slots  134  or blades  138  may be further optimized through experimentation and/or modeling. 
     With reference back to FIG. 1, the displacer drive motor  25  is provided for rotating the displacer  16 . The displacer drive motor may  25  be of any suitable type, such as, for example, a DC servo motor or a high torque stepper motor. Preferably, the motor  25  is operatively connected to and controlled by a microprocessor. 
     The illustrated motor has an output shaft (not shown), which extends through the end assembly  50  and is coupled to the shaft  20 . To prevent leakage of the working fluid, the connection between the motor  25  and the end assembly  50  may be suitably sealed as described above. The motor  25  preferably is enclosed within motor cover  140 , which may be attached to the end assembly  50 . More preferably, the interior of the motor cover  140  is pressurized to a pressure that is substantially near or above the pressure of the working fluid. 
     In a modified embodiment, the motor may be situated within the engine block  12 . For example, the motor  25  may be situated within the shaft  20 . In such an embodiment, the motor  25  preferably is wirelessly connected to the microprocessor via, by way of example, infrared or RF signals. In another embodiment, the rotary displacer  16  may be rotated via a combination of magnets and/or magnetic materials. For example, magnetic material may be placed on/in the rotary displacer  16  and the rotary displacer  16  can be rotated by alternately subjecting to the rotary displacer  16  to the force of a magnetic field. In yet another embodiment, the rotary displacer  16  can be coupled to an output shaft of a piston, which is driven by the expansion and contraction of the working fluid. 
     As shown in FIG. 1, the illustrated embodiment utilizes a rolling sock piston  150  to convert the expansion and contraction of the working fluid into electricity. The rolling sock piston  150  comprises a piston chamber  152 , which is coupled or connected to the end assembly  50  so as to be in communication with the chamber  14 , a flexible membrane  154  and a piston rod  156 . The membrane  154  is attached to the interior of the chamber  152  to prevent the leakage of working fluid past the piston  150 . The piston rod  156  is coupled at a first end  158  to the membrane  154 . Preferably, a second end  160  of the rod  156  preferably is coupled to a transmission, flywheel and generator. These components are well known in the art and are used to convert the linear movement of the piston rod  156  to electricity. 
     Preferably, the piston chamber  154  is attached to the cold side  66  of the engine block  12  to reduce the heat exposure. It should be appreciated that in modified embodiments the engine  12  can include a plurality of rolling sock pistons  150  or other piston types. Moreover, the rolling sock pistons can be located at other positions on the engine  10 , such as, for example, the sides of the engine block  12 . 
     It should also be appreciated that there are many modified embodiments, which utilize different methods for converting the expansion and compression of the working fluid to electrical energy. For example, a linear alternator or voice coil generator can be used to convert the linear movement of the piston directly to electricity. In another embodiment, the expansion and contraction may be used to stress a piezoelectric material. In yet another embodiment, the expansion and contraction can be used to generate power through a reverse speaker. In such an arrangement, the reverse speaker can include a cone, which expands and contracts with the expansion and the compression of the working fluid. A voice coil is located at the apex of the cone and moves back and forth in accordance with the cone expansion and contraction. The voice coil is positioned within a magnetic field generated, by way of example, by a permanent magnet. The movement of the cone voice coil within the magnetic field causes a current to be generated in the voice coil. 
     In use, the drive motor  25  rotates the rotary displacer  16  to a first position, which is illustrated in FIG.  1 . In this position, the rotary displacer  16  occupies the cold side  66  of the chamber  14 . As such, most of the working fluid is located in the hot side  68  of the chamber  14 . Heat is transferred from the heat source to the working fluid through the fins  22 . This causes the working fluid to expand. As the working fluid expands, the piston is pushed to the left of FIG.  1 . The movement of the piston, in turn, may be converted to electricity as described above. 
     The motor  25  then rotates the displacer  16  from the first position to a second position. In the second position, the displacer  16  occupies the hot side  68  of the chamber  14 . As such, most of the working fluid in the hot side  68  of the chamber  14  is displaced and now occupies the cold side  66  of the chamber  14 . As such, heat is transferred from the working fluid to the heat sink through the internal fins  22 . This causes the working fluid to contract. As the working fluid contracts, the piston  150  is pulled to the right of FIG.  1 . This movement also may be converted to electricity as described above. 
     Preferably, the rotary displacer  16  is continuously rotated between the first and second positions at a rate of approximately 100 to 1000 revolutions per minute. FIG. 9 illustrates the sinusoidal movement of the working fluid from the hot side  68  of the chamber  14  to the cold side  66 . This sinusoidal movement is typical of many prior art Stirling engines. FIG. 9 also illustrates a square curve in which the working fluid is instantaneously moved from the hot side  68  to the cold side  66  of the chamber  14 . In terms of theoretical thermodynamic efficiency, this represents the ideal movement of the working fluid. However, to produce such a square curve would dramatically increase aerodynamic drag and require large amounts of energy to move and stop the rotary displacer  16 . Therefore, the costs associated with the square curve must be balanced with respect to the thermodynamic advantages. 
     In the illustrated embodiment, the displacer  16  can be precisely controlled by the drive motor  25 . For example, the rotational speed of the displacer  16  can be varied within a single revolution. Such precise control of the movement of the displacer  16  is not possible with many prior art Stirling engines. Because the illustrated embodiment provides for such precise control, the motion of the displacer  16  can be varied from the typical sinusoidal movement and optimized using a general or special purpose, computer, or neural net using, by way of example, a predictive adaptive method and/or fuzzy logic algorithm. Preferably, this involves varying the motion of the displacer  16  and using a feedback loop that utilizes measurements of system performance and/or models. For example, (i) a table can be used to lookup the next position and/or velocity of the displacer given the current piston position and/or velocity and/or displacer shaft position and velocity, (ii) a finite-state machine can be used to yield the next displacer positioned and/or velocity a based on the current engine state, (iii) an equation can be used that yields the next displacer position as a function of displacer velocity, current displacer position and/or piston position and (iv) an equation, which synchronizes displacer phase and piston phase with desired generator power output, current wave form phase and frequency can also be used. 
     For corresponding applications, several other features of the engine can be further optimized using experimentation and/or modeling. For example, the aerodynamic shape of the rotary displacer may be further optimized to minimize drag, reduce/enhance turbulence, conductive heat transfer and/or convective heat transfer. The width of the blades, the rotary displace and/or the fins also may be further optimized with respect to, by way of example, the efficient expansion/contraction of the working fluid, movement of the working fluid between hot and cold segments the engine, the thermal transfer and rate of thermal transfer between the fins, the engine block, and the working fluid. 
     An important design parameter is the pressure of the working fluid. In general, increasing the pressure of the working fluid increases the thermal efficiency of the engine. Of course, the pressure of the working fluid must be balanced against, for example, safety and the costs and mechanical complexity of sealing the engine. In one preferred embodiment, the working fluid is at a pressure greater than approximately 20 atmospheres. 
     The working fluid itself preferably has a low coefficient of aerodynamic friction, a low viscosity, a high thermal conductivity, a high coefficient of thermal expansion and is non-reactive with other engine materials, such as, for example, Air, Helium, Hydrogen and Argon. Other embodiments use a liquid-gas phase-changing working fluid with boiling points within the operating range of the engine, such as, for example, Water, fluorocarbons and commercial refrigerants. 
     FIG. 10 illustrates another embodiment of a rotary Stirling engine  170 . In this embodiment, a single rotary displacer  172  is positioned within an engine block  174 . The engine block  176  defines a chamber  178 , which is not divided into sub-chambers by internal fins. As such, heat is transferred to/from the working fluid through the side walls  180  of the engine block  176 . 
     As shown in FIG. 10, the rotary displacer  172  may include turbulence generators  182 , which in the illustrated arrangement comprise a plurality of blades. The turbulence generated by the turbulence generators promote more efficient heat transfer to/from the engine walls  180 . The illustrated embodiment also includes a pair of fans  184 , which force/pull air across the hot and cold sides  68 ,  66  of the chamber  178 . 
     FIGS.  11  and  12 A-C illustrate an embodiment of a four-quadrant Stirling engine  200  having certain features and advantages according to the present invention. In this embodiment, the engine  200  includes an engine block  201  formed by a series of fin plates  203  and chamber plates  205 . The engine block  201  has two cold corners  202  and two hot corners  204 . The cold corners  202  are cooled by cooling passages  206  and the hot corners  204  are heated by heating passages  208  (see FIG. 12A) formed in the fm plates  203  and chamber plates  205 . A rotary displacer  210  is positioned within the engine block  201  and includes a first lobe  212  and a second lobe  214 , which fill opposite corners of a chamber  216 , which is defined by the engine block  201 . To prevent heat transfer between the quadrants, the fin plates  201  are provided with a pair of slots  218 , which partially separate the corners. In a modified arrangement that is illustrated in FIG. 13, each corner  219  is a separate piece, which is separated from the other corners by insulating material  220 . One advantage of the four-quadrant Stirling engine  200  is that the rotary displacer  210  is balanced about a central axis  222  of the engine  200 . 
     It should be appreciated that many of the modified embodiments described above with respect to the rotary Stirling engines  10 ,  80 ,  170  can also be applied to the four-quadrant Stirling engine of FIGS. 11-13. 
     FIG. 14A is a schematic cross-sectional view of an embodiment of a pendulum Stirling engine  250  having certain features and advantages according to the present invention. In this embodiment, the Stirling engine  250  comprises an engine block  252  that has a generally triangular cross-section. The engine block  252  defines a chamber  254  for the working fluid. A pendulum displacer  256  is positioned in the chamber  254  and is journalled for reciprocal motion about a pivot axis  258 , which is positioned at one apex  260  of the engine block  252 . The displacer  256  is generally configured to occupy half of the chamber  254 . The engine block  252  has a hot side  262  and a cold side  264 , which can be heated or cooled in several different ways as described above. For example, cooling and heating passages can be formed in the walls of the engine block  252  and/or the walls of the engine block  252  can be exposed directly to a heat sink and/or heat source. Between the hot side and the cold side is a base  266 , which may be curved, as illustrated, or flat. One or more rolling sock pistons (not shown) may be positioned on the base  266  or any other suitable location for capturing the energy from the expansion and contraction of the working fluid as the pendulum displacer  256  is moved back and forth within the chamber  254 . 
     FIG. 14B illustrates a modified embodiment of a pendulum Stirling engine  270 . In this embodiment, the cold side  264 , hot side  262 , and base  266  are separated by an insulating material  272 . This reduces heat transfer between the cold side  264  and hot side  262 . 
     As with the four-quadrant engine, it should be appreciated that many of the modified embodiments described above with respect to the rotary Stirling engine can also be applied to the pendulum Stirling engine of FIGS. 14A and 14B. For example, the engine block can be formed from a series of chamber plates and fin plates, which define a plurality of sub-chambers. In such an arrangement, the pendulum displacer can include a plurality of blades positioned within the sub-chambers. In another example, the pendulum displacer can include blades and/or slots to promote turbulence and heat transfer to/from the working fluid. 
     FIGS. 15A and 15B illustrates an embodiment of a radial Stirling engine  300  having certain features and advantages according the present invention. As shown in FIG. 15, the engine block comprises a inner cylinder  302  and an outer cylinder  304 . The space between the two cylinders defines a chamber  306  for the working fluid. In this arrangement, the inner cylinder  302  is the hot side of the Stirling engine  300  while the outer cylinder  304  is the cold side of the engine  300 . An iris-type displacer  308  is used to alternately expose the working fluid to the cold side  304  and the hot side  302 . In a modified arrangement, the outer cylinder  304  may be the hot side and inner cylinder  302  may be the cool side. The working fluid is alternately expanded and contracted by expanding and contracting the iris displacer  308 . In the position shown in FIG. 15A, the displacer  308  is contracted and most of the working fluid is in contact with the cold side  304  of the engine  300 . In the position shown in FIG. 15B, the displacer is expanded and most of the working fluid is in contact with the hot side  302  of the engine. 
     As with the previous embodiments, it should be appreciated that many of the modified embodiments described above with respect to the rotary Stirling engine can also be applied to the radial Stirling engine of FIGS. 15A-B. 
     FIG. 16 illustrates another embodiment of a Stirling engine  350  having certain features and advantages according to the present invention. This embodiment uses an air circulator  352  instead of a displacer to move the working fluid from a cold side  360  of the engine  350  to a hot side  358  of the engine  350 . As shown in FIG. 16, the engine  350  includes an engine block  356 , which comprises the hot side  358 , a the cold side  360  and a working fluid section  362 . As explained above, the hot side is  358  exposed to a hot thermal source and the cold side  360  is exposed to a thermal sink. 
     The hot side  358 , cold side  360  and working fluid section  362  respectively define a hot path  364 , a cold path  366  and a working fluid space  368 . The hot path  364  is connected to the working fluid space  368  by an inlet  370  and an outlet  372 . The inlet  370  includes an inlet valve  374 , which, in the illustrated embodiment, is an active valve, such as, for example, electromechanical or pneumatic valve. The active valve  374  preferably is operatively connected to and controlled by a control system  376 , which, by way of example, can be based on a microprocessor as discussed above. The outlet  372  includes an outlet valve  378 , which, in the illustrated embodiment, is a passive valve  378 , such as, for example, a check valve. The passive valve  378  is configured to allow working fluid to flow from the hot path  364  into the working fluid space  368  while preventing working fluid from flowing into the hot path  364  from the working fluid space  368 . In modified embodiments, the inlet valve  374  can be passive while the outlet valve  378  is active. In another embodiment, both the inlet and the outlet valves  374 ,  378  may be active or only one active valve may be provided in the hot path  364 . It should also be appreciated that the valves  374 ,  378  may be moved upstream and/or downstream from the inlet  370  and/or outlet  372 . 
     In a similar manner, the cold path  366  is also connected to the working fluid space  368  by an inlet  380  and an outlet  382 . The inlet  380  includes an inlet valve  384 , which, in the illustrated embodiment, is an active valve, which preferably is operatively connected to and controlled by the control system  386 . The outlet  382  also includes an outlet valve  386 , which, in the illustrated embodiment, is a passive valve, such as, for example, a check valve. The passive valve  386  is configured to allow working fluid to flow from the cold path  366  into the working fluid space  368  while preventing working fluid from flowing into the cold path  366  from the working fluid space  368 . As with the hot path  364 , in modified embodiments, the inlet valve  384  may be passive while the outlet valve  386  is active. In other embodiments, both the inlet and the outlet valves  384 ,  386  can may be active or only one active valve may be provided in the cold path  366 . Moreover, the valves  384 ,  386  may be moved upstream and/or downstream from the inlet and/or outlet. 
     In one embodiment, the hot side  358  and the cold side  360  are formed from U-shaped pipes  390 . In such an embodiment, each end  392  of the U-shaped pipe corresponds to an inlet  370 ,  380  and an outlet  372 ,  382  respectively. In some embodiments, the hot and/or cold side  358 ,  360  may be formed from a single or plurality of cast, extruded and/or milled blocks that are made, by way of example, stainless steel and/or copper. In other embodiments, the hot and/or side  358 ,  360  may be formed from sheets of material that are bent and welded together. 
     Preferably, the working section  362  is insulated from the hot and cold sides  358 ,  360  of the engine  350  and the volume of the working section  362  is significantly larger either than the volume of the hot and/or cold paths  364 ,  366 . A working fluid circulator  352 , such as, for example a fan, impeller and/or pump, is preferably positioned within the working fluid space  368 . As will be explained in more detail below, the working fluid circulator  352  is configured to move the working fluid alternately through the hot path  364  and the cold path  366 . In modified embodiments, the engine  350  may include a plurality of air circulators. In such an arrangement, the air circulators can be located, by way of example, in the hot path  364 , the cold path  366 , and /or the working fluid space  368 . The air circulator  352  preferably is operated in a continuous manner although in modified embodiments the air circulator  352  can be intermittently operated. 
     The illustrated embodiment utilizes a linear alternator piston  380  to convert the expansion and contraction of the working fluid into electricity. The linear alternator piston  380  comprises a piston chamber  382  that is connected to the working fluid space  368 . A piston  384  is suitably journalled for movement within the chamber  382 . As such, the piston  384  moves back and forth with the expansion and contraction of the working fluid. By way of example, a permanent magnet is provided on the piston  384  for generating a magnetic field and a coil  386  is provided around the piston chamber  382 . Thus, the movement of permanent magnet on the piston causes a current to be generated by the coil  386 . Of course, as mentioned above, there are many modified embodiments, which may utilize different methods for converting the expansion and compression of the working fluid to electrical energy. To transfer heat to/from the working fluid in the hot and cold fluid paths  364 ,  366 , both the hot side  358  and the cold side  360  preferably include heat exchangers  392 , such as, by way of example, internal fins that extend from the walls of the engine  350  into the hot or cold paths  364 ,  366  or a fibrous material (e.g., a copper wool). In other embodiments, heat can be transferred to/from the working fluid through the walls of the engine block  352 . 
     In use, the working fluid is circulated within the engine by the air circulator  352 . In a first position, the valve control system  376  the inlet valve  374  to the hot path  364  is open and the inlet valve  384  to the cold path  366  is closed while the check valves  378 ,  386  prevent the flow of working fluid into the outlets  356 ,  382  of the hot and cold paths  364 ,  366 . As such, in this position, most of the working fluid is circulated through the hot path  364  and heat is transferred from the heat source to the working fluid through the heat exchanger  392 . This causes the working fluid to expand. As the working fluid expands, the piston is pushed to the right of FIG.  16 . The movement of the piston, in turn, may be converted to electricity as described above. 
     The valve control system  376  then closes the inlet valve  374  to the hot path  364  and opens the inlet valve  384  to the cold path  366  while the check valves  378 ,  386  continue to prevent the flow of working fluid into the outlets  372 ,  382  of the hot and cold paths  364 ,  366 . As such, in this second position, most of the working fluid is circulated through the cold path  366 . As such, heat is transferred from the working fluid to the heat sink through the heat exchanger  392 . This causes the working fluid to contract. As the working fluid contracts, the piston  384  is pulled to the left of FIG.  16 . This movement also maybe converted to electricity as described above. 
     In a manner similar to the rotary displacer described above, the timing of the opening and closing of the inlet valves  374 ,  384  can be further optimized using a general or special purpose computer, or neural net using, by way of example, a predictive adaptive method and/or fuzzy logic algorithm. In a similar manner, the volume of working fluid circulated by the working fluid circulator  352  can also be further optimized. 
     FIG. 17 illustrates another modified embodiment of a Stirling engine  400  that uses an air circulator  402  instead of a displacer to move the working fluid from the cold side  360  of the engine to the hot side  358  of the engine  400 . In FIG. 17, the same reference numbers will be used to describe components substantially similar to components shown in FIGS.  16 . In this embodiment, the air circulator  402  is a deep impeller squirrel cage fan. The fan  402  is driven by a motor  404 , which may be located within the working fluid space  368 . In a modified embodiment, which is shown in FIG. 18, a deep impeller conical squirrel cage fan  406  can be used as the air circulator  402 . An annular port  408  is preferably located at an inlet  410  of the fan  406  to prevent working fluid from bypassing the fan  406 . 
     In the embodiment illustrated in FIG. 17, the inlet and outlet valves for the hot and cold path  364 ,  366  are replaced with two rotor valves  412 ,  414 , which are also shown in FIG.  19 . As shown in FIG. 19, the rotor valves  412 ,  414  comprise a hollow, cup-shaped, rotor portion  416 , which fits inside a hollow stator portion  418 . The rotor portion  416  includes a passage  420  while the stator portion includes first and second passages  422 ,  424 . Although the illustrated passages  420 ,  422 ,  424  are square, they may be formed into other shapes, such as, for example, a circle. 
     The rotor portion  416  is connected to a rotor shaft  426  such that rotor portion  416  can be rotated with respect to the stator portion  418 . As such, the first and second passages  422 ,  424  of the stator portion  418  can be alternately covered and opened. Preferably, the first passage  422  is in communication with the hot path  364  while the second passage  424  is in communication with the cold path  366 . Correspondingly, an interior space  428  of the stator portion is in communication with the working fluid space  368 . In this manner, by opening and closing the first and second passages  422 ,  424 , the working fluid in the working fluid space  368  can be alternately directed to the hot path  364  and the cold path  366 . 
     With reference back to FIG. 17, the rotor shaft, in the illustrated embodiment, is rotated by the same motor  404  that powers the working fluid circulator  404 . A gear arrangement  430  (e.g., elliptical and/or half gear) can be used to control the timing of the opening and closing of the first and second passages  422 ,  424 . In modified embodiments, either or both of the rotor valves  412 ,  414  may be controlled by a separate motor. In another modified arrangement, the outlet valves  412 ,  414  of the hot and cold paths  364 ,  366  may be replace by a passive valve or an active valve. 
     As with the previous embodiments, it should be appreciated that many of the modified embodiments described above can also be applied to the radial Stirling engine of FIG. 16 or  17 . 
     FIG. 20 shows a modified embodiment of the hot path  364  for the Stirling engines  350 ,  400  of FIG. 16 or  17 . In this embodiment, the hot path  364  includes a manifold portion  434  in which the hot path  364  is divided into a series of smaller paths  436 . By way of example, the smaller paths  436  may be defined by a plurality of ducts and/or pipes  438 , which can be made of a high thermally conductive material, such as, for example, copper. In the illustrated embodiment, the pipes  438  are bundled together in a hexagonal pattern in which each individual pipe  438  is spaced approximately ⅜ of an inch from each other. In such an embodiment, the manifold  434  is formed from 19 tubes  438  with a 0.5 inch outer diameter, which can be arranged within a 4 inch circle. 
     A reflector  440 , which in the illustrated embodiment comprises a thin sheet of stainless steel, is positioned around at least a portion of the manifold  434 . The reflector  440  is configured to reflect heat generated by a heat source  442 , which, by way of example, may be a natural gas flame burner. The reflector  442  improves heat transfer to the tubes  438  furthest from the heat source  442  by reflecting radiation. A thermal insulator  444  preferably is provided on the side of the reflector  442  opposite the tube bundle (i.e., manifold)  434  to minimize heat loss. 
     FIGS. 21A-25B illustrate several embodiments of a regenerator  500  that can be used with the Stirling engines embodiments described above. As will be explained in more detail below, the regenerator  500  is used to store energy from the working fluid as it flows towards the cold side of the engine and gives energy to the working fluid as the working fluid flows through the regenerator  500  to the hot side of the engine. One advantage of the illustrated embodiments is that the regenerator  500  is moveable with respect to the engine. Such an arrangement conserves space and reduces the weight and complexity of the engine. The embodiments described below will be described in the context of an air circulator-type Stirling engine such as is illustrated in FIGS. 16 and 17. However, it should be appreciated that the regenerator  500  may also be used with the rotary and pendulum engines described herein and/or with other Stirling engine configurations. 
     FIGS. 21A-C illustrate one embodiment of a regenerator  500  positioned within the Stirling engine  350  of FIG.  16 . In the illustrated embodiment, the regenerator  500  is positioned at an outlet  502  of the working fluid space  368  and is configured to alternately direct working fluid to the inlets  370 ,  380  of the hot and cold paths  364 ,  366 . 
     The regenerator  500  comprises a valve housing  504 , which defines a generally circular valve chamber  506 . The valve housing  504  includes first  508 , second  510  and third openings  512 , which place the working fluid space  368 , the hot fluid path  364  and the cold path  366  each in communication with the valve chamber  506 . A generally cylindrical valve  514  is positioned within the valve housing  504  and is journalled for movement within the valve housing  504 . Specifically, the valve  514  is journalled for rotation between at least a first position illustrated in FIG. 21A and a second position illustrated in FIG.  21 B. More preferably, the valve  514  is also journalled for rotation between a third position illustrated in FIG.  21 C. Most preferably, the valve  514  can be rotated 360 degrees within the valve housing  504  in an oscillating manner or continuously in one direction. In one embodiment, an electric motor can be coupled to the valve  514  to rotate the valve  514 . In another embodiment, the valve  514  can be coupled by to the piston by a gear arrangement. In still another arrangement, the valve  514  can be rotated by a combination of magnets. 
     The valve  514  includes an inner surface  516 , which defines a flow path  518  that has a first end  520  and a second end  522  positioned on an outer cylindrical surface  523  of the valve  514 . As shown in FIG. 21A, in the first position, the valve  514  is configured to place the working fluid space  368  in communication with the hot path  364 . That is, in the first position the first end  520  is aligned with the first opening  508  and the second end  522  is aligned with the second opening  510 . In this manner, the rotary regenerator  500  directs working fluid from the working fluid space  368  to the hot path  364 . 
     The valve can be rotated in the direction of arrow A from the first position to the second position (see FIG.  21 B). In the second position, the second side  522  of the flow path  518  is aligned with the first opening  508  and the first side  520  is aligned with the third opening  512 . In this manner, the regenerator  500  directs working fluid from the working fluid space  368  to the cold path  366 . 
     As mentioned above, the regenerator  500  can be configured to rotate to a third position, which is illustrated in FIG.  21 C. In this position, the first and second sides  520 ,  522  of the flow path  518  are not aligned with the openings  508 ,  510 ,  512  or are aligned with only one of the openings  508 ,  510 ,  512  as in the illustrated embodiment. In this manner, the working fluid cannot flow through the regenerator  500 . 
     The regenerator  500  preferably includes a heat absorber/transfer device  524  that is configured to absorb heat from the working fluid as it flows from the working space  368  to the cold path  366  and to heat the working fluid as it flows from the working space  368  to the hot path  364 . The heat absorber/transfer device  524  can be formed in a variety of ways. In the illustrated embodiment, the heat absorber/transfer device  524  comprises a matrix of a material that has a high thermal conductivity and a high heat capacity, such as, for example, copper. In one preferred embodiment, the heat absorber/transfer device is a fibrous material (e.g., a copper wool) In other embodiments, internal fins can be placed within the path  518  and the valve  514 . 
     When the regenerator  500  is initially rotated to the first position (FIG.  21 A), the cold working fluid absorbs heat as it passes through the heat absorber/transfer device  524 . As will be apparent from the description below, the heat absorber/transfer device is generally colder near the first end  520  as compared to the second end  522 . As such, the working fluid is gradually heated as it flows from through the regenerator  500 . 
     When the regenerator  500  is rotated to the second position from the first position, the second end or hotter end  522  of the valve  514  is aligned with the working fluid space  368  and the first or colder end  520  is aligned with the cold path  366 . As such, hot working fluid, which is now directed to the cold path  366  is gradually cooled as it flows through the regenerator  500 . That is, the regenerator  500  absorbs heat from the working fluid before the working fluid passes into the cold path  366 . This heat is transferred back to the working fluid when the regenerator  500  is rotated back to the first position as described above. 
     In the third position, FIG. 21C, working fluid cannot flow through the valve  514  and flow through the engine  350  is temporarily stopped or slowed. 
     FIGS. 22A-23B illustrates a modified embodiment of a regenerator  550 . In this embodiment, the regenerator  550  includes a generally cylindrical valve  552 , with at least a first end  554  and an outer cylindrical surface  555 . The valve  552  preferably defines a generally U-shaped internal path  556  with first and second openings  558 ,  560  located on the first end  554  of the valve  552 . The illustrated valve  552  is configured to rotate about a longitudinal axis  562 . Positioned within the path  556  is a heat absorber transfer/device  564  as described above. 
     In a first position, illustrated in FIGS. 22A and 23B, the first opening  558  is aligned with an outlet  566  of the working fluid space  368  and the second opening  260  is aligned with the inlet  568  of the hot path  364 . In this manner, the working fluid is heated as it is transferred to the hot path  364  as described above with respect to FIG.  21 A. In a second position, the second opening  560  is aligned with the outlet of the working fluid space  368  and the first opening  558  is aligned with an inlet  570  to the cold path  366 . In this manner, heat is removed from the working fluid as it is transferred to the cold path  366  as described above with respect to FIG.  21 A. In a modified embodiment, the hot path  364 , cold path  366  and/or the working space  368  or portions thereof can be rotated with respect to the regenerator  550 . 
     FIGS. 24-25B illustrate yet another embodiment of a regenerator  600 . This embodiment includes a valve housing  602 , which defines a generally cylindrical valve chamber  604 . The illustrated housing  602  includes two inlet ports  604   a ,  604   b , which define inlets paths that are in communication with the valve chamber  604  and the working space  368  of the Stirling engine. The housing  602  also includes two outlet ports  606   a ,  606   b , which also define outlet paths that are also in communication with the valve chamber  602 . The first outlet port  606   a  is in communication with the cold path  366  of the engine and the second outlet port  606   b  is in communication with the hot path  364  of the engine. 
     Positioned with the valve chamber  602  is a rotary assembly  610 . The rotary assembly includes a cold side rotor  612 , a hot side rotor  614  and a regenerator housing  616 , which defines a regenerator path  617  in which a heat absorber/transfer device  618  is positioned. The cold side rotor includes an end portion  620 , a side portion  622 , and a channel  624 . As will be explained in more detail below, the cold side rotor  612  is configured to rotate within the housing  602 . As best seen in FIG. 25A in a first position, the side portion  622  blocks the first outlet port  606   a  and the channel  624  is in communication with the regenerator path  617  and the first inlet  604   a . In a second position (FIG.  25 B), the side portion  622  blocks the first inlet  604   a  and the channel  624  is in communication with the regenerator path  617  and the cold side first outlet port  606   a.    
     Similarly, the hot side rotor  614  also includes an end portion  630 , a side  632  portion, and a channel  634  (see FIG.  25 A). As best seen in FIG. 25A, in a first position, the side portion  632  blocks inlet port  606   a  and the channel  634  are in communication with the regenerator  618  and the hot side outlet port  606   b . In a second position (FIG.  25 B), the side portion  632  blocks hot side outlet port  606   b  and the channel  634  are in communication with the regenerator  618  and the cold side outlet  606   a.    
     The regenerator housing  616  is positioned between the hot and cold rotors  612 ,  614 , and the regenerator path  617  connects the channels  624 ,  634  of the hot and cold rotors  612 ,  614 . Preferably, the rotors  612 ,  614  and the regenerator housing  616  are coupled together and rotate about a common axis  640 . In the illustrated embodiment, the end portions  620 ,  630  include shafts  642 , which are journalled for rotation on end assemblies  644 , which close the valve chamber  604 . As such, the hot rotor, the cold rotor, and the regenerator housing  616  define a passage  641  through the rotor assembly  610 . An electric motor or gear arrangement can be coupled to the shafts  642  to rotate the assembly  610 . In a modified embodiment, the regenerator housing  616  can be stationary with respect to the valve housing  602  while the hot and cold rotors  612 ,  614  rotate within the housing  602  either independently or in conjunction with each other. 
     With reference to FIG. 25A, when the rotary valve  610  is in a first position, working fluid can flow from the first port  604   a  into the regenerator  618 , through the hot side outlet  606   b  and into the hot path  364 . In this manner, the working fluid is heated as it is transferred to the hot path  364  as described above with respect to FIG.  21 A. In a second position (FIG.  25 B), the working fluid can flow through the second inlet port  604   a  and into the regenerator  618 , through the cold side outlet  606   a  and into the cold path  366 . aligned with the working fluid space and the first opening is aligned with the cold path. In this manner, heat is removed from the working fluid as it is transferred to the cold path  366  as described above with respect to FIG.  21 A. 
     Although this invention has been disclosed in the context of certain preferred embodiments and examples, it will be understood by those skilled in the art that the present invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof. In addition, while a number of variations of the invention have been shown and described in detail, other modifications, which are within the scope of this invention, will be readily apparent to those of skill in the art based upon this disclosure. It is also contemplated that various combination or sub-combinations of the specific features and aspects of the embodiments may be made and still fall within the scope of the invention. Accordingly, it should be understood that various features and aspects of the disclosed embodiments can be combine with or substituted for one another in order to form varying modes of the disclosed invention. Thus, it is intended that the scope of the present invention herein disclosed should not be limited by the particular disclosed embodiments described above, but should be determined only by a fair reading of the claims that follow.