Patent Publication Number: US-2023142663-A1

Title: Stirling engine

Description:
TECHNICAL FIELD 
     The present invention relates to a Stirling engine. 
     BACKGROUND ART 
     A Stirling engine can recover motive power from a wide variety of high temperature heat sources. In recent years, the Stirling engine has attracted attention as an exhaust heat recovery/power generation technique from existing high-temperature exhaust heat (from waste incineration plants, factory furnaces, and the like). In the Stirling engine, spaces of a heater heat exchanger, a regenerator, and a cooler heat exchanger are connected in this order to a high-temperature space (expansion space) above the piston. The Stirling engine generates motive power by inserting a heater heat exchanger into a high-temperature heat source and absorbing heat therefrom. 
     Conventional Stirling engines (for example, Patent Documents 1 to 3) are structured such that a heater heat exchanger is directly connected to an expansion space and a regenerator, and the heater heat exchanger and the engine (including the expansion space) are arranged in proximity to each other. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: JP-A-7-259646 
     Patent Document 2: JP-A-10-213012 
     Patent Document 3: Japanese Patent No. 5533713 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the conventional Stirling engines, there is a problem that the degree of freedom of installation of the heater heat exchanger is small, and it is difficult to install the engine in accordance with various high temperature heat sources. For example, according to Patent Document 1, since the heater heat exchanger is arranged in the piston sliding direction (on the cylinder axis) of the engine, if the pipe for the high-temperature heat source gas is installed just beside the engine, the heater heat exchanger cannot be inserted into the heat source pipe. 
     The present invention has been made in view of the above problem. An object of the present invention is to provide a Stirling engine having a high degree of freedom in installation of a heater heat exchanger. 
     Solution to Problem 
     In order to solve the above problem, a Stirling engine of the present invention is a Stirling engine including an engine unit, a heater heat exchanger, a regenerator, and a cooler heat exchanger. An engine main body including at least the engine unit and the cooler heat exchanger and a heater structure including at least the heater heat exchanger are separately structured. The engine main body and the heater structure are connected via a coupling pipe portion. 
     According to the above configuration, the positional relationship between the engine main body and the heater structure can be easily changed by altering the shape of the coupling pipe portion (for example, by replacing the coupling pipe portion). As a result, the degree of freedom in installation of the heater heat exchanger is increased, so that the heater heat exchanger can be easily installed in a wide variety of high temperature heat sources. 
     In the Stirling engine, the regenerator and the cooler heat exchanger may be arranged behind a cylinder, and an upper end position of the regenerator may be above an upper end position of the cylinder. 
     According to the above configuration, setting the upper end position of the regenerator above the upper end position of the cylinder makes it easy to secure the arrangement space of the coupling pipe portion among the regenerator, the cylinder, and the heater heat exchanger. 
     The Stirling engine may be a double-acting engine in which a plurality of cylinders is arranged linearly with respect to a crankshaft of the engine unit. 
     Further, in the Stirling engine, in the heater heat exchanger, the heater thin tube group for a plurality of cylinders may be arranged in an annular shape. 
     According to the above configuration, the compact arrangement of the heater thin tube group can be realized by annularly arranging the heater thin tube group for the plurality of cylinders in the heater heat exchanger. 
     The Stirling engine may be arranged such that a longitudinal direction of the heater heat exchanger intersects a sliding direction of a piston in the cylinder. 
     The Stirling engine may include a first support member that holds the heater structure. 
     The Stirling engine may include a second support member that holds the regenerator. 
     The Stirling engine may include an on-off valve on a working fluid path connecting a low-temperature chamber of the cylinder and the cooler heat exchanger, and the working fluid path may be partially closed by the on-off valve during stoppage of the engine. 
     According to the above configuration, the stop control of the engine can be safely performed using an inexpensive valve such as a butterfly valve. In addition, since the on-off valve partially closes the working fluid path, it is possible to prevent a load (compression pressure) applied to the closed path from becoming too large, and avoid occurrence of damage to the components and the like. 
     The Stirling engine may include a bypass path that connects low-temperature chambers of cylinders with a phase shift of 180°, and a communication valve provided on the bypass path, and the communication valve may be closed to close the bypass path during operation of the engine, and the communication valve may be opened to conduct the bypass path during stoppage of the engine. 
     According to the above configuration, since the low-temperature chambers of the cylinders with a phase shift of 180° communicate with each other, it is possible to promptly stop the engine without applying an overload to the components or the like when the engine is to be stopped. 
     In addition, the Stirling engine can be configured such that the engine output is adjustable by controlling the on-off valve or the communication valve to an arbitrary opening degree during operation of the engine. 
     According to the above configuration, since the engine output is adjustable, when the temperature of the high-temperature heat source is excessively increased, for example, the engine output can be reduced to protect the components of the engine. 
     The Stirling engine may include a starter motor for starting the engine, start the starter motor in a state where the communication valve is opened at a time of starting the engine, and close the communication valve after starting the engine to stop the starter motor. 
     According to the above configuration, the engine load is reduced by opening the communication valve at the time of starting the engine, so that a small starter motor can be used. 
     In the Stirling engine, the regenerator may be included in the engine main body. 
     According to the above configuration, since both the regenerator and the cooler heat exchanger have a cylindrical similar shape, the regenerator is included in the engine main body, and the regenerator and the cooler heat exchanger are connected to each other in a constant manner, which is advantageous for downsizing the Stirling engine. 
     In the Stirling engine, each of the coupling pipes configuring the coupling pipe portion may be configured such that a heat storage is provided in the coupling pipe wall over the entire pipeline. 
     According to the above configuration, the connection pipe can have the same function as the regenerator by the heat storage action of the heat storage, so that the output of the Stirling engine can be improved by effectively using the heat. 
     In the Stirling engine, the heat storage may have a cavity portion in the center. 
     According to the above configuration, the cavity portion serves as a passage for the working fluid, so that it is possible to restrain an increase in pressure loss due to the heat storage in the coupling pipe. 
     In the Stirling engine, the coupling pipe portion may be attachable to and detachable from the engine main body and the heater structure, and have a metal O-ring arranged on a sealing surface between the coupling pipe portion and a member to be connected. 
     According to the above configuration, since the coupling pipe portion is attachable to and detachable from the engine main body and the heater structure, the positional relationship between the engine main body and the heater structure can be easily changed, and the use of the metal O-ring enables sealing at a place requiring resistance to high temperatures. 
     Advantageous Effects of Invention 
     In the Stirling engine of the present invention, the engine main body and the heater structure have separate structures, and the engine main body and the heater structure are connected to each other via the coupling pipe portion, so that the degree of freedom in installation of the heater heat exchanger is increased, and the heater heat exchanger can be easily installed in a wide variety of high temperature heat sources. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a perspective view of an embodiment of the present invention, which illustrates an outer appearance of a Stirling engine; 
         FIG.  2    is a perspective view of the Stirling engine of  FIG.  1    as viewed from a different direction; 
         FIG.  3    is a schematic diagram illustrating a schematic configuration of the Stirling engine; 
         FIG.  4    is an explanatory diagram illustrating an example of a positional relationship between the Stirling engine and a high-temperature heat source; 
         FIG.  5    is an explanatory diagram illustrating an example of a couple of forces acting on a crankshaft; 
         FIG.  6    is an explanatory diagram illustrating a preferred example of a couple of forces acting on the crankshaft; 
         FIG.  7    is a schematic diagram illustrating a schematic configuration of the Stirling engine; 
         FIG.  8    is an enlarged perspective view of a coupling pipe portion in the Stirling engine; 
         FIG.  9    is a diagram illustrating an arrangement relationship among a regenerator, a cooler heat exchanger, and cylinders in the Stirling engine; 
         FIG.  10    is a plan view illustrating an arrangement example of a heater thin tube group in the heater heat exchanger; 
         FIG.  11    is a perspective view illustrating an outer appearance example of a coupling pipe; 
         FIG.  12    is a schematic diagram illustrating a schematic configuration of the Stirling engine; and 
         FIG.  13    is a cross-sectional view of the coupling pipe. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.  FIGS.  1  and  2    are perspective views illustrating an outer appearance of a Stirling engine  10  according to a first embodiment.  FIG.  3    is a schematic diagram illustrating a schematic configuration of the Stirling engine  10 . 
     As illustrated in  FIGS.  1  and  2   , the Stirling engine  10  includes an engine unit  11 , a heater heat exchanger  12 , a regenerator  13 , a cooler heat exchanger  14 , and a coupling pipe portion  15 . In addition, in the Stirling engine  10  illustrated in  FIGS.  1  and  2   , a generator  20  is connected to a crankshaft  115  (see  FIG.  3   ) of the engine unit  11 , and the generator  20  can generate power by driving the Stirling engine  10 . 
     In the Stirling engine  10 , the heater heat exchanger  12  is inserted into a high-temperature heat source (for example, a high-temperature pipe through which a high-temperature fluid flows), and the working fluid is heated in the heater heat exchanger  12 . In the cooler heat exchanger  14 , the working fluid is cooled by cooling water (a cooling water supply unit is not illustrated). The Stirling engine  10  is designed to drive the engine unit  11  by the movement of the working fluid thus heated/cooled. Although the engine unit  11  may be a single-cylinder type engine or a multi-cylinder type engine, the four-cylinder type engine unit  11  is exemplified in the first embodiment. 
     As illustrated in  FIG.  3   , the engine unit  11  includes four cylinders  111 A to  111 D (simply referred to as cylinders  111  unless otherwise distinguished). Here, four cylinders arranged linearly with respect to the crankshaft  115  are designated as the cylinders  111 A to  111 D according to the arrangement order. Each cylinder  111  includes a piston  112 , a high-temperature chamber  113  on one side (upper side in  FIG.  3   ) with respect to a sliding direction (up-down direction in  FIG.  3   ) of the piston  112 , and a low-temperature chamber  114  on the other side (lower side in  FIG.  3   ). The high-temperature chamber  113  is connected to the heater heat exchanger  12 , and the low-temperature chamber  114  is connected to the cooler heat exchanger  14 . The heater heat exchanger  12  and the cooler heat exchanger  14  are connected with the regenerator  13  interposed therebetween. The regenerator  13  serves as a heat storage means between the heater heat exchanger  12  and the cooler heat exchanger  14 , and stores heat from the working fluid when the working fluid moves from the heater heat exchanger  12  to the cooler heat exchanger  14  and causes the working fluid to recover the heat to the heater heat exchanger  12  in the opposite flow, thereby effectively utilizing the heat. The Stirling engine  10  illustrated in  FIG.  3    is a four-cylinder double-acting engine, and the heater heat exchanger  12  and the cooler heat exchanger  14  connected with the same regenerator  13  interposed therebetween are connected to different cylinders  111 . 
     The operation of the Stirling engine  10  is established by repeating a cycle in which the pistons  112  in the cylinders  111  sequentially take a first position (a top dead center position: the cylinder  111 A in  FIG.  3   ), a second position (a position at which the crankshaft  115  is rotated by 90° from the top dead center position while the piston  112  moves downward: the cylinder  111 D in  FIG.  3   ), a third position (a bottom dead center position: the cylinder  111 C in  FIG.  3   ), and a fourth position (a position at which the crankshaft  115  is rotated 90° from the bottom dead center position while the piston  112  moves upward: the cylinder  111 B in  FIG.  3   ). 
     The Stirling engine  10  according to the first embodiment is structurally characterized in that an engine main body E (see  FIG.  4   ) including at least the engine unit  11  and the cooler heat exchanger  14  and a heater structure H (see  FIG.  4   ) including at least the heater heat exchanger  12  are formed as separate structures, and are connected together via the coupling pipe portion  15 . The regenerator  13  may be included in the engine main body E or may be included in the heater structure H. In the first embodiment, the regenerator  13  is included in the engine main body E as an example. In this case, the coupling pipe portion  15  includes a plurality of coupling pipes connecting the heater heat exchanger  12  and the regenerator  13  and a plurality of coupling pipes connecting the heater heat exchanger  12  and the high-temperature chambers  113  of the cylinders  111 . 
     If the regenerator  13  is included in the heater heat exchanger  12 , the coupling pipe portion  15  includes a plurality of coupling pipes connecting the regenerator  13  and the cooler heat exchanger  14  and a plurality of coupling pipes connecting the heater heat exchanger  12  and the high-temperature chambers  113  of the cylinders  111 . However, since both the regenerator  13  and the cooler heat exchanger  14  have similar cylindrical shapes, integrally connecting them is advantageous to downsize the Stirling engine  10 , and the regenerator  13  is preferably included in the engine main body E. 
     As described above, in the Stirling engine  10  in which the engine main body E and the heater structure H are connected via the coupling pipe portion  15 , the positional relationship between the engine main body E and the heater structure H can be easily changed by changing the shape of the coupling pipe portion  15  (for example, by replacing the coupling pipe portion  15 ). That is, the heater heat exchanger  12  can be easily installed in a wide variety of high-temperature heat sources. 
     For example, in the example illustrated in  FIG.  4   , the heater structure H is arranged so as to extend laterally from the engine main body E. In a case where the high-temperature heat source in which the heater heat exchanger  12  is to be arranged is a high-temperature pipe  50 A present on the side of the engine main body E, the heater heat exchanger  12  can be easily installed in the high-temperature heat source. However, in the case where the high-temperature heat source in which the heater heat exchanger  12  is to be arranged is a high-temperature pipe  50 B existing above the engine main body E, the heater structure H preferably extends upward rather than laterally from the engine main body E. In the Stirling engine  10  according to the first embodiment, the heater structure H can be easily arranged to extend upward from the engine main body E by changing the shape of the coupling pipe portion  15 . 
     In the Stirling engine  10 , if the engine main body E and the heater structure H are supported only by the coupling pipe portion  15 , there is a problem of strength. If the support strength in the Stirling engine  10  is weak, the vibrations of the plurality of cylinders  111  cannot be restrained, and the vibration of the entire engine increases. In addition, for example, as illustrated in  FIGS.  1  and  2   , when the heater structure H has a lateral structure extending laterally from the engine main body E (in other words, a structure in which the longitudinal direction of the heater heat exchanger  12  is orthogonal to the sliding direction of the piston  112  in the cylinder  111 ), an unbalanced load may be generated on the coupling pipe portion  15  due to the weight of the heater structure H. 
     Therefore, the Stirling engine  10  according to the present embodiment preferably includes support members (for example, frames  31  and  32  in  FIG.  1   ) that hold the engine main body E and the heater structure H. The frame  31  supports the heater heat exchanger  12  horizontally connected to the engine main body E, from an engine base  33  and the cylinder block of the engine unit  11 . The frame  31  corresponds to the first support member described in the claims. The frame  32  connects the heater heat exchanger  12  and the regenerators  13 , and also connects the regenerators  13  to each other and supports the regenerators  13 . The frame  32  corresponds to the second support member described in the claims. These support members can restrain the vibration of the Stirling engine  10 . In addition, it is possible to adopt a heater lateral structure that cannot be realized by a conventional structure. 
     Second Embodiment 
     In a second embodiment, it is assumed that a Stirling engine  10  is a four-cylinder double-acting engine. That is, as illustrated in  FIG.  3   , in the Stirling engine  10 , pistons  112  in four cylinders  111 A to  111 D are driven with a phase shift of 90° (specifically, the phases of the pistons  112  are delayed by 90° in the order of the cylinders  111 A to  111 D.). When a reference cylinder (for example, the cylinder  111 A) is defined as a first cylinder and the other cylinders are defined as second to fourth cylinders in order of phase delay from the first cylinder, in the example of  FIG.  3   , the cylinder  111 B is the second cylinder, the cylinder  111 C is the third cylinder, and the cylinder  111 D is the fourth cylinder. 
     In the case of a cylinder double-acting engine in which four cylinders are arranged linearly with respect to the crankshaft  115 , a couple of forces is generated between two cylinders with a phase shift of 180°, and the couple of forces causes engine vibration or applies a load (bending stress) to the crankshaft. In the example of  FIG.  3   , the four cylinders  111  are arranged in order from the first cylinder to the fourth cylinder. A couple of forces is generated between the first cylinder and the third cylinder and between the second cylinder and the fourth cylinder. In addition, as illustrated in  FIG.  5   , in a case where the force given to the crankshaft  115  by the cylinder is F and the pitch between two adjacent cylinders is L, the maximum couple of forces N (for example, the couple of forces between the first cylinder and the third cylinder) acting on the crankshaft  115  is N=2 FL. 
     On the other hand, in the second embodiment, the couple of forces generated in the crankshaft  115  is restrained (minimized) by adjusting the arrangement order of the cylinders. Specifically, the cylinders with a phase shift of 180° are arranged close to (adjacent to) each other. For example, as illustrated in  FIG.  6   , when the first cylinder and the third cylinder are arranged adjacent to each other and the second cylinder and the fourth cylinder are arranged adjacent to each other, the maximum couple of forces N (for example, the couple of forces between the first cylinder and the third cylinder) acting on the crankshaft  115  is N=FL. Although the first cylinder, the third cylinder, the fourth cylinder, and the second cylinder are arranged in this order in  FIG.  6   , the order of the fourth cylinder and the second cylinder may be switched. 
     In the four-cylinder double-acting engine, the heater heat exchanger  12 , the regenerator  13 , and the cooler heat exchanger  14  as a set are connected between cylinders with a phase shift of 90°. Taking  FIG.  3    as an example, a set of the heater heat exchanger  12 , the regenerator  13 , and the cooler heat exchanger  14  is connected between the cylinder  111 A that is the first cylinder and the cylinder  111 B that is the second cylinder. Specifically, the cooler heat exchanger  14  is connected to the low-temperature chamber  114  of the cylinder  111 A of which phase is advanced, and the heater heat exchanger  12  is connected to the high-temperature chamber  113  of the cylinder  111 B of which phase is delayed. Similar connection relationships are present between the cylinder  111 B that is the second cylinder and the cylinder  111 C that is the third cylinder, between the cylinder  111 C that is the third cylinder and the cylinder  111 D that is the fourth cylinder, and between the cylinder  111 D that is the fourth cylinder and the cylinder  111 A that is the first cylinder. 
       FIG.  3    illustrates the cylinder arrangement with respect to the crankshaft  115  in the order of the first to fourth cylinders (the arrangement order corresponding to  FIG.  5   ). On the other hand, if the cylinder arrangement is the order of the first cylinder, the third cylinder, the fourth cylinder, and the second cylinder (the arrangement order corresponding to  FIG.  6   ) in order to reduce the load on the crankshaft  115 , the connection of the heater heat exchanger  12 , the regenerator  13 , and the cooler heat exchanger  14  between the cylinders is schematically as illustrated in  FIG.  7   . 
     The heater heat exchanger  12  is configured with a heater thin tube group so that efficient heat exchange can be performed in a state of being inserted into a high-temperature heat source. In a conventional structure in which the engine main body E and the heater structure H have an integrated structure and the heater heat exchanger  12  is directly connected (without the coupling pipe portion  15 ) to both the high-temperature chamber  113  of the engine unit  11  and the regenerator  13 , it is difficult to obtain a connection structure as illustrated in  FIG.  7   . That is, in the heater heat exchanger  12 , compact arrangement of the heater thin tube group for four cylinders (for example, regular arrangement of the heater thin tube groups as illustrated in  FIG.  10   ) becomes impossible. 
     On the other hand, in the Stirling engine  10  according to the second embodiment, as in the first embodiment, the engine main body E and the heater structure H are separate structures and are connected via the coupling pipe portion  15 . Therefore, as illustrated in  FIG.  8   , the plurality of coupling pipes in the coupling pipe portion  15  enables the connection between the heater heat exchanger  12  and the regenerator  13  and the connection between the heater heat exchanger  12  and the high-temperature chamber  113  of each cylinder  111  with a high degree of freedom. As a result, the heater heat exchanger  12  can be compactly arranged in the heater thin tube group for four cylinders, regardless of the connection relationship with the regenerator  13  and the cylinder  111 . 
     More specifically, as illustrated in  FIG.  9   , it is preferable that the regenerator  13  and the cooler heat exchanger  14  are vertically placed close to each other at the rear of the cylinder  111 , and an upper end position P 1  of the regenerator  13  is above an upper end position P 2  of the cylinder  111 . This makes it easy to secure an arrangement space of the coupling pipe portion  15  among the regenerator  13 , the cylinder  111 , and the heater heat exchanger  12 . Further, in the heater heat exchanger  12 , the heater thin tube group for four cylinders is preferably arranged in an annular shape, as illustrated in  FIG.  10   . This realizes compact arrangement of the heater thin tube group in the heater heat exchanger  12 . 
     The coupling pipe portion  15  can be configured such that a coupling pipe  150  as illustrated in  FIG.  11    is individually connected (attachable to and detachable from the engine main body E and the heater structure H) between the heater heat exchanger  12  and the regenerator  13  or between the heater heat exchanger  12  and the cylinder  111 . The coupling pipe  150  is preferably configured to obtain airtightness by arranging a metal O-ring  151  on a sealing surface between the coupling pipe  150  and a member (the heater heat exchanger  12 , the regenerator  13 , or the cylinder  111 ) to be connected. As the metal O-ring  151 , a metallic hollow O-ring gasket having resistance to high temperature or the like can be used. 
     Third Embodiment 
     A Stirling engine  10  is a passive engine and basically continues to operate as long as heat is supplied from a high-temperature heat source (and stops operating when there is no supply of heat). However, it is also conceivable that the operation of the engine needs to be stopped in an emergency or the like. In a third embodiment, a preferred example of a configuration for stopping the Stirling engine  10  will be described. 
     The Stirling engine  10  can stop by stopping the movement of a working fluid. Therefore, the Stirling engine  10  according to the third embodiment can be configured such that an on-off valve  16  (see  FIG.  1   ) is provided in a low-temperature portion path of the working fluid (a working fluid path connecting a low-temperature chamber  114  of a cylinder  111  and a cooler heat exchanger  14 ), the on-off valve  16  is opened during the operation of the engine, and the engine is stopped by closing the on-off valve  16 . In principle, the path provided with the on-off valve  16  is not particularly limited, and the on-off valve can be provided in a high-temperature portion path (a working fluid path connecting a high-temperature chamber  113  of the cylinder  111  and a heater heat exchanger  12 ). However, in the Stirling engine  10 , the high-temperature portion path configures a coupling pipe portion  15 , so that the high-temperature portion path is unsuitable for arrangement of the on-off valve  16 , and the on-off valve  16  is preferably provided on the low-temperature portion path. 
     The type of the on-off valve  16  used is not particularly limited, and for example, an inexpensive valve such as a butterfly valve can be used. In this case, if the on-off valve  16  completely closes the path, a load (compression pressure) applied to the closed path becomes too large, and damage may occur in components and the like. Therefore, it is preferable that the on-off valve  16  does not completely close the path, and is a perforated valve that can allow the working fluid to pass to some extent (partially close the path). That is, even if the on-off valve  16  does not completely close the path, the engine can be stopped only by decreasing the flow path area to reduce the movement amount of the working fluid. More specifically, the path closing area of the on-off valve  16  is set to a maximum area in which the engine is not damaged under the compression pressure generated by the closing the valve and in which the engine can be reliably stopped (engine output≤mechanical loss). 
     The on-off valve  16  may be configured to adjust the flow path area using a rotary solenoid or the like. In this case, it is possible to perform control to gradually reduce the flow path area, and it is possible to avoid a sudden stop of the engine and reduce a load or the like applied to pistons  112  when the engine is stopped. 
     As a modification of the Stirling engine  10  according to the third embodiment, a configuration illustrated in  FIG.  12    is also conceivable. A Stirling engine  10  illustrated in  FIG.  12    is configured such that low-temperature chambers  114  of cylinders  111  with phase shifts of 180° are connected to each other by bypass paths  17 , and communication valves  171  are provided in the bypass paths  17 . In the example of  FIG.  12   , a cylinder  111 A and a cylinder  111 C are connected by the bypass path  17 , and a cylinder  111 B and a cylinder  111 D are connected by the bypass path  17 . 
     In the Stirling engine  10  of  FIG.  12   , the communication valves  171  are closed during the operation of the engine, and the communication valves  171  are opened to make the bypass paths  17  conductive (provide communication between the low-temperature chambers  114  of the cylinders  111  with a phase shift of 180°), whereby the engine can be stopped. In this configuration, it is possible to promptly stop the engine without applying an overload to components or the like. 
     When the engine stop configuration in  FIG.  12    is applied to the Stirling engine  10  employing the cylinder arrangement illustrated in  FIG.  6   , the cylinders with a phase shift of 180° are arranged adjacent to each other, so that the bypass paths  17  can be shortened. As a result, it is possible to restrain generation of an unnecessary volume and a cost increase due to the bypass paths  17 . In addition, it is also possible to reduce the horsepower loss at the time of startup in a case where the bypass paths  17  are long. 
     In addition, in the Stirling engine  10  according to the third embodiment, the opening degree of the on-off valves  16  and the communication valves  171  can be adjusted, so that the Stirling engine  10  can be used for output control of the engine. For example, if the temperature of the high-temperature heat source excessively rises, the on-off valves  16  are somewhat closed, or the communication valves  171  are somewhat opened, so that it is possible to reduce the engine output and protect the components of the engine. 
     Fourth Embodiment 
     In a fourth embodiment, a preferred example of a configuration for startup control of a Stirling engine  10  will be described. 
     The Stirling engine  10  requires a starter motor  40  (see  FIG.  1   ) at its startup. As a matter of course, the larger the engine load (pressure loss) at the startup of the Stirling engine  10 , the larger the size of the starter motor  40  is required. 
     On the other hand, the Stirling engine  10  according to the fourth embodiment is assumed to have the configuration illustrated in  FIG.  12   , and is characterized by reducing the engine load at the time of startup using communication valves  171 . That is, in the Stirling engine  10  according to the fourth embodiment, the starter motor  40  is started with the communication valves  171  opened at the time of startup. In the Stirling engine  10 , since the engine load is reduced by opening the communication valves  171 , the small-sized starter motor  40  can be used. Then, when the rotation speed of the engine reaches a predetermined value, the communication valves  171  are closed and the starter motor  40  is stopped, so that the operation of the engine can be maintained. 
     Fifth Embodiment 
     The Stirling engine  10  described above is characterized in that the engine main body E and the heater structure H are formed as separate structures, and they are connected via the coupling pipe portion  15 . In this configuration, the coupling pipe portion  15  becomes an ineffective volume that does not contribute to the thermal cycle, which may cause a decrease in the output of the Stirling engine  10 . In relation to a fifth embodiment, a preferred example for restraining a decrease in output due to the coupling pipe portion  15  will be described. 
       FIG.  13    is a cross-sectional view of a coupling pipe  150  for use in the coupling pipe portion  15 . In the coupling pipe  150  illustrated in  FIG.  13   , a heat accumulator  153  such as a wire mesh or a metal nonwoven fabric is provided inside the coupling pipe wall  152  over the entire pipeline. In addition, the central portion of the heat accumulator  153  is preferably a cavity portion  154 . The coupling pipe  150  configured in this manner can accumulate heat in the heat accumulator  153  and reduce heat dissipation to the outside when a high-temperature working fluid passes through the inside of the coupling pipe  150 . In addition, since the central portion of the heat accumulator  153  is formed as the cavity portion  154 , the cavity portion  154  serves as a working fluid passage, so that it is possible to restrain an increase in pressure loss due to the heat accumulator  153 . 
     The coupling pipe  150  illustrated in  FIG.  13    can have a function similar to that of a regenerator  13  by the heat accumulation action of the heat accumulator  153 , so that the output of the Stirling engine  10  can be improved by effectively using the heat. 
     The embodiments disclosed herein are illustrative in all respects and do not provide a basis for a limited interpretation. Therefore, the technical scope of the present invention should not be construed only by the above-described embodiments, but is defined based on the description of the claims. In addition, the present invention includes all modifications within a meaning and scope equivalent to the claims. 
     LIST OF REFERENCE SIGNS 
       10  Stirling engine 
       11  Engine unit 
       111  Cylinder 
       112  Piston 
       113  High-temperature chamber 
       114  Low-temperature chamber 
       115  Crank shaft 
       12  Heater heat exchanger 
       13  Regenerator 
       14  Cooler heat exchanger 
       15  Coupling pipe portion 
       150  Coupling pipe 
       151  Metal O-ring 
       152  Coupling pipe wall 
       153  Heat accumulator 
       154  Cavity portion 
       16  On-off valve 
       17  Bypass path 
       171  Communication valve 
       20  Generator 
       31  Frame (first support member) 
       32  Frame (second support member) 
       33  Engine base 
       40  Starter motor 
       50 A High-temperature pipe 
       50 B High-temperature pipe 
     E Engine main body 
     H Heater structure