Patent Publication Number: US-7581393-B2

Title: Stirling engine

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a stirling engine, and more particular, to a stirling engine suitable for increasing a working gas pressure of the stirling engine. 
   2. Description of the Related Art 
   In recent years, stirling engines which have an excellent theoretical thermal efficiency attract attention as an external combustion engine which collects exhaust heat from an internal combustion engine mounted on a vehicle such as an automobile, a bus, and a truck, as well as exhaust heat from factories. 
   Japanese Patent Application Laid-Open No. S64-342 discloses an output control apparatus for a stirling engine which includes a connection tube that connects a working space and a crankcase and an accumulator. 
   An efficient increase in the working gas pressure of the stirling engine is desired. In particular, when a pressuring device such as a pressurizing pump is to be used, reduction of energy used for the pressurization is desired. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, an object of the present invention is to provide a stirling engine which is capable of efficiently increasing a working gas pressure. 
   A stirling engine according to one aspect of the present invention includes a flow path that communicates a working space of the stirling engine and outside of the stirling engine. A working gas is supplied from the outside of the Stirling engine to the working space via the flow path based on a differential pressure of the working space and the outside of the stirling engine. 
   In the stirling engine, the flow path is provided with a filter that prevents an impurity from entering the working space from the outside into via the flow path. 
   In the stirling engine, the working gas is supplied from the outside of the stirling engine into the working space via the flow path when a pressure of the working gas in the working space is lower than a mean value of the pressure of the working gas in the working space in one cycle. 
   In the stirling engine, a working gas of an atmospheric pressure is supplied from the outside of the stirling engine into the working space via the flow path. 
   The stirling engine further includes a pressurized fluid supplying unit that is connected to the flow path at the outside of the stirling engine to supply a pressurized working gas. 
   In the stirling engine, the pressurized fluid supplying unit is a piston pump. 
   In the stirling engine, the piston pump is provided so that a phase of an intra-cylindrical pressure of the piston pump is of an opposite phase with the pressure of the working gas in the working space. 
   The stirling engine further includes a communication tube that communicates the working space with a crankcase of the stirling engine; and an opening and closing unit that opens and closes the communication tube. The opening and closing unit is put into a state where the communication tube is open when the pressure of the working gas in the working space is higher than a pressure of the crankcase. 
   In the stirling engine, the flow path is provided so that the flow path communicates the working space at a low temperature side of the stirling engine and the outside of the stirling engine. 
   The stirling engine further includes a cylinder; and a piston that reciprocates in the cylinder. The piston reciprocates in the cylinder while keeping cylinder airtight with an air bearing provided between the cylinder and the piston. 
   The stirling engine further include an approximately linear mechanism that is connected to the piston so that the approximately linear mechanism makes an approximately linear motion when the piston reciprocates in the cylinder. 
   A hybrid system according to another aspect of the present invention includes a stirling engine according to the present invention; and an internal combustion engine of a vehicle. The stirling engine is mounted on the vehicle and a heater of the stirling engine is provided to receive a heat from an exhaust system of the internal combustion engine. 
   In the hybrid system, the stirling engine includes at least two cylinders, and a heat exchanger including a cooler, a regenerator, and the heater. The heat exchanger is configured so that at least a portion of the heat exchanger forms a curve to connect the two cylinders. The curve is adapted to connect upper portions of the two cylinders where a dimension of an inner diameter of the exhaust tube of the internal combustion engine is approximately same with a distance between an end portion of the heater and an uppermost portion of the heater. 
   In the hybrid system, the stirling engine is attached to the vehicle so that pistons of the stirling engine reciprocate substantially horizontally. 
   In view of the foregoing, an object of the present invention is to provide a stirling engine which is capable of efficiently increasing a working gas pressure. 
   According to the stirling engine of the present invention, an efficient increase of the working gas pressure is allowed. 
   The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic sectional view of a structure of a stirling engine according to a first embodiment of the present invention; 
       FIG. 2  is a graph of intra-cylindrical pressure prior to pressurization of a crankcase in the stirling engine according to the first embodiment of the present invention; 
       FIG. 3  is a graph of intra-cylindrical pressure after the pressurization of the crankcase in the stirling engine according to the first embodiment of the present invention; 
       FIG. 4  is a schematic sectional view of a stirling engine according to a second embodiment of the present invention; 
       FIG. 5  is a schematic sectional view of a structure of a stirling engine according to a third embodiment of the present invention; 
       FIG. 6A  is a graph of intra-cylindrical pressure prior to closing of a valve in the stirling engine according to the third embodiment of the present invention, and  FIG. 6B  is a graph of the intra-cylindrical pressure after the closing of the valve in the stirling engine according to the third embodiment of the present invention; 
       FIG. 7  is a graph of intra-cylindrical pressure prior to pressurization of a crankcase in a stirling engine according to a fourth embodiment of the present invention; 
       FIG. 8  is a schematic sectional view of a structure of the stirling engine according to the fourth embodiment of the present invention; 
       FIG. 9  is a schematic sectional view of a structure of the stirling engine according to a fifth embodiment of the present invention; 
       FIG. 10  is a graph of intra-cylindrical pressure of the stirling engine and the intra-cylindrical pressure of a piston pump in the stirling engine according to the fifth embodiment of the present invention; 
       FIG. 11  is a sectional view of a basic common structure of the stirling engine according to the embodiments of the present invention; 
       FIG. 12  is a plan view of an attachment state of an internal combustion engine and the stirling engine of the embodiments of the present invention; 
       FIG. 13  is a graph of the intra-cylindrical pressure of the stirling engine according to the embodiments of the present invention; and 
       FIG. 14  is an explanatory diagram of approximately linear mechanism applied to the stirling engine according to the embodiments of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the following, stirling engines according to embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
   The embodiments which will be described below relate to pressurization of a working space, i.e., increase in working gas pressure, and pressurization of a crankcase  41 . First, a common structure to all embodiments will be described followed by descriptions of pressurization of the working space (i.e., increase in working gas pressure) and the crankcase  41  according to the respective embodiments. 
     FIG. 11  is a front sectional view of a stirling engine of the embodiments. As shown in  FIG. 11 , the stirling engine of the embodiments is a stirling engine  10  of α-type (two-piston type) and provided with two power pistons  20  and  30 . Two power pistons  20  and  30  are arranged in parallel in line. A piston  31  of the power piston  30  on a low temperature side is arranged so that the piston  31  moves with a phase difference of 90° in a crank angle with respect to a piston  21  of the power piston  20  on a high temperature side as shown in  FIG. 13 . 
   A working fluid heated by a heater  47  flows into a space (expansion space) in an upper section of a cylinder  22  (hereinafter referred to as a high temperature side cylinder) of the power piston  20  on the high temperature side. A working fluid cooled by a cooler  45  flows into a space (compression space) in an upper section of a cylinder  32  (hereinafter referred to as a low temperature side cylinder) of the power piston  30  on the low temperature side. 
   A regenerator (regenerative heat exchanger)  46  stores heat while the working fluid flows back and forth between the expansion space and the compression space. In other words, when the working fluid flows from the expansion space to the compression space, the regenerator  46  receives heat from the working fluid, whereas the stored heat is transferred to the working fluid when the working fluid flows from the compression space to the expansion space. 
   The reciprocating flow of the working fluid caused by the reciprocating movement of two pistons  21  and  31  (also referred to as expansion piston  21  and compression piston  31  hereinbelow), changes the ratio of the working fluid in the expansion space of the high temperature side cylinder  22  and the compression space of the low temperature side cylinder  32 , as well as the total volume of the fluid in the spaces to cause pressure variations. When the relation between the pressure level and the positions of the cylinders  21  and  31  is to be compared, the pressure is substantially higher when the expansion piston  21  is in a lower position than in a higher position, whereas the pressure is substantially lower when the compression piston  31  is in a lower position than in a higher position. Thus, the expansion piston  21  performs a positive work (expansion work) of a substantial amount to the outside, whereas the compression piston  31  needs to receive a work (compression work) from the outside. The expansion work is partly utilized for the compression work and the rest is extracted as an output via a driving shaft  40 . 
   The stirling engine of the embodiments is employed with a main engine  200 , a gasoline engine or an internal combustion engine, for example, in a vehicle as shown in  FIG. 12 , thereby forming a hybrid system. In other words, the stirling engine  10  is an exhaust heat collecting unit which utilizes exhaust gas from the main engine  200  as a heat source. With the heater  47  of the stirling engine  10  placed in an exhaust tube  100  of the main engine  200  of the vehicle, heat energy collected from the exhaust gas heats up the working fluid thereby starting up the stirling engine  10 . 
   Since the stirling engine  10  of the embodiments is placed in a limited space in the vehicle, e.g., the heater  47  is housed in the exhaust tube  100 , the overall structure thereof is preferably made compact to increase the degree of freedom in installation. To this end, in the stirling engine  10 , two cylinders  22  and  32  are not arranged in a “V” configuration but placed in a parallel in-line layout. 
   The heater  47  is arranged inside the exhaust tube  100 , so that a side of the heater  47  on the side of high temperature side cylinder is located at an upstream side  100   a  (a side closer to the main engine  200 ) of the exhaust gas where exhaust gas of a relatively high temperature flows in the exhaust tube  100 , whereas a side of the heater  47  on the side of the low temperature side cylinder  32  is located at a downstream side  100   b  (a side farther from the main engine  200 ) where exhaust gas of a relatively low temperature flows. Such arrangement intends to heat up the side of the heater  47  on the side of the high temperature side cylinder  22  to a higher level. 
   Each of the high temperature side cylinder  22  and the low temperature side cylinder  32  is formed in a cylindrical shape and supported by a base plate  42  which serves as a baseline. In the embodiments, the base plate  42  serves to provide a reference position for respective components of the stirling engine  10 . With such structure, the relative location accuracy of respective components of the stirling engine  10  can be secured. In addition, the base plate  42  can be used as a reference for attachment of the stirling engine  10  to the exhaust tube  100  (exhaust path) which provides exhaust heat to be collected. 
   The base plate  42  is fixed to a flange  100   f  of the exhaust tube  100  via a heat insulating material (spacer not shown). The base plate  42  is also fixed to a flange  22   f  provided in a side face (outer peripheral surface)  22   c  of the high temperature side cylinder  22 . The base plate  42  is also fixed to a flange  46   f  provided in a side face (outer peripheral surface)  46   c  of the regenerator  46 . 
   The exhaust tube  100  is attached to the stirling engine  10  via the base plate  42 . The stirling engine  10  is attached to the base plate  42  so that an end face (an upper face of a top portion  22   b ) of the high temperature side cylinder  22  where the heater  47  is connected, and an end face (a top face  32   a ) of the low temperature side cylinder  32  where the cooler  45  is connected are substantially parallel with the base plate  42 . Alternatively, the stirling engine  10  is attached to the base plate  42  so that the base plate  42  is parallel with a rotation shaft of a crank shaft  61  (or the driving shaft  40 ) or so that a central axis of the exhaust tube  100  is parallel with the rotation shaft of the crank shaft  61 . Thus, the stirling engine  10  can be readily attached to the exhaust tube  100  of an existing type without a major change in design. As a result, the stirling engine  10  can be attached to the exhaust tube  100  without impoverishing the characteristics such as performance, mountability and a noise-reducing feature, of the internal combustion engine of a vehicle from which the exhaust gas is collected. In addition, since the stirling engine  10  of the same specification can be attached to a different exhaust tube only with a change in specification of the heater  47 , the versatility of the stirling engine can be enhanced. 
   The stirling engine  10  is arranged horizontally in a space adjacent to the exhaust tube  100  which is placed under a floor of the vehicle. In other words, the stirling engine  10  is arranged so that the axes of the high temperature side cylinder  22  and the low temperature side cylinder  32  are substantially parallel with the floor (not shown) of the vehicle. Two pistons  21  and  31  reciprocate horizontally. In the embodiments, an upper dead point side and a lower dead point side of two pistons  21  and  31  are referred to as an upper direction and a lower direction, respectively, for the simplicity of description. 
   A higher output can be obtained when a mean pressure (Pmean described later) of the working fluid is higher since a differential pressure at the same temperature difference caused by the cooler  45  and the heater  47  is larger. Hence, as described above, the working fluid in the working space of the high temperature side cylinder  22  and the low temperature side cylinder  32  is maintained in a high pressure. 
   The pistons  21  and  31  are formed in a cylindrical shape. Between the outer peripheral surface of pistons  21  and  31  and the inner peripheral surface of the cylinders  22  and  32 , a minute clearance of a few ten micrometers (μm) is provided. The working fluid (air) of the stirling engine  10  is present in the clearance. The pistons  21  and  31  are supported by an air bearing  48  so that the pistons do not contact with the cylinders  22  and  32 , respectively. Hence, piston rings are not provided around the pistons  21  and  31 , and lubricant which is generally used together with the piston ring is not employed. To the inner peripheral surface of the cylinders  22  and  32 , however, an antifriction is fixed. Though resistance of the air bearing  48  toward sliding movement caused by the working fluid is originally extremely low, the antifriction is provided for further resistance reduction. As described above, the air bearing  48  serves to maintain the expansion space and the compression space airtight with the working fluid (air) and seals the clearance without the piston ring and the lubricant. 
   The heater  47  includes a plurality of heat transfer tubes (tube group)  47   t , each of which is formed generally in a U-shape. A first end portion  47   ta  of each heat transfer tube  47   t  is connected to the upper portion (top portion) (end face at the side of a top face  22   a )  22   b  of the high temperature side cylinder  22 . A second end portion  47   tb  of each heat transfer tube  47   t  is connected to an upper portion (end face at the side of the heater  47 )  46   a  of the regenerator  46 . The reason why the heater  47  is formed generally in U-shape as described above will be described later. 
   The regenerator  46  includes a heat storage material (matrix not shown) and a regenerator housing  46   h  that houses the matrix. Since the regenerator housing  46   h  accommodates the working fluid of high pressure, the regenerator housing  46   h  is formed as a pressure-tight container. The regenerator  46  here includes laminated metallic meshes as the matrix. 
   The regenerator  46  has to meet the following conditions to realize the above described functions. The regenerator  46  is required to have a high heat transfer performance, a high heat storage capacity, a low flow resistance (flow loss, pressure loss), a low heat conductivity in a direction of the working fluid flow, and a large thermal gradient. The metallic mesh may be formed of stainless steel. When the working fluid passes through each of the laminated metallic meshes, heat of the working fluid is transferred and stored in the metallic mesh. 
   A connecting portion (shape of a cross section) of the heater  47  with the high temperature side cylinder  22  is formed in the same shape and size with the shape of an opening (perfect circle) of the upper portion (a connecting portion with the heater  47 ) of the high temperature side cylinder  22 . Similarly, a connecting portion of the heater  47  with the regenerator  46  is formed in the same shape and size with the upper face of the regenerator  46 . Thus, the cross sections of the heater  47  and the regenerator  46  are formed in the same shape and size with the openings of the high temperature side cylinder  22  and the low temperature side cylinder  32 , respectively. With such a structure, resistance of a flow path (flow resistance) of the working fluid is decreased. 
   The crank shaft  61  is rotatably supported by a bearing with respect to the crankcase  41 . In the embodiments, a counterweight  90  is provided on a side of the high temperature side cylinder  22 . The position of the counterweight  90  is selected so as to minimize the influence on the vertical size of the overall stirling engine  10 . A sufficient space can be secured in the space on a side of the high temperature side cylinder  22 . 
   Next, a reason why the heater  47  is formed generally in U-shape (curved shape) as described above will be described. 
   The heat source of the stirling engine  10  is the exhaust gas of the main engine  200  of the vehicle as described above and not a heat source dedicated exclusively to the stirling engine. Hence, the amount of heat to be obtained is not very large. The stirling engine  10  is required to start up with a small amount of heat obtained from the exhaust gas, of approximately 800° C., for example. Thus, the heater  47  of the stirling engine  10  is required to efficiently receive the heat from the exhaust gas in the exhaust tube  100 . 
   A volume of a heat exchanger which includes the heater  47 , the regenerator  46 , and the cooler  45  is a void volume, which does not directly affect the output. When the volume of the heat exchanger increases, the output of the stirling engine  10  decreases. On the other hand, when the heat exchanger is made small in volume, the heat exchange becomes difficult and the received amount of heat decreases, whereby the output of the stirling engine  10  is decreased. Hence, to realize both the decrease in the void volume and the increase in the received amount of heat, the efficiency of the heat exchanger is required to be enhanced. In other words, the efficient receipt of heat by the heater  47  is required. 
   To realize the efficient heat receipt from the exhaust gas in the exhaust tube  100  and the efficient heat exchange, the whole structure of the heater  47  is required to be accommodated in the exhaust tube  100  in just proportion, and the cooler  45  is required to be located outside the exhaust tube  100  to avoid receiving the heat from the exhaust gas. Hence, when the flange  100   f  where the exhaust tube  100  is attached to the stirling engine  10  is taken as a reference, a position of attachment of the low temperature side cylinder  32  is lower than a position of attachment of the high temperature side cylinder  22  at least by the height of the cooler  45 . Thus, a position of the compression space formed in the upper section of the low temperature side cylinder  32  is lower than the position of the expansion space formed in the upper section of the high temperature side cylinder  22 , and an upper dead point of the compression piston  31  is lower than a position of an upper dead point of the expansion piston  21 . 
   In the embodiments, piston pins  60   a  and  60   b  are connected to pistons  21  and  31 , respectively, with extensions (piston supports)  64   a  and  64   b  of different sizes to change the positions of the upper dead points of the pressurizing piston  31  and the expansion piston  21 . Since the position of the upper dead point of the expansion piston  21  is higher than the upper dead point of the compression piston  31 , the extension  64   a  connected to the expansion piston  21  is longer than the extension  64   b  connected to the compression piston  31  by the difference in the height of position of the upper dead point. 
   In the embodiments, the expansion piston  21  and the compression piston  31  are formed so that the lengths thereof are equal. In other words, the distances between the upper faces of pistons  21  and  31  and connection points  21   c  and  31   c  with the extensions  64   a  and  64   b  of the pistons  21  and  31 , respectively, are made equal. Therefore, the extensions  64   a  and  64   b  are formed in different lengths to arrange the upper dead points of the piston  21  and  31  at different positions. Alternatively, the extensions of the expansion piston and the compression piston may be formed in the same length, and the lengths of the expansion piston and the compression piston may be made different. Thus, the positions of the upper dead points of the expansion piston and the compression piston can be made different. A technical advantage of such structure where the vertical length of the expansion piston itself is made longer than that of the compression piston itself will be described below. 
   For the suppression of deterioration in the efficiency of the stirling engine  10 , a space outside the expansion space in the high temperature side power piston  20  and a space outside the compression space in the low temperature side power piston  30 , i.e., a space around the crank shaft  61  in each of the high temperature side power piston  20  and the low temperature side power piston  30  is required to be maintained at a room temperature. Hence, secure sealing must be provided between the high temperature side cylinder  22  and the expansion piston  21 , and between the low temperature side cylinder  32  and the compression piston  31 , so that the working fluid of a high temperature in the expansion space will not flow into the space around the crank shaft  61  at the side of the high temperature side power piston  20  and the working fluid of a low temperature in the compression space will not flow into the space around the crank shaft  61  on the side of the low temperature side power piston  30 . As described later, the air bearing  48  is employed to achieve such sealing. 
   On the other hand, since the top portion  22   b  and the side face  22   c  of the high temperature side cylinder  22  are housed inside the exhaust tube  100  as described above, the upper portion of the high temperature side cylinder  22  and the upper portion of the expansion piston  21  thermally expand. Then, the sealing might not be secured in a section where the upper portions of the high temperature side cylinder  22  and the expansion piston  21  expand. To avoid such inconvenience, the expansion piston  21  and the high temperature side cylinder  22  may be formed longer in the vertical direction to provide a thermal gradient in vertical direction of the expansion piston  21 . Then, the secure sealing can be guaranteed with the section not affected by the thermal expansion, i.e., the lower portion of the expansion piston  21 . Further, since the sealing between the high temperature side cylinder  22  and the expansion piston  21  is provided with the lower portion of the expansion piston  21 , i.e., the section not affected by the thermal expansion, the high temperature side cylinder  22  may be formed longer in the vertical direction to guarantee the sufficient moving distance for the sealing section and to sufficiently pressurize the expansion space. 
   The structure of the embodiments is preferable regardless of the type of the heat source, since such structure allows efficient reception of heat from the heat source and efficient heat exchange by providing the heater with a large heat transfer area for the reception of heat energy and the cooler which can be arranged in a position not heated. 
   In particular, when the exhaust heat is to be utilized, the heat energy is generally supplied by the exhaust gas through a tube. Then, an area where the heat can be received (tube interior, for example) is relatively limited. In such case, the structure of the stirling engine  10  as described above is particularly preferable since it provides a large heat transfer area and a cooler is arranged in a position not heated. A technical advantage of the structure of the stirling engine  10  will be further described below. 
   A smaller void volume (the cooler, the regenerator, and the heater) is preferable as described above. In addition, when the void volume section has a curved shape, the resistance in the flow path becomes large when many such curved portions exist whereas the resistance in the flow path increases when the curvature of the curved portion is small. In other words, with the pressure loss of the working fluid considered, preferably a single curved portion with a large curvature is provided. Though the heater  47  is generally in U-shape, the heater  47  has only one curved portion. In addition, the cooler  45  is formed to have a curved portion for the downsizing of the stirling engine  10  (reduction in vertical dimension), whereby the structure with the features as described above is realized. 
   In addition, as shown in  FIG. 11 , the curvature of the void volume portion in the embodiments is set according to the arrangement where the upper portions of two cylinders  22  and  32  arranged in parallel in line are coupled, and the vertical distance between the top portion  22   b  of the high temperature side cylinder  22  and the upper face  46   a  of the regenerator  46  arranged approximately in the same plane to suppress the increase in flow resistance of the working fluid in the exhaust tube  100  and the upper inner face of the exhaust tube  100  is set to a height h which is approximately equal to the distance between the end portions  47   ta  and  47   tb  and the uppermost portion of a central portion  47   c  of the heater  47 . To secure a large contact area with the fluid heat source such as the exhaust gas in a limited space such as the interior of the exhaust tube  100 , the curved shape as described above is desirable. 
   With such advantages considered, the heater in the void volume portion is preferably formed in a curved shape such as a U-shape or a J-shape, so that the entirety of the heater is housed in a limited space (heat-receiving space) receiving the heat from the heat source such as the interior of the exhaust tube and a maximum area to receive the heat from the heat source can be secured and the resistance of the flow path is minimized in the heat-receiving space. 
   To minimize the resistance of the working fluid in the flow path, the regenerator  46  is arranged linearly (along the same axis) along a direction of extension (direction of axis) of the low temperature side cylinder  32 . Thus, the regenerator  46  connected to a second end portion  47   tb  of the heater  47  is arranged along the direction of extension of the low temperature side cylinder  32 . A first end portion  47   ta  of the heater  47  is seamlessly connected to the upper portion of the high temperature side cylinder  22 . Thus, the heater  47  has portions extending along the directions of extension of the high temperature side cylinder  22  and the low temperature side cylinder  32  at least at the sides of the first end portion  47   ta  and the second end portion  47   tb  of the heater  47 , and the central portion  47   c  of the heater  47 , in many cases, has a curved shape as described above. 
   Due to the technical reasons as described above, the heater  47  is formed in a curved shape between two cylinders  22  and  32  which are arranged in parallel in line. Thus, the heater  47  has a curved portion connecting two cylinders  22  and  32 . 
   Next, a sealing structure of a piston/cylinder section and a mechanism of a piston/crank section will be described. 
   As described above, since the heat source of the stirling engine  10  is the exhaust gas from the internal combustion engine of the vehicle, the obtainable amount of heat is limited and the stirling engine  10  is required to function in the range of obtainable heat amount. Hence, in the embodiments, the internal friction of the stirling engine  10  is minimized as far as possible. In the embodiments, to eliminate the friction loss by the piston ring which generally produces the largest friction loss among the internal friction in the stirling engine, the piston ring is eliminated from the structure. In place of the piston ring, the air bearing  48  is provided between the cylinders  22  and  32  and the pistons  21  and  31 , respectively. 
   The air bearing  48  can significantly reduce the internal friction of the stirling engine  10  since the sliding resistance thereof is extremely small. Since the cylinders  22  and  32  and the pistons  21  and  31  are secured airtight also with the air bearing  48 , the working fluid of a high temperature would not leak out at the time of expansion and contraction. 
   The air bearing  48  utilizes the air pressure generated in the minute clearances between the cylinders  22  and  32  and the pistons  21  and  31  to support the pistons  21  and  31  in a floating position. The air bearing  48  of the embodiments has a clearance of a few ten micrometers (μm) in diameter between the cylinders  22  and  32  and the pistons  21  and  31 . To realize the air bearing that supports a material in a floating position, a mechanism may be structured to have a high air pressure section (thereby creating pressure gradient). Alternatively, a highly-pressurized air may be sprayed as described later. 
   The air bearing used in the embodiments is not the type to which the highly-pressurized air is sprayed but an air bearing which has the same configuration as an air bearing employed between a cylinder and a piston for a glass injection syringe for medical application. 
   In addition, since the use of the air bearing  48  eliminates the lubricant which is used for the piston ring, the deterioration of the heat exchanger (the regenerator  46  and the heater  47 ) of the stirling engine  10  is not caused by the lubricant. Here, as far as the inconvenience accompanying the use of the lubricant and the piston ring, such as the sliding resistance, can be eliminated, any air bearings excluding one type of fluid dynamic bearing called an oil bearing which uses oil may be employed other than the air bearing  48 . 
   Alternatively, a static pressure air bearing may be employed between the pistons  21  and  31  and the cylinders  22  and  32  of the embodiments. The static pressure air bearing floats a material (the pistons  21  and  31  in the embodiments) by spraying a pressurized fluid and utilizing a generated static pressure. Alternatively, a dynamical pressure air bearing may be employed instead of the static pressure air bearing. 
   When the pistons  21  and  31  reciprocate inside the cylinders  22  and  32  with the use of the air bearing  48 , an accuracy of linear motion should be maintained below the clearance in diameter of the air bearing  48 . Further, since the loading capacity of the air bearing  48  is small, a side force applied by the pistons  21  and  31  is required to be substantially zero. In other words, since the air bearing  48  has a little capacity to bear the force applied in a direction of a diameter of the cylinders  22  and  32 , i.e., a lateral direction or a thrust direction, the accuracy of linear motion of the pistons  21  and  31  with respect to axes of the cylinders  22  and  32  is required to be particularly high. In particular, since the air bearing  48  of the embodiments which floats and supports the material with the air pressure produced by the minute clearance has a lower pressure bearing capacity in the thrust direction compared with the type of bearing that sprays the highly-pressurized air, an accordingly higher accuracy of linear motion of the piston is required. 
   Hence in the embodiments, a grasshopper mechanism  50 , i.e., an approximately linear link, is employed for the piston/crank section. The grasshopper mechanism  50  achieves the same accuracy of linear motion in a smaller mechanism compared with other approximately linear mechanism (the Watt mechanism, for example), thereby providing a more compact overall system. In particular, since the stirling engine  10  of the embodiments is installed in a limited space, for example, the heater  47  thereof is housed in the exhaust tube of the vehicle, a more compact overall system increases a degree of freedom in installation. In addition, the grasshopper mechanism  50  can achieve the same accuracy of linear motion in a lighter mechanism compared with other mechanisms, and is advantageous in terms of fuel consumption. Further, the grasshopper mechanism  50  has a relatively simple structure and is easy to build (manufacture/assemble). 
     FIG. 14  shows a schematic structure of a piston/crank mechanism of the stirling engine  10 . In the embodiments, the piston/crank mechanism adopts a common structure for the high temperature side power piston  20  and the low temperature side power piston  30 . A description will be given hereinbelow only on the low temperature side power piston  30  and a description on the high temperature side power piston  20  will be omitted. 
   As shown in  FIGS. 14 and 11 , a reciprocating movement of the pressurizing piston  31  is transferred to the driving shaft  40  via a connecting rod  109  ( 65   a  and  65   b ) and converted into a rotation movement. The connecting rod  109  is supported by the approximately linear mechanism  50  shown in  FIG. 14  to make the low temperature side cylinder  32  reciprocate linearly. With the approximately linear mechanism  50  supporting the connecting rod  109 , the side force F produced by the compression piston  31  is substantially zero. Hence, even the air bearing  48  with a small load bearing capacity can sufficiently support the compression piston  31 . 
   Next, pressurization of the working fluid in the working space of the stirling engine  10  and pressurization of the crankcase  41  will be described. 
   As described above, a high output can be obtained when the mean working gas pressure Pmean of the working fluid in the working space of the stirling engine  10  is maintained at a high level. In addition, in the stirling engine  10  of the embodiments, the pressure in the crankcase  41  is raised up to the mean working gas pressure Pmean inside the cylinder of the stirling engine  10 . The increase in the pressure in the crankcase  41  up to the mean working gas pressure Pmean inside the cylinder of the stirling engine  10  is intended to eliminate the need of a high strength of the components (piston, for example) of the stirling engine  10  in the design thereof. 
   In other words, when the pressure of the crankcase  41  is at the level of the mean working gas pressure Pmean inside the cylinder of the stirling engine  10 , the differential pressure of the intra-cylindrical pressure of the stirling engine  10  and the pressure inside the crankcase  41  can be suppressed to the differential pressure between the maximum (or minimum) intra-cylindrical pressure and the mean working gas pressure Pmean at the maximum. Thus, with the suppression of differential pressure between the intra-cylindrical pressure of the stirling engine  10  and the pressure of the crankcase  41 , the strength of the components of the stirling engine  10  can be low. When the components are not required to have a high strength, lighter components can be realized. 
   In the stirling engine  10  of the embodiments, the crankcase  41  is pressurized prior to a normal operation up to the mean working gas pressure Pmean inside the cylinder of the stirling engine  10 . 
   First, pressurization of the working fluid in the working space of the stirling engine  10  and pressurization of the crankcase  41  will be described according to a first embodiment. 
   Here, the mean working gas pressure Pmean mentioned above will be described with reference to  FIG. 13 . 
     FIG. 13  shows changes of the top position of the high temperature side piston  21  and the top position of the low temperature side piston  31 . As described above, the phase difference is provided so that the low temperature side piston  31  moves 90° later by the crank angle than the high temperature side piston  21 . In  FIG. 13 , a combined wave W of a wave form of the high temperature side piston  21  and a wave form of the low temperature side piston  31  represents the intra-cylindrical pressure (intra-cylindrical pressure P of  FIG. 2 ). In  FIG. 13 , the reference character “Pmean” indicates the mean working gas pressure which is a mean value of the intra-cylindrical pressure. 
     FIG. 2  shows an initial state of the crankcase  41  of the stirling engine  10  according to the first embodiment prior to the pressurization. The graph of  FIG. 2  shows the combined wave W where the vertical axis represents the intra-cylindrical pressure and the horizontal axis represents the crank angle. As shown in  FIG. 2 , prior to the pressurization of the crankcase  41 , the pressure Pc of the crankcase  41  (=mean working gas pressure Pmean) is equal to the atmosphere pressure Po. 
   In the first embodiment, changes in the pressure (intra-cylindrical pressure P) of the working fluid of the stirling engine  10  is utilized for the increase in the pressure Pc of the crankcase  41  as described later. In general, the intra-cylindrical pressure P moves from a lower range than the mean working gas pressure Pmean (from a second half of the expansion process through a first half of the compression process) up to a higher range than the mean working gas pressure Pmean (from a second half of the compression process through a first half of the expansion process) repeatedly as indicated by the reference character W in  FIG. 13 . In the first embodiment, the pressure Pc of the crankcase  41  is increased together with the mean working gas pressure Pmean with the use of the changes in the intra-cylindrical pressure P. 
   In the foregoing, the lower range of the intra-cylindrical pressure P than the mean working gas pressure Pmean corresponds with a period in one cycle of the expansion/pressurization of the working fluid where the working gas pressure is lower than the mean Pmean of the working gas pressure in the pertinent cycle, whereas the higher range of the intra-cylindrical pressure P than the mean working gas pressure Pmean corresponds with a period where the working gas pressure is higher than the mean Pmean of the working gas pressure in the pertinent cycle. The same applies below. 
     FIG. 1  is a schematic diagram of a structure of the first embodiment. In  FIG. 1 , the same components with the components shown in  FIG. 11  are indicated with the same reference characters and the detailed description thereof will not be repeated. 
   As shown in  FIG. 1 , a path  71  is provided at a position corresponding to a position around a lower dead point of the piston  31  in the low temperature side cylinder  32  to communicate with the compression space (inside the cylinder) of the low temperature side cylinder  32 . In the path  71  a filter  72  is provided. The path  71  serves to let the fluid (working fluid) of the atmospheric pressure Po flow from the outside of the stirling engine  10  into the cylinder. The path  71  is configured to let the fluid flow (let the pressure transfer) only in one direction, i.e., from the outside into the cylinder. 
   The filter  72  serves to prevent any impurities from entering the cylinder from outside of the stirling engine  10  via the path  71 . As described above, the path  71  is not provided to the high temperature side cylinder  22 , but is connected to the low temperature side cylinder  32 . Since the thermal difference between the outside of the stirling engine  10 , i.e, of a room temperature, and the working fluid is smaller for the compression space of the low temperature side cylinder  32  than for the expansion space of the high temperature side cylinder  22 , the path  71  is connected to the low temperature side cylinder  32  to cause relative decrease in the thermal loss at the time the outside air comes into the cylinder. 
   As shown in  FIG. 2 , when the intra-cylindrical pressure P drops below the atmospheric pressure Po (becomes a negative pressure) (from the second half of the expansion process through the first half of the compression process), the fluid (air) of the atmospheric pressure Po enters into the cylinder via the path  71 , and is pressurized through the compression process of the stirling engine  10  (from the second half of the compression process in particular). The pressure (fluid) pressurized in the compression process is transferred to the crankcase  41  via the clearance CL between the cylinders  32  and  22  and the pistons  31  and  21 . Thus, the crankcase  41  is pressurized. 
   With the repetition of the above described process, the mean working gas pressure Pmean (which is equal to the pressure Pc in the crankcase  41 ) rises above the atmospheric pressure Po and the mean working gas pressure Pmean attains the level of the pressure Pc of the crankcase  41  as shown in  FIG. 3 . When the stirling engine  10  operates in the raised state of the mean working gas pressure Pmean, the stirling engine  10  can attain a high output. 
   Next, with reference to  FIG. 4 , a pressurizing of the working fluid in the working space of the stirling engine  10  and a pressurizing of the crankcase  41  according to a second embodiment will be described. 
     FIG. 4  shows a schematic structure of a stirling engine according to the second embodiment. The same component with the first embodiment shown in  FIG. 1  is indicated with the same reference character and the description thereof will not be repeated. 
   The second embodiment is different from the first embodiment in that a check valve  73  is provided in the path  71 . The check valve  73  is formed so that the check valve  73  opens only when a pressure at the side of the tip portion  71   a  of the path  71  is higher than a pressure at the side of a base portion  71   b  thereof. Hence, the path  71  has a structure to transfer the pressure (working fluid) only in the direction from the outside into the cylinder. In addition, the second embodiment includes a path  81  which connects the interior of the cylinder of the stirling engine  10  with the crankcase  41 . 
   According to the second embodiment, when the intra-cylindrical pressure P of the stirling engine  10  is lower than the atmospheric pressure Po, the fluid of the atmospheric pressure Po of the outside flows into the cylinder via the path  71  and pressurized in the compression process of the stirling engine  10 . The pressure increased in the compression process is transmitted to the crankcase  41  via the path  81 . Thus, the crankcase  41  is pressurized. With the repetition of the process, the mean working gas pressure Pmean (pressure Pc in the crankcase  41 ) rises above the atmospheric pressure Po and the mean working gas pressure Pmean attains the level of the pressure Pc of the crankcase  41  as shown in  FIG. 3  similar to the first embodiment. 
   In the first embodiment, when the sealing pressure of the minute clearance between the cylinders  32  and  22  and the pistons  31  and  21  is high, the pressure (fluid) increased in the compression process is not readily transferred to the crankcase  41  via the clearance CL (or the transfer takes time). In the second embodiment, however, since the pressure is transferred to the crankcase  41  via the path  81 , such inconvenience will not occur. 
   Next, a pressurizing of the working fluid in the working space of the stirling engine  10  and a pressurizing of the crankcase  41  according to a third embodiment will be described with reference to  FIGS. 5 to 6B . 
     FIG. 5  shows a schematic structure of the third embodiment. The same component with the second embodiment shown in  FIG. 4  is indicated with the same reference character and the detailed description thereof will not be repeated. The third embodiment is different from the second embodiment in that a check valve  82  and a valve  83  are provided in the path  81 . The check valve  82  is formed so that the check valve opens only when a pressure at the tip portion  81   a  on the side of the cylinder is higher than a pressure at the tip portion  81   b  on the side of the crankcase  41 . 
   According to the third embodiment, the crankcase  41  is pressurized via the path  81  when the intra-cylindrical pressure P is higher than the pressure Pc of the crankcase  41  while the valve  83  is open as shown in  FIG. 6A . When the intra-cylindrical pressure P is lower than the atmospheric pressure Po, the fluid of the atmospheric pressure Po flows into the cylinder via the path  71 . With the repetition of the process, the crankcase  41  is pressurized and the valve  83  eventually closes. Then as shown in  FIG. 6B , the mean working gas pressure Pmean rises up to the level of the pressure Pc of the crankcase  41 . 
   When the volume of the working fluid in the cylinder and the volume of the crankcase  41  are compared, the volume of the working fluid is smaller than the volume of the crankcase  41 . Hence, the mean working gas pressure Pmean rises up to the pressure Pc of the crankcase  41 . In the third embodiment, with the check valve  82  and the valve  83  in the path  81 , the flow of the fluid from the side of the crankcase  41  into the cylinder via the path  81  can be securely suppressed. 
   Next, a pressurizing of the working fluid of the working space of the stirling engine  10  and a pressurizing of the crankcase  41  according to a fourth embodiment will be described with reference to  FIGS. 7 and 8 . 
   In the first to the third embodiments described above, the pressure Pc of the crankcase  41  is increased with the use of the atmospheric pressure Po. In the fourth embodiment, the pressure Pc of the crankcase  41  is increased with the use of auxiliary machinery such as a pressure source like a pressurizing pump. In the fourth embodiment, reduction in energy consumption of the auxiliary machinery which is used to increase the pressure Pc of the crankcase  41  and downsizing of the installation scale are intended. 
   In the fourth embodiment, the reduction in energy consumption of the auxiliary machinery and the downsizing of the installation scale are realized through the use of a pumping function accompanying the pressurization/expansion of the stirling engine  10  which is described above with reference to the first to the third embodiments. 
     FIG. 8  shows a structure of a stirling engine according to the fourth embodiment. The same component with the first embodiment shown in  FIG. 1  is indicated with the same reference character and the detailed description thereof will not be repeated. In the fourth embodiment, a branch path  75  is connected to the path  71  so that the branch path  75  diverts from the path  71 . The branch path  75  is provided with a pressurizing pump  91  and a tank  92  arranged at a downstream side of the pressurizing pump  91 . The tank  92  serves to store the fluid pressurized by the pressurizing pump  91  or the like. 
   As shown in  FIG. 7 , in the fourth embodiment, the outside pressure (pressure in the tank  92 , and also the atmospheric pressure Po when the intra-cylindrical pressure P is lower than the atmospheric pressure Po) is introduced into the cylinder. The pressure introduced into the cylinder is further increased in the compression process of the stirling engine  10 . The pressure (fluid) increased in the compression process is transferred to the crankcase  41  via the clearance CL between the cylinders  32  and  22  and the pistons  31  and  21 . Thus, the crankcase  41  is pressurized. 
   In the fourth embodiment, at the pressurization of the crankcase  41 , not only the pressure produced by the pressurizing pump  91  works on the crankcase  41 , but the pressure produced through a further pressurization in the compression process of the stirling engine  10  to the pressure produced by the pressurizing pump  91  works on the crankcase  41 . Hence, the reduction in energy consumption of the pressurizing pump  91  and the downsizing of the installation scale are realized. 
   Next, a pressurizing of the working fluid in the working space of the stirling engine  10  and a pressurizing of the crankcase  41  according to a fifth embodiment will be described with reference to  FIGS. 9 and 10 . The same structure with the embodiments described above will be indicated by the same reference character and the detailed description thereof will not be repeated. 
   As shown in  FIG. 9 , two check valves  76  and  77  are provided in the path  71  arranged in the low temperature side cylinder  32 . The check valves  76  and  77  are formed so that the check valves  76  and  77  open only when a pressure in an upstream side of the path  71  is higher than a pressure in a downstream side of the path  71 . A piston pump  95  is arranged between the check valves  76  and  77 . 
   A crank shaft of the piston pump  95  is integrally formed with a crank shaft of the stirling engine  10  and is structured so that the movement of two pistons in the stirling engine  10  and the piston pump  95  are of opposite phase with each other. A valve  78  is further provided on an still upstream side of the check valve  77  in the path  71 . 
   An upper graph in  FIG. 10  represents the intra-cylindrical pressure P of the stirling engine  10  whereas a lower graph in  FIG. 10  represents the intra-cylindrical pressure of the piston pump  95 . In each graph of  FIG. 10 , the vertical axis represents pressure and the horizontal axis represents crank angle. 
   When the valve  78  of  FIG. 9  is open and the intra-cylindrical pressure of the piston pump  95  is low (or negative), the external pressure is introduced into the cylinder of the piston pump  95  via the path  71  and is further increased in the compression process of the piston pump  95  as shown in  FIG. 10 . 
   In the compression process of the piston pump  95 , the intra-cylindrical pressure P of the stirling engine  10  is in the expansion process (due to the antiphase relation), whereby the differential pressure is large. The fluid pressurized in the compression process of the piston pump  95  is introduced into the cylinder when the intra-cylindrical pressure P of the stirling engine  10  is low (in the expansion process) and further pressurized in the compression process of the stirling engine  10 . The pressure (fluid) increase in the compression process is transferred to the crankcase  41  via the clearance CL between the cylinders  32  and  22  and the pistons  31  and  21 . Thus, the crankcase  41  is pressurized. 
   As described above, in the stirling engine  10  of the fifth embodiment, the pressure of the crankcase  41  is raised up to the mean working gas pressure Pmean inside the cylinder of the stirling engine  10 . Hence, when it is difficult to suppress the pressure leakage to zero through the perfect sealing of the crankcase  41  after the pressurization of the crankcase  41  at the shipping, repressurization of the crankcase  41  is necessary in some manner. Then, a pressurizing source such as the pump  91  or the piston pump  95  may be necessary. These needs considered, it is advantageous to utilize the pumping function of the stirling engine  10  not simply for an original purpose such as acquisition of torque but for the increase of the pressure of the crankcase  41  as described above in order to minimize the installation scale/energy of the pressurizing source. 
   Here, the first to the fifth embodiments can be combined as appropriate. For example, the path  81  may be provided to connect inside the cylinder and the crankcase  41  to the stirling engine of the fourth and/or the fifth embodiments as in the second or the third embodiment. 
   As described above, following features are disclosed according to the above described embodiments. 
   (1) The crankcase is pressurized according to the change in working gas pressure in the stirling engine. 
   (2) In (1), the crankcase is pressurized through the intake of air from the outside into the cylinder in the stirling engine when the intra-cylindrical pressure P of the stirling engine is lower than the atmospheric pressure Po. 
   (3) In (1), the crankcase  41  is pressurized through the transfer of the intra-cylindrical pressure P to the crankcase  41  when the intra-cylindrical pressure P is higher than the pressure Pc in the crankcase  41 . The crankcase  41  is pressurized with the use of the differential pressure between the pressure Pc of the crankcase  41  and the intra-cylindrical pressure P. 
   (4) In (2), the path to take in the air is connected to the low temperature side cylinder (to reduce the thermal loss). 
   (5) In (3), the mean working gas pressure Pmean of the working gas eventually attains the level of the pressure Pc in the crankcase  41 . 
   (6) In (3), the mean working gas pressure Pmean is raised up to the level of the pressure Pc in the crankcase  41  through the closing of the path connecting inside the cylinder with the crankcase  41 . 
   (7) In (2), the impurities are prevented from coming into the cylinder. 
   (8) The pressurization of the crankcase  41  is complemented by the change in the working gas pressure of the stirling engine. 
   (9) In (8), the load on the device that pressurizes the crankcase  41  is reduced via the introduction of external pressure from the outside into the cylinder when the intra-cylindrical pressure P is low. 
   (10) In (8) and (9), the increase of the pressure Pc of the crankcase  41  is speeded up through the introduction of external pressure and pressurization by the stirling engine. 
   (11) With the addition of the piston which is of antiphase with the power piston of the stirling engine for the pressurization of the crankcase  41 , the reduction of energy consumption in the pressurization of the crankcase  41  is achieved. 
   (12) In one of (8) to (11), the path that introduces the external pressure is connected to the low temperature side cylinder (for the reduction in thermal loss). In the above described embodiments, the stirling engine  10  is connected to the exhaust tube  100  to utilize the exhaust gas from the internal combustion engine of the vehicle as the heat source. The stirling engine of the present invention is, however, not limited to a type that is connected to the exhaust tube of the internal combustion engine of the vehicle. 
   Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.