Stirling engine

A stirling engine 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.

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.

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 crankcase41. 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 crankcase41according to the respective embodiments.

FIG. 11is a front sectional view of a stirling engine of the embodiments. As shown inFIG. 11, the stirling engine of the embodiments is a stirling engine10of α-type (two-piston type) and provided with two power pistons20and30. Two power pistons20and30are arranged in parallel in line. A piston31of the power piston30on a low temperature side is arranged so that the piston31moves with a phase difference of 90° in a crank angle with respect to a piston21of the power piston20on a high temperature side as shown inFIG. 13.

A working fluid heated by a heater47flows into a space (expansion space) in an upper section of a cylinder22(hereinafter referred to as a high temperature side cylinder) of the power piston20on the high temperature side. A working fluid cooled by a cooler45flows into a space (compression space) in an upper section of a cylinder32(hereinafter referred to as a low temperature side cylinder) of the power piston30on the low temperature side.

A regenerator (regenerative heat exchanger)46stores 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 regenerator46receives 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 pistons21and31(also referred to as expansion piston21and compression piston31hereinbelow), changes the ratio of the working fluid in the expansion space of the high temperature side cylinder22and the compression space of the low temperature side cylinder32, 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 cylinders21and31is to be compared, the pressure is substantially higher when the expansion piston21is in a lower position than in a higher position, whereas the pressure is substantially lower when the compression piston31is in a lower position than in a higher position. Thus, the expansion piston21performs a positive work (expansion work) of a substantial amount to the outside, whereas the compression piston31needs 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 shaft40.

The stirling engine of the embodiments is employed with a main engine200, a gasoline engine or an internal combustion engine, for example, in a vehicle as shown inFIG. 12, thereby forming a hybrid system. In other words, the stirling engine10is an exhaust heat collecting unit which utilizes exhaust gas from the main engine200as a heat source. With the heater47of the stirling engine10placed in an exhaust tube100of the main engine200of the vehicle, heat energy collected from the exhaust gas heats up the working fluid thereby starting up the stirling engine10.

Since the stirling engine10of the embodiments is placed in a limited space in the vehicle, e.g., the heater47is housed in the exhaust tube100, the overall structure thereof is preferably made compact to increase the degree of freedom in installation. To this end, in the stirling engine10, two cylinders22and32are not arranged in a “V” configuration but placed in a parallel in-line layout.

The heater47is arranged inside the exhaust tube100, so that a side of the heater47on the side of high temperature side cylinder is located at an upstream side100a(a side closer to the main engine200) of the exhaust gas where exhaust gas of a relatively high temperature flows in the exhaust tube100, whereas a side of the heater47on the side of the low temperature side cylinder32is located at a downstream side100b(a side farther from the main engine200) where exhaust gas of a relatively low temperature flows. Such arrangement intends to heat up the side of the heater47on the side of the high temperature side cylinder22to a higher level.

Each of the high temperature side cylinder22and the low temperature side cylinder32is formed in a cylindrical shape and supported by a base plate42which serves as a baseline. In the embodiments, the base plate42serves to provide a reference position for respective components of the stirling engine10. With such structure, the relative location accuracy of respective components of the stirling engine10can be secured. In addition, the base plate42can be used as a reference for attachment of the stirling engine10to the exhaust tube100(exhaust path) which provides exhaust heat to be collected.

The base plate42is fixed to a flange100fof the exhaust tube100via a heat insulating material (spacer not shown). The base plate42is also fixed to a flange22fprovided in a side face (outer peripheral surface)22cof the high temperature side cylinder22. The base plate42is also fixed to a flange46fprovided in a side face (outer peripheral surface)46cof the regenerator46.

The exhaust tube100is attached to the stirling engine10via the base plate42. The stirling engine10is attached to the base plate42so that an end face (an upper face of a top portion22b) of the high temperature side cylinder22where the heater47is connected, and an end face (a top face32a) of the low temperature side cylinder32where the cooler45is connected are substantially parallel with the base plate42. Alternatively, the stirling engine10is attached to the base plate42so that the base plate42is parallel with a rotation shaft of a crank shaft61(or the driving shaft40) or so that a central axis of the exhaust tube100is parallel with the rotation shaft of the crank shaft61. Thus, the stirling engine10can be readily attached to the exhaust tube100of an existing type without a major change in design. As a result, the stirling engine10can be attached to the exhaust tube100without 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 engine10of the same specification can be attached to a different exhaust tube only with a change in specification of the heater47, the versatility of the stirling engine can be enhanced.

The stirling engine10is arranged horizontally in a space adjacent to the exhaust tube100which is placed under a floor of the vehicle. In other words, the stirling engine10is arranged so that the axes of the high temperature side cylinder22and the low temperature side cylinder32are substantially parallel with the floor (not shown) of the vehicle. Two pistons21and31reciprocate horizontally. In the embodiments, an upper dead point side and a lower dead point side of two pistons21and31are 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 cooler45and the heater47is larger. Hence, as described above, the working fluid in the working space of the high temperature side cylinder22and the low temperature side cylinder32is maintained in a high pressure.

The pistons21and31are formed in a cylindrical shape. Between the outer peripheral surface of pistons21and31and the inner peripheral surface of the cylinders22and32, a minute clearance of a few ten micrometers (μm) is provided. The working fluid (air) of the stirling engine10is present in the clearance. The pistons21and31are supported by an air bearing48so that the pistons do not contact with the cylinders22and32, respectively. Hence, piston rings are not provided around the pistons21and31, and lubricant which is generally used together with the piston ring is not employed. To the inner peripheral surface of the cylinders22and32, however, an antifriction is fixed. Though resistance of the air bearing48toward sliding movement caused by the working fluid is originally extremely low, the antifriction is provided for further resistance reduction. As described above, the air bearing48serves 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 heater47includes a plurality of heat transfer tubes (tube group)47t, each of which is formed generally in a U-shape. A first end portion47taof each heat transfer tube47tis connected to the upper portion (top portion) (end face at the side of a top face22a)22bof the high temperature side cylinder22. A second end portion47tbof each heat transfer tube47tis connected to an upper portion (end face at the side of the heater47)46aof the regenerator46. The reason why the heater47is formed generally in U-shape as described above will be described later.

The regenerator46includes a heat storage material (matrix not shown) and a regenerator housing46hthat houses the matrix. Since the regenerator housing46haccommodates the working fluid of high pressure, the regenerator housing46his formed as a pressure-tight container. The regenerator46here includes laminated metallic meshes as the matrix.

The regenerator46has to meet the following conditions to realize the above described functions. The regenerator46is 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 heater47with the high temperature side cylinder22is formed in the same shape and size with the shape of an opening (perfect circle) of the upper portion (a connecting portion with the heater47) of the high temperature side cylinder22. Similarly, a connecting portion of the heater47with the regenerator46is formed in the same shape and size with the upper face of the regenerator46. Thus, the cross sections of the heater47and the regenerator46are formed in the same shape and size with the openings of the high temperature side cylinder22and the low temperature side cylinder32, respectively. With such a structure, resistance of a flow path (flow resistance) of the working fluid is decreased.

The crank shaft61is rotatably supported by a bearing with respect to the crankcase41. In the embodiments, a counterweight90is provided on a side of the high temperature side cylinder22. The position of the counterweight90is selected so as to minimize the influence on the vertical size of the overall stirling engine10. A sufficient space can be secured in the space on a side of the high temperature side cylinder22.

Next, a reason why the heater47is formed generally in U-shape (curved shape) as described above will be described.

The heat source of the stirling engine10is the exhaust gas of the main engine200of 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 engine10is required to start up with a small amount of heat obtained from the exhaust gas, of approximately 800° C., for example. Thus, the heater47of the stirling engine10is required to efficiently receive the heat from the exhaust gas in the exhaust tube100.

A volume of a heat exchanger which includes the heater47, the regenerator46, and the cooler45is a void volume, which does not directly affect the output. When the volume of the heat exchanger increases, the output of the stirling engine10decreases. 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 engine10is 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 heater47is required.

To realize the efficient heat receipt from the exhaust gas in the exhaust tube100and the efficient heat exchange, the whole structure of the heater47is required to be accommodated in the exhaust tube100in just proportion, and the cooler45is required to be located outside the exhaust tube100to avoid receiving the heat from the exhaust gas. Hence, when the flange100fwhere the exhaust tube100is attached to the stirling engine10is taken as a reference, a position of attachment of the low temperature side cylinder32is lower than a position of attachment of the high temperature side cylinder22at least by the height of the cooler45. Thus, a position of the compression space formed in the upper section of the low temperature side cylinder32is lower than the position of the expansion space formed in the upper section of the high temperature side cylinder22, and an upper dead point of the compression piston31is lower than a position of an upper dead point of the expansion piston21.

In the embodiments, piston pins60aand60bare connected to pistons21and31, respectively, with extensions (piston supports)64aand64bof different sizes to change the positions of the upper dead points of the pressurizing piston31and the expansion piston21. Since the position of the upper dead point of the expansion piston21is higher than the upper dead point of the compression piston31, the extension64aconnected to the expansion piston21is longer than the extension64bconnected to the compression piston31by the difference in the height of position of the upper dead point.

In the embodiments, the expansion piston21and the compression piston31are formed so that the lengths thereof are equal. In other words, the distances between the upper faces of pistons21and31and connection points21cand31cwith the extensions64aand64bof the pistons21and31, respectively, are made equal. Therefore, the extensions64aand64bare formed in different lengths to arrange the upper dead points of the piston21and31at 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 engine10, a space outside the expansion space in the high temperature side power piston20and a space outside the compression space in the low temperature side power piston30, i.e., a space around the crank shaft61in each of the high temperature side power piston20and the low temperature side power piston30is required to be maintained at a room temperature. Hence, secure sealing must be provided between the high temperature side cylinder22and the expansion piston21, and between the low temperature side cylinder32and the compression piston31, so that the working fluid of a high temperature in the expansion space will not flow into the space around the crank shaft61at the side of the high temperature side power piston20and the working fluid of a low temperature in the compression space will not flow into the space around the crank shaft61on the side of the low temperature side power piston30. As described later, the air bearing48is employed to achieve such sealing.

On the other hand, since the top portion22band the side face22cof the high temperature side cylinder22are housed inside the exhaust tube100as described above, the upper portion of the high temperature side cylinder22and the upper portion of the expansion piston21thermally expand. Then, the sealing might not be secured in a section where the upper portions of the high temperature side cylinder22and the expansion piston21expand. To avoid such inconvenience, the expansion piston21and the high temperature side cylinder22may be formed longer in the vertical direction to provide a thermal gradient in vertical direction of the expansion piston21. Then, the secure sealing can be guaranteed with the section not affected by the thermal expansion, i.e., the lower portion of the expansion piston21. Further, since the sealing between the high temperature side cylinder22and the expansion piston21is provided with the lower portion of the expansion piston21, i.e., the section not affected by the thermal expansion, the high temperature side cylinder22may 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 engine10as 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 engine10will 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 heater47is generally in U-shape, the heater47has only one curved portion. In addition, the cooler45is formed to have a curved portion for the downsizing of the stirling engine10(reduction in vertical dimension), whereby the structure with the features as described above is realized.

In addition, as shown inFIG. 11, the curvature of the void volume portion in the embodiments is set according to the arrangement where the upper portions of two cylinders22and32arranged in parallel in line are coupled, and the vertical distance between the top portion22bof the high temperature side cylinder22and the upper face46aof the regenerator46arranged approximately in the same plane to suppress the increase in flow resistance of the working fluid in the exhaust tube100and the upper inner face of the exhaust tube100is set to a height h which is approximately equal to the distance between the end portions47taand47tband the uppermost portion of a central portion47cof the heater47. 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 tube100, 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 regenerator46is arranged linearly (along the same axis) along a direction of extension (direction of axis) of the low temperature side cylinder32. Thus, the regenerator46connected to a second end portion47tbof the heater47is arranged along the direction of extension of the low temperature side cylinder32. A first end portion47taof the heater47is seamlessly connected to the upper portion of the high temperature side cylinder22. Thus, the heater47has portions extending along the directions of extension of the high temperature side cylinder22and the low temperature side cylinder32at least at the sides of the first end portion47taand the second end portion47tbof the heater47, and the central portion47cof the heater47, in many cases, has a curved shape as described above.

Due to the technical reasons as described above, the heater47is formed in a curved shape between two cylinders22and32which are arranged in parallel in line. Thus, the heater47has a curved portion connecting two cylinders22and32.

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 engine10is the exhaust gas from the internal combustion engine of the vehicle, the obtainable amount of heat is limited and the stirling engine10is required to function in the range of obtainable heat amount. Hence, in the embodiments, the internal friction of the stirling engine10is 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 bearing48is provided between the cylinders22and32and the pistons21and31, respectively.

The air bearing48can significantly reduce the internal friction of the stirling engine10since the sliding resistance thereof is extremely small. Since the cylinders22and32and the pistons21and31are secured airtight also with the air bearing48, the working fluid of a high temperature would not leak out at the time of expansion and contraction.

The air bearing48utilizes the air pressure generated in the minute clearances between the cylinders22and32and the pistons21and31to support the pistons21and31in a floating position. The air bearing48of the embodiments has a clearance of a few ten micrometers (μm) in diameter between the cylinders22and32and the pistons21and31. 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 bearing48eliminates the lubricant which is used for the piston ring, the deterioration of the heat exchanger (the regenerator46and the heater47) of the stirling engine10is 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 bearing48.

Alternatively, a static pressure air bearing may be employed between the pistons21and31and the cylinders22and32of the embodiments. The static pressure air bearing floats a material (the pistons21and31in 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 pistons21and31reciprocate inside the cylinders22and32with the use of the air bearing48, an accuracy of linear motion should be maintained below the clearance in diameter of the air bearing48. Further, since the loading capacity of the air bearing48is small, a side force applied by the pistons21and31is required to be substantially zero. In other words, since the air bearing48has a little capacity to bear the force applied in a direction of a diameter of the cylinders22and32, i.e., a lateral direction or a thrust direction, the accuracy of linear motion of the pistons21and31with respect to axes of the cylinders22and32is required to be particularly high. In particular, since the air bearing48of 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 mechanism50, i.e., an approximately linear link, is employed for the piston/crank section. The grasshopper mechanism50achieves 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 engine10of the embodiments is installed in a limited space, for example, the heater47thereof 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 mechanism50can 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 mechanism50has a relatively simple structure and is easy to build (manufacture/assemble).

FIG. 14shows a schematic structure of a piston/crank mechanism of the stirling engine10. In the embodiments, the piston/crank mechanism adopts a common structure for the high temperature side power piston20and the low temperature side power piston30. A description will be given hereinbelow only on the low temperature side power piston30and a description on the high temperature side power piston20will be omitted.

As shown inFIGS. 14 and 11, a reciprocating movement of the pressurizing piston31is transferred to the driving shaft40via a connecting rod109(65aand65b) and converted into a rotation movement. The connecting rod109is supported by the approximately linear mechanism50shown inFIG. 14to make the low temperature side cylinder32reciprocate linearly. With the approximately linear mechanism50supporting the connecting rod109, the side force F produced by the compression piston31is substantially zero. Hence, even the air bearing48with a small load bearing capacity can sufficiently support the compression piston31.

Next, pressurization of the working fluid in the working space of the stirling engine10and pressurization of the crankcase41will 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 engine10is maintained at a high level. In addition, in the stirling engine10of the embodiments, the pressure in the crankcase41is raised up to the mean working gas pressure Pmean inside the cylinder of the stirling engine10. The increase in the pressure in the crankcase41up to the mean working gas pressure Pmean inside the cylinder of the stirling engine10is intended to eliminate the need of a high strength of the components (piston, for example) of the stirling engine10in the design thereof.

In other words, when the pressure of the crankcase41is at the level of the mean working gas pressure Pmean inside the cylinder of the stirling engine10, the differential pressure of the intra-cylindrical pressure of the stirling engine10and the pressure inside the crankcase41can 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 engine10and the pressure of the crankcase41, the strength of the components of the stirling engine10can be low. When the components are not required to have a high strength, lighter components can be realized.

In the stirling engine10of the embodiments, the crankcase41is pressurized prior to a normal operation up to the mean working gas pressure Pmean inside the cylinder of the stirling engine10.

First, pressurization of the working fluid in the working space of the stirling engine10and pressurization of the crankcase41will be described according to a first embodiment.

Here, the mean working gas pressure Pmean mentioned above will be described with reference toFIG. 13.

FIG. 13shows changes of the top position of the high temperature side piston21and the top position of the low temperature side piston31. As described above, the phase difference is provided so that the low temperature side piston31moves 90° later by the crank angle than the high temperature side piston21. InFIG. 13, a combined wave W of a wave form of the high temperature side piston21and a wave form of the low temperature side piston31represents the intra-cylindrical pressure (intra-cylindrical pressure P ofFIG. 2). InFIG. 13, the reference character “Pmean” indicates the mean working gas pressure which is a mean value of the intra-cylindrical pressure.

FIG. 2shows an initial state of the crankcase41of the stirling engine10according to the first embodiment prior to the pressurization. The graph ofFIG. 2shows the combined wave W where the vertical axis represents the intra-cylindrical pressure and the horizontal axis represents the crank angle. As shown inFIG. 2, prior to the pressurization of the crankcase41, the pressure Pc of the crankcase41(=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 engine10is utilized for the increase in the pressure Pc of the crankcase41as 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 inFIG. 13. In the first embodiment, the pressure Pc of the crankcase41is 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. 1is a schematic diagram of a structure of the first embodiment. InFIG. 1, the same components with the components shown inFIG. 11are indicated with the same reference characters and the detailed description thereof will not be repeated.

As shown inFIG. 1, a path71is provided at a position corresponding to a position around a lower dead point of the piston31in the low temperature side cylinder32to communicate with the compression space (inside the cylinder) of the low temperature side cylinder32. In the path71a filter72is provided. The path71serves to let the fluid (working fluid) of the atmospheric pressure Po flow from the outside of the stirling engine10into the cylinder. The path71is configured to let the fluid flow (let the pressure transfer) only in one direction, i.e., from the outside into the cylinder.

The filter72serves to prevent any impurities from entering the cylinder from outside of the stirling engine10via the path71. As described above, the path71is not provided to the high temperature side cylinder22, but is connected to the low temperature side cylinder32. Since the thermal difference between the outside of the stirling engine10, i.e, of a room temperature, and the working fluid is smaller for the compression space of the low temperature side cylinder32than for the expansion space of the high temperature side cylinder22, the path71is connected to the low temperature side cylinder32to cause relative decrease in the thermal loss at the time the outside air comes into the cylinder.

As shown inFIG. 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 path71, and is pressurized through the compression process of the stirling engine10(from the second half of the compression process in particular). The pressure (fluid) pressurized in the compression process is transferred to the crankcase41via the clearance CL between the cylinders32and22and the pistons31and21. Thus, the crankcase41is pressurized.

With the repetition of the above described process, the mean working gas pressure Pmean (which is equal to the pressure Pc in the crankcase41) rises above the atmospheric pressure Po and the mean working gas pressure Pmean attains the level of the pressure Pc of the crankcase41as shown inFIG. 3. When the stirling engine10operates in the raised state of the mean working gas pressure Pmean, the stirling engine10can attain a high output.

Next, with reference toFIG. 4, a pressurizing of the working fluid in the working space of the stirling engine10and a pressurizing of the crankcase41according to a second embodiment will be described.

FIG. 4shows a schematic structure of a stirling engine according to the second embodiment. The same component with the first embodiment shown inFIG. 1is 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 valve73is provided in the path71. The check valve73is formed so that the check valve73opens only when a pressure at the side of the tip portion71aof the path71is higher than a pressure at the side of a base portion71bthereof. Hence, the path71has 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 path81which connects the interior of the cylinder of the stirling engine10with the crankcase41.

According to the second embodiment, when the intra-cylindrical pressure P of the stirling engine10is lower than the atmospheric pressure Po, the fluid of the atmospheric pressure Po of the outside flows into the cylinder via the path71and pressurized in the compression process of the stirling engine10. The pressure increased in the compression process is transmitted to the crankcase41via the path81. Thus, the crankcase41is pressurized. With the repetition of the process, the mean working gas pressure Pmean (pressure Pc in the crankcase41) rises above the atmospheric pressure Po and the mean working gas pressure Pmean attains the level of the pressure Pc of the crankcase41as shown inFIG. 3similar to the first embodiment.

In the first embodiment, when the sealing pressure of the minute clearance between the cylinders32and22and the pistons31and21is high, the pressure (fluid) increased in the compression process is not readily transferred to the crankcase41via the clearance CL (or the transfer takes time). In the second embodiment, however, since the pressure is transferred to the crankcase41via the path81, such inconvenience will not occur.

Next, a pressurizing of the working fluid in the working space of the stirling engine10and a pressurizing of the crankcase41according to a third embodiment will be described with reference toFIGS. 5 to 6B.

FIG. 5shows a schematic structure of the third embodiment. The same component with the second embodiment shown inFIG. 4is 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 valve82and a valve83are provided in the path81. The check valve82is formed so that the check valve opens only when a pressure at the tip portion81aon the side of the cylinder is higher than a pressure at the tip portion81bon the side of the crankcase41.

According to the third embodiment, the crankcase41is pressurized via the path81when the intra-cylindrical pressure P is higher than the pressure Pc of the crankcase41while the valve83is open as shown inFIG. 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 path71. With the repetition of the process, the crankcase41is pressurized and the valve83eventually closes. Then as shown inFIG. 6B, the mean working gas pressure Pmean rises up to the level of the pressure Pc of the crankcase41.

When the volume of the working fluid in the cylinder and the volume of the crankcase41are compared, the volume of the working fluid is smaller than the volume of the crankcase41. Hence, the mean working gas pressure Pmean rises up to the pressure Pc of the crankcase41. In the third embodiment, with the check valve82and the valve83in the path81, the flow of the fluid from the side of the crankcase41into the cylinder via the path81can be securely suppressed.

Next, a pressurizing of the working fluid of the working space of the stirling engine10and a pressurizing of the crankcase41according to a fourth embodiment will be described with reference toFIGS. 7 and 8.

In the first to the third embodiments described above, the pressure Pc of the crankcase41is increased with the use of the atmospheric pressure Po. In the fourth embodiment, the pressure Pc of the crankcase41is 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 crankcase41and 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 engine10which is described above with reference to the first to the third embodiments.

FIG. 8shows a structure of a stirling engine according to the fourth embodiment. The same component with the first embodiment shown inFIG. 1is indicated with the same reference character and the detailed description thereof will not be repeated. In the fourth embodiment, a branch path75is connected to the path71so that the branch path75diverts from the path71. The branch path75is provided with a pressurizing pump91and a tank92arranged at a downstream side of the pressurizing pump91. The tank92serves to store the fluid pressurized by the pressurizing pump91or the like.

As shown inFIG. 7, in the fourth embodiment, the outside pressure (pressure in the tank92, 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 engine10. The pressure (fluid) increased in the compression process is transferred to the crankcase41via the clearance CL between the cylinders32and22and the pistons31and21. Thus, the crankcase41is pressurized.

In the fourth embodiment, at the pressurization of the crankcase41, not only the pressure produced by the pressurizing pump91works on the crankcase41, but the pressure produced through a further pressurization in the compression process of the stirling engine10to the pressure produced by the pressurizing pump91works on the crankcase41. Hence, the reduction in energy consumption of the pressurizing pump91and the downsizing of the installation scale are realized.

Next, a pressurizing of the working fluid in the working space of the stirling engine10and a pressurizing of the crankcase41according to a fifth embodiment will be described with reference toFIGS. 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 inFIG. 9, two check valves76and77are provided in the path71arranged in the low temperature side cylinder32. The check valves76and77are formed so that the check valves76and77open only when a pressure in an upstream side of the path71is higher than a pressure in a downstream side of the path71. A piston pump95is arranged between the check valves76and77.

A crank shaft of the piston pump95is integrally formed with a crank shaft of the stirling engine10and is structured so that the movement of two pistons in the stirling engine10and the piston pump95are of opposite phase with each other. A valve78is further provided on an still upstream side of the check valve77in the path71.

An upper graph inFIG. 10represents the intra-cylindrical pressure P of the stirling engine10whereas a lower graph inFIG. 10represents the intra-cylindrical pressure of the piston pump95. In each graph ofFIG. 10, the vertical axis represents pressure and the horizontal axis represents crank angle.

When the valve78ofFIG. 9is open and the intra-cylindrical pressure of the piston pump95is low (or negative), the external pressure is introduced into the cylinder of the piston pump95via the path71and is further increased in the compression process of the piston pump95as shown inFIG. 10.

In the compression process of the piston pump95, the intra-cylindrical pressure P of the stirling engine10is 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 pump95is introduced into the cylinder when the intra-cylindrical pressure P of the stirling engine10is low (in the expansion process) and further pressurized in the compression process of the stirling engine10. The pressure (fluid) increase in the compression process is transferred to the crankcase41via the clearance CL between the cylinders32and22and the pistons31and21. Thus, the crankcase41is pressurized.

As described above, in the stirling engine10of the fifth embodiment, the pressure of the crankcase41is raised up to the mean working gas pressure Pmean inside the cylinder of the stirling engine10. Hence, when it is difficult to suppress the pressure leakage to zero through the perfect sealing of the crankcase41after the pressurization of the crankcase41at the shipping, repressurization of the crankcase41is necessary in some manner. Then, a pressurizing source such as the pump91or the piston pump95may be necessary. These needs considered, it is advantageous to utilize the pumping function of the stirling engine10not simply for an original purpose such as acquisition of torque but for the increase of the pressure of the crankcase41as 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 path81may be provided to connect inside the cylinder and the crankcase41to 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 crankcase41is pressurized through the transfer of the intra-cylindrical pressure P to the crankcase41when the intra-cylindrical pressure P is higher than the pressure Pc in the crankcase41. The crankcase41is pressurized with the use of the differential pressure between the pressure Pc of the crankcase41and 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 crankcase41.

(6) In (3), the mean working gas pressure Pmean is raised up to the level of the pressure Pc in the crankcase41through the closing of the path connecting inside the cylinder with the crankcase41.

(7) In (2), the impurities are prevented from coming into the cylinder.

(8) The pressurization of the crankcase41is complemented by the change in the working gas pressure of the stirling engine.

(9) In (8), the load on the device that pressurizes the crankcase41is 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 crankcase41is 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 crankcase41, the reduction of energy consumption in the pressurization of the crankcase41is 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 engine10is connected to the exhaust tube100to 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.