Stirling engine and control method therefor

In a Stirling engine, a casing houses therein component elements of the Stirling engine, including a high-temperature-side cylinder, a high-temperature-side piston, a connecting rod, a crankshaft, etc. A pressure control device determines whether the pressure of the gas charged in the casing has declined. If the pressure of the gas has declined, the pressure control device drives a pump to pressurize the gas charged in the casing.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2007-001712 filed on Jan. 9, 2007, including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to a piston engine in which a piston reciprocates within a cylinder.

2. Description of Related Art

In recent years, Stirling engines, which are excellent in the theoretical heat efficiency, are drawing attention for the recovery of exhaust heat of an internal combustion engine mounted in a vehicle, such as a passenger car, a bus, a truck, etc., or of factory waste heat. Japanese Patent Application Publication No. 2005-106009 (JP-A-2005-106009) discloses a Stirling engine in which high pressure is maintained within a crankcase in order to obtain high output from the Stirling engine.

However, as for the Stirling engine disclosed in Japanese Patent Application Publication No. 2005-106009 (JP-A-2005-106009), no consideration is given to, for example, the pressure decline of the gas charged in the crankcase (casing) due to leakage of the gas.

SUMMARY OF THE INVENTION

It is an object of the invention to substantially prevent a Stirling engine in which pressurization in a casing is performed from undergoing the decline in the output caused by a decline in the pressure of the gas charged in the casing.

A first aspect of the invention relates to a Stirling engine. This Stirling engine includes: a casing that houses at least one component element of the Stirling engine; a determination device that determines whether pressure of a gas charged within the casing has declined based on an index that represents a targeted pressure of the gas; and a pressure adjustment device that compensates for a decline in the pressure of the gas by pressurizing the gas. Here, the gas may be air.

The at least one component element may include: a cylinder; a piston supported in the cylinder via a gas bearing; and an approximately linear mechanism that supports the piston.

Therefore, even if there occurs a decline in the pressure of the gas within the casing due to a change in the operation environment or leakage, the decline in the pressure can be compensated for by the pressure adjustment device. As a result, it is possible to restrain the decline in the output of the Stirling engine caused by a decline in the pressure of the gas charged in the casing.

The index may be the pressure of the gas charged in the casing which is determined based on temperature of the gas, and the pressure adjustment device may pressurize the gas so that the pressure of the gas reaches the index.

The index may be determined based on a ratio of the pressure of the gas to temperature of the gas or a ratio of temperature of the gas to the pressure of the gas, and the pressure adjustment device may pressurize the gas so that the ratio of the pressure of the gas to the temperature of the gas or the ratio of the temperature of the gas to the pressure of the gas reaches the index.

In the foregoing construction, before the Stirling engine is started, the determination device may determine whether the pressure of the gas has declined.

The index may be the pressure of the gas charged in the casing which occurs when the Stirling engine produces an output that is determined from a specification of the Stirling engine.

A second aspect of the invention relates to a Stirling engine. This Stirling engine includes: a cylinder; a piston supported in the cylinder via a gas bearing; an approximately linear mechanism that supports the piston; a casing that houses the cylinder, the piston and the approximately linear mechanism; a determination device that determines whether pressure of a gas charged in the casing has declined based on an index that represents a targeted pressure of the gas; and a pressure adjustment device that pressurizes the gas.

Therefore, even if there occurs a decline in the pressure of the gas within the casing due to a change in the operation environment or leakage, the decline in the pressure can be compensated for by the pressure adjustment device. As a result, it is possible to restrain the decline in the output of the Stirling engine caused by a decline in the pressure of the gas charged in the casing. Besides, due to the gas bearing and the approximately linear mechanism, the friction loss between the piston and the cylinder can be reduced, and therefore the decline in the output can be more effectively restrained.

A third aspect of the invention relates to a Stirling engine control method. This Stirling engine control method includes: the step of determining whether pressure of a gas charged in a casing that houses at least one component element of a Stirling engine has declined based on an index that represents a targeted pressure of the gas; and the step of compensating for a decline in the pressure of the gas by pressurizing the gas.

According to the Stirling engine and the Stirling engine control method in the foregoing aspects, in a Stirling engine in which the gas charged in the casing is pressurized, it is possible to restrain the decline in the output of the engine caused by a decline in the pressure of the gas charged in the casing.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention will be described in detail hereinafter with reference to the drawings.

An embodiment of the invention is characterized in the following respects. That is, the embodiment is a Stirling engine in which the gas charged within a casing that houses component elements of the Stirling engine is pressurized beforehand. In this Stirling engine, it is determined whether or not the pressure of the gas has declined on the basis of an index that represents a targeted pressure of the gas charged in the casing. Then, if the pressure of the gas is lower than the pressure found from the index, the gas is pressurized to compensate for the amount of decline in the pressure of the gas from the pressure target value. Firstly, a construction of a Stirling engine in accordance with the invention will be described. Incidentally, the following description will be made in conjunction with an example where the Stirling engine is used as an exhaust heat recovery device to recover thermal energy from exhaust gas discharged from an internal combustion engine, which is a heat engine. Incidentally, the heat engine may be of any kind. Here, air may be used as the gas.

FIG. 1is an illustrative diagram showing a section of a Stirling engine in accordance with this embodiment.FIG. 2is an illustrative diagram showing a section of an example of a construction of an air bearing provided in the Stirling engine in accordance with this embodiment.FIG. 3is an illustrative diagram showing an approximately linear mechanism that supports pistons of the Stirling engine in accordance with the embodiment. A Stirling engine100that is an exhaust heat recovery device in accordance with this embodiment is a so-called α-type in-series two-cylinder Stirling engine. That is, a high-temperature-side piston103termed a first piston which is contained in a high-temperature-side cylinder101termed a first cylinder, and a low-temperature-side piston104termed a second piston which is contained in a low-temperature-side cylinder102termed a second cylinder are arranged in series.

The high-temperature-side cylinder101and the low-temperature-side cylinder102are directly or indirectly supported by and fixed to a base board111that is a reference body. In the Stirling engine100in accordance with this embodiment, the base board111serves as a positional reference for various component elements of the Stirling engine100. This construction secures accuracy of the relative positions of the component elements. Besides, in the Stirling engine100in accordance with this embodiment, a gas bearing GB is interposed between the high-temperature-side cylinder101and the high-temperature-side piston103and also between the low-temperature-side cylinder102and the low-temperature-side piston104. More specifically, the high-temperature-side piston103and the low-temperature-side piston104are disposed in series in the direction in which a crankshaft110extends.

In the Stirling engine in accordance with the embodiment, since the high-temperature-side cylinder101and the low-temperature-side cylinder102are mounted directly or indirectly on the base board111, which is the reference body, the clearance between the piston and the cylinder can be accurately maintained. This allows the function of each gas bearing GB to be fully performed. Besides, this facilitates the assembly of the Stirling engine100.

A heat exchanger108constructed of a generally U-shape heater105, a regenerator106and a cooler107is disposed between the high-temperature-side cylinder101and the low-temperature-side cylinder102. Since the heater105has a generally U-shape, the heater105cab easily be disposed even in such a relatively narrow space as an exhaust gas passageway of an internal combustion engine. Besides, as in this Stirling engine100, the series arrangement of the high-temperature-side cylinder101and the low-temperature-side cylinder102makes it relatively easy to dispose the heater105even in such a tubular space as the exhaust gas passageway of the internal combustion engine.

One of two end portions of the heater105is disposed on a high-temperature-side cylinder101side, and the other end portion thereof is disposed on a regenerator106side. One of two end portions of the regenerator106is disposed on the heater105side, and the other end portion thereof is disposed on a cooler107side. One of two end portions of the cooler107is disposed on the regenerator106side, and the other end portion thereof is disposed on a low-temperature-side cylinder102side.

A working fluid (air in this embodiment) is enclosed in the high-temperature-side cylinder101, the low-temperature-side cylinder102and the heat exchanger108. The heat supplied from the heater105and the heat discharged from the cooler107constitute the Stirling cycle, and thus drives the Stirling engine100. It is to be noted herein that, for example, the heater105and the cooler107can each be constructed of bundling a plurality of tubes made of a material that is high in thermal conductivity and excellent in heat resistance. The cooler107may be of an air-cooled type or of a water-cooled type. Besides, the regenerator106can be constructed of a porous thermal storage body. Incidentally, the construction of each of the heater105, the cooler107and the regenerator106is not limited to the foregoing examples. Instead, any preferred construction can be selected depending on the thermal conditions of an object of the exhaust heat recovery, the specifications of the Stirling engine100, etc.

The high-temperature-side piston103and the low-temperature-side piston104are supported within the high-temperature-side cylinder101and the low-temperature-side cylinder102, respectively, via the gas bearings GB. That is, the pistons are supported within the cylinders without using a piston ring therebetween. This structure reduces the friction between the pistons and the cylinders, and improves the exhaust heat recovery efficiency of the Stirling engine100. Besides, if the friction between the pistons and the cylinders is reduced, it becomes easier to operate the Stirling engine100to recover thermal energy from exhaust heat in the form of kinetic energy even under operation conditions of a low heat source and a small temperature difference, such as the conditions in the case of the exhaust heat recovery in an internal combustion engine.

To construct the gas bearing GB, a clearance tc between the high-temperature-side piston103and the high-temperature-side cylinder101is set at several ten μm throughout the circumference of the high-temperature-side piston103or the like as shown inFIG. 2. The low-temperature-side piston104and the low-temperature-side cylinder102are arranged in substantially the same manner. The high-temperature-side cylinder101, the high-temperature-side piston103, the low-temperature-side cylinder102and the low-temperature-side piston104can be constructed, for example, by using a metal material that is easy to machine.

The reciprocating motion of each of the high-temperature-side piston103and the low-temperature-side piston104are transmitted by a connecting rod109to a crankshaft110that is an output shaft, and is thereby converted into rotary motion. In this embodiment, the high-temperature-side piston103and the low-temperature-side piston104are supported by an approximately linear mechanism (e.g., a grasshopper mechanism)113shown inFIG. 3. In this manner, the high-temperature-side piston103and the low-temperature-side piston104can be reciprocated approximately linearly. As a result, the side force F on the high-temperature-side piston103(i.e., the force directed in a direction of the diameter of the piston) becomes substantially zero. Therefore, the piston can be sufficiently supported even by the gas bearing GB, which is low in the ability to withstand the side force.

As shown inFIG. 1, the component elements constituting the Stirling engine100, including the high-temperature-side cylinder101, the high-temperature-side piston103, the connecting rod109, the crankshaft110, etc., are housed in a casing100C. The casing100C of the Stirling engine100includes a crankcase114A and a cylinder block114B. The gas charged in the casing100C (which is the same as the working fluid in this embodiment) is pressurized by a pump115. This pump115can be regarded as a pressure adjustment device in the invention. The pump115may be driven by, for example, an internal combustion engine that is the object of the exhaust heat recovery of the Stirling engine100, or may also be driven by using an electric motor or the like.

As for the Stirling engine100, the higher the average pressure of the working fluid, the greater the pressure difference between the high-temperature-side and the low-temperature-side is, and therefore the higher output is obtained, provided that the temperature difference between the heater105and the cooler107is fixed. The Stirling engine100in accordance with the embodiment is constructed so that a greater amount of output can be taken out from the Stirling engine100by pressurizing the gas charged in the casing100C so as to maintain high pressure of the working fluid. This construction makes it possible to take out a greater amount of output from the Stirling engine100even in the case where only low-quality heat source can be used as in the case of exhaust heat recovery. Incidentally, the output of the Stirling engine100increases substantially in proportion to the pressure of the gas charged in the casing100C.

In the Stirling engine100in accordance with the embodiment, a seal bearing116is mounted on the casing100C, and the seal bearing116supports the crankshaft110. In the Stirling engine100in accordance with the embodiment, although the gas charged in the casing100C is pressurized, the seal bearing116minimizes the leakage of the gas charged in the casing100C. The output of the crankshaft110is taken to the outside of the casing100C via a flexible coupling118, such as an Oldham's coupling.

The operation of the pump115is controlled by a pressure control device30provided in an engine ECU (Electronic Control Unit)50. The pressure of the gas charged in the casing100C is measured by a pressure sensor40that is a pressure detection portion. The temperature of the gas charged in the casing100C is measured by a temperature sensor41that is a temperature detection portion. The pressure P and the temperature T of the gas charged in the casing100C which are measured by the pressure sensor40and the temperature sensor41are taken into the pressure control device30provided in the engine ECU50, and are used for the pressure control of the gas charged in the casing100C.

In the Stirling engine100in accordance with the embodiment, the leakage of the gas charged in the casing100C is minimized by the seal bearing116, but slight leakage occurs. Therefore, as time passes, the pressure P of the gas charged in the casing100C declines. Besides, the pressure P of the gas charged in the casing100C may also decline depending on the operation environment of the Stirling engine100.

For example, if the temperature of the operation environment of the Stirling engine100declines and therefore the temperature T of the gas charged in the casing100C declines, then the pressure P of the gas charged in the casing100C also declines. If the pressure P of the gas charged in the casing100C declines, the output of the Stirling engine100declines. In order to avoid the decline in the output of the Stirling engine100, there is a need to keep the pressure P of the gas charged in the casing100C at a predetermined value.

In this embodiment, in the case where the pressure P of the gas charged in the casing100C declines below a pre-determined pressure target value, an amount of the gas is supplied into the casing100C by the pump115so as to raise the pressure P of the gas charged in the casing100C to the pressure target value. This restrains the decline in the output of the Stirling engine100caused by leakage or a change in the operation environment. The pressure target value is an index that represents a targeted pressure of the gas charged in the casing100C, and may be set, for example, at the pressure of the gas charged in the casing100C in a standard operation state of the Stirling engine100. Incidentally, the standard operation state of the Stirling engine100refers to, for example, a state where the Stirling engine100is producing the output that is determined from the specifications of the Stirling engine100.

FIG. 4is a schematic diagram showing an example of a construction in which a Stirling engine in accordance with the embodiment is employed for the recovery of exhaust heat from an internal combustion engine. In this embodiment, the output of the Stirling engine100is input to an internal combustion engine transmission4via a Stirling engine transmission5, and is therefore combined with the output of the internal combustion engine1, and the combined power is taken out.

In this embodiment, the internal combustion engine1is mounted in, for example, a vehicle such as a passenger car, a truck or the like, to serve as a motive power source of the vehicle. The internal combustion engine1produces output as a main motive power source during run of the vehicle. On the other hand, the Stirling engine100is not able to provide a minimum necessary output until the temperature of exhaust gas EX reaches a certain level of temperature. Therefore, in this embodiment, after the temperature of the exhaust gas EX discharged by the internal combustion engine1exceeds a predetermined temperature, the Stirling engine100recovers thermal energy from the exhaust gas EX of the internal combustion engine1and produces output so as to drive the vehicle in cooperation with the internal combustion engine1. In this manner, the Stirling engine100serves as a subsidiary motive power source of the vehicle.

The heater105of the Stirling engine100is disposed in an exhaust passageway2of the internal combustion engine1. Incidentally, in the exhaust passageway2, the regenerator (seeFIG. 1)106of the Stirling engine100may also be disposed. The heater105of the Stirling engine100is provided in a hollow heater case3that is provided on the exhaust passageway2.

In this embodiment, the thermal energy of exhaust gas EX recovered through the use of the Stirling engine100is converted into kinetic energy by the Stirling engine100. A clutch6that is a power connection-disconnection device is attached to the crankshaft110, which is an output shaft of the Stirling engine100. Thus, the output of the Stirling engine100is transmitted to the Stirling engine transmission5via the clutch6.

The output of the internal combustion engine1is input to the internal combustion engine transmission4via an output shaft1sof the internal combustion engine1. Then, the internal combustion engine transmission4combines the output of the internal combustion engine1and the output of the Stirling engine100input thereto via the Stirling engine transmission5, and outputs the combined power to a transmission output shaft9. The clutch6, which is the power connection-disconnection device, is provided between the internal combustion engine transmission4and the Stirling engine100. In this embodiment, the clutch6is provided between an input shaft5sof the Stirling engine transmission5and the crankshaft110of the Stirling engine100. The clutch6is engaged and disengaged to establish and remove the mechanical connection between the crankshaft110of the Stirling engine100and the input shaft5sof the Stirling engine transmission5. Incidentally, the clutch6is controlled by an engine ECU50.

The Stirling engine100recovers thermal energy of the exhaust gas EX discharged by the internal combustion engine1. Incidentally, in the case where the temperature of the exhaust gas EX is low, for example, at the time of cold start of the internal combustion engine1, or the like, thermal energy cannot be recovered from the exhaust gas EX, and therefore the Stirling engine100does not produce output. Therefore, until it becomes possible for the Stirling engine100to produce output, the clutch6is disengaged to disconnect the Stirling engine100and the internal combustion engine1from each other. Thus, the energy loss due to the Stirling engine100being driven by the internal combustion engine1is restrained.

When the clutch6is engaged, the crankshaft110of the Stirling engine100and the output shaft1sof the internal combustion engine1are directly linked via the Stirling engine transmission5and the internal combustion engine transmission4. As a result of this, the output produced by the Stirling engine100and the output produced by the internal combustion engine1are combined by the internal combustion engine transmission4, and the combined power is taken out via the transmission output shaft9. On the other hand, when the clutch6is disengaged, the output shaft1sof the internal combustion engine1rotates disconnected from the crankshaft110of the Stirling engine100. Next, the construction of the pressure control device30will be described.

FIG. 5is an illustrative diagram showing a pressure control device in accordance with the embodiment. As shown inFIG. 5, the pressure control device30in accordance with the embodiment is incorporated into the engine ECU50. The engine ECU50is constructed of a CPU (Central Processing Unit)50p, a memory portion50m, an input port55, an output port56, an input interface57, and an output interface58.

Incidentally, a pressure control device30in accordance with the embodiment may instead be provided separately from the engine ECU50, and may be connected to the engine ECU50. Then, in order to realize the pressure control of the gas charged in the casing100C of the Stirling engine100in accordance with the embodiment, it is possible to provide a construction in which the control functions the engine ECU50has for the Stirling engine100and the like are allowed to be used by the pressure control device30.

The pressure control device30includes a pressure determination portion31, a control condition determination portion32, and a pressure control portion33. These portions form portions that execute operation controls in accordance with the embodiment. In the embodiment, the pressure control device30is constructed as a portion of the CPU50pthat constitutes the engine ECU50. Besides, the CPU50pis provided with an engine control portion50h, whereby the operation of the internal combustion engine1and the Stirling engine100is controlled.

The CPU50p, the memory portion50m, the input port55and the output port56are interconnected via buses541to543. Therefore, the pressure determination portion31, the control condition determination portion32and the pressure control portion33that constitute the pressure control device30can exchange control data with each other, and can output a command to an appropriate one of these portions. Besides, the pressure control device30can acquire operation control data that the engine ECU50has regarding the internal combustion engine1, the Stirling engine100, etc., and can use the data. Besides, the pressure control device30can interrupt an operation control routine set beforehand in the engine ECU50with the operation control in accordance with the embodiment.

The input interface57is connected to the input port55. Sensors and the like necessary for the control of maintaining a predetermined pressure of the gas charged in the casing100C of the Stirling engine100are connected to the input interface57. In this embodiment, these sensors and the like include the pressure sensor40, and the temperature sensor41. In addition, the sensors and the like connected to the input interface57also include sensors and the like provided for acquiring information necessary for the operation control of the internal combustion engine1and the Stirling engine100, and the control of the internal combustion engine transmission4and the Stirling engine transmission5.

The signals from these sensors and the like are converted by an A/D converter57aand a digital input buffer57din the input interface57into signals usable by the CPU50p, which are sent to the input port55. Therefore, the CPU50pcan acquire information necessary for the operation control of the internal combustion engine1and the pressure control of the gas charged in the casing100C.

The output interface58is connected to the output port56. Control objects necessary for the control of maintaining a predetermined pressure of the gas charged in the casing100C of the Stirling engine100are connected to the output interface58. In this embodiment, these control objects include the pump115. Other control objects connected to the output interface58are control objects (e.g., the clutch6) necessary for the operation control of the internal combustion engine1and the Stirling engine100, and the control of the internal combustion engine transmission4and the Stirling engine transmission5.

The output interface58has control circuits581,582, and the like, causes the control objects to operate on the basis of the control signal generated through the computation performed by the CPU50p. Due to the construction as described above, on the basis of the output signals of the foregoing sensors and the like, the CPU50pof the engine ECU50can control the pump115and the clutch6as well as the Stirling engine100, the internal combustion engine1, etc.

The memory portion50mstores computer programs, including a processing procedure of the pressure control in accordance with the embodiment, as well as data maps and the like. Incidentally, the memory portion50mmay be constructed of a volatile memory, such as a RAM (Random Access Memory), a non-volatile memory, such as a flash memory or the like, or a combination of such memories.

The computer programs may be programs that realize a processing procedure of the pressure control in accordance with the embodiment by combining with a computer program recorded beforehand in the CPU50p. Besides, the pressure control device30may also realize the functions of the pressure determination portion31, the control condition determination portion32and the pressure control portion33by using dedicated hardware devices or the like instead of the computer programs. Next, the pressure control in accordance with the embodiment will be described. The pressure control in accordance with the embodiment can be realized by the pressure control device30. The next description will be best understood with appropriate reference toFIGS. 1 to 5.

FIG. 6is a flowchart showing a procedure of the pressure control in accordance with the embodiment.FIG. 7is a conceptual diagram showing a pressure target value determination map for use in the pressure control in accordance with the embodiment. The pressure control in accordance with the embodiment described below is executed before the Stirling engine100is started. However, the pressure control may also be executed during operation of the Stirling engine100. If the pressure control is executed before the Stirling engine100is started, a pre-established output can be secured immediately after the Stirling engine100is started.

In order to execute the pressure control in accordance with the embodiment, the pressure determination portion31of the pressure control device30, in step S101, acquires the temperature T of the gas charged in the casing100C of the Stirling engine100(hereinafter, termed the gas actual temperature) from the temperature sensor41shown inFIGS. 1 and 4.

In step S102, the pressure determination portion31determines a pressure target value Pc. As described above, the pressure target value is the pressure of the gas charged in the casing100C during the standard operation state. To determine the pressure target value, the pressure determination portion31gives the gas actual temperature T acquired in step S101to a pressure target value determination map45shown inFIG. 7, and thus acquires a corresponding pressure target value Pc. For example, if the actual pressure of the gas charged in the casing100C is a pressure target value Pm in the case where the temperature of the gas charged in the casing100C is Tm, the Stirling engine100can produce a pre-established output. Incidentally, the pressure target value determination map45is stored in the memory portion50mof the engine ECU50.

In the pressure target value determination map45, combinations of the pressure target value Pc and the temperature T of the gas charged in the casing100C during the standard operation state are described in accordance with a plurality of conditions. In this embodiment, for example, the temperature T is described as T1<T2< . . . <Tm< . . . <Tn, and the pressure target value Pc is described as Pc1>Pc2> . . . >Pcm> . . . >Pcn. That is, the greater the temperature T, the smaller the pressure target value Pc is set. Incidentally, the temperature T and the pressure target value Pc of the gas charged in the casing100C during the standard operation state are not limited to the setting provided in the pressure target value determination map45. Besides, since the temperature T and the pressure target value Pc are discretely described, a temperature T that is not described in the pressure target value determination map45requires, for example, linear interpolation, in order to determine a corresponding pressure target value Pc.

By determining the pressure target value Pc through the use of the temperature of the gas charged in the casing100C in this manner, the pressure P of the gas charged in the casing100C can be controlled with higher accuracy. As a result, insufficient pressurization can be restrained, and therefore the decline in the output of the Stirling engine100can be more reliably restrained. Besides, since excessive pressurization can also be restrained, the unnecessary driving of the pump115can be avoided to restrain the energy consumption.

The pressure P of the gas charged in the casing100C may also be controlled on the basis of the ratio between the pressure P and the temperature T of the gas in the casing100C (termed the pressure/temperature ratio) P/T. For example, if a pressure target value Pc_p is targeted at a temperature Tc_p, the ratio P/T is Pc_p/Tc_p=A (constant). This constant A is set beforehand on the basis of the ratio between the pressure P and the temperature T of the gas in the casing100C, and is an index representing a targeted pressure of the pressure P of the gas charged in the casing100C. Here, the temperature T may be expressed as an absolute temperature. Hereinafter, the constant A will be termed the pressure target index.

In the case where the pressure P of the gas charged in the casing100C is controlled through the use of the pressure/temperature ratio P/T, the pressure determination portion31finds the pressure/temperature ratio P/T at the present time point from the pressure P and the temperature T of the gas charged in the casing100C which are acquired from the pressure sensor40and the temperature sensor41. Then, the pressure control portion33of the pressure control device30controls the pressure P of the gas charged in the casing100C so that the ratio P/T at the present time point becomes greater than or equal to the pressure target index A. Therefore, the pressure P of the gas charged in the casing100C can be maintained at or above the foregoing pressure target value Pc.

Besides, the pressure P of the gas charged in the casing100C may also be controlled on the basis of the ratio between the temperature T and the pressure P of the gas in the casing100C (termed the temperature/pressure ratio) T/P. For example, if a pressure Pc_p is targeted at a temperature Tc_p, the ratio T/P is Tc_p/Pc_p=B (constant). This constant B is set beforehand on the basis of the ratio between the pressure P and the temperature T of the gas in the casing100C, and is an index representing a targeted pressure of the pressure P of the gas charged in the casing100C. Hereinafter, the constant B will be termed the pressure target index.

In the case where the pressure of the gas in the casing100C is controlled through the use of the temperature/pressure ratio T/P, the pressure determination portion31finds the temperature/pressure ratio T/P at the present time point from the pressure P and the temperature T of the gas charged in the casing100C which are acquired from the pressure sensor40and the temperature sensor41. Then, the pressure control portion33controls the pressure P of the gas in the casing100C so that the ratio T/P at the present time point becomes less than or equal to the pressure target index B. Therefore, the pressure P of the gas in the casing100C can be maintained at or above the foregoing pressure target value Pc.

Thus, in this embodiment, the pressure P of the gas charged in the casing100C can also be controlled on the basis of the pressure/temperature ratio P/T or the temperature/pressure ratio T/P and on the basis of the pre-set pressure target index. This manner of control eliminates the need to use the pressure target value determination map45, and therefore curbs the use of the memory portion50mprovided in the engine ECU50. Besides, the time and trouble taken to create the pressure target value determination map45can also be lessened.

After the in-casing gas pressure during the standard operation state is determined, the control condition determination portion32of the pressure control device30, in step S103, acquires the pressure P of the gas charged in the casing100C of the Stirling engine100(termed the gas actual pressure) P from the pressure sensor40shown inFIGS. 1 and 4. Incidentally, the control condition determination portion32can be regarded as a determination device in the invention. In step S104, the control condition determination portion32compares the gas actual pressure P acquired in step S103with the pressure target value Pc determined in step S102.

If the answer to the determination in step S104is “YES”, that is, if the control condition determination portion32determines P<Pc, the Stirling engine100cannot produce the pre-established output. Therefore, in step S105, the pressure control portion33of the pressure control device30drives the pump115shown inFIGS. 1 and 4to pressurize the gas charged in the casing100C of the Stirling engine100.

In the case where the gas charged in the casing100C is pressurized, the pressurization by the pump115is continued until the pressure of the gas charged in the casing100C, which is detected by, for example, the pressure sensor40, reaches the pressure target value Pc. The pressurization by the pump115may also be performed on the basis of the necessary amount of pressurization calculated from a difference between the pressure target value Pc and the pressure P of the gas charged in the casing100C at the present time point.

In the case where the pressure P of the gas charged in the casing100C is controlled on the basis of the pressure/temperature ratio P/T and the pre-set pressure target index A, the gas is pressurized by the pump115until the pressure/temperature ratio P/T becomes equal to or greater than the pressure target index. In the case where the pressure P of the gas charged in the casing100C is controlled on the basis of the temperature/pressure ratio T/P and the pre-set pressure target index B, the gas is pressurized by the pump115until the pressure/temperature ratio P/T becomes equal to or less than the pressure target index. Besides, the pressurization by the pump115may also be performed on the basis of the necessary amount of pressurization calculated from a difference between the pressure/temperature ratio P/T and the pressure target index A or from a difference between the temperature/pressure ratio T/P and the pressure target index B.

If the answer to the determination in step S104is “NO”, that is, if the control condition determination portion32determines P≧Pc, the Stirling engine100can produce the pre-established output, and therefore, the pressure control portion33does not pressurize the gas charged in the casing100C of the Stirling engine100. The engine control portion50hof the engine ECU50starts the Stirling engine100for operation. Incidentally, if the state of P<Pc occurs during the operation of the Stirling engine100, the gas charged in the casing100C of the Stirling engine100may be pressurized.

As described above, according to the embodiment, in the Stirling engine in which the gas charged within the casing of the Stirling engine is pressurized beforehand, it is determined whether or not the pressure of the gas charged in the casing has declined with reference to the pressure target value of the gas. If the pressure of the gas is lower than the pressure target value, the gas is pressurized so as to compensate for the decline in the pressure of the gas with reference to the pressure target value. Therefore, the decline in the output of the Stirling engine caused by a decline in the pressure of the gas charged in the casing can be restrained.

As described above, the Stirling engine in accordance with the invention is useful as a Stirling engine in which the gas charged in the casing is pressurized beforehand, and is particularly suitable to restrain the decline in the output caused by a decline in the pressure of the gas charged in the casing.

While the invention has been described with reference to example embodiments thereof, it is to be understood that the invention is not limited to the described embodiments or constructions. On the other hand, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the disclosed invention are shown in various example combinations and configurations, other combinations and configurations, including more, less or only a single element, are also within the scope of the appended claims.