Patent Publication Number: US-2012042645-A1

Title: Control device for stirling engine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Japanese Patent Application No. 2010-185605 filed on Aug. 20, 2010, which is incorporated herein by reference in its entirety including the specification, drawings and abstract. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a control device for a Stirling engine and, more particularly, to a Stirling engine control device that is provided for a Stirling engine constructed so as to obtain a decompression effect of reducing the degree of compression of the working fluid. 
     2. Description of Related Art 
     In recent years, Stirling engines are drawing attention as a measure to recover exhaust heat from an internal combustion engine mounted in a vehicle, such as a passenger car, a bus, a truck, etc., or exhaust heat from a factory. The Stirling engine can be expected to achieve high heat efficiency. Furthermore, since the Stirling engine is an external combustion engine in which working fluid is heated from outside, the Stirling engine also has an advantage of being able to utilize practically any heat source available, that is, being able to utilize varieties of low-temperature-difference alternative energy forms, such as solar heat, terrestrial heat, exhaust heat, etc., and of contributing to energy conservation. A technology that is considered to be relevant to this invention in that, to start a Stirling engine, the working fluid is discharged to the outside of the Stirling engine so as to obtain the decompression effect is disposed in, for example, Japanese Patent Application Publication No. 2009-127476 (JP-A-2009-127476). Furthermore, Japanese Patent Application Publication No. 05-38956 (JP-A-05-38956) is considered to be relevant to the invention in that a technology related to the starting of a Stirling engine. Besides, Japanese Patent Application Publication No. 2005-299594 (JP-A-2005-299594) is considered to be relevant to the invention in that a technology related to the decompression effect is disclosed. 
     In the technology disclosed in Japanese Patent Application Publication No. 2009-127476 (JP-A-2009-127476), when the self-sustaining operation of the Stirling engine has started, the discharge of the working fluid is stopped. Therefore, in this technology, at the time of start of the self-sustaining operation at which the rotational speed of the Stirling engine has risen, the working fluid is suddenly retained in a working space as the decompression effect discontinues. Consequently, it is considered that this technology may sometimes bring about occurrence of a torque fluctuation that exceeds a permissible torque fluctuation. Concretely, as shown in  FIG. 6 , for example, while the permissible torque fluctuation is a torque fluctuation between a maximum torque T max  and a minimum torque T min  with an average torque T m  being the middle point therebetween, a torque fluctuation in which the torque fluctuates to a torque T′ max  that exceeds the maximum torque T max  occurs at the time of an instantaneous stop of the decompression effect. 
     SUMMARY OF THE INVENTION 
     The invention provides a Stirling engine control device capable of preventing or restraining a torque fluctuation greater than a permissible torque fluctuation from occurring in association with discontinuation of the decompression effect. 
     A control device for a Stirling engine in accordance with a first aspect of the invention includes: two cylinder units; and a decompression portion that brings about a decompression effect of reducing a degree of compression of a working fluid that flows back and forth between the two cylinder units, by letting out the working fluid that flow back and forth between the two cylinder units, when the Stirling engine is started; the control device including a control portion that controls the decompression portion so that the decompression effect is gradually weakened after the Stirling engine is started. 
     A control device for a Stirling engine in accordance with a second aspect of the invention includes: two cylinder units made up of a high-temperature cylinder unit that includes a high-temperature cylinder and a high-temperature piston that is gas-lubricated with respect to the high-temperature cylinder, and a low-temperature cylinder unit that includes a low-temperature cylinder and a low-temperature piston that is gas-lubricated with respect to the low-temperature cylinder; a crankcase provided with a crankshaft that converts reciprocating motion of the high-temperature piston and the low-temperature piston into rotational motion; a communication portion that communicates the interior of the crankcase with a working space in which a working fluid that flows back and forth between the two cylinder units is present; and a flow amount adjustment valve that is interposed in the communication portion; the control device including a control portion that executes a control to open the flow amount adjustment valve when the Stirling engine is being started, and to gradually close the flow amount adjustment valve after the Stirling engine is started. 
     According to the foregoing aspects of the invention, it is possible to prevent or restrain a torque fluctuation greater than a permissible torque fluctuation from occurring in association with discontinuation of the decompression effect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is a diagram showing an ECU  80  that is a control device in accordance with an embodiment of the invention, together with a Stirling engine; 
         FIG. 2  is a diagram showing in a flowchart an operation of the ECU in accordance with the embodiment of the invention; 
         FIG. 3  is a diagram showing torque fluctuation in accordance with the embodiment of the invention; 
         FIG. 4  is a diagram showing portions of a Stirling engine in accordance with a modified embodiment of the invention; 
         FIG. 5  is a diagram showing in a flowchart an operation of an ECU to control the Stirling engine in accordance with the modification of the invention; and 
         FIG. 6  is a diagram showing torque fluctuation in association with instantaneous discontinuation of the decompression effect according to the related art. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the invention will be described in detail with reference to the drawings. 
       FIG. 1  is a diagram showing an ECU  80 , which serve as the control device for a Stirling engine in accordance with an embodiment of the invention, together with a Stirling engine  10 . The Stirling engine  10  is a two-cylinder α-type Stirling engine. The Stirling engine  10  includes a pair of cylinder units, specifically, a high-temperature cylinder unit  20  and a low-temperature cylinder unit  30 , which are disposed in an in-line parallel arrangement such that a cylinder arrangement direction X of the cylinders is parallel to an extending direction of a crankshaft line CL. The high-temperature cylinder unit  20  has an expansion piston  21  that serves as a high-temperature piston, and a high-temperature cylinder  22 . The low-temperature cylinder unit  30  has a compression piston  31  that serves as a low-temperature piston, and a low-temperature cylinder  32 . The reciprocating phase of the compression piston  31 , which reciprocates within the low-temperature cylinder  32 , differs from that of the expansion piston  21 , which reciprocates within the high-temperature cylinder  22 , so that the compression piston  31  lags behind the expansion piston  21  by about 90° in crank angle. The reciprocating motion of the pistons  21  and  31  are transmitted via connecting rods  110  to a crankshaft  113  that is provided within the crank case  120 , and is thereby converted into rotational motion. 
     An upper space in the high-temperature cylinder  22  is an expansion space. A working fluid heated by a heater  47  flows into the expansion space. Concretely, in this embodiment, the heater  47  is disposed within an exhaust pipe  100  of a gasoline engine that is mounted in a vehicle. In connection with this respect, the Stirling engine  10  is disposed so that the extending direction of the crankshaft line CL (i.e., the cylinder arrangement direction X) is parallel to a flowing direction V 1  of exhaust gas. In the heater  47 , the working fluid is heated by thermal energy that is recovered from exhaust gas that is a fluid that constitutes a high-temperature heat source. An upper space in the low-temperature cylinder  32  is a compression space. The working fluid cooled by a cooler  45  flows into the compression space. A regenerator  46  exchanges heat with the working fluid that moves back and forth between the expansion space and the compression space, which are working spaces. Specifically, the regenerator  46  absorbs heat from the working fluid when the working fluid flows from the expansion space to the compression space, and releases stored heat to the working fluid when the working fluid flows from the compression space to the expansion space. The working fluid used in this embodiment is air. However, other gasses may also be used as the working fluid, such as He, H 2 , N 2 , etc. 
     Next, the operation of the Stirling engine  10  will be described. As the working fluid is heated by the heater  47 , the working fluid expands to push the expansion piston  21  down, whereby the crankshaft  113  is pivoted. Next, as the expansion piston  21  transitions into an ascending stroke, the working fluid is moved into the regenerator  46  through the heater  47 . The working fluid releases heat to the regenerator  46 , and then flows out into the cooler  45 . The working fluid cooled by the cooler  45  flows into the compression space, and then is compressed as the compression piston  31  ascends. The thus compressed working fluid now flows into the heater  47  through the generator  46  while absorbing heat from the regenerator  46 , so that the temperature of the working fluid rises. In the heater  47 , the working fluid is heated and expands again. That is, through the reciprocating flowage of the working fluid in this manner, the Stirling engine  10  is operated. 
     Accordingly, in this embodiment, because the heat source of the Stirling engine  10  is exhaust gas from the internal combustion engine of the vehicle, the obtainable amount of heat is restricted, and therefore the Stirling engine  10  needs to be operated within the obtainable amount of heat. In the embodiment, therefore, the internal friction of the Stirling engine  10  is reduced as much as possible. Specifically, in order to minimize the frictional losses caused by a piston ring which is the greatest of the losses caused by the internal frictions of the Stirling engines, gas lubrication is adopted between the cylinders  22  and  32  and the pistons  21  and  31 , respectively. 
     In the gas lubrication, the pistons  21  and  31  are floated in air by utilizing the pressure (distribution) of air that occurs in small clearances between the cylinders  22  and  32  and the pistons  21  and  31 . Since the gas lubrication in which an object is floated in air causes only a very small sliding resistance, the internal friction of the Stirling engine  10  can be considerably reduced. For the gas lubrication in which an object is floated in air, it is possible to apply, for example, a hydrostatic gas lubrication in which a pressurized fluid is jetted and the thus produced hydrostatic pressure is used to float the object. However, this is not restrictive, but the gas lubrication may also be, for example, a hydrodynamic gas lubrication. 
     The clearance between the cylinders  22  and  32  and the pistons  21  and  31  in the gas lubrication has a size of several ten micrometers. In the clearance, there exists the working fluid of the Stirling engine  10 . Due to the gas lubrication, the pistons  21  and  31  are supported in a state of non-contact with the cylinders  22  and  32 , respectively, or in a state of allowable contact with the cylinders  22  and  32 . Therefore, a piston ring is not provided around either of the pistons  21  and  31 , and the lubricating oil that is used together with a piston ring in a common lubrication method is not used. In the gas lubrication, the air-tightness of the expansion space and of the compression space is maintained by the small clearances, and the clearance seal is established in a ring-less and oil-less manner. 
     In the Stirling engine  10 , the interior of the crankcase  120  may be pressurized in order to increase output. In order to pressurize the interior of the crankcase  120 , the Stirling engine  10  further includes a pressurizing pump  61 , a pressurization-purpose piping  62  and a pressurization-purpose open-close valve  63 . The pressurizing pump  61  serves as pressurization means for pressurizing the inside of the crankcase  120 , and the pressurization-purpose piping  62  serves as connection means for connecting the pressurizing pump  61  and the crankcase  120 . The pressurization-purpose open-close valve  63  is provided in an intermediate portion of the pressurization-purpose piping  62 , and serves as switch means for switching between the permission and the prohibition of the pressurization of the inside of the crankcase  120 . 
     In the Stirling engine  10 , when the interior of the crankcase  120  is pressurized, the average pressure of the working fluid present in the expansion space and in the compression space gradually becomes equalized with the average pressure the average pressure of the working fluid present in the crankcase  120  due to the small clearances formed between the pistons  21  and  31  and the cylinders  22  and  32 . Therefore, in the Stirling engine  10 , the interior of the crankcase  120  is pressurized to make the pressure of the working fluid high so that the working fluid is sufficiently pressurized. 
     Besides, the Stirling engine  10  is constructed so that a decompression effect is obtained, in order to reduce the engine-starting torque. In order to bring about the decompression effect, the Stirling engine  10  further includes a decompression valve  71  and a bypass pipe  72 . The decompression valve  71  is provided in an intermediate portion of the bypass pipe  72 , and serves as decompression means for bringing about the decompression effect of reducing the degree of compression of the working fluid that flows back and forth between the cylinder units  20  and  30  by letting out the working fluid that flow back and forth between the cylinder units  20  and  30 . Specifically, the decompression valve  71  is constructed so as to allow the working fluid that flows back and forth between the cylinder units  20  and  30  to move back and forth between the working space and the inside of the crankcase  120 . 
     The bypass pipe  72  is communication means for providing communication between the inside of the crankcase  120  and the working space in which the working fluid that flows back and forth between the cylinder units  20  and  30  is present (i.e., a space made up of the compression space, the cooler  45 , the regenerator  46 , the heater  47  and the expansion space). Specifically, in this embodiment, the bypass pipe  72  provides communication between the expansion space and the interior of the crankcase  120 . Therefore, more concretely, the decompression valve  71  allows the working fluid that flows back and forth between the cylinder units  20  and  30  to move back and forth between the expansion space and the inside of the crankcase  120 . The decompression valve  71  provided in the bypass pipe  72 , besides being able to provide communication between the inside of the crankcase  120  and the working space (the expansion space in this example) in which the working fluid that flows back and forth between the cylinder units  20  and  30  is present, also serves as change means capable of changing the state of communication when communication is provided between the working space and the inside of the crank case  120 . Therefore, the decompression effect can be weakened by the change means. In conjunction with this respect, the decompression valve  71  may be a flow amount adjustment valve capable of adjusting the degree of opening of the valve, and concretely in this embodiment, a butterfly valve is used as the decompression valve  71 . 
     The Stirling engine  10  includes the ECU  80 . The ECU  80  includes a microcomputer made up of a CPU, a ROM, a RAM, etc., and also includes an input/output circuit. The ECU  80  is electrically connected to various sensors/switches and the like, for example, a rotational speed N SE  detection sensor  91  for detecting the rotational speed N SE  of the Stirling engine  10 , a pressure sensor  92  for detecting the pressure inside the crankcase  120 , an exhaust gas temperature sensor  93  for detecting the exhaust gas temperature T in  immediately prior to heat exchange of the exhaust gas with the heater  47 , etc. In addition, the ECU  80  is also electrically connected to various control objects, such as the pressurizing pump  61 , the pressurization-purpose open-close valve  63 , the decompression valve  71 , etc. 
     The programs in which various processes that the CPU executes are described and also for storing map data, etc are store in ROM. In the ECU  80 , various control means, determination means, detection means, etc., may be implemented through processes executed by the CPU while utilizing a temporary storage area in the RAM according to need based on the programs stored in the ROM. 
     For example, in the ECU  80 , the control means for controlling the decompression valve  71  so as to bring about the decompression effect when the engine is started. For bringing about the decompression effect, the control means is realized, concretely, so as to control the decompression valve  71  so that the valve  71  opens and, more concretely, so as to control the decompression valve  71  so that the degree of opening thereof becomes a fully open degree. Besides, for discontinuing the decompression effect, the control means is realized so as to control the decompression valve  71  so that after the engine is started, the decompression effect is gradually weakened, that is, the valve  71  is gradually closed. Specifically, the control means is realized so as to control the decompression valve  71  so that the decompression effect is gradually weakened, in the case where the rotational speed N SE  of the Stirling engine  10  has reached a starting rotational speed (i.e. a minimum rotational speed that is needed for the self-sustaining operation of the Stirling engine  10 ). 
     Besides, for gradually weakening the decompression effect, the control means is, concretely, realized so as to firstly control the decompression valve  71  so that the degree of opening of the decompression valve  71  reaches an intermediate degree of opening, and then control the decompression valve  71  so that the decompression valve  71  changes gradually from the state of the intermediate degree of opening to a fully closed state. Concretely, the intermediate degree of opening is a degree of opening at which the decompression effect may be maintained without the occurrence of torque fluctuations in the Stirling engine  10  in excess of the permissible torque fluctuation range once self-sustaining operation of the Stirling engine  10  has started. The torque fluctuation remains within a permissible torque fluctuation range, for example, at the time of a predetermination operation. The predetermination operation is, concretely, an operation that the Stirling engine  10  performs after starting the self-sustaining operation subsequent to the start of the engine. More concretely in this embodiment, the predetermined operation is a rated operation that produces a rated output that is rated as a guaranteed limit in use. The intermediate degree of opening of the decompression valve  71  is set at a degree of opening at which the aperture of the decompression valve  71  exceeds the aperture provided by the small clearances formed between the pistons  21  and  31  and the cylinders  22  and  32 . 
     Furthermore, for gradually weakening and discontinuing the decompression effect, the control means is realized so as to control the decompression valve  71  so that the decompression valve  71  reaches a fully closed state after a predetermined time t 1  has elapsed after the control means begins controlling the decompression valve  71  so as to gradually weakening the decompression effect (concretely, in this example, after the control means has controlled the decompression valve  71  so that the degree of opening of the decompression valve  71  reaches the intermediate degree of opening). In conjunction with this respect, the predetermined time t 1  is pre-set at a length of time that is needed in order to prevent the torque fluctuation of the Stirling engine  10  from exceeding the permissible torque fluctuation when the decompression effect is to be gradually weakened to the discontinuation of the decompression effect. Then, in order to prevent the torque fluctuation of the Stirling engine  10  from exceeding the permissible torque fluctuation, the predetermined time t 1  is pre-set according to the rotational speed N SE  of the Stirling engine  10 . In conjunction with this respect, in setting the intermediate degree of opening of the decompression valve  71  at a degree of opening at which the decompression effect can be maintained without allowing the torque fluctuation to exceed the permissible torque fluctuation, the intermediate degree of opening can also be pre-set according to the rotational speed N SE  of the Stirling engine  10 . 
     Next, an operation executed by the ECU  80  will be described with reference to a flowchart shown in  FIG. 2 . The ECU  80  determines whether it is possible to start the Stirling engine  10  (step S 1 ). Whether it is possible to start the Stirling engine  10  may be determined, for example, by determining whether the exhaust gas temperature T in  is higher than a predetermined temperature that is pre-set as a temperature that allows the self-sustaining operation of the Stirling engine  10 . The self-sustaining operation of the Stirling engine  10  becomes feasible when the state of the working fluid receiving heat at the heater  47  is such that the Stirling engine  10  is able to produce output by overcoming the internal friction thereof and the inertia mass of the drive system. If a negative determination is made in step S 1 , it means that the Stirling engine  10  cannot be started, and the process of the flowchart is temporarily ended. 
     On the other hand, if an affirmative determination is made in step S 1 , it means that the Stirling engine  10  can be started. In this case, the ECU  80  determines whether the crankcase pressure needs to be increased by the pressurization (step S 2 ). Concretely, on the basis of the output of the pressure sensor  92 , the ECU  80  determines whether the crankcase pressure is smaller than a predetermined pressure. If the crankcase pressure is smaller than the predetermined pressure, the ECU  80  determines that pressurization is needed in order to increase the crankcase pressure. If an affirmative determination is made in step S 2 , the ECU  80  opens the pressurization-purpose open-close valve  63 , and turns on the pressurizing pump  61  (step S 3 ). Thus, the pressurization of the inside of the crankcase  120  is started. On the other hand, if a negative determination is made in step S 2 , the ECU  80  closes the pressurization-purpose open-close valve  63 , and turns off the pressurizing pump  61  (step S 4 ). 
     Subsequently, the ECU  80  fully opens the decompression valve  71  (step S 5 ). Then, the ECU  80  starts the Stirling engine  10  by an external start (step S 6 ). The Stirling engine  10  can be started by the external start, for example, by driving the crankshaft  113  through the use of power from the motive power source, such as an engine, an electric motor, etc., and thereby causing the pistons  21  and  31  to reciprocate. Since prior to step S 6 , the decompression valve  71  is fully opened in step S 5 , the decompression effect can be obtained when the Stirling engine  10  is started. 
     Subsequently to step S 6 , the ECU  80  increases the rotational speed N SE  of the Stirling engine  10  to the starting rotational speed (step S 7 ). Subsequently, the ECU  80  closes the decompression valve  71  so that the degree of opening of the decompression valve  71  reaches the intermediate degree of opening, and then gradually closes the decompression valve  71  to the fully closed state over the predetermined time t 1  (i.e., at a closing rate that is slower than the closing rate at which the degree of opening of the decompression valve  71  is reduced to the intermediate degree of opening) (step S 8 ). Subsequently, the ECU  80  determines whether the decompression valve  71  is fully closed (step S 9 ). If a negative determination is made in step S 9 , the process returns to step S 8 . However, if an affirmative determination is made in step S 9 , the ECU  80  then determines whether the Stirling engine  10  is in the self-sustaining operation (step S 10 ). It is possible to determine whether the Stirling engine  10  is in the self-sustaining operation, for example, on the basis of whether the rotational speed N SE  of the Stirling engine  10  has exceeded the starting rotational speed while the decompression valve  71  is in the fully closed state. If a negative determination is made in step S 10 , the process returns to step S 8 . However, if an affirmative determination is made in step S 10 , the ECU  80  ends the external start operation of the Stirling engine  10  (step S 11 ). 
     Next, operation and effect of the ECU  80  will be described. It is to be noted herein that the Stirling engine  10  is constructed so that at the time of starting the engine, the decompression effect is obtained by opening the decompression valve  71  to allow the working fluid to move between the working space and the interior of the crankcase  120 . Therefore, due to the decompression effect, the Stirling engine  10  is able to restrain the starting torque, so that the crankshaft  113  may be driven with a reduced power in order to start the Stirling engine  10 . In contrast, to discontinue the decompression effect, the ECU  80  controls the decompression valve  71  so as to gradually weaken the decompression effect after the Stirling engine  10  is started. Therefore, the ECU  80  is able to prevent or restrain the occurrence of a torque fluctuation that exceeds the permissible torque fluctuation. 
     To gradually weaken the decompression effect, the decompression valve  71  may also be controlled so that the open valve  71  is gradually closed. However, the ECU  80  first controls the decompression valve  71  so that the degree of opening of the decompression valve  71  decreases to an intermediate degree of opening, so that the decompression effect can be reduced in an earlier stage and to a greater extent, and therefore the output can be quickly increased. Besides, in this case, since the intermediate degree of opening of the decompression valve  71  is set at a degree of opening at which the decompression effect can be maintained without allowing the torque fluctuation to exceed the permissible torque fluctuation, the ECU  80  is able to prevent a torque fluctuation greater than the permissible torque fluctuation from occurring when the degree of opening of the decompression valve  71  is changed to the intermediate degree of opening, and is also able to cause the decompression effect to be more effectively realized by setting the intermediate degree of opening at a degree of opening at which the aperture of the decompression valve  71  exceeds the aperture provided by the small clearances that are formed between the pistons  21  and  31  and the cylinders  22  and  32 . Besides, since, to gradually weaken the decompression effect to the discontinuation, the ECU  80  gradually closes the decompression valve  71  to the full closure over the predetermined time t 1  after the ECU  80  has begun to weaken the decompression effect, it is possible to more certainly prevent a torque fluctuation greater than the permissible torque fluctuation from occurring when the decompression effect discontinues. 
     As a result of the ECU  80  preventing the torque fluctuation in this manner, the Stirling engine  10  has torque fluctuation as shown in  FIG. 3 . As shown in  FIG. 3 , as the degree of opening of the decompression valve  71  is changed to the intermediate degree of opening at a discontinuation start time T S  and the decompression valve  71  is gradually closed to the fully closed state over the predetermined time t 1  between the discontinuation start time T S  and a discontinuation end time T E , the torque fluctuations gradually increase within a permissible torque fluctuation (within a range between a maximum torque T max  and a minimum torque T min  with an average torque T m  being the middle point therebetween) starting at the time T S . At the time T E , the torque fluctuations no longer increase and remains within the permissible torque fluctuation range. That is, concretely, in conjunction with the discontinuation of the decompression effect, the ECU  80  is able to prevent the occurrence of a torque fluctuation that exceeds the permissible torque fluctuation. 
     The above embodiment is simply an example embodiment of the invention. The invention is not restricted to the particular of the above embodiment, but may be carried out with various modifications and the like without departing from the gist of the invention. For example, the above embodiment is preferable in that the decompression effect can be more effectively utilized in the gradual weakening of the decompression effect. Therefore, the embodiment is described above in conjunction with the case where when the rotational speed N SE  of the Stirling engine  10  reaches the starting rotational speed, the decompression valve  71  is controlled so that the degree of opening of the decompression valve  71  reaches the intermediate degree of opening, and then the decompression valve  71  is gradually closed from the intermediate degree of opening to the fully closed state over the predetermined time t 1 . However, the invention is not necessarily restricted to this. Instead, the control means may control the decompression means so that the decompression effect begins to be gradually weakened at an appropriate rotational speed of the Stirling engine  10  after the torque passes the first ridge of torque fluctuation that occurs following the start of the Stirling engine. 
     Besides, the embodiment is described above in conjunction with the case where the decompression means is the decompression valve  71 . However, the invention is not necessarily limited to this, but the decompression means may also include, for example, a plurality of valves as shown in  FIG. 4  (two valves in the example shown in  FIG. 4 ), that is, a main valve  75  and a secondary valve  76 .  FIG. 4  shows portions of a modification of the Stirling engine  10  of the embodiment in which the decompression valve  71  and the bypass pipe  72  are replaced with valves  75  and  76  and a multi-step bypass pipe  77 . The multi-step bypass pipe  77  is multi-step bypass means that has a multi-step construction of a plurality of pipes that form a bypass route in providing communication between the interior of a crankcase  120  and a working space (concretely, a cooler  45  and an expansion space). Specifically, the plurality of pipes include a main pipe  77   a  and a secondary pipe  77   b  in the example shown in  FIG. 4 . The main valve  75  and the secondary valve  76  are provided in an intermediate portion of the main pipe  77   a  and an intermediate portion of the secondary pipe  77   b , respectively. The main pipe  77   a  forms a larger flow path than the secondary pipe  77   b.    
     In this modification, the two valves  75  and  76  may be flow amount adjustment valves whose degree of opening can be adjusted. In such a case, the decompression effect can be discontinued, for example, as shown in  FIG. 5 . A flowchart shown in  FIG. 5  is substantially the same as the flowchart shown in  FIG. 3 , except that steps S 8  and S 9  are replaced with steps S 21  to S 24 . Besides, in this modification, the ECU  80  is electrically connected to the valves  75  and  76  as control objects instead of the decompression valve  71 , and control means is functionally realized so as to perform a control as shown in  FIG. 5  in the controlling of the valves  75  and  76 . 
     As shown in  FIG. 5 , next in step S 7 , the ECU  80  controls the main valve  75  so that the degree of opening of the main valve  75  reaches an intermediate degree of opening, and then gradually closes the main valve  75  from the intermediate degree of opening to a fully closed state (at a closing rate that is slower than the closing rate at which the degree of opening of the main valve  75  is reduced to the intermediate degree of opening) over a predetermined time t 2  (step S 21 ). With respect to the main valve  75 , the intermediate degree of opening and the predetermined time t 2  may be similarly pre-set to the intermediate degree of opening and the predetermined time t 1  regarding the decompression valve  71 . Next, the ECU  80  determines whether the main valve  75  is fully closed (step S 22 ). If the determination is negative, the process returns to step S 21 . If the determination is affirmative, the ECU  80  controls the secondary valve  76  so that the degree of opening of the secondary valve  76  reaches an intermediate degree of opening, and then gradually closes the secondary valve  76  from the intermediate degree of opening to the fully closed state (at a closing rate that is slower than the closing rate at which the degree of opening of the secondary valve  76  is reduced to the intermediate degree of opening) over a predetermined time t 3  (step S 23 ). With regard to the secondary valve  76 , the intermediate degree of opening and the predetermined time t 3  can be pre-set similarly to the intermediate degree of opening and the predetermined time t 1  regarding the decompression valve  71 . Next, the ECU  80  determines whether the secondary valve  76  is fully closed (step S 24 ). If the determination is negative, the process returns to step S 23 . If the determination is affirmative, the process proceeds to step S 10 . In this modification, too, the decompression effect is not instantaneously discontinued, but is gradually weakened to the discontinuation, so that it is possible to prevent torque fluctuations in excess of the permissible torque fluctuation from occurring in association with the discontinuation of the decompression effect. 
     Alternatively, instead of implementing the control means by the ECU  80  in the above embodiment may be implemented through hardware, such as an electronic control unit other than that described above, a dedicated electronic circuit, etc., or by any combination of such components.