Patent Publication Number: US-7716928-B2

Title: External combustion engine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an external combustion engine for converting the displacement of a liquid portion of a working fluid caused by a volume change of the working fluid due to the generation and liquefaction of the vapor of the working fluid into mechanical energy and then outputting the mechanical energy. 
     2. Description of the Related Art 
     One type of external combustion engine is known to have the configuration in which a working fluid in a liquid state is sealed in a pipe-like container, the vapor of the working fluid is generated by heating part of the working fluid in the container by a heater, the vapor of the working fluid is cooled and liquefied by a cooler to thereby change the volume of the whole working fluid, and the displacement of the liquid portion of the working fluid generated by the volume change of the working fluid is converted into mechanical energy and then outputted (See, for example, Japanese Unexamined Patent Publication No. 2005-330910). 
     In the above technique, the heater is arranged above the cooler, and when part of the working fluid is heated by the heater, the high-temperature high-pressure vapor of the working fluid is accumulated on the portion of the container where the heater is arranged, so that the liquid level of the working fluid is pushed down toward the cooler. As a result, the liquid portion of the working fluid is displaced downward in the container. 
     The vapor of the working fluid, advancing into the portion of the container where the cooler is arranged, is cooled and liquefied by the cooler. Therefore, the force to push down the liquid level of the working fluid is lost, and the liquid level of the working fluid rises into the heater, and the liquid portion of the working fluid is displaced upward. By repeating this operation, the liquid portion of the working fluid is moved and displaced periodically. In the process, the internal pressure of the container changes periodically. 
     Japanese Patent Application No. 2006-78802 (hereinafter referred to as the prior application) proposes an external combustion engine improved in output and efficiency. This prior application is intended to improve the output and efficiency of the external combustion engine by controlling the average value of the internal pressure of a container toward a target value. 
     More specifically, the working fluid in a liquid state is sealed in an auxiliary container separate from a main container sealed with the working fluid, the main container and the auxiliary container communicate with each other through a choke, and the working fluid in the auxiliary container is compressed or expanded by a piston mechanism thereby to control the internal pressure of the auxiliary container. 
     In this configuration, since the main container and the auxiliary container communicate with each other through the choke, the internal pressure of the auxiliary container does not change periodically with the internal pressure of the main container, and can be stabilized at a level substantially equal to the average value of the internal pressure of the main container. Thus, a target value of the internal pressure of the main container is calculated based on the temperature of a heater, etc., and the internal pressure of the auxiliary container is controlled toward the target value by a piston mechanism. As a result, the average value of the internal pressure of the main container near the target value can be obtained. 
     According to the prior application described above, in the case where the external combustion engine stops and the heater stops heating the working fluid, the temperature of the heater gradually drops to ambient temperature. As long as the vapor of the working fluid is accumulated in the main container when the external combustion engine stops; however, the saturated vapor pressure of the working fluid also drops with the heater temperature, resulting in condensation and liquefaction of the vapor of the working fluid. Thus, the internal pressure of the main container drops. 
     Once the internal pressure of the main container drops below the internal pressure of the auxiliary container, the working fluid in the auxiliary container gradually begins to flow into the main container through the choke, and the volume of the working fluid in the main container increases excessively. This phenomenon is more likely to occur in winter when the ambient temperature is low. 
     In the case where the external combustion engine is restarted and the working fluid is heated by the heater with an excessive volume of the working fluid in the main container as described above, part of the working fluid is gasified and the internal pressure of the main container rises. Once the internal pressure of the main container increases beyond the internal pressure of the auxiliary container, the excess working fluid in the main container is returned to the auxiliary container through the choke. 
     Since only a small amount of the working fluid can flow through the choke at a time, considerable time is required before all of the excess working fluid in the main container returns to the auxiliary container. As a result, a predetermined output cannot be produced, before all of the excess working fluid in the main container can return to the auxiliary container after the engine restarts, thereby posing the problem that the restarting time is lengthened before the predetermined output is obtained. 
     In order to avoid the above engine restart problem, the external combustion engine is required to be stopped at a when the vapor of the working fluid is not accumulated in the main container, thereby greatly complicating the operation to stop the external combustion engine. 
     SUMMARY OF THE INVENTION 
     In view of the points described above, the object of this invention is to provide an external combustion engine capable of producing a predetermined output quickly after engine start. 
     In order to achieve this object, according to a first aspect of the invention, there is provided an external combustion engine comprising: 
     a main container ( 11 ) containing a working fluid ( 12 ) adapted to flow in liquid state; 
     a heater ( 13 ) for heating part of working fluid ( 12 ) in main container ( 11 ) and generating the vapor of working fluid ( 12 ); 
     a cooler ( 14 ) for cooling and liquefying the vapor; 
     an output unit ( 1 ) for converting the displacement of the liquid portion of the working fluid ( 12 ) caused by the volume change of the working fluid ( 12 ) due to the generation and liquefaction of the vapor into mechanical energy and outputting the mechanical energy; and 
     auxiliary containers ( 16 ,  1   a ) communicating with the main container ( 11 ); 
     wherein the heater ( 13 ), the cooler ( 14 ) and the output unit ( 1 ) are arranged in that order in the direction of displacement of the working fluid ( 12 ); 
     wherein the auxiliary containers ( 16 ,  1   a ) containing the working fluid ( 12 ); 
     wherein the auxiliary containers ( 16 ,  1   a ) communicate with the portion of the main container ( 11 ) nearer the output unit ( 1 ) than the cooler ( 14 ); 
     the external combustion engine further comprising communication area adjusting means ( 17   a ,  15   b ,  25 ,  26 ,  30 ,  21 ,  32 ) for establishing the communication between the main container ( 11 ) and the auxiliary containers ( 16 ,  1   a ) through a first communication area in normal operation mode and through a second communication area larger than the first communication area in starting mode. 
     In this configuration, the excess working fluid ( 12 ) in the main container ( 11 ) can be quickly returned to the auxiliary containers ( 16 ,  1   a ) in starting mode, and therefore, a predetermined output can be produced quickly after the engine start. 
     The wording “starting mode” herein is defined as the time before a predetermined output is produced after the engine start. 
     According to a second aspect of the invention, there is provided an external combustion engine, wherein the communication area adjusting means includes a choke ( 17   a ,  15   b ) for establishing communication between the main container ( 11 ) and the auxiliary containers ( 16 ,  1   a ) in normal operation mode, and a path ( 25 ) larger in flow path area than the choke ( 17   a ,  15   b ) for establishing communication between the main container ( 11 ) and the auxiliary containers ( 16 ,  1   a ) in starting mode. 
     According to a third aspect of the invention, there is provided an external combustion engine, wherein the path ( 25 ) has a check valve ( 26 ) for allowing the working fluid ( 12 ) to flow from the main container ( 11 ) to the auxiliary containers ( 16 ,  1   a ) and blocking the reverse flow of the working fluid ( 12 ) from the auxiliary containers ( 16   a ,  1   a ) to the main container ( 11 ), and therefore, the working fluid ( 12 ) in the auxiliary containers ( 16 ,  1   a ) is prevented from flowing back into the main container ( 11 ) through the path ( 25 ) and the volume of the working fluid ( 12 ) in the main container ( 11 ) from increasing excessively in normal operation mode. 
     According to a fourth aspect of the invention, there is provided an external combustion engine, wherein the check valve ( 26 ) is a spring-type check valve including a spring portion ( 26   a ), wherein the spring constant of the spring portion ( 26   a ) is adapted to change with temperature and the working pressure (ΔP) of the check valve ( 26 ) changes with the spring constant, and wherein the spring portion ( 26   a ) is heated by a heating means ( 31 ) controlled by a control means reducing the working pressure (ΔP) in starting mode below the level thereof in normal operation mode. 
     In this configuration, the working fluid ( 12 ) in the auxiliary containers ( 16 ,  1   a ) is prevented from flowing into the main container ( 11 ) through the path ( 25 ) in normal operation mode while at the same time facilitating the engine starting operation by suppressing the working pressure of the check valve ( 26 ) to a low level in starting mode. 
     According to a fifth aspect of the invention, there is provided an external combustion engine, wherein the communication area adjusting means includes a valve ( 30 ) for opening/closing the path ( 25 ) and a control means ( 21 ) for controlling the operation of the valve ( 30 ) so as to be in a closed state in normal operation mode and to be in an open state in starting mode. 
     According to a sixth aspect of the invention, there is provided an external combustion engine, wherein the communication area adjusting means includes a variable choke mechanism ( 32 ) for establishing communication between the main container ( 11 ) and the auxiliary containers ( 16 ,  1   a ) and a control means ( 21 ) for controlling the variable choke mechanism ( 32 ) in such a manner as to increase the opening degree of the variable choke mechanism ( 32 ) in starting mode beyond the opening degree in normal operation mode. 
     According to a seventh aspect of the invention, there is provided an external combustion engine comprising a main container includes a first container ( 11 ), a second container ( 33 ) having the same configuration as the first container ( 11 ) and one auxiliary container ( 16 ) communicating with the first container ( 11 ) and the second container ( 33 ). 
     In this configuration, one auxiliary container ( 16 ) is shared by two external combustion engines, and therefore, the number of the auxiliary containers ( 16 ) can be reduced for a lower cost. 
     According to an eighth aspect of the invention, there is provided an external combustion engine, wherein the output unit ( 1 ) includes a casing ( 1   a ) containing the working fluid ( 12 ), and the casing ( 1   a ) makes up an auxiliary container. 
     In this configuration, the auxiliary container can be integrated with the output unit ( 1 ), and therefore, cost can be reduced. 
     According to a ninth aspect of the invention, there is provided an external combustion engine, wherein the output unit ( 1 ) includes a casing ( 1   a ) containing the working fluid ( 12 ), a cylinder ( 15   a ) for establishing communication between the casing ( 1   a ) and the main container ( 11 ) and a piston ( 15 ) supported slidably in the cylinder ( 15   a ) and driven by the displacement of the working fluid ( 12 ), wherein the casing ( 1   a ) makes up an auxiliary container and a minuscule clearance ( 15   b ) formed between the piston ( 15 ) and the cylinder ( 15   a ) makes up a choke. 
     In this configuration, the choke can be configured of the existing piston ( 15 ) and the cylinder ( 15   a ), and no separate choke is required for a lower cost. 
     The reference numerals inserted in the parentheses followed by the names of the respective means described in this column and the appending claims indicate the correspondence with the specific means included in the embodiments described later. 
     The present invention may be more fully understood from the description of the preferred embodiments of the invention, as set forth below, together with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a general configuration of a power generating system according to a first embodiment of the invention. 
         FIG. 2  is a diagram explaining the operation characteristics of an external combustion engine according to the first embodiment. 
         FIGS. 3A ,  3 B and  3 C are PV diagrams of the external combustion engine according to the first embodiment, in which  FIG. 3A  shows the ideal state,  FIG. 3B  the state in which the peak value of the internal pressure of the main container is lower than the saturated vapor pressure, and  FIG. 3C  the state in which the peak value of the internal pressure of the main container is higher than the saturated vapor pressure. 
         FIGS. 4A and 4B  are diagrams explaining the problems posed by the external combustion engine described in Japanese Unexamined Patent Publication No. 2005-330910, in which  FIG. 4A  shows the state in which the volume of the working fluid is reduced and  FIG. 4B  the state in which the volume of the working fluid is increased. 
         FIG. 5  is a graph showing the relationship between the volume of the working fluid and the efficiency of the external combustion engine. 
         FIG. 6  is a block diagram showing the outline of the control operation according to the first embodiment. 
         FIG. 7  is a graph showing the vapor pressure curve of the working fluid. 
         FIG. 8  is a diagram showing a general configuration of a power generating system according to a second embodiment of the invention. 
         FIG. 9  is a diagram showing a general configuration of a power generating system according to a third embodiment of the invention. 
         FIG. 10  is a diagram showing a general configuration of a power generating system according to a fourth embodiment of the invention. 
         FIG. 11  is a diagram showing a general configuration of a power generating system according to a fifth embodiment of the invention. 
         FIG. 12  is a diagram showing a general configuration of a power generating system according to a sixth embodiment of the invention. 
         FIG. 13  is a diagram showing a general configuration of a power generating system according to a seventh embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     A first embodiment is explained below with reference to  FIGS. 1 to 7 . In this embodiment, the external combustion engine  10  according to the invention is used for a power generating system.  FIG. 1  is a diagram showing a general configuration of the power generating system according to this embodiment. The basic configuration of this power generating system is similar to that of the prior application described above, and therefore, the configuration in common with the prior application is explained below first. 
     The external combustion engine  10  according to this embodiment drives a power generator  1  for generating the electromotive force by vibratory displacement of a movable member  2  with a permanent magnet buried therein, and includes a main container  11  sealed with a working fluid  12  adapted to flow in liquid state, a heater  13  for heating and gasifying the working fluid  12  in the main container  11  and a cooler  14  for cooling the vapor of the working fluid  12  heated and gasified by the heater  13 . According to this embodiment, water is used as the working fluid  12 , and a refrigerant may alternatively be used. 
     The heater  13  according to this embodiment exchanges heat with a high-temperature gas (such as an automotive exhaust gas) and may be configured of an electric heater. Also, the cooling water is circulated in the cooler  14  according to this embodiment. Though not shown, a radiator for radiating heat absorbed from the vapor of the working fluid  12  by the cooling water is arranged in the cooling water circulation circuit. 
     The portion of the main container  11  in contact with the heater  13 , i.e. a heated portion  11   a  and the portion of the main container  11  in contact with the cooler  14 , i.e. a cooled portion  11   b  are desirably formed of a material high in heat conductivity. According to this embodiment, the heated portion  11   a  and the cooled portion  11   b  are formed of copper or aluminum. Incidentally, the heated portion  11   a  may be formed integrally with the heater  13 , and so may the cooled portion  11   b  with the cooler  14 . 
     The intermediate portion  11   c  of the main container  11  between the heated portion  11   a  and the cooled portion  11   b , on the other hand, is desirably formed of a material high in thermal insulation. According to this embodiment in which water is used as the working fluid  12 , the intermediate portion  11   c  is formed of stainless steel. Similarly, the portion of the main container  11  nearer the power generator  1  than the cooled portion  11   b  is formed of stainless steel high in thermal insulation. 
     The main container  11  is a pressure vessel formed in a substantially U-shaped pipe including a bent portion  11   d  located at the bottom thereof and first and second linear portions  11   e ,  11   f  extending vertically. The heater  13  and the cooler  14  are arranged on the first linear portion  11   e  at one horizontal end (right side in  FIG. 1 ) of the main container  11  beyond the bent portion  11   d . The heater  13  is located above the cooler  14 . 
     Though not shown, in order to secure the space for gasifying the working fluid  12 , a gas (air, for example) of a predetermined volume is sealed at the upper end of the first linear portion  11   e.    
     On the other hand, a power generator  1  is arranged on the top of the second linear portion  11   f  of the main container  11  at the other horizontal end (left side in  FIG. 1 ) beyond the bent portion  11   d . In the casing  1   a  of the generator  1  has a piston  15  slidably arranged in a cylinder  15   a . The piston  15  is adapted to be displaced under the pressure applied from the liquid portion of the working fluid  12 . Incidentally, the power generator  1  corresponds to the output unit according to the invention. 
     The piston  15  is coupled to the shaft  2   a  of the movable member  2  in the casing  1   a  of the power generator  1 . A spring  3  constituting an elastic means for generating the elastic force to press the movable member  2  toward the piston  15  is arranged on the other side of the movable member  2  far from the piston  15 . 
     An auxiliary container  16  for adjusting the internal pressure Pc of the main container  11  (hereinafter referred to as the main container internal pressure Pc) is arranged above the bent portion lid of the main container  11 . The bent portion  11   d  and the bottom of the auxiliary container  16  communicate with each other through a first connection pipe  17 . The internal volume of the auxiliary container  16  is smaller than that of the main container  11 . 
     In order to stabilize the internal pressure Pt of the auxiliary container  16  (hereinafter referred to as the auxiliary container internal pressure Pt) at a level substantially equal to the average value Pca (described in detail later) of the main container internal pressure Pc, a choke  17   a  is arranged in the first connection pipe  17 . According to this embodiment, the choke  17   a  is formed by reducing the diameter of the path in the first connection pipe  17 . 
     The lower internal part of the auxiliary container  16  is filled with the working fluid  12  in liquid state, and the upper internal part thereof with a gas  18 . The gas  18  is desirably insoluble in the working fluid  12 , and according to this embodiment, formed of helium hard to solve in water. Incidentally, the auxiliary container  16  may alternatively be filled with only the working fluid  12  in liquid state. 
     The auxiliary container  16  and the first connection pipe  17  are desirably formed of a material high in thermal insulation, and according to this embodiment, formed of stainless steel. 
     The piston mechanism  19  making up the pressure regulation mechanism for adjusting the auxiliary container internal pressure Pt is configured of a pressure regulation piston  19   a  and an electrically-operated actuator  19   b.    
     The pressure regulation piston  19   a  is arranged at the upper end in the auxiliary container  16 , and adapted to reciprocate vertically by the electrically-operated actuator  19   b  arranged on the outside of the auxiliary container  16 . 
     Next, the electronic control unit according to this embodiment will be briefly explained. The control unit  21  is configured of a well-known microcomputer including a CPU, a ROM and a RAM and peripheral circuits, and corresponds to the control means according to this invention. 
     The control unit  21 , in order to control the piston mechanism  19 , is supplied with detection signals from a heated portion temperature sensor  22  for detecting the temperature T 1  of the heated portion  11   a  (hereinafter referred to as the heated portion temperature), a cooled portion temperature sensor  23  for detecting the temperature T 2  of the cooled portion  11   b  (hereinafter referred to as the cooled portion temperature) and a pressure sensor  24  for detecting the auxiliary container internal pressure Pt. The control unit  21  is adapted to control the drive operation of the electrically-operated actuator  19   b  based on the detection signals from the sensors  22  to  24 . 
     To produce a predetermined output quickly after engine start, this embodiment is different from the prior application in the following points. 
     Specifically, this embodiment includes a communication area adjusting means for adjusting the communication area between the main container  11  and the auxiliary container  16 . The communication area adjusting means is configured of a second connection pipe  25  for establishing communication between the main container  11  and the auxiliary container  16 , a check valve  26  arranged in the second connection pipe  25 , and the first connection pipe  17  and the choke  17   a  described above. 
     More specifically, the second connection pipe  25  establishes the communication between the bent portion lid of the main container  11  and the lower portion of the auxiliary container  16  where the working fluid  12  exists in liquid state. Also, the flow path area of the second connection pipe  25  is larger than that of the choke  17   a . According to this embodiment, the second connection pipe  25  is formed of stainless steel like the first connection pipe  17 . Incidentally, the second connection pipe  25  corresponds to the path according to this invention. 
     The check valve  26  is configured to permit the flow of the working fluid  12  from the main container  11  to the auxiliary container  16 , not to permit the flow from the auxiliary container  16  to the main container  11  in the second connection pipe  25 . In this embodiment, the spring check valve having a spring  26   a  is adopted as the check valve  26 . 
     The check valve  26  is adapted to open only in the case where the difference between the main container internal pressure Pc and the auxiliary container internal pressure Pt is not lower than a predetermined pressure (hereinafter referred to as the working pressure) ΔP. According to this embodiment, the working pressure ΔP is set to a level higher than the difference between the maximum value Pcmax of the main container internal pressure Pc in operation (hereinafter referred to as the maximum operating pressure) and the minimum value Ptmin of the auxiliary container internal pressure Pt (ΔP&gt;Pcmax−Ptmin) at the time of operation. 
     Incidentally, the minimum value Ptmin of the auxiliary container internal pressure Pt is defined as the auxiliary container internal pressure Pt with the pressure regulation piston  19   a  operated at the uppermost position in  FIG. 1 . 
     Next, the operation with this configuration will be explained with reference to  FIG. 2 . Upon actuation of the heater  13  and the cooler  14 , the working fluid (water) in the heated portion  11   a  is heated and gasified by the heater  13 , and the high-temperature high-pressure vapor of the working fluid  12  is accumulated in the heated portion  11   a  thereby to push down the liquid level of the working fluid  12  in the first linear portion  11   e  of the main container  11 . 
     Then, the liquid portion of the working fluid  12  in the main container  11  is displaced from the first linear portion  11   e  toward the second linear portion  11   f  and pushes up the piston  15  near to the power generator  1 . In the process, the spring  3  is compressed and elastically deformed by the piston  15 . 
     The liquid level of the working fluid  12  in the first linear portion  11   e  drops to the cooled portion  11   b , and with the advance of the vapor of the working fluid  12  into the cooled portion  11   b , the vapor is cooled into liquid state by the cooler  14  thereby to extinguish the force to push down the liquid level of the working fluid  12  in the first linear portion  11   e.    
     As a result, the piston  15  near the power generator  1  which has been pushed up by the expansion of the vapor of the working fluid  12  drops due to the elastic restitutive force of the spring  3 , and the liquid portion of the working fluid  12  in the main container  11  is displaced from the second linear portion  11   f  to the first linear portion  11   e , resulting in the rise of the liquid level in the first linear portion  11   e.    
     This operation is repeated until the heater  13  and the cooler  14  stop operating. In the meantime, the liquid portion of the working fluid  12  in the main container  11  is periodically displaced (in what is called the self-excited vibration), and the movable member  2  of the power generator  1  is moved up and down. 
     The relationship between the peak value Pc 1  of the main container internal pressure Pc and the performance (output and efficiency) of the external combustion engine  10  will now be explained.  FIG. 3A  is a PV diagram in a given state of the external combustion engine  10 . 
     In this PV diagram, the abscissa represents the volume of the space defined by the main container  11  and the piston  15  (hereinafter referred to as the piston volume), which is changed with the reciprocal motion of the piston  15 . The abscissa of the PV diagram shown in  FIGS. 3B and 3C  also represent such a volume. 
     The PV diagram of  FIG. 3A  represents the case in which the peak value Pc 1  of the main container internal pressure Pc is lower than the saturated vapor pressure Ps 1  of the working fluid  1  at the heated portion temperature T 1  and as nearest to the saturated vapor pressure Ps 1 . This represents the ideal state in which the work done by the external combustion engine  10  per period is maximum and the performance of the external combustion engine  10  is highest. 
       FIG. 3B , on the other hand, is the PV diagram in the state in which the peak value Pc 1  is very low as compared with the saturated vapor pressure Ps 1 . In this state, the work done per period is decreased, and therefore, the performance of the external combustion engine  10  is also decreased. 
     The PV diagram of  FIG. 3C  shows the case in which the peak value Pc 1  is higher than the saturated vapor pressure Ps 1 . Specifically, with the increase in the heated portion temperature T 1 , the high-temperature vapor exists in the heater  12  even in the case where the piston  15  is located at the top dead center (the uppermost position in  FIG. 1 ) where the piston volume is maximum. 
     In the process, with the movement of the piston  15  from the top dead center toward the bottom dead center (the lowest position in  FIG. 1 ) and the resulting reduction in piston volume, the vapor of the working fluid  12  is compressed and the main container internal pressure Pc rises. Also, in view of the fact that the liquid portion of the working fluid  12  advancing into the heated portion  11   a  is heated and gasified, the main container internal pressure Pc further rises. As a result, the peak value Pc 1  exceeds the saturated vapor pressure Ps 1 . 
     As long as the peak value Pc 1  is higher than the saturated vapor pressure Ps 1  as described above, the vapor of the working fluid  12  is partially condensed into liquid state. As a result, the work for moving down the piston  15 , i.e. the negative work is done undesirably, resulting in a deteriorated performance of the external combustion engine  10 . 
     In order to secure the maximum performance of the external combustion engine  10 , therefore, the peak value Pd 1  of the main container internal pressure Pc is required to be kept lower than the saturated vapor pressure Ps 1  of the working fluid  12  at the heated portion temperature T 1  while at the same time maintaining a value as near to the saturated vapor pressure Ps 1  as possible. 
     With the change in the heated portion temperature T 1 , however, the saturated vapor pressure Ps 1  of the working fluid  12  changes (as described later with reference to  FIG. 7 ). The peak value Pc 1  of the main container internal pressure Pc also changes with the heated portion temperature T 1  and the temperature T 2  of the cooled portion  11   b  (hereinafter referred to as the cooled portion temperature) and the leakage of the working fluid  12  from the main container  11 . 
     Specifically, the reduction in the temperature of the high-temperature gas providing the heat source of the heater  13  or the reduction in the temperature of the cooling water circulating in the cooler  14  reduces the heated portion temperature T 1  and the cooled portion temperature T 2 . The resulting reduction in the temperature of the liquid portion of the working fluid  12  thermally compresses and reduces the volume of the liquid portion of the working fluid  12 . The gradual leakage of the working fluid  12  from the main container  11  also reduces the volume of the liquid portion of the working fluid  12 . 
     With the decrease in the volume of the liquid portion of the working fluid  12 , as shown in  FIG. 4A , the liquid-phase working fluid  12  fails to advance sufficiently into the heated portion  11   a  even in the case where the piston  15  is located at the bottom dead center and the piston volume is minimum. 
     As a result, the gasification of the working fluid  12  in the heated portion  11   a  is suppressed, and the peak value Pc 1  of the main container internal pressure Pc is reduced. 
     With the increase in the heated portion temperature T 1  and the cooled portion temperature T 2 , on the other hand, the liquid portion of the working fluid  12  is thermally expanded and increases in volume. With the increase in the volume of the liquid portion of the working fluid  12 , as shown in  FIG. 4B , the vapor of the working fluid  12  fails to advance sufficiently into the cooled portion  11   b  even in the case where the piston  15  is located at the top dead center and the piston volume is maximum. 
     As a result, the liquefaction of the vapor of the working fluid  12  in the cooled portion  11   b  is suppressed, thereby increasing the peak value Pc 1  of the main container internal pressure Pc. 
       FIG. 5  is a graph showing the relationship between the volume of the liquid portion of the working fluid  12  and the efficiency of the external combustion engine  10 . Though not shown, the relationship between the volume of the liquid portion of the working fluid  12  and the output of the external combustion engine  10  is similar to the relationship shown in  FIG. 5 . 
     As understood from  FIG. 5 , the performance of the external combustion engine  10  reaches the maximum in the case where the liquid portion of the working fluid  12  reaches a predetermined volume V 1 . In this case, the PV diagram is as shown in  FIG. 3A . 
     In the case where the liquid portion of the working fluid  12  has a volume V 2  smaller than the predetermined volume V 1 , on the other hand, the PV diagram as shown in  FIG. 3B  is obtained, and the performance of the external combustion engine  10  is decreased. In the case where the volume of the liquid portion of the working fluid  12  is V 3  and larger than the predetermined volume V 1 , the PV diagram is as shown in  FIG. 3C , and the performance of the external combustion engine  10  is decreased. 
     In view of this, according to this embodiment, when the external combustion engine  10  is in operation, the average value Pca of the main container internal pressure Pc is controlled toward the target value PcO thereby to suppress the performance reduction of the external combustion engine  10  which otherwise might be caused by the change in the saturated vapor pressure Ps 1  or the change of the peak value Pc 1  of the main container internal pressure Pc. 
     The average value Pca of the main container internal pressure Pc is defined by a value during the self-excited vibration of the liquid portion of the working fluid  12  for one period. The target value PcO, on the other hand, is defined as a value approximated to the average value ( FIG. 3A ; hereinafter referred to as the ideal average value) Pci of the main container internal pressure Pc in the ideal state where the performance of the external combustion engine  10  is highest, i.e. the state in which the peak value Pc 1  of the main container internal pressure Pc is lower than the saturated vapor pressure Ps 1  of the working fluid  12  at the heated portion temperature T 1  and as near to the saturated vapor pressure Ps 1  as possible. 
       FIG. 6  is a block diagram briefly showing the control operation according to this embodiment. First, the saturated vapor pressure Ps 1  of the working fluid  12  at the heated portion temperature T 1  is calculated based on the heated portion temperature T 1  and the vapor pressure curve of the working fluid  12  shown in  FIG. 7 . 
     Also, the saturated vapor pressure Ps 2  of the working fluid  12  at the cooled portion temperature T 2  is calculated based on the cooled portion temperature T 2  and the vapor pressure curve of the working fluid  12  shown in  FIG. 7 . Incidentally, the saturated vapor pressure Ps 2  of the working fluid  12  at the cooled portion temperature T 2  is equal to the minimum value Pc 2  ( FIGS. 3A to 3C ) of the main container internal pressure Pc during one period. 
     Next, the target value PcO is calculated based on the saturated vapor pressure Ps 1  of the working fluid  12  at the heated portion temperature T 1  and the saturated vapor pressure Ps 2  of the working fluid  12  at the cooled portion temperature T 2 . According to this embodiment, the target value PcO is assumed to be the intermediate value between, or more specifically, a value substantially equal to the average value of the saturated vapor pressure Ps 1  of the working fluid  12  at the heated portion temperature T 1  and the saturated vapor pressure Ps 2  of the working fluid  12  at the cooled portion temperature T 2 . 
     Since the choke  17   a  is formed in the first connection pipe  17 , the auxiliary container internal pressure Pt is prevented from changing with the periodical change of the main container internal pressure Pc, so that the auxiliary container internal pressure Pt is kept stable at a value substantially equal to the average value Pca of the main container internal pressure Pc. 
     As long as the auxiliary container internal pressure Pt is lower than the target value PcO, the electrically-operated actuator  19   b  pushes out the pressure regulation piston  19   a  to reduce the volume of the auxiliary container  16 . As a result, the working fluid  12  in the liquid state is compressed and the auxiliary container internal pressure Pt rises. 
     In the case where the auxiliary container internal pressure Pt is higher than the target value PcO, the pressure regulation piston  19   a  is pulled in to reduce the volume of the auxiliary container  16 . As a result, the working fluid  12  in liquid state is expanded and the auxiliary container internal pressure Pt is reduced. 
     By adjusting the auxiliary container internal pressure Pt in this way, the average value Pca of the main container internal pressure Pc approaches the target value PcO, or in other words, the ideal average value Pci. 
     As a result, the external combustion engine  10  can usually be operated under ideal conditions, and therefore, the performance reduction of the external combustion engine  10  which otherwise might be caused by the change in the saturated vapor pressure Ps 1  or the change in the peak value Pc 1  of the main container internal pressure Pc can be prevented. 
     In the absence of the choke  17   a  in the first connection pipe  17 , the auxiliary container internal pressure Pt would be changed with the periodical change in the main container internal pressure Pc. Unless the period at which the pressure sensor  24  detects the auxiliary container internal pressure Pt is shortened greatly, the average value Pca of the main container internal pressure Pc cannot be calculated accurately. 
     According to this embodiment, the presence of the choke  17   a  in the first connection pipe  17  can stabilize the auxiliary container internal pressure Pt at substantially the same level as the average value Pca of the main container internal pressure Pc without any change with the periodic change of the main container internal pressure Pc. As a result, the average value Pca of the main container internal pressure Pc can be accurately calculated even in the case where the period at which the pressure sensor  24  detects the auxiliary container internal pressure Pt is long. 
     The compressibility of a liquid is lower than that of a gas. In the case where the auxiliary container  18  is filled with only the working fluid  12  in the liquid state, the change mount of the auxiliary container internal pressure Pt with respect to the displacement amount of the pressure regulation piston  19   a  excessively increases and the fine adjustment of the auxiliary container internal pressure Pt becomes difficult. 
     However, according to this embodiment, the auxiliary container  18  is sealed with a gas  18  higher in compressibility than the working fluid  12  in liquid state as well as the working fluid  12  in liquid state. Therefore, the change amount of the auxiliary container internal pressure Pt with respect to the displacement amount of the pressure regulation piston  19   a  can be suppressed. This facilitates the fine adjustment of the auxiliary container internal pressure Pt. 
     In the configuration described above, assume that the external combustion engine  10  stops with the piston  15  located at other than the bottom dead center. Then, the heater  13  stops heating the working fluid  12  with the vapor of the working fluid  12  existing in the first linear portion  11   e  of the main container  11 . 
     Then, the heated portion temperature T 1  gradually drops to the ambient temperature, and with the decrease in the saturated vapor pressure Ps 1 , the vapor of the working fluid  12  is condensed and liquefied, thereby reducing the main container internal pressure Pc. 
     Once the main container internal pressure Pc drops below the auxiliary container internal pressure Pt, the working fluid  12  in liquid state in the auxiliary container  16  flows into the main container  11  through the first connection pipe  17 , so that the volume of the working fluid  12  in the main container  11  becomes excessive. This phenomenon is likely to occur in winter when the ambient temperature is low. 
     As long as the volume of the working fluid  12  in the main container  11  of the external combustion engine  10  remains excessive as described above, a predetermined output cannot be produced. However, according to this embodiment, as explained below, the excess of the working fluid  12  in the main container  11  can be quickly returned to the auxiliary container  16  at the time of restarting the external combustion engine  10 , and therefore, a predetermined output can be produced quickly after restart. 
     Specifically, according to this embodiment, the power generator  1  is driven by the power supplied from an external source and the piston  15  passes through the bottom dead center at least once at the time of starting the external combustion engine  10 . 
     With the movement of the piston  15  from top dead center toward bottom dead center, the working fluid  12  in the main container  11  is compressed and the main container internal pressure Pc rises to more than the maximum operating pressure Pcmax. 
     According to this embodiment, the pressure regulation piston  19   a  is moved to the uppermost position in  FIG. 1  to maintain the auxiliary container internal pressure Pt at the minimum level Ptmin at the time of stopping the external combustion engine  10 . As a result, the main container internal pressure Pc is increased beyond the auxiliary container internal pressure Pt. 
     In the absence of the second connection pipe  25 , an increase in the main container internal pressure Pc beyond the auxiliary container internal pressure Pt would cause the working fluid  12  in liquid state in the main container  11  to flow into the auxiliary container  16  through only the first connection pipe  17 . The presence of the choke  17   a  in the first connection pipe  17  and the resulting fact that only a slight amount of the working fluid  12  in liquid state flows through the choke  17   a  at a time, however, blocks the flow of the working fluid  12 . As a result, considerable time would be taken before the excess of the working fluid  12  in the main container  11  returns to the auxiliary container  16  in its entirety. 
     According to this embodiment, in contrast, an increase in the main container internal pressure Pc beyond the auxiliary container internal pressure Pt opens the check valve  26  arranged in the second connection pipe  25  and the working fluid  12  in liquid state in the main container  11  flows into the auxiliary container  16  through the second connection pipe  25 . 
     In short, according to this embodiment, the main container  11  and the auxiliary container  16  communicate with each other only through the small choke  17   a  in normal operation mode, while the main container  11  and the auxiliary container  16  communicate with each other through the second connection pipe  25  larger in communication area than the choke  17   a  as well as through the choke  17   a  at the time of engine start. As a result, the excess of the working fluid  12  in the main container  11  can be quickly returned to the auxiliary container  16 . 
     If the check valve  26  opens in normal operation mode, the working fluid  12  in liquid state in the main container  11  would flow into the auxiliary container  16  through the second connection pipe  25 , with the result that the volume of the working fluid  12  in the main container  11  would be reduced for a reduced performance of the external combustion engine  10 . 
     According to this embodiment, the working pressure ΔP of the check valve  26  is set to a value larger than the difference between the maximum operating pressure Pcmax of the main container internal pressure Pc and the minimum level Ptmin of the auxiliary container internal pressure Pt. In normal operation mode, therefore, the check valve  26  is not opened and the working fluid  12  in the main container  11  is prevented from flowing into the auxiliary container  16  through the second connection pipe  25 . 
     Second Embodiment 
     In the second embodiment, unlike in the first embodiment, a valve  30  for opening/closing the second connection pipe  25  is added as shown in  FIG. 8 . The operation of the valve  30  is controlled by the control unit  21 . The valve  30  and the control unit  21 , together with the first connection pipe  17 , the choke  17   a , the second connection pipe  25  and the check valve  26 , makes up the communication area adjusting means. 
     The valve  30  is controlled by the control unit  21  to be closed in normal operation mode and open only at the time of starting the external combustion engine  10 . Even in the case where the check valve  26  is open in the normal operation mode of the external combustion engine  10 , therefore, the working fluid  12  is prevented by the valve  30  from flowing into the auxiliary container  16  through the second connection pipe  25 . 
     As a result, unlike in the first embodiment, the operating pressure ΔP of the check valve  26  is not required to be set to a value larger than the difference between the maximum operating pressure level Pcmax of the main container internal pressure Pc and the minimum level Ptmin of the auxiliary container internal pressure Pt. 
     According to this embodiment, the operating pressure ΔP of the check valve  26  is set to a level higher than zero but not higher than the difference between the maximum operating pressure Pcmax of the main container internal pressure Pc and the minimum level Ptmin of the auxiliary container internal pressure Pt (0&lt;ΔP≦Pcmax−Ptmin). 
     In the first embodiment, the check valve  26  is not opened at the main container internal pressure Pc not higher than the maximum operating pressure Pcmax, and therefore, the power generator  1  is required to be driven with a large drive power at the time of stating the external combustion engine  10 . 
     However, according to this embodiment, the check valve  26  is opened at the main container internal pressure Pc larger than the auxiliary container pressure Pt but not higher than the maximum operating pressure Pcmax, and therefore, as compared with the first embodiment, the driving force of the power generator  1  at the time of starting the external combustion engine  10  can be reduced. As compared with the first embodiment, therefore, the external combustion engine can be started easily. 
     Third Embodiment 
     According to the third embodiment, unlike in the first embodiment, the operating pressure ΔP of the check valve  26  can be controlled variably as shown in  FIG. 9 . 
     Specifically, the spring portion  26   a  of the check valve  26  is formed of a shape memory alloy or bimetal so that the spring constant of the spring portion  26   a  changes with temperature. Further, the operating pressure ΔP of the check valve  26  changes in accordance with the change in the spring constant of the spring portion  26   a . Alternatively, the spring portion  26   a  may not have such a characteristic as to change the spring constant thereof with temperature, but a thermostat adapted to expand/contract with temperature may be provided to change the operating pressure ΔP of the check valve  26 . 
     The spring portion  26   a  is heated by the heater  31  which in turn is controlled by the control unit  21 . 
     According to this embodiment, the heater  31  is configured of an actuator for energizing the spring portion  26   a , which is heated by Joule heat upon energization. 
     The heater  31  is controlled by the control unit  21  in such a manner that the operating pressure ΔP of the check valve  26  at the time of starting the external combustion engine is reduced below the operating pressure ΔP of the check valve  26  in normal operation mode. 
     As a result, the check valve  26  is prevented from opening in the normal operation mode of the external combustion engine  10 , while the check valve  26  can be opened even at the main container internal pressure Pc not higher than the maximum operating pressure Pcmax at the time of starting the external combustion engine  10 . Thus, the same effects as in the second embodiment are produced. 
     Fourth Embodiment 
     According to the fourth embodiment, as shown in  FIG. 10 , the check valve  26  included in the second embodiment is omitted. The valve  30  is kept open until the piston  15  first reaches the bottom dead center at the time of starting the external combustion engine  10  and closed the instant the piston  15  reaches the bottom dead center for the first time. 
     Without the provision of the check valve  26 , therefore, the working fluid  12  is prevented from flowing into the auxiliary container  16  through the second connection pipe  25  in the normal operation mode of the external combustion engine  10 . As a result, the same effects as in the second and third embodiments described above can be produced. 
     Fifth Embodiment 
     According to the fifth embodiment, as shown in  FIG. 11 , the second connection pipe  25  and the check valve  26  included in the first embodiment are omitted, and the choke  17   a  of the first connection pipe  17  is replaced by an electrically-operated variable choke mechanism  32 . 
     The opening degree of the variable choke mechanism  32  is controlled by the control unit  21  in such a manner as to be larger than in normal operation mode before the piston  15  first reaches the bottom dead center at the time of starting the external combustion engine  10  and reaches the same level as in normal operation mode the instant the piston  15  first reaches the bottom dead center. 
     As a result, the excess of the working fluid  12  in the main container  11  can be quickly returned to the auxiliary container  16  at the time of starting the engine, while at the same time preventing the working fluid  12  from flowing into the auxiliary container in normal operation mode. Thus, this embodiment exhibits the same effects as the second to fourth embodiments. 
     According to this embodiment, the first connection pipe  17 , the variable choke mechanism  32  and the control unit  21  make up the communication area adjusting means. 
     Sixth Embodiment 
     The power generating system according to the sixth embodiment, as shown in  FIG. 12 , includes two external combustion engines  10  of the fourth embodiment. According to this embodiment, these two external combustion engines  10  are designated by the same reference numeral. In  FIG. 12 , the control unit  21  and the sensors  22  to  24  are not shown for the reason of illustration. 
     The configuration of the main containers of the two external combustion engines are similar to that of the main container according to each embodiment described above. For convenience the main container of one of the two external combustion engines  10  is referred to as a first container  11  and that of the other external combustion engine  10  as a second container  33 . 
     Incidentally, in  FIG. 12 , the heated portion, the cooled portion, the intermediate portion, the bent portion and the first and second linear portions of the first container  11  are designated by the same reference numerals as the corresponding parts, respectively, of the main container according to each embodiment described above, while the heated portion, the cooled portion, the intermediate portion, the bent portion and the first and second linear portions of the second container  33  are designated by the reference numerals  33   a  to  33   f , respectively. 
     The two external combustion engines  10  are so configured as to have the same target value PcO of the main container internal pressure Pc. Therefore, one auxiliary container  16  is shared by the two external combustion engines  10 . 
     More specifically, only one auxiliary container  16  available communicates with both the first container  11  and the second container  33  through the first connection pipe  17  and the second connection pipe  25 , respectively. As a result, the number of the auxiliary containers  16  is reduced for a lower cost. 
     This embodiment is also applicable to the configuration having three or more external combustion engines  10  with equal effect. Further, this embodiment is of course applicable also to the external combustion engine  10  according to any of the first to the third and fifth embodiments. In the application to the external combustion engine  10  according to the fifth embodiment, the second connection pipe  25  is of course not required. 
     Seventh Embodiment 
     According to the seventh embodiment shown in  FIG. 13 , as compared with the first embodiment described above, the auxiliary container  16  is configured of the power generator  1 . More specifically, the working fluid  12  in liquid state and the gas  18  are sealed in the casing  1   a  of the power generator  1 , and the piston mechanism  19  for adjusting the auxiliary container internal pressure Pt is arranged above the generator  1 . The second connection pipe  25  is arranged between the bottom surface of the casing  1   a  of the generator  1  and the second linear portion  11   f  of the main container  11 . 
     According to this embodiment, the cylinder  15   a  functions as the first connection pipe  17  according to the first embodiment, and the minuscule clearance existing between the piston  15  and the cylinder  15   a  functions as the choke  17   a  according to the first embodiment. 
     As a result, this embodiment produces the same effects as the first embodiment. Also, according to this embodiment, the auxiliary container  16 , the first connection pipe  17  and the choke  17   a  according to the first embodiment are not required. Therefore, both the number of the parts and the cost are reduced. 
     This embodiment is of course applicable also to the external combustion engine  10  according to the second to fourth embodiments. 
     Other Embodiments 
     Although one end of the second connection pipe  25  is connected to the lower part of the auxiliary container  16  and the other end of the second connection pipe  25  to the bent portion  11   d  of the main container  11  according to the first to fourth and sixth embodiments, one end of the second connection pipe  25  may be connected to the portion of the first connection pipe  17  nearer the auxiliary container  16  than the choke  17   a , and the other end of the second connection pipe  25  to the portion of the first connection pipe  17  nearer the main container  11  than the choke  17   a.    
     At the time of starting the engine, therefore, the working fluid  12  in the main container  11  can be introduced into the auxiliary container  16  while bypassing the choke  17   a . Thus, this configuration produces the same effects as the other embodiments. 
     Also, in each of the embodiments described above, the main container  11  has a substantially U-shaped configuration. Nevertheless, the main container  11  may alternatively be linear in shape. For example, the linear main container  11  may be arranged vertically, while the heater  13 , the cooler  14  and the generator  1  may be arranged in that order top down. In such a case, the auxiliary container  16  is formed to communicate with the portion of the main container  11  nearer the generator  1  than the cooler  14 . Also, any configuration can be employed in which the vapor generated by the heater  13  is prevented from reaching the generator  1  by arranging, for example, the heater  13 , the cooler  14  and the generator  1  at an angle to the vertical direction or horizontally. 
     The external combustion engine according to each of the embodiments described, though explained as an application as a drive source of the power generating system, may alternatively be used as a drive source for systems than the power generating system. 
     While the invention has been described by reference to specific embodiments chosen for purposes of illustration, it should be apparent that numerous modification could be made thereto by those skilled in the art without departing from the basic concept and scope of the invention.