Abstract:
An external combustion engine is disclosed, comprising a container ( 11 ) for sealing a working liquid ( 12 ) in a way adapted to allow the liquid to flow therein, a heater ( 13 ) for heating and vaporizing the working liquid ( 12 ) in the container ( 11 ), and a cooler ( 14 ) for cooling and liquefying the vapor of the working liquid ( 12 ) heated and vaporized by the heater ( 13 ). The displacement of the working liquid ( 12 ) caused by the volume change of the vapor of the working liquid ( 12 ) is output by being converted into mechanical energy. In the heated portion ( 11   d ) of the container ( 11 ) for vaporizing the working liquid ( 12 ), the direction of displacement of the working liquid ( 12 ) at the parts ( 17, 19 ) far from the cooler ( 14 ) is changed with respect to the direction of displacement at the part ( 16 ) near to the cooler ( 14 ).

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to an external combustion engine for converting the displacement of a working liquid caused by the vapor volume change thereof into, and outputting it as, mechanical energy. 
   2. Description of the Related Art 
   A conventional external combustion engine is disclosed in Japanese Unexamined Patent Publication No. 2004-84523, in which a working liquid is sealed in a container and partly heated and vaporized by a heater, and the vapor of the working liquid thus vaporized is cooled and liquefied by a cooler, so that the displacement of the working liquid caused by the vapor volume change thereof is output by being converted into mechanical energy. 
   In this conventional external combustion engine, a heated portion of the container, in which the working liquid is vaporized, is formed of a straight tube and the heater is arranged on the outer peripheral surface of the heated portion thereby to heat and vaporize the working liquid. 
   SUMMARY OF THE INVENTION 
   In the conventional combustion engine in which the heated portion is formed of a straight tube, however, the working liquid, if changed in vapor volume, uniformly flows in the heated portion and is displaced. During the heat transfer from the heater to the working liquid before vaporization thereof, therefore, a thermal boundary layer is developed undesirably in the neighborhood of the inner wall surface of the heated portion. As a result, the problem is posed that the heat transfer rate from the heater to the working liquid is reduced. 
   In view of this problem, the object of this invention is to improve the heat transfer rate from the heater to the working liquid. 
   In order to achieve this object, according to a first aspect of the invention, there is provided an external combustion engine comprising: 
   a container ( 11 ) for sealing a working liquid ( 12 ) in a way adapted allow the liquid to flow therein; 
   a heater ( 13 ) for heating and vaporizing the working liquid ( 12 ) in the container ( 11 ); and 
   a cooler ( 14 ) for cooling and liquefying the vapor of the working liquid ( 12 ) heated and vaporized by the heater ( 13 ); 
   wherein the displacement of the working liquid ( 12 ) caused by the volume change of the vapor of the working liquid ( 12 ) is converted into mechanical energy and output, and 
   wherein the heated portion ( 11   d ) of the container ( 11 ) for vaporizing the working liquid ( 12 ) is so formed that the direction of displacement of the working liquid ( 12 ) at the part ( 17 ,  19 ) of the heated portion ( 11   d ) far from the cooler ( 14 ) is changed with respect to the direction of displacement of the working liquid ( 12 ) at the part ( 16 ) near to the cooler ( 14 ). 
   With this configuration, when the direction of displacement of the working liquid ( 12 ) is changed in the heated portion ( 11   d ), the working liquid ( 12 ) collides with the inner wall surface of the heated portion ( 11   d ). Thus, the working liquid ( 12 ) is agitated and a turbulence is generated, so that the thermal boundary layer in the neighborhood of the inner wall surface of the heated portion ( 11   d ) can be destroyed. As a result, the heat transfer rate from the heater ( 13 ) to the working liquid ( 12 ) is improved. 
   Specifically, according to the invention, the heated portion ( 11   d ) is formed of a first path portion ( 16 ) extending toward the cooler ( 14 ) and a second path portion ( 17 ,  19 ) extending in the direction, across the first path portion ( 16 ), from the end of the first path portion ( 16 ) far from the cooler ( 14 ). 
   With this simple configuration, the direction of displacement of the working liquid ( 12 ) at the part ( 17 ,  19 ) of the heated portion ( 11   d ) far from the cooler ( 14 ) can be changed with respect to the direction of displacement of the working liquid ( 12 ) at the part ( 16 ) near to the cooler ( 14 ). 
   Specifically, according to the invention, the angle formed between the direction in which the first path portion ( 16 ) extends and the direction in which the second path portion ( 17 ,  19 ) extends is set to the range not less than 15 degrees but not more than 90 degrees. 
   With this configuration, it has been found that the working liquid ( 12 ) is effectively agitated, and the heat transfer rate from the heater ( 13 ) to the working liquid ( 12 ) can be effectively improved, as described in detail later. 
   Specifically, according to the invention, the second path portion ( 17 ,  19 ) extends in horizontal direction. 
   With this configuration, the working liquid ( 12 ) agitated by colliding with the inner wall surface of the heated portion ( 11   d ) can advance into the second path portion ( 17 ,  19 ) smoothly in spite of gravity. As a result, the advance of the agitated working liquid ( 12 ) into the second path portion ( 17 ,  19 ) is facilitated, thereby improving the heat transfer rate from the heater ( 13 ) to the working liquid ( 12 ). 
   Specifically, according to the invention, the sectional area of the second path portion ( 17 ,  19 ) is smaller than that of the first path portion ( 16 ). It is possible, therefore, to effectively heat the working liquid ( 12 ) far from the inner wall surface of the second path portion ( 17 ,  19 ) as well as the working liquid ( 12 ) in the neighborhood of the inner wall surface of the second path portion ( 17 ,  19 ). Thus, the heat transfer rate from the heater ( 13 ) to the working liquid ( 12 ) is improved. 
   Specifically, according to the invention, a plurality of the second path portions ( 17 ,  19 ) are formed. 
   Specifically, according to the invention, the second path portion ( 17 ) is formed as a tube. 
   Specifically, according to the invention, the second path portion ( 17 ) is formed as a hollow cylinder having the inner diameter (d 2 ) not more than the heat penetration depth (δ). 
   With this configuration, the working liquid ( 12 ) far from the inner wall surface of the second path portion ( 17 ,  19 ) as well as the working liquid ( 12 ) in the neighborhood of the inner wall surface of the second path portion ( 17 ,  19 ) can be positively heated, and therefore the heat transfer rate from the heater ( 13 ) to the working liquid ( 12 ) is improved. 
   The heat penetration depth (δ), which is an index of the extent to which the periodic temperature change, if any, of the working liquid ( 12 ) in the second path portion ( 17 ,  19 ) is transmitted, is expressed by Equation ( 1 ) below.
 
δ=√(2·α/ω)  (1)
 
where α is the thermal diffusivity (JIS Z8202-4) and ω the angular frequency.
 
   Specifically, according to the invention, the second path portion ( 19 ) is formed as a planar hollow portion. 
   Specifically, according to the invention, the size (c) of the cavity ( 20 ) of the second path portion ( 19 ) in the direction perpendicular to the direction in which the second path portion ( 19 ) extends is set to not more than the heat penetration depth (δ). 
   With this configuration, the working liquid ( 12 ) far from the inner wall surface of the second path portion ( 19 ) as well as the working liquid ( 12 ) in the neighborhood of the inner wall surface of the second path portion ( 19 ) can be positively heated, and therefore the heat transfer rate from the heater ( 13 ) to the working liquid ( 12 ) is further improved. 
   According to a second aspect of the invention, there is provided an external combustion engine comprising: 
   a container ( 11 ) for sealing a working liquid ( 12 ) in a way adapted to allow the liquid to flow therein; 
   a heater ( 13 ) for heating and vaporizing the working liquid ( 12 ) through the container ( 11 ); and 
   a cooler ( 14 ) for cooling and liquefying the vapor formed by being heated by the heater ( 13 ); 
   wherein the periodic flow displacement of the working liquid ( 12 ) caused by the vaporization and the liquefaction of the working liquid ( 12 ) is output by being converted into mechanical energy; 
   wherein the inner wall surface of the heated portion ( 11   d ) of the container ( 11 ) for vaporizing the working liquid ( 12 ) has a stepped collision surface in which a first inner wall surface portion ( 24 ) far from the cooler ( 14 ) is projected inward of the heated portion ( 11   d ) more than a second inner wall surface portion ( 25 ) near to the cooler ( 14 ). 
   With this configuration, the vapor of the working liquid ( 12 ) is cooled and liquefied by the cooler ( 14 ), and the working liquid ( 12 ), advancing into the heated portion ( 11   d ) from the cooler ( 14 ), collides with the collision surface ( 23 ) of the heated portion ( 11   d ). 
   As a result, the working liquid ( 12 ) is agitated and a turbulence is formed, thereby making it possible to destroy the thermal boundary layer in the neighborhood of the inner wall surface of the heated portion ( 1   d ). Thus, the heat transfer rate from the heater ( 13 ) to the working liquid ( 12 ) is improved. 
   Specifically, according to the invention, the collision surface ( 23 ) is formed over the entire periphery of the heated portion ( 11   d ). 
   With this configuration, a greater amount of the working liquid ( 12 ) can be agitated by collision with the inner wall surface of the heated portion ( 11   d ), and therefore the heat transfer rate from the heater ( 13 ) to the working liquid ( 12 ) is improved. 
   Specifically, according to the invention, the heated portion ( 11   d ) may be arranged above the cooled portion ( 11   e ) for liquefying the vapor of the working liquid ( 12 ) in the container ( 11 ). 
   Specifically, according to the invention, a gas ( 18 ) always exists in the heated portion ( 11   d ), and therefore, a space for vaporizing the working liquid ( 12 ) heated by the heater ( 13 ) can be secured in the heated portion ( 11   d ). 
   Specifically, according to the invention, a gas sealing portion ( 21 ) for sealing the gas ( 18 ) and communicating with the heated portion ( 11   d ) may be formed in the container ( 11 ). 
   Specifically, according to the invention, a gas sealing portion ( 21 ) for sealing the gas ( 18 ) and communicating with the second path portion ( 17 ) may be formed in the container ( 11 ). 
   Specifically, the external combustion engine according to the invention includes a heating means ( 13 ) for heating the gas sealing portion ( 21 ) to at least the temperature of the vapor of the working liquid ( 12 ). Therefore, the vapor of the working liquid ( 12 ), which may advance into the gas sealing portion ( 21 ) at the time of heating and vaporizing the working liquid ( 12 ) by the heater ( 13 ), is prevented from being cooled and liquefied by the gas sealing portion ( 21 ). 
   Specifically, according to the invention, the heating means constitutes the heater ( 13 ) so that the gas sealing portion ( 21 ) can be heated to not lower than the vapor temperature of the working liquid ( 12 ) with a simple configuration. 
   Specifically, according to the invention, the container ( 11 ) is formed to extend from an end for outputting the mechanical energy toward the other end, and the gas sealing portion ( 21 ) is arranged nearer to the other end than the heated portion ( 11   d ). 
   Specifically, according to the invention, the air may be employed as the gas ( 18 ). 
   Specifically, according to the invention, the vapor of the working liquid ( 12 ) can be employed as the gas ( 18 ). 
   The reference numerals inserted in the parentheses following the names of each means described above and in the claims indicate the correspondence with the specific means described in the embodiments described later. 
   The present invention may be more fully understood from the description of 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 unit according to a first embodiment of the invention. 
       FIG. 2  is a diagram for explaining the operation characteristics of an external combustion engine according to the first embodiment. 
       FIG. 3A  is a diagram showing a general configuration of the power generating unit according to a second embodiment of the invention, and  FIG. 3B  a sectional view taken in line A-A in  FIG. 3A . 
       FIG. 4A  is a diagram showing a general configuration of the power generating unit according to a third embodiment of the invention, and  FIG. 4B  a sectional view taken in line B-B in  FIG. 4A . 
       FIG. 5  is a diagram showing a general configuration of the power generating unit according to a fourth embodiment of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   First Embodiment 
   The first embodiment of the invention is explained below with reference to  FIGS. 1 and 2 .  FIG. 1  is a diagram showing a general configuration of a power generating unit including an external combustion engine  10  according to the invention and a power generator  1 . In  FIG. 1 , the up arrow indicates “up” in vertical direction and the down arrow “down” in vertical direction. 
   As shown in  FIG. 1 , the external combustion engine  10  according to this embodiment, which is for driving the generator  1  to generate the electromotive force by the vibratory displacement of a movable element  2  embedded with a permanent magnet, includes a container  11  for sealing a working liquid (water in this embodiment)  12  in a way adapted to allow the liquid to flow therein, a heater  13  making up a heating means for heating the working liquid  12  in the container  11 , and a cooler  14  for cooling the vapor of the working liquid  12  heated and vaporized by the heater  13 . 
   According to this embodiment, a high-temperature gas is used as a heat source of the heater  13 . Also, the cooling water is circulated in the cooler  14  according to this embodiment. Though not shown, a radiator for radiating the heat deprived of by the cooling water from the vapor of the working liquid  12  is arranged in the cooling water circulation circuit. 
   The container  11  is a tubular pressure vessel formed substantially in the shape of U having first and second straight portions  11   b ,  11   c  with a bent portion  11   a  at the lowest position. The first straight portion  11   b  at one horizontal end (right side on the page) following the bent portion  11   a  of the container  11  includes the heater  13  and the cooler  14  with the former located above the latter. 
   According to this embodiment, the heated portion  11   d  of the container  11  in contact with the heater  13  and the cooled portion  11   e  of the container  11  in contact with the cooler  14  are formed of copper or aluminum high in heat conductivity. 
   The intermediate portion  11   f  between the heated portion  11   d  and the cooled portion  11   e  of the container  11 , on the other hand, is formed of stainless steel high in heat insulating properties. The portion of the container  11  nearer to the generator  1  than the cooled portion  11   e  is also formed of stainless steel high in heat insulating properties. 
   A piston  15  adapted to be displaced under the pressure of the working liquid is arranged slidably in a cylinder unit  15   a  at the upper end of the second straight portion  11   c  at the other horizontal end (left side on the page) of the container following the bent portion  11   a.    
   The piston  15  is coupled to the shaft  2   a  of the movable element  2 , and a spring  3  making up an elastic means for generating the elastic force to press the movable element  2  against the piston  15  is arranged on the other side of the generator  1  far from the piston  15  beyond the movable element  2 . 
   In order to improve the heat transfer rate from the heater  13  to the working liquid  12 , the heated portion  11   d  formed at the upper end of the first straight portion  11   b  is formed as a bent tube. Specifically, the heated portion  11   d  is formed of a cylindrical first path portion  16  extending in parallel to the first straight portion  11   b  near to the cooled portion  11   e  and a cylindrical second path portion  17  extending in the direction across the direction in which the first path portion  16  extends from the end (upper end in  FIG. 1 ) of the first path portion  16  far from the cooled portion  11   e.    
   According to this embodiment, the first path portion  16  extends in vertical direction, and the angle between the direction in which the first path portion  16  extends and the direction in which the second path portion  17  extends is set at 90 degrees. Thus, the second path portion  17  extends in horizontal direction. 
   The inner diameter d 2  of the second path portion  17  is smaller than the inner diameter d 1  of the first path portion  16 . The sectional area of the second path portion  17 , therefore, is smaller than that of the first path portion  16 . 
   Further, the inner diameter d 2  of the second path portion  17  is set to not more than the heat penetration depth δ. The heat penetration depth δ is an indicator of the extent to which the periodic temperature change, if any, of the working liquid  12  in the second path portion  17  is transmitted. Specifically, the heat penetration depth δ is the indicator for determining the radial distribution of the entropy change in the second path portion  17  from the thermal diffusivity α(m/s) and the angular frequency ω(rad/s), and expressed by Equation (1) below.
 
δ=√(2·α/ω)  (1)
 
where the thermal diffusivity α is a value obtained by dividing the heat conductivity of the working liquid  12  by the specific heat and density thereof (JIS Z8202-4).
 
   In order to secure the internal space of the container  11  to vaporize the working liquid  12  heated by the heater  13 , the gas  18  of a predetermined volume is sealed in the second path portion  17 . This gas  18  may be, for example, air or a pure vapor of the working liquid  12 . 
   The gas  18  in  FIG. 1  assumes the state at the moment when the liquid level of the working liquid  12  in the first straight portion  11   b  is highest. In this state, the gas  18  exists in the deepest part (left side in  FIG. 1 ) of the second path portion  17 . 
   Next, the operation with the aforementioned configuration is explained with reference to  FIG. 2 . With the activation of the heater  13  and the cooler  14 , the working liquid (water)  12  in the heated portion  11   d  is heated and vaporized by the heater  13 , and the high-temperature high-pressure vapor of the working liquid  12  is accumulated in the heated portion  11   d  thereby to press down the liquid level of the working liquid  12  in the first straight portion  11   b . Then, the working liquid  12  sealed in the container  11  is displaced from the first straight portion  11   b  to the second straight portion  11   c  and pushes up the piston  15  in the generator  1 . 
   Also, if the liquid level of the working liquid  12  in the first straight portion  11   b  of the container  11  drops to the cooled portion  11   e  and the vapor of the working liquid  12  advances into the cooled portion  11   e , the vapor of the working liquid  12  is cooled and liquefied by the cooler  14 . Therefore, the force to push down the liquid level of the working liquid  12  in the first straight portion  11   b  is lost, and the liquid level of the working liquid  12  in the first straight portion  11   b  rises. As a result, the piston  15  in the power generator  1  which has been pushed up by the expansion of the vapor of the working liquid  12  falls. 
   This operation is repeated until the heater  13  and the cooler  14  stop the operation. In the process, the working liquid  12  in the container  11  is periodically displaced (by what is called the self-excited vibration) thereby to move the movable element  2  of the power generator  1  vertically. 
   According to this embodiment, the heated portion  11   d  is formed as a bent tube. In the heated portion  11   d , therefore, the direction of displacement of the working liquid  12  is changed along the bend of the heated portion  11   d.    
   More specifically, assume that the vapor of the working liquid  12  is cooled and liquefied by the cooler  14  and the liquid level in the first straight portion  11   b  rises. Then, the working liquid  12 , after being displaced upward and advancing into the first path portion  16  of the heated portion  11   d , changes the direction of displacement toward the second path portion  17  (left side in  FIG. 1 ) and enters the second path portion  17 . In the process, as indicated by arrow a in  FIG. 1 , the working liquid  12  collides with the inner wall surface of the heated portion  11   d.    
   The working liquid  12 , colliding with the inner wall surface of the heated portion  11   d  as described above, is agitated and generates turbulence. As a result, the thermal boundary layer is destroyed in the neighborhood of the inner wall surface of the heated portion  11   d  collided by the working liquid  12 , and therefore the heat transfer rate from the heater  13  to the working liquid  12  is improved. 
   In the case where the angle of bend of a fluid path in which a fluid flows is set in the range of not less than 15 degrees but not more than 90 degrees, the fluid is effectively agitated and the heat transfer rate is improved, as reported in K. P. Perry, “Heat Transfer By Convection from a Hot Gas Jet to a Plane Surface”, Proceedings of Institution of Mechanical Engineers, Vol. 168 (1954, Great Britain), pp. 775 to 780. 
   Thus, in the case where the angle of bend of the heated portion  11   d  forming the flow path of the working liquid  12 , i.e. the angle between the direction in which the first path portion  16  extends and the direction in which the second path portion  17  extends is set to between 15 degrees and 90 degrees inclusive, then the heat transfer rate from the heater  13  to the working liquid  12  can be effectively improved. 
   Also, according to this embodiment, the second path portion  17  extends in horizontal direction, and therefore, the agitated working liquid  12  can advance into the second path portion  17  smoothly in spite of gravity. As a result, the working liquid, while kept agitated, can easily enter the second path portion  17 . Thus, the heat transfer rate from the heater  13  to the working liquid  12  is more effectively improved. 
   Further, according to this embodiment, the inner diameter d 2  of the second path portion  17  is smaller than the inner diameter d 1  of the first path portion  16 , and the sectional area of the second path portion  17  is smaller than that of the first path portion  16 . Therefore, the working liquid  12  along the center (the part far from the inner wall surface) as well as in the neighborhood of the inner wall surface the second path portion  17  can be effectively heated. As a result, the heat transfer rate from the heater  13  to the working liquid  12  can be more effectively improved. 
   Furthermore, as the inner diameter d 2  of the second path portion  17  is not more than the heat penetration depth δ, the working liquid  12  along the center as well as in the neighborhood of the inner wall surface of the second path portion  17  can be positively heated. In the second path portion  17 , therefore, the heat transfer rate from the heater  13  to the working liquid  12  can be more effectively improved. 
   As described above, according to this embodiment, the heat transfer rate from the heater  13  to the working liquid  12  is improved with a simple configuration in which the heated portion  11   d  is formed as a bent tube. 
   Second Embodiment 
   According to the second embodiment, unlike in the first embodiment with the heated portion  11   d  formed as a bent tube, the heated portion  11   d  has a plurality of tubular branches on the side thereof far from the cooled portion  11   e  as shown in  FIGS. 3A ,  3 B. 
     FIG. 3A  is a diagram showing a general configuration of a power generating unit according to this embodiment, and  FIG. 3B  a sectional view taken in line A-A in  FIG. 3A . 
   According to this embodiment, unlike in the first embodiment, a plurality of cylindrical second path portions  17  are formed. More specifically, four second path portions  17  extend radially in horizontal direction from the upper end of the first path portion  16 . 
   The inner diameter d 2  of the four second path portions  17 , as in the first embodiment, is set to a value smaller than the inner diameter d 1  of the first path portion  16  and not larger than the heat penetration depth δ. 
   According to this embodiment, in the case where the vapor of the working liquid  12  is cooled and liquefied by the cooler  14  and the liquid level in the first straight portion  11   b  rises, then the working liquid  12  collides with the inner wall surface of the heated portion  11   d  as shown by arrow b in  FIG. 3A . 
   As a result, the working liquid  12  in the heated portion  11   d  is agitated and a turbulence is generated. Thus, the heat transfer rate from the heater  13  to the working liquid  12  is improved in the neighborhood of the inner wall surface of the heated portion  11   d  collided by the working liquid  12 . 
   The working liquid  12  that has collided with the inner wall surface of the heated portion  11   d  advances into the four second path portions  17  in agitated state, and therefore the heat transfer rate from the heater  13  to the working liquid  12  is improved in the four second path portions  17 . 
   As a result, the effects similar to those of the first embodiment are achieved. 
   Third Embodiment 
   According to this third embodiment, unlike in the first and second embodiments in which the second path portion  17  is formed as a cylinder, the second path portion  19  is formed as a flat hollow portion as shown in  FIGS. 4A ,  4 B. 
     FIG. 4A  is a diagram showing a general configuration of the power generating unit according to this embodiment, and  FIG. 4B  a sectional view taken in line B-B in  FIG. 4A . The flat hollow second path portion  19 , in the shape of a circle having the center on the first path portion  16 , extends horizontally. Therefore, the direction in which the first path portion  16  extends and the direction in which the second path portion  19  extends form an angle of 90 degrees with each other. 
   The cavity  20  of the second path portion  19  also assumes a circle extending in horizontal direction. The vertical size c of the cavity  20  is smaller than the inner diameter d 1  of the first path portion  16  and not larger than the heat penetration depth δ. 
   A flat hollow gas sealing portion  21  sealed with the gas  18  is formed above the second path portion  19 . The gas sealing portion  21  is in the shape of a circle concentric with the second path portion  19 , and communicates with the second path portion  19  through a plurality of communication pipes  22  arranged along the circumference thereof. 
   Also, the gas sealing portion  21  is heated to at least the temperature of the second path portion  19  by the heater  13 . According to this embodiment, the gas sealing portion  21  is formed of copper or aluminum high in heat conductivity. 
   According to this embodiment, the vapor of the working liquid  12  is cooled and liquefied by the cooler  14 , and with the rise of the liquid level in the first straight portion  11   b , the working liquid  12  comes to collide with the inner wall surface of the heated portion  11   d  as shown by arrow e in  FIG. 4A . 
   As a result, the working liquid  12  in the heated portion  11   d  is agitated and a turbulence generated. The thermal boundary layer can thus be destroyed in the neighborhood of the inner wall surface of the heated portion  11   d  with which the working liquid  12  collides. As a result, the heat transfer rate from the heater  13  to the working liquid  12  is improved. 
   The working liquid  12  that has collided with the inner wall surface of the heated portion  11   d , while kept agitated, advances into the second path portion  19 . Therefore, the heat transfer rate from the heater  13  to the working liquid  12  is effectively improved. 
   Also, according to this embodiment, the vertical size c of the second path portion  19  is smaller than the inner diameter d 1  of the first path portion  16 . Therefore, the working liquid  12  far from the inner wall surface of the second path portion  19  as well as in the neighborhood of the inner wall surface of the second path portion  19  can be effectively heated. As a result, the heat transfer rate from the heater  13  to the working liquid  12  is effectively improved in the second path portion  19 . 
   Further, in view of the fact that the vertical size c of the second path portion  19  is not larger than the heat penetration depth δ, the working liquid  12  far from the inner wall surface of the second path portion  19  as well as in the neighborhood of the inner wall surface of the second path portion  19  can be positively heated. As a result, the heat transfer rate from the heater  13  to the working liquid  12  is even more effectively improved in the second path portion  19 . 
   Also, according to this embodiment, the gas sealing portion  21  is heated by the heater  13  to at least the temperature of the second path portion  19 , i.e. at least the temperature of the vapor of the working liquid  12 . Therefore, the vapor of the working liquid  12 , heated and vaporized by the heater  13  and advancing into the gas sealing portion  21 , is prevented from being cooled and liquefied by the gas sealing portion  21 . 
   Fourth Embodiment 
   In the embodiments described above, the working liquid  12  is caused to collide with the inner wall surface of the heated portion  11   d  by changing the direction in which the working liquid  12  is displaced in the heated portion  11   d . According to the fourth embodiment, on the other hand, as shown in  FIG. 5 , a collision surface  23  is formed as a stepped inner wall surface of the heated portion  11   d , with which the working liquid  12  is caused to collide. 
     FIG. 5  is a diagram showing a general configuration of the power generating unit according to this embodiment. In this embodiment, the heated portion  11   d  is formed of a cylinder as a whole extending in parallel to the first straight portion  11   b  without being bent. 
   As shown in  FIG. 5 , the stepped collision surface  23  is formed on the inner wall surface of the heated portion  11   d . Specifically, the first inner wall surface portion  24  of the inner wall surface of the heated portion  11   d , which is far from the cooled portion  11   e , is projected inward of the heated portion  11   a  as compared with the second inner wall surface portion  25  near to the cooled portion  11   e.    
   An annular collision surface  23  facing the cooled portion  11   e  is formed between the first inner wall surface portion  24  and the second inner wall surface portion  25 . Also, the heated portion  11   d  is sealed with the gas  18  of a predetermined volume. 
   According to this embodiment, assume that the vapor of the working liquid  12  is cooled and liquefied by the cooler  14 , and the liquid level in the first straight portion  11   b  rises. Then, as shown by arrow f in  FIG. 5 , the working liquid  12  advances into the heated portion  11   d , and collides with the collision surface  23  of the heated portion  11   d.    
   Thus, the working liquid  12  in the heated portion  11   d  is agitated and a turbulence is generated. Thus, the thermal boundary layer in the neighborhood of the collision surface  23  can be destroyed. As a result, the heat transfer rate from the heater  13  to the working liquid  12  is improved. 
   The gas  18  may be, for example, air or a pure vapor of the working liquid  12 , as is in the embodiments described above. 
   OTHER EMBODIMENTS 
   (1) The second path portion  17 , though formed to extend in horizontal direction in the first and second embodiments described above, may alternatively be formed to extend in other than the horizontal direction. 
   (2) The angle between the direction in which the first path portion  16  extends and the direction in which the second path portion  17  extends, though set to 90 degrees in the first and second embodiments described above, may alternatively be set in the range between 15 degrees and 90 degrees inclusive. 
   (3) The first path portion  16  and the second path portion  17 , though formed as a cylinder in the first and second embodiments described above, may alternatively be formed as a rectangular tube, for example, other than a cylinder. 
   (4) The second path portion  19 , though formed to extend in horizontal direction in the third embodiment described above, may alternatively be formed in other than the horizontal direction. 
   (5) The angle between the direction in which the first path portion  16  extends and the direction in which the second path portion  17  extends, though set to 90 degrees in the third embodiment described above, may alternatively be set in the range between 15 and 90 degrees inclusive. 
   (6) Unlike in the third embodiment described above in which only one second path portion  19  is formed, a plurality of the second path portions  19  branching from the first path portion  16  may be formed. 
   (7) The heated portion  11   d  as a whole, though formed as a circular cylinder in the fourth embodiment described above, may alternatively be formed as other than a circular cylinder such as a rectangular cylinder. 
   (8) The heated portion  11   d , though formed as a straight tube in the fourth embodiment described above, may alternatively be formed as a bent tube. 
   (9) The gas sealing portion  21 , though communicating with the second path portion  19  in the third embodiment described above, may alternatively communicate with the first path portion  16 . 
   (10) The gas sealing portion  21 , though arranged at a position nearer to the end of the container  11  than the heated portion  11   d  in the third embodiment, may alternatively be arranged between the heated portion  11   d  and the power generator  1 . 
   (11) The gas  18 , though sealed in the heated portion  11   d  in the first, second and fourth embodiments described above, may alternatively be sealed in the gas sealing unit communicating with the heated portion  11   d.    
   (12) The heated portion  11   d , though arranged above the cooled portion  11   e  in the embodiments described above, may alternatively be arranged under the cooled portion  11   e.    
   (13) The heater  13  and the heated portion  11   d , though formed as separate members in the embodiments described above, may alternatively be formed integrally with each other. 
   (14) Although a high-temperature gas is used as a heat source of the heater  13 , an electric heater may be used as the heater  13 . 
   (15) Although an application of the invention to the drive source of the power generating unit is explained above, the external combustion engine according to the invention may also be used as a drive source of other than a power generating unit. 
   While the invention has been described by reference to specific embodiments chosen for purposes of illustration, it should be apparent that numerous modifications could be made thereto by those skilled in the art without departing from the basic concept and scope of the invention.