Patent Publication Number: US-2009217667-A1

Title: External combustion engine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an external combustion engine using evaporation and condensation of a working medium to cause displacement of a liquid part of the working medium and converting displacement of the liquid part of the working medium to mechanical energy for output. 
     2. Description of the Related Art 
     In the past, in this type of external combustion engine, also called a “liquid piston steam engine”, a pipe-shaped container is sealed with a working medium flowable in a liquid state, a heater arranged at one end of the container (heated part) is used to heat part of the liquid state working medium to cause it to evaporate, and a cooler arranged at the middle of the container (cooled part) is used to cool the steam of the working medium to cause it to condense. By this evaporation and condensation of this working medium, the liquid part of the working medium is cyclically made to displace (so-called “self-excited vibration”), then this self-excited vibration of the working medium is taken out at an output part communicated with the other end of the container. (For example, see Japanese Patent Publication (A) No. 2004-84523). 
     In this regard, Japanese Patent Application No. 2007-174065 (hereinafter referred to as the “prior application”) proposes applying a liquid piston steam engine to an electric power generation system mounted in a vehicle. In this prior application, the exhaust gas of the engine for driving the vehicle, in this case a water-cooled type internal combustion engine (E/G), is utilized as the heat source of the heater so as to heat the working medium. The waste heat of the water-cooled type internal combustion engine is therefore recovered and utilized to generate electric power in this electric power generation system. 
     Further, in this prior application, the cooling water of the water-cooled type internal combustion engine is circulated to the cooler of the liquid piston steam engine so as to combine the cooling water circulation circuit of the water-cooled type internal combustion engine and the cooling water circulation circuit of the liquid piston steam engine and thereby streamline the configuration. 
     According to this prior application, when the vehicle ignition switch (I/G) is turned off and the water-cooled type internal combustion engine stops, exhaust gas is no longer produced. For this reason, in the liquid piston steam engine, right after the water-cooled type internal combustion engine stops, the excess heat stored in the heater causes the heated part to become high in temperature, but after that the temperature of the heated part gradually falls so the working medium can no longer be sufficiently heated and the liquid piston steam engine stops operating. 
     However, in general, in the cooling water circulation circuit of a water-cooled type internal combustion engine, the water pump for circulating the cooling water is driven by the power of the water-cooled type internal combustion engine, so if the water-cooled type internal combustion engine stops, the water pump also stops so the circulation of the cooling water ends up stopping and the amount of heat radiated from the cooling water becomes extremely small. 
     Furthermore, as shown in  FIG. 7 , after the water-cooled type internal combustion engine stops, if the amount of heat transferred from the heater having the excess heat to the cooling water in the cooler is larger than the amount of heat radiated from the cooling water, the temperature of the cooling water inside the cooler ends up rising. 
     If, as a result, the temperature of the cooling water in the cooler ends up rising to the boiling point or more, the cooling water in the cooler will boil and therefore the internal pressure of the cooling water circulation circuit will end up abnormally rising. In the worst case, there is the problem that the various pipes and devices in the cooling water circulation circuit will break and leakage of cooling water will end up being caused. 
     Note that this problem is liable to similarly occur not only after the water-cooled type internal combustion engine stops, but also in rapid transition of the water-cooled type internal combustion engine from the high load state to the low load state. That is, the water pump is driven by the power of the water-cooled type internal combustion engine, so in rapid transition of the water-cooled type internal combustion engine from the high load state with a large amount of waste heat to the low load state where the amount of circulation of the cooling water is small, the amount of circulation of the cooling water becomes insufficient and the cooling water in the cooler ends up rising in temperature. 
     Further, this problem is liable to similarly occur not only when applying a liquid piston steam engine to an electric power generation system mounted in a vehicle, that is, when using the exhaust gas of a water-cooled type internal combustion engine (engine for driving a vehicle) as a heat source to heat and evaporate a working medium and using the cooling water cooling the water-cooled type internal combustion engine as a cooling source to cool and condense the steam of the working medium, but also when using the waste heat of various heat engines as a heat source to heat and evaporate a working medium and using the cooling fluid cooling the heat engine as a cooling source to cool and condense the steam of the working medium. 
     SUMMARY OF THE INVENTION 
     The present invention, in consideration of the above point, has as its object to suppress the boiling of the cooling water in the cooler. 
     To achieve the above object, in the aspect of the invention described in claim  1 , there is provided an external combustion engine provided with 
     a pipe-shaped container ( 11 ) in which a working medium is sealed flowably in a liquid state, 
     a heater ( 12 ) having a heated part ( 12   a ) communicating with one end of the container ( 11 ) and heating the working medium at the heated part ( 12   a ) to make it evaporate, 
     a cooler ( 13 ) arranged at a middle of the container ( 11 ) and cooling the steam of the working medium produced by the heater ( 12 ) to make it condense, and 
     an output part ( 14 ) communicated with the other end of the container ( 11 ) and converting the displacement of the liquid part of the working medium produced due to the change in volume of the working medium accompanying evaporation and condensation of the working medium to mechanical energy for output, 
     the heater ( 12 ) being designed to use waste heat of a heat engine ( 1 ) as a heat source to heat the working medium and make it evaporate, 
     the cooler ( 13 ) being designed to use a cooling fluid cooling the heat engine ( 1 ) as a cooling source to cool the steam of the working medium and make it condense, and 
     the engine provided with a heated part temperature reducing means for reducing the temperature (Th) of the heated part ( 12   a ) when the amount of heat radiated from the cooling fluid to the outside becomes smaller than the amount of heat transferred from the working medium to the cooling fluid. 
     Due to this, it is possible to suppress the rise of temperature (Tw) of the cooling fluid in the cooler ( 13 ), so it is possible to suppress boiling of the cooling water at the cooler ( 13 ). 
     In the aspect of the invention as set forth in claim  2 , there is provided an external combustion engine as set forth in claim  1 , wherein the heated part temperature reducing means has a pump means ( 21 ,  31 ) for circulating the cooling fluid to the cooler ( 13 ). 
     In the aspect of the invention as set forth in claim  3 , there is provided an external combustion engine as set forth in claim  2 , wherein the heated part temperature reducing means controls the pump means ( 21 ,  31 ) based on at least one of the amount of waste heat of the heat engine ( 1 ), the temperature (Th) of the heated part ( 12   a ), and the temperature (Tw) of the cooling fluid. 
     In the aspect of the invention as set forth in claim  4 , there is provided an external combustion engine as set forth in claim  2  or  3 , wherein the pump means ( 21 ) is driven by electric power. 
     In the aspect of the invention as set forth in claim  5 , there is provided an external combustion engine as set forth in claim  2 , wherein the pump means ( 21 ) is coupled with an output part ( 14 ) so as to be driven by output from the output part ( 14 ). 
     In the aspect of the invention as set forth in claim  6 , there is provided an external combustion engine as set forth in any one of claims  2  to  5 , wherein the length of the flow path of the cooling fluid becomes shorter by providing a bypass flow path ( 20 ) for circulating the cooling fluid bypassing the heat engine ( 1 ), and 
     the bypass flow path ( 20 ) is provided with a pump means ( 21 ) and a radiating means ( 22 ) for radiating the heat of the cooling fluid to the outside. 
     According to this, the length of the flow path of the cooling fluid becomes shorter, so the drive force of the pump means ( 21 ) can be reduced. 
     In the aspect of the invention as set forth in claim  7 , there is provided an external combustion engine as set forth in claim  6 , wherein the radiating means ( 22 ) is provided with a heat storing material for storing the heat discharged from the cooling fluid. According to this, the heat storing material makes it possible to effectively utilize the heat. 
     In the aspect of the invention as set forth in claim  8 , there is provided an external combustion engine as set forth in claim  1 , wherein the heated part temperature reducing means has cooling means ( 40 ,  41 ) for cooling the heater ( 12 ) from the outside. 
     In the aspect of the invention as set forth in claim  9 , there is provided an external combustion engine as set forth in claim  8 , wherein the cooling means ( 40 ) is a means for blowing air to the heater ( 12 ). 
     In the aspect of the invention as set forth in claim  10 , there is provided an external combustion engine as set forth in claim  8 , wherein the cooling means ( 41 ) is a means for spraying water to the heater ( 12 ). 
     In the aspect of the invention as set forth in claim  11 , there is provided an external combustion engine as set forth in any one of claims  1  to  10  wherein the heated part temperature reducing means has judging means (S 100 , S 300 ) for judging when the amount of heat radiated from the cooling fluid to the outside becomes smaller than the amount of heat transferred from the working medium to the cooling fluid based on at least the temperature (Th) of the heated part ( 12   a ). 
     In the aspect of the invention as set forth in claim  12 , there is provided an external combustion engine as set forth in claim  11 , wherein the judging means (S 100 , S 300 ) deems that the amount of heat radiated from the cooling fluid to the outside has become smaller than the amount of heat transferred from the working medium to the cooling fluid when the heat engine ( 1 ) has stopped and the temperature (Th) of the heated part ( 12   a ) exceeds the boiling point of the cooling fluid. 
     Note that “when the temperature (Th) of the heated part ( 12   a ) exceeds the boiling point of the cooling fluid” in the present invention does not mean only when the temperature (Th) of the heated part ( 12   a ) strictly exceeds the boiling point of the cooling fluid and means also when the temperature (Th) of the heated part ( 12   a ) exceeds a temperature near the boiling point of the cooling fluid. 
     In the aspect of the invention as set forth in claim  13 , there is provided an external combustion engine as set forth in claim  11  or  12 , wherein the judging means (S 300 ) calculates the temperature (Th) of the heated part ( 12   a ) based on the pressure inside the heated part ( 12   a ). 
     In the aspect of the invention as set forth in claim  14 , there is provided an external combustion engine as set forth in claim  11  or  12 , wherein the judging means (S 300 ) calculates the temperature (Th) of the heated part ( 12   a ) based on the radiant heat radiated from the heater ( 12 ). 
     Note that the reference numerals after the means described in this section and the claims show the correspondence with specific means described in the later explained embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects and features of the present invention will become clearer from the following description of the preferred embodiments given with reference to the attached drawings, wherein: 
         FIG. 1  is a schematic view of the overall configuration of a vehicle electric power generation system in a first embodiment of the present invention; 
         FIG. 2  is a flow chart showing an outline of the control processing executed by an electronic control unit of a first embodiment; 
         FIG. 3  is a flow chart showing an outline of the control processing executed by an electronic control unit at the time of a cooling mode of  FIG. 2 ; 
         FIG. 4  is a timing chart showing an example of control in the first embodiment; 
         FIG. 5  is a schematic view of the overall configuration of a vehicle electric power generation system in a second embodiment of the present invention; 
         FIG. 6  is a schematic view of the overall configuration of a vehicle electric power generation system in a third embodiment of the present invention; and 
         FIG. 7  is a graph showing the problem in the prior application. 
     
    
    
     In the figures,  1  indicates a water-cooled type internal combustion engine (heat engine),  11  a container,  12  a heater,  12   a  a heated part,  13  a cooler,  14  an output part,  20  a bypass flow path,  21  an auxiliary water pump (pump means), and  22  an auxiliary radiator (radiating means). 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     Below, a first embodiment of the present invention will be explained. This embodiment applies the external combustion engine according to the present invention (liquid piston steam engine) to an electric power generation system mounted in a vehicle.  FIG. 1  is a schematic view of the overall configuration of a vehicle electric power generation system according to the present embodiment. 
     First, a liquid piston steam engine  10  will be simply explained. The liquid piston steam engine  10  may be roughly divided into a pipe-shaped container  11  in which a working medium (in this example, water) is sealed flowably in a liquid state, a heater  12  heating the working medium to make it evaporate, a cooler  13  cooling the steam of the working medium evaporated by the heater  12  to make it condense, and an output part  14  converting the displacement of the liquid part of the working medium caused by the change in volume of the working medium accompanying evaporation and condensation of the working medium to mechanical energy for output. 
     In this embodiment, a plurality of containers  11  (in the example of  FIG. 1 , three) are arranged in parallel to form a so-called multi-cylinder type liquid piston steam engine, but it is also possible to provide just one container  11  to form a so-called single-cylinder type liquid piston steam engine. 
     The heater  12  is a heat exchanger exchanging heat with exhaust gas of the engine driving the vehicle, in this case a water-cooled type internal combustion engine (heat engine)  1 , and is arranged at one end of the container  11 . Inside the heater  12  is formed a space communicated with the container  11 . This space forms a heated part  12   a  using the exhaust gas of the water-cooled type internal combustion engine  1  as a heat source to heat the working medium. 
     The cooler  13  circulates cooling water cooling the water-cooled type internal combustion engine  1  (cooling fluid) through it so as to cool the steam of the working medium and is arranged so that the middle of the container  11  runs through it. Therefore, the middle of the container  11  forms a cooled part cooling the working medium. 
     The output part  14  converts the reciprocating displacement of the liquid part of the working medium (linear motion) to rotary motion and outputs it from an output shaft  14   a.  For example, it may be comprised by an expansion mechanism made of a piston, crankshaft, etc. Further, a linear motor may also be used to switch to linear motion. 
     The output shaft  14   a  of the output part  14  has a motor generator  15  coupled with it. This motor generator  15  can generate electric power by the output from the output part  14  and charge a storage battery  16  when the liquid piston steam engine  10  is operating and can receive electric power from the storage battery  16  and drive the output shaft  14   a  of the output part  14  when the liquid piston steam engine  10  has stopped. 
     For example, when the liquid piston steam engine  10  is started up, the motor generator  15  functions as a starter for starting the liquid piston steam engine  10 . In this embodiment, an existing battery (not shown) mounted in the vehicle is used as the storage battery  16 , but a specialized storage battery separate from the existing battery may also be used. 
     Next, the circuit for circulating the cooling water cooling the water-cooled type internal combustion engine  1  (hereinafter referred to as a “cooling water circulation circuit”) will be explained. The cooling water circulation circuit may be roughly divided into a radiator circuit  2  radiating the heat of the cooling water flowing out from the water-cooled type internal combustion engine  1  to the outside (outside air) and a heater circuit  3  heating the air-conditioning air inside the passenger compartment by the heat of the cooling water flowing out from the water-cooled type internal combustion engine  1 . 
     The radiator circuit  2  has a radiator  4  cooling the cooling water by heat exchange between the cooling water and the outside air. An electric blower fan  5  blowing air to the radiator  4  is driven by the electric power supplied from the above-mentioned battery. 
     In the heater circuit  3 , a heater core  6  of a vehicle air-conditioning system is arranged. This heater core  6  is a heating use heat exchanger heating the air-conditioning air inside the passenger compartment by heat exchange between the cooling water and the air-conditioning air inside the passenger compartment. The heater circuit  3  merges with the radiator circuit  2  at the cooling water inlet side of the water-cooled type internal combustion engine  1 . 
     In the cooling water inlet side of the water-cooled type internal combustion engine  1 , a water pump  7  for circulating the cooling water to the radiator circuit  2  and heater circuit  3  is arranged. This water pump  7  is driven by the power of the water-cooled type internal combustion engine  1 . 
     At the upstream side from the part merging with the heater circuit  3  in the radiator circuit  2 , a thermostat  8  regulating the ratio of the flow rate of the cooling water circulating through the radiator circuit  2  and the flow rate of the cooling water circulating through the heater circuit  3  is arranged. 
     In the heater circuit  3 , a cooler  13  of the liquid piston steam engine  10  is arranged between the water-cooled type internal combustion engine  1  and the heater core  6 . Further, in the heater circuit  3 , a bypass flow path  20  is provided between the cooling water inlet side and cooling water outlet side of the cooler  13 . 
     This bypass flow path  20  is a bypass for making the cooling water flowing out from a cooling water outlet part of the cooler  13  bypass the water-cooled type internal combustion engine  1  and heater core  6  etc. and flow toward a cooling water inlet part of the cooler  13  and is provided so as to shorten the length of the flow path of the cooling water in the heater circuit  3 . 
     In the bypass flow path  20 , an auxiliary water pump (pump means)  21  circulating cooling water to the bypass flow path  20  and an auxiliary radiator (radiating means)  22  cooling the cooling water flowing through the bypass flow path  20  by heat exchange between the cooling water and the outside air are arranged. 
     The auxiliary water pump  21  is an electric water pump driven by electric power supplied from the storage battery  16 . The electric blower fan  23  blowing air to the auxiliary radiator  22  is also driven by the electric power supplied from the storage battery  16 . The auxiliary water pump  21  and the electric blower fan  23  of the auxiliary radiator  22  are controlled by a not shown electronic control unit. 
     The details will be explained later, but these bypass flow path  20 , auxiliary water pump  21 , auxiliary radiator  22 , electric blower fan  23 , and electronic control unit configure a heated part temperature reducing means reducing the temperature Th of the heated part  12   a  (hereinafter referred to as the “heated part temperature”) when the amount of heat radiated from the cooling water to the outside (the outside air) becomes smaller than the amount of heat transferred from the working medium to the cooling water. Note that the specific configuration of the heated part temperature reducing means is not limited to this and can be modified in various ways as explained later. 
     In the heater circuit  3 , electric type shutoff valves  24  to  27  opening and closing the cooling water flow path are arranged. Specifically, these are arranged between a branch part  3   a  of the bypass flow path  20  and the heater core  6 , between a merged part  3   b  of the bypass flow path  20  and the water-cooled type internal combustion engine  1 , at the inlet part of the bypass flow path  20 , and at an outlet part of the bypass flow path  20 . 
     The operations of these shutoff valves  24  to  27  are also controlled by the electronic control unit. In this embodiment, the above-mentioned water pump  7  and electric blower fan  5  of the radiator  4  are controlled by the electronic control unit. 
     The electronic control unit is comprised of a known microcomputer including a CPU, ROM, RAM, etc. and its peripheral circuits and performs various computations and processing based on a control program stored in the ROM to control the operations of the various equipment connected to the output side. 
     At the input side of this electronic control unit, detection signals from a heated part temperature sensor  28  detecting the heated part temperature Th, a cooling water temperature sensor  29  detecting the cooling water temperature Tw at the cooling water outlet side of the water-cooled type internal combustion engine  1 , and a cooling water flow rate sensor  30  detecting the cooling water flow rate Fw at the cooling water outlet side of the water-cooled type internal combustion engine  1  are input. 
     The heated part temperature sensor  28  is arranged at the heated part  12   a.  The cooling water temperature sensor  29  and cooling water flow rate sensor  30  are arranged in the heater circuit  3  at the cooling water outlet side of the water-cooled type internal combustion engine  1 . 
     It is possible to use a thermocouple as the heated part temperature sensor  28 . Instead of using the heated part temperature sensor  28  to detect the heated part temperature Th, it is also possible to provide a pressure sensor for detecting the internal pressure of the heated part  12   a  and have the electronic control unit calculate the heated part temperature Th based on the internal pressure of the heated part  12   a.  Further, it is also possible to provide a radiant heat sensor detecting the radiant heat radiated from the heater  12  and have the electronic control unit calculate the heated part temperature Th based on the radiant heat radiated from the heater  12 . 
     Next, the operation of the present embodiment in the above configuration will be explained. The operation of the vehicle electric power generation system may be roughly divided into the normal operation mode performed at the time of operation of the water-cooled type internal combustion engine  1  and the cooling mode performed after the water-cooled type internal combustion engine  1  stops. 
     The normal operation mode recovers the waste heat of the exhaust gas of the water-cooled type internal combustion engine  1  and generates electric power. The first and second cooling modes cool the liquid piston steam engine  10  after the water-cooled type internal combustion engine  1  stops. 
     In the normal operation mode, in the liquid piston steam engine  10 , the heater  12  uses the exhaust gas of the water-cooled type internal combustion engine  1  as a heat source to heat and evaporate the working medium. Due to this, high temperature, high pressure steam of the working medium is stored at one end of the container  11  and the liquid part of the working medium is pushed out and displaced to the other end of the container  11  (output part  14  side). 
     Here, at the time of operation of the water-cooled type internal combustion engine  1 , the water pump  7  is driven and cooling water is circulated in the heater circuit  3 . For this reason, when the steam of the working medium  14  stored at one end of the container  11  reaches the middle of the container  11  (location where cooler  13  is arranged), the steam of the working medium is cooled and condensed by the cooling water circulating through the cooler  13 . Due to this, the liquid part of the working medium pushed out by the steam of the working medium to the output part  14  is pushed back to the heater  12  side. 
     This operation is repeatedly performed until stopping the operations of the heater  12  and cooler  13 . During that time, the working medium inside the container  11  cyclically displaces (so-called self-excited vibration). The self-excited vibration of the working medium (linear motion) is converted at the output part  14  to rotary motion of the output shaft  14   a  and output. Due to this, electric power is generated at the motor generator  15  and the storage battery  16  is charged. 
     The cooling mode may further be roughly divided into a first cooling mode and a second cooling mode. In the first cooling mode, the excess heat stored in the heater  12  is used to operate the liquid piston steam engine  10  and take out output from the output part  14 . This output is used by the motor generator  15  to generate electric power. At this time, the cooling water is made to flow through only the bypass flow path  20  and is prevented from flowing to the water-cooled type internal combustion engine  1  and heater core  6  side by the electronic control unit operating the shutoff valves  24  to  27 . 
     Furthermore, the electric power generated by the motor generator  15  is used to drive the auxiliary water pump  21  to circulate cooling water in the cooler  13 . At the same time, the electric blower fan  23  of the auxiliary radiator  22  is also driven by the electric power generated by the motor generator  15 . Due to this, the cooler  13  cools the working medium, the excess heat of the heater  12  is gradually robbed, and the heated part temperature Th gradually falls. 
     In the second cooling mode, the electric power supplied from the storage battery  16  is used to drive the motor generator  15  whereby the output shaft  14   a  of the output part  14  is driven to forcibly displace the working medium in the container  11 . At the same time as this, the auxiliary water pump  21  and the electric blower fan  23  of the auxiliary radiator  22  are also driven by the electric power supplied from the storage battery  16 . 
     Due to this, the cooler  13  cools the working medium and the cooled working medium enters the heated part  12   a  and cools the heater  12 , so the excess heat of the heater  12  is further robbed and the heated part temperature Th further falls. 
     The switching control of the normal operation mode and first and second cooling modes will be explained next based on  FIG. 2  and  FIG. 3 .  FIG. 2  is a flow chart showing the outlines of the control processing executed by the electronic control unit. This control processing is started when a not shown vehicle start switch (ignition switch) is turned on. 
     First, at step S 100 , it is judged if the water-cooled type internal combustion engine  1  is operating. Note that step S 100  and the later explained step S 300  correspond to the judging means in the present invention. 
     When it is judged at step S 100  that the water-cooled type internal combustion engine  1  is operating (ON), the routine proceeds to step S 200  where the above-mentioned normal operation mode is executed. When it is judged at step S 100  that the water-cooled type internal combustion engine  1  has stopped (OFF), the routine proceeds to step S 300  where it is judged if the heated part temperature Th exceeds a first predetermined temperature Tb. 
     Here, as the first predetermined temperature Tb, for example the boiling point of the cooling water or a temperature near the boiling point may be set. As the temperature near the boiling point of the cooling water, the temperature when the internal pressure of the cooling water circulation circuit becomes the upper limit pressure in the pressure resistant performance may be set. 
     When it is judged at step S 300  that the heated part temperature Th exceeds the first predetermined temperature Tb, the routine proceeds to step S 400  where the above-mentioned cooling mode is shifted to. When it is judged at step S 300  that the heated part temperature Th is the first predetermined temperature Tb or less, the control processing is ended without performing the cooling mode. 
       FIG. 3  is a flow chart showing the outline of the control processing executed by the electronic control unit at the time of the cooling mode. In the cooling mode, first, at step S 410 , it is judged if the heated part temperature Th exceeds a second predetermined temperature To. Here, as the second predetermined temperature To, the lower limit temperature at which output can be taken out from the liquid piston steam engine  10  is set. 
     That is, the liquid piston steam engine  10  can no longer maintain the self-excited vibration of the working medium and can no longer take out output when the heated part temperature Th becomes less than a certain temperature. Therefore, the lower limit of the heated part temperature Th enabling the self-excited vibration of the working medium to be maintained and output to be taken out is set as the second predetermined temperature To. Note that the second predetermined temperature To is a temperature higher than the first predetermined temperature Tb (To&gt;Tb). 
     When it is judged at step S 410  that the heated part temperature Th exceeds the second predetermined temperature To, the routine proceeds to step S 420  where the above-mentioned first cooling mode is performed. The first cooling mode is performed until it is judged at step S 410  that the heated part temperature Th is the second predetermined temperature To or less. 
     When it is judged at step S 410  that the heated part temperature Th is the second predetermined temperature To or less, the routine proceeds to step S 430  where the above-mentioned second cooling mode is shifted to. The second cooling mode is performed until it is judged at step S 440  that the heated part temperature Th is the first predetermined temperature Tb or less. 
     Furthermore, when it is judged at step S 440  that the heated part temperature Th is the first predetermined temperature Tb or less, the second cooling mode is ended and the cooling mode itself is ended. 
       FIG. 4  is a timing chart showing an example of the control in the present embodiment. Note that the amount of heat radiated in  FIG. 4  means the amount of heat recovered from the heater  12  and radiated to the outside (the outside air). Further, the cooling water temperature in  FIG. 4  means the temperature of the cooling water in the cooler  13 . 
     As will be understood from  FIG. 4 , according to the present embodiment, when the water-cooled type internal combustion engine  1  has stopped and the heated part temperature Th exceeds the first predetermined temperature Tb (for example, the boiling point of the cooling water), the first and second cooling modes are performed, so even if the water-cooled type internal combustion engine  1  stops, cooling water continues to be circulated to the cooler  13  of the liquid piston steam engine  10  until the heated part temperature Th falls to the first predetermined temperature Tb or less. 
     For this reason, after the water-cooled type internal combustion engine  1  stops, the heater  12  with the excess heat is quickly cooled and the heated part temperature Th quickly falls. As a result, after the water-cooled type internal combustion engine  1  stops, it is possible to keep the excess heat stored in the heater  12  of the liquid piston steam engine  10  from causing the temperature of the cooling water in the cooler  13  to rise. 
     That is, the present embodiment deems that the amount of heat radiated from the cooling water to the outside (the outside air) has become smaller than the amount of heat transferred from the working medium to the cooling water and reduces the temperature Th of the heated part  12   a  by the above-mentioned heated part temperature reducing means when the water-cooled type internal combustion engine  1  has stopped and the heated part temperature Th exceeds the first predetermined temperature Tb (for example, the boiling point of cooling water). 
     For this reason, it is possible to keep the temperature of the cooling water in the cooler  13  from rising to the boiling point or more and thereby the cooling water from ending up boiling. As a result, it is possible to prevent the problem of the boiling of the cooling water from causing the internal pressure of the cooling water circulation circuit to abnormally rise, the various pipes and devices of the cooling water circulation circuit to break, and cooling water to leak. 
     Further, in the first cooling mode, the excess heat of the heater  12  is used to take out output from the liquid piston steam engine  10 , so all or part of the drive power of the auxiliary water pump  21  and electric blower fan  23  of the auxiliary radiator  22  in the first and second cooling modes can be met by the output from the liquid piston steam engine  10 . For this reason, in the first and second cooling modes, it is possible to reduce the amount of electric power supplied from the storage battery  16  and possible to save energy. 
     Further, in the first and second cooling modes, the cooling water bypasses the water-cooled type internal combustion engine  1  and heater core  6  etc. and flows through the bypass flow path  20 , so the length of the flow path of the cooling water is shortened, so it is possible to reduce the drive power of the auxiliary water pump  21  and possible to further save energy. 
     Further, as shown in  FIG. 4 , in the first and second cooling modes, if reducing the drive power of the auxiliary water pump  21  along with the fall in the heated part temperature Th, it is possible to further save energy. This is true not only for the drive power of the auxiliary water pump  21 , but also the drive power of the electric blower fan  23  of the auxiliary radiator  22 . 
     Note that in the present embodiment, in the first cooling mode, the output taken out from the output part  14  is used by the motor generator  15  to generate electric power, but as a modification, it is also possible not to generate electric power by the motor generator  15  even if taking out output from the output part  14  in the first cooling mode. 
     That is, when not generating electric power by the motor generator  15  despite taking out output from the output part  14 , the output part  14  becomes load-less, so the operating frequency of the liquid piston steam engine  10  rises. 
     For this reason, in this modification, it is possible to raise the operating frequency of the liquid piston steam engine  10  in the first cooling mode, so the amount of heat robbed from the heated part  12   a  by the working medium is increased, the cooling of the working medium in the cooler  13  is promoted, and the heated part temperature Th can be reduced in a short time. As a result, the time during which the cooling mode is performed can be shortened. 
     Second Embodiment 
     In the above first embodiment, the auxiliary water pump  21  and the electric blower fan  23  of the auxiliary radiator  22  are driven by the electric power supplied from the storage battery  16 , but in the second embodiment, as shown in  FIG. 5 , the auxiliary water pump  31  and the electric blower fan  32  of the auxiliary radiator  22  are coupled with the output shaft  14   a  of the output part  14 . 
     Due to this, the auxiliary water pump  31  and the electric blower fan  32  of the auxiliary radiator  22  are directly driven by the output from the output part  14 . 
     In the present embodiment as well, advantageous effects similar to the first embodiment can be obtained. 
     Third Embodiment 
     In the above embodiments, after the water-cooled type internal combustion engine  1  stops, cooling water is circulated through the cooler  13  to suppress boiling of the cooling water at the cooler  13 , but in the third embodiment, as shown in  FIG. 6 , after the water-cooled type internal combustion engine  1  stops, the heater  12  is cooled from the outside to suppress boiling of the cooling water at the cooler  13 . 
       FIG. 6  is a schematic view of the configuration of a vehicle electric power generation system in the present embodiment. The heater  12  of the liquid piston steam engine  10  is arranged in the middle of an exhaust pipe  40  through which the exhaust gas of the water-cooled type internal combustion engine  1  flows. 
     In this embodiment, as the cooling means for cooling the heater  12  from the outside, an air duct (not shown) introducing air blown by the electric blower fan  5  of the radiator  4  to the heater  12 , an electric blower fan  40  directly blowing air to the heater  12 , and a water spray mechanism  41  spraying water on the heater  12  are provided. 
     In this embodiment, the electric blower fan  40  is designed to be driven by the electric power supplied from the storage battery  16 . The water spray mechanism  82  has a not shown water tank and an injector  41   a  injecting water inside the water tank toward the heater  12 . The electric blower fan  40  and injector  41   a  are controlled by the above-mentioned electronic control unit. 
     The cooling of the heater  12  by these means is started when the water-cooled type internal combustion engine  1  stops and the heated part temperature Th exceeds a first predetermined temperature Tb and is stopped when the heated part temperature Th falls to the first predetermined temperature Tb or more. Due to this, it is possible to obtain advantageous effects similar to the above first embodiment. 
     As will be understood from the above explanation, in the present embodiment, the above-mentioned heated part temperature reducing means is comprised by a cooling means for cooling the heater  12  from the outside. Note that as the cooling means for cooling the heater  12  from the outside, it is not necessarily required to provide all of the above-mentioned air duct, electric blower fan  40 , and water spray mechanism  41 . It is sufficient to provide even one of these. 
     Other Embodiments 
     Note that in the above first and second embodiments, an electric blower fan  23  for blowing air to the auxiliary radiator  22  is arranged, but the electric blower fan  23  may also be eliminated. In this case, a heat storing material may be arranged at the auxiliary radiator  22 . Due to this, it is possible to avoid a drop in the heat radiating performance accompanying elimination of the electric blower fan  23  and possible to effectively utilize the heat recovered by the cooling water. 
     Further, in the above embodiments, an auxiliary water pump  21  and an auxiliary radiator  22  are arranged in the bypass flow path  20 , but it is also possible to eliminate the bypass flow path  20  and arrange the auxiliary water pump  21  and auxiliary radiator  22  in the main circuit of the heater circuit  3 . 
     Further, in the above embodiments, the cooler  13  of the liquid piston steam engine  10  is arranged in the heater circuit  3 , but it is also possible to arrange the cooler  13  of the liquid piston steam engine  10  in the radiator circuit  2 . In this case, in the first and second cooling modes, heat can be radiated from the cooling water by the radiator  4  of the heater circuit  3 , so the auxiliary radiator  22  can be eliminated. 
     Further, in the above embodiments, the heated part temperature Th is reduced to suppress boiling of the cooling water at the cooler  13  only after the water-cooled type internal combustion engine  1  is stopped, but in the same way as the above embodiments, it is also possible to reduce the heated part temperature Th to suppress boiling of the cooling water at the cooler  13  when the water-cooled type internal combustion engine  1  rapidly transits from a high load state to a low load state. 
     That is, in the above embodiments, the water pump  7  is driven by power of the water-cooled type internal combustion engine  1 , so in rapid transition of the water-cooled type internal combustion engine  1  from the high load state with a large amount of waste heat to a low load state with little amount of circulation of the cooling water, the amount of circulation of the cooling water is liable to become insufficient and the cooling water is liable to end up boiling at the cooler  13 . 
     Further, in the above embodiments, the water pump  7  was driven by power of the water-cooled type internal combustion engine  1 , but if using a water pump  7  comprised of an electric water pump and using electric power supplied from the storage battery  16  to drive this water pump  7  after the water-cooled type internal combustion engine  1  stops, it is possible to suppress boiling of the cooling water at the cooler  13  in the same way as the above embodiments. 
     In this case, it is possible to eliminate the bypass flow path  20 , auxiliary water pump  21 , auxiliary radiator  22 , electric blower fan  40 , water spray mechanism  41 , etc. in the above embodiments. 
     Further, in the above embodiments, the external combustion engine was configured to use the exhaust gas of an engine driving a vehicle, in this case a water-cooled type internal combustion engine  1 , as the heat source to heat and evaporate the working medium and to use the cooling water of the water-cooled type internal combustion engine  1  as the cooling source to cool and condense the steam of the working medium, but the invention is not limited to this. The external combustion engine may also be configured to use the waste heat of various heat engines as the heat source to heat and evaporate the working medium and use the various cooling fluids cooling the heat engines (for example oil etc.) as the cooling source to cool and condense the steam of the working medium. 
     Further, the above embodiments only show examples of the configuration of the liquid piston steam engine  10 . For example, it is of course possible to change the configuration of the liquid piston steam engine  10  as shown in Japanese Patent Publication (A) No. 2004-84523. 
     While the invention has been described with reference to specific embodiments chosen for purpose 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.