Abstract:
Gaseous-phase portion of a condenser contains vapor and a non-condensing gas, such as air, that impedes condensation of the vapor, and a non-condensing gas discharge device of the condenser is arranged to discharge only the non-condensing gas from the condenser. The non-condensing gas discharge device includes a valve device, in the form of an air vent, for separating the non-condensing gas from the vapor and selectively discharging only the non-condensing gas from the condenser.

Description:
This Non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No(s). 2003-344492 and 2003-359850 filed in Japan on Oct. 2, 2003 and Oct. 20, 2003, respectively, the entire contents of which are hereby incorporated by reference. 
   FIELD OF THE INVENTION 
   The present invention relates generally to non-condensing (or non-condensable) discharge devices of condensers. More particularly, the present invention relates to an improved non-condensing gas discharge device of a condenser in a Rankine cycle apparatus which is used, for example, as a vehicle-mounted apparatus for converting exhaust heat energy of a vehicle-mounted engine into mechanical energy. 
   BACKGROUND OF THE INVENTION 
   Rankine cycle apparatus have been known as systems for converting heat energy into mechanical work. The Rankine cycle apparatus include a structure for circulating water as a working medium, in the liquid- and gaseous-phase states within a sealed piping system forming a circulation system in the apparatus. Generally, the Rankine cycle apparatus include a water supplying pump unit, an evaporator, an expander, a condenser, and pipes connecting between these components to provide circulation circuitry. 
     FIG. 17  hereof is a schematic block diagram of a general setup of a conventionally-known Rankine cycle apparatus (e.g., vehicle-mounted Rankine cycle apparatus) and certain details of a condenser employed in the Rankine cycle apparatus. The Rankine cycle apparatus of  FIG. 17  includes a water supplying pump unit  110 , an evaporator  111 , an expander  107 , and the condenser  100 . These components  110 ,  111 ,  107  and  100  are connected via pipes  108  and  115 , to provide circulation circuitry in the apparatus. 
   Water (liquid-phase working medium), which is supplied, a predetermined amount per minute, by the water supplying pump unit  110  via the pipe  115 , is imparted with heat by the evaporator  111  to turn into water vapor (gaseous-phase working medium). The vapor is delivered through the next pipe  115  to the expander  107  that expands the water vapor. Mechanical device (not shown) is driven through the vapor expansion by the expander  107  so as to perform desired mechanical work. 
   Then, the expanded water vapor is delivered through the pipe  108  to the condenser  100 , where the vapor is converted from the vapor phase back to the water phase. After that, the water is returned through the pipe  115  to the water supplying pump unit  110 , from which the water is supplied again for repetition of the above actions. The evaporator  111  is constructed to receive heat from an exhaust pipe extending from the exhaust port of the engine of the vehicle. Among various literatures and documents showing structural examples of the Rankine cycle apparatus is Japanese Patent Application Laid-open Publication No. 2002-115504. 
   The following paragraphs detail a structure and behavior of the condenser  100  in the conventional vehicle-mounted Rankine cycle apparatus, with reference to  FIGS. 17 to 19 . 
   The condenser  100  includes a vapor introducing chamber  101 , a water collecting chamber  102 , and a multiplicity of cooling pipes  103  vertically interconnecting the two chambers  101  and  102 . In  FIG. 17 , only one of the cooling pipes  103  is shown in an exaggerative manner. Substantial upper half of the interior of each of the cooling pipes  103  is a vapor (gaseous-phase) portion  104  (i.e., portion occupied with the vapor  104 ), while a substantial lower half of the interior of the cooling pipe  103  is a water (liquid-phase) portion  105  (i.e., portion occupied with the water  105 ). In the vapor (gaseous-phase) portion  104 , most of the working medium introduced via the vapor introducing chamber  101  to the cooling pipe  103  is in the gaseous phase, while, in the water portion  105 , most of the working medium flowing through the cooling pipe  103  is kept in the liquid (condensed water) phase. Boundary between the vapor  104  and the water  105  (i.e., gas-liquid interface) is a liquid level position  112 . 
   One cooling fan  106  is disposed behind the cooling pipes  103  (to the right of the cooling pipes  103  in  FIG. 17 ). The cooling fan  106  is surrounded by a cylindrical shroud  106   a . Normally, operation of the cooling fan  106  is controlled by an electronic control unit on the basis of a water temperature at an outlet port of the condenser  100 . The single cooling fan  106  sends air to the entire region, from top to bottom, of all of the cooling pipes  103  to simultaneously cool the cooling pipes  103 . 
   The condenser  100  operates as follows during operation of the Rankine cycle apparatus. Water vapor of a relatively low temperature, discharged from the expander  107  with a reduced temperature and pressure, is sent into the vapor introducing chamber  101  of the condenser  100  via the low-pressure vapor pipe  108  and then directed into the cooling pipes  103 . Cooling air  109  drawn into the cooling fan  106  is sent to the condenser  100 . 
   Strong cooling air is applied by the cooling fan  106  to the upstream vapor portion  104  of the condenser  100 , i.e. a portion of each of the cooling pipes  103  where a mixture of the vapor and water exists, and thus latent heat emitted when the vapor liquefies can be recovered effectively by the cooling air. Cooling air is also applied by the cooling fan  106  to the downstream water portion  105  of the condenser  100 , i.e. a portion of each of the cooling pipes  103  where substantially only the water exists. Water condensed within the cooling pipes  103  of the condenser  100 , is collected into the water collecting chamber  102  and then supplied by the water supplying pump unit  110  to the evaporator  111  in a pressurized condition as noted above. 
   In  FIG. 17 , reference numeral  116  represents a surface area of a condensing heat transmission portion, and  117  represents a surface area of a heat transmission portion of the condensed water. The surface areas  116  and  117  of the heat transmission portions and the liquid level position  112  have the following relationship. 
   The conventional Rankine cycle apparatus  100  inherently has the characteristic that the liquid fluid position  112  varies. Namely, because the engine output varies in response to traveling start/stop and transient traveling velocity variation of the vehicle, the amount of water supply to the evaporator  111  also varies, in response to which the liquid level position  112  within the condenser  100  varies. Namely, in the condenser  100 , the liquid level position  112  rises when the amount of the vapor flowing into the condenser  100  (i.e., inflow amount of the vapor) is greater than the amount of the condensed water discharged from the condenser  100  (i.e., discharge amount of the condensed water), but lowers when the inflow amount of the vapor is smaller than the discharge amount of the condensed water. In this way, the vapor-occupied portion  104  in the cooling pipes  103  of the condenser  100  increases or decreases. Because the condensed water (in the portion  105 ) is discharged from the water supplying pump unit  110  subjected to predetermined flow rate control, a pressure from an outlet port  113  of the expander  107  to an inlet port  114  of the water supplying pump unit  110  is determined by a pressure within the condenser  100 . The pressure within the condenser  100  is determined by an amount of condensing heat exchange caused by cooling of the vapor portion  104  of the condenser, and the amount of condensing heat exchange is determined by a flow rate of the medium to be cooled and a surface area of the condensing heat transmission portion  116 . Thus, if the portion occupied with the vapor increases or decreases due to variation (rise or fall) of the liquid level position  112 , the surface area  116  of the condensing heat transmission portion increases or decreases and so the pressure within the condenser  100  and the flow rate of the medium to be cooled do not uniformly correspond to each other any longer. 
   Similarly, the temperature of the condensed water at the outlet port of the condenser  100  is determined by an amount of heat exchange caused by cooling of the water portion  105  of the condenser, and the amount of the heat exchange of the condensed water is determined by the flow rate of the medium to be cooled and a surface area  117  of a heat transmission portion of the condensed water. Thus, if the portion occupied with the condensed water  105  increases or decreases due to variation (rise or fall) of the liquid level position  112 , the surface area  117  of the heat transmission of the condensed water portion increases or decreases and so the temperature of the condensed water and the flow rate of the medium to be cooled do not uniformly correspond to each other any longer. 
   In the Rankine cycle apparatus where water is used as the working medium, the saturation pressure, at an atmospheric temperature, of the water within the circulation system is lower than the atmospheric pressure, and so the interior of the circulation system would assume a negative pressure after deactivation of the Rankine cycle apparatus as the entire apparatus is cooled. Thus, a non-condensing (i.e., non-condensable) gas, such as air, would enter the interior of the circulation system through sealed portions of various components and joints between the pipes. Further, where the working medium used has a saturation pressure at an atmospheric temperature greater than the atmospheric pressure, and if the working medium is contained in the circulation system in poor filling condition, the non-condensing gas, such as air, would remain within the circulation system of the apparatus. 
   If the Rankine cycle apparatus is operated with the non-condensing gas present or contained within the circulation system of the Rankine cycle apparatus, the non-condensing gas would enter the condenser  100  along with a flow of vapor. In such a case, the vapor  104  having entered the condenser  100  condenses within the condenser  100  and is discharged as condensed water  105 , as illustrated in  FIG. 18 . On the other hand, the non-condensing gas  121 , having flown into the condenser  100 , would build up or accumulate within the condenser  100  due to its con-condensable characteristic. Because the flow of the vapor  104  from the expander  107  to the condenser  100  is present in an upstream region of the condenser  100 , the non-condensing gas  121  is carried, by the flow of the vapor  104 , to a lower area of the vapor portion  104  within the condenser  100 . In other words, the circulation system is formed systematically, in the Rankine cycle apparatus, by the flows of the water and vapor as illustrated in  FIG. 17 , and the non-condensing gas too flows into the circulation system in accordance with the flow of the vapor  104  through the pipe  108  extending from the expander  107  to the condenser  100 . 
   The condensable vapor  104  condenses by the condensing operation of the condenser  100  and is discharged from the condenser  100  as condensed water  105 . The non-condensing gas, on the other hand, does not condense and would therefore remain within the condenser  100  in the gaseous-phase state while being subjected to the vapor flow. As a consequence, the non-condensing gas would remain in the lower area of the vapor portion  104  within the condenser  100  as denoted at  121  in  FIGS. 18 and 19 . 
   Further, because the interior of the condenser  100  is placed in conditions such that the air density is greater than the vapor density, air would accumulate in the lower area of the vapor portion  104  due to the action of gravity. Actually, in the boundary between the gaseous-phase portion  118  (corresponding to the vapor portion  104 ) and the liquid-phase portion  119  (corresponding to the water portion  105 ), there would be produced water and condensate liquid membrane  105   a  as illustrated in  FIG. 19 . The non-condensing gas  121  is surrounded by the water  105  and condensate liquid membrane  105   a  and pressed in an upstream-to-downstream direction by the flow of saturated vapor  104 . As a consequence, the non-condensing gas (i.e., air)  121  having a greater density than the vapor  104  would be accumulated in the lower area of the gaseous-phase portion (i.e., condensing heat trans-mission portion) within the condenser  100 . Thus, in the lower area of the gaseous-phase portion within the condenser  100 , as illustrated in  FIGS. 18 and 19 , the non-condensing gas  121  would become a resistance to impede passage of the saturated vapor  104  supplied from upstream, and so there would be formed an area  122  where the saturated vapor  104  can never reach or can only reach with difficulty. In the area  122 , no heat exchange can be effected, so that the heat transmission area  116  for the vapor  104  to condense would decrease. As a consequence, the operating efficiency of the condenser  104  would decline significantly. 
   Therefore, a particular mechanism is required to discharge the non-condensing gas  121  accumulated within the condenser  100 . Japanese Utility Model Publication No. SHO-63-47751 discloses a heat exchange apparatus for an automotive vehicle engine, which is designed to reduce a temperature difference between upwind and downwind portions of cooling air of the heat exchanger and control opening/closing of an electronic magnetic valve, provided in a tank beneath the heat exchanger, to discharge the non-condensing gas when the working medium has reached a high temperature. However, in the disclosed heat exchange apparatus, the opening/closing of the electronic magnetic valve is controlled on the basis of the temperature condition alone. Therefore, even vapor that can not be differentiated on the basis of the temperature condition would be undesirably discharged, and thus it was difficult to selectively discharge only the non-condensing gas accumulated in the lower are of the gaseous-phase portion  116 . 
   For the foregoing reasons, there has been a great demand for an improved non-condensing gas discharge device of a condenser which can reliably separate the non-condensing gas, remaining within the condenser and impeding condensation of the vapor, from the vapor and thus selectively discharge only the non-condensing gas so that the gaseous-phase portion of the condenser is filled only with the vapor, to thereby achieve an enhanced condensing efficiency and permit efficient heat exchange on the entire heat transmitting surface of the gaseous-phase portion. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the present invention, there is provided a non-condensing gas discharge device of a condenser, which comprises: a gaseous-phase portion containing a condensable gaseous-phase working medium and a non-condensing gas; a liquid-phase portion containing a liquid-phase working medium; a cooling section for cooling the working media to convert the gaseous-phase working medium back to the liquid phase; and a valve device, operatively connected with (or provided in correspondence with) a part of the gaseous-phase portion near a liquid level of the liquid-phase working medium along a boundary between the gaseous-phase portion and the liquid-phase portion, for separating the non-condensing gas from the gaseous-phase working medium and thereby discharging only the non-condensing gas from the condenser. 
   The non-condensing gas discharge device of the present invention separates the non-condensing gas (such as air), remaining within the condenser, from the gaseous-phase working medium (such as vapor) and thus selectively discharges only the separated non-condensing gas from the condenser. Thus, the non-condensing gas discharge device allows the gaseous-phase portion to be filled with the vapor and thereby allows a high condensing efficiency of the condenser to be maintained and even enhanced effectively utilizing the heat transmission area of the heat transmission portion. Namely, by means of the valve device, the non-condensing gas discharge device of the present invention can selectively discharge, on a timely basis, only the non-condensing gas (such as air) in separated relation from the gaseous-phase working medium (such as vapor), thereby allowing the high condensing efficiency to be maintained with good response. 
   Preferably, the valve device in the present invention is shifted to its opened position (i.e., valve-opening position) at a temperature lower than the boiling temperature of the liquid-phase working medium (such as water), so as to discharge the non-condensing gas of a temperature lower than the boiling temperature. Thus, only the non-condensing gas can be discharged via the valve device, without the gaseous-phase working medium (vapor), higher in temperature than the boiling temperature of the liquid-phase working medium (water), being discharged via the valve device. Further, if, for example, the valve device is set at a low valve-opening temperature with a predetermined difference from a saturated vapor temperature, the present invention can selectively discharge only the non-condensing gas from the condenser, without discharging the vapor, while effectively preventing the non-condensing gas from accumulating within the condenser. 
   Further, preferably, the condenser includes a plurality of condensing pipes and an intermediate chamber communicating with the plurality of condensing pipes, and the valve device is operatively connected with (or provided in correspondence with) the intermediate chamber. This inventive arrangement can minimize the number of the valve device to be used and permits shared use of the valve device among the plurality of condensing pipes, with the result that the present invention can effectively prevent the non-condensing gas from staying and accumulating non-uniformly across the condenser and can thereby discharge the non-condensing gas in a stabilized manner. 
   In another preferred embodiment of the invention, the condenser includes a plurality of condensing pipes, and the valve device is provided in each of the plurality of condensing pipes. 
   In an embodiment of the invention, the liquid-phase working medium is water, the gaseous-phase working medium is vapor, and the non-condensing gas is air. 
   Further, preferably, the valve device in the present invention has an outlet for discharging the water, and the liquid level of the water lies below the outlet. The valve device is constantly operatively connected with the gaseous-phase portion lying immediately above and close to the liquid level and set to operate at any time, so that it can be brought to its opened position on a timely basis and thereby allows the condenser to keep up its high condensing efficiency. 
   Further, in the present invention, the valve device is preferably in the form of an air vent. 
   As set forth above, the present invention is characterized by separating, from the vapor, the non-condensing gas that impedes condensation of the vapor and discharging only the thus-separated non-condensing gas from the condenser, to thereby allow the gaseous-phase portion of the condenser to be filled with the vapor; thus, the condenser can maintain its high condensing efficiency and even enhance the condensing efficiency. The non-condensing gas having a higher density than the gaseous-phase working medium (vapor) would stay and accumulate in the lower area of the gaseous-phase portion of the condenser near the liquid level due to the flow of the gaseous-phase working medium (vapor) and action of gravity. However, in the present invention, the valve device, which is brought to its opened position to selectively discharge only the non-condensing gas at a temperature lower than the boiling temperature of the liquid-phase working medium (e.g., temperature lower than the temperature of saturated vapor), is operatively connected with the lower area of the gaseous-phase portion. In this way, the present invention allows the condenser to maintain its high condensing efficiency with good response while effectively preventing undesired discharge of the gaseous-phase working medium (vapor) from the condenser. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Certain preferred embodiments of the present invention will hereinafter be described in detail, by way of example only, with reference to the accompanying drawings, in which: 
       FIG. 1  is a block diagram showing a general system setup of a Rankine cycle apparatus in accordance with an embodiment of the present invention; 
       FIG. 2  is a sectional view illustrating an inner structure of a water supplying pump unit of  FIG. 1 ; 
       FIG. 3  is a view illustrating example layout of various components of the Rankine cycle apparatus of  FIG. 1  when mounted on a vehicle; 
       FIG. 4  is a block diagram showing a system setup of the Rankine cycle apparatus, which particularly shows flows of a working medium in the apparatus of  FIG. 1 ; 
       FIG. 5  is a side view showing an inner structure of a condenser and other components peripheral to the condenser in the Rankine cycle apparatus of  FIG. 1 ; 
       FIG. 6  is a sectional view showing a structure of an air vent in its closed position; 
       FIG. 7  is a sectional view of the air vent taken along the A—A lines of  FIG. 6 ; 
       FIG. 8  is a sectional view of the air vent in an opened position; 
       FIG. 9  is a graph showing respective saturation curves of a temperature-sensitive liquid and water; 
       FIG. 10  is a front view of a modification of the condenser; 
       FIG. 11  is a graph showing variation characteristics of vapor and air densities relative to pressure variation; 
       FIG. 12  is a graph showing variation characteristics of vapor and air densities relative to temperature variation; 
       FIG. 13  is an enlarged sectional view of the air vent and portions peripheral to the air vent; 
       FIGS. 14A and 14B  are a view and table explanatory of details of liquid level position settings; 
       FIG. 15  is a flow chart showing an operational flow of a liquid level position control of the condenser; 
       FIG. 16  is a timing chart showing variation in a traveling velocity of the vehicle having the Rankine cycle apparatus mounted thereon, variation in an engine output, variation in an amount of water supply to an evaporator and variation in the liquid level position within the condenser; 
       FIG. 17  is a schematic view of a conventional vehicle-mounted Rankine cycle apparatus; 
       FIG. 18  is a front view of a conventional condenser; and 
       FIG. 19  is an enlarged sectional view of an “A” portion of the condenser shown in  FIG. 18 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   First, a description will be made about an example general setup of a Rankine cycle apparatus in accordance with an embodiment of the present invention, with reference to  FIG. 1 . 
   The Rankine cycle apparatus  10  includes an evaporator  11 , an expander  12 , a condenser  13 , and a water supplying pump unit  14  provided with a supply pump. 
   The evaporator  11  and the expander  12  are interconnected via a pipe  15 , and the expander  12  and the condenser  13  are interconnected via a pipe  16 . Further, the condenser  13  and the water supplying pump unit  14  are interconnected via a pipe  17 , and the water supplying pump unit  14  and the evaporator  11  are interconnected via a pipe  18 . With such a piping structure, there is formed closed circulation circuitry (circulation system) through which a working medium is circulated within the Rankine cycle apparatus  10  in the gaseous or liquid phase. The working medium in the Rankine cycle apparatus  10  is in water (liquid) and water vapor (gaseous) phases. 
   The circulation circuitry of the Rankine cycle apparatus  10  has a circulating structure hermetically sealed from the outside, which allows water or vapor to circulate therethrough. 
   In the circulation circuitry of the Rankine cycle apparatus  10 , the water (liquid-phase working medium) travels from a liquid level position, indicated by a broken line P 1 , within the condenser  13 , through the water supplying pump unit  14 , to the evaporator  11 . In  FIG. 1 , the pipes  17  and  18 , through which the water travels, are indicated by thick solid lines. The vapor (gaseous-phase working medium) travels from the evaporator  11 , through the expander  12 , to the liquid level position P 1  within the condenser  13 . The pipes  15  and  16 , through which the vapor travels, are indicated by thick broken lines. 
   The Rankine cycle apparatus  10  is constructed to phase-convert water into water vapor using heat from a heat source, and produce mechanical work using expansion of the water vapor. The evaporator  11  is a mechanism for converting water into vapor. 
   As will be later described in detail, the Rankine cycle apparatus  10  is constructed as a vehicle-mounted apparatus suitable for mounting on an automotive vehicle. For that purpose, the evaporator  11  uses heat of exhaust gas from the vehicle engine as the heat source. Namely, the evaporator  11  uses heat of the exhaust gas, flowing through an exhaust pipe  45  of the engine (internal combustion engine), to heat and superheat water supplied from the water supplying pump unit  14 , so as to produce high-temperature and high-pressure water vapor. The high-temperature and high-pressure water vapor produced by the evaporator  11  is supplied to the expander  12 . 
   Needless to say, the evaporator  11  may use higher-temperature exhaust gas from an exhaust port, exhaust manifold (not shown) or the like located downstream of an exhaust valve of the engine, rather than from the exhaust pipe  45 . 
   The expander  12  has an output shaft  12   a  connected to the rotor (not shown) or the like of a motor/generator (M/G)  19  so as to allow the motor/generator (M/G)  19  to operate as a generator. The expander  12  is constructed to expand the high-temperature and high-pressure water vapor supplied from the evaporator  11  and rotates the output shaft  12   a  through the expansion of the vapor. The rotation of the output shaft  12   a  rotates the rotor of the motor/generator  19  to cause the motor/generator  19  to make predetermined mechanical rotation or perform predetermined power generation operation. The output shaft  12   a  of the expander  12  is also connected to a hydraulic pump  25  to drive the pump  25 . 
   As noted above, the expander  12  produces mechanical work through the expansion of the high-temperature and high-pressure water vapor supplied from the evaporator  11  via the pipe  15  and thereby drives various loads, such as the motor/generator  19  and hydraulic pump  25 . The vapor  12  discharged from the evaporator  12  decreases in temperature and pressure and is delivered via the pipe  16  to the condenser  13  with the decreased temperature and pressure. 
   The condenser  13  cools and liquefies the vapor delivered from the evaporator  12 . Water produced through the liquefaction by the condenser  13  (i.e., condensed water) is returned via the pipe  17  to the water supplying pump unit  14 . 
   High-pressure pump  44  of the water supplying pump unit  14  pressurizes the water liquefied by the condenser  13  (i.e., condensed water from the condenser  13 ) and re-supplies or replenishes the pressurized condensed water to the evaporator  11 . 
   The Rankine cycle apparatus  10  having the above-described general system setup includes the following as other relevant components. 
   In a portion of the pipe  18  near the evaporator  11 , there is provided a pressure relief valve  22  for adjusting a pressure within the pipe  18  in response to pressure variation within the pipe  18 . 
   Within a casing  21  of the expander  12 , there is provided a breather (separator)  23  for returning leaked water vapor to the pipe  16 . Further, within the casing  21 , an oil pan  24  is disposed under the expander  12 . Oil built up in the oil pan  24  with water mixed therein is delivered by the hydraulic pump  25  to an oil coalescer  27  via a pipe  26 . 
   The oil and water are separated from each other by the oil coalescer  27 , and the separated water is stored in a lower portion of an oil tank  28  due to a difference in specific gravity. Valve mechanism  30  operating on the basis of a float sensor  29  is mounted in the oil tank  28 . 
   The oil separated from the water by the oil coalescer  27  and stored in an upper portion of the oil tank  28  is supplied, through a pipe  31 , to various sections of the expander  12  by way of an oil path (not shown) formed in the output shaft  12   a.    
   The water stored or accumulated in the lower portion of the oil tank  28  is supplied, via a pipe  33 , to an open tank  32  of the water supplying pump unit  14  through operation of the valve mechanism  30 . The open tank  32  is so named because it is open to the atmospheric air, and it accumulates or stores therein the working medium, leaked or discharged out of the circulation circuitry, in the liquid-phase state. 
   The open tank  32  of the water supplying pump unit  14  and the condenser  13  are interconnected by a pipe  35  via a water supplying return pump  37  and check valve  34 . 
   The condenser  13  includes a liquid level sensor  38  and air vent  39  provided near the liquid level position. Water supply from the open tank  32  to the condenser  13  is performed by the water supplying return pump  37  that is driven by a motor  36  turned on/off in response to a signal from the liquid level sensor  38 . Further, the open tank  32  and the condenser  13  are inter-connected by a pipe  40  that discharges the water via the air vent  39 . 
   The pipe  17  for returning the condensed water discharged from the condenser  13  is connected to a water coalescer  42  within a sealed tank  41  of the pump unit  14 . Water in the sealed tank  41  is supplied, by the high-pressure water supplying pump  44  driven by a motor  43 , to the evaporator  11  via the pipe  18 . 
   Further, in association with the condenser  13 , there are provided a plurality of cooling fans  46 – 48  for generating cooling air independently for different portions of the condenser  13 . 
   In the above-described arrangements, a working medium supply device is constituted by elements pertaining to the liquid level position within the condenser  13  and lower section of the condenser  13  and by the water supplying pump unit  14 . 
   In a sealed working medium circulation system of the Rankine cycle apparatus  10 , a working medium leaked from the breather  23  of the expander  12  is returned via an outlet port P 2  to the pipe  16  of the circulation system. 
     FIG. 2  is a view showing an example specific structure of the water supplying pump unit  14 . 
   The water supplying pump unit  14  comprises the water coalescer  42 , sealed tank  41 , high-pressure water supplying pump  44  driven by the drive motor  43 , open tank  32 , return pump  37 , and check valve  34 . 
   Although a rotation shaft  49  of the drive motor  43  is shown in the figure as being parallel to the surface of the sheet of the drawing, this is just for convenience of illustration; in practice, the rotation shaft  49  is disposed perpendicularly to the sheet of the drawing. The rotation shaft  49  of the drive motor  43  is held in engagement with a cam mechanism  49   a , so as to function as a cam shaft. 
   The water coalescer  42  separates oil and water, and the sealed tank  41  directly collects leaked water from the high-pressure water supplying pump  44 . The high-pressure water supplying pump  44  supplies a required amount of water by performing water amount control based on the number of pump rotations. 
   The open tank  32  is provided for temporarily storing water leaked out of the circulation circuitry. The return pump  37  returns the leaked water to the sealed tank  41  or to a supercooler of the condenser  13 . Namely, the return pump  37  returns the leaked water from the open tank  32  to the closed tank  41  through a pipe  152  equipped with a check valve  151 , or delivers the water to the supercooler of the condenser  13  through the pipe  35  equipped with the check valve  34  as necessary. The check valve  151  of the pipe  152  prevents a reverse flow of the water from the sealed tank  41 , and the check valve  34  of the pipe  35  prevents a reverse flow of the water from the supercooler of the condenser  13 . 
   Water discharged from the outlet port  13   a  (see  FIG. 1 ) of the condenser  13  is passed through the water coalescer  42  via the pipe  17  so that the water is separated from oil and only the water is fed to the high-pressure water supplying pump  44  driven by the drive motor  43 . The high-pressure water supplying pump  44  delivers the water to the evaporator  11  via the pipe  18 . Leaked water is returned via the pipe  40  to the open tank  32 . 
   The following paragraphs describe the Rankin cycle apparatus  10  when mounted on the vehicle, with reference to  FIG. 3 . 
   In  FIG. 3 , reference numeral  201  indicates a front body of the vehicle, and  202  a front road wheel. Engine room  203  is formed within the front body  201 , and the engine  50  is mounted in the engine room  203 . The exhaust manifold  51  is provided on a rear surface of the engine  50 , and the above-mentioned exhaust pipe  45  is connected to the exhaust manifold  51 . 
   The evaporator  11  is mounted on a portion of the exhaust pipe  45  near the exhaust manifold  51 . The pipe  18  extending from the high-pressure water supplying pump  44  is coupled to the evaporator  11 , and the pipe  18  supplies water to the evaporator  11  using, as its heat source, the heat of exhaust gas from the high-pressure water supplying pump  44 . The evaporator  11  phase-converts the water into water vapor using the heat of the exhaust gas and supplies the converted vapor to the expander  12  via the pipe  15  connected to a vapor inlet port  52  of the expander  12 . The expander  12  converts expansion energy of the water vapor into mechanical energy. 
   The expander  12  has a vapor outlet port  53  connected to the pipe  16 , and the condenser  13  for cooling/condensing water vapor into water is disposed between the pipe  16  and the sealed tank  41  leading to an inlet side of the high-pressure water supplying pump  44 . The condenser  13  is located in a front area of the engine room  203 . In  FIG. 3 , there is also shown a layout of the open tank  32 , water coalescer  42 , return pump  37 , oil coalescer  27 , super cooler  54  (liquid-phase portion of the condenser  13 ), air vent  39 , check valve  34 , etc. As note above, the high-pressure water supplying pump  44 , evaporator  11 , expander  12 , condenser  13 , etc. together constitute the Rankine cycle apparatus for converting heat energy into mechanical energy. 
   Behavior of the Rankine cycle apparatus is explained below in the order that corresponds to flows of water and water vapor within the Rankine cycle apparatus. 
   Water cooled and condensed in the condenser  13  is supplied, in a pressurized condition, by the high-pressure water supplying pump  44  to the evaporator  11  via the pipe  18 . 
   The water, which is a liquid-phase working medium, is heated by the evaporator  11  imparting the water with heat energy until it becomes high-temperature and high-pressure water vapor, and the resultant high-temperature and high-pressure water vapor is supplied to the expander  12 . The expander  12  converts the heat energy into mechanical energy through expanding action of the high-temperature and high-pressure water vapor, and the mechanical energy is supplied to the motor/generator  19  annexed to the expander  12 . 
   The water vapor let out from the expander  12  assumes a lowered temperature and pressure, which is then delivered to the condenser  13 . The water vapor of lowered temperature and pressure delivered to the condenser  13  is again cooled and condensed in the condenser  13 , and the resultant condensed water is supplied via the water coalescer  42  to the high-pressure water supplying pump  44 . After that, the water, which is a liquid-phase working medium, repeats the above circulation, so that the expander  12  continues to be supplied with water vapor of high temperature and pressure. 
   Next, with reference to  FIGS. 5–16 , a description will be made about a mechanism for discharging air (non-condensable or non-condensing gas) remaining within the condenser  13  of the Rankine cycle apparatus  10 , as well as control of the liquid level position of water accumulated in the condenser  13  of the Rankine cycle apparatus  10 . 
     FIG. 4  shows the system of the Rankine cycle apparatus  10  with a central focus on the condenser  13 , which particularly shows a front view of the condenser  13  as taken from before the vehicle; more specifically, states of the working medium (water or condensed water W 1  and water vapor W 2 ) within the condenser  13  are illustrated.  FIG. 5  is a side view of the cooling device condenser  13 , which shows positional relationship among cooling fans  46 ,  47  and  48  provided for the condenser  13  as well as inner states of the condenser  13 . 
   The condenser  13  includes a vapor introducing chamber  13 A in its upper end portion, a water collecting chamber  13 B in its lower end portion, and an intermediate chamber  56 . A plurality of cooling pipes  55  are provided between the vapor introducing chamber  13 A and the intermediate chamber  56  and between the intermediate chamber  56  and the water collecting chamber  13 B, and these three chambers  13 A,  13 B and  56  are in fluid communication with each other. Cooling fins  55   a  are provided on the outer periphery of the cooling pipes  55 . 
   The vapor introducing chamber  13 A of the condenser  13  is connected via the pipe  16  to the vapor outlet port  53  of the expander  12 , and the water collecting chamber  13 B is connected via the pipe  17  to the water supplying pump unit  14 . As noted earlier, the expander  12  is connected via the pipe  15  to the evaporator  11 , and the water supplying pump unit  14  is connected via the pipe  18  to the evaporator  11 . 
   The evaporator  11  receives heat  50 A from the exhaust gas of the engine (heat source)  50  via the exhaust pipe  45  (see  FIG. 1 ). Within the water supplying pump unit  14 , there are included various components, such as the sealed tank  41 , water coalescer  42 , high-pressure water supplying pump  44 , drive motor  43 , open tank  32 , return pump  37  and motor  36 . 
   In the condenser  13 , water vapor W 2  is cooled and condensed to turn to water (condensed water) W 1  and accumulated in a lower inner portion of the condenser  13 . Horizontal line drawn in the figure within the intermediate chamber  56  represents a liquid level  65  (corresponding to the liquid level position P 1  of  FIG. 1 ) that indicates a liquid level position of the water W 1  accumulated in the condenser  13 . 
   The liquid level sensor  38  and intermediate discharge port  59  are provided at a position corresponding to the position of the liquid level  65 . The liquid level sensor  38  outputs a detection signal, representative of the liquid level position detected thereby, to a control device  60 . The control device  60  generates a motor control instruction signal on the basis of the liquid level position detection signal from the sensor  38  and sends the motor control instruction signal to the motor  36  of the return pump  37 . 
   The air vent  39  for water vapor is coupled to the intermediate discharge port  59 , and it has an output end communicating with the open tank  32  via the pipe  40  equipped with a check valve  58 . Exhaust pump  57  is annexed to the pipe  40  in parallel relation thereto. 
   Further, as seen in  FIG. 6 , the cooling fan  46  is disposed adjacent the rear surface (right side surface in the figure) of the condenser  13  in corresponding relation to a gaseous-phase portion or vapor condensing portion  70  of the condenser  13  where the vapor W 2  is accumulated, and the cooling fans  47  and  48  are disposed adjacent the rear surface of the condenser  13  in corresponding relation to a liquid-phase portion or condensed water cooling portion  71  of the condenser where the water W 1  is accumulated. 
   The cooling operation by the cooling fan  46  is controlled by a pressure control device  62  on the basis of a vapor pressure detection signal output by a pressure sensor  61  mounted, for example, on the pipe  16  through which the vapor W 2  flows. Namely, the cooling fan  46  is a vapor-condensing cooling fan to be used for vapor pressure adjustment. Further, the cooling operations by the cooling fans  47  and  48  are controlled by a temperature control device  64  on the basis of a water temperature detection signal output by a temperature sensor  63  mounted, for example, on the pipe  17  through which the water W 1  flows. Namely, the cooling fans  47  and  48  are water-cooling fans to be used for cooling of the condensed water. 
   In  FIG. 5 , A 1  indicates a flow of cooling air applied from before the gaseous-phase portion  70  of the condenser  13  on the basis of the rotation of the cooling fan  46 , while A 2  indicates a flow of cooling air applied from before the liquid-phase portion  71  of the condenser  13  on the basis of the rotation of the cooling fans  47  and  48 . 
   As apparent from the foregoing, the gaseous-phase portion or vapor condensing portion  70  and the liquid-phase portion or condensed water cooling portion  71  in the condenser  13  are cooled independently of each other. Reference numeral  72  represents shrouds that zone or define the individual cooling regions. 
   Referring back to  FIG. 4 , the water vapor discharged from the vapor outlet port  53  of the expander  12  is substantially equivalent in pressure to the atmospheric pressure. In the intermediate chamber  56  into which the respective outlets of the upper cooling pipes (condensing pipes)  55  open, water is discharged via the air vent  39  in order to adjust the liquid level  65  to lie within the intermediate chamber  56 . Further, the high-pressure water supplying pump  44  functions, as a water supplying pump of a main circulation circuit in the Rankine cycle apparatus  10 , to supply a necessary amount of water to the evaporator  11 . 
   The reserving open tank  32 , which is open to the atmosphere, retains reserve water for the sealed circulation circuitry in the system. The return pump  37  supplies water into the condenser  13  in response to the detection signal from the liquid level sensor  38 . The exhaust pump  57  sucks in air from the downstream end of the air vent  39  when the condenser  13  is to be operated at a negative pressure. 
   The above-mentioned exhaust pump  57  may be constructed to operate in response to detection of a negative pressure by the pressure sensor  61  and pressure control device  62  shown in  FIG. 5 , or by the control device  60  detecting via the liquid level sensor  38  when the position of the liquid level  65  rises above a predetermined upper limit. 
   The check valve  58  prevents a reverse flow of the atmospheric air when the interior pressure of the condenser  13  turns to a negative pressure, and the check valve  34  prevents a reverse flow of water from the condenser  13  to the return pump  37 . The air vent  39  is constructed to allow water and air to pass therethrough, but prevent water vapor from passing therethrough. The intermediate discharge port  59  functions to limit variation in the position of the liquid level  65  of the condensed water, through emission of non-condensing (non-condensable) gas or overflow of the water, so that the liquid level position varies only within a predetermined vertical range. 
   The liquid sensor  38  outputs a position detection signal, representative of an actual current position of the liquid level  65 , to the control device  60 , and the control device  60  controls the return pump  37  so that the position of the liquid level  65  constantly lies within the intermediate chamber  56 . More specifically, the position of the liquid level  65  is controlled to lie within a predetermined vertical range between the air vent  39  and the liquid level sensor  38 . The liquid level sensor  38  may be, for example, in the form of a capacitance-type level sensor or float-type level switch. 
   In  FIG. 5 , the pressure sensor  61  detects an interior pressure of the condenser  13 ; basically, it detects a pressure of the water vapor W 2 . The pressure control device  62  operates the cooling fan  46  in such a manner that the interior pressure of the condenser  13  equals a predetermined pressure setting. The temperature sensor  63  detects a current temperature of the condensed water W 1 . The temperature control device  64  operates the cooling fans  47  and  48  in such a manner that the condensed water temperature equals a predetermined temperature setting. 
   Next, construction and behavior of the air vent  39  employed in the instant embodiment will be detailed with reference to  FIGS. 6 to 8 .  FIG. 6  is a vertical sectional view of the air vent  39  and  FIG. 7  is a sectional view of the air vent  39  taken along the A—A lines of  FIG. 6 , both of which show the air vent  39  in a closed position.  FIG. 8  is a vertical sectional view of the air vent  39  in an opened position (i.e., valve-open position). In these figures, the left side of the air vent  39  is a side communicating with the condenser  13  (i.e., “condenser side”), while the right side of the air vent  39  is a side communicating with the atmosphere (i.e., “atmosphere side”). The air vent  39  is hermetically sealed when its interior is filled with saturated vapor ( FIG. 6 ), automatically opened when water or non-condensing gas is present in the interior, and again hermetically sealed by discharging the water or non-condensing gas ( FIG. 8 ). 
   In  FIG. 6 , the air vent  39  includes a valve  66  located generally centrally therein, a valve support  67  supporting the valve  66 , and a valve port (packing)  68 . 
   The valve  66  supported by the valve support  67  is positioned to close up the valve port  68  when necessary. The valve  66  comprises a pair of opposed diaphragms  66   a  combined to form a hermetically-sealed space therebetween, and temperature-sensitive liquid  69  is held in the sealed space. The temperature-sensitive liquid  69  has characteristics such that, like water, it is kept in the liquid phase under less than a predetermined pressure or temperature but expands as a gas once the temperature exceeds a predetermined level. 
     FIG. 9  shows respective saturation curves C 1  and C 2  of the temperature-sensitive liquid  69  and water. The temperature at which the temperature-sensitive liquid  69  turns to the gaseous state is lower by ΔT (about 10° C.) than the temperature at which water turns to water vapor. Thus, when the interior of the air vent  39  is filled with the water vapor W 2 , the temperature-sensitive liquid  69  is kept in the gaseous state, so that the sealed space containing the expanded temperature-sensitive liquid  69  presses the opposed diaphragms  66   a  outwardly away from each other so as to close up a gap between the valve port  68  and the valve  66  comprised of the diaphragms  66   a  (see  FIG. 6 ). Conversely, when the interior of the air vent  39  is at a low temperature (e.g., when non-condensing gas A 3 , such as air, is present in the ambient environment around the valve  66 ), the temperature-sensitive liquid  69  is kept in the liquid state, the opposed diaphragms  66   a  are pressed inwardly toward each other, so that air etc. is discharged through the gap between the valve  66  and the valve port  68  (see  FIG. 8 ). 
   As apparent from the foregoing, the control device  60  shown in  FIG. 4  is constructed to control the position of the liquid level  65  to vary only within the predetermined vertical range (variation width) in the condenser  13  that cools the water vapor W 2  via the cooling fan  46  to convert the vapor W 2  back to the water (condensed water) W 1 . When the detection signal output from the liquid level sensor  38 , which detects a current position of the liquid level  65  that corresponds to the boundary between the gaseous-phase portion  70  and the liquid-phase portion  71  (see  FIG. 4 ) in the condenser  13 , indicates that the position of the liquid level  65  is lower than the lower limit of the predetermined range, the control device  60  controls the motor  36  of the return pump  37  that supplies water into the condenser  13 , to thereby re-supply or replenish a deficient amount of water from the open tank  32  via the pipe  35  to the condenser  13 . 
   Further, when the position of the liquid level  65  is higher than the upper limit of the predetermined range, the control device  60  discharges an excessive water to the open tank  32  via the intermediate discharge port  59 , air vent  39 , etc. In this way, a desirable range of the position of the liquid level  65  can be set in accordance with the range determined by the lower limit based on the detection by the liquid level sensor  38  and the upper limit based on the operation of the air vent  39 . 
   The intermediate discharge port  59  for discharging the water (condensed water) W 1  is provided in the intermediate chamber  56  of the condenser  13 , in order to control the position of the liquid level  65 . When the liquid level  65  is higher than the intermediate discharge port  59 , the intermediate discharge port  59  causes the water to flow out therethrough to the reserving open tank  32  so that the liquid level  65  can be lowered. When the liquid level  65  is lower than the intermediate discharge port  59 , the air vent  39  coupled to the intermediate discharge port  59  prevents the vapor from escaping via the water outlet  59 . 
   As seen in  FIGS. 6–8 , the air vent  39  for preventing the vapor from escaping via the intermediate discharge port  59  automatically closes the valve when vapor is present or contained in its interior, but automatically opens the valve when air (non-condensing gas) or water is present. 
   Further, as seen in  FIG. 4 , the liquid level sensor  38  is provided at a position lower than the intermediate discharge port  59 , and, when the position of the liquid level  65  has lowered below the liquid level sensor  38 , a deficient amount of water is re-supplied or replenished from the open tank  32  by means of the return pump  37 , so as to raise the liquid level  65  to the position of the liquid level sensor  38 . 
   As set forth above, the position of the liquid level  65  is constantly kept within the vertical range between the intermediate discharge port  59  and the liquid level sensor  38 . If the interval is distance between the intermediate discharge port  59  and the liquid level sensor  38  is increased, an error in heat transmission area between the vapor portion W 2  and the water (condensed water) portion W 1  will become greater. Conversely, if the interval between the intermediate discharge port  59  and the liquid level sensor  38  is decreased, the return pump  37  and air vent  39  have to operate very often. Therefore, it is preferable that the interval between the intermediate discharge port  59  and the liquid level sensor  38  be set within a moderate range such that both of the above two adverse influences or inconveniences can be lessened to an appropriate degree. Further, in order to keep constant the heat transmission areas, it is desirable that the interval between the intermediate discharge port  59  and the liquid level sensor  38  be as small as possible or zero. 
   In the instant embodiment, the air vent  39  also functions as a non-condensing gas discharge device of the condenser  13 . As noted earlier, the air vent  39  includes the valve  66  attached to, i.e. operatively connected with, a part of the gaseous-phase portion  70  near the boundary between the gaseous-phase portion  70  (filled with the vapor W 2 ) and the liquid-phase portion  71  (filled with the water W 1 ). The valve  66  opens at a temperature below the boiling temperature of the water, so as to discharge the accumulated air (non-condensing gas) A 3  staying at a temperature equal to or lower than the boiling temperature of the water. Preferably, the valve-opening temperature of the air vent  39  is lower than the boiling temperature of the water as the liquid-phase working medium. 
   As described above in relation to  FIG. 4 , the condenser  13  includes the intermediate chamber  56 , and the air vent  39  is operatively connected with the intermediate chamber  56 . The air vent  39  discharges the water W 1  so that the liquid level  65  within the condenser  13  is kept at least below the air vent  39 . 
   Alternatively, the intermediate chamber  56  may be dispensed with as illustrated in  FIG. 10 , in which case it is preferable that a separate air vent  39  be provided for each of the plurality of cooling pipes (condensing pipes)  55 . 
   The reason why the non-condensing gas (i.e., air in this case) accumulates beneath the vapor W 2  within the cooling pipes  55  is explained below with reference to  FIGS. 11 and 12 . In  FIG. 11 , the horizontal axis represents pressure, while the vertical axis represents density. In  FIG. 12 , the horizontal axis represents temperature, while the vertical axis represents density. Specifically,  FIG. 11  shows a curve G 1  representative of variation in the vapor density relative to variation in the pressure, and a curve G 2  representative of variation in the air density relative to variation in the pressure.  FIG. 12  shows a curve G 3  representative of variation in the vapor density relative to variation in the temperature, and a curve G 4  representative of variation in the air density relative to variation in the temperature. 
   As illustrated in  FIG. 11 , the vapor density is greater than the air pressure when the pressure is higher than a pressure level P 1 . When the pressure is lower than the level P 1 , the air density is greater than the vapor density. As illustrated in  FIG. 12 , the vapor density is greater than the air pressure when the temperature pressure is higher than a temperature level T 1 . When the temperature is lower than the level T 1 , the air density is greater than the vapor density. 
   The pressure within the condenser  13  is set to be lower than the level P 1 , and the temperature within the condenser  13  is set to be lower than the level T 1 . Thus, within the condenser  13 , the air is heavier than the vapor and thus accumulates beneath the vapor. 
   Because the non-condensing gas or air A 3  accumulates in the lower area of the gaseous-phase portion  70  within the condenser  13 , the air vent  39  has an air outlet that is located in the lowermost area of the gaseous-phase portion  70  (as close to the liquid level of the condensed water as possible) in order to discharge the air from the condenser  13  efficiently. 
   Further, as explained in relation to  FIGS. 6 to 8 , the air vent  39  is a selective discharge valve which is automatically brought to its closed position (valve-closed position) when its predetermined installed portion is filled with the vapor W 2  and automatically brought to its opened position (valve-open position) when the water W 1  or non-condensing gas A 3  is present in the predetermined installed portion. When the condenser  13  is to be operated at a pressure higher than the atmospheric pressure, the downstream side of the air vent  39  is opened to the atmospheric pressure so that the air is automatically discharged into the atmospheric air. When the condenser  13  is to be operated at a negative pressure, on the other hand, the exhaust pump  57  located downstream of the air vent  39  sucks in the air from the downstream side of the air vent  39 , so that the air can be discharged into the atmospheric air. 
     FIG. 13  is an enlarged schematic view of the intermediate chamber  56  with the air vent  39  attached thereto, which particularly shows how the air A 3  is discharged via the air vent  39  functioning as the non-condensing gas discharge device of the condenser  13 . 
   In  FIG. 13 , the vapor W 2  condenses on the inner wall surface of the cooling pipes  55  (only one of which is shown) to form a condensate liquid membrane W 1 - 1 , and then moves downward as condensate liquid drops W 1 - 2  so that it is received in the intermediate chamber  56  as water W 1 . 
   The air A 3  present or contained within the cooling pipes  55  is discharged through the air vent  55  located above the liquid level  65 . Because the air A 3  is present within all of the cooling pipes  55 , it is necessary that the air A 3  be discharged from all of the cooling pipes  55 . 
   Alternatively, where the number of the cooling pipes  55  is relatively small, a separate air vent  39  may be provided in correspondence with, or operatively connected with, each of the cooling pipes  55 , as illustrated in  FIG. 10 . Where, on the other hand, a relatively great number of the cooling pipes  55  are employed, a single air vent  39  may be operatively connected with the intermediate chamber  56  in such a manner that the air A 3  is discharged collectively through the same or common air vent  39 , as described above illustrated in  FIGS. 4 and 13 . In this way, the gaseous-phase portion  70  of the condenser  13  is filled only with the vapor, which can thereby achieve an enhanced condensing efficiency. 
   In order to discharge the air A 3  from the condenser  13 , the liquid level  65  of the condensed water W 1  is adjusted to be in the intermediate chamber  56  and lower than the installed portion of the air vent  39 . Control of the liquid level position will be described in greater detail later. 
   In the above-described manner, the gaseous-phase portion  70  of the condenser  13  can be completely filled with the vapor, so that the condenser  13  can constantly operate with an enhanced heat exchange performance; as a result, the condenser  13  can be effectively reduced in size and can operate with reduced power consumption. 
     FIG. 14A  shows positional relationship among the liquid level sensor  38 , the air vent  39  and the liquid level  65  in the Rankine cycle apparatus, and  FIG. 14B  shows relationship among the liquid level  65  and operational states of the air vent  39  and return pump  37 . 
   In  FIG. 14A , H A , H B  and H L  represent the upper-limit position of the liquid level, lower-limit liquid level and position of the liquid level  65 , respectively. When the actual position H L  of the liquid level  65  is higher than the upper-limit position H A , the air vent  39  is set in its opened position, and the return pump  37  (see  FIG. 4 ) is set in its OFF state. When the position H L  of the liquid level  65  is between the upper-limit and lower-limit positions H A  and H B  of the liquid level, the air vent  39  is set in its closed position (i.e., valve-closed position), and the return pump  37  (see  FIG. 4 ) is set in its OFF state. When the position H L  of the liquid level  65  is lower than the lower-limit positions H B , the air vent  39  is set in its closed position, and the return pump  37  (see  FIG. 4 ) is set in its ON state. In this way, variation in the liquid level  65  can be reliably confined within the range between the upper-limit and lower-limit positions H A  and H B . 
   Also, even when the inflow amount (mass flow rate) of water vapor or the amount of water discharge (mass flow rate) to the high-pressure water supplying pump  44  varies at the time of activation/deactivation or transient variation of the Rankine cycle apparatus  10 , the described arrangements of the instant embodiment can effectively restrict or control variation of the position of the liquid level  65  within the condenser  13  and thereby permits stable operation of the condenser  13 . 
   Further, as illustrated in  FIG. 4 , the Rankine cycle apparatus  10  includes the reserving open tank  32  open to the atmosphere and provided separately from the main circulation circuit. This open tank  32  is connected to the condenser  13 , via the air vent  39  coupled to the intermediate discharge port  59  and the check valve  58 . Lower portion of the open tank  32  is connected to the outlet port  13   a  of the condenser  13  via the return pump  37 , pipe  35  and check valve  34 . 
   When the liquid level  65  is higher in position than the intermediate discharge port  59 , the water overflows out of the condenser  13  to be directed into the open tank  32 , while, when the liquid level  65  is lower in position than the liquid level sensor  38 , the return pump  37  is activated to replenish water to the condenser  13 . Because the amount of water supply by the high-pressure water supplying pump  44 , located downstream of the condenser  13 , is controlled in the instant embodiment, the activation of the return pump  37  causes the liquid level  65  to rise up to the position of the liquid level sensor  38  due to the water supply into the condenser  13 , upon which the return pump  37  is deactivated. 
   Further, because the intermediate chamber  56 , into which the plurality of cooling pipes (condensing pipes)  55  open, is provided in the region including the intermediate discharge port  59  and liquid sensor  38 , the liquid level  65  is allowed to vary with improved response and in a stabilized manner during water discharge from the intermediate discharge port  59  or water supply from the return pump  37 . 
   Note that the provision of the intermediate chamber  56  is not necessarily essential to the present invention if the vapor introducing chamber  13 A and water collecting chamber  13 B are in fluid communication with each other via the plurality of cooling pipes (condensing pipes)  55 . 
   Operational sequence of the liquid level position control performed by the control device  60  is explained below with reference to a flow chart of the  FIG. 15 . 
   At step S 10 , the control device  60  reads the current position HL of the liquid level  65  via the liquid level sensor  38 . 
   At step S 11 , it is determined whether the liquid level position H L  is higher than the upper-limit position H A  of the liquid level, and, if so, control proceeds to step S 12 , where the air vent  39  is brought to its opened position to discharge the excessive water so as to lower the liquid level  65 . After that, the control device  60  reverts to step S 10 . When the liquid level position H L  is lower than the upper-limit position H A  of the liquid level, control proceeds to step S 13  in order to close the air vent  39 . 
   At step S 14 , it is determined whether the liquid level position HL is lower than the lower-limit position H B  of the liquid level, and, if so, control proceeds to step S 15 , where the return pump  37  is turned on for re-supply or replenishment of deficient water. Further, if the liquid level position H L  is higher than the lower-limit position H B  of the liquid level, the return pump  37  is turned off to not replenish water. After that, the control device  60  reverts to step S 10 . 
     FIG. 16  is a timing chart showing variation in the velocity of the vehicle having the Rankine cycle apparatus  10  mounted thereon, variation in the engine output, variation in the amount of water supply to the evaporator and variation in the liquid level position within the condenser, in contradistinction to the conventional apparatus. More specifically, section (A) of  FIG. 16  shows variation in the traveling velocity of the vehicle, (B) variation in the engine output of the vehicle, (C) variation in the amount of water supply to the evaporator in the conventional apparatus, (D) variation in the liquid level position within the condenser in the conventional apparatus, and (E) variation in the liquid level position within the condenser in the embodiment of the present invention. 
   As the velocity of the vehicle, having the Rankine cycle apparatus mounted thereon, varies as illustrated in (A) of  FIG. 16 , the engine output of the vehicle varies as illustrated in (B) of  FIG. 16 , in response to which the amount of water supply to the evaporator varies in a manner as illustrated in (C) of  FIG. 16  and also the liquid level position within the condenser varies in a manner as illustrated in (D) of  FIG. 16 . In other words, as the vehicle starts traveling at time points t 1 , t 3  and t 5  and stops traveling at time points t 2 , t 4  and t 6  along the time axis, the engine output varies and the amount of water supply to the evaporator also varies, so that the liquid level position within the condenser varies. 
   With the condenser  100  of the conventional vehicle-mounted Rankine cycle apparatus shown in  FIG. 17 , the amount of water supply to the evaporator  111  varies because the engine output varies as illustrated in (B) of  FIG. 16  in response to the start/stop of the vehicle and transitional vehicle velocity variation as illustrated in (A) of  FIG. 16 , so that the liquid level position  112  in the cooling pipes  103  of the condenser  100  would vary. Namely, in the condenser  100 , the liquid level position  112  rises when the inflow amount of vapor is greater than the discharge amount of condensed water, but falls when the inflow amount of vapor is smaller than the discharge amount of condensed water. 
   By contrast, according to the instant embodiment, the above-described liquid level position control is performed when the vehicle varies in traveling velocity as illustrated in (A) of  FIG. 16 , and thus, the liquid level position can be controlled to vary between the upper-limit and lower-limit positions H A  and H B  at the time of a start/stop of traveling of the vehicle. As a consequence, the instant embodiment can reliably prevent great variation or fluctuation in the liquid level position within the condenser  13 . 
   In the present invention, as set forth above, the positional variation in the liquid level  65  of the water (condensed water) W 1  accumulated in the condenser  13  is confined to the predetermined range, so that respective variation of the heat transmission areas of the gaseous-phase portion and liquid-phase portion, corresponding to vapor and condensed water, in the condenser  13  can be effectively reduced. As a consequence, the present invention can perform the necessary cooling without regard to variation in the heat transmission areas and achieve an enhanced accuracy of the control. Also, the present invention can reduce cavitations in the pump device and extra heat energy consumption during re-heating in the evaporator  11 . 
   Further, the present invention can keep a variation width of the heat transmission areas within a permissible range and impart a hysteresis to switching between discharge and replenishment of the liquid-phase working medium, to thereby lower the frequency of the switching operation. As a result, the present invention can achieve stabilized operation of the condenser  13  and enhanced durability of devices involved in the discharge and replenishment of the liquid-phase working medium. 
   Moreover, because the present invention can appropriately control the liquid level by discharging the liquid-phase working medium (water) from within the condenser  13  while preventing discharge of the gaseous-phase working medium (vapor), it can achieve even further stabilized operation of the condenser  13 . 
   Furthermore, the present invention can replenish the liquid-phase working medium directly up to the set liquid level from the reserving open tank, accumulating the liquid-phase working medium, via the return pump, so that the liquid level position can be appropriately adjusted and accurately stabilized promptly through high-response and high-precision supply amount control of the pump. 
   In addition, the present invention can perform the liquid level position control while keeping the necessary total mass flow rate of the working medium in the circulation circuitry, and thus, the circulation circuitry need not be equipped with particular devices indented for working medium discharge and replenishment to and from the outside. 
   Furthermore, the present invention can reduce differences in the liquid level position among the cooling pipes of the condenser and thereby accurately stabilize the liquid level promptly during the discharge and replenishment of the liquid-phase working medium, as a result of which the present invention can achieve even further stabilized operation of the condenser  13 . 
   Obviously, various minor changes and modifications of the present invention are possible in the light of the above teaching. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.