Patent Publication Number: US-2011067383-A1

Title: Working gas circulation engine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to a gas circulation engine that has a combustion chamber and a circulation path that connects an intake-side portion and an exhaust-side portion of the combustion chamber to each other. More specifically, the invention relates to a working gas circulation engine that has a combustion chamber which is supplied with an oxidant, fuel, the combustion (oxidation) of which is promoted by the oxidant, and working gas that generates power with the use of combustion of the fuel, and a circulation path that connects an intake-side portion and an exhaust-side portion of the combustion chamber to each other, and that is formed in such a manner that the working gas is circulated back to the combustion chamber through the circulation path without being released into the atmosphere. 
     2. Description of the Related Art 
     A working gas circulation engine of this type is a so-called closed-cycle engine, and described in, for example, Japanese Patent Application Publication No. 11-93681 (JP-A-11-93681). In the working gas circulation engine described in JP-A-11-93681, oxygen and hydrogen are supplied to a combustion chamber as an oxidant and fuel, respectively, and argon is circulated as working gas in order to enhance the thermal efficiency. In the working gas circulation engine, argon is thermally expanded due to the combustion of hydrogen that takes place in the combustion chamber. The thermal expansion of argon pushes a piston down so that power is produced. Because water vapor is generated due to the combustion of hydrogen that takes place in the combustion chamber, the water vapor is discharged into a circulation path along with the argon. Therefore, in the working gas circulation engine, a condenser, which liquefies the water vapor to remove it, is provided in the circulation path so that only argon, which is used as the working gas, is circulated back to the combustion chamber. 
     In the condenser described above, exhaust gas that contains the argon, which is used as the working gas, and the water vapor are cooled by coolant, whereby the water content is separated from the exhaust gas in the form of condensed water. In this way, the water content is removed from the exhaust gas. In order to condense the water vapor, the exhaust gas needs to be cooled to a normal temperature. However, the temperature of the exhaust gas that is just discharged from the combustion chamber is very high. Therefore, a large-capacity condenser and a large-capacity radiator that cools the coolant are needed to cool the high-temperature exhaust gas to the normal temperature. However, such large-capacity condenser and large-capacity radiator may increase the vehicle weight, and may be too large to be mounted in an engine compartment that has a limited space. Therefore, there are limitations to increases in the capacities of the condenser and the radiator, which makes it difficult to obtain required cooling performance. Accordingly, the water vapor may not be entirely removed from the exhaust gas (the water vapor may partially remain in the exhaust gas) if the temperature of the exhaust gas or the ambient temperature is not appropriate. 
     In the working gas circulation engine, the high-temperature exhaust gas discharged from the combustion chamber is not released into the atmosphere, unlike in a so-called open-cycle engine. Therefore, the engine is repeatedly operated in the state where the exhaust gas is not sufficiently cooled in the condenser. Accordingly, the circulation path is warmed and the temperature of the working gas that circulates through the circulation path increases. As a result, the pressure in the circulation path increases, which may cause various inconveniences. For example, an increase in the pressure in the circulation path may reduce the durability of the circulation path. Also, an increase in the pressure in the circulation path may cause leakage of the working gas through a portion at which an engine body and a circulation pipe, which defines the circulation path, are connected to each other, or a portion at which the circulation pipe and the condenser are connected to each other. In the circulation path at a portion from the combustion chamber to the condenser, because the temperature of the working gas is especially high and the pressure is likely to be especially high, the pressure in the combustion chamber may also be increased abruptly. 
     In the working gas circulation engine, it is important to ensure sufficient exhaust gas cooling performance. 
     SUMMARY OF THE INVENTION 
     The invention provides a working gas circulation engine provided with improved exhaust gas cooling performance. 
     An aspect of the invention relates to a working gas circulation engine that includes: a combustion chamber that is supplied with fuel, the combustion product of which is condensed, and working gas that generates power with the use of combustion of the fuel and that has a specific heat ratio higher than a specific heat ratio of the air; a circulation path that connects an inlet and an outlet of the combustion chamber to each other in such a manner that the working gas is circulated back to the combustion chamber without being released into the atmosphere; and at least two condensers that are provided in the circulation path, that are supplied with exhaust gas which is discharged from the combustion chamber and which contains the combustion product and the working gas, and that condense and remove the combustion product. 
     In the working gas circulation engine according to the aspect of the invention, the capacities of the respective condensers may be set in such a manner that, if the exhaust gas discharged from the combustion chamber has the highest possible temperature that may be achieved during an operation of the engine, the combustion product is substantially entirely condensed and removed by the time the exhaust gas finishes passing through the condenser that is farthest from the outlet of the combustion chamber among all the condensers. 
     In the working gas circulation engine according to the aspect of the invention, at least two condensers are provided. Therefore, it is possible to efficiently cool the high-temperature exhaust gas without reducing the ease in mounting the condensers in the engine compartment and without significantly increasing the vehicle weight. 
     In the working gas circulation engine according to the aspect of the invention, an exhaust gas inlet of the condenser that is closest to the outlet of the combustion chamber among all the condensers may be positioned near the outlet of the combustion chamber. 
     With the structure described above, the amount of exhaust gas that is present in the circulation path at a portion from the outlet of the combustion chamber to the condenser closest to the outlet is reduced. Therefore, it is possible to more appropriately suppress increases in the temperature and the pressure in the circulation path that may be caused by the high-temperature exhaust gas. 
     The condenser of which the exhaust gas inlet is positioned near the outlet of the combustion chamber may be arranged adjacent to an engine body, formed integrally with the engine body, or arranged at an exhaust pipe gathering portion of an exhaust manifold. 
     Coolant used to cool an engine body may be used in the condenser of which the exhaust gas inlet is positioned near the outlet of the combustion chamber. 
     With the working gas circulation engine according to the aspect of the invention described above, the water vapor (H 2 O), which is the combustion product contained in the exhaust gas, is substantially entirely converted into the condensed water (H 2 O) and discharged to the outside of the engine, and the working gas that has a higher specific heat ratio is supplied into the combustion chamber due to the improved exhaust gas cooling performance. Therefore, it is possible to prevent a decrease in the thermal efficiency that may be caused if the water vapor (H 2 O) having a low specific heat ratio is supplied into the combustion chamber. In addition, with the working gas circulation engine, it is possible to prevent an excessive increase in the pressure in the circulation path due to the improved exhaust gas cooling performance. Accordingly, with the working circulation engine, it is possible to maintain the durability of the circulation path, and to prevent leakage of the exhaust gas through portions at which the circulation path is connected to various elements. As a result, it is possible to prevent a decrease in the thermal efficiency that may be caused due to shortage of the working gas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features, advantages and technical and industrial significance of this invention will be described in the following detailed description of example embodiments of the invention with reference to the accompanying drawings, wherein the same or corresponding portions will be denoted by the same reference numerals and wherein: 
         FIG. 1  is a view showing the structure of a working gas circulation engine according to a first embodiment of the invention; 
         FIG. 2  is a view showing the structure of a working gas circulation engine according to a second embodiment of the invention; and 
         FIG. 3  is a view showing the structure of a working gas circulation engine according to a modification of the second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereafter, gas circulation engines according to example embodiments of the invention will be described in detail with reference to the accompanying drawings. Note that the invention is not limited to the example embodiments described below. 
     Hereafter, a working gas circulation engine according to a first embodiment of the invention will be described with reference to  FIG. 1 . 
     The working gas circulation engine according to the first embodiment of the invention is a so-called closed-cycle engine that has a combustion chamber which is supplied with fuel, a combustion-product of which is condensed, and working gas that generates power with the use of combustion of the fuel and that has a specific heat ratio higher than that of air, and a circulation path which connects an inlet and an outlet of the combustion chamber to each other, and that is formed in such a manner that the working gas is circulated back to the combustion chamber through the circulation path without being released into the atmosphere. In the working gas circulation engine, the fuel is burned in the combustion chamber, whereby the working gas is thermally expanded to generate power. As the fuel the combustion-product of which is condensed, fuel of which the combustion is promoted by an oxidant, for example, hydrogen is used. After hydrogen is burned, water vapor is produced as a combustion-product, as described later in detail. In this case, the oxidant is also supplied into the combustion chamber. 
     First, the structure of the working gas circulation engine according to the first embodiment of the invention will be described with reference to  FIG. 1 . 
     The working gas circulation engine includes an engine body  10  in which a combustion chamber CC is formed, a circulation path  20  that connects an inlet and an outlet of the combustion chamber CC to each other, an oxidant supply device  30  that supplies an oxidant into the combustion chamber CC, and a fuel supply device  40  that supplies the fuel into the combustion chamber CC. The combustion chamber CC and the circulation path  20  are filled with the working gas, and the working gas discharged from the combustion chamber CC is circulated back to the combustion chamber CC through the circulation path  20 .  FIG. 1  shows only one cylinder. However, the invention may be applied even when the engine body  10  includes multiple cylinders. 
     First, the engine body  10  will be described. 
     The engine body  10  includes a cylinder head  11  in which the combustion CC is formed, a cylinder block  12 , and a piston  13 . The piston  13  is connected to a crankshaft (not shown) via a connecting rod  14 , and is arranged in such a manner that the piston  13  may reciprocate within a space that is defined by a wall face of a recess  11   a  formed in a bottom face of the cylinder head  11  and a wall face of a cylinder bore  12   a  of the cylinder block  12 . The combustion chamber CC is a space defined by the wall face of the recess  11   a  of the cylinder head  11 , the wall face of the cylinder bore  12   a,  and a top face  13   a  of the piston  13 . 
     An intake port  11   b  and an exhaust port  11   c  that constitute part of the circulation path  20  are formed in the cylinder head  11 . The intake port  11   b  and the exhaust port  11   c  open at first ends into the combustion chamber CC. At an opening of the intake port  11   b  into the combustion chamber CC, there is provided an intake valve  15 , which opens the opening when the intake valve  11   b  is opened and which closes the opening when the intake valve  11   b  is closed. At an opening of the exhaust port  11   c  into the combustion chamber CC, there is provided an exhaust valve  16 , which opens the opening when the exhaust valve  11   c  is opened and which closes the opening when the exhaust valve  16  is closed. 
     For example, valves that are opened and closed in accordance with rotation of a camshaft (not shown) and elastic forces of elastic members (coil springs) may be used as the intake valve  15  and the exhaust valve  16 . When this type of valves are used as the intake valve  15  and exhaust valve  16 , a power transmission mechanism that is formed of, for example, a chain or a sprocket is provided between the camshaft and the crankshaft. With the power transmission mechanism, rotation of the camshaft is linked to rotation of the crankshaft. In this way, the intake valve  15  and the exhaust valve  16  are opened at prescribed opening timing and closed at prescribed closing timing. The engine body  10  may be provided with a variable valve mechanism, for example, a so-called variable valve timing and lift mechanism, which is able to change the opening timing/closing timing and the lift amounts of the intake valve  15  and the exhaust valve  16 . With this structure, the opening timing/closing timing and the lift amounts of the intake valve  15  and the exhaust valve  16  may be changed appropriately based on an engine operating state. Alternatively, a so-called electromagnetically-driven valve that opens and closes the intake valve  15  and the exhaust valve  16  using an electromagnetic force may be used in the engine body  10 . In this case as well, it is possible to obtain the same effects as those obtained by the variable valve operation mechanism. 
     Next, the circulation path  20  will be described. 
     The circulation path  20  is formed of the intake port  11   b,  the exhaust port  11   c , and a circulation passage  21  that connects a second end of the intake port  11   b  and a second end of the exhaust port  11   c  to each other. With this structure, a closed space is formed within the circulation path  20  and the combustion chamber CC. 
     In the working gas circulation engine, the working gas is supplied into the closed space, and the working gas is circulated in such a manner that the working gas is supplied from the circulation path  20  into the combustion chamber CC through the intake port  11   b,  from the combustion chamber CC to the circulation path  20  through the exhaust port  11   c,  and from the exhaust port  11   c  to the intake port  11   b  through the circulation passage  21 . When the intake valve  15  is opened, the working gas in the circulation passage  21  is supplied into the combustion chamber CC through the intake port  11   b . When the exhaust valve  16  is opened, the working gas in the combustion chamber CC is discharged together with the gas, which is obtained after the fuel is burned, to the circulation passage  21  through the exhaust port  11   c.  That is, the working gas discharged from the combustion chamber CC is circulated back to the combustion chamber CC through the circulation path  20  without being released into the atmosphere. 
     As the working gas, monatomic gas (more specifically, rare gas, for example, argon or helium) that has a specific heat ratio higher than that of air is used. In the first embodiment of the invention, argon (Ar) is used as the working gas. 
     More specifically, the circulation passage  21  according to the first embodiment of the invention is formed of a first circulation passage  21   a,  a second circulation passage  21   b,  a third circulation passage  21   c  and a fourth circulation passage  21   d.  The first circulation passage  21   a  connects the second end of the intake port  11   b  to an outlet  32   a  of an oxidant supply unit  32 , described later in detail, of the oxidant supply device  30 . The second circulation passage  21   b  connects the second end of the exhaust port  11   c  to an exhaust gas inlet  61   a  of an upstream-side condenser  61 , described later in detail. The third circulation passage  21   c  connects a working gas outlet  61   b  of the upstream-side condenser  61  to an exhaust gas inlet  62   a  of a downstream-side condenser  62 . The fourth circulation passage  21   d  connects a working gas outlet  62   b  of the downstream-side condenser  62  to a working gas inlet  32   b  of the oxidant supply unit  32 . 
     Note that, the expressions “upstream” and “downstream” in this specification mean “upstream” and “downstream” in the direction in which the exhaust gas discharged from the combustion chamber CC flows. 
     Next, the oxidant supply device  30  will be described. 
     The oxidant supply device  30  includes an oxidant storage tank  31  in which the oxidant is stored at high pressure, the oxidant supply unit  32  that supplies the oxidant into the circulation passage  21 , an oxidant supply passage  33  that connects the oxidant storage tank  31  to the oxidant supply unit  32 , a regulator  34  that is provided in the oxidant supply passage  33 , and an oxidant flowmeter  35 . In the oxidant supply passage  33 , the regulator  34  is provided at a position upstream of the oxidant flowmeter  35  (the regulator  34  is closer to the oxidant storage tank  31  than the flowmeter  35 ). 
     In the first embodiment of the invention, the oxidant is mixed with the working gas in the circulation passage  21  and then supplied to the circulation passage  21 , instead of being supplied alone to the circulation passage  21 . Therefore, the oxidant supply unit  32  used in the first embodiment of the invention is an oxidant mixing unit that mixes the oxidant from the oxidant supply passage  33  with the working gas from the circulation passage  21  and delivers the oxidant and the working gas from the outlet  32   a  to the circulation passage  21  at a portion that is downstream of the oxidant supply unit  32  (that is close to the intake port  11   b ). Therefore, the oxidant is supplied into the combustion chamber CC together with the working gas through the intake port  11   b  when the intake valve  15  is opened. 
     The regulator  34  regulates the pressure in the oxidant supply passage  33  at a portion downstream of the regulator  34  (at a portion close to the oxidant flowmeter  35 ) to a target pressure according to a command from an electronic control unit (ECU)  50 . In other words, the regulator  34  is used to control the flow rate of the oxidant in the oxidant supply passage  33 . The oxidant flowmeter  35  is a device that measures the flow rate of the oxidant in the oxidant supply passage  33 , which is regulated by the regulator  34 . A signal indicating the result of measurement performed by the oxidant flowmeter  35  is transmitted to the electronic control unit  50 . 
     In the first embodiment of the invention, oxygen (O 2 ) is used as the oxidant. Therefore, oxygen (O 2 ) is stored in the oxidant storage tank  31  at high pressure, for example, 70 MPa. 
     Next, the fuel supply device  40  will be described. 
     The fuel supply device  40  includes a fuel storage tank  41  in which the fuel is stored at high pressure, a fuel injection device  42  that injects the fuel, a fuel supply passage  43  that connects the fuel storage tank  41  to the fuel injection device  42 , a regulator  44  that is provided in the fuel supply passage  43 , a fuel flowmeter  45 , and a surge tank  46 . In the fuel supply passage  43 , the regulator  44 , the fuel flowmeter  45  and the surge tank  46  are provided in this order from the upstream side (fuel storage tank  41 -side). 
     In the first embodiment of the invention, the fuel injection device  42  is provided at the cylinder head  11  so that the fuel is injected directly into the combustion chamber CC. The fuel injection device  42  is a so-called fuel injection valve that is controlled by the electronic control unit  50 . For example, the electronic control unit  50  controls the timing at which the fuel is injected and the injection amount of fuel based on the engine operating state, for example, the engine speed. 
     The regulator  44  regulates the pressure in the fuel supply passage  43  at a portion downstream of the regulator  44  (at a portion close to the fuel flowmeter  45  and the surge tank  46 ) to a prescribed pressure. In other words, the regulator  44  is used to control the flow rate of the fuel in the fuel supply passage  43 . The fuel flowmeter  45  is a device that measures the flow rate of the fuel in the fuel supply passage  43 , which is regulated by the regulator  44 . A signal indicating the result of measurement performed by the fuel flowmeter  45  is transmitted to the electronic control unit  50 . The surge tank  46  is used to reduce pulsations generated in the fuel supply passage  43  when the fuel injection device  42  injects the fuel. 
     In the first embodiment of the invention, hydrogen (H 2 ) is used as the fuel. Therefore, hydrogen (H 2 ) is stored in the fuel storage tank  41  at high pressure, for example, 70 MPa. 
     In the working gas circulation engine according to the first embodiment of the invention, hydrogen (H 2 ), used as the fuel, and oxygen (O 2 ), used as the oxidant, are supplied into the combustion chamber CC, and diffusion combustion of the hydrogen (H 2 ) is performed. Therefore, in this working gas circulation engine, high-pressure hydrogen (H 2 ) is injected into high-temperature compressed gas (oxygen (O 2 ) and argon (Ar)) formed in the combustion chamber CC, whereby part of the hydrogen (H 2 ) self-ignites. Then, the hydrogen (H 2 ) and the compressed gas (oxygen (O 2 )) are burned while being diffusively mixed together. Due to the combustion of the hydrogen (H 2 ), the hydrogen (H 2 ) and the oxygen (O 2 ) bind together to form water vapor (H 2 O) and thermal expansion of argon (Ar) that has high specific heat ratio takes place in the combustion chamber CC. Therefore, in the working gas circulation engine, the piston  13  is pushed down due to the diffusion combustion of hydrogen (H 2 ) and the thermal expansion of argon (Ar), whereby power is generated. 
     When the combustion of hydrogen (H 2 ) and the thermal expansion of argon (Ar) are completed (e.g. when the piston  13  is near the bottom dead center), the water vapor (H 2 O) and the argon (Ar) are discharged from the combustion chamber CC into the exhaust port  11   c  when the exhaust valve  16  is opened. The discharged argon (Ar) needs to be circulated back to the combustion chamber CC through the circulation path  20  and the intake port  11   b  so that the thermal efficiency of the engine body  10  is enhanced. However, the water vapor (H 2 O) that is discharged together with the argon (Ar) is triatomic, and has a specific heat ratio that is lower than that of the argon (Ar). Therefore, if the water vapor (H 2 O) is circulated back to the combustion chamber CC together with the argon (Ar), the thermal efficiency of the engine body  10  may be reduced. Therefore, a device that removes the water vapor (H 2 O) contained in the exhaust gas is provided in the circulation path  20 . 
     As the device that removes the water vapor (H 2 O) contained in the exhaust gas, there is used a condenser that cools the exhaust gas, which contains the argon (Ar) used as the working gas and the water vapor (H 2 O), with the use of a cooling medium (coolant, in this case), and condenses the water vapor (H 2 O) contained in the exhaust gas to separate the argon (Ar) and the condensed water (H 2 O) from each other and to remove the condensed water (H 2 O). In the first embodiment of the invention, at least two condensers of the above-mentioned type are provided in the circulation path  20  so that the high-temperature exhaust gas is cooled more efficiently while the condensers are more easily mounted in the engine compartment. 
     More specific description will be provided below. In the first embodiment of the invention, the upstream-side condenser  61 , which is at a position close to the outlet of the combustion chamber CC, and the downstream-side condenser  62 , which is at a position distant from the outlet of the combustion chamber CC than the upstream-side condenser  61  is, are provided in the circulation passage  21 , as shown in  FIG. 1 . 
     In this case, the condensers  61  and  62  are provided in the circulation passage  21  at positions upstream of the oxidant supply unit  32 . At least the upstream-side condenser  61  is arranged in such a manner that the exhaust gas inlet thereof is at a position close to the outlet of the combustion chamber CC. With this structure, the high-temperature exhaust gas is cooled at an early stage. In other words, the upstream-side condenser  61  is provided at a position close to the engine body  10 . Note that, in  FIG. 1 , the upstream-side condenser  61  is at a position distant from the engine body  10  (outlet of the combustion chamber CC) just for convenience in illustration. 
     The exhaust gas inlet  61   a  of the upstream-side condenser  61  is connected to the second circulation passage  21   b,  and the working gas outlet  61   b  of the upstream-side condenser  61  is connected to the third circulation passage  21   c.  In the upstream-side condenser  61 , the high-temperature exhaust gas flowing from the exhaust gas inlet  61   a  is cooled by the circulating coolant, whereby the water vapor (H 2 O) contained in the exhaust gas is condensed and the argon (Ar) and the condensed water (H 2 O) are separated from each other. The coolant is circulated between the upstream-side condenser  61  and a radiator  64  by a water pump  63 . In the upstream-side condenser  61 , if the temperature of the exhaust gas flowing therein is low, the entirety of the water vapor (H 2 O) contained in the exhaust gas is condensed into the condensed water (H 2 O). However, if the temperature of the exhaust gas flowing in the upstream-side condenser  61  is high, part of the water vapor (H 2 O) may remain uncondensed. Therefore, the argon (Ar), or the argon (Ar) and the water vapor (H 2 O) is/are discharged from the upstream-side condenser  61  into the third circulation passage  21   c  through the working gas outlet  61   b , while the condensed water (H 2 O) is discharged into a condensed water passage  22  through a condensed water outlet  61   c.  The condensed water (H 2 O) is discharged to the outside of the working gas circulation engine when the electronic control unit  50  opens an on-off valve  65  which has been fully closed. Note that, in at least one of the two condensers, the coolant used to cool the engine body  10  is used. 
     Next, the downstream-side condenser  62  will be described. The structure of the downstream-side condenser  62  is similar to that of the upstream-side condenser  61 . The exhaust gas inlet  62   a  of the downstream-side condenser  62  is connected to the third circulation passage  21   c,  and the working gas outlet  62   b  of the downstream-side condenser  62  is connected to the fourth circulation passage  21   d.  The exhaust gas (the argon (Ar), or the argon (Ar) and the water vapor (H 2 O)), which is cooled in the upstream-side condenser  61 , flows into the downstream-side condenser  62  through the exhaust gas inlet  62   a.  Therefore, in the downstream-side condenser  62 , if the water vapor (H 2 O) remains in the exhaust gas, the exhaust gas is cooled by the circulating coolant, whereby the water vapor (H 2 O) in the exhaust gas is condensed and the argon (Ar) and the condensed water (H 2 O) are separated from each other. The coolant is circulated between the downstream-side condenser  62  and a radiator  67  by a water pump  66 . In this case, the exhaust gas is cooled to the normal temperature because the capacities of the condensers  61  and  62  and the radiators  64  and  67  are appropriately set as described later in detail. Therefore, the entirety of the water vapor (H 2 O) that remains in the exhaust gas is condensed into the condensed water (H 2 O). In the downstream-side condenser  62 , if the water vapor (H 2 O) does not remain in the exhaust gas, the argon (Ar) is delivered to the working gas outlet  62   b  without being cooled or after being cooled. Therefore, the argon (Ar) is discharged from the downstream-side condenser  62  into the fourth circulation passage  21   d  through the working gas outlet  62   b , while the condensed water (H 2 O), if it is formed in the downstream-side condenser  62 , is discharged into a condensed water passage  23  through a condensed water outlet  62   c . The condensed water (H 2 O) is discharged to the outside of the working gas circulation engine when the electronic control unit  50  opens an on-off valve  68  which has been fully closed. 
     The capacities (i.e., exhaust gas cooling performance) of the condensers  61  and  62  and the radiators  64  and  67  are set in such a manner that, if the exhaust gas, which has the highest possible temperature that may be achieved during engine operation, is discharged from the combustion chamber CC, the temperature of the exhaust gas is finally reduced to the temperature (normal temperature) at which the entirety of the water vapor (H 2 O) in the exhaust gas is condensed. That is, the capacities of the condensers  61  and  62  and the radiators  64  and  67  are set in such a manner that the water vapor (H 2 O) in the exhaust gas is almost entirely entirely removed by the time the exhaust gas finishes passing through the downstream-side condenser  62 . Thus, when the exhaust gas is circulated back to the combustion chamber CC, the water vapor (H 2 O) that has a low specific heat ratio is not supplied into the combustion chamber CC, and the argon (Ar) that is used as the working gas having a high specific heat ratio is supplied into the combustion chamber CC. Therefore, the engine is operated while the high heat efficiency is maintained by the working gas. 
     These capacities are set based on the results of experiments and simulations. Preferably, the capacities of the condensers  61  and  62  and the radiators  64  and  67  are set in such a manner that the capacity of the upstream-side condenser  61  is larger than the capacity of the downstream-side condenser  62 . In this way, the condensers  61  and  62  are mounted in the engine compartment more easily. With this structure, the exhaust gas discharged from the combustion chamber CC is cooled greatly in the upstream-side condenser  61 , which has a larger capacity, at an early stage. Therefore, a significant temperature increase in the circulation path  20  due to the high-temperature exhaust gas is prevented, and an excessive increase in the pressure in the circulation path  20  is also prevented. Accordingly, in the working gas circulation engine, it is possible to maintain sufficient durability of the circulation path  20  and to prevent leakage of the exhaust gas through portions at which the circulation path  20  is connected to the engine body  10 , etc. 
     The exhaust gas discharged from the combustion chamber CC may contain not only the water vapor (H 2 O) and the argon (Ar) but also hydrogen (H 2 ) or oxygen (O 2 ). For example, when the amount of hydrogen (H 2 ) supplied into the combustion chamber CC is larger than the amount of oxygen (O 2 ) supplied into the combustion chamber CC, part of the hydrogen (H 2 ) is left unburned and the unburned hydrogen (H 2 ) is discharged to the circulation path  20 . On the other hand, when the amount of oxygen (O 2 ) supplied into the combustion chamber CC is larger than the amount of hydrogen (H 2 ) supplied into the combustion chamber CC, part of the oxygen (O 2 ) is left unused and the unused oxygen (O 2 ) is discharged to the circulation path  20 . The hydrogen (H 2 ) or the oxygen (O 2 ) in the exhaust gas is separated from the water vapor (H 2 O) in the condensers  61  and  62 , and then discharged to the fourth circulation passage  21   d  together with the argon (Ar). Therefore, the hydrogen (H 2 ) or the oxygen (O 2 ) is also circulated back to the combustion chamber CC. 
     Therefore, in the working gas circulation engine, in order to prevent the amount of hydrogen (H 2 ) or oxygen (O 2 ) in the combustion chamber CC from being excessive, the amount of hydrogen (H 2 ) or the amount of oxygen (O 2 ) in the exhaust gas is determined, and the amount of hydrogen (H 2 ) that is injected from the fuel supply device  40  or the amount of oxygen (O 2 ) that is supplied from the oxidant supply device  30  is adjusted with the timing, at which the hydrogen (H 2 ) or the oxygen (O 2 ) reaches the combustion chamber CC, taken into account. In the first embodiment of the invention, a hydrogen concentration detection device (a hydrogen concentration sensor  71 ) that detects the hydrogen concentration in the exhaust gas and an oxygen concentration detection device (an oxygen concentration sensor  72 ) that detects the oxygen concentration in the exhaust gas are provided in the fourth circulation passage  21   d  of the circulation passage  21 . Then, the hydrogen concentration sensor  71  and the oxygen concentration sensor  72  transmit signals indicating detection results to the electronic control unit  50 . In this way, the electronic control unit  50  determines the amount of hydrogen (H 2 ) or oxygen (O 2 ) that remains in the exhaust gas based on the detection signal, and controls the amount of hydrogen (H 2 ) that is injected from the fuel injection device  42  or the target pressure for the regulator  34  (that is, the supply amount of oxygen (O 2 )) with the timing, at which the hydrogen (H 2 ) or the oxygen (O 2 ) reaches the combustion chamber CC, taken into account. 
     As described above, in the working gas circulation engine according to the first embodiment of the invention, at least two condensers are provided. Therefore, it is possible to efficiently cool the high-temperature exhaust gas without reducing the ease in mounting the condensers in the engine compartment (i.e., without upsizing the condenser and the radiator in order to increase the capacities of the condenser and the radiator), and without significantly increasing the vehicle weight. Therefore, the exhaust gas that has passed through the downstream-side condenser  62  contains no water vapor (H 2 O) or only a small amount of water vapor (H 2 O). As a result, in the working gas circulation engine, it is possible to prevent a decrease in the thermal efficiency that may be caused by the water vapor (H 2 O) having a low specific heat ratio. 
     Further, in the working gas circulation engine, it is possible to prevent an excessive increase in the pressure in the circulation path  20  because the exhaust gas cooling performance is improved. Therefore, sufficient durability of the circulation path  20  of the working gas circulation engine is maintained. Further, in the working gas circulation engine, leakage of the exhaust gas through the portions at which the circulation path  20  is connected to the engine body  10 , etc. is prevented. As a result, it is possible to prevent a decrease in the thermal efficiency that may be caused due to shortage of the working gas. 
     Next, a working gas circulation engine according to a second embodiment of the invention will be described with reference to  FIGS. 2 and 3 . 
     In the working gas circulation engine according to the second embodiment of the invention, an upstream-side condenser  161  is used instead of the upstream-side condenser  61  used in the working gas circulation engine according to the first embodiment of the invention. In the second embodiment of the invention, a circulation path  120 , which is formed by changing part of the structure of the circulation path  20  in the first embodiment of the invention, is used because the upstream-side condenser  161  is used instead of the upstream-side condenser  61 . 
     In the first embodiment of the invention described above, the upstream-side condenser  61  is provided at a position close to the engine body  10  (outlet of the combustion chamber CC). However, in the first embodiment of the invention, there is the second circulation passage  21   b  between the engine body  10  (outlet of the combustion chamber CC) and the upstream-side condenser  61 . Therefore, the circulation path  20  may be warmed by the high-temperature exhaust gas in the second circulation passage  21   b,  and the exhaust gas (working gas, etc.) may leak through the portions at which the second circulation passage  21   b  is connected to the engine body  10 , etc. That is, there is room for improvement in the exhaust gas cooling performance in the working gas circulation engine according to the first embodiment of the invention. 
     Therefore, according to the second embodiment of the invention, the exhaust gas cooling performance is further improved by using the upstream-side condenser  161 , the circulation path  120 , etc. shown in  FIG. 2  instead of the upstream-side condenser  61 , the circulation path  20 , etc. 
     The upstream-side condenser  161  according to the second embodiment of the invention is provided at a position as close as possible to the combustion chamber CC. 
     For example, in an example shown in  FIG. 2 , an exhaust gas inlet  161   a  is arranged at the downstream-side end of the exhaust port  11   c,  and the upstream-side condenser  161  is fitted to the engine body  10  or formed integrally with the engine body  10 . A working gas outlet  161   b  of the upstream-side condenser  161  is connected to a second circulation passage  121   b,  and the exhaust gas (the argon (Ar), or the argon (Ar) and the water vapor (H 2 O)), which is quickly cooled in the upstream-side condenser  161  after being discharged from the exhaust port  11   c,  is delivered to the downstream-side condenser  62  through the second circulation passage  121   b.    
     As in the first embodiment of the invention, the fourth circulation passage  21   d  is connected to the working gas outlet  62   b  of the downstream-side condenser  62 . 
     That is, the circulation path  120  according to the second embodiment of the invention is formed by replacing the circulation passage  21  in the first embodiment of the invention with a circulation passage  121  in  FIG. 2 . The circulation passage  121  is formed of the first circulation passage  21  a and the fourth circulation passage  21   d , which are used in the first embodiment of the invention as well, and the second circulation passage  121   b  between the upstream-side condenser  161  and the downstream-side condenser  62 . 
     In the upstream-side condenser  161 , the high-temperature exhaust gas, which is supplied from the exhaust port  11   c  into the upstream-side condenser  161  through the exhaust gas inlet  161   a,  is cooled by the circulating coolant, whereby the water vapor (H 2 O) contained in the exhaust gas is condensed and the argon (Ar) and the condensed water (H 2 O) are separated from each other. That is, the exhaust gas is cooled at an earlier stage in the upstream-side condenser  161  in the second embodiment of the invention than in the upstream-side condenser  61  in the first embodiment of the invention. Accordingly, it is possible to more appropriately suppress increases in the temperature and the pressure in the circulation path due to the high-temperature exhaust gas. If the temperature of the exhaust gas flowing into the upstream-side condenser  161  is low, the water vapor (H 2 O) in the exhaust gas is almost entirely entirely condensed into the condensed water (H 2 O). On the other hand, if the temperature of the exhaust gas flowing into the upstream-side condenser  161  is high, part of the water vapor (H 2 O) may remain uncondensed. Therefore, only the argon (Ar), or the argon (Ar) and the water vapor (H 2 O) is/are discharged from the working gas outlet  161   b  of the upstream-side condenser  161 , while the condensed water (H 2 O) is discharged into a condensed water passage  122  through a condensed water outlet  161   c.  The condensed water (H 2 O) is discharged to the outside of the working gas circulation engine when the electronic control unit  50  opens an on-off valve  165  which has been fully closed. 
     In the upstream-side condenser  161 , the coolant may be circulated and cooled by the water pump  63  and the radiator  64  which are arranged in the same manner as that in the first embodiment of the invention. However, the coolant for the engine body  10  is used in this case. 
     In order to form a circulation path through which the coolant from the engine body  10  is circulated, there are provided a first coolant passage  81  through which the coolant is introduced from a coolant passage in the engine body  10  (more specifically, the cylinder head  11 ) into the upstream-side condenser  161 , a second coolant passage  82 . through which the coolant discharged from the upstream-side condenser  161  is returned to a coolant passage in the engine body  10  (more specifically, the cylinder block  12 ), and a water pump  163  that is provided in the second coolant passage  82  and that delivers the coolant in the second coolant passage  82  toward the engine body  10  (cylinder block  12 ), as shown in  FIG. 2 . In this case, the coolant is circulated between the engine body  10  and the upstream-side condenser  161  by the water pump  163 . 
     If the coolant that is warmed by passing through the upstream-side condenser  161  is returned to the engine body  10  without being cooled, the engine body  10  is not cooled sufficiently. Therefore, there are provided a radiator  164  that cools the coolant which has passed through the upstream-side condenser  161 , and a thermostat  169  that controls the manner in which the coolant that has passed through the upstream-side condenser  161  is returned to the engine body  10  based on the temperature of the coolant. That is, it is determined whether the coolant that has passed through the upstream-side condenser  161  should be returned to the engine body  10  without passing through the radiator  164  or the coolant should be returned to the engine body  10  after passing through the radiator  164 . 
     The radiator  164  is connected to the second cooling passage  82  via two coolant passages (a third coolant passage  83  and a fourth coolant passage  84 ) shown in  FIG. 2 . Through the third coolant passage  83 , the coolant in the coolant passage  82  is introduced into the radiator  164 . Through the fourth coolant passage  84 , the coolant is returned from the radiator  164  to the second coolant passage  82 . 
     The thermostat  169  is provided at a position at which the second coolant passage  82  and the fourth coolant passage  84  are connected to each other. The thermostat  169  determines, based on the temperature of the coolant, whether the coolant should be returned to the engine body  10  without being cooled in the radiator  164  or the coolant should be returned to the engine body  10  after being cooled in the radiator  164 . Such a determination may be made based on whether the engine body cooling efficiency will be reduced if the coolant, which is warmed by passing through the upstream-side condenser  161 , is returned to the engine body  10  without being cooled. 
     In the second embodiment of the invention, the capacities (i.e., exhaust gas cooling performance) of the condensers  161  and  62  and the downstream-side radiator  67  are set in such a manner that, if the exhaust gas, which has the highest possible temperature that may be achieved during engine operation, is discharged from the combustion chamber CC, the temperature of the exhaust gas is finally reduced to the temperature (normal temperature) at which the entirety of the water vapor (H 2 O) in the exhaust gas is condensed. That is, the capacities of the condensers  161  and  62  and the radiator  67  are set in such a manner that the entirety of the water vapor (H 2 O) in the exhaust gas is removed by the time the exhaust gas finishes passing through the downstream-side condenser  62 . Thus, when the exhaust gas is circulated back to the combustion chamber CC, the water vapor (H 2 O) that has a low specific heat ratio is not supplied into the combustion chamber CC, and the argon (Ar) that is used as the working gas having a high specific heat ratio is supplied into the combustion chamber CC. Therefore, the engine is operated while the high heat efficiency is maintained by the working gas. 
     These capacities are set based on the results of experiments and simulations. Preferably, the capacities of the condensers  161  and  62  and the radiator  67  are set in such a manner that the capacity of the upstream-side condenser  161  is larger than the capacity of the downstream-side condenser  62 . In this way, the condensers  161  and  62  are mounted in the engine compartment more easily. With this structure, the exhaust gas discharged from the combustion chamber CC is cooled greatly in the upstream-side condenser  161 , which has a larger capacity, at an early stage. In addition, the distance between the upstream-side condenser  161  and the outlet of the combustion chamber CC is shorter than the distance between the upstream-side condenser  61  and the outlet of the combustion chamber CC in the first embodiment of the invention. Therefore, a significant temperature increase in the circulation path due to the high-temperature exhaust gas is prevented more reliably, and an excessive increase in the pressure in the circulation path is also prevented more reliably according to the second embodiment of the invention. Accordingly, in the working gas circulation engine according to the second embodiment of the invention, it is possible to maintain sufficient durability of the circulation path  120  and to prevent leakage of the exhaust gas through portions at which the circulation path  120  is connected to the engine body  10 , etc. 
     As described above, in the working gas circulation engine according to the second embodiment of the invention, at least two condensers are provided, and one of the condensers is provided in such a manner that the distance between this condenser and the engine body  10  (outlet of the combustion chamber CC) is shorter than the distance between the upstream-side condenser  61  and the engine body  10  in the first embodiment of the invention. Therefore, it is possible to cool the high-temperature exhaust gas more efficiently in the second embodiment of the invention than in the first embodiment of the invention, without reducing the ease in mounting the condensers in the engine compartment (i.e., without upsizing the condenser and the radiator in order to increase the capacities of the condenser and the radiator), and without significantly increasing the vehicle weight. The amount of high-temperature exhaust gas that is present in the circulation path at a portion between the combustion chamber CC and the upstream-side condenser is smaller in the working gas circulation engine according to the second embodiment of the invention than in that according to the first embodiment of the invention. Accordingly, higher exhaust gas cooling performance is achieved according to the second embodiment of the invention. The exhaust gas that has passed through the downstream-side condenser  62  contains no water vapor (H 2 O). As a result, in the working gas circulation engine according to the second embodiment of the invention as in the working gas circulation engine according to the first embodiment of the invention, it is possible to prevent a decrease in the thermal efficiency that may be caused by the water vapor (H 2 O) having a low specific heat ratio. 
     Further, it is possible to prevent an excessive increase in the pressure in the circulation path more reliably by providing higher exhaust gas cooling performance in the working gas circulation engine according to the second embodiment of the invention than in the working gas circulation engine according to the first embodiment of the invention. Therefore, sufficient durability of the circulation path of the working gas circulation engine is maintained more reliably according to the second embodiment of the invention. Further, in the working gas circulation engine according to the second embodiment of the invention, leakage of the exhaust gas through the portions at which the circulation path  120  is connected to the engine body  10 , etc. is prevented. As a result, it is possible to prevent a decrease in the thermal efficiency that may be caused due to shortage of the working gas. 
     The upstream-side condenser  161  may be provided in such a manner that the distance between the upstream-side condenser  161  and the outlet of the combustion chamber CC is shorter than that described above. With this structure, the above-described effect is obtained more reliably. For example, the upstream-side condenser  161  may be formed in such a manner that the coolant passage is formed within the cylinder head  11  at a position around the exhaust port  11   c  and the passage through which the condensed water (H 2 O) is discharged to the outside of the working gas circulation engine is formed within the cylinder head  11  so as to branch from the exhaust port  11   c.    
     When the engine body  10  has multiple cylinders, a condenser  261  may be provided at a gathering portion  17   a  of an exhaust manifold  17 , as shown in  FIG. 3 . With this structure, the above-described effect is obtained more reliably. In this case, the condenser  261  is fitted to the exhaust manifold  17  by bonding an exhaust gas inlet  261   a  and an end of the gathering portion  17   a  together, and the second circulation passage  121   b  is connected to a working gas outlet  261   b  of the condenser  261 . Although not shown in  FIG. 3 , the downstream-side condenser  62  and the radiator  67 , which are the same as those described above, are provided downstream of the condenser  261 . 
     In the condenser  261 , the high-temperature exhaust gas, which is supplied from the cylinders through the exhaust manifold  17  and the exhaust gas inlet  261   a  into the condenser  261 , is cooled by the circulating coolant, and the water vapor (H 2 O) contained in the exhaust gas is condensed and the argon (Ar) and the condensed water (H 2 O) are separated from each other. The exhaust gas is cooled at an earlier stage in the condenser  261  than in the upstream-side condenser  61  according to the first embodiment of the invention. Accordingly, it is possible to more appropriately suppress increases in the temperature and the pressure in the circulation path that may be caused by the high-temperature exhaust gas. If the temperature of the exhaust gas that flows in the condenser  261  is low, the entirety of the water vapor (H 2 O) in the exhaust gas is condensed into the condensed water (H 2 O). However, if the temperature of the exhaust gas that flows in the condenser  261  is high, part of the water vapor (H 2 O) may remain uncondensed. Therefore, only the argon (Ar), or the argon (Ar) and the water vapor (H 2 O) is/are discharged from the working gas outlet  261   b  of the condenser  261 , while the condensed water (H 2 O) is discharged into a condensed water passage  222  through a condensed water outlet  261   c.  The condensed water (H 2 O) is discharged to the outside of the working gas circulation engine when the electronic control unit  50  opens an on-off valve  265  which has been fully closed. 
     The coolant that flows in the condenser  261  is circulated between the condenser  261  and a radiator  264  by a water pump  263 . Alternatively, the coolant for the engine  10  may be used as the coolant that flows in the condenser  261 , as in the example shown in  FIG. 2 . In this case, the radiator  164 , etc. may be used, as in the example shown in  FIG. 2 . 
     The capacities (i.e., exhaust gas cooling performance) of the condensers  261  and  62  and the radiators  264  and  67  are set in such a manner that, if the exhaust gas having the highest possible temperature that may be achieved during engine operation is discharged from the combustion chamber CC, the temperature of the exhaust gas is finally reduced to the temperature (normal temperature) at which the water vapor (H 2 O) in the exhaust gas is almost entirely entirely condensed. That is, the capacities of the condensers  261  and  62  and the radiators  264  and  67  are set in such a manner that the entirety of the water vapor (H 2 O) in the exhaust gas is removed by the time the exhaust gas has passed through the downstream-side condenser  62 . Thus, when the exhaust gas is circulated back to the combustion chamber CC, the water vapor (H 2 O) that has a low specific heat ratio is not supplied into the combustion chamber CC, and the argon (Ar) that is used as the working gas having a high specific heat ratio is supplied into the combustion chamber CC. Therefore, the engine is operated while the high heat efficiency is maintained by the working gas. 
     In the first and second embodiments of the invention, the fuel injection device  42  is provided in such a manner that the fuel is injected, directly into the combustion chamber CC. Alternatively, the fuel injection device  42  may be fitted to the cylinder head  11  so that the fuel is injected into the intake port  11   b.  That is, the invention described in the first and second embodiments may be applied to a so-called port injection working gas circulation engine. In this case as well, the same effects as those in the first and second embodiments may be obtained. 
     In the working gas circulation engine according to the first and second embodiments of the invention, diffuse combustion of hydrogen (H 2 ), used as fuel, is performed. Alternatively, the fuel may be ignited by a spark plug (not shown) and so-called spark ignition combustion may be performed. Further alternatively, a spark plug may be used to assist ignition and diffuse combustion may be performed. In various working gas circulation engines that differ in combustion manner, it is possible to obtain the effects that are the same as those in the first and second embodiments of the invention. 
     As described above, the gas circulation engines according to the embodiments of the invention are useful in improving the exhaust gas cooling performance.