Patent Publication Number: US-2021172656-A1

Title: Gas heat-pump system

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority to Korean Patent Application No. 10-2019-0161649, filed Dec. 6, 2019, the entire contents of which is incorporated herein for all purposes by this reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to a gas heat-pump system and, more particularly, to a gas heat-pump system in which switching to a first-stage turbocharge mode or a second-stage turbocharge mode takes place according to a required load ratio of an air conditioning module, and in which a first turbocharger and a second turbocharger, which are drivable independently of each other, operate in the first-stage turbocharge mode or the second-stage turbocharge mode. Thus, the first and second turbochargers can actively operate in a manner that corresponds to a change in load, and the efficiency of the gas heat-pump system can be improved. 
     Description of the Related Art 
     A heat-pump system is a system that is capable of performing a cooling or heating operation through a refrigeration cycle, and operates in cooperation with a hot water supply apparatus or a cooling and heating apparatus. 
     That is, hot water is produced or air conditioning for cooling and heating is performed using a heat source that is obtained as a result of heat exchange occurring between cooling refrigerant in the refrigeration cycle and a predetermined heat storage medium. 
     Generally, a configuration for the refrigeration cycle requires that a compressor compressing refrigerant, a condenser condensing the refrigerant compressed by the compressor, an expansion device decompressing the refrigerant condensed by the condenser, and an evaporator evaporating the decompressed refrigerant are included. 
     The heat-pump systems are categorized into electric heat-pump systems and gas heat-pump systems according to a type of drive source for driving the compressor. 
     The electric heat-pump systems, which have a lower load capacity, are suitable for family use. 
     The gas heat-pump systems, which have a high load capacity, are suitable for industrial use or for large buildings. 
     Therefore, instead of an electric motor, the gas heat-pump system uses a gas engine in order to drive a high capacity compressor suitable for this high load capacity. 
     The gas heat-pump system is configured to include an engine that burns a mixture of gaseous and air and (hereinafter referred to as a “a fuel-to-air mixture) and thus generates a motive force, a fuel supply device, a mixer for mixing air and gaseous fuel, and a device for supplying the fuel-to-air mixture to the engine. 
     Regarding the device for supplying the fuel-to-air mixture, a turbocharger that applies pressure to the fuel-to-air mixture to increase the efficiency and output of the engine and supplies the mixture of air and gaseous to the engine is generally used. 
     Patent Document 1 discloses a turbocharger that rotates an impeller using a turbine, as a drive source, which is rotated with exhaust gas. 
     Furthermore, Patent Document 1 discloses a configuration of a two-level pressure application-type turbocharger that applies first-level and second-level pressure to the fuel-to-air mixture to secure a high output of the engine. 
     However, in the configuration disclosed in Patent Document 1, because the driving of the turbocharger depends only on the exhaust gas from the engine, operation cannot be actively performed according to a change in load on an air conditioning module. 
     In addition, in the configuration disclosed in Patent Document 1, because the exhaust gas is supplied in a divided manner to a first turbine and a second turbine in a second-stage turbocharge mode, the efficiency of the turbocharger is inevitably lowered in a low rpm section for the engine. 
     The foregoing is intended merely to aid in the understanding of the background of the present disclosure, and is not intended to mean that the present disclosure falls within the purview of the related art that is already known to those skilled in the art. 
     DOCUMENT OF RELATED ART 
     Patent Document 
     
         
         (Patent Document 1) Korean Patent No. 10-2005093 
       
    
     SUMMARY OF THE INVENTION 
     An objective of the present disclosure is to provide a gas heat-pump system in which switching to a first-stage turbocharge mode or a second-stage turbocharge mode takes place according to a required load ratio of an air conditioning module, and in which a first turbocharger and a second turbocharger, which are drivable independently of each other, operate in the first-stage turbocharge mode or the second-stage turbocharge mode. Thus, the first and second turbochargers can actively operate in a manner that corresponds to a change in load, and the efficiency of the gas heat-pump system can be improved. 
     Another objective of the present disclosure is to provide a gas heat-pump system in which exhaust gas is guided to a fuel-to-air mixture by a turbocharger and in which a portion of the exhaust gas is recirculated to an engine. In the gas heat-pump system, an amount of discharged exhaust gas is decreased, the exhaust gas is forced to be discharged by the turbocharger, and thus an output of the engine is increased. 
     According to an aspect of the present disclosure, there is provided a gas heat-pump system including: a compressor of an air conditioning module; a gas engine generating a drive force of the compressor; and a turbocharger applying first-level pressure to a fuel-to-air mixture and supplying the resulting fuel-to-air mixture to the gas engine or applying second-level pressure to the fuel-to-air mixture to which the first-level pressure is applied and supplying the resulting fuel-to-air mixture to the gas engine. 
     In the gas heat-pump system, the turbocharger may include: a first turbocharger including: a first motor; and a first impeller driven by the first motor; and a second turbocharger including: a second motor; and a second impeller driven by the second motor, wherein, when applying the first-level pressure for supply, the first-level pressure may be applied to the fuel-to-air mixture by the second impeller, and when applying the second-level pressure for supply, the first-level pressure may be applied to the fuel-to-air mixture by the first impeller, and the second-level pressure may be applied to the fuel-to-air mixture by the second impeller. 
     The gas heat-pump system may further include: a mixture device guiding the fuel-to-air mixture to the turbocharger; a connection pipe connecting a discharge port of the first turbocharger and an inlet port of the second turbocharger to each other for a fluid flow, the mixture device being connected to the connection pipe at a position between the discharge port of the first turbocharger and the inlet port of the second turbocharger; a first turbocharger entrance pipe connected to the mixture device upstream from a position where the connection pipe is connected to the mixture device and guiding the fuel-to-air mixture to a first inlet port of the first turbocharger; and a first switch valve arranged at a position where the first turbocharger entrance pipe is connected to the mixture device and switching a flowing direction of the fuel-to-air mixture flowing within the mixture device. 
     In the gas heat-pump system, the first switch valve may include: a first port connected to the mixture device; a second port connected to the mixture device in a manner that faces the first port; and a third port connected to the first turbocharger entrance port. 
     The gas heat-pump system may further include; an exhaust pipe guiding exhaust gas discharged from the gas engine to the outside; a first bypass pipe connecting the exhaust pipe and a second inlet port of the first turbocharger to each other for the fluid flow and at least partly guiding the exhaust gas to the first turbocharger; a second switch valve arranged at a position where the first bypass pipe is connected to the exhaust pipe and allowing or blocking the fluid flow between the first bypass pipe and the exhaust pipe; a second bypass pipe of which a first end portion is connected to the exhaust pipe downstream from the position where the first bypass pipe is connected to the exhaust pipe and of which a second end portion is connected to the connection pipe somewhere between a position where the mixture device is connected to the connection pipe and the discharge port of the first turbocharger; and a third switch valve arranged at a position where the second bypass pipe and the connection pipe are connected to each other and allowing or blocking the fluid flow between the second bypass pipe and the connection pipe, wherein the second switch valve may include: a fourth port connected to the exhaust pipe; a fifth port connected to the exhaust pipe in a manner that faces the fourth port; and a sixth port connected to the first bypass pipe, and the third switch valve may include: a seventh port connected to the connection pipe; an eighth port connected to the connection pipe in a manner that faces the seventh port; and a ninth port connected to the second bypass pipe. 
     The gas heat-pump system may further include: a controller adjusting an output rpm of the gas engine in a manner that corresponds to a required load ratio of the air conditioning module, wherein when it is determined that the required load ratio is lower than a first reference load ratio, the controller may perform control in such a manner that the turbocharger operates in a first turbocharger operation mode, when it is determined that the required load ratio is equal to or higher than the first reference load ratio and is lower than a second reference load ratio, the controller may perform control in such a manner that the turbocharger operates in a second turbocharger operation mode, when it is determined that the required load ratio is equal to or higher than the second reference load ratio, the controller may perform control in such a manner that the turbocharger operates in a third turbocharger operation mode, and the second reference load ratio may be higher than the first reference load ratio. 
     In the gas heat-pump system, the first reference load ratio may be 30% of a maximum amount of load on the air conditioning module, and the second reference load ratio may be 70% of the maximum amount of load. 
     In the gas heat-pump system, in the first turbocharge operation mode, the controller may set a flow path with respect to the first switch valve in such a manner that the first port and the second port are open and that the third port is closed, may set the flow path with respect to the second switch valve in such a manner that the fourth port and the fifth port are open and that the sixth port is closed, and may set the flow path with respect to the third switch valve in such a manner that at least two of the seventh port, the eighth port, and the ninth port are closed. 
     In the gas heat-pump system, in the first turbocharger operation mode, the controller may supply electric power to the second motor and thus may rotate the second impeller, and may block the electric power from being supplied to the first motor and thus may stop the first impeller. 
     In the gas heat-pump system, while operation in the first turbocharger operation mode is in progress, the controller may determine whether or not the output rpm of the gas engine reaches a target rpm corresponding to the required load ratio, and when it is determined that the output rpm of the gas engine reaches the target rpm, the controller may perform control in such a manner that the exhaust gas is at least partly introduced into the gas engine. 
     In the gas heat-pump system, in order to at least partly introduce the exhaust gas into the gas engine, the controller may maintain the flow path with respect to the first switch valve, may switch the flow path with respect to the second switch valve in such a manner that the fourth port and the fifth port are open and that the sixth port is partly open, and may switch the flow path with respect to the third switch valve in such a manner that the seventh port and the eighth port are open and that the ninth port is closed. 
     In the gas heat-pump system, the degree of opening to which the sixth port is partly open may range from 5 to 30%. 
     In the gas heat-pump system, in the second turbocharger operation mode, the controller may set the flow path with respect to the first switch valve in such a manner that the first port and the second port are open and that the third port is closed, may set the flow path with respect to the second switch valve in such a manner that the fourth port and the sixth port are open and that the fifth port is closed, and may set the flow path with respect to the third switch valve in such a manner that the seventh port and the ninth port are open and that the eighth port is closed. 
     In the gas heat-pump system, in the second turbocharger operation mode, the controller may supply electric power to the first motor and the second motor and thus may rotate the first impeller and the second impeller. 
     In the gas heat-pump system, while operation in the second turbocharger operation mode is in progress, the controller may determine whether or not the output rpm of the gas engine reaches a target rpm corresponding to the required load ratio, and when it is determined that the output rpm of the gas engine reaches the target rpm, the controller may perform control in such a manner that the exhaust gas is at least partly introduced into the gas engine. 
     In the gas heat-pump system, in order to at least partly introduce the exhaust gas into the gas engine, the controller may maintain the settings of the flow paths with respect to first switch valve and the second switch valve, and may switch the flow path with respect to the third switch valve in such a manner that the seventh port and the ninth port are open and that the eighth port is partly open. 
     In the gas heat-pump system, the degree of opening to which the eighth port is partly open may range from 5 to 30%. 
     In the gas heat-pump system, in the third turbocharger operation mode, the controller may set a flow path with respect to the first switch valve in such a manner that the first port and the third port are open and that the second port is closed, may set the flow path with respect to the second switch valve in such a manner that the fourth port and the fifth port are open and that the sixth port is closed, and may set the flow path with respect to the third switch valve in such a manner that the seventh port and the eighth port are open and that the ninth port is closed. 
     In the gas heat-pump system, in the third turbocharger operation mode, the controller may supply electric power to the first motor and the second motor and thus may rotate the first impeller and the second impeller. 
     In the gas heat-pump system, while operation in the third turbocharger operation mode is in progress, the controller may determine whether or not the output rpm of the gas engine reaches a target rpm corresponding to the required load ratio, and when it is determined that the output rpm of the gas engine reaches the target rpm, the controller may perform control in such a manner that the exhaust gas is at least partly introduced into the gas engine. 
     In the gas heat-pump system, in order to at least partly introduce the exhaust gas into the gas engine, the controller may maintain the settings of the flow paths with respect to the first switch valve and the third switch valve, and may switch the flow path with respect to second switch valve in such a manner that the fourth port and the fifth port are open and the sixth port is partly open. 
     In the gas heat-pump system, the degree of opening to which the sixth port is partly open may range from 5 to 30%. 
     In the gas heat-pump system according to the present disclosure, switching to a first-stage turbocharge mode or a second-stage turbocharge mode takes place according to the required load ratio of the air conditioning module, and the first turbocharger and the second turbocharger, which are drivable independently of each other, operate in the first-stage turbocharge mode or the second-stage turbocharge mode. Thus, the first and second turbochargers can actively operate in a manner that corresponds to a change in load, and the efficiency of the gas heat-pump system can be improved. 
     In addition, in the gas heat-pump system, the exhaust gas is guided to the fuel-to-air mixture by the turbocharger and a portion of the exhaust gas is recirculated to the engine. Thus, an amount of discharged exhaust gas can be decreased, the exhaust gas can be forced to be discharged by the turbocharger, and thus an output of the engine can be increased. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objectives, features, and other advantages of the present disclosure will be more clearly understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a view schematically illustrating a configuration of a gas heat-pump system according to an embodiment of the present disclosure; 
         FIG. 2  is a view illustrating a detailed configuration of an engine module in  FIG. 1 ; 
         FIG. 3  is a graph illustrating a process of switching a turbocharger operation mode according to a load ratio of an air conditioning module and a compressor operation stage; 
         FIG. 4  is a table illustrating compressor operations stages that are entered when using a plurality of the compressors in  FIG. 3 ; 
         FIGS. 5 to 10  are views each schematically illustrating a state where flow paths for a mixture of air and gaseous and exhaust gas are set for each turbocharger operation mode according to the embodiment of the present disclosure; 
         FIG. 11  is a block diagram illustrating a configuration of a controller of a gas heat-pump system according to the embodiment of the present disclosure; and 
         FIGS. 12 to 16  are flow charts each illustrating a method of controlling the gas heat-pump system according to the embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Exemplary embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings. 
     Various modifications can be made to the present disclosure, and thus various embodiments can be implemented. The resulting specific embodiments will also be described in detail below with reference to the drawings. This description is not intended to limit the present disclosure to the specific embodiments. All alterations, equivalents, and substitutes that are included within the technical idea of the present disclosure should be construed as falling within the scope of the present disclosure. 
     The terms first and second, and so on are used to describe various constituent elements, but should not be construed as imposing any limitation on the various constituent elements. These terms are used only to distinguish one element from another. For example, a first constituent element may be expressed as a second constituent element without departing from the scope of the present disclosure. In the same manner, the second constituent element may also be expressed as the first constituent element. 
     The phrase “and/or” is used to join two words, phrases, and sentences or to refer to one of the two words, phrases, and sentences. 
     It should be understood that, when a constituent element is referred to as being “coupled to” or “connected to” a different constituent element, this means that the constituent element may be directly coupled to or directly connected to the different constituent element or means that an intervening constituent element may be present therebetween. In contrast, it should be understood that, when a constituent element is referred to as being “directly coupled to” or “directly connected to” a different constituent element, this means that no intervening constituent element is present therebetween. 
     The terms used in the present specification are only for describing specific embodiments and are not intended to impose any limitation on the present disclosure. The indefinite article “a/an” is used to mean “one or more”, not only one, except as distinctively expressed in context. 
     The term “include,” “have” or the like in the present application is intended to indicate that a feature, a number, a step, an operation, a constituent element, a component, or combinations of these, which is described in the specification, is present, and thus should be understood not to preclude in advance the possibility that one or more other features, numbers, steps, operations, constituent elements, components, or combinations of these will be present or added. 
     Unless otherwise defined, each of the terms, including technical and scientific terms, which are used in the present specification, has the same meaning as is normally understood by a person of ordinary skill in the art to which the present disclosure pertains. The term as defined in commonly used dictionaries should be construed as having the same meaning in context in the art and, unless otherwise explicitly defined in the present specification, is not construed as having an ideal meaning or an excessively-formal meaning. 
     The embodiments will be provided below for illustrative purpose to help a person of ordinary skill in the art to get a full understanding of the present disclosure, and shapes, sizes, and the like of elements in the drawings can be exaggerated for clearer description. 
       FIG. 1  is a view schematically illustrating a configuration of a gas heat-pump system according to an embodiment of the present disclosure.  FIG. 2  is a view illustrating a detailed configuration of an engine module in  FIG. 1 . 
     With reference to  FIG. 1 , a gas heat-pump system  10  according to an embodiment of the present disclosure includes an air conditioning module, an engine module, and a cooling module. 
     The air conditioning module includes a plurality of components that are necessary for a refrigerant cycle. 
     For example, the air conditioning module includes a compressor  110  and a four-way valve  115 . The compressor  110  compresses refrigerant. The four-way valve  115  switches a direction of the refrigerant compressed in the compressor  110 . 
     The compressor  110  operates with a drive force generated by an engine  201  that will be described below, and serves to compress the refrigerant in a gas state and to discharge the resulting refrigerant. 
     A pulley and clutch assembly  112  is provided on a drive shaft of the compressor  110 . The drive force generated by the engine  210  is transferred to the compressor  110  through a belt  111  and the pulley and clutch assembly  112 . 
     In  FIG. 1 , a configuration in which the air conditioning module includes one compressor  110  is illustrated, but the air conditioning module may include a plurality of compressors  110 , depending on a load capacity of an indoor air conditioning condenser unit. 
     The plurality of compressors  110  each have the pulley and clutch assembly  112  to which the drive force generated by the engine  210  is selectively transferred. 
     Furthermore, the air conditioning module may further include an outdoor heat exchanger  120  and an indoor heat exchanger  140 . 
     The outdoor heat exchanger  120  is arranged within an outdoor air conditioning condenser unit that is installed outdoors, and the indoor heat exchanger  140  is arranged within the indoor air conditioning condenser unit that is installed indoors. 
     The refrigerant that passes through the four-way valve  115  flows to the outdoor heat exchanger  120  or the indoor heat exchanger  140 . 
     Components other than the indoor heat exchanger  140  and an indoor expansion device  145  of the gas heat-pump system  10 , which are illustrated in  FIG. 1 , are arranged outdoors, that is, are arranged within the outdoor air conditioning condenser unit. 
     In a case where the gas heat-pump system  10  operates in a cooling operation mode, the refrigerant passing through the four-way valve  115  flows toward the indoor heat exchanger  140  through the outdoor heat exchanger  120 . 
     In contrast, in a case where the gas heat-pump system  10  operates in a heating operation mode, the refrigerant passing through the four-way valve  115  flows toward the outdoor heat exchanger  120  through the indoor heat exchanger  140 . 
     The air conditioning module may further include a refrigerant pipe  170  (a flow path indicated by a solid line) that connects the compressor  110 , the outdoor heat exchanger  120 , the indoor heat exchanger  140 , and the like to each other and guides a flow of the refrigerant. 
     First, the configuration of the gas heat-pump system  10  operating in the cooling operation mode will be described below. 
     The refrigerant flowing to the outdoor heat exchanger  120  exchanges heat with outside air and thus is condensed. An outdoor fan  122  that blows the outside air into the outdoor heat exchanger  120  is arranged on one side thereof. 
     A main expansion device  125  for decompressing the refrigerant is provided to the exit side of the outdoor heat exchanger  120 . For example, the main expansion device  125  may include an electronic expansion valve (EEV). The electronic expansion valve (EVV) is controlled using a pulse-width modulation method. In a case where a pulse increases (by a positive value), the degree of opening to which the main expansion device  125  is open is increased. In a case where the pulse decreases (by a negative value), the degree of opening to which the main expansion device  125  is open is decreased. 
     When performing a cooling operation, the main expansion device  125  is fully open, and thus an operation of decompressing the refrigerant is not performed. 
     A supercooling heat exchanger  130  for additionally cooling the refrigerant is provided to the exit side of the main expansion device  125 . Then, a supercooling flow path  132  is connected to the supercooling heat exchanger  130 . The supercooling flow path  132  branches off from the refrigerant pipe  170  and is connected to the supercooling heat exchanger  130 . 
     Then, a supercooling expansion device  135  is installed on the supercooling flow path  132 . The refrigerant flowing along the supercooling flow path  132  is decompressed while passing through the supercooling expansion device  135 . 
     In the supercooling heat exchanger  130 , heat exchange occurs between the refrigerant in the refrigerant pipe  170  and the refrigerant on the supercooling flow path  132 . In a heat exchange process, the refrigerant in the refrigerant pipe  170  is supercooled, and the refrigerant on the supercooling flow path  132  absorbs heat. 
     The supercooling flow path  132  is connected to a gas-liquid separator  160 . The refrigerant on the supercooling flow path  132 , which exchanges heat in the supercooling heat exchanger  130 , flows into the gas-liquid separator  160 . 
     The refrigerant in the refrigerant pipe  170 , which passes through the supercooling heat exchanger  130 , flows toward the indoor air conditioning condenser unit, is decompressed in the indoor expansion device  145 , and then evaporates in the indoor heat exchanger  140 . The indoor expansion device  145  is installed within the indoor air conditioning condenser unit and is configured as the electronic expansion valve (EEV). 
     In addition, the refrigerant evaporating in the indoor heat exchanger  140  may pass through the four-way valve  115  and then may flow right into the gas-liquid separator  160 . Gaseous-phase refrigerant, resulting from refrigerant separation, is absorbed into the compressor  110 . 
     Next, the configuration of the gas heat-pump system  10  operating in the heating operation mode will be described below. 
     In a heating process, the refrigerant compressed in the compressor  110  flows to the indoor heat exchanger  140 , and the refrigerant condensed in the indoor heat exchanger  140  flows to an auxiliary heat exchanger  150 . A refrigerant branch pipe  151  is connected to the auxiliary heat exchanger  150 . 
     An expansion valve  152  is provided on a portion, positioned to the entrance side of the auxiliary heat exchanger  150 , of the refrigerant branch pipe  151 . The expansion valve  152  decompresses the refrigerant while adjusting the flow of the refrigerant. 
     The auxiliary heat exchanger  150  is a heat exchanger in which heat exchange occurs between low pressure refrigerant and high temperature cooling water. Examples of the auxiliary heat exchanger  150  include a plate-type heat exchanger. 
     The refrigerant passing through the auxiliary heat exchanger  150  may flow into the gas-liquid separator  160 . 
     In the gas-liquid separator  160 , the refrigerant passing through the auxiliary heat exchanger  150  is separated into gas and liquid. The gaseous-phase refrigerant, resulting from the refrigerant separation, is absorbed into the compressor  110 . 
     The cooling module includes a cooling water pipe  360  (a flow path indicated by a dotted line) that guides a flow of cooling water for cooling the engine  210  that will be described below. 
     A cooling water pump  300 , a plurality of flow switch units  310  and  320 , and a radiator  330  are installed on the cooling water pipe  360 . The cooling water pump  300  generates a flow force of the cooling water. The plurality of flow switch units  310  and  320  switch a flow direction of the cooling water. The radiator  300  cools the cooling water. 
     The plurality of flow switch units  310  and  320  include a first flow switch unit  310  and a second flow switch unit  320 . As an example, the first flow switch unit  310  and the second flow switch unit  320  each have a three-way valve. 
     The radiator  330  is positioned to one side of the outdoor heat exchanger  120 . The cooling water in the radiator  330  exchanges heat with the outside air by driving the outdoor fan  122  and, during this heat exchange process, is cooled. 
     When the cooling water pump  300  is driven, the cooling water passes through the engine  210  and an exhaust gas heat exchanger  280  and selectively flows into the radiator  330  or the auxiliary heat exchanger  150  through the first flow switch unit  310  and the second flow switch unit  320 . 
     The engine module includes the engine  210  and various components for supplying a fuel-to-air mixture to the engine  210 . 
     The engine module includes a mixer  230  that is arranged to the entrance side of the engine  210  and mixes air and gaseous fuel. 
     An air filter  220  and a zero governor  240  are installed upstream from the mixer  230 . The air filter  220  supplies purified air to the mixer  230  through an air pipe  220   a . The zero governor  240  supplies gaseous fuel at predetermined pressure or lower through a fuel pipe  200   b.    
     The zero governor  240  is a device that uniformly adjusts output pressure regardless of a magnitude of entrance pressure of the gaseous fuel or a change in an amount of flow and supplies the resulting gaseous fuel. 
     In the mixer  230 , the air passing through the air filter  220  and the gaseous fuel discharged from the zero governor  240  are mixed to generate the fuel-to-air mixture. The generated fuel-to-air mixture is supplied to the engine  210  through a mixture device  200   c.    
     The engine module may further include a turbocharger  250  and an adjustment unit  270  that are arranged between the mixer  230  and the engine  210 . 
     The turbocharger  250  applies pressure to the fuel-to-air mixture to increase the density of the fuel-to-air mixture and supplies the resulting fuel-to-air mixture to the engine  210 . The turbocharger  250  is used to provide a higher output than in a natural aspirated engine. 
     As illustrated in  FIG. 1 , the turbocharger  250  applies pressure to the fuel-to-air mixture that is discharged after being generated as a result of the mixer  230  mixing air and gaseous fuel, and discharges the resulting fuel-to-air mixture toward the engine  210  through a turbocharger exit pipe  252   e  in  FIG. 2 . 
     For example, as illustrated in  FIG. 2 , the turbocharger  250  includes a first turbocharger  251  and a second turbocharger  252 . The first turbocharger  251  and the second turbocharger  252  apply first-level pressure to the fuel-to-air mixture generated by the mixer  230 , and directly supply the resulting fuel-to-air mixture to the engine  210 , or apply second-level pressure to the fuel-to-air mixture to which the first-level pressure is applied and supply the resulting fuel-to-air mixture to the engine  210 . 
     The first turbocharger  251  and the second turbocharger  252  have motors  251   b  and  252   b  and impellers  251   a  and  252   a , respectively. The motors  251   b  and  252   b  have the same shape and structure. The impellers  251   a  and  252   a  have the same shape and structure. The turbocharger  250  is configured to include the motors  251   b  and  252   b  having the same shape and structure and the impellers  251   a  and  252   a  having the same shape and structure. Thus, the turbocharger  250  has the advantages of possibly achieving miniaturization and efficiency over a turbocharger configured to include one motor and one impeller. 
     A detailed configuration in which the turbocharger  250  includes the first turbocharger  251  and the second turbocharger  252  will be described below with reference to  FIG. 2 . 
     The adjustment unit  270  is arranged between the turbocharger  250  and the engine  210  and adjusts an amount of the compressed fuel-to-air mixture that is to be supplied to the engine  210 . 
     Examples of the adjustment unit  270  include a valve that employs an electronic throttle control (ETC) scheme. An embodiment in which the ETC valve capable of being electronically controlled is used as the adjustment unit  270  will be described below. However, the present disclosure is not limited to this embodiment. 
     In this manner, the mixer  230  mixes gaseous fuel and air to generate the fuel-to-air mixture. The turbocharger  250  applies high pressure to the generated fuel-to-air mixture, and then the resulting fuel-to-air mixture is supplied to the engine  210 . 
     At this point, an amount of the high pressure fuel-to-air mixture that is to be supplied to the engine  210  is precisely controlled with the ETC valve  270 , and thus an output of the engine  210  is controlled. 
     As described above, the fuel-to-air mixture passing through the turbocharger  250  is in high temperature and high pressure states. For this reason, an intercooler  260  is provided between the turbocharger  250  and the adjustment unit  270 . The intercooler  260  lowers the temperature and pressure of the fuel-to-air mixture and supplies the resulting fuel-to-air mixture to a cylinder  211  of the engine  210 . 
     For example, the intercooler  260  is configured in such a manner that heat exchange occurs partly between the fuel-to-air mixture to be supplied to the engine  210  and a portion of the cooling water to flow to the engine  210  or in such a manner that, as illustrated in  FIG. 2 , heat exchange occurs between the cooling water circulating through a separate water pump  261  in  FIG. 2  and the fuel-to-air mixture. 
     The engine module may further include the exhaust gas heat exchanger  280  which is arranged to the exhaust outlet side of the engine  210  and in which heat exchange occurs between the cooling water and exhaust gas. 
       FIG. 2  is a view schematically illustrating a configuration of an engine module  200  in  FIG. 1 . A detailed configuration of the engine module  200  according to an embodiment of the present disclosure will be described below with reference to  FIG. 2 . 
     The turbocharger  250  of the engine module  200  according to the embodiment of the present disclosure operates in a plurality of operation modes that vary according to a load ratio of the air conditioning module. 
     More specifically, the turbocharger  250  operates in a first turbocharger operation mode for a low load ratio section, a second turbocharger operation mode for a medium load ratio section, and a third turbocharger operation mode for a high load ratio section. 
     In the first turbocharger operation mode and the second turbocharger operation mode, the first-level pressure is applied to the fuel-to-air mixture, and the resulting fuel-to-air mixture is supplied to the engine  210 . In the third turbocharger operation mode, the second-level pressure is applied to the fuel-to-air mixture to which the first-level pressure is applied, and the resulting fuel-to-air mixture is supplied to the engine  210 . 
     To this end, the engine module  200  includes the turbocharger  250 , a pipe, and a plurality of switch valves. The turbocharger  250  includes the first turbocharger  251  and the second turbocharger  252  that operate independently of each other. The pipe connects the first turbocharger  251  and the second turbocharger  252  to each other for a fluid flow. The plurality of switch valves switch a flowing path for the fuel-to-air mixture that flows along the pipe. 
     For independent operation, the first turbocharger  251  includes the first motor  251   b , the first impeller  251   a  that is driven by the first motor  251   b , and a first housing  251   c  that accommodates the first impeller  251   a , and the second turbocharger  252  includes the second motor  252   b , the second impeller  252   a  that is driven by the second motor  252   b , and a second housing  252   c  that accommodates the second impeller  252   a.    
     The first motor  251   b  and the second motor  252   b  are accommodated in a separate motor housing  254 . 
     When applying the first-level pressure for supply, the fuel-to-air mixture is guided to the second turbocharger  252  along the mixture device  200   c , and the first-level pressure is applied to the fuel-to-air mixture only by the second impeller  252   a  of the second turbocharger  252 . Thereafter, the resulting fuel-to-air mixture is supplied to the cylinder  211  of the engine  210  through an intake manifold  212  along the turbocharger exit pipe  252   e  connected to an exhaust outlet  252   c - 2 . At this point, the pressure fuel-to-air mixture is not supplied toward the first turbocharger  251 . 
     When applying the second-level pressure for supply, the fuel-to-air mixture is guided to a first turbocharger entrance pipe  251   d  of the first turbocharger  251  along the mixture device  200   c , the first-level pressure is primarily applied to the fuel-to-air mixture by the first impeller  251   a , and then the resulting fuel-to-air mixture is discharged through a discharge port  251   c - 3 . 
     The discharged fuel-to-air mixture is guided to an inlet port  252   c - 1  of the second turbocharger  252 , the second-level pressure is secondarily applied to the fuel-to-air mixture by the second impeller  252   a , and then the resulting fuel-to-air mixture is supplied to the cylinder  211  of the engine  210  along the turbocharger exit pipe  252   e.    
     A pipe is configured to include the mixture device  200   c , a connection pipe  253 , the first turbocharger entrance pipe  251   d , a first bypass pipe  200   e , and a second bypass pipe  200   f.    
     The mixture device  200   c  serves to guide the fuel-to-air mixture, generated by the mixer  230  mixing air and gaseous fuel, to the turbocharger  250 . 
     As illustrated, the mixer  230  is connected upstream from the mixture device  200   c , and the air pipe  200   a  and the fuel pipe  200   b  are connected to the mixer  230 . 
     The first turbocharger entrance pipe  251   d  and the connection pipe  253  are connected downstream from the mixture device  200   c.    
     The connection pipe  253  serves to connect the discharge port  251   c - 3  of the first turbocharger  251  and the inlet port  252   c - 1  of the second turbocharger  252  for the fluid flow. The fuel-to-air mixture to which the first-level pressure is primarily applied in the first turbocharger  251  is guided to the second turbocharger  252  along the connection pipe  253 . 
     The mixture device  200   c  is connected to the connection pipe  253  at a position between the discharge port  251   c - 3  of the first turbocharger  251  and the inlet port  252   c - 1  of the second turbocharger  252 . 
     The first turbocharger entrance pipe  251   d  serves to guide the fuel-to-air mixture to a first inlet portion  251   c - 1  of the first turbocharger  251  and is configured in such a manner that the connection pipe  253  is connected to the mixture pipe  200   c  upstream from a position where the connection pipe  253  is connected to the mixture device  200   c.    
     At this point, a first switch valve V 1  is arranged at a position where the first turbocharger entrance pipe  251   d  is connected to the mixture device  200   c.    
     The first switch valve V 1  performs a function of switching a flow direction of the fuel-to-air mixture flowing in the mixture device  200   c  to a direction of the connection pipe  253  or a direction of the first turbocharger entrance pipe  251   d . It is desirable that the first switch valve V 1  is configured as a three-way valve including a first port V 1   a , a second port V 1   b , and a third port V 1   c . The first port V 1   a  is connected to the mixture device  200   c . The second port V 1   b  is connected to the mixture device  200   c  in a manner that faces the first port V 1   a . The third port V 1   c  is connected to the first turbocharger entrance pipe  251   d.    
     The first port V 1   a , the second port V 1   b , and the third port V 1   c  are controlled in such a manner that a flow path is open or closed independently of each other. 
     The first bypass pipe  200   e  connects an exhaust pipe  200   d  and a second inlet port  251   c - 2  of the first turbocharger  251  to each other for the fluid flow and serves to at least partly guide the exhaust gas to the first turbocharger  251 . 
     As will be described below, the first bypass pipe  200   e  has the purpose of recirculating a portion of the exhaust gas according to the above-described operation modes of the turbocharger  250  to the engine  210  and the purpose of wholly or partly forcing the exhaust gas to be discharged through the first turbocharger  251 . 
     The second switch valve V 2  is provided to selectively introduce the exhaust gas into the first turbocharger  251  along the first bypass pipe  200   e  according to the operation modes of the turbocharger  250 . 
     As illustrated, the second switch valve V 2  is arranged at a position where the first bypass pipe  200   e  is connected to the exhaust pipe  200   d.    
     The second switch valve V 2  is configured as a three-way valve in the same way as the first switch valve V 1 . The second switch valve V 2  includes a fourth port V 2   a , a fifth port V 2   b , and a sixth port V 2   c . The fourth port V 2   a  is connected to the exhaust pipe  200   d . The fifth port V 2   b  is connected to the exhaust pipe  200   d  in a manner that faces the fourth port V 2   a . The sixth port V 2   c  is connected to the first bypass pipe  200   e.    
     Likewise, the fourth port V 2   a , the fifth port V 2   b , and the sixth port V 2   c  are controlled in such a manner that the flow path is open or closed independently of each other. 
     The second bypass pipe  200   f  is configured to connect the exhaust pipe  200   d  and the above-described connection pipe  253  to each other for the fluid flow. 
     More specifically, a first end portion of the second bypass pipe  200   f  is connected to the exhaust pipe  200   d  downstream from a position where the first bypass pipe  200   e  is connected to the exhaust pipe  200   d . Then, a second end portion of the second bypass pipe  200   f  is connected to the connection pipe  253  somewhere between a position where the mixture device  200   c  is connected to the connection pipe  253  and the discharge port  251   c - 3  of the first turbocharger  251 . 
     In this case, a third switch valve V 3  that allows or blocks the fluid flow between the second bypass pipe  200   f  and the connection pipe  253  according to the operation modes of the turbocharger  250  is provided. 
     The third switch valve V 3  is configured as a three-way valve in the same way as the first switch valve V 1  and the second switch valve V 2 . The third switch valve V 3  includes a seventh port V 3   a , an eighth port V 3   b , and a ninth port V 3   c . The seventh port V 3   a  is connected to the connection pipe  253 . The eighth port V 3   b  is connected to the connection pipe  253  in a manner that faces the seventh port V 3   a . The ninth port V 3   c  is connected to the second bypass pipe  200   f.    
     Likewise, the 7-seventh port V 3   a , the 8-eighth port V 3   b , and the 9-ninth port V 3   c  are controlled in such a manner that the flow path is open or closed independently of each other. 
     As illustrated, check valve  290  is included in the connection pipe  253  at a position adjacent to the first switch valve V 1   
     The check valve  290  serves to limit a flow direction of fluid in such a manner that in the first and second turbocharger operation modes, the fuel-to-air mixture passing through the first switch valve V 1  does not flow toward the third switch valve V 3 . 
     Accordingly, in the first and second turbocharger operation modes, the fuel-to-air mixture is effectively blocked from flowing toward the first turbocharger  251  along the connection pipe  253 . Thus, the fuel-to-air mixture can be prevented from being wasted. 
       FIG. 3  is a graph illustrating a process of switching the turbocharger operation mode according to a load ratio of the air conditioning module and stages in which the compressor  110  operates. 
     Stages in which a first compressor  110  and a second compressor  110  are to operate are preset in a manner that corresponds to the load ratio of the air conditioning module. 
     As illustrated in  FIG. 4 , the first stage refers to a state where the air conditioning module operates only with the first compressor  110 . In the first stage, a first clutch of the first compressor  110  operates, the drive force of the engine  210  is transferred to the first compressor  110 , and a first capacity valve is controlled to be partly open. 
     The second stage refers to a state where the air conditioning module operates with the first compressor  110  and the second compressor  110 . In the second stage, the first clutch of the first compressor  110  and a second clutch of the second compressor  110  operate, and the drive force of the engine  210  is transferred to both the first compressor  110  and the second compressor  110  at the same time. In this case, the first capacity valve and a second capacity valve are controlled to be partly open, and thus an amount of refrigerant discharged by the compressor  110  is controlled. 
     The third stage refers to a state where the air conditioning module operates at a higher load ratio than in the first and second stages. In the third stage, the first compressor  110  and the second compressor  110  are controlled to discharge a larger amount of refrigerant. Unlike in the second stage, the first capacity valve is controlled to be fully open. 
     The fourth stage refers to a state where the air conditioning module operates at the highest load ratio. Unlike in the third stage, control is performed in such a manner that the first capacity valve and the second capacity valve are both controlled to be open and that the amount of refrigerant discharged by the compressor  110  is thus maximized. 
     An output of the engine  210 , that is, an rpm of the engine  210  is controlled to be increased or decreased in a manner that corresponds to the stage in which the compressor  110  operates. 
     In a case where the output of the engine  210  is increased or decreased, in order to adjust the mount of the fuel-to-air mixture to be supplied to the engine  210 , there is also a need to adjust an output of the turbocharger  250 . 
     To this end, the gas heat-pump system  10  according to the embodiment of the present disclosure regulates the turbocharger operation modes for efficiently operating the engine  210  according to an amount of load on the air conditioning module. 
     More specifically, sections for load ratios are set to be divided into a section for a low load ratio that is lower than 30%, a section for a medium load ratio that is equal to or higher than 30% and is lower than 70%, and a section for a high load ratio that is equal to or higher than 70%. 
     In this case, with the configuration illustrated in  FIG. 2 , the turbocharger  250  operates in the first turbocharger operation mode in the section for the low load ratio, operates in the second turbocharger operation mode in the section for the medium load ratio, and operates the third turbocharger operation mode in the section for the high load ratio. Thus, the turbocharger  250  can effectively operate in a manner that corresponds to a change in load on the engine  210 . The operational efficiency of the engine  210  and the turbocharger  250  can thus be improved. 
     First, the first turbocharger operation mode in the section for the low load ratio will be described with reference to  FIGS. 5 and 6 . 
     As illustrated in  FIG. 5 , because the rpm and torque of the engine  210  are maintained to a relatively low level in the section for the low load ratio that is lower than 30%, the engine  210  operates only with a small amount of the fuel-to-air mixture. 
     Therefore, the turbocharger  250  operates in the first turbocharger operation mode in which the first-level pressure is applied to the fuel-to-air mixture and in which the resulting fuel-to-air mixture is then supplied to the engine  210 . 
     In the first turbocharger operation mode, the flow path is set in such a manner that the fuel-to-air mixture is supplied only to the second turbocharger  252 . 
     More specifically, the first port V 1   a  and the second port V 1   b  of the first switch valve V 1  are set to be open, and the third port V 1   c  is set to be closed. 
     Therefore, the fuel-to-air mixture flowing along the mixture device  200   c  does not flow toward the first turbocharger  251  and is all guided only to the second turbocharger  252  along a path indicated by a solid line. 
     At this point, electric power is supplied to the second motor  252   b  of the second turbocharger  252 , the second impeller  252   a  applies pressure to the fuel-to-air mixture, and the resulting fuel-to-air mixture is discharged to the turbocharger exit pipe  252   e . Control is performed in such a manner that electric power is not supplied to the first motor  251   b  of the first turbocharger  251 . 
     In the first turbocharger operation mode, the fourth port V 2   a  and the fifth port V 2   b  of the second switch valve V 2  are set to be open, and the sixth port V 2   c  is set to be closed. 
     Therefore, the exhaust gas that is discharged from the engine  210  through an exhaust manifold  213  is all naturally discharged to the outside along the exhaust pipe  200   d  without being introduced into the first bypass pipe  200   e.    
     In addition, in the first turbocharger operation mode, at least two of the 7-seventh port V 3   a , the 8-eighth port V 3   b , and the 9-ninth port V 3   c  of the third switch valve V 3  are set to be closed. In  FIG. 5 , for example, a state where the 8-eighth port V 3   b  and the 9-ninth port V 3   c  are closed is illustrated. An embodiment in which the 8-eighth port V 3   b  and the 9-ninth port V 3   c , as illustrated, are closed in the first turbocharger operation mode will be described in a focused manner. However, the present disclosure is not limited to this embodiment. 
     The third switch valve V 3  is set in this manner, and thus the exhaust gas is prevented from being introduced through the second bypass pipe  200   f , or the fuel-to-air mixture is prevented from being discharged through the second bypass pipe  200   f.    
     When it is determined that the engine  210  reaches a target rpm while operation in the first turbocharger operation mode is in progress and thus the engine  210  is stabilized, as illustrated in  FIG. 6 , an exhaust gas recirculation state where a portion of the exhaust gas is recirculated to the engine  210  is set to be entered. 
     For the exhaust gas recirculation, the sixth port V 2   c  of the second switch valve V 2  switches from a closed state to a partly-open state, and the 8-eighth port V 3   b  of the third switch valve V 3  switches from a closed state to a fully open state. 
     An exhaust gas flow path is formed in a direction indicated by a dotted line within the first bypass pipe  200   e  through the partly-open sixth port V 2   c  and within the connection pipe  253  through the 7-seventh port V 3   a  and the 8-eighth port V 3   b  of the third switch valve V 3 . 
     At this point, the degree of opening to which the sixth port V 2   c  is partly open ranges from 5 to 30%. 
     When the second switch valve V 2  and the third switch valve V 3  switch the flow path, electric power is supplied to the first motor  251   b  of the first turbocharger  251 , and the first impeller  251   a  is controlled to be rotated. 
     The first impeller  251   a  applies pressure to recirculation exhaust gas introduced into the first bypass pipe  200   e , and the resulting recirculation exhaust gas is discharged to the connection pipe  253 . The discharged recirculation exhaust gas passes through the 7-seventh port V 3   a  and the 8-eighth port V 3   b  of the third switch valve V 3 . 
     The recirculation exhaust gas passing through the third switch valve V 3 , along with the fuel-to-air mixture, is guided to the second turbocharger  252 , and pressure is applied to the recirculation exhaust gas and the fuel-to-air mixture by the second impeller  252   a . Then, the resulting recirculation exhaust gas and the resulting fuel-to-air mixture are guided to the engine  210 . 
     A portion of the exhaust gas is recirculated in this manner in a state where the operation of the engine  210  is stabilized in the first turbocharger operation mode. Thus, the effects of lowering a combustion temperature within the cylinder  211  and thus reducing an amount of discharged exhaust gas are achieved. 
     Next, the second turbocharger operation mode in the section for the medium load ratio will be described with reference to  FIGS. 7 and 8 . 
     As illustrated in  FIG. 7 , in the section for the medium load ratio that is equal to or higher than 30% and is lower than 70%, in the same manner as in the first turbocharger operation mode, the turbocharger  250  is controlled to apply the first-level pressure to the fuel-to-air mixture, and to supply the resulting fuel-to-air mixture to the engine  210 . 
     However, unlike in the first turbocharger operation mode in which the exhaust gas is discharged in a natural exhaust manner, in the second turbocharger operation mode, the exhaust gas is forced to be discharged using the first turbocharger  251 . 
     That is, the exhaust gas is forced to be discharged through the first turbocharger  251 . Thus, the effects of remarkably lowering exhaust gas resistance of the engine  210  and making a slightly greater improvement in the output and efficiency of the engine  210  than in the first turbocharger operation mode can be achieved. 
     More specifically, as illustrated in  FIG. 7 , in the same manner as in the first turbocharger operation mode, the first port V 1   a  and the second port V 1   b  of the first switch valve V 1  are maintained in an open state, and the third port V 1   c  thereof is maintained in a closed state. 
     However, in the second turbocharger operation mode, the fourth port V 2   a  and the sixth port V 2   c  of the second switch valve V 2  are set to be open, and the fifth port V 2   b  is set to be closed. 
     Therefore, the exhaust gas discharged from the engine  210  is all guided to the first turbocharger  251  along the first bypass pipe  200   e.    
     In addition, in the second turbocharger operation mode, the 7-seventh port V 3   a  and the 9-ninth port V 3   c  of the third switch valve V 3  are set to be open, and the 8-eighth port V 3   b  thereof is set to be closed. 
     When the settings of the second switch valve V 2  and the third switch valve V 3  are completed, electric power is supplied to the first motor  251   b , and the first impeller  251   a  is rotated. 
     Therefore, the first impeller  251   a  applies pressure to the exhaust gas introduced along the first bypass pipe  200   e . Then, the resulting exhaust gas is discharged to the connection pipe  253 , is guided to the second bypass pipe  200   f  through the third switch valve V 3 , and is finally forced to be discharged through the exhaust pipe  200   d.    
     When it is determined that the engine  210  reaches a target rpm while operation in the second turbocharger operation mode is in progress and that the engine  210  is thus stabilized, as illustrated in  FIG. 8 , the exhaust gas recirculation state where a portion of the exhaust gas is recirculated to the engine  210  is set to be entered. 
     For the exhaust gas recirculation, the 8-eighth port V 3   b  of the third switch valve V 3  switches from a closed state to a partly-open state. 
     The exhaust gas flow path is formed in a direction indicated by a dotted line within the connection pipe  253  through the partly-open 8-eighth port V 3   b.    
     At this point, the degree of opening to which the sixth port V 2   c  is partly open ranges from 5 to 30%. 
     The recirculation exhaust gas passing through the third switch valve V 3 , along with the fuel-to-air mixture, is guided to the second turbocharger  252 , and pressure is applied to the recirculation exhaust gas and the fuel-to-air mixture by the second impeller  252   a . Then, the resulting recirculation exhaust gas and the resulting fuel-to-air mixture are guided to the engine  210 . 
     A portion of the exhaust gas is recirculated in this manner in the state where the operation of the engine  210  is stabilized in the second turbocharger operation mode. Thus, the effects of lowering the combustion temperature within the cylinder  211  and thus reducing the amount of discharged exhaust gas are achieved. 
     Next, the third turbocharger operation mode in the section for the high load ratio will be described with reference to  FIGS. 9 and 10 . 
     As illustrated in  FIG. 9 , in the section for the high load ratio that exceeds 70%, the turbocharger  250  switches to the third turbocharger operation mode in which the second-level pressure is applied to the fuel-to-air mixture and in which the resulting fuel-to-air mixture is then supplied to the engine  210 . 
     That is, in order to raise the output of the engine  210  to a maximum, the second-level pressure is applied to the fuel-to-air mixture by the first turbocharger  251  and the second turbocharger  252 , and then the resulting fuel-to-air mixture is supplied to the engine  210 . 
     More specifically, as illustrated in  FIG. 9 , the first port V 1   a  and the third port V 1   c  of the first switch valve V 1  are open, and the second port V 1   b  thereof is fully closed through switching. 
     Therefore, the fuel-to-air mixture guided through the mixture device  200   c  is all guided to the first turbocharger  251 . 
     In addition, in the third turbocharger operation mode, the fourth port V 2   a  and the fifth port V 2   b  of the second switch valve V 2  are set to be open, and the sixth port V 2   c  thereof is set to be closed. 
     Therefore, the exhaust gas discharged from the engine  210  is all naturally discharged along the exhaust pipe  200   d  without being introduced into the first bypass pipe  200   e.    
     In addition, in the third turbocharger operation mode, the 7-seventh port V 3   a  and the 8-eighth port V 3   b  of the third switch valve V 3  are set to be open, and the 9-ninth port V 3   c  thereof is set to be closed. 
     Accordingly, the fuel-to-air mixture to which the first-level pressure is primarily applied by the first turbocharger  251  is blocked from flowing to the second bypass pipe  200   f , and is all guided to the second turbocharger  252  along the connection pipe  253 . 
     When the settings of the first switch valve V 1 , the second switch valve V 2 , and the third switch valve V 3  are completed, electric power is supplied to the first motor  251   b , and the first impeller  251   a  is rotated. 
     Therefore, the first impeller  251   a  applies the pressure to the fuel-to-air mixture that is introduced through the mixture device  200   c , and then the resulting fuel-to-air mixture is discharged to the connection pipe  253  and is guided to the second turbocharger  252  through the third switch valve V 3 . Finally, the second impeller  252   a  of the second turbocharger  252  secondarily applies the second-level pressure to the fuel-to-air mixture. Thereafter, the resulting fuel-to-air mixture is discharged and then is guided to the engine  210 . 
     When it is determined that the engine  210  reaches a target rpm while operation in the third turbocharger operation mode is in progress and that the engine  210  is thus stabilized, as illustrated in  FIG. 10 , the exhaust gas recirculation state where a portion of the exhaust gas is recirculated to the engine  210  is set to be entered. 
     For the exhaust gas recirculation, the sixth port V 2   c  of the second switch valve V 2  switches from a closed state to a partly-open state. 
     The exhaust gas flow path is formed in a direction indicated by a dotted line within the first bypass pipe  200   e  through the sixth port V 2   c.    
     At this point, the degree of opening to which the sixth port V 2   c  is partly open ranges from 5 to 30%. 
     The recirculation exhaust gas passing through the second switch valve V 2  is guided to the second inlet port  251   c - 2  of the first turbocharger  251 . Then, the first impeller  251   a  primarily applies the first-level pressure to the recirculation exhaust gas, along with the fuel-to-air mixture, and then the resulting recirculation exhaust gas and the resulting fuel-to-air mixture are discharged to the connection pipe  253 . 
     The fuel-to-air mixture and the recirculation exhaust gas to which the first-level pressure is primarily applied by the first turbocharger  251  and which are then discharged are guided to the second turbocharger  252  through the connection pipe  253 . The second impeller  252   a  secondarily applies the second-level pressure to the fuel-to-air mixture and the recirculation exhaust gas, and then the resulting fuel-to-air mixture and the resulting recirculation exhaust gas are guided to the engine  210 . 
     A portion of the exhaust gas is recirculated in this manner in the state where the operation of the engine  210  is stabilized in the third turbocharger operation mode. Thus, the effects of lowering the combustion temperature within the cylinder  211  and thus reducing the amount of discharged exhaust gas are achieved. 
       FIG. 11  is a block diagram illustrating a controller  300  of the gas heat-pump system  10  according to the embodiment of the present disclosure.  FIGS. 12 to 16  are flow charts each illustrating a method of controlling the gas heat-pump system  10  according to the present disclosure. 
     A method of controlling the gas heat-pump system  10  according to the present disclosure will be described below with a focus on the controller  300 . 
     As illustrated, the controller  300  is electrically connected to the air conditioning module, a cooling module, the power supply unit  400 , and the engine module  200  and generates signals for controlling these components. 
     First, in a state where the gas heat-pump system  10  is stopped, when a system operation signal is input through an operation unit that is not illustrated, the controller  300  generates signals for operating the air conditioning module, the cooling module, and the engine module  200 , receives necessary electric power from the power supply unit  400 , and supplies the received necessary electric power to the air conditioning module, the cooling module, and the engine module  200 . 
     A specific method and configuration in which the controller  300  controls the air conditioning module and the cooling module are known in the art, and therefore detailed descriptions thereof are omitted. 
     An operational condition for operating the engine module  200  is read from a memory  310 . Specifically, control is performed in such a manner that the air pipe  200   a  and the fuel pipe  200   b , which are not illustrated, are open, and that air and fuel are thus introduced from the air pipe  200   a  and the fuel pipe  200   b , respectively, and are mixed in the mixer  230 . 
     In addition, in order to drive the turbocharger  250 , the controller  300  performs control in such a manner that electric power is supplied to the first motor  251   b  and the second motor  252   b . As described above, the controller  300  performs control in such a manner that the first motor  251   b  and the second motor  252   b  operate independently of each other and that the first turbocharger operation mode, the second turbocharger operation mode, and the third turbocharger operation mode are thus enabled. 
     When the fuel-to-air mixture is supplied to the engine  210  according to each of the turbocharger operation modes, the controller  300  transmits an ignition signal to an ignition plug in accordance with a stroke of each cylinder  211  and ignites the fuel-to-air mixture supplied to each cylinder  211 . 
     Actuators (not illustrated) for driving the first switch valve V 1 , the second switch valve V 2 , and the third switch valve V 3  are all electrically connected to the controller  300 . According to a control signal of the controller  300 , the actuators are driven, and the ports of each of the first switch valve V 1 , the second switch valve V 2 , and the third switch valve V 3  are controlled to be open or closed independently of each other. 
     In addition, the controller  300  is electrically connected to a sensor module SM and in real time monitors an operational state and the like of the engine  210  through an electric signal received from the sensor module SM. 
     In addition, the controller  300  is electrically connected to an actuator of the ETC valve  270 . When the output of the engine  210  is increased or decreased, the actuator adjusts the degree of opening to which the ETC valve  270  is open, according to a control signal of the controller  300 , and thus increases or decreases the output of the engine  210 . 
     A specific process of controlling the engine module  200  according to a required load ratio of the air conditioning module will be described below. 
     First, the controller  300  receives a load change signal of the air conditioning module (S 1 ). 
     The load change signal is input through the above-described operation unit or is input through a cooling load automatic detection unit or the like of the air conditioning module. 
     When receiving the load change signal, the controller  300  computes a required load ratio and a target rpm of the engine  210 , which corresponds to the required load ratio (S 2 ). 
     At this time, in order to compute the target rpm of the engine  210  for the required load ratio, the controller  300  reads pre-mapped data from the memory  310  and performs the computation. 
     Next, the controller  300  determines whether or not the required load ratio excesses a first reference load ratio (S 3 ). 
     It is desirable that the first reference load ratio is 30% of a maximum amount of load. 
     When it is determined that the required load ratio is lower than the first reference load ratio, the controller  300  performs control in such a manner that the turbocharger  250  operates in the first turbocharger operation mode (S 4 ). 
     More specifically, as illustrated in  FIG. 13 , in order for the turbocharger  250  to operate in the first turbocharger operation mode, the controller  300  sets a flow path with respect to the first switch valve V 1  in such a manner that the first port V 1   a  and the second port V 1   b  are open and that the third port V 1   c  is closed (S 4   a ). 
     In addition, the controller  300  sets the flow path with respect to the second switch valve V 2  in such a manner that the fourth port V 2   a  and the fifth port V 2   b  are open and that the sixth port V 2   c  is closed (S 4   b ). 
     In addition, the controller  300  sets the flow path with respect to the third switch valve V 3  in such a manner that at least two of the 7-seventh port V 3   a , the 8-eighth port V 3   b , and the 9-ninth port V 3   c  are closed (S 4   c ). 
     It is desirable that the 8-eighth port V 3   b  and the 9-ninth port V 3   c  are closed. 
     Next, the controller  300  supplies electric power to the second motor  252   b  and thus rotates the second impeller  252   a , and blocks electric power from being supplied to the first motor  251   b  and thus keeps the first motor  251   b  in a stopped state (S 4   d ). 
     When the setting to the first turbocharger operation mode is completed in this manner, the controller  300  adjusts the degree of opening to which a throttle valve is open, in a manner that corresponds to the target rpm of the engine  210  (S 5 ). 
     Next, the controller  300  determines whether or not an output rpm of the engine  210  reaches the above-described target rpm while the operation in the first turbocharger operation mode is progress (S 6 ). 
     When it is determined that the output rpm of the engine  210  reaches the target rpm, the controller  300  determines that the operation of the engine  210  is stabilized, and performs control in such a manner that the exhaust gas is recirculated at least partly to the cylinder  211  of the engine  210  (S 7 ). 
     More specifically, as illustrated in  FIG. 16 , the controller  300  determines which turbocharger operation mode is currently enabled (S 7   a ). 
     Because the operation in the first turbocharger operation mode is currently in progress, the controller  300  performs control in such a manner that the setting of the flow path with respect to the first switch valve V 1  is maintained (S 7   b ). 
     Next, the controller  300  switches the flow path with respect to the second switch valve V 2  in such a manner that the fourth port V 2   a  and the fifth port V 2   b  are all open and that the sixth port V 2   c  is partly open (S 7   c ). 
     At this point, the degree of opening to which the sixth port V 2   c  is partly open ranges from 5 to 30%. 
     In addition, the controller  300  switches the flow path with respect to the third switch valve V 3  in such a manner that the 7-seventh port V 3   a  and the 8-eighth port V 3   b  are open and that the 9-ninth port V 3   c  is closed (S 7   d ). 
     Accordingly, the exhaust gas flow path is formed within the first bypass pipe  200   e  through the partly-open sixth port V 2   c  and within the connection pipe  253  through the 7-seventh port V 3   a  and the 8-eighth port V 3   b  of the third switch valve V 3 . 
     When the switching of the flow paths with respect to the second switch valve V 2  and the third switch valve V 3  is completed in this manner, the controller  300  maintains a current operational condition and causes the engine  210  to operate in a normal state (S 8 ). 
     When it is determined in Step S 3  that the required load ratio is equal to or higher than the first reference load ratio, the controller  300  additionally determines whether or not the required load ratio exceeds a second reference load ratio (S 9 ). 
     It is desirable that the first reference load ratio is 70% of the maximum amount of load. 
     When it is determined that the required load ratio is equal to or higher than the first reference load ratio and is lower than the second reference load ratio, the controller  300  performs control in such a manner that the turbocharger  250  operates in the second turbocharger operation mode (S 10 ). 
     More specifically, as illustrated in  FIG. 14 , in order for the turbocharger  250  to operate in the second turbocharger operation mode, the controller  300  sets the flow path with respect to the first switch valve V 1  in such a manner that the first port V 1   a  and the second port V 1   b  are open and that the third port V 1   c  is closed (S 10   a ). 
     In addition, the controller  300  sets the flow path with respect to the second switch valve V 2  in such a manner that the fourth port V 2   a  and the sixth port V 2   c  are open and that the fifth port V 2   b  is closed (S 10   b ). 
     In addition, the controller  300  sets the flow path with respect to the third switch valve V 3  in such a manner that the 7-seventh port V 3   a  and the 9-ninth port V 3   c  are open and that the 8-eighth port V 3   b  is closed (S 10   c ). 
     Next, the controller  300  performs control in such a manner the first motor  251   b  and the second motor  252   b  operate at the same time and that the first turbocharger  251  and the second turbocharger  252  are thus all in an operating state (S 10   d ). 
     Accordingly, the first impeller  251   a  applies pressure to the exhaust gas introduced through the first bypass pipe  200   e , and then the resulting exhaust gas is discharged to the connection pipe  253 , is guided to the second bypass pipe  200   f  through the third switch valve V 3 , and is finally forced to be discharged through the exhaust pipe  200   d.    
     When the setting to the second turbocharger operation mode is completed in this manner, Steps S 5  and S 6  are performed. 
     When it is determined that the output rpm of the engine  210  reaches the target rpm while the operation in the second turbocharger operation mode is in progress, the controller  300  determines that the operation of the engine  210  is stabilized, and performs control in such a manner that the exhaust gas is recirculated at least partly to the cylinder  211  of the engine  210  (S 7 ). 
     More specifically, as illustrated in  FIG. 16 , the controller  300  determines which turbocharger operation mode is currently enabled (S 7   a ). 
     Because the operation in the second turbocharger operation mode is currently in progress, the controller  300  performs control in such a manner that the settings of the flow paths with respect to the first switch valve V 1  and the second switch valve V 2  are maintained (S 7   e ). 
     The controller  300  switches the flow path the third switch valve V 3  in such a manner that the 7-seventh port V 3   a  and the 9-ninth port V 3   c  are open and that the 8-eighth port V 3   b  is partly open (S 7   d ). 
     At this point, the degree of opening to which the 8-eighth port V 3   b  is partly open ranges from 5 to 30%. 
     Accordingly, the recirculation exhaust gas passing through the third switch valve V 3 , along with the fuel-to-air mixture, is guided to the second turbocharger  252 , and the second impeller  252   a  applies the pressure to the recirculation exhaust gas and the fuel-to-air mixture. Then, the resulting recirculation exhaust gas and the resulting fuel-to-air mixture are guided to the engine  210 . 
     When the switching of the flow path with respect to the third switch valve V 3  is completed, the controller  300  maintains the current operational condition and causes the engine  210  to operate in the normal state (S 8 ). 
     When it is determined in Step S 9  that the required load ratio is equal to or higher than the second reference load ratio, the controller  300  performs control in such a manner that the turbocharger  250  operates in the third turbocharger operation mode (S 11 ). 
     More specifically, as illustrated in  FIG. 15 , in order for the turbocharger  250  to operate in the third turbocharger operation mode, the controller  300  sets the flow path with respect to the first switch valve V 1  in such a manner that the first port V 1   a  and the third port V 1   c  are open and that the second port V 1   b  is closed (S 11   a ). 
     In addition, the controller  300  sets the flow path with respect to the second switch valve V 2  in such a manner that the fourth port V 2   a  and the fifth port V 2   b  are open and that the sixth port V 2   c  is closed (S 11   b ). 
     In addition, the controller  300  sets the flow path with respect to the third switch valve V 3  in such a manner that the 7-seventh port V 3   a  and the 8-eighth port V 3   b  are open and the 9-ninth port V 3   c  is closed (S 11   c ). 
     Next, the controller  300  performs control in such a manner the first motor  251   b  and the second motor  252   b  operate at the same time and that the first turbocharger  251  and the second turbocharger  252  are thus all in the operating state (S 11   d ). 
     Accordingly, the first impeller  251   a  applies the pressure to the fuel-to-air mixture that is introduced through the mixture device  200   c , and then the resulting fuel-to-air mixture is discharged to the connection pipe  253  and is guided to the second turbocharger  252  through the third switch valve V 3 . Finally, the second impeller  252   a  of the second turbocharger  252  secondarily applies the second-level pressure to the fuel-to-air mixture. Thereafter, the resulting fuel-to-air mixture is discharged and then is guided to the engine  210 . 
     When the setting to the third turbocharger operation mode is completed in this manner, Steps S 5  and S 6  are performed. 
     When it is determined that the output rpm of the engine  210  reaches the target rpm while the operation in the second turbocharger operation mode is in progress, the controller  300  determines that the operation of the engine  210  is stabilized, and performs control in such a manner that the exhaust gas is recirculated at least partly to the cylinder  211  of the engine  210  (S 7 ). 
     More specifically, as illustrated in  FIG. 16 , the controller  300  determines which turbocharger operation mode is currently enabled (S 7   a ). 
     Because the operation in the third turbocharger operation mode is currently in progress, the controller  300  performs control in such a manner that the settings of the flow paths with respect to first switch valve V 1  and the third switch valve V 3  are maintained (S 7   g ). 
     The controller  300  switches the flow path with respect to the second switch valve V 2  in such a manner that the fourth port V 2   a  and the fifth port V 2   b  are open and that the sixth port V 2   c  is partly open (S 7   h ). 
     At this point, the degree of opening to which the 8-eighth port V 3   b  is partly open ranges from 5 to 30%. 
     Accordingly, the fuel-to-air mixture and the recirculation exhaust gas to which the first-level pressure is primarily applied by the first turbocharger  251  and which are thus discharged are guided to the second turbocharger  252  through the connection pipe  253 . The second impeller  252   a  secondarily applies the second-level pressure to the fuel-to-air mixture and the recirculation exhaust gas, and then the resulting fuel-to-air mixture and the resulting recirculation exhaust gas are guided to the engine  210 . 
     When the switching of the flow path with respect to the second switch valve V 2  is completed, the controller  300  maintains the current operational condition and causes the engine  210  to operate in the normal state (S 8 ). 
     Accordingly, from the above-described technical configurations of the embodiments of the present disclosure, it would be apparent to a person of ordinary skill to which the present disclosure pertains that specific embodiments other than the above-described embodiment of the present disclosure will be implemented without departing the technical idea and necessary features of the present disclosure. 
     Therefore, it should be understood that the above-described embodiments are illustrative and non-restrictive in all respects. The scope of the present disclosure is defined in the following claims rather than the detailed description provided above. All alterations, modifications, and the like that are derived from the following claims and equivalents thereof should be interpreted as being included within the scope of the present disclosure.