Abstract:
An internal combustion engine with a supercharger comprises an internal combustion engine including a plurality of cylinders; an intake passage that supplies gas to the internal combustion engine; a turbocharger-type supercharger including a turbine portion having a plurality of exhaust gas introduction passages; and an exhaust passage including a plurality of connecting passages that connect the plurality of cylinders and the plurality of exhaust gas introduction passages, wherein exhaust gas discharged from the internal combustion engine flows through the exhaust passage. The internal combustion engine comprises a bridge passage that connects two or more of the plurality of connecting passages to each other; a branch passage connected to the bridge passage; and a first opening and closing device provided into the bridge passage to open and close the bridge passage.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a National Stage of International Application No. PCT/JP2010/065272, filed on Sep. 7, 2010, claiming priority based on Japanese Patent Application Nos. 2009-206953, filed Sep. 8, 2009 and JP 2010-197796, filed Sep. 3, 2010, the contents of all of which are incorporated herein by reference in their entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to an internal combustion engine with a supercharger comprising a turbocharger-type supercharger including a turbine portion having a plurality of exhaust gas introduction passages, and an exhaust passage including a plurality of connecting passages that connect a plurality of cylinders and a plurality of exhaust gas introduction passages, wherein exhaust gas discharged from the internal combustion engine flows through the exhaust passage. 
     BACKGROUND OF THE INVENTION 
     Exhaust-driven turbocharger-type superchargers that perform supercharging using flow strength of an exhaust stream are frequently used as superchargers to improve the intake efficiency of an internal combustion engine. For example, see Patent Document 1. Patent Document 1 discloses an exhaust system in which two or more collecting tubes joining a plurality of exhaust passages, are connected with a bridge passage serving as a communication tube. The communication tube is adapted to be opened and closed by a valve. Patent Document 1 describes the communication tube as contributing to an improvement in thermal efficiency in an internal combustion engine. Patent Document 1 also describes that the internal diameter of the communication tube may be set to 20 to 100% of the internal diameter of each collecting tube to ensure the contribution to an improvement in thermal efficiency.
     Patent Document 1: Japanese Laid-open Patent Publication No. 2001-164934   

     SUMMARY OF THE INVENTION 
     However, even in the case where a turbocharger-type supercharger is driven by using flow strength of exhaust gas to improve engine power, the pressure of the exhaust gas, i.e., the exhaust pressure should be set lower than the strength against exhaust pressure in exhaust system parts such as a sealing structure of an exhaust system. To set the exhaust pressure lower than the strength against the exhaust pressure in exhaust system parts, the internal diameter of the communication tube cannot be much smaller than the internal diameter of each collecting tube. Meanwhile, when the internal diameter of the communication tube is increased, the size of the valve for opening and closing the communication tube must be increased. For this reason, it is necessary to reduce the internal diameter of the communication tube so as to miniaturize the valve for opening and closing the communication tube. However, it is impossible for the exhaust system disclosed in Patent Document 1 to achieve both a reduction in the exhaust pressure to be lower than the strength against the exhaust pressure and a reduction in the internal diameter (passage sectional area) of the communication tube (bridge passage). 
     It is an object of the present invention to achieve an exhaust pressure smaller than the strength against exhaust pressure, and a smaller passage sectional area of a bridge passage. 
     In one aspect of the invention, an internal combustion engine with a supercharger comprising: an internal combustion engine including a plurality of cylinders; an intake passage that supplies gas to the internal combustion engine; a turbocharger-type supercharger including a turbine portion having a plurality of exhaust gas introduction passages; and an exhaust passage including a plurality of connecting passages that connect the plurality of cylinders and the plurality of exhaust gas introduction passages, wherein exhaust gas discharged from the internal combustion engine flows through the exhaust passage, is provided. The internal combustion engine comprises a bridge passage that connects two or more of the plurality of connecting passages to each other; a branch passage connected to the bridge passage; and a first opening and closing device provided into the bridge passage to open and close the bridge passage. 
     In one embodiment, the connecting passage includes a joining passage that joins the plurality of connecting passages that connects the plurality of cylinders. 
     In another embodiment, the branch passage serves as an EGR passage having one end connected to the bridge passage and the other end connected to the intake passage, and the internal combustion engine further comprises: a heat exchanger provided on the EGR passage to cool exhaust gas flowing through the EGR passage; and a second opening and closing device provided downstream of the heat exchanger in the EGR passage to open and close the EGR passage. 
     In another embodiment, the internal combustion engine further comprises a third opening and closing device that opens and closes the EGR passage upstream of the heat exchanger. 
     In another embodiment, the third opening and closing device is provided on the EGR passage upstream of the heat exchanger. 
     In another embodiment, the first opening and closing device and the third opening and closing device configure a single switch valve that switches communication and blocking between the EGR passage and the bridge passage upstream of the heat exchanger, and that switches opening and closing of the bridge passage, the EGR passage and the bridge passage are connected via the switch valve, and the EGR passage and the bridge passage are located upstream of the heat exchanger. 
     In another embodiment, the internal combustion engine comprises a rate-of-revolution detection device that detects the rate of revolution of the internal combustion engine; a load detection device that detects load of the internal combustion engine; and a control device that controls opening and closing of the second opening and closing device and the first opening and closing device, wherein the control device controls opening and closing of the first opening and closing device and the second opening and closing device in accordance with the rate of revolution detected by the rate-of-revolution detection device and the load detected by the load detection device. 
     In a further embodiment, the internal combustion engine comprises a rate-of-revolution detection device that detects the rate of revolution of the internal combustion engine; a load detection device that detects load of the internal combustion engine; and a control device that controls opening and closing of the second opening and closing device, the first opening and closing device, and the third opening and closing device, wherein the control device controls opening and closing of the first opening and closing device, the second opening and closing device, and the third opening and closing device in accordance with the rate of revolution detected by the rate-of-revolution detection device and the load detected by the load detection device. 
     In another embodiment, the control device opens all of the first opening and closing device, the second opening and closing device, and the third opening and closing device in a low load region, the control device closes all of the first opening and closing device, the second opening and closing device, and the third opening and closing device in a low-revolution-rate high-load region with a load higher than that of the low load region and with low revolution rate, the control device opens the first opening and closing device and closes the second opening and closing device and the third opening and closing device in an intermediate-revolution-rate high-load region with a load higher than that of the low load region and with a higher rate of revolution than that of the low-revolution-rate high-load region, and the control device opens the first opening and closing device and the third opening and closing device and closes the second opening and closing device in a high-revolution high-load region with a load higher than that of the low load region and with a higher rate of revolution than that of the intermediate-revolution-rate high-load region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an overall structural view of an internal combustion engine illustrating a first embodiment; 
         FIG. 2A  is a side sectional view of a turbocharger-type supercharger; 
         FIG. 2B  is a sectional view taken along the line  2 B- 2 B of  FIG. 2A ; 
         FIG. 3  is a region graph represented by the rate of revolution of an engine and engine load; 
         FIG. 4  is a flowchart illustrating an opening and closing control program; 
         FIG. 5A  is a graph illustrating a relation between passage diameter and output torque; 
         FIG. 5B  is a graph illustrating a relation between passage diameter and maximum value of an exhaust pulse; 
         FIG. 6A  is a graph illustrating a relation between passage diameter and output torque; 
         FIG. 6B  is a graph illustrating a relation between passage diameter and maximum value of an exhaust pulse; 
         FIG. 7  is a graph illustrating pressure fluctuation in a connecting passage; 
         FIG. 8A  is a graph illustrating change in pressures within an intake passage and an EGR passage; 
         FIG. 8B  is a graph illustrating change in fluid flow rate within the EGR passage; 
         FIG. 8C  is a graph illustrating change in fluid flow rate within the EGR passage; 
         FIG. 9  is an overall structural diagram of an internal combustion engine illustrating a second embodiment; 
         FIG. 10  is a flowchart illustrating an opening and closing control program; 
         FIG. 11A  is an overall structural view of an internal combustion engine illustrating a third embodiment; 
         FIG. 11B  is a sectional view illustrating an internal structure of a three-way valve V 4 ; 
         FIG. 11C  is a sectional view illustrating the internal structure of the three-way valve V 4 ; and 
         FIG. 11D  is a sectional view illustrating the internal structure of the three-way valve V 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A four-cylinder diesel engine of a first embodiment of the present invention will be described with reference to  FIGS. 1 to 8 . 
     As illustrated in  FIG. 1 , a diesel engine  10  serving as an internal combustion engine includes a plurality of cylinders  11 A,  11 B,  11 C, and  11 D each of which houses a non-illustrated piston. A cylinder head  12  is connected to a cylinder block (not illustrated) that forms the cylinders  11 A,  11 B,  11 C, and  11 D. Fuel injection nozzles  13  are attached to the cylinder head  12  to correspond to the cylinders  11 A,  11 B,  11 C, and  11 D. A light oil serving as fuel is supplied to the fuel injection nozzles  13  via a fuel pump  14  and a common rail  15 . The fuel injection nozzles  13  inject the fuel into each of the cylinders  11 A,  11 B,  11 C, and  11 D. 
     An intake manifold  16  is connected with the cylinder head  12 . An intake passage  17  is connected with the intake manifold  16 . A compressor portion  20  of a turbocharger-type supercharger  19  is provided in the middle of the intake passage  17 . The turbocharger-type supercharger  19  is a variable nozzle type supercharger that is activated by an exhaust gas stream. Air within the intake passage  17  on the upstream side of the compressor portion  20  of the turbocharger-type supercharger  19  is drawn in and fed from the compressor portion  20 . 
     Exhaust passages  22 A,  22 B,  22 C, and  22 D are connected with the cylinder head  12 . The exhaust passages  22 A and  22 D are merged and connected to a joining passage  23 AD. The exhaust passages  22 B and  22 C are merged and connected to a joining passage  23 BC. The joining passage  23 AD and the joining passage  23 BC are connected to a turbine portion  21  of the turbocharger-type supercharger  19 . The exhaust passages  22 A and  22 D and the joining passage  23 AD configure a first connecting passage connected to the turbine portion  21 . The exhaust passages  22 B and  22 C and the joining passage  23 BC configure a second connecting passage connected to the turbine portion  21 . The first connecting passage and the second connecting passage configure an exhaust gas passage for guiding exhaust gas discharged from the diesel engine  10  to the turbine portion  21 . 
     Exhaust gas discharged from the cylinders  11 A and  11 D is directed to the joining passage  23 AD via the exhaust passages  22 A and  22 D, and exhaust gas discharged from the cylinders  11 B and  11 C is directed to the joining passage  23 BC via the exhaust passages  22 B and  22 C. The exhaust gas directed from the joining passages  23 AD and  23 BC to the turbine portion  21  is discharged to the atmosphere via an exhaust passage  24 . 
       FIG. 2A  illustrates the internal structure of the turbocharger-type supercharger  19 . The compressor portion  20  includes a compressor housing  25  and a compressor wheel  27  which is fixedly attached to a rotor shaft  26 . The turbine portion  21  includes a turbine housing  28  and a turbine wheel  29  which is fixedly attached to the rotor shaft  26 . The compressor housing  25  and the turbine housing  28  are connected via a center housing  30 . 
     As illustrated in  FIG. 2B , a pair of scroll passages  31 AD and  31 BC each serving as an exhaust gas introduction passage is provided in the turbine housing  28 . The exhaust gas discharged from the cylinders  11 A and  11 D to the turbine portion  21  via the joining passage  23 AD is fed into the scroll passage  31 AD and a swirling passage  32  and is directed against blades  291  of the turbine wheel  29 . The exhaust gas discharged from the cylinders  11 B and  11 C to the turbine portion  21  via the joining passage  23 BC is fed into the scroll passage  31 BC and the swirling passage  32  and is directed against the blades  291  of the turbine wheel  29 . This allows the turbine wheel  29 , the rotor shaft  26 , and the compressor wheel  27  to rotate in an integrated manner. 
     The compressor wheel  27  introduces the air within the intake passage  17  on the upstream side of the compressor portion  20  into a compressor passage  251  provided in the compressor housing  25 , and directs the air to the intake passage  17  downstream of the compressor portion  20 . 
     A plurality of nozzle vanes  33  is disposed in the middle of the swirling passage  32 . As illustrated in  FIG. 2B , the nozzle vanes  33  are rotatably supported with a nozzle ring  34 . The nozzle vanes  33  may change a sectional area of a flow passage between the adjacent nozzle vanes  33 . 
     As illustrated in  FIG. 2A , an arm  36  is fixedly attached to a spindle  35  which is rotatable with respect to the nozzle ring  34 , and a unison ring  37  is inseparably engaged with the arm  36 . A spindle  38  is rotatably supported on the center housing  30 . A drive arm  39  is fixedly attached to one end of the spindle  38 . The drive arm  39  is engaged with the unison ring  37 . Rotation of the drive arm  39  about the spindle  38  allows the unison ring  37  to be rotated. 
     A drive lever  40 , which is fixedly attached to the other end of the spindle  38 , is rotated about the spindle  38  by an operation of a non-illustrated actuator. When the drive lever  40  is rotated, the drive arm  39  and the unison ring  37  rotate and the arm  36  and the nozzle vanes  33  rotate. That is, a vane opening degree is changed. An increase in the vane opening degree causes a decrease in turbine rotational speed, which results in a decrease in the flow rate of the air within the intake passage  17  on the downstream side of the compressor portion  20 . A decrease in the vane opening degree causes an increase in turbine rotational speed, which results in an increase in the flow rate of the air within the intake passage  17  on the downstream side of the compressor portion  20 . 
     As illustrated in  FIG. 1 , a bridge passage  41  is connected to the middle of the joining passage  23 AD and to the middle of the joining passage  23 BC. An electric first opening and closing valve V 1  is provided in the middle of the bridge passage  41 . The bridge passage  41  is connected with one end of an EGR passage  42  serving as a branch passage. The other end of the EGR passage  42  is connected to the intake passage  17 . When the first opening and closing valve V 1  is in a closed state, the communication between the joining passage  23 AD and the joining passage  23 BC via the bridge passage  41  is blocked. When the first opening and closing valve V 1  is in an open state, the joining passage  23 AD and the joining passage  23 BC communicate with each other via the bridge passage  41 . The first opening and closing valve V 1  serves as a first opening and closing device that is provided to the bridge passage  41  to open and close the bridge passage  41 . 
     An intercooler  46  and a throttle valve  47  are each provided in the middle of the intake passage  17 . The intercooler  46  cools the air flowing within the intake passage  17 . The throttle valve  47  regulates the flow rate of the air to be fed to the cylinders  11 A,  11 B,  11 C, and  11 D. The opening degree of the throttle valve  47  is controlled in accordance with depression of a non-illustrated accelerator pedal. 
     The opening degree of the throttle valve  47  is detected by a throttle opening detector  45 . A rotation angle (crank angle) of a non-illustrated crank shaft is detected by a crank angle detector  48 . Throttle opening degree detection information detected by the throttle opening detector  45  and crank angle detection information detected by the rank angle detector  48  are sent to a control computer C. The control computer C calculates and controls a fuel injection time (an injection start time and an injection end time) of the fuel injection nozzles  13  based on the throttle opening degree detection information and the crank angle detection information. The control computer C also calculates the rate of revolution N of the engine based on the crank angle detection information obtained by the crank angle detector  48 . The control computer C also calculates engine load from the fuel injection time (or the amount of fuel injection) described above, for example. 
     The control computer C and the crank angle detector  48  configure a rate-of-revolution detection device that detects the rate of revolution of the internal combustion engine. The control computer C, the throttle opening detector  45 , and the crank angle detector  48  configure a load detection device that detects load of the internal combustion engine. 
     The intake manifold  16  is provided with a pressure detector  44 . The pressure detector  44  detects pressure within the intake manifold  16 , i.e., supercharging pressure. Information regarding the supercharging pressure detected by the pressure detector  44  is provided to the control computer C. 
     The control computer C determines a target supercharging pressure from a preliminarily set map based on the rate of revolution of the engine, engine load, and the like. Further, the control computer C controls the vane opening degree of the turbine portion  21  of the turbocharger-type supercharger  19  so that the supercharging pressure detected by the pressure detector  44  reaches the target supercharging pressure. 
     A heat exchanger  43  is provided in the middle of the EGR passage  42 . An electric second opening and closing valve V 2  is provided in the middle of the EGR passage  42  on the downstream side of the heat exchanger  43 . An electric third opening and closing valve V 3  is provided in the middle of the EGR passage  42  on the upstream side of the heat exchanger  43 . When the second opening and closing valve V 2  is in the closed state, communication between the heat exchanger  43  and the intake passage  17  is blocked. When the second opening and closing valve V 2  is in the open state, the heat exchanger  43  and the intake passage  17  communicate with each other via the EGR passage  42 . When the third opening and closing valve V 3  is in the closed state, communication between the heat exchanger  43  and the bridge passage  41  is blocked. When the third opening and closing valve V 3  is in the open state, the heat exchanger  43  and the joining passage  23 AD communicate with each other via the EGR passage  42  and the bridge passage  41 . 
     The second opening and closing valve V 2  serves as a second opening and closing device that is provided downstream of the heat exchanger  43  in the EGR passage  42  to open and close the EGR passage  42 . The third opening and closing valve V 3  serves as a third opening and closing device that is upstream of the heat exchanger  43  to open and close the EGR passage  42 . 
     The control computer C controls opening and closing of the first opening and closing valve V 1 , the second opening and closing valve V 2 , and the third opening and closing valve V 3 . 
       FIG. 3  is a region graph represented by the rate of revolution N of the engine and engine load F. A region G 1  is a region where it is desirable that a turbine driving force in the turbocharger-type supercharger  19  is increased when the rate of revolution N of the engine is low. A region G 2  is a region where it is desirable that the turbine driving force in the turbocharger-type supercharger  19  is increased while preventing the pressure within each of the cylinders  11 A,  11 B,  11 C, and  11 D from exceeding allowable maximum pressure. A region G 3  is a region where it is desirable that the turbine driving force in the turbocharger-type supercharger  19  is increased while preventing a peak value of an exhaust pulse from exceeding an allowable maximum value. A region G 4  is a region where it is desirable that exhaust gas is sent to the EGR passage  42  to thereby clean the exhaust gas. 
     The region G 4  is a low load region. The region G 1  is a low-revolution-rate high-load region with a higher load and a lower rate of revolution than those of the low-load region G 4 . The region G 2  is an intermediate-revolution-rate high-load region with a load higher than that of the low-load region G 4  and with a higher rate of revolution than that of the low-revolution-rate high-load region G 1 . The region G 3  is a high-revolution-rate high-load region with a load higher than that of the low-load region G 4  and with a higher rate of revolution than that of the intermediate-revolution-rate high-load region G 2 . 
       FIG. 4  is a flowchart illustrating an opening and closing control program for controlling opening and closing of the first opening and closing valve V 1 , the second opening and closing valve V 2 , and the third opening and closing valve V 3 . Hereinafter, a control for opening and closing the first opening and closing valve V 1 , the second opening and closing valve V 2 , and the third opening and closing valve V 3  will be described with reference to this flowchart. 
     The control computer C judges if a pair (N, F) of the calculated rate of revolution N of the engine and the calculated engine load F is present in the low-revolution-rate high-load region G 1  (step S 1 ). When the pair (N, F) is present in the low-revolution-rate high-load region G 1  (YES in step S 1 ), the control computer C controls all of the first opening and closing valve V 1 , the second opening and closing valve V 2 , and the third opening and closing valve V 3  to be brought into the closed state (step S 2 ). This control enables a large turbine driving force even when the rate of revolutions N of the engine is low, while preventing the exhaust gas within the joining passages  23 AD and  23 BC from being sent to the intake passage  17  via the bridge passage  41 , the EGR passage  42 , and the heat exchanger  43 . 
     In step S 1 , when the pair (N, F) is not present in the low-revolution-rate high-load region G 1 , the control computer C judges if the pair (N, F) is present in the intermediate-revolution-rate high-load region G 2  (step S 3 ). When the pair (N, F) is present in the intermediate-revolution-rate high-load region G 2  (YES in step S 3 ), the control computer C controls the first opening and closing valve V 1  to be brought into the open state and controls the second opening and closing valve V 2  and the third opening and closing valve V 3  to be brought into the closed state (step S 4 ). This control allows the joining passage  23 AD and the joining passage  23 BC to communicate with each other via the bridge passage  41 , while preventing the exhaust gas within the bridge passage  41  from being directed to the intake passage  17  via the EGR passage  42  and the heat exchanger  43 . In this state, a large turbine driving force can be obtained, while preventing the pressure within each of the cylinders  11 A,  11 B,  11 C, and  11 D from exceeding the allowable maximum pressure. 
     In step S 3 , when the pair (N, F) is not present in the intermediate-revolution-rate high-load region G 2 , the control computer C judges if the pair (N, F) is present in the high-revolution-rate high-load region G 3  (step S 5 ). When the pair (N, F) is present in the high-revolution-rate high-load region G 3  (YES in step S 5 ), the control computer C controls the first opening and closing valve V 1  and the third opening and closing valve V 3  to be brought into the open state, and controls the second opening and closing valve V 2  to be brought into the closed state (step S 6 ). This control allows the joining passage  23 AD and the joining passage  23 BC to communicate with each other via the bridge passage  41  and allows the heat exchanger  43  to communicate with the bridge passage  41  via the EGR passage  42 , while preventing the exhaust gas within the bridge passage  41  from being sent to the intake passage  17  via the EGR passage  42 . In this state, a large turbine driving force can be obtained, while preventing the peak value of the exhaust pulse from exceeding the allowable maximum value. 
     In step S 5 , when the pair (N, F) is not present in the high-revolution-rate high-load region G 3 , i.e., when the pair (N, F) is present in the low-load region G 4 , the control computer C controls all of the first opening and closing valve V 1 , the second opening and closing valve V 2 , and the third opening and closing valve V 3  to be brought into the open state (step S 7 ). This control allows the joining passage  23 AD and the joining passage  23 BC to communicate with each other via the bridge passage  41 , and allows the intake passage  17  to communicate with the bridge passage  41  via the EGR passage  42 . Accordingly, the exhaust gas within the bridge passage  41  is directed to the intake passage  17  via the EGR passage  42 , and the exhaust gas is cleaned using recirculation of the exhaust gas. 
     The control computer C is a control device that controls opening and closing of the first opening and closing valve V 1 , the second opening and closing valve V 2 , and the third opening and closing valve V 3  in accordance with the rate of revolution N detected by the rate-of-revolution detection device and the load F detected by the load detection device. 
     A curve P 1  in the graph of  FIG. 7  represents pressure fluctuation in the joining passage  23 AD when the rate of revolution N of the engine is high (for example, 3600 rpm) and when the first opening and closing valve V 1  is in the open state. The abscissa axis represents crank angle, and the ordinate axis represents pressure. A curve P 2  represents pressure fluctuation in the joining passage  23 AD when the rate of revolution N of the engine is high as described above, and when the first opening and closing valve V 1  is in the closed state. When the first opening and closing valve V 1  is in the closed state, the maximum value of the exhaust pulse is excessively large. On the other hand, when the first opening and closing valve V 1  is brought into the open state, the maximum value of the exhaust pulse can be lowered to be less than compressive strength of exhaust parts (for example, a sealing structure of the exhaust system). 
     Step S 4  in the flowchart is a control step of opening only the first opening and closing valve V 1  to lower the maximum value of the exhaust pulse to be less than the compressive strength. As a result, in the intermediate-revolution-rate high-load region G 2  in which the rate of revolution N of the engine is intermediate, a large turbine driving force can be obtained, while preventing the pressure within each of the cylinders  11 A,  11 B,  11 C, and  11 D from exceeding the allowable maximum pressure. 
     On the contrary, the low-revolution-rate high-load region G 1  is a region where the rate of revolution N of the engine is low is a region where it is desirable that the maximum value of the exhaust pulse is set to be approximate to the compressive strength of the exhaust parts (for example, sealing structure of an exhaust system) to thereby increase the turbine driving force. Step S 2  in the flowchart is a control step therefor. This enables a large turbine driving force also in the low-revolution-rate high-load region G 1  in which the rate of revolution N of the engine is low. 
     When the sectional area of the bridge passage  41  is small in the high-revolution-rate high-load region G 3  in which the rate of revolution N of the engine is high, there is a possibility that the maximum value of the exhaust pulse cannot be set to be less than the compressive strength with opening only the first opening and closing valve V 1 . 
     Each of curves T 1 , T 2 , T 3 , and T 4  in the graph illustrated in  FIG. 5A  represents a change in output torque when the joining passage  23 AD and the joining passage  23 BC communicate with each other via the bridge passage  41  and when the heat exchanger  43  communicates with the bridge passage  41  via the EGR passage  42 . The axis of abscissa represents the passage diameter of the bridge passage  41 , and the axis of ordinate represents an output torque. The curve T 1  represents a change in output torque when a vane opening degree ratio in the turbocharger-type supercharger  19  is 50%. The curve T 2  represents a change in output torque when the vane opening degree ratio in the turbocharger-type supercharger  19  is 60%. The curve T 3  represents a change in output torque when the vane opening degree ratio in the turbocharger-type supercharger  19  is 70%. The curve T 4  represents a change in output torque when the vane opening degree ratio in the turbocharger-type supercharger  19  is 80%. 
     Each of curves E 1 , E 2 , E 3 , and E 4  in the graph illustrated in  FIG. 5B  represents a change in maximum value of the exhaust pulse when the joining passage  23 AD and the joining passage  23 BC communicate with each other via the bridge passage  41  and when the heat exchanger  43  communicates with the bridge passage  41  via the EGR passage  42 . The abscissa axis represents the passage diameter of the bridge passage  41 , and the ordinate axis represents the maximum value of the exhaust pulse. The curve E 1  represents change in maximum value of the exhaust pulse when the vane opening degree ratio in the turbocharger-type supercharger  19  is 50%. The curve E 2  represents change in maximum value of the exhaust pulse when the vane opening degree ratio in the turbocharger-type supercharger  19  is 60%. The curve E 3  represents a change in maximum value of the exhaust pulse when the vane opening degree ratio in the turbocharger-type supercharger  19  is 70%. The curve E 4  represents a change in maximum value of the exhaust pulse when the vane opening degree ratio in the turbocharger-type supercharger  19  is 80%. 
       FIGS. 5A and 5B  each illustrate the case where the rate of revolution of the engine is high (for example, 3600 rpm). A passage diameter So represents the passage diameter of the bridge passage  41 . 
     In this embodiment, assuming that a minimum value of a required output torque is 300 Nm and an allowable maximum value of an exhaust pulse is 450 kPa when the vane opening degree ratio is 60%, when the passage diameter So of the bridge passage  41  is set to a necessary value, the minimum value of the output torque can be obtained and the maximum value of the exhaust pulse can be set to be equal to or lower than the allowable value. 
     On the other hand, each of curves t 1 , t 2 , t 3 , and t 4  in the graph illustrated in  FIG. 6A  represents a change in output torque when the joining passage  23 AD and the joining passage  23 BC communicate with each other via the bridge passage  41 , while the heat exchanger  43  does not communicate with the bridge passage  41  via the EGR passage  42 . The abscissa axis represents the passage diameter of the bridge passage  41 , and the ordinate axis represents an output torque. The curve t 1  represents a change in output torque when the vane opening degree ratio in the turbocharger-type supercharger  19  is 50%. The curve t 2  represents a change in output torque when the vane opening degree ratio in the turbocharger-type supercharger  19  is 60%. The curve t 3  represents a change in output torque when the vane opening degree ratio in the turbocharger-type supercharger  19  is 70%. The curve t 4  represents a change in output torque when the vane opening degree ratio in the turbocharger-type supercharger  19  is 80%. 
     Each of curves e 1 , e 2 , e 3 , and e 4  in the graph illustrated in  FIG. 6B  represents a change in maximum value of the exhaust pulse when the joining passage  23 AD and the joining passage  23 BC communicate with each other via the bridge passage  41 , while the heat exchanger  43  does not communicate with the bridge passage  41  via the EGR passage  42 . The abscissa axis represents the passage diameter of the bridge passage  41 , and the ordinate axis represents the maximum value of the exhaust pulse. The curve e 1  represents a change in maximum value of the exhaust pulse when the vane opening degree ratio in the turbocharger-type supercharger  19  is 50%. The curve e 2  represents a change in maximum value of the exhaust pulse when the vane opening degree ratio in the turbocharger-type supercharger  19  is 60%. The curve e 3  represents a change in maximum value of the exhaust pulse when the vane opening degree ratio in the turbocharger-type supercharger  19  is 70%. The curve e 4  represents a change in maximum value of the exhaust pulse when the vane opening degree ratio in the turbocharger-type supercharger  19  is 80%. 
     In both  FIGS. 6A and 6B , the rate of revolution of the engine is high (for example, 3600 rpm). The passage diameter So represents the passage diameter of the bridge passage  41 . 
     In the case of  FIGS. 6A and 6B , if the passage diameter So of the bridge passage  41  is set larger than when the heat exchanger  43  communicate with the bridge passage  41  via the EGR passage  42 , the minimum value of the output torque can be obtained. Furthermore, the maximum value of the exhaust pulse can be set to be equal to or lower than the allowable value. 
     The difference between the case of  FIGS. 5A and 5B  and the case of  FIGS. 6A and 6B  resides in whether the passage volume of the heat exchanger  43  is used or not. 
     Step S 6  in the flowchart is a control step of opening not only the first opening and closing valve V 1  but also the third opening and closing valve V 3  to thereby set the maximum value of the exhaust pulse to be less than the compressive strength. When the third opening and closing valve V 3  is brought into the open state, the heat exchanger  43  communicates with the bridge passage  41  via the EGR passage  42 , and the passage volume in the heat exchanger  43  is used to lower the maximum value of the exhaust pulse. As a result, even when the passage diameter of the bridge passage  41  is small, the maximum value of the exhaust pulse can be lowered and a large turbine driving force can be obtained in the high-revolution-rate high-load region G 3 . 
     The low-load region G 4  in which recirculation of the exhaust gas is carried out is a region where it is desirable that recirculation of the exhaust gas is carried out to clean the exhaust gas. However, there is a possibility that the air within the intake passage  17  backflows into the EGR passage  42 . 
     Curve Q in the graph illustrated in  FIG. 8A  represents pressure within the intake passage  17  on the downstream side of the intercooler  46  when the exhaust gas is sent only from the joining passage  23 AD to the EGR passage  42  and the intake passage  17 . The abscissa axis represents crank angle, and the ordinate axis represents pressure. A curve V represents a change in the pressure within the EGR passage  42  on the downstream side of the heat exchanger  43  when the exhaust gas is sent only from the joining passage  23 AD to the EGR passage  42  and the intake passage  17 . As indicated by the curve Q, the pressure within the intake passage  17  may become higher than the pressure within the EGR passage  42  on the downstream side of the heat exchanger  43 . In such a case, the air within the intake passage  17  backflows into the EGR passage  42 . 
     The curve U in the graph illustrated in  FIG. 8B  represents change in fluid flow rate (units of kg/s) of the EGR passage  42  on the downstream side of the heat exchanger  43 . The abscissa axis represents crank angle, and the ordinate axis represents fluid flow rate. The curve U represents change in fluid flow rate of the exhaust gas in the case corresponding to the curve Q illustrated in  FIG. 8A  (i.e., when the exhaust gas is provided only from the joining passage  23 AD to the EGR passage  42  and the intake passage  17 ). The curve U below the abscissa axis represents backflow of the air within the intake passage  17  to the EGR passage  42 . 
     Step S 7  in the flowchart is a control step of opening all of the first opening and closing valve V 1 , the second opening and closing valve V 2 , and the third opening and closing valve V 3 , to provide the exhaust gas from both the joining passages  23 AD and  23 BC to the EGR passage  42  and the intake passage  17 . This control prevents backflow from the intake passage  17  to the EGR passage  42  as indicated by the curve W in the graph illustrated in  FIG. 8C . That is, step S 7  is a control step for preventing backflow from the intake passage  17  to the EGR passage  42 . 
     The first embodiment has the following effects. 
     (1) By closing the second opening and closing valve V 2  and opening the first opening and closing valve V 1  and the third opening and closing valve V 3 , the passage volume in the heat exchanger  43  can be used to reduce the maximum value of the exhaust pulse. As a result, even when the passage diameter of the bridge passage  41  is small, the maximum value of the exhaust pulse can be lowered and a large turbine driving force can be obtained. Accordingly, the first opening and closing valve V 1  can be downsized. 
     (2) When the third opening and closing valve V 3  is omitted, in the state where the second opening and closing valve V 2  is closed and the first opening and closing valve V 1  is opened, the passage volume within the heat exchanger  43  is constantly used to reduce the maximum value of the exhaust pulse. While such control is possible, the presence of the third opening and closing valve V 3  allows finer control of the turbine driving force in accordance with the rate of revolution N of the internal combustion engine and the load F, as in the case where the internal combustion engine is present in the intermediate-revolution-rate high-load region G 2 , for example. 
     Next, a second embodiment will be described with reference to  FIGS. 9 and 10 . The same components as those of the first embodiment are denoted by the same reference numerals, and the detailed description thereof is omitted. 
     In the second embodiment, the third opening and closing valve V 3  of the first embodiment is omitted. In this case, the control of opening and closing the first opening and closing valve V 1  and the second opening and closing valve V 2  is carried out as in steps S 8 , S 9 , S 10 , S 11 , and S 12  in the flowchart of  FIG. 10 . The control computer C is a control device that controls opening and closing of the first opening and closing valve V 1  and the second opening and closing valve V 2  in accordance with the rate of revolution N detected by the rate-of-revolution detection device and the load F detected by the load detection device. 
     Even when the third opening and closing valve V 3  is omitted, the same effects as those described in the item (1) of the first embodiment can be obtained. 
     Next, a third embodiment will be described with reference to  FIGS. 11A ,  11 B,  11 C, and  11 D. The same components as those of the first embodiment are denoted by the same reference numerals, and the detailed description thereof is omitted. 
     As illustrated in  FIG. 11A , an electric three-way valve V 4  is provided in the bridge passage  41 . The rotational position of the three-way valve V 4  is controlled by the control computer C. As illustrated in  FIG. 11B , the three-way valve V 4  includes a rotation valve body  50  in a valve housing  49 , and three ports  501 ,  502 , and  503  are provided in the rotation valve body  50  so as to communicate with one another. Three valve holes  491 ,  492 , and  493  are provided in the valve housing  49 . The valve hole  491  communicates with the joining passage  23 AD via the bridge passage  41 , and the valve hole  492  communicates with the joining passage  23 BC via the bridge passage  41 . The valve hole  493  communicates with the EGR passage  42 . 
     When the pair of the rate of revolution N of the engine and the engine load F is present in the low-revolution-rate high-load region G 1  (see  FIG. 3 ), the three-way valve V 4  is controlled to be brought into the state illustrated in  FIG. 11D , and the second opening and closing valve V 2  is controlled to be brought into the closed state. In this state, the communication between the joining passage  23 AD and the joining passage  23 BC via the bridge passage  41  is blocked, as in step S 2  of the flowchart of  FIG. 4 . 
     When the pair of the rate of revolution N of the engine and the engine load F is present in the intermediate-revolution-rate high-load region G 2  (see  FIG. 3 ), the three-way valve V 4  is controlled to be brought into the state illustrated in  FIG. 11C , and the second opening and closing valve V 2  is controlled to be brought into the closed state. In this state, the joining passage  23 AD and the joining passage  23 BC communicate with each other via the bridge passage  41 , while the communication between the EGR passage  42  and the bridge passage  41  is blocked, as in step S 4  of the flowchart of  FIG. 4 . 
     When the pair of the rate of revolution N of the engine and the engine load F is present in the high-revolution-rate high-load region G 3  (see  FIG. 3 ), the three-way valve V 4  is controlled to be brought into the state illustrated in  FIG. 11B , and the second opening and closing valve V 2  is controlled to be brought into the closed state. In this state, the joining passage  23 AD and the joining passage  23 BC communicate with each other via the bridge passage  41 , and the heat exchanger  43  communicates with the bridge passage  41  via the EGR passage  42 , as in step S 6  of the flowchart of  FIG. 4 . 
     When the pair of the rate of revolution N of the engine and the engine load F is present in the low-load region G 4  (see  FIG. 3 ), the three-way valve V 4  is controlled to be brought into the state illustrated in  FIG. 11B , and the second opening and closing valve V 2  is controlled to be brought into the open state. In this state, the joining passage  23 AD and the joining passage  23 BC communicate with each other via the bridge passage  41 , and the bridge passage  41  and the intake passage  17  communicate with each other via the EGR passage  42 , as in step S 7  of the flowchart of  FIG. 4 . 
     The three-way valve V 4  is a single switch valve that switches communication and blocking between the EGR passage  42  and the bridge passage  41  on the upstream side of the heat exchanger  43 , and that switches opening and closing of the bridge passage  41 . The EGR passage  42  and the bridge passage  41  on the upstream side of the heat exchanger  43  are connected via the three-way valve V 4 . That is, the three-way valve V 4 , which is a switch valve, serves as the first opening and closing device and the third opening and closing device. Use of such a three-way valve V 4  having combined configuration contributes to simplification of the piping configuration of the exhaust gas passage. 
     In the present invention, the following embodiments can also be implemented. 
     In the first embodiment, one of the exhaust passages  22 A and  22 D configuring the first connecting passage and one of the exhaust passages  22 B and  22 C configuring the second connecting passage may be connected to each other with a bridge passage. 
     The present invention can be applied to a six-cylinder engine disclosed in Patent Document 1, or a V-shaped eight-cylinder engine. 
     For example, cylinders in the six-cylinder engine may be divided into three groups, and each connecting passage may be guided to a turbocharger-type supercharger from each group. In this case, each connecting passage is connected to the corresponding bridge passage in the middle of each connecting passage, and each bridge passage is provided with the first opening and closing device. 
     Two or more of a plurality of connecting passages for connecting a plurality of exhaust gas introduction passages with a plurality of cylinders in one-to-one correspondence may be connected to each other via a bridge passage. 
     In an internal combustion engine with no EGR passage, the exhaust passage  24  and the bridge passage  41  on the downstream side of the turbine portion  21  may be connected together via a branch passage, and the third opening and closing device may be provided on the branch passage. 
     The present invention can also be applied to a gasoline engine.