Abstract:
There is provided an exhaust heat recovery structure that may suppress boiling of coolant in a heat exchanger. The exhaust heat recovery structure includes a first pipe, a second pipe, a valve and a thermostat. Exhaust gas from an engine flows in the first pipe. The second pipe branches from the first pipe and a heat exchanger that exchanges heat between the coolant and exhaust gas is provided at the second pipe. The valve is provided at the first pipe or the second pipe. The valve adjusts a flow amount of exhaust gas flowing into the second pipe by opening and closing. The thermostat is equipped with a heat-sensing portion that is disposed inside the heat exchanger. When the temperature of the heat-sensing portion is high, the thermostat opens or closes the valve to reduce the flow amount of exhaust gas flowing into the second pipe.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2015-126882 filed on Jun. 24, 2015, which is incorporated by reference herein. 
       BACKGROUND 
       [0002]    Technical Field 
         [0003]    The present invention relates to an exhaust heat recovery structure. 
         [0004]    Related Art 
         [0005]    An exhaust heat recovery structure is known that heats a coolant and accelerates warm-up of an engine, by a heat exchanger being disposed on an exhaust pipe through which exhaust gas from the engine flows and heat being exchanged between coolant that flows in the heat exchanger and the exhaust gas. A configuration of this exhaust heat recovery structure is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2006-312884 (Patent Document 1), which is equipped with a bypass pipe through which exhaust gas flows, a valve body (valve) provided inside the bypass pipe, and a branch pipe that branches from the bypass pipe and is connected to a heat exchanger. A further configuration is recited in which a temperature of the coolant circulating through the heat exchanger is sensed and the valve body is controlled to open or close in accordance with this temperature. 
         [0006]    However, the technology disclosed in Patent Document 1 gives no consideration to the location at which the temperature of the coolant is sensed, and it may not be possible to accurately acquire a temperature of the coolant in the heat exchanger. Consequently, when the temperature of the coolant in the heat exchanger rises, the coolant may boil. 
       SUMMARY 
       [0007]    In consideration of the circumstances described above, an object of the present invention is to provide an exhaust heat recovery structure that may suppress boiling of coolant in a heat exchanger. 
         [0008]    An exhaust heat recovery structure according to a first aspect of the present invention includes: a first pipe through which exhaust gas from an engine flows; a second pipe that branches from the first pipe and at which a heat exchanger is provided, the heat exchanger exchanging heat between a coolant and exhaust gas; a valve provided at one of the first pipe or the second pipe, the valve adjusting a flow amount of exhaust gas flowing into the second pipe by opening and closing; and a thermostat equipped with a heat-sensing portion that is disposed at an interior of the heat exchanger, the thermostat opening or closing the valve and reducing the flow amount of exhaust gas flowing into the second pipe when a temperature of the heat-sensing portion is high. 
         [0009]    In the exhaust heat recovery structure according to the first aspect, the second pipe branches from the first pipe and the heat exchanger is provided at the second pipe. The valve is provided at the first pipe or the second pipe, and the first pipe or second pipe is opened and closed by this valve being operated. Thus, flow amounts of exhaust gas flowing in the second pipe can be adjusted. If, for example, the valve is provided in the first pipe, in the state in which the first pipe is closed by the valve, almost all of the exhaust gas from the engine flows into the second pipe, heat is exchanged between the coolant and the exhaust gas by the heat exchanger, and the coolant is heated. Hence, warm-up of the engine at a time of cold starting may be accelerated. In contrast, in the state in which the first pipe is opened by the valve, most of the exhaust gas from the engine flows in the first pipe, and hardly any of the exhaust gas flows in the second pipe. Hence, there is hardly any heat exchange between the coolant and the exhaust gas, and a rise in temperature of the coolant after warm-up of the engine or the like may be suppressed. On the other hand, if the valve is provided in the second pipe, in the state in which the second pipe is closed by the valve, hardly any of the exhaust gas flows in the second pipe. In contrast, in the state in which the second pipe is opened by the valve, the exhaust gas flows into the second pipe. Therefore, heat is exchanged between the coolant and the exhaust gas by the heat exchanger and the coolant may be heated. 
         [0010]    The thermostat equipped with the heat-sensing portion is provided with the heat-sensing portion of the thermostat disposed inside the heat exchanger. When the temperature of the heat-sensing portion rises, the thermostat opens or closes the valve to reduce the flow amount of exhaust gas flowing into the second pipe. Therefore, in contrast to a structure in which a heat-sensing portion is disposed outside a heat exchanger, the valve may be operated before a temperature of coolant in the heat exchanger rises excessively. That is, the flow amount of exhaust gas flowing into the second pipe may be reduced and a rise in the temperature of the coolant may be suppressed before the coolant in the heat exchanger boils. 
         [0011]    In an exhaust heat recovery structure according to a second aspect, in the first aspect, the heat-sensing portion is disposed at a vehicle upper side of the interior of the heat exchanger. 
         [0012]    In the exhaust heat recovery structure according to the second aspect, because the temperature of coolant in the heat exchanger that is at the vehicle upper side—and is at a higher temperature among temperatures of the coolant—is sensed, boiling of the coolant may be suppressed effectively. That is, higher-temperature exhaust gas flows in the upper portions of the first pipe and the second pipe more than in the lower portions. Therefore, the coolant flowing through the vehicle upper side of the heat exchanger is subject to heat exchange from the higher-temperature exhaust gas and is likely to be at a higher temperature than coolant at the vehicle lower side of the heat exchanger. Therefore, if the temperature of the coolant at the vehicle upper side of the heat exchanger is sensed and flow amounts of the exhaust gas flowing into the second pipe are adjusted in accordance with this temperature, boiling of the coolant may be suppressed effectively. 
         [0013]    In an exhaust heat recovery structure according to a third aspect, in the first aspect or the second aspect, a circulation pump that circulates the coolant between the engine and the heat exchanger is provided, the circulation pump being temporarily stopped at a time of cold starting. 
         [0014]    In the exhaust heat recovery structure according to the third aspect, the temperature of coolant in the heat exchanger may be raised, within a range in which the coolant does not boil, by the circulating pump being temporarily stopped at a time of cold starting. When the circulation pump is then re-started, the engine may be warmed up by the high-temperature coolant and warm-up performance of the engine may be improved. 
         [0015]    As described hereabove, according to the exhaust heat recovery structure according to the first aspect, there is an excellent effect in that boiling of the coolant in the heat exchanger may be suppressed. 
         [0016]    According to the exhaust heat recovery structure according to the second aspect, there is an excellent effect in that boiling of the coolant may be suppressed effectively. 
         [0017]    According to the exhaust heat recovery structure according to the third aspect, there is an excellent effect in that warm-up performance of the engine may be improved. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0018]      FIG. 1  is a diagram schematically showing an exhaust heat recovery structure in accordance with a first exemplary embodiment. 
           [0019]      FIG. 2  is a sectional diagram showing principal portions of the exhaust heat recovery structure in accordance with the first exemplary embodiment, showing a state in which a valve is closed. 
           [0020]      FIG. 3  is a sectional diagram corresponding to  FIG. 2 , showing a state in which the valve is opened. 
           [0021]      FIG. 4  is a sectional diagram showing principal portions of an exhaust heat recovery structure in accordance with a second exemplary embodiment, showing a state in which a valve is closed. 
           [0022]      FIG. 5  is a sectional diagram corresponding to  FIG. 4 , showing a state in which the valve is opened. 
           [0023]      FIG. 6  is a sectional diagram showing a state in which  FIG. 4  is cut along line  6 - 6 . 
       
    
    
     DETAILED DESCRIPTION 
     First Exemplary Embodiment 
       [0024]    An exhaust heat recovery structure according to a first exemplary embodiment is described with reference to  FIG. 1  to  FIG. 3 . The arrow FR that is shown as appropriate in the drawings indicates the vehicle front side of a vehicle in which the exhaust heat recovery structure is employed, the arrow UP indicates the vehicle upper side, and the arrow RH indicates the vehicle right side. In the following descriptions, where the directions front, rear, up, down, left and right are used without being particularly specified, the same represent the front and rear in the vehicle front-and-rear direction, up and down in the vehicle up-and-down direction, and left and right if facing in the running direction. For convenience of depiction, pipes are shown in section in the drawings. 
         [0025]    As shown in  FIG. 1 , an exhaust system in which the exhaust heat recovery structure according to the present exemplary embodiment is employed has a structure in which a first pipe  12  is connected to an engine  10  and exhaust gas from the engine  10  flows in the first pipe  12 . 
         [0026]    The first pipe  12  extends toward the vehicle lower side from the engine  10  and then extends toward the vehicle rear side. A catalytic converter  14  is disposed at the first pipe  12 , at the downstream side from the engine  10 . The term “downstream” as used herein refers to downstream in a flow direction of exhaust gas. Where the terms “upstream” and “downstream” are used in the descriptions below, these refer to upstream and downstream in the flow direction of exhaust gas. 
         [0027]    The catalytic converter  14  is a tubular member, both end portions of which are open. A catalytic carrier for cleaning gases is provided inside the catalytic converter  14 . The catalytic carrier is formed by a thin plate, which is formed in a honeycomb pattern, a wave shape or the like, being structured into a spiral shape, concentric rings or the like. Thus, the catalytic carrier is formed in a circular rod shape or a circular tube shape that enlarges the surface area of the material of the catalytic carrier. The catalytic carrier carries a catalyst (platinum, palladium, rhodium or the like) in a state in which the catalyst is adhered to the surface of the catalytic carrier. 
         [0028]    A branch point  12 A is provided in the first pipe  12 , at the downstream side relative to the catalytic converter  14 . A second pipe  18  branches from the first pipe  12  at the branch point  12 A. In other words, the second pipe  18  is connected to the branch point  12 A of the first pipe  12 . 
         [0029]    The second pipe  18  extends to the vehicle upper side from the branch point  12 A, then inflects to the vehicle front-and-rear direction and extends substantially in parallel with the first pipe  12 . The second pipe  18  joins the first pipe  12  at a junction point  12 B at the downstream side relative to a valve  16 , which is described below. The second pipe  18  is formed to be wider in the vehicle width direction than the first pipe  12 . 
         [0030]    A heat exchanger  20  is provided at the second pipe  18 . The heat exchanger  20  is disposed inside the second pipe  18 . A flow channel along which coolant flows is provided inside the heat exchanger  20 . An inlet portion  20 A and an outlet portion  20 B are provided at the heat exchanger  20 . Coolant that has circulated through the engine  10  is fed in through the inlet portion  20 A, and the coolant is fed out through the outlet portion  20 B. Accordingly, in the heat exchanger  20 , heat is exchanged between exhaust gas flowing along the second pipe  18  and the coolant. 
         [0031]    In this exemplary embodiment, a thermostat  22  is mounted at the heat exchanger  20 . The thermostat  22  is described below. 
         [0032]    Now, a circulation channel  25  for the coolant is described. The circulation channel  25  includes a circulation pipe  26  and a recovery pipe  28 . The circulation pipe  26  is provided so as to circulate coolant between the engine  10  and a radiator  24 . The recovery pipe  28  includes an inlet flow channel  28 A and an outlet flow channel  28 B. The inlet flow channel  28 A feeds a portion of coolant into the heat exchanger  20  from partway along the circulation pipe  26 . The outlet flow channel  28 B returns coolant from the heat exchanger  20  to the circulation pipe  26 . 
         [0033]    To be specific, the inlet flow channel  28 A branches from the flow channel of the circulation pipe  26  running from the engine  10  to the radiator  24 . A circulation pump  30  is provided on the inlet flow channel  28 A, and the inlet flow channel  28 A is structured such that coolant may be circulated between the engine  10  and the heat exchanger  20 . In this exemplary embodiment, the portion of coolant flowing along the circulation pipe  26  is caused to flow into the inlet flow channel  28 A by the circulation pump  30 , and the coolant is fed from the inlet flow channel  28 A through the inlet portion  20 A into the heat exchanger  20 . 
         [0034]    The outlet flow channel  28 B is also connected to the outlet portion  20 B of the heat exchanger  20 . Thus, the outlet flow channel  28 B is structured such that coolant that has passed through the heat exchanger  20  is fed out through the outlet portion  20 B to the outlet flow channel  28 B. The coolant flowing through the outlet flow channel  28 B is then returned to a flow channel from the radiator  24  toward the engine  10 . 
         [0035]    Now, the valve  16  and the thermostat  22  are described. As shown in  FIG. 2 , the valve  16  is provided between the branch point  12 A and junction point  12 B of the first pipe  12 . The valve  16  is formed by a member with a substantially flat plate shape. Viewed from the vehicle front side thereof, the valve  16  is formed in a substantially circular shape that corresponds with a cross-sectional shape of the first pipe  12 . In this exemplary embodiment, an upper end portion of the valve  16  is fixed to a turning rod  32  that extends in the vehicle width direction. Thus, the valve  16  is made swingable about the turning rod  32 . The first pipe  12  is opened and closed by the valve  16  swinging. In the closed state, the valve  16  is disposed in a direction (the vehicle up-and-down direction) that is orthogonal to an axial direction of the first pipe  12  (the vehicle front-and-rear direction). The valve  16  is urged in the direction of closing by an urging member such as a spring or the like, which is not shown in the drawings. 
         [0036]    The turning rod  32  extends outside the first pipe  12 . A driving cam  34  is fixed to the turning rod  32 . The driving cam  34  is formed substantially in an “L” shape in a side view seen in the vehicle width direction. The driving cam  34  includes a vertical portion  34 A that extends in the vehicle up-and-down direction and a horizontal portion  34 B that extends to the vehicle rear side. A lower end portion of the vertical portion  34 A and a front end portion of the horizontal portion  34 B are joined together. For convenience of explanation, the driving cam  34  and the thermostat  22 , which is described below, are shown in  FIG. 1  to  FIG. 3 . In practice, however, the driving cam  34  and the thermostat  22  would be disposed to the paper surface side of the drawing relative to the first pipe  12 . 
         [0037]    A penetrating hole is formed in a rear end portion of the horizontal portion  34 B of the driving cam  34 . The turning rod  32  penetrates through this penetrating hole, and the horizontal portion  34 B is fixed to the turning rod  32 . Thus, in this structure, the turning rod  32  is turned and the valve  16  is swung by the driving cam  34  swinging about the rear end portion of the horizontal portion  34 B. 
         [0038]    The thermostat  22  is disposed above the vertical portion  34 A of the driving cam  34 . In the present exemplary embodiment, a thermostat utilizing a wax thermostatic element is employed as the thermostat  22 . The thermostat  22  principally includes a heat-sensing portion  22 A, a large diameter portion  22 B, a shaft cover  22 C and a shaft  22 D. 
         [0039]    The heat-sensing portion  22 A is formed in a substantially circular tube shape of which an upper end portion is closed off. The heat-sensing portion  22 A extends as far as the vehicle upper side of the interior of the heat exchanger  20 . That is, an upper end portion of the heat-sensing portion  22 A is disposed at the vehicle upper side of the interior of the heat exchanger  20 . Thus, the heat-sensing portion  22 A senses a temperature of coolant at the vehicle upper side inside the heat exchanger  20 . Paraffin wax, which is not shown in the drawings, is accommodated inside the heat-sensing portion  22 A. Although the heat-sensing portion  22 A is disposed within the heat exchanger  20 , the heat-sensing portion  22 A is represented by solid lines in  FIG. 1  to  FIG. 3  for convenience of depiction. 
         [0040]    The large diameter portion  22 B is formed at the vehicle lower side of the heat-sensing portion  22 A. The large diameter portion  22 B is formed in a substantially circular tube shape with a larger diameter than the heat-sensing portion  22 A. The large diameter portion  22 B is disposed at the exterior of the heat exchanger  20 . A diaphragm, which is not shown in the drawings, extends across the interior of the large diameter portion  22 B and seals in the paraffin wax accommodated in the heat-sensing portion  22 A. The shaft cover  22 C and shaft  22 D are provided at the vehicle lower side of the large diameter portion  22 B. The shaft cover  22 C is formed in a substantially circular tube shape with a smaller diameter than the large diameter portion  22 B. The shaft  22 D, with a substantially circular rod shape, is accommodated inside the shaft cover  22 C. 
         [0041]    In the present exemplary embodiment, at a usual temperature (for example, 25° C.), only a distal end portion of the shaft  22 D protrudes from the shaft cover  22 C. When the heat-sensing portion  22 A is heated, the paraffin wax expands and pushes the diaphragm down, moving the shaft  22 D toward the vehicle lower side. As a result, a protrusion amount of the shaft  22 D from the shaft cover  22 C increases. In other words, the shaft  22 D elongates to the vehicle lower side from the shaft cover  22 C. 
         [0042]    As shown in  FIG. 3 , in a state in which the shaft  22 D has elongated and is pushing down on the driving cam  34 , the turning rod  32  turns in the counterclockwise direction. Consequently, via the turning rod  32 , the valve  16  swings to the vehicle rear side. As a result, the first pipe  12  is mechanically opened. When the first pipe  12  has been opened, a portion of the exhaust gas from the engine  10  passes the valve  16  and flows toward the vehicle rear side. Therefore, a flow amount of exhaust gas flowing into the second pipe  18  decreases. That is, flow amounts of exhaust gas flowing into the second pipe  18  are adjusted by the valve  16  opening and closing. In the present exemplary embodiment, the composition and the like of the wax are adjusted such that the shaft  22 D pushes the driving cam  34  down at a stage at which the heat-sensing portion  22 A has been heated to approximately 70° C. 
         [0043]    Now, a warm-up process of the engine  10  at a time of cold starting is described. First, a process for warming up the engine  10  in a state in which the circulation pump  30  is operated is described. 
         [0044]    As shown in  FIG. 1 , the circulation pump  30  circulates coolant between the engine  10  and the heat exchanger  20 . When the engine  10  is started, exhaust gas from the engine  10  is exhausted into the first pipe  12 . 
         [0045]    As shown in  FIG. 2 , after the cold start, the valve  16  is kept in the closed state. Therefore, as illustrated by arrow F 1 , almost all of the exhaust gas from the engine  10  flows into the second pipe  18 . Because the exhaust gas flows through the second pipe  18 , heat is exchanged between coolant flowing in the heat exchanger  20  and the exhaust gas, heating the coolant. The heated coolant is fed out through the outlet portion  20 B to the outlet flow channel  28 B and flows into the circulation pipe  26 . Warm-up of the engine  10  is accelerated by this heated coolant flowing inside the engine  10  (see  FIG. 1 ). 
         [0046]    As the warm-up of the engine  10  proceeds, the temperature of the coolant rises, and the temperature of the heat-sensing portion  22 A of the thermostat  22  disposed inside the heat exchanger  20  also rises. As the temperature of the heat-sensing portion  22 A rises, the shaft  22 D elongates to the vehicle lower side. 
         [0047]    At a time at which the temperature of the heat-sensing portion  22 A rises above a predetermined temperature (for example, 70° C.), the shaft  22 D abuts against the driving cam  34  and the driving cam  34  is pushed down by the shaft  22 D. As a result, the valve  16  swings toward the vehicle rear side via the turning rod  32  and the first pipe  12  is opened. Hence, as illustrated by arrow F 2  in  FIG. 3 , a portion of the exhaust gas flows through the first pipe  12 . Thus, a flow amount of the exhaust gas flowing into the second pipe  18  is reduced. 
         [0048]    Because the flow amount of exhaust gas flowing into the second pipe  18  is reduced, heat exchange between the exhaust gas and the coolant ceases and the temperature of the coolant falls. Hence, the coolant takes heat from the engine  10  and the temperature of the engine  10  is kept constant. 
         [0049]    Now, a case in which the circulation pump  30  is stopped at a time of cold starting is described. 
         [0050]    When the circulation pump  30  is stopped, the flow of coolant circulating between the engine  10  and the heat exchanger  20  stops. Therefore, coolant is heated in the engine  10 . Thus, warm-up performance of the engine  10  may be improved. 
         [0051]    Meanwhile, coolant that is in the heat exchanger  20  is also heated. If the temperature of the coolant in the heat exchanger  20  rises above the predetermined temperature while the circulation pump  30  is stopped, the shaft  22 D of the thermostat  22  pushes down on the driving cam  34  as described above. As a result, the first pipe  12  is opened, a flow amount of exhaust gas flowing into the second pipe  18  is reduced, and boiling of the coolant is suppressed. 
         [0052]    —Operation and Effects— 
         [0053]    Now, operation and effects of the exhaust heat recovery structure according to the present exemplary embodiment are described. 
         [0054]    In the present exemplary embodiment, because the heat-sensing portion  22 A of the thermostat  22  is disposed inside the heat exchanger  20 , the valve  16  may be pushed open by the shaft  22 D earlier than in a structure in which the heat-sensing portion  22 A is disposed outside the heat exchanger  20 . Therefore, boiling of coolant in the heat exchanger  20  may be suppressed. 
         [0055]    In the present exemplary embodiment, because the heat-sensing portion  22 A of the thermostat  22  is disposed at the vehicle upper side of the interior of the heat exchanger  20 , the temperature of coolant whose temperature is higher may be sensed. That is, of exhaust gas flowing through the second pipe  18 , higher temperature exhaust gas flows at the vehicle upper side of the second pipe  18 . Therefore, coolant that is at the vehicle upper side of the interior of the heat exchanger  20  is at a higher temperature. As a result, boiling of the coolant may be suppressed effectively. 
         [0056]    In the present exemplary embodiment, at a time of cold starting, the coolant in the heat exchanger  20  may be heated further by the circulation pump  30  being temporarily stopped. As a result, warm-up performance of the engine  10  may be improved. 
         [0057]    In the present exemplary embodiment, because the thermostat  22  is mounted to be oriented downward, maintenance, replacement and the like in the vehicle-mounted state may be made easier than in a structure in which the thermostat  22  is mounted to be oriented in the vehicle front-and-rear direction. Thus, serviceability may be improved. 
       Second Exemplary Embodiment 
       [0058]    Now, an exhaust heat recovery structure according to a second exemplary embodiment is described with reference to  FIG. 4  to  FIG. 6 . Overall structure of the exhaust system is similar to the structure in  FIG. 2 , apart from the principal portions shown in  FIG. 4  to  FIG. 6 . Structures that are the same as in the first exemplary embodiment are assigned the same reference numerals and, as appropriate, are not described. For convenience of explanation, a driving cam  56  and the thermostat  22  are shown in  FIG. 4  and  FIG. 5 . In practice, however, the driving cam  56  and the thermostat  22  would be disposed to the paper surface side of the drawing relative to a first pipe  52 . 
         [0059]    As shown in  FIG. 4 , the exhaust heat recovery structure according to the present exemplary embodiment is a coaxial-type structure in which the first pipe  52  and a second pipe  54  are provided concentrically. The first pipe  52  extends in the vehicle front-and-rear direction. Exhaust gas from the engine flows in the first pipe  52  (see  FIG. 1 ). 
         [0060]    In this exemplary embodiment, the first pipe  52  is separated into a front side first pipe  52 A and a rear side first pipe  52 B. The front side first pipe  52 A is at the vehicle front side of a branching region between the first pipe  52  and the second pipe  54 , and the rear side first pipe  52 B is at the vehicle rear side of the branching region. The front side first pipe  52 A and rear side first pipe  52 B are spaced apart in the vehicle front-and-rear direction and disposed to be coaxial. 
         [0061]    The second pipe  54  is formed in a substantially circular tube shape with a larger diameter than the first pipe  52 . The second pipe  54  is provided to branch from the first pipe  52  and extend between the front side first pipe  52 A and the rear side first pipe  52 B. As shown in  FIG. 6 , the second pipe  54  is disposed at the outer periphery side of the first pipe  52 . The heat exchanger  20  is provided between the second pipe  54  and the first pipe  52 . 
         [0062]    The heat exchanger  20  is formed in a substantially circular tube shape. As shown in  FIG. 4 , the heat exchanger  20  is disposed at the outer periphery side of a distal end portion of the front side first pipe  52 A. The inlet portion  20 A that feeds in coolant that has circulated through the engine and the outlet portion  20 B that feeds out coolant are provided at the heat exchanger  20 . Thus, in the heat exchanger  20 , heat is exchanged between exhaust gas flowing through the second pipe  54  and the coolant. 
         [0063]    The valve  16  is provided in the vicinity of an aperture portion of the rear side first pipe  52 B. An upper end portion of the valve  16  is mounted at the turning rod  32  that extends in the vehicle width direction, in the same manner as in the first exemplary embodiment. In this structure, the rear side first pipe  52 B is opened by the valve  16  swinging. The valve  16  is urged in the direction of closing by an urging member such as a spring or the like, which is not shown in the drawings. 
         [0064]    The turning rod  32  extends outside the first pipe  52 , and the driving cam  56  is mounted at the turning rod  32 . The driving cam  56  is formed substantially in an “L” shape that is flipped left-to-right in a side view seen in the vehicle width direction. The driving cam  56  includes a horizontal portion  56 A that extends in the vehicle front-and-rear direction and a vertical portion  56 B that extends to the vehicle upper side. A rear end portion of the horizontal portion  56 A and a lower end portion of the vertical portion  56 B are joined together. 
         [0065]    A penetrating hole is formed in an upper end portion of the vertical portion  56 B. The turning rod  32  penetrates through this penetrating hole, and the driving cam  56  is fixed to the turning rod  32 . Thus, in this structure, the turning rod  32  is turned and the valve  16  is opened and closed by the driving cam  56  swinging about the upper end portion of the vertical portion  56 B. 
         [0066]    The thermostat  22  is provided to the vehicle front side of the driving cam  56 . In the present exemplary embodiment, the heat-sensing portion  22 A of the thermostat  22 , the large diameter portion  22 B and a portion of the shaft cover  22 C are disposed inside the heat exchanger  20 . The shaft  22 D opposes the horizontal portion  56 A of the driving cam  56  in the vehicle front-and-rear direction. When the heat-sensing portion  22 A is heated, the paraffin wax expands and the shaft  22 D moves toward the vehicle rear side. 
         [0067]    —Operation and Effects— 
         [0068]    Now, operation and effects of the exhaust heat recovery structure according to the present exemplary embodiment are described. 
         [0069]    In the present exemplary embodiment, at a time of cold starting, the valve  16  is closed and flows of exhaust gas from the front side first pipe  52 A into the rear side first pipe  52 B are blocked. Therefore, as shown by arrows F 3  in  FIG. 4 , most of the exhaust gas flows toward the vehicle front side and is blown against the heat exchanger  20 . The exhaust gas passes along the heat exchanger  20  through a gap between the second pipe  54  and the heat exchanger  20 . As a result, coolant flowing in the heat exchanger  20  may be heated and warm-up of the engine may be accelerated. 
         [0070]    The exhaust gas that has flowed to the vehicle front side turns back at a front end portion of the second pipe  54  and flows toward the vehicle rear side along the inner wall of the second pipe  54 . The exhaust gas flows into the rear side first pipe  52 B through one or a plural number of vent apertures  52 C that are formed in an outer periphery face of the rear side first pipe  52 B (see arrows F 4  in  FIG. 4 ). 
         [0071]    When the temperature of the coolant in the heat exchanger  20  rises and the heat-sensing portion  22 A is heated up to the predetermined temperature, as shown in  FIG. 5 , the shaft  22 D pushes the driving cam  56  to the vehicle rear side, causing the valve  16  to swing to the vehicle rear side. As a result, the rear side first pipe  52 B is opened and exhaust gas flows linearly from the front side first pipe  52 A into the rear side first pipe  52 B (see arrow F 5  in  FIG. 5 ). Hence, flow amounts of exhaust gas flowing into the second pipe  54  are reduced and boiling of the coolant may be suppressed. In the present exemplary embodiment, the exhaust heat recovery structure may be structured more compactly than in the first exemplary embodiment. 
         [0072]    The first and second exemplary embodiments of the present invention have been described hereabove but the present invention is not limited by the structures described above and it will be clear that the present invention may be embodied in numerous modes beside the above structures within a scope that does not depart from the gist of the present invention. For example, in the first exemplary embodiment, the valve  16  is provided at the first pipe  12  as shown in  FIG. 2 , but the present invention is not limited thus. A structure is possible in which the valve  16  is provided at the second pipe  18 . In this case, exhaust gas may be allowed to flow in the second pipe  18  by the valve  16  provided at the second pipe  18  being urged in a direction of opening by an urging member such as a spring or the like. If the valve  16  is closed by the shaft  22 D of the thermostat  22  at a time at which the temperature of the heat-sensing portion  22 A of the thermostat  22  rises beyond the predetermined temperature, flows of exhaust gas in the second pipe  18  cease and boiling of the coolant may be suppressed. 
         [0073]    In the exemplary embodiments described above, driving cams with substantial “L” shapes are used, but the present invention is not limited thus and driving cams with alternative shapes may be used. Further, structures are possible in which no driving cam is used but a valve is pushed open directly by a shaft. 
         [0074]    In the second exemplary embodiment, as shown in  FIG. 4 , the thermostat  22  is disposed a little to the upper side of a vehicle up-and-down direction central region of the heat exchanger  20 , but this is not limiting. For example, the thermostat  22  may be disposed at an upper end portion of the interior of the heat exchanger  20 . In this case, the first pipe  52  may be controlled to open in accordance with the temperature of the coolant flowing in the heat exchanger  20  that is at the highest temperature. Thus, boiling of the coolant may be suppressed effectively. 
         [0075]    In the exemplary embodiments described above, the thermostat  22  that utilizes expansion of a wax is employed, but this not limiting. A thermostat that utilizes a bimetallic strip in which metal plates of two kinds with different thermal expansion coefficients are stuck together. In this case, when the bimetallic element disposed at the heat-sensing portion is heated, the bimetallic element bends, pushing out the shaft and causing the driving cam to swing. 
         [0076]    The shape and size of the thermostat  22  may be freely modified, and a number of the thermostat  22  is not particularly limited.