Patent Publication Number: US-2022220883-A1

Title: Exhaust device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The disclosure of Japanese Patent Application No. 2021-002010 filed on Jan. 8, 2021, including specification, drawings and claims is incorporated herein by reference in its entirety. 
     BACKGROUND 
     The present invention relates to an exhaust device. 
     In the related art, as an exhaust device of a straddle-type vehicle, a device in which a primary catalyst case is disposed in front of an engine and a secondary catalyst case is disposed on a lower side of the engine is known (for example, see Patent Literature 1). A starter catalyst is accommodated in the primary catalyst case, and a main catalyst is accommodated in the secondary catalyst case. Exhaust gas is sent from a pair of exhaust ports to the primary catalyst case through an exhaust pipe on an upstream side, and the exhaust gas is sent from the primary catalyst case to the secondary catalyst case through an exhaust pipe on a downstream side. The primary catalyst case is brought close to the exhaust port and activated at an early stage, and the secondary catalyst case is activated at an early stage by the exhaust gas heated by a catalyst reaction.
     Patent Literature 1: JP-A-2020-041520   

     SUMMARY 
     According to one advantageous aspect of the present invention, there is provided an exhaust device configured to guide exhaust gas from an exhaust pipe in front of an engine to a muffler in a rear of the engine, the exhaust device including:
         a primary catalyst case accommodating a primary catalyst configured to purify the exhaust gas at a downstream side from the exhaust pipe;   a secondary catalyst case accommodating a secondary catalyst configured to purify the exhaust gas at a downstream side from the primary catalyst; and   a chamber formed with a muffling chamber configured to reduce an exhaust noise at a downstream side from the secondary catalyst, wherein   the primary catalyst case is disposed in a front space of the engine,   the secondary catalyst case is disposed on a front part of a lower space of the engine, and the chamber is disposed so as to occupy at least a rear part of the lower space of the engine.       

    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a right side view of a straddle-type vehicle according to a present embodiment. 
         FIG. 2  is a front view of an engine according to the present embodiment. 
         FIG. 3  is a side view of the engine according to the present embodiment. 
         FIG. 4  is a bottom view of the engine according to the present embodiment. 
         FIGS. 5A and 5B  are cross-sectional views of a bent pipe according to the present embodiment. 
         FIG. 6  is a cross-sectional view of a secondary catalyst case according to the present embodiment. 
         FIG. 7  is a cross-sectional view of a chamber according to the present embodiment. 
         FIG. 8  is a bottom view of an engine according to a modification. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLIFIED EMBODIMENTS 
     However, in the exhaust device disclosed in Patent Literature 1, the lower side of the engine is occupied by the secondary catalyst case, and it is difficult to secure a muffling chamber with a sufficient volume on an upstream side of a muffler. Therefore, a volume of the muffling chamber in the muffler is increased, and a size of the muffler is increased. 
     The present invention has been made in view of this point, and an object of the present invention is to provide an exhaust device capable of securing a muffling chamber on a lower side of an engine and reducing a size of a muffler. 
     An exhaust device according to one aspect of the present invention guides exhaust gas from an exhaust pipe in front of an engine to a muffler in a rear of the engine. A primary catalyst case is provided downstream of the exhaust pipe, and a primary catalyst that purifies the exhaust gas downstream of the exhaust pipe is accommodated in the primary catalyst case. A secondary catalyst case is provided downstream of the primary catalyst case, and a secondary catalyst that purifies the exhaust gas downstream of the primary catalyst is accommodated in the secondary catalyst case. A chamber is provided downstream of the secondary catalyst case, and a muffling chamber that reduces an exhaust noise is formed in the chamber. The primary catalyst case is disposed in a front space of the engine, the secondary catalyst case is disposed on a front part of a lower space of the engine, and the chamber is configured to occupy at least a rear part of the lower space of the engine. Since the primary catalyst case and the secondary catalyst case are compactly disposed in front of the engine, the lower space of the engine is not occupied by the secondary catalyst case. The chamber is disposed in a wide range of the lower space of the engine, and the exhaust noise is reduced by the muffling chamber of the chamber. By securing the muffling chamber on an upstream side of the muffler, it is possible to reduce a size of the muffler. 
     Hereinafter, a present embodiment will be described in detail with reference to the accompanying drawings.  FIG. 1  is a right side view of a straddle-type vehicle according to the present embodiment. In the following drawings, an arrow FR indicates a vehicle front, an arrow RE indicates a vehicle rear, an arrow L indicates a vehicle left side, and an arrow R indicates a vehicle right side. 
     As shown in  FIG. 1 , a straddle-type vehicle  1  is formed by mounting various components such as an engine  30  and an electrical system on a cradle type vehicle body frame  10 . The vehicle body frame  10  includes a main tube  12  that extends rearward from a head pipe  11  and then bends downward, and a down tube  13  that extends downward from the head pipe  11  and then bends rearward. A rear side of the engine  30  is supported by the main tube  12 , and a front side and a lower side of the engine  30  are supported by the down tube  13 . A fuel tank  17  is supported on the main tube  12 , and a rider seat  18  and a pillion seat  19  are provided in a rear of the fuel tank  17 . 
     A pair of front forks  21  are supported by the head pipe  11  via a steering shaft (not shown), and a front wheel  22  is rotatably supported on lower portions of the front forks  21  so as to be steered. A swing arm (not shown) is swingably supported at a rear half portion of the main tube  12 , and a rear wheel  23  is rotatably supported at a rear end of the swing arm. The engine  30  is connected to the rear wheel  23  via a transmission mechanism, and power from the engine  30  is transmitted to the rear wheel  23  via the transmission mechanism. An exhaust device  50  is connected to the engine  30 , and exhaust gas from the engine  30  is discharged to the outside through the exhaust device  50 . 
     A method of expanding a catalyst capacity of a catalyst case disposed on a lower side of the engine, a method of disposing the primary catalyst case in front of the engine and disposing the secondary catalyst case on the lower side of the engine, and the like are used in order to comply with exhaust gas regulations in recent years. In these methods, the lower side of the engine is used to expand the catalyst capacity, and it is difficult to secure a muffling chamber with a sufficient volume upstream of a muffler unless a basic structure of the vehicle body frame or the engine is changed. Therefore, the size of the muffler is increased, a degree of freedom in designing a muffler appearance is reduced, and an influence of heat damage to a rider and other components is increased. 
     Therefore, in the exhaust device  50  of the present embodiment, a front space of the engine  30  and a front part of a lower space of the engine  30  are effectively used, and a catalyst case is compactly disposed in front of the engine  30  in comparison with a general exhaust device. In the lower space of the engine  30 , a chamber  71  (see  FIG. 3 ) having only a muffling function without a catalyst is disposed, and a muffling chamber having a sufficient volume is secured on an upstream of a muffler  82  by the chamber  71 . Accordingly, it is possible to reduce a size of the muffler  82  by reducing the muffling chamber in the muffler  82  while minimizing changes to a basic structure of the vehicle body frame  10  or the engine  30 . 
     Hereinafter, the engine and the exhaust device will be described with reference to  FIGS. 2 to 4 .  FIG. 2  is a front view of the engine according to the present embodiment.  FIG. 3  is a side view of the engine according to the present embodiment.  FIG. 4  is a bottom view of the engine according to the present embodiment.  FIGS. 5A and 5B  are cross-sectional views of a bent pipe according to the present embodiment.  FIG. 5A  shows a state where the bent pipe of  FIG. 3  is cut along a line A-A, and  FIG. 5B  shows a state where the bent pipe of  FIG. 4  is cut along a line B-B. 
     As shown in  FIGS. 2 and 3 , the engine  30  is a parallel two-cylinder engine and is formed by assembling a cylinder block  32 , a cylinder head  33 , and a head cover  34  on a crankcase  31 . A drive component such as a crankshaft  35  is accommodated in the crankcase  31 , and the cylinder block  32  is attached to an upper portion of the crankcase  31 . A pair of aligned cylinder bores (not shown) disposed in a left-right direction are formed in the cylinder block  32 , and a piston (not shown) connected to the crankshaft  35  is disposed in each cylinder bore. The cylinder head  33  is attached to an upper portion of the cylinder block  32 . 
     A pair of intake ports (not shown) connected to the pair of cylinder bores are formed on a rear surface side of the cylinder head  33 , and a pair of exhaust ports  36 L and  36 R connected to the pair of cylinder bores are formed on a front surface side of the cylinder head  33 . The head cover  34  is attached to an upper portion of the cylinder head  33 , and a valve gear or the like is accommodated in the cylinder head  33  and the head cover  34 . An oil pan  37  that stores oil for lubrication and cooling is attached to a lower portion of the crankcase  31 . An oil filter  38  that removes a foreign matter from the oil is attached to a lower portion of a front surface of the crankcase  31 . 
     The engine  30  is assembled inside the vehicle body frame  10 . The down tube  13  of the vehicle body frame  10  includes an upper down tube  14  extending downward from the head pipe  11  (see  FIG. 1 ) in a center of the engine  30  in the left-right direction, and a pair of lower down tubes  15 L and  15 R branching left and right from a lower end of the upper down tube  14  and extending obliquely downward. The lower down tubes  15 L and  15 R are bent rearward on the lower side of the engine  30 , and are joined to the main tube  12  at rear end portions of the lower down tubes  15 L and  15 R. The oil filter  38  is positioned between the lower down tubes  15 L and  15 R. 
     A pair of exhaust pipes  51 L and  51 R extend from a front surface of the cylinder head  33  so as to avoid the upper down tube  14  and the lower down tubes  15 L and  15 R, and the exhaust device  50  that guides exhaust gas from the exhaust pipes  51 L and  51 R to a muffler  82  in a rear of the engine  30  is provided. The exhaust device  50  is provided with a small primary catalyst  54  that functions as a starter catalyst and a large secondary catalyst  62  that functions as a main catalyst. The exhaust gas enters the exhaust device  50  from the exhaust ports  36 L and  36 R, and the primary catalyst  54  and the secondary catalyst  62  purify air pollutants such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen compounds (NOx) in the exhaust gas. 
     In the exhaust device  50 , the exhaust pipes  51 L and  51 R, a collecting pipe  52 , a primary catalyst case  53 , a bent pipe  55 , a secondary catalyst case  61 , the chamber  71 , an exhaust pipe  81 , and the muffler  82  form an exhaust passage that wraps around from the front to the lower side of the engine  30  and extends to the rear. The exhaust pipe  51 L extends forward from the exhaust port  36 L and is connected to the collecting pipe  52 , and the exhaust pipe  51 R extends forward from the exhaust port  36 R and then extends to a left and is connected to the collecting pipe  52 . Thus, the exhaust pipe  51 L is shorter than the exhaust pipe  51 R, and pipe lengths of the exhaust pipes  51 L and  51 R are different. The exhaust pipes  51 L and  51 R have a circular cross section. 
     An upstream side of the collecting pipe  52  is bifurcated, and a downstream side of the collecting pipe  52  is formed in a cylindrical shape. The exhaust pipes  51 L and  51 R are connected to the upstream side of the collecting pipe  52 , and the primary catalyst case  53  is connected to a downstream end of the collecting pipe  52 . The exhaust gas that passes through the exhaust pipes  51 L and  51 R is collected by the collecting pipe  52  and sent to the primary catalyst case  53 . A first oxygen sensor  83  is disposed between the exhaust pipes  51 L and  51 R on a wall surface of the collecting pipe  52 , and an average oxygen concentration of the exhaust gas flowing in from the exhaust pipes  51 L and  51 R is detected by the first oxygen sensor  83 . A detection result of the first oxygen sensor  83  is used for feedback control of a fuel injection amount. 
     The primary catalyst case  53  is formed in a cylindrical shape, and is connected to the collecting pipe  52  in a substantially vertical posture. Since the exhaust pipe  51 L is shorter than the exhaust pipe  51 R, the primary catalyst case  53  and the collecting pipe  52  are disposed on a left side (one side in the left-right direction) of a center line C 1  of the engine  30  extending in an upper-lower direction. The primary catalyst  54  for purifying the exhaust gas that passes through the exhaust pipes  51 L and  51 R is accommodated in the primary catalyst case  53 . The primary catalyst  54  is formed by adhering a catalyst substance to a surface of a honeycomb-shaped or lattice-shaped partition plate, and when the exhaust gas flowing in from the collecting pipe  52  passes through the primary catalyst  54 , the air pollutant reacts with oxygen and is purified. 
     Since the pipe length of the exhaust pipe  51 L is short, high-temperature exhaust gas flows into the primary catalyst case  53  from the exhaust port  36 L, the primary catalyst  54  in the primary catalyst case  53  is warmed up in a short time, and a purification performance of the exhaust gas from the exhaust port  36 L is improved. In this case, the pipe length of the exhaust pipe  51 R is long and a temperature of the exhaust gas is likely to decrease, but the primary catalyst  54  is warmed up in a short time by the high-temperature exhaust gas from the exhaust pipe  51 L, and a purification performance of the exhaust gas from the exhaust port  36 R is also improved. Thus, an early activation of the primary catalyst  54  is implemented by intentionally adding a difference in the pipe lengths to the exhaust pipes  51 L and  51 R. 
     The bent pipe  55  is formed in an L-shaped tubular shape in which a vertical tubular portion  56  on an upstream side and a horizontal tubular portion  57  on a downstream side are connected. A cross-sectional shape of the bent pipe  55  gradually changes from a circular shape to an elliptical shape from an upstream end toward a downstream end (see  FIGS. 5A and 5B ). In this case, a major axis of an elliptical cross section of the downstream side of the bent pipe  55  is larger than a diameter of a circular cross section of the upstream end of the bent pipe  55 , and a minor axis of the elliptical cross section of the downstream side of the bent pipe  55  is smaller than the diameter of the circular cross section of the upstream end of the bent pipe  55 . At a bent portion at a boundary between the vertical tubular portion  56  and the horizontal tubular portion  57 , a major axis of the bent pipe  55  is oriented substantially horizontally, and a minor axis of the bent pipe  55  faces a bending radial direction of the bent pipe  55 . 
     The primary catalyst case  53  is connected to the upstream end of the vertical tubular portion  56  having a circular cross section, the secondary catalyst case  61  is connected to the downstream end of the horizontal tubular portion  57  having an elliptical cross section. The exhaust gas that passes through the primary catalyst  54  is guided to the secondary catalyst  62  on the lower side of the engine  30  by the bent pipe  55 . Compared with a configuration in which the primary catalyst case  53  and the secondary catalyst case  61  are connected by a plurality of exhaust pipes, a gap between the exhaust pipes is not required, and left and right spaces are not compressed. A second oxygen sensor  84  is disposed on a wall surface of the horizontal tubular portion  57 , and an oxygen concentration of the exhaust gas that passes through the primary catalyst case  53  is detected by the second oxygen sensor  84 . A detection result of the second oxygen sensor  84  is used for feedback control of the fuel injection amount and diagnosis of catalyst deterioration. 
     As shown in  FIGS. 3 and 4 , the secondary catalyst case  61  is formed in an elliptical cylindrical shape, and is connected to the bent pipe  55  in a substantially horizontal posture. At this time, the secondary catalyst case  61  extends obliquely rearward from the left side toward a right side (the one side to the other side in the left-right direction). The secondary catalyst  62  for purifying the exhaust gas that passes through the bent pipe  55  is accommodated in the secondary catalyst case  61 . The secondary catalyst  62  is formed by adhering the catalyst substance to a surface of a honeycomb-shaped or lattice-shaped partition plate, and when the exhaust gas flowing in from the bent pipe  55  passes through the secondary catalyst  62 , the air pollutant reacts with oxygen and is purified. 
     The chamber  71  is formed in an elliptical cylindrical shape, and is connected to the secondary catalyst case  61  in a substantially horizontal posture. A muffling chamber  72  (see  FIG. 7 ) for reducing the exhaust noise is formed in the chamber  71 . The chamber  71  extends in a front-rear direction, and an upstream end of the chamber  71  is joined to an outer wall surface of the secondary catalyst case  61  so as to cover the outer wall surface of the secondary catalyst case  61 . The chamber  71  is disposed on a right side (the other side in the left-right direction) of a center line C 2  of the engine  30  extending in the front-rear direction. A tapered pipe  73  connected to a downstream end of the secondary catalyst case  61  and a punching pipe  74  connected to a downstream end of the tapered pipe  73  are provided inside the chamber  71 . 
     A cross-sectional shape of the tapered pipe  73  gradually changes from an elliptical shape to a circular shape from an upstream end toward the downstream end. A large number of small holes are formed in a peripheral surface of the punching pipe  74 , and an inner side of the punching pipe  74  and the muffling chamber  72  are connected to each other through the large number of small holes. When the exhaust gas enters the muffling chamber  72  from the punching pipe  74 , the exhaust gas is expanded in the muffling chamber  72 , so that the exhaust noise is reduced. An outer wall of the chamber  71  has a double-cylinder structure, and a gap between an inner cylinder and an outer cylinder is filled with glass wool for sound absorption. The chamber  71  is supported by the vehicle body frame  10  via a bracket  75 . 
     The exhaust pipe  81  is formed in a cylindrical shape and extends rearward from a downstream end of the chamber  71 . The muffler  82  (see  FIG. 1 ) is positioned on a right side of the rear wheel  23 , and is connected to a downstream end of the exhaust pipe  81 . A muffling chamber (not shown) for reducing the exhaust noise is formed in the muffler  82 , and a rear end of the muffling chamber is connected to the outside through an exhaust port. Although a structure of the muffler  82  is not particularly limited, one muffling chamber may be formed, or a plurality of muffling chambers may be formed inside the muffler  82 . Thus, the exhaust noise is reduced in two stages by the chamber  71  and the muffler  82  in the exhaust device  50 . 
     An arrangement configuration of the catalyst case and the chamber will be described with reference to  FIGS. 2 to 7 .  FIG. 6  is a cross-sectional view of the secondary catalyst case according to the present embodiment, and shows a state where the secondary catalyst case of  FIG. 4  is cut along a line C-C.  FIG. 7  is a cross-sectional view of the chamber according to the present embodiment, and shows a state where the chamber of  FIG. 4  is cut along a line C-C. In  FIG. 6 , the primary catalyst is omitted. 
     As shown in  FIG. 2 , the lower down tubes  15 L and  15 R branching from the lower end of the upper down tube  14  extend obliquely downward. A branching point of the lower down tubes  15 L and  15 R are positioned below the exhaust ports  36 L and  36 R and above a height position O 1  at a center of the crankshaft  35 . Accordingly, a space is formed on left and right sides of an upper portion of the engine  30  avoiding the upper down tube  14 , and the exhaust pipes  51 L and  51 R are easily extended forward from the exhaust ports  36 L and  36 R. A space is formed between the lower down tubes  15 L and  15 R in a lower portion of the engine  30 , and the bent pipe  55 , the secondary catalyst case  61 , and the chamber  71  are easily disposed. 
     A V-shaped space is formed between the lower down tubes  15 L and  15 R on the front surface of the crankcase  31 . An oil filter  38  is provided in the V-shaped space, and the oil filter  38  is positioned on the center line C 1  of the engine  30  extending in the upper-lower direction. Since the oil filter  38  is positioned at a center of a vehicle body, an oil passage inside the engine  30  is formed simply. Since the exhaust device  50  passes through a left side and a lower side of the oil filter  38  avoiding a front of the oil filter  38 , an entry path for a tool to the oil filter  38  is secured. 
     The oil pan  37  is attached to a lower surface of the crankcase  31 . A bottom surface of the oil pan  37  is formed at a deep bottom on the left side (the one side in the left-right direction), and is inclined so that the bottom surface of the oil pan  37  becomes shallow from a deep bottom portion  41  toward the right side (the other side in the left-right direction) (see  FIG. 7 ). A right side of the bottom surface of the oil pan  37  is recessed in an arch shape, and the exhaust device  50  passes through a recess of the oil pan  37 . Thus, on the lower side of the engine  30 , a space for disposing the exhaust device  50  is formed on a right side of the deep bottom portion  41  of the oil pan  37 . A positional relationship between the exhaust device  50  and the oil pan  37  will be described in detail later. 
     As shown in  FIGS. 2 and 3 , the primary catalyst case  53  overlaps the lower down tube  15 L in a front view. An upstream end of the primary catalyst case  53  substantially coincides with the height position O 1  at the center of the crankshaft  35 . The bent pipe  55  is bent in an L shape from the downstream end of the primary catalyst case  53  to the right side, and a downstream end of the bent pipe  55  is positioned below the oil filter  38 . Since the bent pipe  55  extends obliquely to reduce an occupied area in the front and rear, the secondary catalyst case  61  connected to the downstream end of the bent pipe  55  is compactly disposed in the front. A rearmost portion of the downstream end of the secondary catalyst case  61  substantially coincides with a front-rear position O 2  at the center of the crankshaft  35 . 
     As shown in  FIG. 4 , the secondary catalyst case  61  is disposed between the lower down tubes  15 L and  15 R so as to overlap the oil pan  37 . The secondary catalyst case  61  extends obliquely rearward from the left side to the right side, and the secondary catalyst case  61  intersects the center line C 2  of the engine  30  extending in the front-rear direction. Since the secondary catalyst case  61  is obliquely disposed, an occupied area of the secondary catalyst case  61  in the front-rear direction can be reduced, and an occupied area of the chamber  71  can be widely secured in a rear of the secondary catalyst case  61 . The secondary catalyst  62  accommodated inside the secondary catalyst case  61  is also obliquely disposed similarly to the secondary catalyst case  61 . 
     The chamber  71  extends in the front-rear direction, and is disposed on the right side of the center line C 2  of the engine  30  extending in the front-rear direction. The upstream end of the chamber  71  is positioned in front of the front-rear position O 2  at the center of the crankshaft  35 , and the downstream end of the chamber  71  extends to a bridge  16  connecting the lower down tubes  15 L and  15 R. An entire length of the chamber  71  is set to a size that does not interfere with a center stand (not shown). The catalyst is not accommodated in the muffling chamber  72  in the chamber  71 , and the muffling chamber  72  is an expansion space of the exhaust gas. The chamber  71  functions as a primary muffler that assists a muffling function of the muffler  82 . 
     The upstream end of the chamber  71  is joined to the outer wall surface of the secondary catalyst case  61 , and the muffling chamber  72  in the chamber  71  is widely secured to improve the muffling performance. More specifically, the chamber  71  is continuously connected to the secondary catalyst case  61 , and a space around the tapered pipe  73  (outside in a radial direction) disposed inside the chamber  71  is also used as the muffling chamber  72 . The exhaust gas flows smoothly from the secondary catalyst case  61  toward the chamber  71  by directly connecting the secondary catalyst case  61  and the chamber  71  without interposing another member such as a connecting pipe between the secondary catalyst case  61  and the chamber  71 . 
     Thus, the primary catalyst case  53  is disposed in the front space of the engine  30 , the secondary catalyst case  61  is disposed on the front part of the lower space of the engine  30 , and the chamber  71  is disposed on the rear part of the lower space of the engine  30 . More specifically, most of the primary catalyst case  53  is disposed in front of the engine  30  below the center of the crankshaft  35 , and most of the secondary catalyst case  61  is disposed on a front side of the center of the crankshaft  35  (see  FIG. 3 ). Then, most of the chamber  71  is disposed on the lower side of the engine  30  and on a rear side of the center of the crankshaft  35 , and the muffling chamber  72  (see  FIG. 7 ) having a sufficient volume is secured on the rear part of the lower space of the engine  30  in the exhaust passage. 
     In the exhaust device  50 , a required volume of the muffling chamber is secured by the chamber  71  and the muffler  82 . Since the chamber  71  serves as the primary muffler, a volume of the muffler  82  downstream of the chamber  71  can be reduced. In a state where a muffling performance of the exhaust device  50  is maintained, a risk of heat damage is reduced by reducing a surface area of the muffler  82 , and a degree of freedom in designing of the muffler  82  is improved. In addition, the risk of the heat damage is reduced by intensively disposing the high-temperature catalyst in the vicinity of the engine  30 . Further, a weight balance is optimized by positioning a center of gravity of the secondary catalyst  62  on the front side of the center of the crankshaft  35 . 
     As shown in  FIG. 6 , the secondary catalyst case  61  is positioned inside an arch-shaped recess of the oil pan  37 . The secondary catalyst case  61  has an elliptical cross-sectional shape having a width larger than a height of the secondary catalyst case  61 . A major axis of the secondary catalyst case  61  is oriented in a substantially horizontal direction, and a minor axis of the secondary catalyst case  61  is oriented in a substantially vertical direction. An upper surface  63  of the secondary catalyst case  61  faces a bottom surface  42  of the oil pan  37  in a wide range, and heat dissipation from the upper surface  63  of the secondary catalyst case  61  is propagated to the bottom surface  42  of the oil pan  37 . In addition, a side surface  64  of the secondary catalyst case  61  faces an inclined portion  43  of the oil pan  37 , and heat dissipation from the side surface  64  of the secondary catalyst case  61  is propagated to the inclined portion  43  of the oil pan  37 . 
     As shown in  FIG. 7 , the chamber  71  is positioned inside the arch-shaped recess of the oil pan  37 . The chamber  71  has an elliptical cross-sectional shape having a width dimension larger than a height dimension. A major axis of the chamber  71  is oriented in a substantially horizontal direction, and a minor axis of the chamber  71  is oriented in a substantially vertical direction. An upper surface  76  of the chamber  71  faces the bottom surface  42  of the oil pan  37  in a wide range, and heat dissipation from the upper surface  76  of the chamber  71  is propagated to the bottom surface  42  of the oil pan  37 . In addition, a side surface  77  of the chamber  71  faces the inclined portion  43  of the oil pan  37 , and heat dissipation from the side surface  77  of the chamber  71  is propagated to the inclined portion  43  of the oil pan  37 . 
     In a front view, the secondary catalyst case  61  overlaps the deep bottom portion  41  of the oil pan  37  (see  FIG. 2 ). More specifically, the secondary catalyst case  61  crosses a front of the deep bottom portion  41  of the oil pan  37  (see  FIG. 4 ), and heat dissipation of the secondary catalyst case  61  is propagated to the oil pan  37  by running wind. Since the heat dissipation from the secondary catalyst case  61  and the chamber  71  is propagated to the oil pan  37 , the oil in the oil pan  37  is increased to a suitable temperature in a short time. Since the minor axes (heights) of the secondary catalyst case  61  and the chamber  71  are small, the engine  30  is low, vehicle body stability during traveling is improved, and capacities of a fuel tank and an air cleaner are easily secured. 
     Next, an arrangement configuration of the first and second oxygen sensors will be described with reference to  FIGS. 2 and 3 . 
     As shown in  FIGS. 2 and 3 , the first oxygen sensor  83  is disposed in the collecting pipe  52  in front of the engine  30 . The first oxygen sensor  83  is erected on the collecting pipe  52  in a state of facing the rear (the engine  30  side) between the exhaust pipes  51 L and  51 R. Since the first oxygen sensor  83  is sandwiched between the exhaust pipes  51 L and  51 R and the first oxygen sensor  83  is brought close to the engine  30 , early activation of the first oxygen sensor  83  is achieved by heat dissipation from the exhaust pipes  51 L and  51 R and the engine  30 . When the exhaust gas from the exhaust pipes  51 L and  51 R is substantially uniform at a detection end of the first oxygen sensor  83 , a detection accuracy of the oxygen concentration by the first oxygen sensor  83  is improved. 
     Since the first oxygen sensor  83  is disposed on a rear side of the collecting pipe  52 , the first oxygen sensor  83  is protected from a flying object from the front by the collecting pipe  52 . Since the bent pipe  55  is present below the first oxygen sensor  83 , the first oxygen sensor  83  is protected from a flying object from below by the bent pipe  55 . The upper down tube  14  is positioned on a right side of the first oxygen sensor  83 , and the first oxygen sensor  83  overlaps the upper down tube  14  in a side view. Accordingly, since wiring is laid along the upper down tube  14 , the wiring is easily connected to the first oxygen sensor  83 . 
     The second oxygen sensor  84  is disposed on the horizontal tubular portion  57  downstream of a bent portion of the bent pipe  55 . The second oxygen sensor  84  is erected on the horizontal tubular portion  57  in a state of facing upward (the engine  30  side). When the second oxygen sensor  84  is brought close to the engine  30 , early activation of the second oxygen sensor  84  is achieved by the heat dissipation from the engine  30 . In the front view, the second oxygen sensor  84  overlaps the vertical tubular portion  56  upstream of the bent portion of the bent pipe  55 . The second oxygen sensor  84  is protected from a flying object from below by the horizontal tubular portion  57 , and the second oxygen sensor  84  is protected from a flying object from above by the vertical tubular portion  56 . 
     The second oxygen sensor  84  is positioned between the lower down tubes  15 L and  15 R, and the second oxygen sensor  84  overlaps the lower down tubes  15 L and  15 R in the side view. The second oxygen sensor  84  is protected from a flying object from the left and right sides by the lower down tubes  15 L and  15 R. Since the second oxygen sensor  84  is closer to the left side and wiring is laid along the lower down tube  15 L, the wiring is easily connected to the second oxygen sensor  84 . Since the first and second oxygen sensors  83  and  84  are disposed on a left side of the engine  30 , a deviation of the first and second oxygen sensors  83  and  84  in the left-right direction is reduced, and the wiring is easily collected. 
     The oil filter  38  is positioned on a right side of the second oxygen sensor  84 , and the second oxygen sensor  84  overlaps the oil filter  38  in the side view. The second oxygen sensor  84  is protected from a flying object from the right by the oil filter  38 . The oil filter  38  protrudes toward the front from the front surface of the crankcase  31 , and the second oxygen sensor  84  is positioned in a rear of a front end of the oil filter  38 . Accordingly, the second oxygen sensor  84  protruding directly upward from the horizontal tubular portion  57  does not cross front space of the oil filter  38 , and the entry path for the tool to the oil filter  38  is secured. 
     As described above, according to the present embodiment, the primary catalyst case  53  is disposed in the front space of the engine  30 , and the secondary catalyst case  61  is disposed on the front part of the lower space of the engine  30 . Since the primary catalyst case  53  and the secondary catalyst case  61  are compactly disposed in front of the engine  30 , the lower space of the engine  30  is not occupied by the secondary catalyst case  61 . The chamber  71  is disposed in a wide range of the lower space of the engine  30 , and the exhaust noise is reduced by the muffling chamber  72  of the chamber  71 . By securing the muffling chamber  72  on an upstream side of the muffler  82 , it is possible to reduce the size of the muffler  82 . 
     In the present embodiment, the chamber is positioned on the right side of the deep bottom portion of the oil pan, but a shape of the chamber is not particularly limited. For example, as long as an attachment portion of the center stand can be relocated, as shown in a modification of  FIG. 8 , a rear portion of a chamber  90  protrudes to a left side in a rear side of the oil pan  37 , and the muffling chamber in the chamber  90  may be further widened. In this case, the muffler on a side of the rear wheel can be eliminated to centralize a mass, the reduction of the risk of the heat damage due to a surface of the muffler, and an improvement of the degree of freedom in designing the muffler appearance can be achieved. 
     In the present embodiment, most of the secondary catalyst case is disposed on the front side of the center of the crankshaft, but the secondary catalyst case may be disposed on the front part of the lower space of the engine. Accordingly, by disposing the chamber in the wide range of the lower space of the engine, it is possible to further improve the muffling performance. The front part of the lower space of the engine refers to a space on a front side of an intermediate position in the front-rear direction in the lower space of the engine. 
     In the present embodiment, most of the chamber is disposed on the rear side of the center of the crankshaft, but the chamber may occupy at least the rear part of the lower space of the engine. For example, the chamber may be disposed on the front side and the rear part of the lower space of the engine. The rear part of the lower space of the engine refers to a space on a rear side of the intermediate position in the front-rear direction in the lower space of the engine. 
     In the present embodiment, a press component may be used for a pipe of the exhaust device. The number of components and welding points can be reduced. 
     In the present embodiment, the left side of the bottom surface of the oil pan is formed at the deep bottom, but a part of the bottom surface of the oil pan may be formed at the deep bottom. For example, if the exhaust device does not interfere with the oil pan, the right side of the bottom surface of the oil pan may be formed at the deep bottom. 
     In the present embodiment, the upstream end of the chamber is joined to the outer wall surface of the secondary catalyst case, but the chamber and the secondary catalyst case may be separated from each other, and the chamber and the secondary catalyst case may be connected to via a connecting pipe. 
     In the present embodiment, the engine is the parallel two-cylinder engine, but a type of engine is not particularly limited, and for example, the engine may be a single-cylinder engine. 
     In the present embodiment, the cross-sectional shape of the secondary catalyst case and the chamber is formed in an elliptical shape, but the cross-sectional shape of the secondary catalyst case and the chamber may be formed in a cross-sectional shape having the width larger than the height. Depending on a shape of the frame, a shape of a bottom surface of the engine, and a minimum ground clearance, the cross-sectional shape of the secondary catalyst case and the chamber may be formed in a circular shape. 
     In the present embodiment, the secondary catalyst case crosses the front of the deep bottom portion of the oil pan, but a positional relationship between the secondary catalyst case and the deep bottom portion of the oil pan is not particularly limited. When the heat dissipation from the secondary catalyst case is propagated to the oil pan by the running wind, the secondary catalyst case may overlap the deep bottom portion of the oil pan in the front view. 
     In the present embodiment, the secondary catalyst case extends obliquely rearward from the left side to the right side, but the secondary catalyst case may extend in the front-rear direction. 
     In the present embodiment, the oxygen sensor is illustrated as a gas sensor, but the gas sensor may be any sensor that can detect an average characteristic of the exhaust gas, and may be, for example, an exhaust noise sensor that detects the exhaust noise of the exhaust gas. 
     The exhaust device of the present embodiment is not limited to the engine of the above straddle-type vehicle, but may be adopted for an engine of another type of straddle-type vehicle. The straddle-type vehicle is not limited to a motorcycle, and may be any vehicle on which an engine is mounted. The straddle-type vehicle is not limited to general vehicles on which a driver rides in a posture of straddling a seat, and includes a scooter-type vehicle on which the driver rides without straddling the seat. 
     As described above, the exhaust device ( 50 ) of the present embodiment is an exhaust device that guides the exhaust gas from the exhaust pipe ( 51 L,  51 R) in front of the engine ( 30 ) to the muffler ( 82 ) in the rear of the engine, and includes the primary catalyst case ( 53 ) in which the primary catalyst ( 54 ) that purifies the exhaust gas downstream of the exhaust pipe is accommodated; the secondary catalyst case ( 61 ) in which the secondary catalyst  62  that purifies the exhaust gas downstream of the primary catalyst is accommodated; and the chamber ( 71 ) in which the muffling chamber ( 72 ) that reduces the exhaust noise is formed is provided downstream of the secondary catalyst, in which the primary catalyst case is disposed in the front space of the engine, the secondary catalyst case is disposed on the front part of the lower space of the engine, and the chamber is configured to occupy at least the rear part of the lower space of the engine. According to this configuration, the primary catalyst case is disposed in the front space of the engine, and the secondary catalyst case is disposed on the front part of the lower space of the engine. Since the primary catalyst case and the secondary catalyst case are compactly disposed in front of the engine, the lower space of the engine is not occupied by the secondary catalyst case. The chamber is disposed in the wide range of the lower space of the engine, and the exhaust noise is reduced by the muffling chamber of the chamber. By securing the muffling chamber on the upstream side of the muffler, it is possible to reduce the size of the muffler. 
     In the exhaust device of the present embodiment, the front part of the lower space of the engine is on the lower side of the engine and on the front side of the center of the crankshaft ( 35 ), and the rear part of the lower space of the engine is on the lower side of the engine and on the rear side of the center of the crankshaft. According to this configuration, the chamber can be disposed in the wide range of the lower space of the engine by an amount that the secondary catalyst case is disposed on the front side of the center of the crankshaft. 
     In the exhaust device of the present embodiment, the secondary catalyst case extends obliquely rearward from the one side toward the other side in the left-right direction of the engine. According to this configuration, since the secondary catalyst case is obliquely disposed, the occupied area of the secondary catalyst case in the front-rear direction can be reduced, and the occupied area of the chamber can be widely secured in the rear of the secondary catalyst case. As compared with a case where the secondary catalyst case is oriented in the left-right direction of the engine, it is possible to prevent a pipe connected to the secondary catalyst case from protruding in the left-right direction of the engine. 
     In the exhaust device of the present embodiment, the engine is assembled to the vehicle body frame ( 10 ), the upper down tube ( 14 ) extends downward from the head pipe ( 11 ) of the vehicle body frame in the center of the engine in the left-right direction, and the pair of lower down tubes ( 15 L,  15 R) branching left and right from the lower end of the upper down tube extend downward, and the primary catalyst case overlaps one of the lower down tubes in the front view, and the secondary catalyst case is positioned between the pair of lower down tubes. According to this configuration, the space is formed on the left and right sides of the upper portion of the engine avoiding the upper down tube, and the space is formed between the pair of lower down tubes in the lower portion of the engine. Therefore, the secondary catalyst case and the chamber are easily disposed in the space between the pair of lower down tubes. 
     Although the present embodiment has been described, the above-described embodiment and modification may be combined entirely or partially as another embodiment. 
     The technique of the present disclosure is not limited to the above-described embodiment, and various changes, substitutions, and modifications may be made without departing from the spirit of the technical idea of the present disclosure. Further, the present invention may be implemented using other methods as long as the technical idea can be implemented by the methods through advance of the technology or other derivative technology. Accordingly, the claims cover all embodiments that may be included within the scope of the technical concepts.