Patent Publication Number: US-9885325-B2

Title: Air intake chamber for saddled vehicle

Description:
CROSS REFERENCE TO THE RELATED APPLICATION 
     This application is a continuation application, under 35 U.S.C § 111(a) of international application No. PCT/JP2014/051563, filed Jan. 24, 2014, which claims priority to Japanese patent application No. 2013-105484, filed May 17, 2013, the entire disclosure of which is herein incorporated by reference as a part of this application. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an air intake chamber in a saddle-riding type vehicle, in which an intake air having been pressurized by a supercharger is accumulated. 
     Description of Related Art 
     Hitherto in a saddle-riding type vehicle equipped with a supercharger, a surge tank or air intake chamber is disposed on an upstream side of a throttle body so that air pressurized by the supercharger can be introduced into the surge tank prior to a distributed supply of such pressurized air from the surge tank into a cylinder through the throttle body. A discharge port of the supercharger and an inlet port of the surge tank are fluidly connected with a tubular connecting pipe. In this respect, see the patent document 1 listed below. 
     PRIOR ART LITERATURE 
     Patent Document 1: JP Laid-open Patent Publication No. H02-006289 
     In the saddle-riding type vehicle of such a type as disclosed in the above mentioned patent document 1, demands have been arisen to increase the engine output. 
     SUMMARY OF THE INVENTION 
     The present invention has been devised in view of the foregoing problems and inconveniences and is intended to provide an air intake chamber for use in the saddle-riding type vehicle, which can enhance an engine output. 
     In order to accomplish the foregoing object of the present invention, an air intake chamber for use in a saddle-riding type vehicle in accordance with the present invention is disposed downstream of a discharge port of a supercharger, which is operable to pressurize and supply an intake air to an engine, and upstream of a throttle body to control the amount of intake air to be supplied to an air intake port of the engine. The air intake chamber referred to above includes a confronting portion having an outlet of the air intake chamber and facing the throttle body; and a connecting portion to connect the confronting portion and the discharge port of the supercharger. The connecting portion referred to above has an outlet side opening that is set to be greater than an inlet side opening. 
     The inventors of the present invention have found that if the capacity for accumulating the pressurized air at a location upstream of the throttle body is increased, the output of the combustion engine increases. However, it has also been found that if the widthwise dimension and the vertical dimension in the vicinity of the throttle body are increased, the dimension of the saddle-riding type vehicle itself increases or lowering of the degree of freedom in configurational design is accompanied. In view of this, the capacity of the air intake chamber is increased by the utilization of the connecting tube, which fluidly connects between the supercharger and the air intake chamber, as a part of the air intake chamber. According to this structural feature, an appropriate chamber shape can be maintained in the vicinity of a throttle body and, while the increase in size of the vehicle and the reduction in degree of freedom of designing are suppressed, the output of the combustion engine can be increased. 
     In a preferred embodiment of the present invention, the outlet side opening of the connecting portion may have a widthwise dimension that is set to the same dimension as a widthwise dimension of the confronting portion, in which case the inlet side opening of the connecting portion has a widthwise dimension that is set to the same dimension as a diameter of the discharge port of the supercharger. According to this structural feature, the capacity of the air intake chamber can be increased effectively and the output of the combustion engine can be increased. 
     In another preferred embodiment of the present invention, the connecting portion may have a transverse width that gradually increase from the inlet side opening towards the outlet side opening. According to this structural feature, since the flow velocity of the intake air decreases gradually, the turbulence of a stream of the intake air is suppressed, and, therefore, the air intake efficiency increases. 
     In a different preferred embodiment of the present invention, the discharge port of the supercharger may be disposed at a location separated rearwardly of the confronting portion. According to this structural feature, the capacity of the air intake chamber can be increased by increasing a forward and rearward direction dimension of the connecting portion. 
     In a further preferred embodiment of the present invention, where the air intake chamber and the supercharger are disposed juxtaposed relative to each other in the forward and rearward direction or longitudinal direction of the vehicle, an impeller shaft of the supercharger may be positioned above a rear portion of the crankcase. In this case, the discharge port of the supercharger is preferably positioned rearwardly of the impeller shaft. According to these structural features, the size of the connecting portion increased by increasing the longitudinal direction dimension between the discharge port of the supercharger and the confronting portion, and therefore, the capacity of the air intake chamber can be increased. 
     In a yet further preferred embodiment of the present invention, the air intake chamber may be disposed above a cylinder block of the engine, and the supercharger may be disposed rearwardly of the cylinder block of the engine and above the crankcase of the combustion engine, in which case an intake air duct for guiding an incoming wind, then flowing forwardly of the engine, towards the supercharger, is disposed to pass laterally of the cylinder block. According to this structural feature, with the interference between the air intake chamber and the air intake duct avoided, the capacity of the air intake chamber can be increased. 
     In a still further preferred embodiment of the present invention, the supercharger may be disposed rearwardly of a cylinder block of the engine, in which case the air intake chamber has a front wall protruding to be positioned forwardly of a throttle body mounting portion defined in the confronting portion. According to this structural feature, by a quantity corresponding to the front wall protruding forwards, the air intake chamber can be expanded in the longitudinal direction. 
     In the practice of the present invention, the dimension from an upstream end portion to a downstream end portion of the air intake chamber is preferably equal to or greater than three times an inner diameter of the outlet of the air intake chamber. According to this dimensional feature, the output of the combustion engine suitably increases. 
     According to a yet further preferred embodiment of the present invention, the engine has a plurality of cylinders and a plurality of outlets corresponding to the respective cylinders are juxtaposed in a transverse direction of the air intake chamber, in which case, there is provided a suppressing member to suppress a movement of an intake air from a region adjacent to one of the adjoining two outlets to the neighborhood of the other of the adjoining two outlets. According to this feature, since the use of the suppressing member is effective to suppress the movement of the intake air which is a pressurized air, the intake air can be uniformly supplied to the respective cylinders. 
     In a yet still preferred embodiment of the present invention, the use may be made of a rectifying member to avoid a bias occurring in the flow of an intake air within an interior space of the air intake chamber. According to the use of the rectifying member, the bias of the flow of the intake air within the interior space of the air intake chamber can be suppressed, and therefore, the intake air can be stably supplied to an air intake port. 
     Any combination of at least two constructions, disclosed in the appended claims and/or the specification and/or the accompanying drawings should be construed as included within the scope of the present invention. In particular, any combination of two or more of the appended claims should be equally construed as included within the scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In any event, the present invention will become more clearly understood from the following description of preferred embodiments thereof, when taken in conjunction with the accompanying drawings. However, the embodiments and the drawings are given only for the purpose of illustration and explanation, and are not to be taken as limiting the scope of the present invention in any way whatsoever, which scope is to be determined by the appended claims. In the accompanying drawings, like reference numerals are used to denote like parts throughout the several views, and: 
         FIG. 1  is a schematic side view showing a motorcycle which is a sort of saddle-riding type vehicle and which is equipped with an air intake chamber designed in accordance with a preferred embodiment of the present invention; 
         FIG. 2  is a perspective view of a combustion engine used in the motorcycle as viewed from rear in a diagonally downward direction; 
         FIG. 3  is a schematic side view showing an arrangement of the air intake chamber and a supercharger employed in the motorcycle; 
         FIG. 4  is a schematic rear view showing the arrangement of the air intake chamber and the supercharger; 
         FIG. 5  is a cross sectional view taken along the line V-V in  FIG. 4 ; 
         FIG. 6  is a schematic top plan view showing a chamber main body of the air intake chamber; 
         FIG. 7  is a schematic top plan view showing a holder for the air intake chamber; and 
         FIG. 8  is a perspective view showing the air intake chamber according to a different example of the present invention as viewed in a direction diagonally rightward and upwardly. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereinafter, a preferred embodiment of the present invention will be described with particular reference to the accompanying drawings. In describing the present invention, however, the terms “left and right” that are used hereinabove and hereinafter are to be understood as relative terms descriptive of positions and/or directions as viewed from a motorcycle rider occupying the seat during the forward travel of the motorcycle. 
       FIG. 1  is a left side view of a motorcycle, which is a sort of saddle-riding type vehicle and which is equipped with an air intake chamber designed in accordance with the preferred embodiment of the present invention. The illustrated motorcycle includes a vehicle body frame structure FR, which includes a main frame  1  forming a front half unit thereof and a rear frame  2  fitted to a rear portion of the main frame  1  and forming a rear half unit thereof. A head pipe  4  formed integrally with a front end of the main frame  1  has a front fork  8  rotatably supported thereby through a steering shaft (not shown), and a front wheel  10  is fitted to the front fork  8 . A handlebar  6  for steering purpose is fixed to an upper end portion of the front fork  8 . 
     On the other hand, a swingarm  12  is supported by a rear end portion of the main frame  1  for movement up and down about a pivot pin  16 , and a rear wheel  14  is supported by a rear end portion of this swingarm  12 . A combustion engine E, which forms a drive source of the motorcycle, is fitted to a lower portion of the main frame  1  and on a front side of the swingarm  12 , and is used to drive the rear wheel  14  through a power transmission mechanism  11  such as, for example, a substantially endless chain. The combustion engine E is in the form of a four cylinder, four cycle parallel multi-cylinder engine of a water cooled type, and a radiator  13  for an engine cooling liquid is disposed forwardly of the combustion engine E. The four cylinders is juxtaposed relative to each other in a widthwise direction of the combustion engine E, which is in coincide with a vehicle widthwise direction. In any event, it is to be noted that the type of the combustion engine E that can be utilized in the practice of the present invention is not necessarily limited to that shown and described. 
     A fuel tank  15  is disposed above the main frame  1 , and a rider&#39;s seat  18  and a fellow passenger&#39;s seat  20  are supported by the rear frame  2 . A fuel pump  17  for supplying fuel towards the combustion engine E is disposed in a lower area of a rear end of the interior of the fuel tank  15 . Also, a front fairing  22  made of a resinous material and used to cover a region forwardly of the head pipe  4  is mounted on a vehicle body front portion. The front fairing  22  is formed with an air intake opening  24  through which an intake air can be captured from the outside so as to supply it towards the combustion engine E. 
     The combustion engine E includes a crankshaft  26  extending in a vehicle widthwise direction, a crankcase  28  for accommodating therein the crankshaft  26  and a transmission, a cylinder block  30  protruding upwardly from an upper surface of a front portion of the crankcase  28 , a cylinder head  32  positioned above the cylinder block  30 , and an oil pan  34  provided below the crankcase  28 . The cylinder head  32  has a head covering  32   a . The cylinder block  30  and the cylinder head  32  are each tilted somewhat forwardly. More specifically, a piston axis line of the combustion engine E extends upwards in a forwardly tilted fashion. Four exhaust pipes  36  connected with an exhaust port at a front face of the cylinder head  32  are merged together at a location beneath the combustion engine E and is in turn fluidly connected with an exhaust muffler  38  that is disposed on a right side of the rear wheel  14 . 
     As shown in  FIG. 2 , on an upper surface of the crankcase  28  and rearwardly of the cylinder block  30 , an air cleaner  40  and a supercharger  42  are disposed juxtaposed relative to each other in a leftward and rightward direction (vehicle widthwise direction). The air cleaner  40  substantially purifies the outside air, and the supercharger  42  pressurizes and then supplies a substantially purified air from the air cleaner  40  to the combustion engine E. The supercharger  42  is of a centrifugal type supercharger that is driven by a driving power transmitted from the combustion engine E through a mechanical power transmitting system. 
     The supercharger  42  is disposed on a right side of the air cleaner  40  while adjoining thereto, and is fixed to the upper surface of the crankcase  28  by means of bolts (not shown). The supercharger  42  has a rotational axis  44  that extends in the vehicle widthwise direction above a rear portion of the crankcase  28 . The supercharger  42  has a leftwardly oriented suction port  46  defined therein and positioned above the crankcase  28  and at an intermediate portion of the combustion engine E with respect to the vehicle widthwise direction. The supercharger  42  also has a discharge port  48  defined therein at the intermediate portion of the combustion engine E with respect to the vehicle widthwise direction. More specifically, the discharge port  48  is positioned at a vehicle widthwise direction intermediate site of a cylinder side opening  68  (best shown in  FIG. 4 ) as will be discussed in detail later. As shown in  FIG. 3 , the discharge port  48  of the supercharger  42  is positioned rearwardly of the rotational axis  44 . 
     The supercharger  42  shown in  FIG. 2  includes an impeller  50  for pressurizing the intake air, an impeller housing  52  enclosing the impeller  50  therein, a transmission mechanism  54  forming a part of the power transmitting system for transmitting the power of the combustion engine E to the impeller  50 , and a transmission mechanism housing  56  enclosing the transmission mechanism  54  therein. The transmission mechanism  54  and the air cleaner  40  are disposed on respective sides of the impeller housing  52  with respect to the vehicle widthwise direction. The impeller housing  52  is connected with the transmission mechanism housing  56  and the air cleaner  40  by means of bolts (not shown). The transmission mechanism  54  is disposed to be displaced on one side of the vehicle widthwise direction with respect to a vehicle widthwise direction intermediate point. In the embodiment now under discussion, the transmission mechanism  54  is disposed having been displaced rightwards, and a chain  58 , that forms a part of the power transmitting system for the supercharger  42 , is disposed rightwardly of the combustion engine E 
     The air cleaner  40  referred to above has a cleaner outlet  62  fluidly connected with the suction port  46  of the supercharger  42  and also has a cleaner inlet  60  fluidly connected, from outside of the vehicle widthwise direction, with an air intake duct  70  for introducing the outside air into the supercharger  42 . The air intake duct  70  is disposed on a left side opposite to the right side where the chain  58  is disposed. Accordingly, it is possible to prevent the air intake duct  70  from protruding in the outside of the vehicle widthwise direction. As shown in  FIG. 1 , the air intake duct  70  passes (extends) a lateral region of the cylinder block  30 . 
     As shown in  FIG. 3 , the discharge port  48  of the supercharger  42  is oriented upwardly. An air intake chamber  74  is disposed intermediate between the discharge port  48  and an air intake port  47  of the combustion engine E with respect to a forward and rearward direction or longitudinal direction of the motorcycle. This air intake chamber  74  forms a part of an air passage extending from the discharge port  48  of the supercharger  42  to the cylinder head  32 . The discharge port  48  of the supercharger  42  and an inlet  77  of the air intake chamber  74  are directly fluidly connected with each other. The air intake port  47  is formed in a rear portion of the cylinder head  32 . The air intake chamber  74  is made of a metallic material, and in the practice of the embodiment now under discussion, the air intake chamber  74  is made of an aluminum alloy. Connection between the discharge port  48  of the supercharger  42  and the inlet  77  of the air intake chamber  74  is accomplished, for example, by the use of a rubber hose (not shown) having its opposite open ends mounted on respective outer peripheries of the discharge port  48  and the inlet  77  while corresponding open end faces of the discharge port  48  and the inlet  77  are butted together. 
     A throttle body  76  for controlling the amount of intake air to be supplied towards the air intake port  47  of the combustion engine E is disposed between the air intake chamber  74  and the cylinder head  32 . In other words, the air intake chamber  74  is disposed downstream of the supercharger  42  and upstream of the throttle body  76 , and is operable to accumulate the intake air which has been pressurized, that is, the pressurized intake air. As shown in  FIG. 5 , the throttle body  76  is provided with a throttle valve  105 . The throttle valve  105  is operable to adjust the amount of the intake air to be supplied to the air intake port  47 . This throttle valve  105  may be electrically controlled on the basis of various sensor values, or may be driven and operated manually, or may be provided with both of a valve manually operated and a valve electrically controlled. 
     The throttle valve  105  has a valve shaft  106   a  that extends in the vehicle widthwise direction and, when opening from a fully closed condition (the throttle valve  105  being in a horizontal condition), the throttle valve  105  is preferably rotated in a direction with a rear end of the throttle valve  105  moving downwardly, that is, in a clockwise direction as viewed in  FIG. 5 . Accordingly, when the throttle valve  105  is opened, an injection port  75   a  of an injector  75 , as will be discussed in detail later, and a rear end portion  105   b  of the throttle valve  105  separate away from each other. Therefore, deposition of fuel on the throttle valve  105  is suppressed. 
     In this throttle body  76 , the fuel is injected into the intake air from the injector  75  with an air and fuel mixture formed consequently, and this air and fuel mixture is then supplied from each air intake port  47  into a corresponding combustion chamber (not shown) within the cylinder bore of the combustion engine E. In addition to the injector  75  referred to above, a main injector  49  is provided on a downstream side of the throttle body  76  for adjusting the air and fuel mixing ratio. Accordingly, variation of the air and fuel mixing ratio can be suppressed. The throttle body  76  is so disposed as to incline upwardly in a direction rearwardly of the air intake port  47 . The injector  75  is provided for each cylinder and is mounted on an upper surface of the air intake chamber  74 . The injector  75  is preferably of a type capable of injecting the fuel in an atomized form and, more specifically, of a type capable of spray injecting the fuel in a radial direction with respect to an axis line IX. 
     The main injector  49  is disposed on one side of an axis (an axis line of the throttle body  76 ) AX of an outlet  73  of the air intake chamber  74  opposite to the injector  75 . In the illustrated embodiment, the injection port  75   a  of the injector  75  is disposed forwardly with respect to the axis line AX of the throttle body  76  as viewed from lateral side and an injection port  49   a  of the main injector  49  is disposed rearwardly with respect to the axis line AX of the throttle body  76  as viewed from lateral side. Accordingly, any undesirable bias in fuel distribution at locations forwardly and rearwardly with respect to the axis line AX of the throttle body  76  can be suppressed. More specifically, the axis line IX of the injector  75  is inclined relative to the axis line AX of the throttle body  76  in a rearward direction towards the air intake port  47  (below). An axis line MX of the main injector  49  is inclined relative to the axis line AX of the throttle body  76  in a forward direction towards the air intake port  47  (below). 
     As shown in  FIG. 2 , the injector  75  has a fuel tube  19  extending in a rearward direction, after having extended forwardly from a delivery tube  21  that extends above the injector  75  in a widthwise direction, and fluidly connected with the fuel pump  17 . The fuel tube  19  extends on a left side of the air intake chamber  74  opposite to a relief tube  83 , as will be described later, and then extends from a region forwardly of the air intake chamber  74  in the rearward direction. Accordingly, it is possible to avoid an undesirable interference between the fuel tube  19  and the relief tube  83 , resulting in increase of the assemblability. With the fuel tube  19  passing laterally of the air intake chamber  74 , an undesirable interference between the fuel tube  19  and a member above the air intake chamber  74 , for example, the fuel tank  15  can be avoided, and the degree of freedom in designing can increase. The details of the injector  75  will be described later. 
     The supercharger  42 , shown in  FIG. 3 , has a rear end disposed in the vicinity of a rear end of the crankcase  28 . The air intake chamber  74  is disposed at a location diagonally upwardly and forwardly of the supercharger  42  and diagonally upwardly and rearwardly of the cylinder head  32  and the throttle body  76 . The air cleaner  40  is disposed below the air intake chamber  74 . The fuel tank  15  shown in  FIG. 1  is disposed above the air intake chamber  74  and the throttle body  76 . The air intake chamber  74  and the injector  75  are overlapped one above the other in a lower portion of the fuel tank  15  when viewed from lateral side. 
     The air intake chamber  74  has an upper surface which, when viewed from lateral side, is inclined upwardly in a forward direction. The injector  75  is disposed at a location displaced from an uppermost portion of the upper surface of the air intake chamber  74  in the longitudinal direction. In the practice of the embodiment now under discussion, the injector  75  is disposed at a location displaced in the forward direction. Accordingly, it is possible to suppress the amount over which the injector  75  protrudes from the air intake chamber  74  in an upward direction. 
     A supercharged air passage  46 , through which the intake air having been pressurized by the supercharger  42  is supplied towards the combustion engine E, is formed by the air intake chamber  74 , shown in  FIG. 3 , and the throttle body  76 . A relief valve  80  for adjusting the air pressure inside the air intake chamber  74  is fluidly connected with a front portion of the air intake chamber  74  through a connecting tube  79  that is formed integrally with the air intake chamber  74 . As shown in  FIG. 2 , the relief tube  83 , forming a relief passage  82  through which the high pressure air A is supplied to the air cleaner  40 , is fluidly connected with the relief valve  80 . The relief tube  83  extends diagonally downwardly and rearwardly after having passed through a right lateral side of the air intake chamber  74 , and then extend on a left lateral side between the cylinder block  30  (or the cylinder head  32 ) and the supercharger  42  before the relief tube  83  is fluidly connected with the air cleaner  40 . 
     The air intake duct  70  shown in  FIG. 1  is disposed on a left lateral side, which is one lateral side of the combustion engine E, and is supported by the head pipe  4  while held in position with a front end opening  70   a  of the air intake duct  70  positioned adjacent to the air intake opening  24  in the front fairing  22 . The air intake duct  70  is operable to increase the pressure of the air, introduced from the front end opening  70   a , by the well known ram effect. The air intake duct  70  shown in  FIG. 2  has a rear end portion  70   b  with which the air cleaner  40  is fluidly connected. Thus, the air intake duct  70  introduces an incoming wind as the intake air to the supercharger  42  after having passed from forwardly of the combustion engine E and through the left lateral outside of the cylinder block  30  and the cylinder head  32 . 
     As shown in  FIG. 3 , the air intake chamber  74  includes a chamber main body  64 , forming a principal portion thereof, and a holder  66  having a joint portion fluidly connected with the throttle body  76 . The holder  66  is formed with the cylinder side opening  68  (best shown in  FIG. 4 ) on which a funnel  69  towards each cylinder of the combustion engine E is mounted. The funnel  69  has a hollow defined therein, which hollow defines the outlet  73  of the air intake chamber  74 . 
       FIG. 4  illustrates a rear view of the air intake chamber  74  and the supercharger  42  as viewed from rear of the vehicle body, with a portion of the chamber main body  64  of the air intake chamber  74  cut out. As shown in  FIG. 4 , the cylinder side opening  68  is made up of four cylinder openings which are equal in number to the number of the engine cylinders and which are juxtaposed in the vehicle widthwise direction (in the leftward and rightward direction). In this way, a downstream side of the air intake chamber  74  is so configured as to have a dimension that is substantially the same as the widthwise dimension of the cylinder head  32 . 
     The inlet  77  referred to previously is positioned at a location separated in a chamber longitudinal direction, which is in coincide with a vehicle longitudinal direction, from a site P between the neighboring cylinder side openings  68  and  68  at an intermediate portion in a chamber transverse direction. Accordingly, the intake air can be equally supplied to each cylinder side opening  68 . Also, the transverse widthwise dimension W 2  passing across the outlet  73  (best shown in  FIG. 3 ), that is, the cylinder side opening  68  is so chosen as to be greater than the transverse widthwise dimension W 1  passing across the inlet  77  in an interior space of the air intake chamber  74 . The inlet  77  and outlet  73  of the air intake chamber  74  are disposed having been separated from each other in the longitudinal direction and are formed at a substantially the same heightwise position. In other words, the air intake chamber  74  extends a large distance in the longitudinal direction as compared with that in the vertical direction. 
     Within the interior of the air intake chamber  74 , a suppressing member  88  in the form of a partition wall is formed, which wall extends from a position adjacent the inlet  77  to the site P so as to divide the interior space into two. The suppressing member  88  suppresses a movement of the pressurized air from an area adjacent one of the neighboring two cylinder side openings  68  and  68  to an area adjacent the other of the neighboring two cylinder side openings  68  and  68 . As shown in  FIG. 5 , the suppressing member  88  is provided in the holder  66  and has a substantially same height as an upper end (inlet end) of the funnel  69 . 
     It may happen that as a result of the time lag occurring in intake strokes of the cylinders, a bias may occur in pressure distribution even within the air intake chamber  74 . In the event that the intake stroke takes place sequentially in the neighboring cylinders, the suppressing member  88  is preferably disposed between respective two chamber outlets that are disposed in correspondence with the neighboring cylinders. Accordingly, it is possible to avoid the possibility that, when an intake air I within the air intake chamber  74  is sucked in the vicinity of the outlet  73  of the air intake chamber  74  corresponding to one cylinder, an intake air I in the vicinity of the outlet  73  corresponding to the other cylinder may be sucked into such one cylinder. As a result thereof, the variation of the intake air I sucked into the cylinders can be avoided. 
     Also, by way of example, the outlet  73  positioned on both sides in the vehicle widthwise direction is apt to exhibit a larger amount of intake air as compared with that in the outlet  73  positioned inwardly in the widthwise direction because of neighboring to a side wall. In view of this, the suppressing member  88  may be disposed intermediate between the outlet  73 , which is positioned inwardly in the vehicle widthwise direction, and the outlet  73  positioned on both sides in the vehicle widthwise direction. By disposing the suppressing member  88  capable of suppressing the flow of the intake air I moving between the outlets  73  in the widthwise direction, the bias or deviation of the intake air I among the cylinders can be suppressed. 
     In addition, so that the capacity or volume in the air intake chamber  74  and in the vicinity of the outlet  73  at which the amount of the intake air is small may become large, the suppressing member  88  may be so disposed as to differentiate the respective capacities in the vicinities of the outlets  73 . The suppressing member  88  may be disposed only in a confronting portion  84  or may be so disposed as to cover a region from the confronting portion  84  to a connecting portion  86 . 
     The suppressing member  88  employed in the practice of the present invention is formed by the sheet material, and is disposed in a form crossing in the chamber widthwise direction and extends in the chamber longitudinal direction and the chamber vertical direction. The suppressing member  88  may be formed with a transit area through which the intake air I passes across the suppressing member  88  in the widthwise direction. In the practice of the embodiment now under discussion, the suppressing member  88  is disposed spaced a distance in the longitudinal direction between a front wall and a rear wall of the air intake chamber  74 , and this distance of separation forms the transit area referred to above. Besides, the suppressing member  88  may be formed with a throughhole that permits a predetermined amount of the intake air I to move in the widthwise direction. With the suppressing member  88  disposed at a location except for the vicinity of the inlet  77  of the air intake chamber  74 , the intake air I flowing from the inlet  77  into the interior of the air intake chamber  74  can be distributed towards the cylinders. It is, however, to be noted that the use of the suppressing member  88  may be dispensed with. 
     The air intake chamber  74  includes the confronting portion  84 , which defines an interior space S 1  of a portion confronting and overlaps the throttle body  76  in a direction along an axis AX of the outlet of the air intake chamber  74 , and the connecting portion, which define an interior space S 2  of a portion connecting the interior space S 1  and the inlet  77 . More specifically, the confronting portion  84  is disposed above the throttle body  76  and the connecting portion  86  is disposed rearwardly of the confronting portion  84 . The confronting portion  84  has an inlet side opening which is of the same open area as that of an outlet side opening of the connecting portion  86 , and they are serially formed continuously in the direction of flow of the intake air I. The inlet side opening of the confronting portion  84  is disposed in a rear end of the confronting portion  84 , and the confronting portion  84  extends in the forward and rearward direction in the same dimension as that of the inlet side opening thereof. 
     In the practice of the embodiment now under discussion, a front wall  74   a  of the air intake chamber  74  is protruding to be disposed forwardly of the throttle body  76  and this protruding portion thereof is also included in the confronting portion  84 . In other words, the term “confronting portion” employed hereinabove and hereinafter in connection with the embodiment now under discussion is to be understood as meaning a portion that defines the interior space S 1  in that portion, which confronts the throttle body  76  when viewed from the direction of the axis X of the outlet of the air intake chamber  74 , and an interior space on a downstream side (left side as viewed in  FIG. 5 ) thereof. More specifically, the confronting portion  84  is positioned adjacent to the throttle body  76 , has a plurality of outlets of the air intake chamber  74  and also has a function of distributing the intake air I, which has been accumulated, to each cylinder. 
     The connecting portion  86  referred to above is positioned intermediate between the confronting portion  84  and the discharge port  48  of the supercharger  42 . The connecting portion  86  has the inlet  77  of the air intake chamber  74  and also has a function of guiding the intake air I from the discharge port  48  of the supercharger  42  to the confronting portion  84 . The connecting portion  86  has an upper surface inclined downwardly in a rearward direction. Accordingly, the upper surface of the connecting portion  86  can be easily adapted to the shape of the posture of a rider when such rider gets on the automotive vehicle. Also, since the intake air I passes smoothly through the connecting portion  86  along the upper wall, the output of the combustion engine E increases. Also, the passage surface area of the inlet  77 , which is the inlet side opening of the connecting portion  86 , is so chosen as to be smaller than that of a communicating opening  81  with the confronting portion  84  which is the outlet side opening. 
     More specifically, as shown in  FIG. 2 , the outlet side opening of the connecting portion  86  has a widthwise dimension W 3  which is so chosen as to be the same as the widthwise dimension W 2  of the air intake chamber  74 , and the widthwise dimension W 1  of the inlet side opening of the connecting portion  86  is so chosen as to be the same as the diameter of the discharge port  48  of the supercharger  42 . The widthwise dimension W 3  of the outlet side opening of the connecting portion  86  may be so chosen as to be greater than the widthwise dimension of the cylinder head  32 . Also, the connecting portion  86  is so constructed as to represent a sector shape with its transverse width continuously increasing from the inlet to the outlet. The shape of the connecting portion  86  may, however, not be limited to the sector shape but may be, for example, a triangular shape or a trapezoidal shape. 
     In the practice of the embodiment now under discussion, the outlet side opening of the connecting portion  86  is so formed as to have a transverse dimension and a dimension (longitudinal dimension) perpendicular to the widthwise direction, both which are greater than those of the inlet side opening of the connecting portion  86 . More specifically, the outlet side opening of the connecting portion  86  is so shaped as to represents a transversely elongated shape in the widthwise direction, and the inlet side opening of the connecting portion  86  is so shaped as to represent a round shape. Both of the widthwise dimension and the longitudinal dimension of the outlet side opening of the connecting portion  86  are so formed as to be greater than the diameter of the inlet side opening of the connecting portion  86 . 
     As shown in  FIG. 5 , within the interior of the air intake chamber  74 , there is provided a rectifying member  90  operable to avoid a bias of the flow of the pressurized air within the interior space of the air intake chamber  74 . In the practice of the embodiment now under discussion, the rectifying member  90  is in the form of a punched metal sheet having a multiplicity of throughholes (a flow passage resistance portion) defined therein and extending over the entire width of the interior space of the air intake chamber  74 . It is, however, to be noted that the rectifying member  90  may not be necessarily limited to the punched metal sheet. 
     The use of the rectifying member  90  is effective to suppress the bias in the amount of the intake air I that is guided towards the outlet  73  even when the distance between the inlet  77  and outlet  73  of the air intake chamber  74  is small. This rectifying member  90  is disposed intermediate between the inlet  77  and the outlet  73  so as to traverse the intake air passage. By disposing a plurality of flow passage resistance portions in a fashion spaced in the widthwise direction, the bias of the flow in the widthwise direction can be suppressed. 
     In the practice of the embodiment now under discussion, the rectifying member  90  is so disposed as to incline downwardly in the chamber rearward direction, and therefore, the bias of the intake air pressure at a region which confronts from above with the outlet  73  can be suppressed. It is, however, to be noted that by applying a bias or variation to the shape of the flow passage resistance portion of the rectifying member  90  in the vehicle widthwise direction, the intake air pressure distribution at the outlet  73  can be biased to thereby suppress variation of the intake air amount for each cylinder. More specifically, it is to be noted that arrangement may be made to increase the pressure in the vicinity of the outlet  73  corresponding to the cylinder which otherwise tends to generate low output. It is, however, to be noted that the use of the rectifying member  90  may not be essential in the practice of the present invention and may therefore be dispensed with. 
     The dimension L from the inlet  77  to the outlet  73  within the interior space of the air intake chamber  74  shown in  FIG. 3  is chosen to be three times the inner diameter D of the outlet  73 . This dimension L is a dimension in a direction perpendicular to the axis AX of the outlet  73 . 
       FIG. 6  illustrates a schematic top plan view of the chamber main body  64  of the air intake chamber  74 , and  FIG. 7  illustrates a schematic top plan view of the holder  66  in the air intake chamber  74 . By the chamber main body  64  and the holder  66 , the confronting portion  84  and the connecting portion  86  are integrally molded. The chamber main body  64  shown in  FIG. 6  includes a flange portion  92  formed in a peripheral edge of an opening  91  open downwardly. A plurality of insertion holes  92   a  oriented in the vertical direction are formed in this flange portion  92 . 
     The chamber main body  64  has a rear end formed integrally with an inlet tube  94  which forms the inlet  77  of the air intake chamber  74 . An upper surface of a front portion of the chamber main body  64 , which is an outer surface of the air intake chamber  74 , is formed with an inclined portion  93  (See  FIG. 5 ) that is inclined downwardly towards forwards, and also with a recessed mounting area  95  (See  FIG. 5 ) that extends from a front end of the inclined portion  93  to a front end of the upper surface of the chamber main body  64 . As shown in  FIG. 5 , the recessed mounting area  95  is downwardly depressed as compared with the upper surface of the front portion of the chamber main body  64 . 
     As shown in  FIG. 6 , the recessed mounting area  95  is formed with a plurality of, for example, four injector seating faces  97  that are juxtaposed side by side in the widthwise direction and are employed in correspondence with the number of the cylinders of the combustion engine E. The injector  75  is disposed on each injector seating face  97 . With the injector  75  employed for each cylinder, a variation in air and fuel mixing ratio can be suppressed, and also the amount of fuel that is simultaneously injected can be increased. The injector seating face  97  is further downwardly depressed from the recessed mounting area  95 . Accordingly, with the injector seating face  97  disposed downwardly of the upper surface of the front portion of the chamber main body  64 , it is possible to suppress the amount of protrusion of the injector  75  in an upward direction while the capacity of the air intake chamber  74  is secured. 
     Each injector seating face  97  is formed with an injector mounting hole  96  in which the corresponding injector  75  is mounted. In the vicinity of each injector seating face  97  in the recessed mounting area  95 , a fuel tube mounting hole  99  in the form of a threaded hole that is oriented upwards is formed. 
     The holder  66  shown in  FIG. 7  includes a flanged portion  98  formed in a peripheral edge of an upwardly oriented opening  101 . At a position of the flanged portion  98  corresponding to the insertion hole  92   a  ( FIG. 6 ) in the chamber main body  64 , a treaded hole  98   a  that is oriented in the vertical direction is formed. 
     The two connecting tubes  79  formed in the front end of the holder  66  are communicated with the interior space of the air intake chamber  74  through respective throughholes  100  defined in the front wall of the holder  66 . In order to increase the interior space around the outlet  73  of the air intake chamber  74 , a vehicle widthwise direction intermediate portion of the front portion of the air intake chamber  74  may be swelled forwards as compared with a vehicle widthwise direction outer side portion. By so doing, an undesirable reduction of the intake air pressure in the vicinity of the outlet  73  can be suppressed. In the practice of the embodiment now under discussion, a space formed by the two connecting tubes  79  are utilized to increase the interior space on a vehicle widthwise direction inner side around the outlet  73 . 
     The four cylinder side opening  68  that are lined in the widthwise direction are formed in intermediate portion of the holder  66  with respect to the forward and rearward direction. On front and rear sides of each cylinder side opening  68 , a funnel mounting holes  102  in the form of a threaded hole oriented in the vertical direction are formed. Also, on both sides of each cylinder side opening  68 , bolt insertion holes  104  in the form of a throughhole oriented in the vertical direction are formed. 
     The details of the injector  75  will be discussed. As shown in  FIG. 3 , the injector  75  is disposed with the axis IX thereof inclined relative to the axis AX of the output of the air intake chamber  74 . More specifically, the axis Ax is disposed having been inclined somewhat rearwardly from the vertical direction, and the axis IX of the injector  75  is disposed having been inclined so as to extend in a rearwardly downwardly. Also, the fuel injection port  75   a  of the injector  75  is provided at a position displaced from the axis AX of the outlet of the air intake chamber  74 . 
     The injector  75  has an upper end positioned at a location substantially the same height of an upper end of the air intake chamber  74 . The upper end of the injector  75  is preferably disposed so as to assume a position lower than the upper end of the air intake chamber  74 . The front surface of the air intake chamber  74  is positioned forwardly of the front surface of the injector  75 . 
     Assemblage and mounting of the air intake chamber  74  will now be described. At the outset, the holder  66  is fixed to the throttle body  76 . More specifically, in a condition that the cylinder side opening  68  in the holder  66  shown in  FIG. 7  is aligned with the inlet of the throttle body  76 , the bolt (not shown) is inserted from above into the bolt insertion hole  104 . Subsequently, the bolt is threaded into a threaded hole (not shown) provided in the throttle body  76 , shown in  FIG. 5 , to thereby fix the holder  66  to the throttle body  76 . 
     Thereafter, the funnel  69  is fitted to the holder  66 . More specifically, from the inner side of the holder shown in  FIG. 7 , the funnel  69  is fitted to the cylinder side opening  68 , and a bolt (not shown) is threaded into the funnel mounting hole  102 . 
     Following the mounting of the funnel  69  over the cylinder side opening  68 , the relief valve  80  shown in  FIG. 2  is fitted to the connecting tube  79  of the holder  66 . More specifically, the relief valve  80  is mounted in an outer peripheral to surface of the connecting tube  79  and, with the use of a pressing means such as, for example, a rubber tube, a mounting portion is urged against an outer peripheral surface of the relief valve  80 . Thereby, the relief valve  80  is fitted to the connecting tube  79 . 
     Furthermore, the injector  75  and the delivery tube  21  are fitted to the chamber main body  64 . At the outset, the four injectors  75  have been fitted to the delivery tube  21 . Starting from this condition, each of the injector  75  is mounted in the corresponding injector mounting hole  96  and, with the use of a bolt (not shown), the delivery tube  21  (shown in  FIG. 4 ) is fitted from above to the fuel tube mounting hole  99 . 
     Further, the holder  66  shown in  FIG. 3  and the chamber main body  64  are connected together. At the outset, as hereinabove described, the discharge port  48  of the supercharger  42  and the inlet  77  of the air intake chamber  74  provided in the chamber main body  64  are connected together, and the front portion of the chamber main body  64  is fixed to the supercharger  42 . Thereafter, the holder  66  and the chamber main body  64  are threadingly connected together. At the last, the delivery tube  21  shown in  FIG. 2  and the fuel tube  19  are connected together. 
     In the following description, the operation of the air intake system for the combustion engine E will be discussed. When the motorcycle starts running, the incoming wind is introduced as the intake air I from the air intake opening  24  into the air intake duct  70 . The intake air I flows rearwardly within the air intake duct  70  and is guided towards the air cleaner  40 , shown in  FIG. 2 , after having been deflected towards a vehicle widthwise direction inner side. 
     The intake air I so guided into the air cleaner  40  is, after having been substantially purified by the air cleaner  40 , introduced into the supercharger  42 . The intake air I so introduced into the supercharger  42  is, after the pressure thereof has been increased by the impeller  50 , discharged from the discharge port  48 . The high pressure intake air I so discharged from the supercharger  42  flows, as shown in  FIG. 4 , towards the cylinder side opening  68  while having been expanded within the air intake chamber  74 . During the flow towards the cylinder side opening  68 , the temperature of the intake air I is lowered. 
     The intake air I flows towards the cylinder side opening  68  after the bias in flow thereof has been suppressed by the rectifying member  90  shown in  FIG. 5 . The fuel F is injected from the injector  75  to the intake air I having passed through the rectifying member  90 . The intake air I with the fuel F, to which the fuel F has been injected, has its temperature further lowered by heat of evaporation of the fuel F, and is then supplied from the funnel  69  to the air intake port  47  (shown in  FIG. 1 ) of the combustion engine E by way of the throttle body  76 . The injector  75 , since the fuel is injected in an atomized form, is effective to achieve a temperature reduction over a wide range by the effect of the heat of evaporation. In the practice of the embodiment now under discussion, at the timing at which the intake air I is introduced from the supercharger  42  to the air intake chamber  74 , the temperature of the intake air I, which has been about 150° C., is lowered down to about 130° C. during the passage through the air intake chamber  74 . 
     Also, when the pressure inside the supercharged air passage on the downstream side of the supercharger  42  becomes higher than a predetermined value, the relief valve  80  (shown in  FIG. 2 ) provided in the air intake chamber  74  is opened to thereby adjust the pressure inside the supercharged air passage including the air intake chamber  74 . As shown in  FIG. 2 , a high pressure air A vented from the relief valve  80  is introduced into the air cleaner  40  after having flows through the relief tube  83  forming the relief passage  82 . 
     As described above, the use of the air intake chamber structure designed in accordance with the preferred embodiment is effective to provide a required output through the reduction of the intake air temperature, even though any cooling means for cooling the intake air I, that is, a so-called intercooler is not employed. As a result thereof, in a quantity corresponding to the intercooler having been dispensed with, the structure can be simplified to reduce the cost of manufacture. 
     The capacity V of the interior space of the air intake chamber  74  will now be discussed. The wording “capacity of the interior space of the air intake chamber” referred to hereinabove and hereinafter means the volume defined by the chamber in its entirety when the outlet  73  of the air intake chamber  74  is traversed or cut out by the plane of extension of the inner surface of the air intake chamber  74  or the cleaner inlet  60 . Where the inlet  77  of the air intake chamber  74  is connected directly with the discharge port  48  of the supercharger  42  as is the case with the embodiment now under discussion, the capacity of the air intake chamber  74  may be a capacity from the discharge port  48  of the supercharger  42  to the inlet of the throttle body  76 . The capacity V of the interior space of the air intake chamber  74  is equal to or greater than twice the displacement X of the combustion engine E. If the capacity V of the air intake chamber  74  is not greater than twice the displacement, there is the possibility that the intake air temperature cannot be sufficiently lowered. 
     The capacity V of the interior space referred to above is preferably within three to eight times the displacement of the combustion engine. In the practice of the embodiment now under discussion, the displacement of the combustion engine E is 1,000 cc and the capacity V of the air intake chamber  74  is 6,000 cc. If the capacity V of the air intake chamber  74  is of a value that is three or more times the displacement, the intake air temperature can be further lowered. Also, if the capacity V of the air intake chamber  74  is chosen to be of a value greater than eight times the displacement, a time lag required to reach from a non-pressurized condition to a pressurized condition is rendered to be so large and the case may occur that the output response relative to a rider&#39;s manipulation will be lowered. 
     In other words, when the number of the cylinders in the combustion engine is of a value expressed by n and the displacement of the combustion engine is of a value expressed by X, the capacity V of the interior space of the chamber is preferably set to (V/n)≧0.5X. The capacity V of the interior space of the chamber is set to (V/n)≧X, more preferably. 
     More specifically, as shown in  FIG. 7 , the interior space of the air intake chamber  74  is divided into four regions R 1  to R 4  which are obtained by dividing by the plane PL that contains the geometric center of each neighboring cylinder side opening  68 , that is, the intermediate point P 1  between the neighboring outlets  73  and lies perpendicular to the direction in which the cylinder side openings  68  are laid (in the widthwise direction) and parallel to the axis AX of the cylinder side opening  68 . The capacity V of the air intake chamber  74  is so defined that the smallest value of one of volumes V 1  to V 4  of respective interior spaces of the four divided regions R 1  to R 4  may become larger than half the displacement X of the combustion engine. In the practice of the embodiment now under discussion, the smallest value of the capacity V 1  of the interior space of the divided region R 1  is set to be larger than half the displacement X of the combustion engine. The four divided regions R 1  to R 4  are lined up in the sequence of the divided regions R 1  to R 4  from left to right. The volume V of the interior space of the air intake chamber  74  is preferably defined so that the minimum value V 1  may become greater than the displacement X. 
     Since the outlet  73  of the air intake chamber  74  is provided for each cylinders, in total the passage area of the outlet  73  is rendered to be large. Therefore, the passage resistance during the suction from the cylinder is reduced and, correspondingly, the need arises to increase the capacity V of the air intake chamber  74 . In view of this, the capacity V of the air intake chamber  74  is preferably of a value greater than 8 times the displacement for each cylinder. Also, the capacity V of the air intake chamber  74  is more preferably of a value greater than 12 times the cylinder capacity for each cylinder but smaller than 32 times the cylinder capacity for each cylinder. 
     The discharge rate of the supercharger per unit time is so set as to be larger than the displacement of the combustion engine per unit time. Accordingly, the air of a pressure higher than 1 atmospheric pressure can be led to the cylinder. More specifically, at the standard state (SATP) of a gas, where the discharge rate of the supercharger  42  exhibited upon one complete rotation of the impeller  50  of the supercharger  42 , when gas in the doorway of the supercharger is at the standard state, is expressed by α liters, the speed increasing ratio of the supercharger  42  during one complete rotation of the crankshaft is expressed by β and the displacement of the combustion engine is expressed by γ, the discharge rate is set to (2×α×(3)&gt;γ. By adjusting the discharge rate of the supercharger  42  and the air-intake of the combustion engine, the interior of the air intake chamber  74  is so set as to attain a pressure not higher than a predetermined pressure. In the practice of the embodiment under discussion, with the incoming wind led to the supercharger, the pressure on the inlet side can be increased and the discharge rate of the supercharger can be further increased. 
     The pressure inside the air intake chamber  74  is preferably set to a value equal to or higher than 1.5 bar and, more preferably, to a value equal to or higher than 2 bar. In the practice of the embodiment now under discussion, the pressure inside the air intake chamber  74  is set to 2.5 bar. With the pressure of the interior of the air intake chamber  74  set to the pressure higher than the atmospheric pressure, the charging volume in the cylinder is increased and the engine output increased accordingly. Where the interior of the air intake chamber  74  is set to a predetermined pressure Pa (bar), it is preferred to set the capacity V of the air intake chamber  74  to be V (≧(Pa/Pb)×Q) that is a value equal to or higher than the product of the displacement Q multiplied by the value of the predetermined pressure Pa (bar) divided by the atmospheric pressure Pb, that is, (Pa/Pb). If the capacity V of the air intake chamber  74  is set to a value smaller than the value discussed above, it may occur that a bias in the pressure distribution within the interior of the air intake chamber  74  will be caused by the intake air to be supplied to the combustion engine E and/or a sufficient amount of the intake air I will not be supplied to the combustion engine E. 
     Also, where the maximum pressure at which the relief valve  80  operates is set to, for example, Pc, it is preferred to set the capacity V to be V (≧(Pc/Pb)×Q) of the air intake chamber  74 , that is a value equal to or higher than the product of the displacement Q multiplied by the value of the maximum pressure Pc divided by the atmospheric pressure Pb, that is, (Pc/Pb). 
     Assuming that the speed increasing ratio of the supercharger relative to the crankshaft is expressed by N, the number of rotations of the supercharger at the time of the maximum output is expressed by A (rpm), and the discharge mass flow rate of the supercharger at the time of the maximum output is expressed by B (kg/sec), the mass M of the air discharged per one complete rotation of the crankshaft can be obtained from M=(B×60)/(A/N). In this equation, the mass C of the air within the air intake chamber  74  at the time of the maximum output is preferably C≧4M. 
     In other words, the air intake chamber  74  is so formed to have such a size enough to accommodate a mass of the air that is four or more times the mass M of the air discharged per one complete rotation of the crankshaft at the time of the maximum output. Preferably, the air intake chamber  74  is so formed to have a size enough to accommodate the mass of air that is within the range of 5 times to 15 times the mass M, that is, (15M≧C≧5M), but in the practice of the embodiment now under discussion the air intake chamber  74  can accommodate the mass of air which is 10 times the mass, that is, (C=10M). 
     If the capacity of the air intake chamber is too large, there is a possibility that the output response to a throttle opening operation performed by the driver will be lowered. In view of this, when the supercharger  42  is rotated from a condition, in which the pressure within the air intake chamber  74  is equal to the atmospheric pressure, at the rotational number at the maximum output, the pressure within the air intake chamber  74  attains, at a speed not lower than, for example, 0.1 second, a predetermined pressure that is set at the time of the maximum output, and is more preferably lower than 0.05 second. In the practice of the embodiment now under discussion, it is set to about 0.025 second. 
     Even though the capacity V of the air intake chamber  74  is so set as hereinabove discussed, the required engine output can be obtained with no intercooler employed. Therefore, by a quantity corresponding to the intercooler so dispensed with, the structure can be simplified to reduce the cost of manufacture. Also, if the injector  75  capable of injecting the fuel in the atomized form into the interior of the air intake chamber  74  as hereinbefore described is used in combination, the intake air temperature within the interior of the air intake chamber  74  can be further lowered. 
     In the construction hereinabove described, the inventors of the present invention have found that when the capacity for accumulating the intake air at a location upstream of the throttle body  76  is increased, the output of the combustion engine increases. However, if the widthwise dimension and/or the vertical direction dimension are increased in the vicinity of the throttle body  76 , the dimension of the motorcycle comes to be large and the degree of freedom in configurational design is lowered, Accordingly, as shown in  FIG. 2 , the connecting portion  86 , which is a portion hitherto used as a connecting tube between the supercharger  42  and the air intake chamber  74 , is used as a part of the air intake chamber  74  to thereby increase the capacity V of the air intake chamber  74 . Accordingly, an appropriate chamber shape can be maintained in the vicinity of the throttle body and the output of the combustion engine E can be increased while suppressing the increase in size of the motorcycle and the reduction in degree of freedom in designing. 
     Also, the widthwise dimension W 3  of the outlet side opening of the connecting portion  86  is set to the same value as that of the widthwise dimension of the air intake chamber  74 , and the widthwise dimension W 1  of the inlet opening of the connecting portion  86  is set to the same dimension as the diameter of the discharge port  48  of the supercharger  42 . Further, the connecting portion  86  has a transverse width increasing gradually from the inlet to the outlet. Accordingly, since the flow velocity of the intake air becomes gradually small, the turbulence of flow of the intake air incident to deceleration is suppressed and the air intake efficiency increases. 
     As shown in  FIG. 5 , since the discharge port  48  of the supercharger  42  is disposed spaced rearwardly from the confronting portion  84 , the capacity of the air intake chamber  74  can be increased by increasing the longitudinal dimension of the connecting portion  86 . 
     The impeller shaft  51  of the supercharger  42  shown in  FIG. 5  is positioned above a rear portion of the crankcase  28  (shown in  FIG. 3 ) and the discharge port  48  of the supercharger  42  is positioned rearwardly of the impeller shaft  51 . Thus, by increasing the longitudinal dimension of the connecting portion  86  between the confronting portion  84  and the discharge port  48  of the supercharger  42 , the capacity of the air intake chamber  74  can be increased. 
     As shown in  FIG. 1 , the air intake duct  70 , through which the incoming wind flowing forwardly of the combustion engine E is guided towards the supercharger  42 , passes laterally of the cylinder block  30 . Accordingly, while the interference between the air intake chamber  74  and the air intake duct  70  is avoided, the capacity of the air intake chamber  74  can be increased. 
     The cylinder head  32  of the combustion engine E is disposed having been tilted forwards, and the confronting portion  84  (shown in  FIG. 5 ) is disposed diagonally rearwardly and above the cylinder block  30 . Accordingly, in a quantity corresponding to the cylinder head  32  having been tilted forwards, the air intake chamber  74  can be expanded in the longitudinal direction. 
     The front wall  74   a  of thee air intake chamber  74  shown in  FIG. 5  is disposed having protruded forwardly of a throttle body mounting portion in the confronting portion  84 . Accordingly, in a quantity corresponding to the front wall  74   a  having been protruded forwards, the air intake chamber  74  can be expanded in the longitudinal direction. 
     The distance (dimension L) from the inlet  77  to the outlet  73  can be increased when the dimension from the upstream end portion on the inlet side to the downstream end portion on the exist side of the interior space of the air intake chamber  74  is chosen to be three or more times the inner diameter D of the outlet  73  shown in  FIG. 7 . Accordingly, by the effect of heat dissipation from an outer wall of the air intake chamber  74 , the intake air temperature can be lowered and the output of the combustion engine increases. 
     As shown in  FIG. 4 , the four outlets  73  corresponding to the respective cylinders, that is, the cylinder side openings  68  are disposed having been juxtaposed in the transverse direction. In the intermediate neighboring two cylinder side openings  68 , which are positioned intermediate between the remaining two cylinder side openings  68 , the suppressing member  88  is provided for suppressing the movement of the intake air I from the region, adjacent to one of the cylinder side openings  68 , to the vicinity of the other of the cylinder side openings  68 . Accordingly, the intake air I can be uniformly supplied to the cylinders. 
     As shown in  FIG. 5 , since the rectifying member  90  for avoiding the bias in the flow of the intake air I is provided inside the air intake chamber  74 , the intake air I can be stably supplied to the air intake port  47 . 
     The inventors of the present invention have found that when the capacity of the interior space of the air intake chamber  84  is increased, the temperature of the intake air I discharged from the supercharger  42  has been lowered. Thus, with the temperature of the intake air lowered, the ignition timing of the combustion engine can be advanced while avoiding the occurrence of knocking. Hence, an improvement in output of the combustion engine can be accomplished. If the capacity V of the interior space of the air intake chamber  74  is of a value equal to or larger than twice the displacement of the combustion engine, a sufficient improvement in engine output can be achieved. Also, if the capacity V of the interior space of the air intake chamber  74  exceeds eight times the displacement of the combustion engine, there is a possibility that the output response to the throttle opening operation by the driver will be lowered. Therefore, the capacity V is preferably within the three to eight times the engine displacement. In the case of the motorcycle, the capacity V of the air intake chamber  74  is preferably set to a value equal to or smaller than 8,000 cc (8 liters). 
     The dimension from the upstream end portion on the inlet side to the downstream end portion on the outlet side of the interior space of the air intake chamber  74  is equal to or greater than three times the inner diameter D forming the outlet  73  shown in  FIG. 7 . Therefore, the distance from the inlet to the outlet is rendered to be long. Accordingly, by the effect of heat dissipation from the outer wall of the air intake chamber  74 , the intake air temperature can be lowered and the output of the combustion engine increases. 
     In other words, the capacity V of the interior space of the air intake chamber  74  is set to (V/n)≧0.5X when the number of the cylinders of the combustion engine is n and the displacement of the combustion engine is X. Also, the capacity V is preferably set to (V/n)≧X. Accordingly, since a capacity higher than the capacity to which the air is sucked during one intake stroke of the combustion engine is secured, reduction in pressure can be further prevented. 
     In addition, the capacity V of the interior space of the air intake chamber  74  is so set that, of the respective capacities V 1  to V 4  of the corresponding interior spaces of the four divided regions R 1  to R 4 , the minimum value V 1  may become larger than half the engine displacement X. The capacity V is preferably so set that the minimum value V 1  may become larger than the engine displacement X. Accordingly, the sufficient capacity V of the interior space of the air intake chamber can be secured and the reduction in intake air pressure can be further suppressed. 
     Also, relative to the mass M of the intake air I discharged from the supercharger  42  per one complete rotation of the crankshaft, the mass C of the intake air I accumulated within the interior of the air intake chamber during a maximum output generating condition is defined C≧4M. By so defining, the output increase of the combustion engine can be achieved. 
     By the effect of heat of evaporation of the fuel F injected from the injector  75  shown in  FIG. 5 , the intake air I is cooled, and therefore, the ignition timing of the combustion engine can be brought to a desired timing. As a result thereof, the engine output increases. Also, the fuel injecting port  75   a  of the injector  75  is disposed at a location displaced from the axis AX of the outlet  73  of the air intake chamber  74 . Accordingly, the distance between the fuel injecting port  75   a  and the outlet  73  is rendered to be great and the heat of evaporation can be effectively utilized, resulting in increase of the cooling effect. 
     Moreover, on the upper surface of the air intake chamber  74  the injector  75  is disposed, and the axis IX of the injector  75  is inclined relative to the outlet axis Ax. Therefore, the amount of protrusion of the injector  75  upwardly from the air intake chamber  74  is suppressed. As a result thereof, a space for the fuel tank  15  disposed above the air intake chamber  74  is not oppressed. 
     Yet, the inlet  77  is formed in the rear portion of the air intake chamber  74  and the cylinder side opening  68  (shown in  FIG. 4 ), which forms the outlet, is formed in the front portion thereof, with the axis IX of the injector  75  disposed to incline in a diagonally downward and rearward direction. Accordingly, since the injector  75  does not oppress the upper surface of the front portion of the air intake chamber  74 , the fuel tank  15  is easily disposed above the air intake chamber  74 . 
     Furthermore, since the injector  75  is mounted in the recessed mounting area  95  in the upper surface of the air intake chamber  74 , the length over which the injector  75  protrudes outwardly from an outer surface of the air intake chamber  75  can be suppressed. 
     Since the air intake chamber  74  is made of an aluminum alloy of a kind having a high heat dissipating property, the injector  75  can be supported stably by the air intake chamber  74 , and also heat dissipation of the intake air I from the outer wall of the air intake chamber  74  can be promoted, to thereby further effectively lower the intake air temperature. 
     As shown in  FIG. 3 , the air intake chamber  74  is disposed at a location diagonally upwardly and rearwardly of the cylinder block  30  of the combustion engine E, the injector  75  is disposed on the upper surface of the air intake chamber  74 , and the air intake duct  70  passes laterally of the cylinder block  30 . Accordingly, as compared with the case in which the air intake duct  70  passes above the combustion engine E, interference of the air intake duct  70  with the injector  75  or a member disposed above the combustion engine, for example, the fuel tank  15  can be avoided. As a result thereof, the degree of freedom in configurational designing above the combustion engine can be improved. 
     Also, when viewing the air intake chamber  74  from lateral side, since the air intake chamber  74  is inclined rearwardly downwardly, the rear half portion of the air intake chamber  74  is lowered in the vertical direction. As a result thereof, the capacity of a rear portion of the fuel tank  15  disposed above the air intake chamber  74  can be increased. 
     In addition, since the upper end of the injector  75  disposed on the upper surface of the air intake chamber  74  is positioned at the substantially same height as that of the upper end of the air intake chamber  74 , it is possible to prevent the injector  75  from protruding upwardly from the air intake chamber  74 . 
     As shown in  FIG. 2 , the fuel tube  19  for the injector  75  is, after extending forwards from the delivery tube  21 , connected with the fuel pump  17 . Accordingly, the fuel tube  19  does not protrude upwardly from the upper surface of the air intake chamber  74 . 
     Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings which are used only for the purpose of illustration, those skilled in the art will readily conceive numerous changes and modifications within the framework of obviousness upon the reading of the specification herein presented of the present invention. By way of example, although in describing the preferred embodiment of the present invention the discharge port  48  of the supercharger  42  has been shown and described as oriented upwards, the discharge port  48  may be oriented diagonally upwardly and rearwards. In this case, the inlet  77  of the air intake chamber  74  comes to be positioned further rearwardly and, therefore, the longitudinal dimension of the air intake chamber  74  becomes large, resulting in securing the large capacity of the air intake chamber  74 . 
     Also, the connecting portion  86  may suffice to be disposed between the discharge port  48  of the supercharger  42  and the confronting portion  84 . For example, where the discharge port  48  is positioned below the confronting portion  84  as shown in  FIG. 8 , the connecting portion  86  is disposed at a location above the discharge port  48  and below the confronting portion  84 . Moreover, the discharge port  48  of the supercharger  42  may be disposed having been displaced in one vehicle widthwise direction, besides a location intermediate of the crankcase  28  in the vehicle widthwise direction. 
     Yet, the present invention can be applied to any saddle-riding type vehicle other than the motorcycle, for example, a buggy having more than two wheels. In addition, the present invention may be particularly applied to any automotive vehicle of a type having a limited vehicle widthwise dimension. Furthermore, the supercharger may be driven by the power other than the power of the combustion engine, for example, with the utilization of exhaust gas energies of the combustion engine and/or with the use of a separate electric motor. Yet, other than the centrifugal type, the Roots type supercharger may be equally used. Also, an intercooler for cooling the intake air I within the air intake chamber  74  may be employed. 
     Accordingly, such changes and modifications are, unless they depart from the scope of the present invention as delivered from the claims annexed hereto, to be construed as included therein. 
     REFERENCE NUMERALS 
     
         
           28  . . . Crankcase 
           30  . . . Cylinder block 
           32  . . . Cylinder head 
           42  . . . Supercharger 
           47  . . . Air intake port 
           48  . . . Discharge port 
           51  . . . Impeller shaft 
           70  . . . Air intake duct 
           73  . . . Outlet of the air intake chamber 
           74  . . . Air intake chamber 
           74   a  . . . Front wall 
           76  . . . Throttle body 
           84  . . . Confronting portion 
           86  . . . Connecting portion 
           88  . . . Suppressing member 
           90  . . . Rectifying member 
         AX . . . Axis of the outlet of the air intake chamber 
         E . . . Combustion engine 
         I . . . Intake air