Abstract:
It is intended to implement an electric supercharger that has a simplified architecture, is easy to assemble, produces reduced vibration and noise, and has a motor inverter, making it possible to minimize losses in motor output and rotary-shaft output. The electric supercharger is provided with the following: an integrated housing with a built-in electric motor and motor inverter; and a ball bearing and damper-sleeve structure arranged on both sides of the electric motor. The damper-sleeve structure comprises a large-diameter sleeve, a spring guide, a coil spring, and a ball bearing. A gap is formed between the ball bearings and a sleeve and the large-diameter sleeve. The inner ring or outer ring of the ball bearings are supported by various support members disposed on both sides. An elastic O-ring that elastically supports the sleeve and large-diameter sleeve is provided on the outside of the sleeves.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an electric supercharger which is, for instance, mounted on a vehicle and is driven by an electric motor, an assembling method of the same and an internal combustion engine in which the electric supercharger is installed. 
       BACKGROUND ART 
       [0002]    In a vehicle, a turbocharger equipped with a compressor and a turbine are provided in an intake passage and an exhaust passage, respectively, to improve thermal efficiency of an internal combustion engine. In recent years, as one means to improve turbo lag and responsiveness of the turbocharger, the electric supercharger driven by the electric motor has attracted attention. However, the electric supercharger has many challenges in durability, ease of assembly and ease of installation at the time of installation to the vehicle. 
         [0003]    From the viewpoint of reducing axial vibration, the electric supercharger is normally supported at both ends of a motor rotor, which is heavy in weight. The electric supercharger of this two end support method is disclosed in Patent Document 1. In reference to  FIG. 8 , this supporting method is described. A housing of the electric supercharger  100 A is composed of a compressor housing  102 , a motor housing  104  and bearing housings  106   a  and  106   b  provided on both sides of the motor housing  104 . In the compressor housing  102 , a compressor wheel  108  is accommodated. A rotation shaft  110  extends toward the motor housing  104  from the center of the compressor wheel  108 . 
         [0004]    A motor rotor  112  is attached to the rotating shaft  110 . A stator (not shown) is provided inside the motor housing  104 . The stator is disposed at a position facing the motor rotor  112 . Inside the bearing housings  106   a  and  106   b,  bearings  114  for supporting the rotating shaft  110  are provided on both sides of the motor housing  104 . 
         [0005]    An inlet  102   a  of the compressor housing  102  is connected to an intake passage of the internal combustion engine, and by rotation of the compressor wheel  108 , supply air a is drawn into the compressor wheel  108  to compress and discharge the supply air a to the outlet side of the intake passage. 
         [0006]      FIG. 9  illustrates an electric supercharger  100 B for supporting the rotation shaft  110  by one end. In the electric supercharger  100 B, a bearing housing  106  is provided between the motor housing  104  and the compressor housing  102 . A plurality of bearings  114  is provided inside the bearing housing  106  so as to support the rotation shaft  110  by these bearings  114 . 
       CITATION LIST 
     Patent Document 
     [Patent Document 1] 
       [0000]    
       
         JP 2002-369474 A 
       
     
       SUMMARY 
     Technical Problem 
       [0008]    The electric motor can be supported in a stable fashion by the electric supercharger of two end supporting type for supporting the electric motor by both ends. This is effective in reducing vibration, but assembly thereof is complicated as the bearings are provided on both sides of the electric motor. In contrast, the electric supercharger of single end supporting type is easy to assemble compared with the electric supercharger of two end supporting type. However, it is not advantageous from the point of view of vibration reduction. 
         [0009]    Further, in the electric supercharger of Patent Document 1, the motor inverter for controlling the rotation speed of the electric motor is provided at a position away from the motor housing. In this isolated arrangement, it is advantageous from the viewpoint of the ease of assembly and ease of installation to the vehicle. However, as the distance between the electric motor and the motor inverter increases, the current resistance passing through between the electric motor and the motor inverter increases. Therefore, loss of the current flowing through between the electric motor and the motor inverter increases, and the motor output is reduced. Especially with limited power generation capacity of a power generating apparatus installed in the vehicle, reduction in the motor output is disadvantageous in terms of performance. 
         [0010]    As the rotation speed of the electric motor increases, the unbalanced load applied to the rotation shaft and the influence of the magnetic attraction force of the electric motor increases. Therefore, there is a problem that, as vibration generated in the rotation shaft becomes larger, the output of the rotation shaft declines and the noise becomes louder. 
         [0011]    In view of the problems of the related art, it is an object of the present invention to realize an electric supercharger and an assembling method thereof, which make it possible to simplify the construction, facilitate the assembling, reduce vibration and noise, and, when provided with a motor inverter, suppress the output loss of the motor and the rotation shaft. 
       Solution to Problem 
       [0012]    To achieve the above object, an electric supercharger according to embodiments of the present invention comprises: a compressor wheel housed in a compressor housing; an electric motor for driving a rotation shaft of the compressor wheel; a motor inverter for controlling rotation speed of the rotation shaft, the motor inverter being arranged adjacent to the electric motor in series on the rotation shaft; a rolling bearing provided on each side of the electric motor to rotatably support the rotation shaft; a sleeve provided outside the rolling bearing and forming a gap between the sleeve and the rolling bearing; a supporting portion clamping an outer ring or inner ring of the rolling bearing from both sides to support the rolling bearing; and a damper mechanism arranged outside the sleeve to elastically support the sleeve. 
         [0013]    In the electric supercharger of the embodiments of the present invention, the motor inverter is arranged adjacent to the electric motor in series on the rotation shaft and thus, it is possible to minimize electric current loss between the electric motor and the motor inverter. Further, in addition to the vibration reduction effect by supporting the electric motor by both ends thereof, the gap is formed between the rolling bearing and the sleeve provided outside the rolling bearing and thus, it is possible to absorb thermal expansion of the rolling bearing and cut off vibration of the rotation shaft, thereby preventing transmission of the vibration from the rotation shaft to the electric motor and the inverter. In addition, by forming the gap, it is possible to suppress eccentric load generated in the rotation shaft and the rolling bearing. Thus, it is possible to lower cost and simplify the device configuration and it is also possible to suppress the reduction in the output of the rotation shaft. 
         [0014]    Further, by providing the damper mechanism outside the sleeve to elastically support the sleeve, it is possible to suppress transmission of the vibration to the electric motor and the inverter. Further, by clamping the outer ring or inner ring of the rolling bearing from both sides to support the rolling bearing, it is possible to fix the rolling bearing in a stable fashion. Thus, a thrust force applied to the rotation shaft from the compressor wheel or the like can be received by the rolling bearing in a stable fashion. This contributes to reduction of vibration and noise, simplification of the device configuration and cost reduction. 
         [0015]    In the above electric supercharger, the rolling bearing may comprise a grease enclosed rolling bearing. As the rolling bearing is configured by a grease enclosed rolling bearing, it is no longer necessary to provide a device such as a lubricating oil piping for supplying the lubricating oil to the rolling bearing and thus, it is possible to simplify the configuration of the electric supercharger and lower its cost. 
         [0016]    In the above electric supercharger, the damper mechanism may comprise a damper member having elasticity, the damper member being provided in a gap between the sleeve and an inner surface of a housing facing the sleeve. With this configuration, it is possible to realize simple and low-cost damper mechanism. 
         [0017]    In the above electric supercharger, the damper mechanism may be configured so that an oil film of lubricating oil is formed in a gap between the sleeve and an inner surface of a housing facing the sleeve. By forming the oil film of lubricating oil, it is possible to realize the long-life damper mechanism whose damper member does not age. 
         [0018]    In the above electric supercharger, the rolling bearing arranged on a motor inverter side and the sleeve arranged on an outer side of the rolling bearing may comprise a damper sleeve structure. The damper sleeve structure may comprise: a large diameter sleeve having a pair of inward engagement portions which projects inwardly from each ends; a cylindrical spring guide provided inside the large diameter sleeve and having an outward engagement portion which projects outward from one end; the rolling bearing arranged inside the large diameter sleeve and forming the gap between the large diameter sleeve and the rolling bearing, the rolling bearing being clamped between one of the inward engagement portions of the large diameter sleeve and the outward engagement portion of the spring guide; and a spring member arranged in a cylindrical space formed between the large diameter sleeve and the spring guide to impart an elastic clamping force to the rolling bearing via the outward engagement portion. 
         [0019]    As the damper sleeve structure is configured to impart an elastic clamping force to the rolling bearing by means of the spring member, it is possible to always maintain the clamping force to the rolling bearing with respect to positional change of the rolling bearing. Thus, it is possible to maintain the clamping force on the rolling bearing while allowing dimensional error of the rolling bearing and a mounting error of each member (the motor rotor, sleeve, etc.) fixed to the rotation shaft. As a result, it is possible to relax the allowable dimensional tolerance of each member and hence save the production cost of each member. Further, by using the damper sleeve structure having been integrally assembled in advance, assembly of the bearing mechanism is facilitated. 
         [0020]    In the above electric supercharger, a spring member may be provided between the rolling bearing arranged on a compressor side and a spacer. When the supply air flows around the back face side of the compressor wheel, the supply air pressure is added to the back face side. As a result, in the axial direction of the rotation shaft, tensile force acting on the compressor side is generated. By providing the spring member between the rolling bearing arranged on the compressor side and the spacer, it is possible to elastically receive the tensile force. Therefore, it is possible to prevent generation of excessive load between the rolling bearing and the spacer and prevent damage to the rolling bearing and the spacer. 
         [0021]    In the above electric supercharger, an intake passage may be connected to the compressor housing, and an intake air bypass passage configured so that intake air bypasses the compressor wheel and a bypass valve provided in the intake air bypass passage may be formed integrally in the compressor housing. As a result, it is no longer necessary to provide an intake bypass passage separately, and the electric supercharger becomes compact. Therefore, mounting of the electric supercharger to the vehicle is facilitated. 
         [0022]    An internal combustion engine according to embodiments of the present invention comprises the electric supercharger described above, and the electric supercharger is provided in an intake passage. By incorporating the electric supercharger of the present invention into the internal combustion engine, it is possible to make the internal combustion engine compact and facilitate the mounting of the internal combustion engine to the vehicle. Further, with the compact internal combustion engine, it is possible to improve the fuel efficiency. Furthermore, it is possible to reduce the noise and vibration of the electric supercharger. 
         [0023]    The internal combustion engine of the present invention may comprise: a turbocharger comprising a compressor provided on an upstream or downstream side of the intake passage with respect to the electric supercharger and a turbine provided in an exhaust passage; and an exhaust gas recirculation mechanism for returning a part of exhaust gas to the intake passage from the exhaust passage. In addition to the above-described effects, by combining the turbocharger and the electric supercharger of the present invention, it is possible to improve the output of the internal combustion engine and also improve responsiveness at the transient operation. Further, by providing the exhaust gas recirculation mechanism, it is possible to suppress generation of NOx in the exhaust gas. 
         [0024]    Moreover, an assembling method of the electric supercharger of the present invention comprises: a first step of fitting a motor rotor to the rotation shaft from an end face of the rotation shaft that is on a motor inverter side and then fixing the motor rotor to an enlarged diameter part formed on the rotation shaft in a state where the motor rotor is engaged to the enlarged diameter part; a second step of fitting the rolling bearing to the rotation shaft from an end face of the rotation shaft that is on a compressor side and then fixing the rolling bearing to the enlarged diameter part in a state where the rolling bearing is engaged to the enlarged diameter part, and fitting the damper sleeve structure to the rotation shaft from the end face of the rotation shaft that is on the motor inverter side and then fixing the damper sleeve structure to the rotation shaft at a position adjacent to the motor rotor; a third step of fitting a bearing housing and the compressor wheel to the rotation shaft from the end of the rotation shaft that is on the compressor side, wherein the sleeve is provided on an inner surface of the bearing housing and a first damper mechanism is provided outside the sleeve, and then fixing the compressor wheel to the rotation shaft, and arranging the sleeve so that the gap is formed between the sleeve and an outer periphery of the rolling bearing; and a fourth step of, wherein a second damper mechanism is provided on an outer surface of an integrated housing which functions as housings for the electric motor and the inverter, covering a rotation shaft assembly unit from both sides with the compressor housing and the integrated housing, the rotation shaft assembly unit being assembled by performing the first to third steps, and joining the compressor housing and the integrated housing together in such a state that the bearing housing is clamped between the compressor housing and the integrated housing so that the second damper is arranged outside the large diameter sleeve. 
         [0025]    According to the above assembling method of the present invention, by engaging the motor rotor and the rolling bearing to the enlarged diameter part formed on the rotation shaft from each side of the rotation shaft, positioning of the motor rotor and the rolling bearing is facilitated. Further, by prefabricating the damper sleeve structure and arranging the damper sleeve structure adjacent to the motor rotor, mounting of the bearing mechanism on the motor inverter side is facilitated. Furthermore, by fabricating the integrated housing serving as housings for the electric motor and the motor inverter and joining the compressor housing and the integrated housing together in such a state that the bearing housing is clamped between the compressor housing and the integrated housing, mounting of the electric supercharger housing can be completed in one step. 
         [0026]    Thus, according to the assembling method of the present invention, it is possible to shorten the assembling process of the electric supercharger of the present invention which has the advantage of reducing vibration and other advantages, and facilitate assembling thereof. 
       Advantageous Effects 
       [0027]    According to the electric supercharger of the present invention, the electric supercharger comprises: a compressor wheel housed in a compressor housing; an electric motor for driving a rotation shaft of the compressor wheel; a motor inverter for controlling rotation speed of the rotation shaft, the motor inverter being arranged adjacent to the electric motor in series on the rotation shaft; a rolling bearing provided on each side of the electric motor to rotatably support the rotation shaft; a sleeve provided outside the rolling bearing and forming a gap between the sleeve and the rolling bearing; a supporting portion clamping an outer ring or inner ring of the rolling bearing from both sides to support the rolling bearing; and a damper mechanism arranged outside the sleeve to elastically support the sleeve. Thus, it is possible to significantly reduce vibration of the electric motor and the motor inverter. In addition, as there is no generation of the eccentric load at the rotation shaft or the rolling bearing, it is possible to simplify and lower the cost of the configuration of the supercharger and also to suppress the reduction in the output of the rotation shaft. Further, against the thrust force added on the rotation shaft, it is possible to fix the rolling bearing in a stable fashion with respect to the thrust force added on the rotation shaft. 
         [0028]    According to the internal combustion engine of the present invention, the electric supercharger of the present invention is provided in the intake passage and thus, it is possible to make the internal combustion engine compact. Thus, it is possible to improve the fuel efficiency and facilitate mounting to the vehicle. Furthermore, it is possible to reduce the noise and vibration of the electric supercharger. 
         [0029]    According to the assembling method of the electric supercharger of the present invention, the assembling process includes the first step to the fourth step. Thus, the assembling process of the electric supercharger of the present invention can be shortened and facilitated. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0030]      FIG. 1  is a sectional front view of an electric supercharger according to a first embodiment. 
           [0031]      FIG. 2  is an enlarged sectional view of a part of a bearing mechanism for the electric supercharger. 
           [0032]      FIG. 3  is an enlarged sectional view of a part of a compressor wheel for the electric supercharger. 
           [0033]      FIG. 4  is an enlarged sectional view of a part of the bearing mechanism according to a second embodiment for the electric supercharger. 
           [0034]      FIG. 5  is a sectional view of a part of a compressor housing according to a third embodiment for the electric supercharger. 
           [0035]      FIG. 6  is a system diagram of an internal combustion engine according to a first embodiment of the present invention. 
           [0036]      FIG. 7  is an assembly flow chart according to the first embodiment of the present invention. 
           [0037]      FIG. 8  is a sectional front view of an electric supercharger of a both end support type of the related art. 
           [0038]      FIG. 9  is a sectional front view of an electric supercharger of a cantilever support type of the related art. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. It is intended, however, that unless particularly specified, dimensions, materials, shapes, relative positions and the like of components described in the embodiments shall be interpreted as illustrative only and not limitative of the scope of the present invention. 
       First Embodiment 
       [0040]    An electric supercharger according to a first embodiment of the present invention is described in reference to  FIG. 1  to  FIG. 3 . The electric supercharger  10  of this embodiment is provided in an intake passage (not shown) of an internal combustion engine installed in a vehicle. A housing for the electric supercharger  10  is composed of a compressor housing  12 , an integrated housing  14  functioning as a housing for an electric motor and a motor inverter, and a bearing housing  16 . Inside the compressor housing  12 , a compressor wheel  18  is housed. A through-hole is provided on an axial center line of the compressor wheel  18 . An end of a rotation shaft  20  is fitted into the through-hole and a nut  26  is screwed to a screw portion  20   a  formed at the end of the rotation shaft  20  so as to join the compressor-side end of the rotation shaft  20  to the compressor wheel  18 . The rotation shaft  20  is arranged on the axial center line of the compressor housing  12 . 
         [0041]    A plurality of impellers  28  projects radially from an outer peripheral surface of the compressor wheel  18 . When the compressor wheel  18  rotates, supply air a is drawn from the intake passage, compressed and then supplied to an inlet manifold (not shown) of the internal combustion engine. In the compressor housing  12  and the integrated housing  14 , flange portions  12   a  and  14   a  having flat faces abutting each other are formed. A flange portion  16   a  of the bearing housing  16  is clamped between the flange portions  12   a  and  14   a,  and the flange portions  12   a  and  14   a  are joined by bolts  50  so as to assemble all the housings of the electric supercharger  10  in one step. 
         [0042]    An enlarged diameter part  20   b  of a disk shape is formed at a center of the rotation shaft  20 . Inside the bearing housing  16 , a ball bearing  24  which is one form of the rolling bearing is press-fit to the rotation shaft  20  at the position which contacts the enlarged diameter part  20   b.  The ball bearing  24  contains grease G packed therein and is provided with a seal member  248  for preventing grease leak (see  FIG. 2 ). One side face of an inner ring  240  of the ball bearing  24  is in contact with the enlarged diameter part  20   b.  Between the ball bearing  24  and the compressor wheel  18 , a sleeve  40  is provided and the other side face of the inner ring  240  is in contact with an end face of the sleeve  40 . In this manner, both of the side faces of the inner ring  240  of the ball bearing  240  are clamped by the enlarged diameter part  20   b  and the sleeve  40 . 
         [0043]    A spacer  30  is joined to the bearing housing  16  by bolts  32 . The spacer  30  is arranged between the compressor wheel  18  and the ball bearing  24 , and a tip of the spacer  30  contacts a side face of an outer ring of the ball bearing  24  to support the ball bearing  24 . 
         [0044]    As illustrated in  FIG. 2 , a sleeve  34  is provided outside the ball bearing  24 . A gap S 1  is formed between the outer peripheral surface of the ball bearing  24  and an inner peripheral surface of the sleeve  34 . The gap s 1  is set to the minimum amount necessary for absorbing vibration of the rotation shaft  20  and thermal expansion at the maximum temperature of the ball bearing (e.g. 100° C.). Another gap s 2  is formed between the sleeve  34  and an inner surface of the bearing housing  16 . The gap s 2  is set to the minimum amount necessary for elastically supporting the sleeve  34  by taking into account a safety factor based on a strain amount of an elastic O ring  38  calculated with respect to a load acting on the ball bearing  24 ,  60  due to vibration of the rotation shaft  20 , and magnetic attraction on the rotation shaft  20 . 
         [0045]    On an inner surface of the bearing housing  16 , two grooves  36  having a rectangular cross sectional shape are provided. In the grooves  36 , elastic O rings  38  made of rubber are fitted. The elastic O rings  38  are arranged to fill the gap S 2  and has elastic supporting action for elastically contacting the sleeve  34  to the inner surface of the bearing housing  16 . 
         [0046]    The ball bearing  24  comprises a spherical roller element  244  provided between the inner ring  240  and the outer ring  242 , a retainer  246  for retaining the roller element  244 , and a seal member  248  fixed to the outer ring  242  to enclose grease G. In such a state that a motor rotor  22  is in contact with the other side face of the enlarged diameter part  20   b,  the motor rotor  22  is press-fit to the rotations shaft  20 . A motor coil  42  is arranged outside the motor rotor  22  with distance from the motor rotor  22 . Outside the motor coil  42 , a motor stator  44  is arranged. 
         [0047]    The motor rotor  22 , the motor coil  42  and the motor stator  44  configure the electric motor  46 . The motor coil  42  and the motor stator  44  are accommodated in the integrated housing  14  which functions as housings for the electric motor  46  and the motor inverter  48 . 
         [0048]    A damper sleeve structure  54  is arranged adjacent to the motor rotor  22  via a ring-shape spacer  52  having a rectangular cross-sectional shape. As illustrated in  FIG. 7 , the damper sleeve structure  54  is configured by a cylindrical large diameter sleeve  56 , a cylindrical spring guide  58 , a grease sealing type bearing  60  and a coil spring  62 . On an inner peripheral face of one end of the large diameter sleeve  56 , an inward engagement portion  56   a  of a ring shape is provided to project inward, and on an inner peripheral face of the outer end of the large diameter sleeve  56 , a plurality of fixing pins  64  which projects inward is attached in the circumferential direction. At one end of the outer peripheral surface of the spring guide  58 , an outward engagement portion  58   a  of a ring shape projects outward. 
         [0049]    In a cylindrical space formed by the large diameter sleeve  56  and the spring guide  58 , the coil spring  62  is housed. Between the fixing pins  64  and the outward engagement portion  58   a,  the outer ring  242  of the ball bearing  60  is fitted. In this manner, the outer ring  242  of the ball bearing  24  is clamped from both sides by the fixing pins  64  and the outward engagement portion  58   a.  To the outer ring  242  of the ball bearing  24 , a spring force of the coil spring  62  is applied via the outward engagement portion  58   a.    
         [0050]    The gap s 1  is formed between the outer peripheral surface of the outer ring  242  of the ball bearing and the inner surface of the large diameter sleeve  56 . The gap s 1  is set to the minimum amount necessary for absorbing vibration of the rotation shaft and thermal expansion at the maximum temperature of the ball bearing (e.g. 100° C.). The gap s 2  is formed between the outer peripheral surface of the large diameter sleeve  56  and the inner peripheral surface of the integrated housing  14 . On the inner surface of the integrated housing  14 , as illustrated in  FIG. 2 , two grooves  36  having a rectangular cross sectional shape are provided. In the grooves  36 , the elastic O rings  38  made of rubber are fitted. The elastic O rings  38  are arranged to fill the gap S 2  and has elastic supporting action for bringing the sleeve  34  into elastic contact with the inner surface of the integrated housing  14 . 
         [0051]    Adjacent to the damper sleeve structure  54 , a sleeve  66  is press-fitted to the rotation shaft  20 . One end of the sleeve is in contact with a side face of the inner ring  240  of the ball bearing  60  to fix the ball bearing  60 . At the other end of the sleeve  66 , a sensor target  68  projects outward. At an inverter side end of the rotation shaft  20 , a screw portion  20   c  is formed, to which a nut  70  is screwed. In the integrated housing  14 , a motor inverter  48  is arranged adjacent to the electric motor  46 . To the inner surface of the integrated housing  124 , a rotation sensor  72  for detecting the rotation speed of the rotation shaft  20  is attached at a position that faces a sensor target  68 . 
         [0052]    According to this embodiment, by supporting the rotation shaft  20  by the grease sealing type bearings  24  and  60 , it is no longer necessary to provide a lubricating oil supply means such as a lubricating oil supplying piping for supplying lubricating oil to the ball bearing. Therefore, it is possible to simplify the configuration of the electric supercharger  10  and lower the cost. 
         [0053]    The electric motor  46  being heavy in weight is supported by the ball bearings  24  and  60  that are arranged at both ends of the electric motor  46  and thus, it is possible to reduce vibration of the rotation shaft  20 . In addition to this, the inner or outer ring of the ball bearing  24  and  60  is clamped by a member arranged at its both ends, and the gap s 1  for absorbing vibration of the rotation shaft  20  and thermal expansion of the ball bearing is formed between the sleeve  34  arranged outside the ball bearing  24 ,  60  and the large diameter sleeve  56  and thus, it is possible to effectively prevent vibration of the rotation shaft  20  from transmitting to the electric motor  46  and the motor inverter  48 . Further, by forming the gap s 1 , it is possible to suppress uneven load generated at the rotation shaft  20  and the ball bearings  24  and  60  and thus to suppress reduction in the output of the rotation shaft  20 . 
         [0054]    By taking into account the magnetic attraction acting on the rotation shaft  20  and the load applied to the ball bearings  24 ,  60  by vibration of the rotation shaft  20 , the gap s 2  is formed between the sleeve  34  and the inner surface of the bearing housing  16  or between the large diameter sleeve  56  and the inner surface of the integrated housing  14  to elastically support the outer peripheral surface of the sleeve  34  or the large diameter sleeve  56  by the elastic O ring  38 . As a result, it is possible to significantly reduce the vibration of the electric motor  46  and the motor inverter  48 . Further, the inner or outer ring of the ball bearing  24 ,  60  is clamped by a member arranged at its both ends and thus, it is possible to receive the thrust force applied to the rotation shaft  20  from the compressor wheel  18 , etc. using the ball bearing  24 ,  60  in a stable fashion. This contributes to reduction of the vibration. By significantly reducing vibration of the electric supercharger  10  in this manner, it is possible to significantly reduce the noise generated from the electric supercharger  10 . 
         [0055]    The integrated housing  14  is configured to house the electric motor  46  and the motor inverter  48 , and the electric motor  46  and the motor inverter  48  are arranged adjacent to each other. Thus, current loss between the electric motor  46  and the motor inverter  48  can be minimized. As a result, the reduction of the motor output can be suppressed to the minimum. 
         [0056]    Further, by providing the damper sleeve structure  54  in the integrated housing  14 , the inner ring  240  and the outer ring  242  of the ball bearing  60  can be clamped at both sides thereof by the elastic force of the coil spring  62  while absorbing installation errors and dimension tolerance at each part in the axial direction of the rotation shaft  20 . As a result, even when dimension tolerance or installation error occurs at each part, the ball bearing can be fixed in a stable fashion. Further, as the elastic O ring  38 , it is preferable to use the one with hardness of at least 70 of JIS standard. Further, by setting the spring force applied to the ball bearing  60  by the coil spring  62  to 70 to 90N (newton), the ball bearing  60  can be fixed in a stable fashion. 
         [0057]    Further, by changing the material, configuration or the like of the compressor wheel  18 , it is possible to reduce an inertia moment, improve responsiveness at the transient operation, and reduce the vibration of the rotation shaft  20 . For instance, instead of aluminum alloy which is normally used, magnesium, resin, etc. may be used so as to reduce the inertia moment. Further, as illustrated in  FIG. 3 , by configuring a back face  18   a  of the compressor wheel  18  to be a flat surface which is orthogonal to the rotation shaft  20 , the overall length of the compressor wheel  18  can be reduced. Furthermore, by providing in the back face  18   a  a circular or oval groove  18   b  having half-moon or semicircular section, it is possible to reduce the compressor wheel  18  in weight. 
         [0058]    When the supply air a enters the back face  18   a  of the compressor wheel  18 , the supply air pressure is applied to the back face  18   a  and the tensile force acting toward the compressor side is generated in the axial direction of the rotation shaft  20 . By this tensile force, an excessive load may be generated between the spacer  30  and the ball bearing  24 . In view of this, in the first embodiment, a spring member such as a coil spring may be placed between the spacer  30  and the ball bearing  24 . As a result, it is possible to prevent generation of the excessive load between the spacer  30  and the ball bearing  24  and prevent damage to the spacer  30  and the ball bearing  24 . 
       Second Embodiment 
       [0059]    Next, the electric supercharger according to a second embodiment of the present invention is described in reference to  FIG. 4 . In this embodiment, a groove  74  is provided on the inner surface of the bearing housing  16 . The groove  74  has a rectangular cross section with width wider than the groove  36 . In the groove  74 , a flat rubber damper  76  which is wide and has a rectangular cross section is fitted to the groove  74 . The second embodiment is substantially the same as the first embodiment except that the elastic damper  76  is in contact with the sleeve  34 , and the gaps s 1  and s 2  are formed on both sides of the sleeve  34 . 
         [0060]    According to this embodiment, in addition to the effects obtained by the first embodiment, by providing one wide groove  74 , it facilitates formation of the groove and also by providing the wide elastic damper  76 , it enhances the elastic support function of the rotation shaft  20  and further reduces the vibration of the electric motor  46  and the motor inverter  48 . Further, the elastic damper  76  may have wave-like shape. Alternatively, the elastic O ring or the elastic damper  76  may be of resin or metal material having unevenness (e.g. a tolerance ring). 
         [0061]    As another alternative to the damper mechanism, it may be configured so that the lubricating oil is supplied to the gap s 2  and an oil film is formed in the gap s 2  to obtain the elastic supporting effect. According to the oil film formation method, it is not necessary to consider deterioration of the elastic damper and the elastic supporting effect can be maintained longer. 
       Third Embodiment 
       [0062]    Next, the electric supercharger according to a third embodiment of the present invention is described in reference to  FIG. 5 . This embodiment illustrates the exemplary case where a bypass passage is provided in the intake passage of the internal combustion engine, the bypass passage being configured so that the intake air bypasses the compressor wheel  18 . The compressor housing  12  has an exhaust port  12   b  for the supply air a. Further, the bypass passage  78  is integrally formed in the compressor housing  12 , and a bypass valve  79  is provided in the bypass passage  78 . 
         [0063]    According to this embodiment, it is no longer necessary to install the bypass pipe for the supply air a when installing the electric supercharger in the engine mounted on a vehicle. Therefore, it is possible to obtain the electric supercharger which is contact and it becomes easier to install the electric supercharger in the vehicle. 
       Fourth Embodiment 
       [0064]    Next, the internal combustion engine according to a fourth embodiment of the present invention is described in reference to  FIG. 6 . In  FIG. 6 , an intake passage  82  and an exhaust passage  84  are provided in an engine  80  mounted on the vehicle. The electric supercharger  10  is installed in the intake passage  82 . A turbocharger  86  is provided astride the intake passage  82  and the exhaust passage  84 . The turbocharger  86  comprises a compressor  862  provided on an upstream side of the electric supercharger  10  in the intake passage  82 , a turbine  864  provided in the exhaust passage  84 , and a rotation shaft  860  connecting the compressor  862  and the turbine  864 . 
         [0065]    In the intake passage  82 , a bypass passage  820  for bypassing the compressor wheel  18  of the electric supercharger  10 , a bypass valve  822  provided in the bypass passage  820 , and an intercooler  824  provided on a downstream side of the compressor wheel  18  are provided. In the exhaust passage  84 , a bypass passage  840  for bypassing the turbine  864  and a bypass valve  842  provided in the bypass passage  840  are provided. 
         [0066]    Further, a low-pressure exhaust gas recirculation passage  88  is provided to connect a downstream section of the exhaust passage  84  which is downstream from the turbine  864  and an upstream section of the intake passage  82  which is upstream from the compressor  862 . In the low pressure exhaust gas recirculation passage  88 , a flow regulating valve  880  and an intercooler  882  are provided. Furthermore, a high-pressure exhaust gas recirculation passage  90  is provided to an upstream section of the exhaust passage  84  which is upstream from the turbine  864  and a downstream section of the intake passage  82  which is downstream from the compressor  862 . In the high pressure exhaust gas recirculation passage  90 , a flow regulating valve  900  and an intercooler  902  are provided. 
         [0067]    Moreover, ECU (engine control unit)  02  is provided to control the opening degrees of the bypass valves  822 ,  864  and the opening degrees of the flow regulating valves  880 ,  900  and also to control operation of the engine  80 . By controlling the opening degree of the bypass valve  822  or  864 , it is possible to control the flow rate in the intake passage  82  or the exhaust passage  84 . By controlling the opening degree of the flow regulating valve  880  or  900 , it is possible to control the exhaust gas recirculation rate. The intake passage  82  splits upstream from the compressor  862 . A branch passage  82   a  is installed in the electric motor  46  and the motor inverter  48  of the electric supercharger  10  to cool the electric motor  46  and the motor inverter  48 . The branch passage  82   a  joins a downstream side of the intake passage  82  which is downstream from the electric motor  46  and the motor inverter  48 . 
         [0068]    In this embodiment, the supply air a supplied to the intake passage  82  is pressurized in two stages by the turbocharger  86  and the electric supercharger  10  and then supplied to the engine  80 . To reduce NOx in the exhaust gas, a part of the exhaust gas is returned to the intake passage  82  from the low pressure exhaust gas recirculation passage  88  or the high pressure exhaust gas recirculation passage  90 . Depending on the operating state, the ECU  92  controls the opening degrees of the bypass valve and the flow regulating valve. For instance, at the low load operation, the pressure of the exhaust gas exhausted to the exhaust passage  84  is low and thus, the exhaust gas is returned to the intake passage  82  through the low pressure exhaust gas recirculation passage  88 . At the high load operation, the pressure of the exhaust gas exhausted to the exhaust passage  84  is high and thus, the exhaust gas is returned to the intake passage  82  through the high pressure exhaust gas recirculation passage  90 . 
         [0069]    At the transient operation for transition from the low load operation to the high load operation, a large amount of the exhaust gas is returned to the intake passage  82  mainly from the low pressure exhaust gas recirculation passage  88  and is supercharged at high pressure by the electric supercharger  10  to suppress deterioration of fuel consumption and reduce the NOx amount in the exhaust gas. 
         [0070]    According this embodiment, by returning the exhaust gas to the intake passage  82  from the low pressure exhaust gas recirculation passage  88  and the high pressure exhaust gas recirculation passage  90 , it is possible to reduce the amount of NOx in the exhaust gas. Further, by arranging the turbocharger  86  and the electric supercharger  10  in two stages in the intake passage  82 , it is possible to increase the output of the engine  80 . This makes it possible to downsize the engine  80 , which results in enhanced fuel efficiency and improved responsiveness at the transient operation. 
         [0071]    Moreover, in this embodiment, only one of the low pressure exhaust gas recirculation passage  88  or the high pressure exhaust gas recirculation passage  90  may be provided. 
       Fifth Embodiment 
       [0072]    Next, a method according to a fifth embodiment of the present invention is described in reference to  FIG. 7 . The enlarged diameter part  20   b  is formed at the center of the rotation shaft  20 , and a through hole  22   a  is formed in the center of the motor rotor  22  in the axial direction. First, the motor rotor  22  is inserted from the inverter side end of the rotation shaft  20 , and then fixed to the rotation shaft  20  in such a state that a small diameter end face is contact with the enlarged diameter part  20   b,  by cooling fitting, press-fitting or the like. Next, the ball bearing  24  is inserted from the compressor end of the rotation shaft  20  and then fixed to the rotation shaft  20  in such a state that the ball bearing  24  is in contact with the enlarged diameter part  20   b,  by cooling fitting, press-fitting or the like. 
         [0073]    Next, the spacer  52  having a rectangular cross section and the damper sleeve structure  54  are inserted from the inverter side end of the rotation shaft  20 . The damper sleeve structure  54  is configured so that the large diameter sleeve  56 , the small diameter spring guide  58 , the ball bearing  60  and the coil spring  62  are integrally formed with the fixing pins  64 . The spacer  52  is interposed between the motor rotor  22  and the damper sleeve structure  54 . In such a state that the spacer  52 , the motor rotor  22  and the damper sleeve structure  54  are in contact, the ball bearing  24  is fixed to the rotation shaft  20  by cold fitting, press-fitting or the like. Next, the sleeve  66  having the sensor target  68  is mounted on the rotation shaft  20  and then the nut  70  is screwed to the screw part  20   c.    
         [0074]    Next, the bearing housing  16  and the compressor wheel  18  are mounted to the rotation shaft  20  in this order, and the nut  26  is screwed to the screw part  20   c.  In the bearing housing  16 , the elastic O ring  38 , the sleeve  34  and the spacer  30  are joined together in advance by the bolts  32 . The elastic O ring  38  and the sleeve  34  are arranged such as to form the gap s 1  on the outer circumferential side of the ball bearing  24 . Further, the gap s 2  is formed between the inner surface of the bearing housing  16  and the sleeve  34 . The inner ring of the ball bearing  24  is clamped by the spacer  52  and the sleeve  66  from both sides. 
         [0075]    Next, to the rotation shaft assembly structure which is assembled in the above manner, the compressor housing  23  and the integrated housing  14  are mounted from both sides. The flange portion  16   a  of the bearing housing  16  is inter posed between the flange portions  12   a  of the compressor housing  12  and the flange portion  14   a  of the integrated housing  14  which face each other, and the flange portion  12   a  and the flange portion  14   a  are joined together by the bolts  50 . 
         [0076]    According to this embodiment, the motor rotor  22  and the ball bearing  24  are retained from both sides by the enlarged diameter part  20   b  formed on the rotation shaft  20 . This facilitates positioning of the motor rotor  22  and the ball bearing  24 . Further, the damper sleeve structure  54  is prepared in advance and then arranged adjacent to the motor rotor  22 . This facilitates mounting of the bearing mechanism on the motor inverter side. Further, the integrated housing  14  functioning as the housing for the electric motor  46  and the motor inverter  48  is manufactured, and the compressor housing  12  and the integrated housing  14  are joined together by clamping the flange portion of the bearing housing  16  by the flange portions of the compressor housing  12  and the integrated housing  14  and joining the flange portions by the bolts  50 . In this manner, assembling of the housings for the electric supercharger can be completed in one step. 
         [0077]    As described above, according to this embodiment, it is possible to shorten the assembly process of the electric supercharger  10  which is advantageous in reducing the vibration, etc, and also to facilitate the assembly. 
       INDUSTRIAL APPLICABILITY 
       [0078]    According to the present invention, it is possible to realize an electric supercharger which is compact and easy to install and is capable of reducing vibration and noise. Further, by installing the electric supercharger in the internal combustion engine, it is possible to make the internal combustion engine compact and facilitates installation of the internal combustion engine to the vehicle. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           10 ,  100 A,  110 B Electric supercharger 
           12 ,  102  Compressor housing 
           12   a,    14   a,    16   a  Flange portion 
           14  Integrated housing 
           16 ,  106 ,  106   a,    106   b  Bearing housing 
           18 ,  108  Compressor wheel 
           20 ,  110  Rotation shaft 
           20   b  Enlarged diameter part 
           22 ,  112  Motor rotor 
           24 ,  60  Ball bearing 
           240  Inner ring 
           242  Outer ring 
           244  Roller element 
           246  Retainer 
           248  Seal member 
           26 ,  70  Nut 
           28  Impeller 
           30 ,  52  Spacer 
           32 ,  50  Bolt 
           34 ,  40 ,  66  Sleeve 
           36 ,  74  Groove 
           38  Elastic O ring 
           42  Motor coil 
           44  Motor stator 
           46  Electric motor 
           48  Motor inverter 
           54  Damper sleeve structure 
           56  Large diameter sleeve 
           56   a  Inward engagement portion 
           58  Spring guide 
           58   a  Outward engagement portion 
           62  Coil spring 
           64  Fixing pin 
           68  Sensor target 
           72  Rotation sensor 
           76  Elastic damper 
           78  Bypass passage 
           79  Bypass valve 
           80  Engine 
           82  Intake passage 
           84  Exhaust passage 
           86  Turbocharger 
           88  Low pressure exhaust gas recirculation passage 
           90  High pressure exhaust gas recirculation passage 
           92  ECU 
           104  Motor housing 
           114  Bearing 
         G Grease 
         a Supply air 
         s 1 , s 2  Gap