Abstract:
A centrifugal supercharger is provided. One embodiment of the supercharger comprises a two-piece housing wherein a parting area is substantially aligned with a rotational axis of a drive or impeller shaft. Another embodiment comprises a sleeve, or intermediate member disposed substantially between the housing and a bearing assembly(s) located within the supercharger housing. Another embodiment comprises a disengagement device located between the supercharger impeller and the engine. The disengagement device allows selective disengagement of the impeller from the engine. This Abstract is provided for the sole purpose of complying with the Abstract requirement rules that allow a reader to quickly ascertain the subject matter of the disclosure contained therein. This Abstract is submitted with the explicit understanding that it will not be used to interpret or to limit the scope or the meaning of the claims.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuation application of co-pending U.S. application Ser. No. 11/470,114 filed Sep. 5, 2006 entitled “Supercharger,” which is a continuation of U.S. application Ser. No. 10/698,192 filed Oct. 31, 2003, entitled “Supercharger,” now U.S. Pat. No. 7,128,061, issued Oct. 31, 2006. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to superchargers. More particularly, the invention concerns a centrifugal supercharger. 
       BACKGROUND OF THE INVENTION 
       [0003]    Superchargers have become pervasive in automobiles, boats, aircraft, and commercial stationary engines as the need to maximize power output has increased due to the use of smaller engines. Centrifugal superchargers employ a high-speed impeller to develop their boost pressure. Although such high-speed machinery places extreme demands on the associated drive machinery, e.g., bearings, seals, shafts, housing components, and the like, centrifugal compressors benefit from very high thermodynamic efficiencies, resulting in optimum engine outputs. 
         [0004]    Most centrifugal superchargers employ some sort of speed increasing mechanism to provide the rotation speed for the centrifugal compressor portion of the device to work. This mechanism, which is usually comprised of two parallel shafts with either a belt or gear system connecting them, requires matching cylindrical bores for the shafts and bearings. In the current art, a minimum of two bearing bores and two locating pin bores are machined in each part that comprise the supercharger case and cover, and the two are assembled like two halves of a clam shell, e.g. the separating plane of the individual case components is orthogonal to both shafts. This process requires eight (8) precision boring operations. A significant problem exists in manufacturing the very precise bores of the case components. For example, the accuracy needed to obtain the desired relationship between the two shafts requires true position and parallelism tolerances of 0.0005 inches. These extremely tight tolerances challenge the capabilities of even the newest and best state-of-the-art computer-controlled machining centers. Manufacturing these assemblies requires expensive and time-consuming set-up, machining, measuring and matching procedures. Even with very careful manufacturing procedures, a significant component rejection rate exists, due to parts that do not meet the strict tolerance requirements. 
         [0005]    In view of the above, there exists a need for an efficient supercharger that is easy to manufacture and service. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides a very efficient supercharger that is easy to manufacture and service. 
         [0007]    One feature of the present invention comprises a supercharger that has a case, or housing that is split into a primary section and a removable section. This two-piece housing greatly enhances and simplifies the ability to attain the required precision manufacturing tolerances. 
         [0008]    Another feature of the present invention comprises a sleeve, or intermediate member disposed substantially around a shaft located within the supercharger housing. The intermediate member may be used on the driveshaft, the impeller shaft, or may be used on both shafts. Between the intermediate member and the shaft are bearing assemblies that allow the shafts to rotate. One feature of the intermediate member is that it has a coefficient of thermal expansion (CTE) that is substantially similar to the CTE of the bearing assemblies. 
         [0009]    Yet another feature of the present invention comprises a disengagement device located between the supercharger impeller and the engine, or motor that drives the supercharger. The disengagement device allows selective disengagement of the impeller from the engine. 
         [0010]    A further feature of the present invention comprises an impeller shaft support designed to reduce mechanical stress and associated rotodynamic instabilities. 
         [0011]    Yet another feature of the present invention comprises a supercharger impeller having at least three sets of blades. A first set of primary blades has a first height and a set of secondary, or splitter blades have a second, shorter height. A third set of splitter blades has a third height that is less than the height of the second set of splitter blades. 
         [0012]    Another feature of the present invention comprises a supercharger having a modular compressor housing. The modular compressor housing includes two or more modular components. In one embodiment, the compressor housing includes a main housing and a shroud. In other embodiments, the compressor housing includes a main housing, a shroud and a diffuser. 
         [0013]    These and other features and advantages of the present invention will be appreciated from review of the following detailed description of the invention, along with the accompanying figures in which like reference numerals refer to like parts throughout. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a side view showing a supercharger, its driveshaft, and pulley arrangement attached to an engine; 
           [0015]      FIGS. 2A and 2B  are cross-sectional and exploded views, respectively, of a supercharger in accordance with the principles of the present invention; 
           [0016]      FIG. 2C and 2D  are plan, and elevation views, respectively, of an oil reservoir cover for use with the supercharger of the present invention; 
           [0017]      FIGS. 3A and 3B  are cross-sectional views of a sleeve assembly for use with the supercharger of the present invention; 
           [0018]      FIG. 3C  is an isometric view of an impeller shaft cartridge assembly for use with the supercharger of the present invention; 
           [0019]      FIG. 3D  is cross-sectional view of a portion of the supercharger of the present invention, illustrating a lubrication conduit and an end view of the impeller shaft and sleeve; 
           [0020]      FIGS. 4A and 4B  are exploded and cross-sectional views, respectively, depicting a disengagement device for use with the supercharger of the present invention; 
           [0021]      FIG. 4C  illustrates a graph of an impeller shaft acceleration/deceleration rate of a conventional supercharger; 
           [0022]      FIG. 4D  illustrates a graph of an impeller shaft acceleration/deceleration rate of a supercharger constructed according to one embodiment of the present invention; 
           [0023]      FIGS. 5A and 5B  are cross-sectional views of a spacer assembly for use with the supercharger of the present invention; 
           [0024]      FIGS. 6A and 6B  are perspective and side views, respectively, of an impeller for use with the supercharger of the present invention; and 
           [0025]      FIGS. 7A and 7B  are side and exploded views, respectively, of a modular compressor housing for use with the supercharger of the present invention. 
       
    
    
       [0026]    It will be recognized that some or all of the Figures are schematic representations for purposes of illustration and do not necessarily depict the actual relative sizes or locations of the elements shown. The Figures are provided for the purpose of illustrating one or more embodiments of the invention with the explicit understanding that they will not be used to limit the scope or the meaning of the claims. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    In the following paragraphs, the present invention will be described in detail by way of example with reference to the attached drawings. Throughout this description, the preferred embodiment and examples shown should be considered as exemplars, rather than as limitations on the present invention. As used herein, the “present invention” refers to any one of the embodiments of the invention described herein, and any equivalents. Furthermore, reference to various feature(s) of the “present invention” throughout this document does not mean that all claimed embodiments or methods must include the referenced feature(s). 
         [0028]    Referring to  FIG. 1 , a supercharger  10  constructed according to the present invention includes a driveshaft  12  for receiving rotational force from an engine  14  via a pulley and belt assembly  16 . More particularly, one end of the driveshaft  12  is attached to supercharger  10  and the opposite end is attached to the pulley and belt assembly  16 . In the illustrated embodiment, driveshaft  12  is depicted as relatively long with respect to the other engine components. However, driveshaft  12  may be considerably shorter such that the supercharger is in close proximity to the pulley and belt assembly  16  without departing from the scope of the present invention. Furthermore, driveshaft  12  may by comprised of an additional shaft member with supporting bearing structure such as described in U.S. Pat. No. 6,092,511 without departing from the scope of the present invention. 
         [0029]    Referring to  FIGS. 2A and 2B , supercharger  10  comprises driveshaft  12 , impeller shaft  20 , impeller  22 , compressor housing  24 , gear housing  26  and lubrication reservoir  28 . In operation, air is drawn through opening  24   a  in the compressor housing  24  and into impeller  22 . Impeller  22 , in conjunction with the compressor housing  24 , compresses the air before discharging it out of the compressor housing  24 . Preferably, impeller  22  is designed to discharge the air smoothly into compressor housing  26 , without substantial discontinuity or aerodynamic perturbation that may reduce performance. 
         [0030]    Driveshaft  12  is mechanically coupled to impeller shaft  20  such that rotation of the driveshaft imparts rotation on the impeller shaft  20 , thereby causing rotation of impeller  22 . The mechanical coupling between the input drive and impeller shafts includes a drive gear  30  disposed about driveshaft  12  and an impeller gear (not shown) disposed about impeller shaft  20 . In a preferred embodiment, the drive gear  30  has a larger circumference than the impeller gear, thereby causing the impeller gear to rotate faster than the drive gear  30 . 
         [0031]    As shown in  FIGS. 2A and 2B , the gear housing  26  defines a chamber that contains the drive gear and impeller gear. Gear housing  26  includes a primary section  26   a  and a removable section  26   b  configured to mate with the lower section. Removable section  26   b  is attached to primary section  26   a  by way of conventional removable fasteners  34  such as screws or bolts, which pass through apertures  34   a  in the removable section and corresponding apertures  34   b  in primary section. Gear housing  26  also contains driveshaft bearing assemblies  38   a,    38   b  disposed on either side of drive gear  30  and impeller shaft bearing assemblies  40   a,    40   b  disposed on either side of the impeller gear (not shown). Bearing assemblies  40   a,    40   b  may comprise single or multiple bearing elements. The bearing elements may be deep-groove or angular contact types, without departing from the scope of this invention. Advantageously, and in the case of multiple angular contact bearing elements, the bearing assemblies  40   a,    40   b  may be configured in tandem pairs (shown), or may be rigidly preloaded duplex sets, configured in either “DF” or “DB” arrangements. 
         [0032]    The impeller gear (not shown) is coupled to impeller shaft  20  such that the rotation of impeller gear imparts rotation to the impeller shaft and impeller  22 . Drive gear  30  is connected to driveshaft  12  such that the rotation of drive gear  30  imparts rotation to the impeller shaft  20 . 
         [0033]    As best seen in  FIG. 2B , removable gear housing section  26   b  includes a semicircular recess  31 , which, in combination with a corresponding recess  33  in primary gear housing section  26   a,  provides an opening dimensioned for the passage of driveshaft  12 . Gear housing  26  is thereby split in two sections along a dividing plane that is substantially parallel with the rotational axis of driveshaft  12 . In the illustrated embodiment, the dividing plane is substantially coplanar with the rotational axis of the driveshaft  12 . Removing the removable gear housing section  26   b  provides access to driveshaft  12 , drive gear  30  and driveshaft bearing assemblies  38   a,    38   b.  It will be appreciated that the gear housing  26  may be split in any number of different ways. For example, the gear housing  26  maybe split along a dividing plane that is substantially parallel with the impeller shaft  20 . Alternatively, the gear housing  26  may be split along multiple dividing planes that may be substantially parallel with both the impeller shaft  20  and the driveshaft  12 . Or, the gear housing  26  may be split along other suitable planes. 
         [0034]    One feature of this aspect of the invention is that the demanding manufacturing tolerances for the gear housing  26  are much easier to achieve, thereby increasing manufacturability, and decreasing waste generated by parts that are out-of-tolerance. In addition, the number of precision machining operations required to manufacture the gear housing  26  can be significantly reduced, e.g., from 8 individual boring operations to two. Advantageously, this reduces manufacturing costs. In addition, this invention feature adds rigidity to the supercharger  10 , and maximizes the manufacturing precision, thereby resulting in improved alignments between gears and shafts for smoother, quieter operation, simplified manufacturing processes, and reduced overall manufacturing costs. 
         [0035]    Again referring to  FIGS. 2A ,  2 B and  4 A, gear housing  26  preferably includes a cover plate  42 , that when removed provides access to the impeller shaft  20 , impeller gear (not shown) and impeller shaft bearing assemblies  40   a,    40   b.  The cover plate  42  includes an aperture  44  dimensioned for the passage of the driveshaft. The cover plate  42  is removably attached to the gear housing primary section  26   a  by way of cover plate fasteners  46  such as screws, bolts or equivalents, which pass through cover plate apertures  48 , and into corresponding gear housing apertures  50  in the gear housing primary section  26   a.  In addition, the cover plate  42  is attached to the gear housing removable section  26   b  by way of conventional fasteners  46  such as screws, bolts or equivalents, which pass through cover plate apertures  48 , and into corresponding gear housing apertures  52  in the gear housing removable section  26   b.    
         [0036]    Some centrifugal superchargers employ the existing lubrication system of the host engine for the supercharger lubrication. However, there exist several advantages of having a self-contained supercharger lubrication system, wherein the supercharger&#39;s lubricating fluid is separate from the engine&#39;s lubricating fluid. One advantage of a self-contained lubrication system is simplification and ease of installation. Some existing supercharger self-contained lubrication systems utilize a splash system wherein one or more gears are dipped into an oil bath. However, these designs suffer from the disadvantage that built-up heat cannot be discharged. 
         [0037]    Referring again to  FIG. 2A , according to another embodiment of the present invention, lubrication reservoir  28  is self-contained within the gear housing  26  such that the supercharger  10  does not require lubrication to be drawn from an external source, such as the engine  14 . Additionally, in another embodiment of the present invention, the lubrication reservoir  28  is preferably separate and detachable from the gear housing  26 , thereby reducing service and repair costs. Lubrication reservoir  28  further includes at least one lubrication inlet  54  and at least one lubrication outlet  56 . The lubrication is preferably either in the form of oil, such as engine oil, or in the form of an oil-air mist delivered by appropriate means such as an atomizer (not shown). Advantageously, in a preferred embodiment, hot lubricating fluid is drained into the lubrication reservoir  28  via the lubrication inlet  54  and allowed to cool before being recirculated. 
         [0038]    Some superchargers provide an air-assist approach to augmenting lubricating oil circulation within the supercharger gearcase. Generally, the air assist approach results in an air-oil mist lubrication, which aids in achieving reliable operation and the minimization of bearing assembly failure. 
         [0039]    In one embodiment of the present invention, the supercharger  10  preferably includes an air assist approach, wherein compressed air from the supercharger  10  is introduced into the lubricating oil by use of a mixing air-assist nozzle assembly (not shown). Such an air-assist assembly may be similar to one described in U.S. Pat. No. 6,293,263. In operation, engine oil, under pressure, mixes with supercharger discharge air, also under pressure, and introduces an air-oil lubricating mist into the supercharger. The lubricating mist is preferably directed towards the supercharger  10  internal gear, shaft, and bearing components. 
         [0040]    One advantage of using an oil/air mist is that the oil can be readily sprayed onto the gears and bearings, thereby maximizing gear and bearing life. Further, the pressurized air atomizes the oil and improves distribution and also assists in driving the oil out of the gear housing  26  after use (and into the lubrication reservoir  28 ), thereby minimizing the oil cycle time in the gear housing  26 , and providing improved lubrication and cooling of the gears and bearings. 
         [0041]    Referring to  FIGS. 2A , and  2 C-D, some embodiments of the present invention may include a reservoir  28  having a reservoir baseplate  29  that may include inlet and outlet ports  32  for the circulation of cooling fluid or water. As shown in  FIG. 2D , such an embodiment incorporates passageways communicating with the inlet and outlet ports  32 , but that do not communicate with reservoir  28 . The passageways supply cooling fluid to the heat transfer elements  35 , that are in contact with any lubricating oil within reservoir  28 . The cooling fluid can be provided from a variety of sources including the engine cooling system, or in the case of a marine application, lake or sea water. Advantageously, as shown in  FIGS. 2C-D , the heat transfer elements  35 , are attached-to or cast-into the baseplate  29  and provide improved cooling performance. 
         [0042]    Referring now to  FIGS. 3A-D , the precision bearing fit and alignment required for high-speed supercharger operation is often difficult to maintain. One problem stems from the intrinsic difference in the coefficient of thermal expansion (CTE) between the bearing assemblies, which are typically ferrous-based, and the gear housing, which is usually made of aluminum. For example, the CTE for aluminum is relatively high (0.00001244 unit length change, per degree Fahrenheit) when compared to ferrous materials such as cast iron (0.00000655), carbon steel (0.00000533), and 440C stainless steel (0.0000056). Most bearing assemblies, such as those used by the present invention, are comprised of steel or ceramic (Silicon Nitride) rolling elements, retained in angular position and alignment by a cage, and interposed between inner and outer steel races. Typical material of the steel races would be SAE52100 ferrous-based steel, although other ferrous-based materials may be used including 440C, and martensitic Chromium steels with homogeneous carbonitride microstructure. 
         [0043]    As shown in  FIGS. 3A-D , according to another aspect of the present invention, an intermediate member, sheath, or sleeve  60  is disposed around the impeller shaft bearing assemblies  40   a,    40   b.  Sleeve  60  preferably comprises a ferrous-based material having a CTE that is substantially similar to the CTE of the bearing assemblies  40   a,    40   b.  According to some embodiments, the CTE of the sleeve preferably includes a CTE that may range between about 0.000004 and about 0.000007 in/in-° F. (i.e., 4.0×10 −6 , and 7.0×10 −6  in/in-° F.). Suitable ferrous-based materials for the sleeve  60  include, but are not limited to, grade G2 gray iron, DURA-BAR®, free-machining steels such as 12L14, and all other ferrous-based materials having a CTE that is substantially similar to the CTE of the bearing assemblies  40   a,    40   b  (DURA-BAR is a registered trademark of Wells Manuf. Co. of Skokie, Ill.). 
         [0044]    As shown in  FIGS. 3A-D , the sleeve  60  includes an opening  62  for gear engagement. Additionally, the sleeve  60  includes a lubrication conduit  64  in fluid communication with a lubrication oil supply conduit  51 , and lubrication apertures  65  in fluid communication with lubrication conduit  64 . Lubricating oil may then drain back to reservoir  28  via drain port  66 , which is aligned to be in communication with port  54  (shown in  FIG. 2A ). It will be appreciated that the sleeve, or sheath  60  may comprise any configuration that results in the sleeve, or intermediate member being positioned between the bearing assemblies  40   a,    40   b  and the gear housing  26 . The intermediate-member may also be comprised of more than one component. 
         [0045]    According to some embodiments, the intermediate member, or sleeve  60  is pressed or shrink-fitted into the gear housing  26 . In other embodiments, sleeve  60  may be installed with a clearance fit into housing  26 , and retained thereto by a fastener, or other suitable device. 
         [0046]    Referring now to  FIG. 3D , in the illustrated embodiment, a replaceable shaft-bearing cartridge  68  comprises sleeve  60 , bearing assemblies  40   a,    40   b,  and impeller shaft  20 . The shaft-bearing cartridge  68  installs into supercharger primary section  26   a  with a slight clearance fit, resulting in an annular gap  67 , interposed between sleeve  60  and primary section  26   a.  In one embodiment, the annular gap  67  may range from about 0.0015 inch to about 0.0002 inch. This gap may change with any change in temperature of the sleeve  60  or the primary section  26   a.    
         [0047]    In a preferred embodiment, lubricating oil, supplied under pressure via conduit  51 , which is in communication with conduit  63 , is forced into annular gap  67  and creates a hydrostatic supporting force, which reacts to gear loads during supercharger  10  operation. Advantageously, this hydrostatic load supporting mechanism also promotes vibration damping characteristics, resulting in quieter operation of the supercharger  10 . 
         [0048]    One feature of the sleeve  60  is that it maintains the bearing assemblies  40   a,    40   b  securely in the gear housing  26  during a range of supercharger  10  operating temperatures. More importantly, the fit between bearing races  40   a,    40   b  and sleeve  60  are maintained regardless of operating temperature. This is achievable because the CTE&#39;s of the sleeve  60  and the bearing assemblies  40   a,    40   b  are substantially matched, thereby expanding and contracting in unison. This feature is especially beneficial to the high-speed impeller shaft  20  bearings  40   a,    40   b,  which may operate at speeds exceeding 60,000 RPM. It will be appreciated that a sleeve(s)  60  may also be placed around the driveshaft bearing assemblies  38   a,    38   b.    
         [0049]    Referring now to  FIG. 3C , the shaft-bearing cartridge  68  may be employed as an insertable device that lends itself to manufacturing and assembly advantages in addition to the aforementioned thermal stability advantage. For example, the shaft-bearing cartridge  68  permits pre-assembly which allows it to be inserted and/or removed as a single unit, thereby reducing service and repair costs. Additionally, the use of a pre-assembled, replaceable shaft-bearing cartridge  68  allows repairs to be performed in the field. 
         [0050]    Referring to  FIGS. 4C-D , superchargers can experience very fast drive- and impeller shaft acceleration rates. The acceleration rates are amplified by the step-up ratio between the driveshaft  12  and the impeller shaft  20 , which is typically in the range of 3:1 to 5:1 (i.e., 3 to 1 and 5 to 1). That is, the impeller shaft  20  may rotate five times faster than the driveshaft  12 . High acceleration and deceleration forces, generally caused by “blipping” the engine, can stress the impeller shaft  20  and its related components, and cause de-stabilizing effects of bearings  40   a,    40   b,  sufficient to cause catastrophic failure. However, the most severe stresses and bearing instabilities generally occur during the transition from very high to relatively slow impeller shaft  20  rotational speeds. An extreme example would be a very rapid rotational acceleration immediately followed by a very rapid deceleration. Such an acceleration rate with the peak point of destabilization is depicted in  FIG. 4C . 
         [0051]    Again referring to  FIGS. 4A and 4B , according to another feature of the present invention, the supercharger  10  preferably includes a disengagement device  70  for disengaging the impeller  22  from the engine  14 . In the illustrated embodiment, the driveshaft  12  is disengageable from the engine  14 . As best seen in  FIG. 4B , the disengagement device  70  is disposed between the driveshaft  12  and the primary drive pulley  72 . According to some embodiments, the disengagement device  70  comprises a one-way clutch, such as a sprag, overrunning clutch, or other suitable device. In a preferred embodiment, the disengagement device  70  is preferably integrated into the primary drive pulley  72 , which may also comprise part of belt and pulley system  16 , as described in  FIG. 1 . 
         [0052]    As shown in  FIG. 4B , in a preferred embodiment, the disengagement device  70  comprises a sprag clutch  71  located between the primary drive pulley  72  and the driveshaft  12 . A sprag clutch employs sprags (not shown), that due to their oblong shape, wedge between driveshaft  12  and the outer sprag bearing race  78 , when rotation occurs in a first direction, but allow driveshaft  12  and outer sprag race  78  to move independently of each other when rotation occurs in the opposite direction. Furthermore, upon rapid deceleration of the primary drive pulley  72  rotational speed, the sprag clutch  71  disengages and allows driveshaft  12 , drive gear  30 , impeller shaft  20 , and impeller  22  to overrun and gently coast to a reduced rotational speed. As shown in  FIG. 4D , the feature of the present invention dramatically reduces the peak destabilizing event, or rapid deceleration. The wedging action of the sprags locks driveshaft  12  and the outer sprag bearing races  78  together, thereby enabling the transfer of rotational force, or torque between the engine  14  and the driveshaft  12 . 
         [0053]    By way of example, a FORMSPRAG® sprag clutch (part number CL42875) can be used as the clutch in the present invention (FORMSPRAG is a registered trademark of Dana Corporation of Toledo, Ohio). Of course, other types of clutches, including, but not limited to roller clutches, spring clutches, centrifugal clutches, friction clutches, non-friction clutches, mechanical clutches, pneumatic clutches, hydraulic clutches, electrical clutches, diaphragm clutches and hysteresis clutches, can be employed without departing from the scope of the present invention. It will be appreciated that the disengagement device  70  may be located anywhere between the engine  14  and the impeller  22 . For example, the disengagement device  70  may be located between the driveshaft  12  and the impeller shaft  20 , or between the impeller shaft  20  and the impeller  22 . 
         [0054]    According to other embodiments, the disengagement device  70  may comprise a speed-sensitive engagement mechanism such as a traditional centrifugal clutch. Alternatively, the disengagement device  70  may comprise both a speed-sensitive engagement feature and an overrunning or disengaging feature. Advantageously, the speed-sensitive engagement feature permits the supercharger  10  to be substantially disengaged from the engine  14  during very low speed operation and engine idle, when supercharger  10  noise maybe objectionable. 
         [0055]    High-performance superchargers (such as for competitive drag racing applications) require high rotational speeds that create high air-flow and pressure ratios, thereby creating significant rotordynamic problems and challenges. One such problem is the inherent lack of stiffness at the impeller-to-impeller shaft shoulder connection point. In a typical supercharger, the impeller abuts against a spacer, which in turn abuts against a shoulder on the impeller shaft. The diameter of the impeller shaft shoulder is normally only slightly larger than the diameter of the impeller shaft, thereby resulting in a relatively low bending stiffness in the region between the impeller and the adjacent support bearing. Low stiffness in this region may result in impeller shaft bending at rotational speeds that are within the range of the supercharger&#39;s high-speed operation, giving rise to rotordynamic critical speeds, identified by dynamic instabilities and/or excessive vibration. Excessive impeller shaft bending and associated dynamic instabilities frequently results in the impeller contacting the compressor housing, causing catastrophic failure of the impeller. 
         [0056]    Referring to  FIGS. 5A and 5B , another feature of the present invention is illustrated. A spacer assembly  80  is disposed around the impeller shaft  20  between the impeller  22  and the impeller shaft inner bearing race  81 . The impeller shaft  20  comprises a distal section  20   a,  which is adjacent to the impeller  22 , and has a first diameter. A proximal section  20   b  is adjacent to the impeller shaft inner bearing race  81 , and has a second, larger diameter. The first and second impeller shaft sections  20   a,    20   b  meet at a transition section  20   c.  The spacer assembly  80  comprises a tubular spacer  84  disposed between the impeller  22  and the transition section  20   c  and an impeller spacer  82  disposed between the tubular spacer  84  and the base of the impeller  22 . The two spacers  82 ,  84  mechanically couple the distal impeller shaft section  20   a  to the impeller shaft inner bearing race  81 , resulting in a much stiffer construction and a significant reduction in vibration between components. Put differently, the tubular spacer  84  adds additional support to the distal impeller shaft section  20   a  by contacting, and supporting the impeller spacer  82  at a diameter that is approximate to the diameter of the impeller shaft inner bearing race  81 . 
         [0057]    As best seen in  FIG. 5A , transition section  20   c  preferably comprises a curvilinear taper providing a gradual transition between the first and second impeller shaft sections. In the illustrated embodiment, transition section  20   c  is substantially concave. However, as would be understood to those of ordinary skill in the art, transition section  20   c  may also be substantially convex or substantially straight, without departing from the scope of the present invention. Advantageously, the transition section  20   c  is configured to significantly reduce impeller shaft stress at critical rotational speeds. More particularly, the tubular spacer  84  allows the transition section  20   c  to be shaped in a preferred configuration, e.g., a fillet with generous radius, thereby dramatically increasing the fatigue resistance of the impeller shaft  20 . This is because the transition section  20   c  can be shaped to minimize localized stresses, thereby eliminating or minimizing the formation of fatigue cracks. 
         [0058]    Referring now to  FIG. 5B , other advantages of replaceable shaft-bearing cartridge  68  become apparent. In this preferred embodiment, bearings  40   a,    40   b  are of the angular contact type, and are mounted as duplex tandem pairs, known in the art as “DT”, with the pairs, in turn mounted “back-to-back” to each other. Bearings  40   a  are firmly retained to impeller shaft  20  proximal section  20   b  by retaining washer  86  and threaded fastener  87 , which engages a mating threaded receptacle in proximal section  20   b.  Bearings  40   b  are retained by spacers  84 ,  82 , impeller  22 , washer  88  and impeller fastener  89 , which engages a mating threaded portion of distal section  20   a.  Preferably, a static preload force should be applied in order to maintain stability of  40   a,    40   b.  Preload is provided by spring elements  83 , which generate a preload force against retainers  85 . In this preferred embodiment, the preload force may range from about 50 lbf to about 400 lbf. 
         [0059]    Alternative embodiments are also possible, and these are described and incorporated herein as within the scope of the present invention. In one such embodiment, angular contact bearings  40   a,    40   b  may be configured as rigidly preloaded duplex sets, and mounted either back-to-back (known in the art as “DB”) or face-to-face (known in the art as “DF”). Advantageously, the clamping forces acting on bearings  40   a,    40   b  inner races are developed by threaded fastener  87  and impeller fastener  89 , which in turn enable the rigid preloading of bearings  40   a,    40   b.    
         [0060]    Referring now to  FIGS. 6A and 6B , high performance superchargers often have air, or gas flow rates that exceed 200 lbm/min. and pressure ratios exceeding 3.0 (i.e., pressures greater than three times ambient atmospheric pressure). Of course, this places extraordinary demands on most centrifugal superchargers and their associated impellers. Proper impeller design is critical for the overall performance of the supercharger. A primary impeller design challenge involves attaining sufficient airflow performance without resorting to undesirable designs. An example of an undesirable design is an impeller having excessively large passageways, which preclude aerodynamic choke, but result in poor blade loading and other deleterious effects. 
         [0061]    On one hand, it is desirable to have a low blade count at the impeller inlet to decrease aerodynamic blockage and increase airflow. On the other hand, in order to increase impeller efficiency, a high blade count is preferred further along the airflow passageway (especially near the impeller outlet). Such a design allows the specific impeller work (e.g., total work per unit blade) to be reduced, thereby reducing blade loading effects to more efficient levels. 
         [0062]    Referring to  FIGS. 6A and 6B , another feature of the present invention is illustrated. An impeller  22  suitable for use with the supercharger  10  is shown. Impeller  22  preferably comprises at least three sets of blades including primary blades  22   a,  secondary blades  22   b,  and tertiary blades  22   c.  In the illustrated embodiment, the impeller  22  comprises a set of primary blades  22   a  having a first height, a set of secondary blades  22   b  having a second height, and a set of tertiary blades  22   c  having a third height. The blade heights are configured such that the first height is greater than the second height, which is greater than the third height. As would be understood to those of ordinary skill in the art, the impeller  22  may consist of additional or fewer sets of blades having different heights without departing from the scope of the present invention. 
         [0063]    As depicted in  FIG. 2A , air or other gasses are drawn into the impeller through opening  24   a  in the compressor housing  24 . Referring to  FIG. 6B , the air enters the impeller  22  through the inlet region  90 , which has a relatively low blade count since the secondary blades  22   b  and tertiary blades  22   c  do not extend up to the top of the impeller  22 . The air is compressed as it travels through a middle region  92  having a relatively medium blade count and a lower region  94  having a relatively high blade count since all three sets of blades extend through this region. 
         [0064]    Specifically, in a preferred embodiment (as shown in  FIGS. 6A and 6B ) of the present invention, the impeller  22  would include five primary blades  22   a,  five secondary blades  22   b,  and 10 tertiary blades  22   c.  Alternative embodiment impellers  22  may have a range of 3 to 9 primary blades  22   a,  with 3 to 9 secondary blades  22   b,  and 6 to 18 tertiary blades  22   c.  It will be appreciated that other blade numbers and/or arrangements may be employed without departing from the scope of the present invention. 
         [0065]    One feature of this aspect of the invention is that the relatively low blade count within inlet region  90  induces a low density air flow that minimizes aerodynamic blockage. Conversely, the relatively high blade count within outlet region  96  provides excellent aerodynamic performance by minimizing blade loading. 
         [0066]    Referring now to  FIGS. 7A and 7B ; centrifugal compressors for superchargers commonly employ an exit assembly such as a compressor housing or volute. Compressor housings are often complex structures that pose both design and manufacturing difficulties. By way of example, one manufacturing problem involves providing access to the inner flow path passage for cleaning (e.g., polishing) and/or maintenance. Other manufacturing problems relate to installing and supporting the core in the mold when casting the compressor housing. Complex cores result in unacceptably high reject rates, but simpler cores limit design options in the critical diffuser region. 
         [0067]    As shown in  FIGS. 7A and 7B , another feature of the present invention is illustrated. A modular compressor housing  24  suitable for use with the supercharger  10  is depicted. The modular compressor housing  24  comprises at least two modular components as opposed to a single casting. In the illustrated embodiment, modular compressor housing  24  comprises three modular components including a main housing or scroll  98 , a shroud  100  and a backplate  102 . As an assembly, shroud  100  and backplate  102  form an annular space or diffuser passageway  104 . Alternatively, two of the three components can be combined into a single component, thereby forming a, modular compressor housing  24  having two components. For example, the shroud  100  and scroll  98  may be combined into a single component. 
         [0068]    As shown in a preferred embodiment of  FIG. 7A , diffuser passageway  104  is curved approximately 45° toward the axial direction, resulting in a more compact overall dimension of compressor housing  24 . Advantageously, curved diffuser passageway  104  affords a reduction in compressor housing  24  dimension without unduly shortening the length of diffuser passageway  104 . Shortening the length of the diffuser passageway reduces the maximum pressure recovery attainable from the diffuser, which deleteriously affects performance of the compressor stage. The amount of curvature toward the axial may range from 20° to 60° without departing from the scope of this feature of the invention. 
         [0069]    Referring to  FIG. 7B , shroud  100  may be cast and machined separately and attached to the main housing  98  using fasteners such as screws, bolts, or other suitable fasteners. The backplate  102  may be attached to the main housing  98  by way of force-fit or friction fit, thereby covering the shroud  100 . Alternatively, the backplate  102  may be attached using suitable removable fasteners. Advantageously, by removing the backplate  102  and shroud  100  components, the interior of the compressor housing  24  is accessible for blending, de-burring, polishing, cleaning, and/or maintenance. Additionally, the compressor housing  24  may incorporate alternative diffusers including, but not limited to, vaneless diffusers, channel or wedge diffusers and low-solidity vane diffusers. Advantageously, the modular design of the compressor housing  24  permits different diffusers to be installed, thereby enabling compressor “tuning.” This reduces the number of parts that must be maintained in stock, thus reducing costs. Also advantageously, the modular design affords ease of manufacture of the curved diffuser passageway  104 . 
         [0070]    Thus, it is seen that a centrifugal supercharger is provided. One skilled in the art will appreciate that the present invention can be practiced by other than the above-described embodiments, which are presented in this description for purposes of illustration and not of limitation. The description and examples set forth in this specification and associated drawings only set forth preferred embodiment(s) of the present invention. The specification and drawings are not intended to limit the exclusionary scope of this patent document. Many designs other than the above-described embodiments will fall within the literal and/or legal scope of the following claims, and the present invention is limited only by the claims that follow. It is noted that various equivalents for the particular embodiments discussed in this description may practice the invention as well.