Abstract:
In the generic exhaust-gas turbocharger, the turbine-side and compressor-side radial bearing bush are of identical design. A more compact bearing space, and improvements in acoustics and rotor dynamics, are achieved by providing different bearings on the turbine and compressor sides.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to an exhaust-gas turbocharger according to the preamble of claim  1 . 
     Description of the Related Art 
     In the generic exhaust-gas turbocharger, the outer and inner bearing widths of the turbine-side radial bearing bush are arranged axially within the bearing bore of the bearing housing. Here, the bearing spacing is the spacing between the axial centers of the two bearing bushes. Said bearing spacing has a significant influence on the rotor stability, which is also determined to a large extent by the design of the inner and outer lubricating oil gaps. 
     On the other hand, owing to the demand for ever more compact installation spaces, as small a bearing space as possible is required, which adversely affects the stability of the bearing. 
     It is therefore an object of the present invention to provide an exhaust-gas turbocharger of the type specified in the preamble of claim  1  which permits an improvement in rotor stability and also in acoustic properties while maintaining at least approximately the same bearing housing dimensions. 
     BRIEF SUMMARY OF THE INVENTION 
     Said object is achieved by means of the features of claim  1 . 
     In contrast to the prior art, in which at least substantially the compressor-side bearing and the turbine-side bearing are of identical design, the present invention is based on the realization that, to obtain an improvement in the bearing arrangement of the rotor shaft in terms of acoustics and at the same time rotor dynamics, different bearings are provided on the turbine and compressor sides as a result of unequal loadings. On account of the different masses of the compressor wheel and turbine wheel and the spacings thereof from the bearing point, the overall rotor has a center of mass which does not lie in the geometric center between the bearing points, and therefore leads to unequal bearing loadings. 
     Here, the acoustic properties of the bearing are determined by subharmonic vibrations generated in the oil film. The characteristics of the oil film in turn are determined by the geometric dimensions of the bearings. The variable components of the bearings are basically the rotor shaft to be mounted, the bearing bushes of the bearings, and the bearing housing or those bearing housing regions in which the bearing bushes of the bearing arrangement of the exhaust-gas turbocharger according to the invention are arranged. 
     For this purpose, it is firstly basically possible for the shaft diameter of the rotor shaft in the region in which the bearing bushes are arranged to be varied, in particular increased in relation to the shaft section between the bearings. For this purpose, a slightly beveled surface may be provided proceeding from the region between the bearings, which beveled surface permits a gradual increase in the shaft diameter. It is alternatively likewise possible to provide a shaft shoulder which permits a stepped increase in shaft diameter. 
     The bearing bushes themselves may be of different design at the compressor side and turbine side in terms of their outer diameter and/or their inner diameter. It is accordingly possible for the outer diameter and/or the inner diameter either on the compressor side or on the turbine side to be increased or decreased in relation to the in each case other side. 
     It is also possible for the bearing width at the inside and/or at the outside to be designed differently. 
     Finally, the shape of the bearing surfaces may be designed differently. It is for example possible for grooves to be formed in one of the bearing surfaces (for example on the compressor side) while the other bearing surface (in this case the turbine-side bearing surface) can be formed without grooves, that is to say with a smooth surface. It is of course likewise possible to provide or omit the grooves on the in each case other side. It is likewise conceivable for grooves to be provided on the outer circumferential surface of the bearing bushes. In this case, too, either the compressor side or the turbine side is provided with grooves or formed without grooves on the outer circumferential surface. 
     Finally, the receiving bores of the bearing housing for the bearing bushes may be designed differently on the compressor side and on the turbine side. This again relates to the diameter of the receiving bores, the width of the receiving bores and the shape of the receiving bores, wherein it is again possible for grooves to be provided either on the compressor side or on the turbine side and for no grooves to be formed on the other side. For this purpose, it is preferable for sickle-shaped grooves to be provided in the bearing housing bore in that partial circumferential region in which the oil supply bore opens out. 
     The subclaims relate to advantageous refinements of the invention. 
     Since the axial extent of the turbine-side oil collecting chamber is utilized for mounting the rotor, it is possible to increase the inner bearing spacing (between the rotor and bearing bushes) without having to change the bearing housing dimensions. Here, the inner overall bearing surface width of the bearing bush is unchanged, but is preferably split into two bearing surface regions. The installation space required for this purpose on the rotor can be obtained by modifying the contact shoulder of the rotor. 
     It is also possible to use the improved rotor stability to reduce the bearing housing dimensions (reduction of the bearing spacing). 
     It is also possible for the bearing bush to be of symmetrical design in order to simplify the assembly of the bearing arrangement. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Further details, advantages and features of the present invention will emerge from the following description of exemplary embodiments on the basis of the appended drawing, in which: 
         FIG. 1  shows a schematically slightly simplified illustration of a first embodiment of an exhaust-gas turbocharger according to the invention, 
         FIG. 2  shows an enlarged perspective illustration of a turbine-side bearing bush circled in  FIG. 1 , 
         FIG. 3  shows a sectional illustration of the bearing bush according to  FIG. 2 , 
         FIG. 4  shows an illustration, corresponding to  FIG. 1 , of a second embodiment of the exhaust-gas turbocharger according to the invention, 
         FIG. 5  shows an illustration, corresponding to  FIG. 2 , of a turbine-side bearing bush circled in  FIG. 4 , 
         FIG. 6  shows a sectional illustration, corresponding to  FIG. 3 , of the bearing bush according to  FIG. 5 , 
         FIG. 7  shows an illustration, corresponding to  FIGS. 1 and 4 , of a third embodiment of the exhaust-gas turbocharger according to the invention, 
         FIG. 8  shows an illustration, corresponding to  FIGS. 2 and 5 , of an alternative embodiment of the turbine-side bearing bush, 
         FIG. 9  shows a sectional illustration, corresponding to  FIGS. 3 and 6 , of the bearing bush according to  FIG. 8 , 
         FIG. 10  shows an illustration, corresponding to  FIG. 1 , of a fourth embodiment of the exhaust-gas turbocharger according to the invention, 
         FIG. 11  shows an illustration, corresponding to  FIG. 1 , of a fifth embodiment of the exhaust-gas turbocharger according to the invention, 
         FIG. 12  shows an illustration, corresponding to  FIG. 1 , of a sixth embodiment of the exhaust-gas turbocharger according to the invention, 
         FIG. 13  shows an illustration, corresponding to  FIG. 1 , of a seventh embodiment of the exhaust-gas turbocharger according to the invention, 
         FIG. 14  shows an illustration, corresponding to  FIG. 1 , of an eighth embodiment of the exhaust-gas turbocharger according to the invention, 
         FIG. 15  shows an illustration, corresponding to  FIG. 1 , of a ninth embodiment of the exhaust-gas turbocharger according to the invention, and 
         FIG. 16  shows an illustration, corresponding to  FIG. 11 , but with the two bearings and bearing spacer combined into a unitary bearing and spacer. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A first embodiment of the exhaust-gas turbocharger  1  according to the invention will be explained below on the basis of  FIGS. 1 to 3 . The exhaust-gas turbocharger  1  is illustrated in schematically simplified form in  FIG. 1 . Said exhaust-gas turbocharger  1  has a compressor  2 , a turbine  3  and a bearing housing  4 . The bearing housing  4  is connected at one end to the compressor  2  and at the other, turbine-side end  5  to the turbine  3 . The bearing housing  4  comprises an oil collecting chamber  6  at the turbine-side end  5  and a bearing arrangement  7  for a rotor shaft  8 , wherein the bearing arrangement  7  has a compressor-side bearing bush and, spaced apart axially from the latter, a turbine-side bearing bush  10 . As can be seen from  FIGS. 2 and 3 , the turbine-side bearing bush  10  has an inner bearing surface  11  and an outer circumferential surface  12 . The bearing is a plain or conventional bearing, e.g., a radial, hydrodynamic, rotating or non-rotating, with one or two bushes) as distinguished from a roller bearing. 
     As illustrated by a circled region of  FIG. 1 , the turbine-side bearing bush  10  extends into the region of the oil collecting chamber  6 . The utilization of the oil collecting chamber  6  for the axial extent of the bearing bush  10  has the advantage that the rotor stability is improved as a result of the increase in the inner bearing spacing between the rotor and bearing bushes. 
     As illustrated in  FIG. 3 , the inner bearing surface  11  is split into two spaced-apart bearing surface regions  13 ,  14 , wherein the bearing surface regions  13 ,  14  are separated from one another by a radially outwardly recessed clearance  15 . Despite the split into two bearing surface regions  13 ,  14 , the overall bearing surface width of the bearing bush remains unchanged. The definition “radially outwardly” is to be understood here to mean an offset in the direction of the arrow R which is at right angles to the longitudinal axis L of the turbocharger  1  (see  FIG. 1 ). 
     In a second embodiment, the turbine-side bearing bush  10  is of symmetrical design with a continuous outer circumferential surface  12 , as shown in  FIG. 8 . 
     As shown in the illustration of  FIG. 5 , in a further embodiment, the turbine-side bearing bush  10  is formed with an outer circumferential surface  12  divided by a shoulder  16  into two outer circumferential surface regions  17 ,  18 . 
     In the following embodiments of the exhaust-gas turbocharger  1  according to the invention, explained on the basis of  FIGS. 10 to 15 , the corresponding technical features are denoted by the same reference numerals as in the preceding embodiments, such that with regard to said corresponding features, reference may be made to the description above. 
       FIG. 10  shows a variation of the inner and outer diameters of the bearing bushes  9  and  10 . As a result of the increases in diameter on the compressor side (bearing bush  9 ), the inner lubricating gap is made larger and the outer gap is made smaller. 
       FIG. 11  illustrates the bearing bushes  9  and  10  with a modified form of the inner bearing surface  11 , by means of the formation of an axially parallel axial groove or a groove which runs in spiral fashion in the axial direction. 
       FIG. 12  shows the possible influences of the bearing housing bore on the oil film. Firstly, the diameter at the bearing points  9 ′ and  10 ′ of the compressor side and turbine side may be selected to be different, and secondly, the outer bearing point width can be influenced, as illustrated in  FIG. 13 .  FIG. 14  shows the modification of the bearing surface form by means of a sickle-shaped groove  20 . Here, in one circumferential segment of the bearing point bore, a groove for better distribution of the supplied oil is formed in the region of the oil supply bore by means of eccentric machining. 
       FIG. 15  shows different geometries of the rotor shaft  8  for influencing the inner lubricating gap. Here, the shaft  8  may have a changed diameter at the bearing points  9 ′,  10 ′, which changed diameter is realized either by a step  21  or by a conical transition  22 . 
       FIG. 16  shows the an exhaust-gas turbocharger ( 1 ) corresponding to  FIG. 11 , but wherein the bearing bushes ( 9 ′″,  10 ′″) and spacer arrangement ( 19 ′) are combined to form one component. 
     All the illustrated possible variations may be combined without restriction, such that different designs are realized in each case on the compressor side and on the turbine side. 
     Furthermore, the bearing bushes  9 ,  10  are held axially spaced apart along the rotor shaft  8  by means of a spacer  19 , as illustrated in  FIGS. 1, 4 and 7 . 
     In addition to the above written disclosure of the invention, reference is made explicitly to the diagrammatic illustration in  FIGS. 1 to 15 . 
     LIST OF REFERENCE SYMBOLS 
     
         
           1  Exhaust-gas turbocharger 
           2  Compressor 
           3  Turbine 
           4  Bearing housing 
           5  Turbine-side end 
           6  Oil collecting chamber 
           7  Bearing arrangement/plain bearing arrangement 
           8  Rotor shaft 
           9  Compressor-side bearing bush 
           9 ′ Compressor-side bearing/bearing point 
           9 ″ Compressor-side receiving bore in the bearing housing 
           10  Turbine-side bearing bush 
           10 ′ Turbine-side bearing/bearing point 
           10 ″ Turbine-side receiving bore in the bearing housing 
           11  Inner bearing surface 
           12  Outer circumferential surface 
           13 ,  14  Spaced-apart bearing surface regions 
           15  Recessed clearance 
           16  Shoulder 
           17 ,  18  Outer circumferential surface regions 
           19  Spacer arrangement 
           20  Sickle-shaped groove 
           21  Step 
           22  Conical transition 
         R Direction of the offset of the clearance  15   
         L Longitudinal axis