In exhaust-gas turbochargers with rotating or static floating-bush-type radial bearings, noises can arise as a result of the excitation of vibration modes of the rotor. In particular, the so-called constant frequency noise (also subsynchronous tone, whis- tling, miaowing) is a noise generated by the cylindrical eigenmode (sub 2) of the rotor. During the operation of such exhaust-gas turbochargers in vehicles, said noises represent a considerable noise nuisance for the vehicle driver.

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 at least a substantial elimination of said noise nuisance.

This object is achieved by the features of claim 1.

Tests carried out within the context of the invention have shown that the main excitation mechanism for the stated noises is a revolving squeeze oil film. The exhaust- gas turbocharger according to the invention permits a disruption of the rotation of the oil film and/or an increase of the bearing force, which leads to a disruption of the oil rotational movement, and the generation of constant frequency noise is thus significantly reduced.

In detail, the truncated-cone shape of the inner or outer geometry of the bearing bush results, in accordance with Bernoulli's principle, in a pressure gradient in the axial direction, which pressure gradient causes the oil to be accelerated axially through the bearing and the rotational movement of the oil film to be disrupted. The rotational symmetry and the uninterrupted bearing surface have the advantage over known spline groove bushes (see DE 39 36 069 C2) that the bearing forces can be absorbed without interruption. The truncated cone narrowing (that is to say the direction of the truncated cone) of the inner and/or outer geometry may be directed toward the same axial side or different axial sides. It is likewise conceivable according to the invention for only the inner geometry or only the outer geometry to be formed as a truncated cone. The radial bearing of the exhaust-gas turbocharger according to the invention may advantageously be used either as a single-bush bearing or as a two-bush bearing. Furthermore, use may also be made of floating bushes or else static bearing bushes.

Dependent claims 2 to 8 relate to advantageous developments of the invention. Claims 9 and 10 define a radial bearing arrangement according to the invention as an object which can be marketed independently.

Further details, advantages and features of the present invention become apparent from the following description of exemplary embodiments with reference to the drawing, in which:

Figure 1 shows a schematically simplified illustration of an exhaust-gas turbo- charger according to the invention, and

Figures 2 to 4 show three different embodiments of a bearing bush of the radial bearing arrangement according to the invention.

Figure 1 shows an exhaust-gas turbocharger 1 according to the invention in a schematically greatly simplified illustration. The exhaust-gas turbocharger 1 has a compressor 2 and a turbine 3 which are connected by means of a bearing housing 4. A shaft 6 is mounted in the bearing housing 4 by means of a radial bearing arrangement 5, to the ends of which shaft are fastened a compressor wheel 11 and a turbine wheel 12 respectively, which together form a rotor. In the particularly preferred embodiment illustrated in figure 1 , the radial bearing arrangement 5 has two bearing bushes 7 and 8 arranged spaced apart from one another. Said bearing bushes 7 and 8 may be floating bearing bushes or static bearing bushes. It is also possible according to the invention for the radial bearing arrangement 5 to comprise only one bearing bush 7 or 8.

Figures 2 to 4 illustrate different embodiments of the bearing bushes 7 and 8. All of the embodiments have in common the fact that the bearing bushes 7, 8 each have an inner bearing surface 9 and an outer bearing surface 10.

In the embodiment of figure 2, the inner bearing surface 9 is of truncated-cone- shaped form, whereas the outer bearing surface 10 is of cylindrical form.

In the embodiment of figure 3, the inner bearing surface 9 is of cylindrical form, and the outer bearing surface 10 is of truncated-cone-shaped form.

In the embodiment of figure 4, the inner bearing surface 9 and the outer bearing surface 10 are both of truncated-cone-shaped form, wherein the bearing surfaces 9 and 10 are arranged concentrically with respect to one another. In this connection, concen- trie means that cross sections through the bearing bush 7 or 8 as per figure 4 transversely with respect to the charger longitudinal axis L yield in each case concentric circles for the inner bearing surface 9 and the outer bearing surface 10, proceeding from the smallest diameter at the left-hand end region in figure 4 to the largest diame- ter at the right-hand end region.

In addition to the above written disclosure of the invention, reference is hereby explicitly made to the illustrative representation in figures 1 to 4 to supplement the disclosure of the invention.

LIST OF REFERENCE SIGNS

1 Exhaust-gas turbocharger

2 Compressor

3 Turbine

4 Bearing housing

5 Radial bearing arrangement

6 Shaft

7, 8 Bearing bushes

9 Inner bearing surface

10 Outer bearing surface

11 Compressor wheel

12 Turbine wheel

L Charger longitudinal axis