An exhaust-gas turbocharger of said type has a bearing arrangement for the mounting of a rotor shaft in a bearing housing. Here, the bearing arrangement may be either in the form of a floating-bushing plain bearing arrangement or in the form of a plain bearing arrangement with non-rotating bearing bushings. The floating-bushing plain bearing arrangement is distinguished by the fact that the rotor shaft rotates within two bearing bushings of the bearing arrangement, and said bearing bushings in turn rotate within the bearing housing. Plain bearing arrangements with non-rotating bearing bushings are distinguished by the fact that the bearing bushings are fixed in the bearing housing so as to be prevented from rotating.

A floating-bushing plain bearing arrangement in this case duly exhibits lower friction losses than a plain bearing arrangement with a non-rotating bearing bushing, but in the case of a floating-bushing plain bearing arrangement, an additional, subsynchronous vibration mode arises which may have a considerably greater amplitude than, for example, an imbalance-induced vibration.

It is therefore an object of the present invention to provide an exhaust-gas turbocharger of the type indicated in the preamble of claim 1 which is provided with an optimized bearing arrangement.

This object is achieved by the features of claim 1.

The exhaust-gas turbocharger according to the invention is distinguished by the fact that its bearing arrangement has a floating-bushing bearing and a non-rotating bearing bushing. This combination advantageously makes it possible for one of the subsynchronous vibrations of the exhaust-gas turbocharger to be suppressed.

Dependent claims 2 and 3 relate to advantageous developments of the exhaust- gas turbocharger according to the invention.

A further optimization of the bearing arrangement according to the invention can be achieved if the compressor-side bearing is the floating-bushing bearing, and the turbine-side bearing is the bearing with the non-rotating bearing bushing. For further stabilization of the bearing arrangement, a spacer bushing is advantageously provided between the floating-bushing bearing and the non-rotating bearing bushing.

Claims 4 to 6 define a 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 an exemplary embodiment with reference to the figures of the drawing, in which:

Figure 1 shows a schematic illustration of an exhaust-gas turbocharger according to the invention, and

Figures 2 and 3 show frequency analyses in which amplitudes of individual frequencies are plotted against the rotational speed.

Figure 1 shows an exhaust-gas turbocharger 1 according to the invention, which has a compressor 2 which comprises a compressor housing 2A in which a compressor wheel 2B is arranged.

The exhaust-gas turbocharger 1 also has a turbine 3 with a turbine housing 3A and a turbine wheel 3B arranged in the turbine housing 3A.

The compressor wheel 2B and the turbine wheel 3B are fixed to the ends of a rotor shaft 7 which is mounted by means of a bearing arrangement in a bearing housing 4 which, as indicated in the figure, is arranged between, and connected to, the compressor housing 2A and the turbine housing 3 A.

The bearing arrangement of the exhaust-gas turbocharger 1 according to the invention has a floating-bushing bearing 5 and a non-rotating bearing bushing 6. As shown in Figure 1, in the case of the particularly preferred embodiment illustrated therein, the compressor- side bearing is the floating-bushing bearing 5, whereas the turbine-side bearing is formed by the non-rotating bearing bushing 6.

Furthermore, Figure 1 illustrates that the bearing arrangement according to the invention has a spacer bushing 8 which is arranged on the rotor shaft 7 between the floating-bushing bearing 5 and the non-rotating bearing bushing 6. As has already been explained in the introduction, the floating-bushing bearing 5 is a bearing in which the rotor shaft 7 rotates and the floating-bushing bearing 5 in turn rotates within the bearing housing 4. By contrast, the fixed bearing bushing 6 is secured so as to be prevented from rotating within the bearing housing 4. Figures 2 and 3 show diagrams (so-called waterfall diagrams) and frequency analyses in which the amplitudes of individual frequencies are plotted against the rotational speed. Here, the diagrams show the radial displacement at the compressor wheel nut.

Here, Figure 2 shows a diagram for the radial displacement at the compressor wheel nut, said diagram showing the dominant subsynchronous vibration in the case of a floating-bushing bearing arrangement.

Figure 3 shows a diagram for the radial displacement at the compressor wheel nut, said diagram showing the case of the combination of a floating bushing mounted at the compressor side and a non-rotating bushing mounted at the turbine side.

As can be seen directly from a comparison of Figures 2 and 3, it is possible by means of the combination of features according to the invention for the subsynchronous vibrations of the exhaust-gas turbocharger to be fully suppressed. The bearing arrangement according to the invention with floating-bushing bearing and non- rotating bearing bushing (Figure 3) thus makes it possible to realize an optimized bearing arrangement.

In addition to the above written description of the invention, reference is hereby explicitly made, for additional disclosure thereof, to the diagrammatic illustration of the invention in the single figure.

LIST OF REFERENCE SIGNS

Exhaust-gas turbocharger

Compressor

A Compressor housing

B Compressor wheel

Turbine

A Turbine housing

B Turbine wheel

Bearing housing

Floating-bushing bearing

Non-rotating bearing bushing

Rotor shaft

Spacer bushing