Not only cast metals are favoured for the turbine housing. Because exhaust manifolds, turbine and catalytic converters are always being mounted closer to one another, one tries, as much as one can, to keep the heat from flowing off, which is particularly important after cold-starting. There are already casings made of die-cut sheet-metal (steel), which as a double-walled casing, make a continuous insulation possible.
Roller bearings are possible at speeds of up to 140.000 RPM in the racing field, above this however, and generally speaking, hydrodynamic bushings which are lubricated by the engine-oil circulation, are normal. They do somewhat reduce the performance, but are more durable.
Apart from this, the engine-oil cools the shaft. A fundamental change would be possible, if the shaft could be centered by using magnetic forces or an air-cushion. Perhaps one day, there will be ceramic bearings which are not lubricated. The relatively high axial forces are a result of the axial entry of the fresh gases and the exiting of the exhaust gases, which are taken up separately from the radial forces.
Together with the charging pressure, the torque increases largely linearly. The highest charging pressure concentrates, and has done for quite a while now, on the lower RPM region, should e.g., the maximum torque be achieved, then the pressure no longer increases. From now on, the turbo-charger will operate with the best possible efficiency.
In theory, the distribution of the charge through two chargers, is possible in all multi-cylinder engines. In practice it more than likely appears in engines having two banks of cylinders. Because of the smaller turbo-chargers, the response is improved. All together, and at the moment, four chargers are possible in one engine (e.g., the Bugatti Veyron).
The so-called turbo-lag can also be reduced by mounting a smaller- and a larger charger in series. The former provides better starting-off, whereupon the latter, kicks in by switching over the air supply. In place of the smaller charger there could also be a compressor. Should the larger take over, for reasons of efficiency, the compressor could be switched off.
Turbo-chargers can be removed by loosening the bolts on the compressor-side. The turbine wheel is, as a rule, welded to the shaft. The main sealing between the casing and the shaft is done with piston rings on the shaft. Apart from sealing- and mechanical damages, in turbo-charging leakages in the air supply and particularly in the air removal are possible. The best way to determine this, is by measuring the charging pressure during a test-drive or on the test-bench. 01/14