Turbocharger rotor-bearing system needs to be optimized for durability, quietness, low power loss and low cost. The optimization is complicated because turbochargers are required to operate over a wide range of speed and bearing inlet pressure and temperature. The rotor and bearing impedance needs to be matched to obtain the best compromise between the shaft motion and bearing housing vibration. Impedance is the vector sum of the displacement dependent force, called “stiffness” and the velocity dependent resisting force called “damping”. These forces are at 90 degrees to each other. The shaft motion directly affects durability while the bearing housing motion impacts on turbocharger noise. The bearing friction loss can be reduced by reducing journal diameter and bearing length. Tests on bearing friction rig confirm the earlier findings that the difference in friction loss between semi-floating (non-rotating) bearing and fully floating (rotating) bearing is very small. Ball bearings have the best performance but their present cost limits their use as an alternative to the fully floating and the semi-floating bearings.
The present optimization procedure heavily relies on experiments. The cycle time and cost for developing an optimized rotor-bearing system may be significantly reduced by the use of computer programs. But accurate modeling of the bearing oil film characteristics i.e., dynamic stiffness, damping and mass properties, is very difficult due to non linear behavior of the bearing oil film. Experimentally determined bearing characteristics, combined with current Rotordynamics computer programs may provide acceptable results. A bearing test rig is under development at the author's company to improve the experimental determination of bearing characteristics under simulated operating conditions.
