Abstract
Automotive turbochargers (TCs) use an engine oil lubricated bearing system to produce acceptable performance (as per the engine volumetric efficiency) and proven reliability. However, the bearings also cause TC rotordynamic responses that are rich in subsynchronous whirl motions through reaching stable limit cycles. The paper describes the lubrication model for a finite length semifloating ring bearing (SFRB) system and its coupling to the rotor and ring structure models for prediction of both linear and nonlinear system responses and their characterization in terms of motion amplitudes and whirl frequency content. The SFRB model includes a thermal energy transport network for the inner and outer films in both radial bearings and the thrust bearings located on the end sides of the ring. The large temperature difference between the hot shaft and a cold housing induces a three-dimensional thermal gradient in the fluid films and the floating ring, further exacerbated by the heat generated from drag power losses in the inner films adjacent to the rotor. The temperature gradients affect the lubricant viscosity and the bearing system operating clearances. The integration of the rotor bearing system (RBS) equations of motion accounts for the SFRB nonlinear forces and starts from a static equilibrium position, if existing. Analysis of the startup speed response of a commercial TC, from 500 Hz (30 krpm) to 4000 Hz (240 krpm), and for particular mass imbalance conditions, shows dominant subsynchronous vibrations (SSVs) with frequencies ranging from approximately ¼ to ∼ ½ of shaft speed and a transition from conical to cylindrical-bending rotor mode shapes. The model predictions of nonlinear behavior are accurate when benchmarked to a set of measurements procured in a gas stand test rig. The analysis also investigates the influence of the bearing inner clearance and rotor mass imbalance distribution on the onset, persistence, and severity of SSV.
