| dc.description.abstract | Automotive turbochargers (TCs) rely on engine oil lubricated floating ring bearing systems to produce acceptable performance as per the engine volumetric efficiency, and along with proven reliability. However, these TCs produce rotordynamic responses that are rich in subsynchronous whirl motions through reaching stable limit cycles.
This dissertation describes the integration of a finite length (semi) floating ring bearing (SFRB) model into a rotor and ring structural model for prediction of both the linear and nonlinear rotor responses and their characterization in terms of motion amplitudes and whirl frequencies. The SFRB model includes a thermal energy transport network for the inner and outer radial films and the thrust bearings on each side of the ring. The floating ring show significant thermal gradients along the radial direction due to the large temperature difference, from a hot shaft to a cold housing, and exacerbated by the heat generated from drag power losses in the inner films adjacent to the rotor. The thermal gradients affect the operating clearances and oil viscosity in each lubricated element, radial and axial.
The time-step numerical integration of the rotor equations of motion accounts for (nonlinear) fluid film bearing forces. The predictive process starts from a static equilibrium position of the TC rotor-bearing system, if existing. Analysis of the transient response of a commercial TC for operation at increasing shaft speeds, from 500 Hz (30 krpm) to 4,000 Hz (240 krpm), and for particular mass imbalance conditions, shows dominant subsynchronous whirl motions with frequencies ranging from approximately ¼ to ~½ of shaft speed. The analysis also investigates the influence of bearing parameters and rotor mass imbalance distribution on the onset, persistence and severity of subsynchronous rotor whirl motions. From comparisons to one set of measurements, the model produces accurate predictions of the nonlinear behavior hence aiding to identify physical parameters that could delay the onset and severity of multi-frequency whirl motions.
Predictions of linear and nonlinear rotordynamic responses for a commercial TC-SFRB system are obtained with nominal operating parameters: bearing inner film clearance ci* = 7.5 μm at 20 ºC, inner film length Li*= 4.3 mm, shaft diameter Di*= 6 mm, and lubricant viscosity μ* = 6.39 c-Poise at 120 ºC. The predictions show the rotor subsynchronous motion amplitudes decrease when the SFRB is supplied with a lubricant of large viscosity (μ =1.8 μ*), and is constructed to have a small inner film clearance (ci =0.8 ci*), a long bearing inner film length (Li =1.3 Li*) and a shaft with a thick diameter (Di =1.3 Di*). However, the recommended TC-SFRB system will aggravate the TC thermal performance as the modified system demands less lubricant flow and increases the viscous drag power loss and the heat in the film.
Integration of a physically accurate model for the lubricated bearings is required for the TC rotor structural model to reproduce actual measurements and field behavior. The predictive model enables an assessment of the lubricated bearings’ system static and dynamic performance as well as both the rotor linear and nonlinear time responses. The predictive tool can save substantial resources (time and cost) in TC production development and prototype testing. | en |
