Thermal effects on the performance of floating ring bearings for turbochargers

Abstract

Physical models and fast computational programs aim to improve the design and performance of turbocharger (TC) rotordynamics. Most commercial automotive TCs incorporate floating ring bearings (FRBs) owing to their low cost and reduced power losses. However, persistent subsynchronous motions afflict this type of rotor/bearing system, albeit reaching limit cycles that enable their continuous operation. FRBs comprise two fluid films in series and are prone to show one or two subsynchronous instabilities over extended speed ranges of operation. A flow model for prediction of FRB forced response is detailed here. The model incorporates a lumped-parameter thermal energy balance for estimation of the lubricant viscosity and thermal growth of the rotor, bearing and floating ring. The FRB model, fully integrated into a non-linear rotordynamics computational program, predicts the floating ring speed, journal and ring eccentricities, power loss and the rotordynamic force coefficients of the inner and outer films as a function of the load applied at a given rotor speed. Knowledge of the actual load conditions, static and dynamic, and the changes in operating clearance and effective lubricant viscosity are most important for accurate estimation of a TC dynamic forced response. Predictions for the exit lubricant temperature, power losses and floating ring speeds agree well with measurements obtained in an automotive turbocharger test rig. The rotordynamic stability characteristics of the test TC are also highlighted.