Prediction of Gas Thrust Foil Bearing Performance for Oil-Free Automotive Turbochargers

Green technologies are a mandate in a world concerned with saving resources and protecting the environment. Oil-free turbocharger systems for passenger and commercial vehicles dispense with the lubricant in the internal combustion engine, hence eliminating not just oil coking, but also suppressing nonlinear behavior, instability and excessive noise; all factors to poor reliability and premature mechanical failure. The work hereby presented is a stepping stone in a concerted effort towards developing a computational design tool integrating both radial and thrust foil gas bearings for oil-free automotive turbochargers. The paper presents the physical analysis and numerical model for prediction of the static and dynamic forced performance of gas thrust foil bearings (GTFBs). A laminar flow, thin film flow model governs the generation of hydrodynamic pressure and a finite element plate model determines the elastic deformation of a top foil and its support bump strip layers. For a specified load, the analysis predicts the minimum gas film thickness, deformation and pressure fields, the drag torque and power loss, and the axial stiffness and damping force coefficients, respectively. Open-source archival test data on load capacity and drag torque serves to benchmark some of the model predictions. Next, predictions are obtained for a GTFB configuration designed for an oil-free turbocharger operating at increasing gas temperatures, axial loads, and shaft rotational speeds. The largest drag torque occurs at the highest temperature since the gas viscosity is also highest, whereas the largest load determines operation with a minute film thickness that sets a limit for the manufacturing tolerance. While airborne, the drag friction factor for the bearing is small, ranging from 0.009 to 0.015, thus demonstrating the advantage of an air bearing technology over engine oil lubricated bearings. The synchronous speed axial stiffness increases with operating speed (and load), whereas the axial damping coefficient remains nearly invariant. The operating gas temperature plays an insignificant role on the variation of the force coefficients with frequency whereas the operating speed and the ensuing applied thrust load determine the largest changes. The model predicts, as an excitation frequency (ω) increases, a GTFB axial stiffness (K_z) that hardens and a damping coefficient (C_z) that quickly vanishes. The most important finding is that C_zΩ/K_z ≈ γ = the material loss factor for the bearing. Hence, the success of foil bearing technology relies on the selection of a metal underspring structure that offers the largest mechanical energy dissipation characteristics.