Behavior of a Variable Geometry Turbine Wheel to High Cycle Fatigue

Abstract Maximization of the turbocharger efficiency is fundamental to the reduction of the internal combustion engine back-pressure. Specifically, in turbochargers with a variable geometry turbine (VGT), energy losses can be induced by the aerodynamic profile of both the nozzle vanes and the turbine blades. Although appropriate considerations on material limits and structural performance of the turbine wheel are monitored in the design and aero-mechanical optimization phases, in these stages, fatigue phenomena might be ignored. Fatigue occurrence in VGT wheels can be categorized into low and high cycle behaviors. The former would be induced by the change in turbine rotational speed in time, while the latter would be caused by the interaction between the aerodynamic excitation and blades resonating modes. In this paper, an optimized turbine stage, including unique nozzle vanes design and turbine blades profile, has been assessed for high cycle fatigue (HCF) behavior. To estimate the robustness of the turbine wheel under several powertrain operations, a procedure to evaluate HCF behavior has been developed. Specifically, the HCF procedure tries to identify the possible resonances between the turbine blades frequency of vibrations and the excitation order induced by the number of variable vanes. Moreover, the method evaluates the turbine design robustness by checking the stress levels in the component against the limits imposed by the Goodman law of the material selected for the turbine wheel. In conclusion, both the VGT design and the HCF approach are experimentally assessed.