Experiments on the influence of oil temperature and pressure on subsynchronous bifurcation and shaft orbit behaviour in automotive turbochargers

This work presents a comprehensive experimental study of the nonlinear dynamic behavior of a high-speed automotive turbocharger rotor system supported by semi-floating bearings. The investigation focuses on how oil supply temperature and pressure influence subsynchronous bifurcation, dynamic instability, and shaft orbit evolution under realistic lubrication environments. Experiments were performed on a dedicated test bench that enabled precise control and monitoring of oil parameters while decoupling rotor dynamics from engine-related disturbances. Shaft-end displacements were measured using dual radial probes, with simultaneous high-frequency vibration and speed tracking. Order spectra and Fourier analysis were conducted at key operational points, and orbit patterns were visualized through whirl plots to capture changes in rotor motion across a full range of speeds. The results reveal that elevated oil temperature markedly lowers the bifurcation threshold, causing nonlinear instabilities—marked by earlier onset of complex, multi-loop orbits—at reduced speeds. These transitions are dominated by two principal subsynchronous spectral components, Sub 1 (cylindrical and conical modes) and Sub 2 (translational and bending modes), whose evolution is mapped in detail using both spectral and orbit-based methods. A distinctive trend toward thermal saturation in the lubrication system was identified, as outlet oil temperature increases plateau at higher inlet temperatures, indicating a thermal energy absorption limit. Comparative analysis demonstrated that the system’s dynamic response is highly sensitive to both viscosity and supply pressure, with higher temperatures and lower viscosities amplifying instability and orbit complexity. Filtered vibration signals provided consistent results with displacement data, validating their use for practical bifurcation detection. By combining advanced order-domain diagnostics with direct orbit visualization, this study establishes clear experimental benchmarks for understanding turbocharger instability mechanisms, justifies the focus on the dominant subsynchronous modes, and highlights the importance of oil rheology and thermal effects in governing rotor dynamics. These insights support the development of improved predictive models and informed maintenance or control strategies for higher efficiency and reliability in turbocharger systems.