Dynamics and stability analysis of unbalance responses in mid-positioned electrically assisted turbocharger rotor

Abstract

Electrically assisted turbochargers (EAT) improve intake efficiency by motor-assisted compressor impeller rotation, enhancing the system's transient response. However, the addition of motor rotor components has increased the number of unbalanced positions in the shaft system, leading to problems such as excessive compressor end vibration and complex changes in oil film stability. To evaluate the effects of unbalance in the motor rotor, along with the parameters of floating ring bearings (FRB), on the dynamic response of EAT, a finite element model of an EAT rotor supported by nonlinear FRB is developed, and the vibration response of the compressor end bearing is obtained by numerical integration. The results indicate: (1) In contrast to the effect of compressor and turbine unbalance, proper motor rotor unbalance is more effective in suppressing oil whirl instability in the high-speed operating range. However, a new inner oil film whirl “instability interval” is also induced in the low-speed operating range, leading to an increase in the Y₁ compressor-end amplitude at low and medium speeds, and this “instability interval” increases with the amount of unbalance. (2) When an oil whirl occurs in the oil film, the maximum eccentricity of the bearing surges and is greater than 0.3, which can be used as an effective threshold for determining whether the oil film is unstable in engineering applications. (3) A suitable outer oil-film clearance range should be 35-40 µm, otherwise, a wide range of outer oil-film whirl instability occurs. Controlling the amount of unbalance and oil-film clearance to suppress the subsynchronous vibration of the EAT, provides a theoretical basis for the design of the dynamics of the nonlinear rotor bearing system and improves the stability of the turbocharger's operation.

Keywords

electrically assisted turbocharger / motor unbalance / oil-film clearance / rotor dynamics / nonlinear vibration

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2024 The Author(s). International Journal of Mechanical System Dynamics published by John Wiley & Sons Australia, Ltd on behalf of Nanjing University of Science and Technology.