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Abstract
Due to operational wear and uneven carbon absorption in compressor and turbine wheels, the unbalance (me) vibration is induced and could lead to sub-synchronous vibration accidents for high-speed turbocharger (TC). There are very few research works that focus on the magnitude effects on such induced unbalance vibration. In this paper, a finite element model (FEM) is developed to characterize a realistic automotive TC rotor with floating ring bearings (FRBs). The nonlinear dynamic responses of the TC rotor system with different levels of induced unbalance magnitude in compressor and turbine wheels are calculated. From the results of waterfall and response spectral intensity plots, the bifurcation and instability phenomena depend on unbalance magnitude during the startup of TC. The sub-synchronous component 0.12× caused rotor unstable is the dominant frequency for small induced unbalance. The nonlinear effects of induced unbalance in the turbine wheel is obvious stronger than the compressor wheel. As the unbalance magnitude increases from 0.05 g·mm to 0.2 g·mm, the vibration component changes from mainly 0.12× to synchronous vibration 1×. When unbalance increases continuously, the rotor vibration response amplitude is rapidly growing and the 1× caused by the large unbalance excitation becomes the dominant frequency. A suitable un-balance magnitude of turbine wheel and compressor wheel for the high-speed TC rotor with FRBs is proposed: the value of induced un-balance magnitude should be kept around 0.2 g·mm.
Highlights
High-speed turbochargers (TCs) have gained significant attention in recent years
Because of high nonlinear characteristics of the inner and outer oil films, it was observed that the loss of instability can induce the onset of stable limit cycles under sub-synchronous frequency via a Hopf bifurcation [8], which can always ensure the safe operation of a TC [9]
Appropriate unbalance can suppress the sub-synchronous vibration and improve the response behavior of TC as many researchers mentioned, which is determined by the special structure, high speed operation conditions, and nonlinear floating ring bearings (FRBs)
Summary
High-speed turbochargers (TCs) have gained significant attention in recent years. They already have been widely used in commercial vehicles. Appropriate unbalance can suppress the sub-synchronous vibration and improve the response behavior of TC as many researchers mentioned, which is determined by the special structure, high speed operation conditions, and nonlinear FRB. 2.1 FRB Model Unlike conventional plain journal bearing, the floating ring rotates with the sum of speed of inner and outer hydrodynamic oil films, which is a key parameter to the analysis of TC rotor’s dynamic characteristics. For FRB, the clearance ratio is probably the most important parameter, the optimal value depends on the operating conditions and ratios of Ro/Ri and Lo/Li. Smaller clearance ratio will decrease the ring speed which is not desirable from the lubrication point of view, it may increase the rotor stability. Considering the practical structure and the locations probability of unbalance on gasoline engine TC, the Fub (Ω, t) usually can be decomposed
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