Dynamics and Control Simulations of Turbocharger Rotor-Bearing Systems

Abstract

In the current global scenario, automotive industries struggle for fuel consumption and emission reductions, while constantly improving the engine power output and performance. These require turbocharger units which make easy long maintenance intervals along with improved reliability and efficiency. A Turbocharger basically consists of a radial inflow turbine, a centrifugal compressor and a central housing of rotating assembly. The present work focuses on the modelling and dynamic analysis of the turbocharger rotor supported on floating ring and foil bearings for parametric identification, stability analysis and vibration reduction aspects. The flexible rotor-bearing system is modelled using the finite element approach and the nonlinear dynamic responses due to unbalance and aerodynamic loads are obtained. Numerical and experimental modal analysis is performed on a real time automotive turbocharger test rig. After validating the response data, the model is employed to identify the bearing parameters using inverse analysis methodology. The optimization based identification scheme of bearing dynamic stiffness and damping coefficients is presented with the measured dynamic responses as input signatures. The robustness of the approach is illustrated with noisy input signals. Later-on, the coupled field fluid dynamic study is conducted for fully-floating ring bearing rotor system with consideration of fluid film properties and structural data of the ring, casing and journal. The pressure and temperature distributions in the floating ring bearing are obtained with appropriate boundary conditions. Further, the effect of the speed ratio on the pressure and temperature distribution is evaluated. The integrated thermo-hydrodynamic (THD) analysis of rotor- bearing system with temperature dependent oil viscosity in floating ring bearings is presented. Furthermore, the stability issues are discussed under accelerating/decelerating conditions. Effects of various parameters including unbalance and phase angle difference at the discs on the dynamics of the overall system are also studied. The influence of nonlinear external gas forces on the dynamics of the rotor is studied. Exhaust gas excitations are modelled with Muszynska’s force model along with axial engine order harmonic excitations. In turbochargers, in order to provide a stable connection between static and rotating parts, often one axial or thrust bearing is provided on compressor side in addition to two radial bearings. Effect of the thrust bearing forces on the stability of the rotor system is studied with a linear spring element model as well as by using a finite difference scheme. In order to minimize the vibration amplitudes at critical and sub-critical operations, semiactive and active control approaches are implemented. In semi-active control scheme, electrorheological fluid lubricants are used in the floating ring bearings and their effectiveness is studied under different input voltage conditions. For, the rotor supported on the foil bearings, a vibration attenuation methodology is proposed to generate a momentary preload by reducing the angular clearances which alters the film thickness via a wedge mechanism. In active control approach, conventional electromagnetic actuator with PD feedback configuration is used to provide the control forces in vibration reduction. In overall, the dynamic modeling and control studies via computer simulations saved the time with reliable results resulting in the better rotor designs.