Ali A. Alsaeed

Stability Analysis of a High-Speed Automotive Turbocharger

Automotive turbochargers are known to have operation into the linear unstable region. The operation in the nonlinear limit cycle has been tolerated on most applications to date. The need for a quieter, smoother operation and reduced emissions has prompted new evaluations of the rotor-bearing design for these systems. In this research, a commercial rotordynamics computer program is used to evaluate the stability and transient response of a high-speed automotive turbocharger. Various models with varying bearing designs and properties have been solved to obtain the linear stability threshold speeds and also the nonlinear transient response. The predicted whirl speed map shows two modes of instability and is very similar to the limited test results in the literature. The calculation process is discussed in detail and the results of the current research will be compared to the literature. An experimental research project is currently in progress at Virginia Tech and those results will be documented in a future publication.

Experimental Test Results for Vibration of a High Speed Diesel Engine Turbocharger

Diesel engine turbochargers are known to have operation into the self-excited unstable region. The operation in the nonlinear limit cycle has been tolerated on most applications to date. The need for a quieter and a smoother operation and in addition reduced emission levels have prompted new evaluations of the rotor bearing design for these systems. In this research, a commercial rotor-bearing dynamics computer program was used to evaluate the stability and transient response of a high-speed diesel engine turbocharger. Various models with varying bearing designs and properties were solved to obtain the linear stability threshold speeds and also the nonlinear transient response. The predicted whirl speed map shows two modes of instability and is very similar to recent on-engine test results conducted at Virginia Tech as part of a recent senior capstone design project. The development of the diesel engine test stand and the associated data acquisition system will be discussed. The results from the unloaded and the loaded engine testing will be compared to the analytical results. Future research will be devoted to the elimination of the large subsynchronous excitation.

Turbocharger On-engine Experimental Vibration Testing:

Many automotive turbochargers operate in the self-excited unstable region. In the past these instabilities have been accepted as unavoidable, but recent developments in analysis and instrumentation may make it possible to reduce or eliminate them. A test stand being developed at Virginia Tech has been used to measure the vibrations of a 3.9 liter diesel engine stock turbocharger with floating bushing journal bearings. Vibration spectrum content clearly identifies the shaft instabilities and provides the basis for additional evaluation of future bearing design modifications. This paper provides additional experimental vibration data reduction that will be useful for future research direction to fully understand the turbocharger dynamic instability.

Induced Unbalance as a Method for Improving the Dynamic Stability of High-Speed Turbochargers

The high-speed diesel engine turbocharger is known to have subsynchronous vibrations for a wide speed range. The bearing fluid-film instability is the main source of the vibration. The nonlinear forces inside the bearings are causing the rotor to whirl in a limit cycle. This study presents a new method for improving the dynamic stability by inducing the turbocharger rotor unbalance in order to suppress the subsynchronous vibration. The finite-element model of the turbocharger with floating-ring bearings is numerically solved for the nonlinear time-transient response. Both compressor and turbine added unbalance are induced and the dynamic stability is computed. The turbocharger model with linearized floating-ring bearings is also solved for eigenvalues to predict the modes of instability. The linear analysis demonstrates that the forward whirling mode of the floating-ring at the compressor end also becomes unstable at the higher turbocharger speeds, in addition to the unstable forward conical and cylindrical modes. The numerical predictions are also compared to the former experimental results of a similar turbocharger. The results of the study show that the subsynchronous frequency amplitude of the dominant first mode is reduced when inducing either the compressor or the turbine unbalance at a certain level.

Turbocharger vibration shows nonlinear jump

Automotive turbochargers are known to operate into the self-excited unstable region. In the past these instabilities have been accepted as unavoidable, but recent developments in analysis and instrumentation may make it possible to reduce or eliminate them. A test stand has been developed at Virginia Tech to measure the vibrations of a 3.9 liter diesel engine stock turbocharger with both stock floating ring journal bearings and also custom design fixed geometry bearings. Vibration spectrum content clearly identifies the shaft instabilities and provides the basis for additional evaluation of future improved bearing design modifications. The current results, for custom fixed journal bearings, have clearly revealed a distinct jump with associated shift in the spectrum frequency content. This paper will document the recent tests of custom design fluid film bearings that have experienced this nonlinear jump condition.

Stability Analysis of a High Speed Automotive Turbocharger

Automotive turbochargers are known to have operation into the linear unstable region. The operation in the nonlinear limit cycle has been tolerated on most applications to date. The need for quieter, smoother operation and reduced emissions has prompted new evaluations of the rotor bearing design for these systems. In this research a commercial rotordynamics computer program is used to evaluate the stability and transient response of a high speed automotive turbocharger. Various models with varying bearing designs and properties have been solved to obtain the linear stability threshold speeds and also the non- linear transient response. The predicted whirl speed map shows two modes of instability and is very similar to the limited test results in the literature. The calculation process is discussed in detail and the results of the current research will be compared to the literature. An experimental research project is currently in progress at Virginia Tech and those results will be documented in a future publication.

Effects of radial aerodynamic forces on rotor-bearing dynamics of high-speed turbochargers

This study investigates the radial aerodynamic forces that may develop inside the centrifugal compressor and the turbine volutes due to pressure variation of the circulating gas. The forces are numerically predicted for magnitudes, directions, and locations. The radial aerodynamic forces are numerically simulated as static forces in the turbocharger finite element model with floating ring bearings and solved for nonlinear time-transient response. The numerical predictions of the radial aerodynamic forces are computed with correlation to earlier experimental results of the same turbocharger. The outcomes of the investigation demonstrate a significant influence of the radial aerodynamic loads on the turbocharger dynamic stability and the bearing reaction forces. The numerical predictions are also compared with experimental results for validation.

Effect of Flexible Damped Bearing-Supports on the Dynamic Stability of High-Speed Turbochargers

The aim of this study is to analytically design flexible damped bearing-supports in order to improve the dynamic characteristics of the rotor-bearing system. The finite-element model of the turbocharger rotor with linearized bearing dynamic coefficients is used to solve for the logarithmic decrements and hence the stability map. The design process attempts to find the optimum dynamic characteristics of the flexible damped bearing-support that would give best dynamic stability of the rotor-bearing system. The method is successful in greatly improving the dynamic stability of the turbocharger and may also lead to a total linear stability throughout the entire speed range when used besides the enhanced-performance hydrodynamic bearings.Copyright