Turbocharger's Base Excitation Identification And Rotor Dynamics Investigation

A turbocharger has a broad application on the engine because it can improve the property of an engine by charging more air into the cylinders, burning more fuels and more sufficiently, and subsequently carrying out more power. As an important component of an engine, the turbocharger has a high command on the working performance, reliability and stability. The turbocharger is a rotational machine with extremely high rotational speed and under high temperature situation. As the turbocharger is always founded on the engine, low frequency, with respect to engine's running speed, large deflection vibrations are transferred from the engine to the turbocharger rotor through the hydrodynamic bearings. Even though these low frequency vibrations are far below from the rotor's running speed, they do affect its operation in a nonlinear way through the journal bearing clearance variation. Uptodate, many efforts have been made to investigate the nonlinear behaviors of rotor dynamics, including bifurcation, chaos, oil whirl and whip, stability, and so on. However, the traditional rotor dynamics mainly aims at the ground rotational machine, and presumes the base is stationary and the supporting stiffness is efficiently large. This presumption is obviously unreasonable for the turbocharger rotor system. The influence of the engine's base excitation must be taken into account in the turbocharger rotor dynamics.In this effort, this dissertation deals with the engine's base excitation identification and the turbocharger rotor dynamics investigation with the base excitation. The base excitation identification method is presented, the modal analysis of the engine-manifold-turbocharger system is carried out, the dynamic model of turbocharger rotor-bearing system is established, including the engine's base excitation and nonlinear oil film force, the nonlinear dynamic behavior of unbalance mass is investigated, and the fault diagnosis and state monitor with the base excitation are studied.Three methods are applied for the engine's base excitation identification in this dissertation.Firstly, based on the fact that the engine's base excitation affects the turbocharger rotor dynamics through the center housing and the hydrodynamic bearings, if the vibration responses of bearing block (center housing inner) when the engine is running on can be obtained, we can use these responses as base excitation for the turbocharger rotor dynamics. However, the responses of center housing inner can hardly be measured directly due to its high temperature and too small room to place accelerometers. Only the responses of center housing lubricant nut (center housing outer) can be measured owing to the cyclic lubricant cool. Considering that the center housing is an extremely stiff hollow cylinder, we assume that the vibration responses of center housing outer are identical to the vibration responses of center housing inner, and the base excitation can be obtained by measuring the responses of center housing outer directly. In order to validate this assumption, the modal analysis of center housing is carried out, the test and comparison of frequency response functions (FRF) of center housing inner and center housing outer are processed, and the test and comparison of vibration responses of the engine on the vibrator table are made. The results show that the responses of center housing outer are identical to the responses of center housing inner, and the base excitation can be obtained by measuring the responses of center housing outer directly.Secondly, an FRF matrix inverse method is applied to acquire the vibration responses of turbocharger center housing inner by means of testing the responses of measurable points on the compressor housing shell and their FRFs. This method is verified by free hanging experiment and vibrator table experiment, and thus proved to be efficient and applicable. The factors that influence the accuracy are analyzed, and the more measurable points that are used to inverse, the more accurate the inversed immeasurable responses of center housing inner.Thirdly, a theoretical dynamic model of the engine-manifold-turbocharger is constructed, and the responses of center housing inner can be derived from this model. The whole engine system is divided into two subsystems: engine main body subsystem and manifold-turbocharger subsystem. The engine main body subsystem is modeled by rigid body movement, and the manifold-turbocharger subsystem is modeled by the combination of finite element method (FEM) and experiment method. These two subsystems are synthesized by free interface component modal synthesis method. The vibration modes and vibration responses are obtained from the synthesized system equations. The calculated results agreed well with the experiment results. Through this study, the response of turbocharger bearing block, which is regard as the base excitation for turbocharger rotor dynamics, can be determined.On the investigation of the turbocharger rotor dynamics with base excitation, there are three aspects of contents.Firstly, the linear dynamic model of the turbocharger bearing-rotor is constructed for the modal analysis and critical speeds calculation, so that the working speed of turbocharger can be far away from the critical speeds. The predicted natural frequencies and mode shapes of the free rotor agree with the experiment results, which validates the dynamic model of turbocharger rotor. Further more, the critical speeds of base-bearing-rotor system are analyzed, and the base mass and supporting stiffness variation influences on the critical speeds are investigated. The results show that the base mass and supporting stiffness variation can only affect the base's natural frequency, and cannot affect the rotor's critical speeds.Secondly, in order to investigate the influence of the base excitation on the nonlinear rotor dynamic behavior of turbocharger, a dynamic model of turbocharger rotor-bearing system including the engine's base excitation and nonlinear lubricant force is established. The rotor vibration response of unbalance mass is simulated by numerical calculation of Runge-Kutta method. The bifurcation disciplinarian and chaos behaviors of nonlinear rotor dynamics with various rotational speeds are studied. The results obtained by numerical simulation show that the difference of dynamic behavior between the turbocharger rotor systems with/without base excitation is obvious. The base excitation will affect the rotor dynamic behavior in a complicated way, e.g. the base excitation will change the rotor dynamic behavior from period-1 motion to period-2 motion at a low rotational speed, and the base excitation will lower the rotational speeds at which the oil whirl begins and chaos motion occurs. Since the frequency of base excitation is much lower than the rotational speed of the turbocharger rotor, the influences of base excitation on the rotor dynamic behavior are mainly at the low rotational speed. The influences of base excitation lessen gradually with the increase of rotor rational speed.Thirdly, on the purpose of revealing more influences of base excitation on the rotor dynamics, the crack rotor dynamics and rub-impact rotor dynamics with base excitation are investigated, and the characteristic information of fault with base excitation are abstracted. The investigations show that base excitation will complicate the cracked rotor dynamics. Especially at the rotational speed that the oil whirl begins, the base excitation will make the crack forces more distinct. The base excitation will increase the possibility and extent that the rub-impact fault of rotor happens.Finally, the dissertation is summarized, and the prospect is presented.