Nonlinear Mechanics Analysis Of Turbocharger

In recent years, turbochargers have increased in importance and have become indispensable to environmental protection and improved output, accompanied by further tightening of emissions standards. The success is reflected in the fact that almost all heavy trucks use turbo Charged diesel engines and this trend is continued into the small diesel for automotive application, particularly in China where their popularity has boomed due to low fuel consumption, durability and low emissions. The primary design objective of a turbocharger is wider mass flow range at a relatively moderate pressure ratio with emphasis on compactness, low cost and good acceleration characteristics to keep size, weight and inertia small. A turbocharger is a device in which heat energy from engine exhaust gas turns a radial inflow turbine along with a centrifugal compressor on the same shaft and supported by two inboard mounted fully floating shaft sleeve, such that inflow air is pressurized by the compressor and supplied to the engine, thereby improving the engine's combustion efficiency. Thanks to this relatively simple principle, nearly 75 percent of the total output of an engine is the result of turbocharging. So the running stability of turbocharger is a precondition for other components to run safely. The centrifugal compressor is perhaps the most common example of high speed turbomachinery. In the long distance transport of material, centrifugal compressors are playing more important roles. The high rotational speed, high centrifugal force and high dependability are basic characteristic of compressor. The exhaust flow from the engine is directed over the blades of the turbine to provide the force to turn the shaft and compressor. The interference fit design ensures stable balance behavior and allows for positive contact between the impeller and shaft assembly throughout the operating speed. In addition to maintaining radial contact, sufficient net radial interface pressure must remain in order to transmit torque when the rotational speed is very high. Therefore, the interference fit between impeller and shaft assembly is one of the most important factors influencing the performance of the turbo unit in the design of turbocharger compressor. Hence the compressor impeller is mainly subjected to centrifugal forces caused by high speed rotation when operating and the impeller is highly stressed component.Different with conventional works, in this paper a turbocharger compressor under combined centrifugal and interference-fit loading is used for the numerical analysis. On the basis of parametric variational principle (PVP), the analysis of the impeller-shaft sleeve-shaft elastic and elastoplastic frictional contact problem is performed. Gyroscopic systems can be guided to Hamilton systems which construct a perfect theoretical frame. Based on the characteristics of rotor dynamics, an adjoint symplectic subspace iteration method is applied to turbocharger rotor dynamics. This methodology is different from the traditional ones and is used to study the nonlinear mechanics problems of turbocharger from a new viewpoint.The main research work covers the following aspects:In chapter 1, the background and motivation of this dissertation are introduced together with a review of some basic concepts and existing methods for contact mechanics and rotor dynamics. In the last section, the brief description of the research in the dissertation is presented.In chapter 2, some advances and applications in parametric quadratic programming (PQP) method for numerical modeling of elastoplastic contact problems are briefly reviewed. The parametric variational principle (PVP) and the corresponding finite element model for numerical simulation of three-dimensional (3D) elastoplastic frictional contact problems with isotropic friction law are introduced. The discretized frictional contact problems can be reduced as a linear complementarity problem.In chapter 3, a locomotive-type turbocompressor with 24 blades under combined centrifugal and interference-fit loading is computed and analyzed in detail. The finite element (FE) parametric quadratic programming (PQP) method developed based on the parametric variational principle (PVP) is used for the analysis of the stress distribution in the 3D contact problem of impeller. To save time in the computation, a multi-substructure technique is adopted for structural modeling, which not only simplifies the calculation, but also provides a convenient service for process computer aided design (CAD) by means of FE simulation. The effects of the fit tolerance, coefficient of friction and rotational speed (centrifugal force), the wall thickness of shaft sleeve and the contact stress on the interference-fitting surfaces are studied in detail in the numerical computation. The numerical results demonstrate the high accuracy and good convergence of the algorithm presented here, which provides an effective approach that achieves more reliable interference-fitted connections and more precise assembly accuracy with lower manufacturing cost in the structural design.In chapter 4, the principle of the over speed technology of the compressor impellers is introduced. Based on the impeller model in chapter 3, the elastoplastic analysis is used to analyse overspeed impeller. It analyses relativities between over speeds and maximum residual stresses, maximum residual strains on compressor after pre-over loading technology. It also investigates contact stress distributing and magnitude of compressor under working condition after pre-over loading technology. The optimal over rotational velocity is proposed and the influence of pre-over loading technology on compressor is discussed. It supplies reference data to improve design and processing of compressor impellers, indicating the necessity ofpre-over loading technology in compressor impeller manufacture.In chapter 5, firstly, using the idea of subspace iteration method of symmetric matrix and many parallel points in the characters of Hamilton matrix and symmetric matrix, an adjoint symplectic subspace iteration method of indefinite gyroscopic systems is proposed to solve the eigenvalue problem of indefinite gyroscopic systems. On the research of the methods in this dissertation, the computer program is developed and used to investigate the rotor dynamic characteristics such as critical speeds of rotor systems very well. Finite element model of turbocharger rotor system is established which includes the gyroscopic moment, and stiffness of floating bearings. The varying trend of critical speed and mode shape with the variation of bearing stiffness, bearing locations and shaft diameters are obtained. The analysis results could be used as the basis for both the design and structure updating of mrbocharger rotor system.In the conclusion, the main contributions of the dissertation are summarized and the further work is remarked.The research of the dissertation is supported by the National Natural Science Foundation of China (50679013, 10225212 and 10421202), the Program for Changjiang Scholars and Innovative Research Team in University of China (PCSIRT) and the National Key Basic Research Special Foundation of China (2005CB321704).