Mathematical Problems in Engineering

Volume 2016, Article ID 9212613, 11 pages

http://dx.doi.org/10.1155/2016/9212613

## Distortion Optimization of Engine Cylinder Liner Using Spectrum Characterization and Parametric Analysis

^{1}School of Aeronautics, Northwestern Polytechnical University, Xi’an 710072, China^{2}State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an 710049, China

Received 19 January 2016; Accepted 13 April 2016

Academic Editor: Xiangyu Meng

Copyright © 2016 Zhaohui Yang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

In an automotive powertrain system, the cylinder liner is one of the most critical components which possesses the intricate structural configurations coupled with complex pattern of various operational loads. This paper attempts to develop a concrete and practical procedure for the optimization of cylinder liner distortion for achieving future requirements regarding exhaust emissions, fuel economy, and oil consumptions. First, numerical calculation based on finite element method (FEM) and computational fluid dynamics (CFD) is performed to capture the mechanism of cylinder liner distortion under actual engine operation conditions. Then, a spectrum analysis approach is developed to describe the distribution characteristic of operational loads (thermal and mechanical) around the circumference of a distorted cylinder bore profile; the FFT procedure provides an efficient way to implement this calculation. With this approach, a relationship between the dominant order of distortion and special operational load is obtained; the design features which are critically relative to cylinder liner distortion are also identified through spectrum analysis. After characterizing the variation tendency of each dominant order of distortion through parametric analysis, a new design scheme is established to implement the distortion optimization. Simulation results indicate that a much better solution is obtained by using the proposed scheme.

#### 1. Introduction

In an automotive powertrain system, the cylinder liner is one of the most critical components affecting the operational performance of an engine. With the ever-increasing demand in higher efficiency of engine sealing units which involves both oil consumption and exhaust emission, the demand on improved tightness of contact on piston ring/cylinder liner interface (PRCI) is also increasing [1–3]. In addition, lower friction design is also being aggressively pursued for the tribological system (e.g., piston ring/cylinder liner) which is important for a better fuel economy [4–6]. However, the former demand (i.e., sealing efficiency) requires a higher pretightening force on piston ring which will lead to excessive engine friction losses and subsequently, resulting in an adverse effect on the fuel consumption. Therefore, higher sealing efficiency and lower engine friction losses are two conflicting goals. In this regard, cautions should be placed in the quality of the interface between piston ring and cylinder liner [7]. Due to imperfections in manufacturing and pretightening process, as well as complex pattern of various operational loads, an ideal circular cylinder bore cannot be achieved during engine operation process. Hence, the elastic piston ring should conform to the distorted cylinder bore. However, if such distortions become too large, the piston ring may be unable to fully conform to the cylinder bore, and this imperfection may influence the normal engine operation in terms of increasing component friction, wear, and oil consumption. To this end, the piston rings may achieve better sealing characteristics within a low distorted cylinder bore, and, for unchanged sealing demand, the engine friction losses could be reduced by decreasing the pretightening force on piston rings. That is, a low distorted cylinder liner opens up potentials for resolving the above conflict.

The ability to predict and optimize the geometrical distortions of cylinder liner has generated significant interest in recent years. Many analytical and computational tools have been used to attack this problem. A pioneering analytical work dealing with the presentation of a distorted cylinder bore profile was performed by Gintsburg [8]. In this work, the sealing characteristic of a splitless piston ring was analyzed by approximating distortions from the ideal circular shape with a Fourier series. By describing a distorted bore profile in this form, Müeller [9] further developed a set of bounds for each Fourier order of distortion, and Luenne and Ziemb [10] adopted these bounds in the development of a measurement system for evaluating the distortion characteristics of cylinder liner. Similarly, Dunaevsky [11, 12] explored a random process based scheme to present the tightness of contact on PRCI. In this scheme, a design criterion was proposed to quantify amplitude and order of bore distortions regarding the piston ring conformability. Although these studies enable complex bore geometries to be decomposed into a series of simpler distortion orders, the distribution characteristic of operational loads (thermal and mechanical) around the circumference of a distorted cylinder bore profile was not yet to be fully analyzed. A systematic approach focusing on the relationship between each Fourier order of distortion and various operational loads was not given, though this relationship may be utilized to get a better understanding for the mechanical nature of cylinder liner distortions.

As a different approach, the numerical simulation based on finite element method (FEM) is very useful to predict the three-dimensional distortions of cylinder liner. These approaches take into account pretightening force, heat transfer, and complicated combustion process which the above analytical models neglect. Soua et al. [13] proposed a classical FE model of engine structure to deal with the computation of cylinder liner distortions. However, this model was excessively simplified by using the symmetry assumptions. In fact, Maassen et al. [14] experimentally observed that both the stress fields and distortion patterns on cylinder liner are by no means axis-symmetric and rotational symmetric. With the development of 64-digit computers and refined calculation technique, several studies [15–19] further complemented the full three-dimensional FE model and simulate the physical behavior to evaluate the structural integrity of different design schemes. Although realistic distortions can be precisely captured at any interest point of cylinder liner in these studies, some critical issues for distortion optimization also remain. For instance, keeping the amplitude of the integral distortion to be the minimum does not always guarantee the best solution in the cylinder liner design. Actually, such integral distortion has long been considered as a composite of various orders of bore distortions, and different orders are sensitive to different operational loads. To truly optimize a design, detailed information for the dominant order of distortion is very important.

In summary, the following factors are vital to the success of distortion optimization:(i)Precise prediction of the distortion patterns on cylinder liner.(ii)Clarification of the relationship between each Fourier order of distortion and various operational loads.(iii)Parameter characterization of design features which are critically relative to cylinder liner distortions.

In the scope of this paper, integration of the above stated factors enables reasonable evaluation of distortion patterns at the beginning. Then, information from an extensive rating of spectrum characteristic of bore distortions and operational loads can be utilized to identify the critical design features. Based on this, the parametric analysis is performed to obtain the variation tendency of each dominant order of distortion, and optimal design scheme can be achieved based on these findings.

#### 2. Computational Model

To establish the analytical methodology of the distortion mechanism of cylinder liner, a line style gasoline engine, having 4 cylinders and 4 strokes, is adopted in this paper. A precise structural analysis with appropriate boundary conditions is of critical importance for the prediction of realistic distortions on cylinder liner. The first phase is to define the complex structural configurations of engine components. Figure 1 shows the FE model of the cylinder structure and water jacket. Since high temperature and stress gradients are anticipated in and around the bridge area of both the water jacket and cylinder structure, these sensitive regions are meshed with high resolution (i.e., the element aspect ratio is approximately 2.0) to precisely capture the physical behaviors occurred on the solid-fluid interface. The total numbers of the elements and nodes in the model are 601 736 and 690 912, respectively. The material properties of the components taken from literature [20] are utilized in this analysis.