Modal analysis of 4-cylinder engine crankshaft based on ANSYS Workbench

LI Xue-min, CUI Zhi-qin

(College of Mechatronic Engineering, North University of China, Taiyuan 030051, China)

李学民， 崔志琴

(中北大学 机电工程学院， 山西 太原 030051)



Modal analysis of 4-cylinder engine crankshaft based on ANSYS Workbench

LI Xue-min, CUI Zhi-qin

(CollegeofMechatronicEngineering,NorthUniversityofChina,Taiyuan030051,China)

CATIA V5R20 software was utilized to build up a three-dimensional solid model of inline four-cylinder gasoline engine crankshaft. The free modal analysis of the first six orders from the whole crankshaft model was carried out based on TGrid algorithm by using ANSYS Workbench14.0 software and the natural frequency and vibration modes were obtained. The reliability of the finite element model was verified by comparing with modal test result. This provides a reference for further design and optimization of the crankshaft.

ANSYS workbench; model analysis; natural frequency; vibration mode

0 Introduction

The crankshaft, which is one of the most important part of an internal combustion engine, carries the heaviest load. The crankshaft in operation is mainly affected by gas pressure, reciprocating force and rotating inertia force, which produce alternating bending stress and torsion stress to the crankshaft[1-4]. These torsion and bending stresses result in severe fatigue damage, which is disastrous for input checking equipment (ICE). Therefore, we need to obtain more accurate crankshaft stress, deformation, natural frequency and vibration mode since they have very great significance to design of the crankshaft and overall performance improvement of the internal combustion engine[5-7].

This paper uses three-dimensional modeling software CATIA V5R20 to establish the physical model of an inline four-cylinder gasoline engine crankshaft and then takes advantage of finite element software ANSYS Workbench14.0 to carry out free modal analysis, aiming at obtaining the lower order natural frequencies and vibration modes of the crankshaft.

1 Establishment of crankshaft model

The main dimensions of the inline four-cylinder gasoline engine crankshaft model are shown in Table 1.

Table 1 Main dimensions of crankshaft model

1.1 Simplification of crankshaft

The structure of crankshaft contains many small fillets, chamfers and fine oil holes. If these factors are considered in the process of crankshaft modeling, it not only makes the mesh of the finite element very dense, but also greatly increases the number of nodes equation. Moreover, it needs more time for data preparation and computer processing, which will cause undesirable element shapes and reduce accuracy of the solution. Therefore, when simplifying the crankshaft, we refer to experience of other scholars in some modal calculations, ignoring threaded holes, center holes and the positioning holes of the front and rear crankshaft as well as the characteristics of small fillets, chamfers and fine oil holes.

1.2 Establishment of solid model of crankshaft

The structure of crankshaft is very complex, but the modeling capability of finite element software ANSYS Workbench is relatively weak[8]. Therefore, we choose CATIA software, with the function of three dimensional modeling, to establish the solid model of engine crankshaft. According to the model size, we draw a sketch in the first place, and then map complete three-dimensional solid model of the crankshaft through a series of stretching, plane creating and mirroring operations, as shown in Fig.1.

Fig.1 Three-dimensional solid model of crankshaft

1.3 Establishment of finite element model of crankshaft

Three-dimensional solid model of the four-cylinder gasoline engine crankshaft set up in CATIA is transplanted to ANSYS Workbench. The material of crankshaft is No.45 steel, elastic modulus is 2.06 GPa, Poisson’s ratio is 0.3, and density is 7 850 kg/m3. Grid mesh uses relevance with overall quality precision of 100, span range of the central angle from -36° to 12°, smoothing rate of 0.272 and the expansion of 5 layers. Based on TGrid algorithms of expansion layer, grid mesh is carried out from the edge and refined at the larger curvature. Then surface mesh is generated, finally a volume mesh produces. Eventually, the three-dimensional finite element model of the crankshaft is established and shown in Fig.2.

Fig.2 3D finite element model of crankshaft

2 Principle of crankshaft modal analysis

According to vibration theory and finite element theory, the vibration differential equation with a finite number of free elastic systems is

(1)

Inmodalanalysis,withouttheeffectofexcitationforce,wetake{F(t)}={0}andthengetfreevibrationequation.Thenwesolvenaturalfrequenciesandvibrationmodeofthestructuralfreevibration.Structuraldampingissmall,whichhaslittleinfluenceonnaturalfrequencyandvibrationmodeofthestructure,thereforeitcanbeneglected.Thusthedifferentialequationofmotionofthestructuralundampedfreevibrationisobtainedas

(2)

Thecorrespondingcharacteristicequationis

(3)

InEq.(3),theletterωreferstonaturalfrequencyofthesystem.

Theexpansionofcharacteristicequationcoefficientgetsn-orderpolynomialaboutωanditssolutionobtainsnaturalfrequencyofthestructure.TakingthemintoEq.(3),thecharacteristicvectorandvibrationmodelofagivenfrequencyisgot.

3 Results of crankshaft modal analysis

3.1 Natural frequency of crankshaft

Natural frequencies and natural modes are determined by geometrical structure, material characteristics and forms of constraints. This paper uses free modal analysis method to calculate the free mode of the crankshaft without any restraint and force. Based on the TGrid algorithms of expansion layer, the first 12 order modals of the crankshaft are extracted from ANSYS Workbench software. Because there is no any constraint and force imposed on the crankshaft, the natural frequency of the first sixth modals is zero, and the seventh is the first meaningful natural frequency[9-10]. Therefore, we take the natural frequencies and vibration modes of the 7th-12th order modals of the crankshaft as those of the first six orders, and their natural frequencies and vibration modes are shown in Table 2. By comparing the finite element models with that of the experiment, it can be seen that the errors are not big, so the finite element is effective and reliable.

Table 2 Natural frequencies and vibration modes of the first six orders of crankshaft

3.2 Figures of vibration mode of crankshaft

By using modal analysis to calculate free mode of crankshaft, the natural frequency of the first sixth modal is zero, and the seventh is the first meaningful natural frequency. The first 12th order modal born ration mode is shown in Figs.3-8 by means of ANSYS Workbench software.

Fig.3 Vibration mode of first order modal

Fig.4 Vibration mode of second order modal

Fig.5 Third order modal vibration mode

Fig.6 Vibration mode of forth order modal

Fig.7 Vibration mode of fifth order modal

Fig.8 Vibration mode of sixth order modal

4 Conclusions

1) From dynamic displays of vibration modes of the crankshaft, we can find the first six natural frequencies and vibration model figures of the crankshaft, which show that bending and torsion are the main forms of deformation in the process of vibrating. As the frequency increases, dangerous modes may occur. By analyzing experiment data, the areas where crank arm is connected with rod journal are dangerous, and they also are the biggest deformation areas in the vibration of crankshaft. Therefore, for the crankshaft design, the parameters of the crank arm should be fully considered.

2) By analyzing the finite element model of inline four-cylinder gasoline engine crankshaft and comparing it with the experiment data, it can be known that the errors are not big, which verifies reliability of the finite element model and provides some reference for optimal design of the crankshaft in the future.

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基于ANSYS Workbench的四缸发动机曲轴模态分析

利用CATIA V5R20软件建立了直列四缸汽油机曲轴的三维实体模型， 再利用ANSYS Workbench14.0软件基于TGrid算法对整个曲轴模型进行前6阶自由模态分析， 得出曲轴的固有频率和振型， 并通过模态敲击试验进行对比， 验证了有限元模型的可靠性， 为曲轴的进一步设计以及优化提供了重要参考依据。

ANSYS Workbench; 模态分析； 固有频率； 振型

LI Xue-min, CUI Zhi-qin. Modal analysis of 4-cylinder engine crankshaft based on ANSYS Workbench. Journal of Measurement Science and Instrumentation, 2015, 6(3)： 282-285. [

李学民， 崔志琴

(中北大学 机电工程学院， 山西 太原 030051)

10.3969/j.issn.1674-8042.2015.03.014]

LI Xue-min (xueming666@163.com)

1674-8042(2015)03-0282-04 doi： 10.3969/j.issn.1674-8042.2015.03.014

Received date： 2015-06-07

CLD number： TK44 Document code： A