Abstract: Automobile internal combustion engine connecting rod is a high volume production component subjected to complex loading。 Proper optimization of this component, which  is critical to the engine fuel efficiency and more vigorously pursued by the automotive industry in recent years, necessitates a detailed understanding of the applied loads and resulting stresses under in-service conditions。 In this study, detailed load analysis under service loading con- ditions was performed for a typical connecting rod, followed by quasi-dynamic finite element analysis (FEA) to capture stress variations over a cycle of operation。 On the basis of the resulting stress-time histories, variation of stress ratio, presence of mean and bending stresses, and multi- axiality of stress states in various locations of the connecting rod under service operating con- ditions were investigated。 It was found that even though connecting rods are typically tested and analyzed under axial loading and stress state, bending stresses are significant and a multiaxial stress state exists at the critical regions of connecting rod。 A comparison is also made between stresses obtained using static FEA which is commonly performed and stresses using quasi- dynamic FEA。 It is shown that considerable differences in obtained stresses exist between the two sets of analyses。86275

Keywords: connecting rod load analysis, connecting rod stress analysis

1    INTRODUCTION

Automobile internal combustion engine connecting rod is a high volume production critical component。 It connects reciprocating piston to rotating crank- shaft, transmitting the thrust of piston to crankshaft, and is subjected to complex loading。 It undergoes high cyclic loads of the order of 108 – 109 cycles, which range from high compressive loads because of combustion, to high tensile loads because of inertia。 Therefore, durability of this component  is of critical importance。 Usually, the worst case load is considered in the design process。 Literature review suggests that investigators [1, 2] use maxi- mum inertia load as one extreme load corresponding to the tensile load and compressive gas load produ- cing maximum torque as the other extreme   design

sCorresponding author: Department of Mechanical, Industrial, and Manufacturing Engineering, The University of Toledo, 2801

W。 Bancroft St, Toledo, OH 43606, USA。 email: afatemi@ eng。utoledo。edu

load corresponding to the compressive load。 In recent years, more emphasis has been placed on higher vehicle fuel efficiency。 Optimization of con- necting rods in an engine is critical to fuel efficiency。 Proper optimization of this component, however, necessitates a detailed understanding of the applied loads and resulting stresses under in-service conditions。

Inertia load is a time-varying quantity and can refer to inertia load of the connecting rod or of the piston assembly。 Questions are naturally raised in light of such complex structural behaviour such as: Does the peak load at the ends of a connecting rod represent the worst case loading? Under the effects of bending and axial loads, can one expect higher stresses than that experienced under axial load alone? Moreover, very little information is available in the literature on bending stiffness requirements, or on the magnitude of bending and multiaxial stresses。

Webster  et  al。  [2]  performed three-dimensional

finite element analysis (FEA) of a high-speed diesel

616 P S Shenoy and A  Fatemi

engine connecting rod。 They used maximum com- pressive load which was measured experimentally, and maximum tensile load which is essentially the inertia load of piston assembly mass in their analysis。 Load distributions on the piston pin end and crank end  were  also   determined   experimentally。 Ishida et al。 [3] measured stress variation at the column center and column bottom  of  connecting  rod,  as well as bending stress at the column center。 From their study it was observed that at  high engine speeds, the maximum stress in connecting  rod column bottom does not occur at  the  top  dead center。 It was also observed that the  stress  ratio varies with location, and at a given location it varies with engine speed。 The maximum  bending  stress over one engine cycle at the column center  was found to be about 25 per  cent  of  the  maximum stress  at  that location。