Automotive Science and Engineering

---

This paper presents low cycle fatigue (LCF) life prediction of an engine exhaust manifold. First Solidworks software was used to model the exhaust manifolds. Then Ansys Workbench software was used to determine stress and fatigue life based on Morrow and Smith-Watson-Topper (SWT) approaches. Thermal fatigue (TMF) of the engine components easily happens due to excessive temperature gradient and thermal stress. Modern exhaust systems must withstand severe cyclic mechanical and thermal loads throughout the whole life cycle. The numerical results showed that the temperature and thermal stresses have the most critical values at the confluence region of the exhaust manifolds. This area was under low cycle fatigue. After several cycles the fatigue cracks will appear in this region. The results of the finite element analysis (FEA) correspond with the experimental tests, carried out in references, and illustrate the exhaust manifolds cracked in this region. Finite element (FE) simulation proved a close correlation between Morrow and SWT criterions results. The lifetime of this part can be determined through finite element analysis instead of experimental tests.
 

---

The confluence area cracks is most important durability problems in internal combustion engines. The aim of this article is a thermo-mechanical analysis for exhaust manifold using elasto-viscoplastic chaboche model. The Chaboche model was selected for the elasto-viscoplastic model including a kinematic hardening plastic law coupled with the Norton creep equation. The modeling, meshing and analyzing was performed on a finite element model of the exhaust manifold in ABAQUS software. In order to increase the accuracy of finite element analysis (FEA) results, temperature-dependent of material parameters was considered. The results of mechanical-thermal analysis showed that the temperature maximum and stress is visible in the confluence area. Obtained FEA results proved the manifold gasket leak is another region of critical that has to sustain the expanding and contracting of the heated exhaust manifold metal. The results of the modal analysis proved that the maximum strain energy density and total strain energy exist in the confluence area. The results of the thermo-mechanical analysis are compared with the real sample of the damaged exhaust manifold to evaluate the properly results, and it has been shown that serious identified zones correspond to the failure areas of the real sample.

---

Generally, parts whose geometric form in the final configuration, or before additional operations such as machining, does not require special dimensional accuracy are produced using casting methods. Producing parts with this method results in significant deviations in dimensions and geometric forms from the main designed geometric model. Due to economic considerations, scrapping such parts will result in energy and material waste and high costs. In this study, dimensional deviations at the exhaust manifold outlet and deviations from the defined geometric tolerance limits of the part during the repair process are identified as geometric non-conformities and investigated using computer tools. Considering the harsh operating conditions under which the engine is under full load, the temperature distribution is determined using computational fluid dynamics, and the thermo-elastic stress distribution is calculated using the finite element method, accounting for the loads applied to the structure. The main part model and the non-conforming models with upper and lower limits in geometric dimensions have been investigated with respect to the deviation in thermo-elastic stress relative to the reference value in the part with the nominal size. The results showed that, given the amount of stress changes in the desired area of parts with deviations from the main design, these parts are also usable and have a lifetime almost the same as a part produced with the nominal size.