Thermomechanical Evaluation of Close Coupled Converter System 2005-01-1623

Stricter emission standards are forcing automakers to attach catalytic converters directly to the exhaust manifold. Mounting catalytic converter at or near the exhaust manifold helps to reduce the increase in emissions that occurs during the first few minutes after a cold engine is started. The spike occurs because cold engines require a richer air-fuel mixture to run smoothly. The emission standards can be met only by new designs of exhaust system with the catalyst being as close as possible to the engine, and with the thin walled exhaust manifolds.

With concept-to-customer timing continuously shrinking in the automotive industry, the need to quickly validate the engineering designs is becoming ever more critical. It is no longer acceptable to design a component, produce soft tooling, build and test a prototype, analyze what failed, and then redesign. Instead, heavy use is being made of advanced CAE methods, such as Finite Element Analysis (FEA) and Computational Fluid Dynamics (CFD).

In this paper, a systematic method is developed for analyzing the stresses produced in the close coupled system, consisting of thin walled fabricated tubular exhaust manifold and converter, due to thermo-mechanical loading utilizing both FEA and CFD. The correct calculation of temperature distribution in the manifold and converter is very important for accurate calculation of thermal durability. Hence, first, computational fluid dynamics (CFD) is used to predict transient flow and thermal conditions in the manifold and converter. The converter monolith, mat, air gap between inner and outer cones are considered in the calculations. The temperature distribution predicted in the manifold and converter is compared against the experimentally measured numbers. Next, the temperature distribution obtained is used to calculate the thermal stresses/strains using finite element analysis. Because of the high temperatures involved in exhaust system operation, both material and geometric non-linearity are considered in the structural analysis. Specifically, the calculation of material response behavior under several thermal cycles, with each cycle involving a heating and a cooling stage, that results in temperature gradients in the manifolds and converter and their overall effect on fatigue life are made. The contraction and expansion behavior of the mat during heating and cooling are also included in the analysis. All the data such as stresses/strains, deformation, and contact forces during heating and cooling for the manifold and converter assembly are calculated.