The present paper describes a CAE analysis approach to evaluate the design of exhaust manifold of a turbo charged gasoline engine. It allows design engineers to identify structural weakness at the early stage or to find the root cause of exhaust manifold failures. A transient none-linear finite element method is used to calculate the plastic deformation and thermal mechanical behaviors of the exhaust manifold assembly during thermal shock cycles, which include rated speed full load, rated speed motored and idle speed conditions. A transient heat transfer simulation is performed to provide thermal boundary conditions for the nonlinear stress/strain analysis. The finite element model includes a part of cylinder head, exhaust manifold, gaskets, turbo charger housing, catalytic converter, brackets, bolts and nuts. The results show that plastic deformation is the main cause of manifold cracking and the manifold flange distortion causes the exhaust leakage. The simulation results indicate that predicted crack locations and leak area are in agreement with that from the engine durability test. Based on the baseline calculation results, local geometric modifications are made, which include changed shape of the inlet flange, changed location of anchor bolt hole and removing the internal baffle. For the modified design of the exhaust manifold, the cumulated equivalent plastic strain and the gasket sealing pressure at the end of third cycle meet the guideline limits. The modified exhaust manifold successfully passed all tests. Finally, general design recommendations of exhaust manifold are summarized in the paper.
