Thermal Fatigue Analysis And Optimization Of An Engine Exhaust Manifold

With the rapid development of Chinese automobile industry, car ownership is constantly growing. It brings us a lot of social problems such as air pollution, noise pollution, energy crisis and so on. The key to solving these problems is designing a good performance exhaust system. As an important part of the engine exhaust system, exhaust manifold’s performance affect the capability of the engine directly. Exhaust manifold is connected to the engine cylinder head directly, so it receives greater thermal shock from exhaust that engine emissions. At the same time, it is affected by the engine vibration. Under the action of spalling of alternating load, exhaust manifold is easy to fatigue failure. Therefore, there are important engineering significances to get on Thermal Fatigue Analysis on exhaust manifold. It also provides a theoretical basis for the research and design of exhaust manifold. With the emission regulations become increasingly stringent, it is particularly important to design a exhaust manifold with superior performance and reasonable structure. This is also a part of our self-developed car brands.Combined with an enterprise of a car engine exhaust manifold,Some large FEM software such as HyperMesh、Fluent、Abaqus and so on are used to go on numerical simulation analysis about internal flow field performance, temperature field, thermal strength, thermal modal and thermal fatigue. The main work is as follows:(1) The internal flow channel of the exhaust manifold is extracted according to the three-dimensional model provided by the enterprise and CFD analysis model of the exhaust manifold is established. What’s more, the simulation analysis on the steady flow field of the exhaust manifold is get on to analyze pressure distribution and temperature distribution in the inner surface of the exhaust manifold and the surface convective heat transfer coefficient.(2) The inner surface temperature and heat transfer coefficient that have been computed are mapped onto the structural finite element model of the exhaust manifold. Then, fluid-solid coupling simulation analysis by interpolation is gotten on to calculate the temperature field of the exhaust manifold.(3)Considering that exhaust manifold is in the action by the bolt preload in the actual assembly process, the thermal strength of exhaust manifold is simulated combined with the temperature field of exhaust manifold in two cycle conditions of engine’s normal operation and shutdown cooling. Then, it sets the temperature under normal operating conditions of engine as the temperature field and sets the temperature under engine’s shutdown cooling as the room temperature and gets on six cycle simulation to analyze the exhaust manifold’s thermal stress and heat distortion..(4)As exhaust manifold is affected by engine and chassis vibration, thermal modal analysis is done to analyze its vibration characteristics under the effect of temperature field and the appropriate evaluation is made to the results of analysis.(5) Considering that exhaust manifold is always in the work environment that is under the influence of spalling alternating load, it is easy to fatigue failure. So it is necessary to analyze the thermal fatigue to explore the equivalent plastic strain in the influence of the six cyclic loading and evaluate its results in accordance with the relevant corporate standards.(6)In the end, based on the above analysis results, the structure of the exhaust manifold is improved reasonably and the comparative analysis is made between the improved exhaust manifold and the original model. The results show that overall performance of the improved exhaust manifold is greatly improved and it can achieve the final design requirements of the product, so it also shows that the scheme can be used to solve real engineering problems.In this paper, finite element method has been used to analyze the exhaust manifold heat fatigue analysis and design optimization systematically. It has greatly shorten the product development cycle, reduced the number of test piece, saved a lot of costs for enterprises and provided a reference for the structural design that is similar to the exhaust manifold.