Study On The Thermal Fatigue Performance Of Exhaust Manifold Of Automobile Engine

With increasingly stringent emission regulations and the continuous improvement of engine performance, the temperature of engine exhaust also increases. The exhaust manifold is the first part that the waste gas which come from the engine will flow through, it’s always in the high temperature condition at work. When the exhaust manifold get heated, it will expand. Under the constraint of internal and external part, it will generate heat stress. Constant work on the condition between thermal expansion and cooling contraction cycle, the exhaust manifold is easy to produce plastic deformation and eventually produce crack or damage. The attention to the exhaust manifold thermal load and thermal fatigue must get strengthen. Based on this, study on the thermal fatigue performance of the exhaust manifold used on the Engine 4A15S.This main research contents and conclusions of this issue are as follows:(1)For the inner runner of exhaust manifold, carry on the steady flow field analysis which the inlet velocity is constant. The function of exhaust manifold is to make the engine exhaust flow into the ternary catalytic carrier, finally flow into the air through the exhaust pipe. The liquidity and the uniformity of the flow velocity is its most basic performance. It’s needed to conduct flow field analysis to aware of these two kinds of performance. Firstly, conduct the grid independence test for the flow field, according to the results, it’s determined that the grid size will be used. Using the grid size which is got from the grid independence test, carry on the steady flow field analysis for the inner runner of exhaust manifold, the results show that the liquidity and the velocity uniformity of this exhaust manifold is good.(2)The equivalent scheme of the entrance boundary conditions of the exhaust manifold which can be used for calculating the equivalent plastic strain is proposed. If completely use the actual inward and outward boundary conditions of the exhaust manifold for the thermal FSI to calculate the steady haet load at work, the amount of data need to be calculated is too large for the normal computer to do this. According to the law of conservation of quality and energy, a equivalent scheme of the result of the one-dimensional simulation is proposed. A simplified model is used to vertify that this equivalent scheme can be used to the calculation of the equivalent plastic strain of the structure.(3)A one dimensional engine performance simulation model is established. To get the inward and outward boundary conditions of the calculation of the heat load of the exhaust manifold, establish a one dimensional engine performance simulation model for the Engine 4A15S, which is vertified by the experiment. Verification results show that the calculation result is consistent with the experiment, the simulation model can be used for the further research.(4)The heat load analysis of the exhaust manifold. Using the equivalent inlet boundary, using the body mapping method, build a thermal FSI model for the exhaust manifold, and get the heat load of the exhaust manifold within a work cycle. The most dangerous position of the exhaust manifold is found.(5)The thermal fatigue life analysis of the exhaust manifold. Using the equivalent plastic strain increment(△PEEQ) within a work cycle of the exhaust manifold which is calculated, using the Manson-Coffin formula to calculate the thermal fatigue life of the exhaust manifold. Study on the influence of the value of the C and α in the Manson-Coffin formula to the thermal fatigue life. It’s found that the value of α is very important to thermal fatigue life calculated by the Manson-Coffin formula. Take wall thickness of the exhaust manifold, coefficient of linear expansion, yield strength and elastic modulus of the material as design variables, the sensitivity analysis of the exhaust manifold thermal fatigue life is conducted. The result shows that within these 4 parameters, the thermal fatigue life of the exhaust manifold is the most sensitive to the wall thickness. Within the material parameter, the thermal fatigue life is the most sensitive to the coefficient of linear expansion. The bigger the wall thickness and the yield strength are, the bigger the thermal fatigue life are.The innovation points of this topic are listed as follows:(1) The equivalent scheme of the entrance boundary conditions of the exhaust manifold is proposed. A simplified model is used to vertify that this equivalent scheme can be used to the calculation of the heat load of the structure. Compared with the commonly used time domain average method, this equivalent scheme is simpler and more convenient to calculate, the feasibility is higher. When it’s used to calculate the thermal FSI, it can realize the dynamic delivery of physical quantities such as the displacement and the pressure between fluid and solid.(2)Carry on the analysis of the parameter sensitivity of the thermal fatigue life of exhaust manifold, and find the most sensitive parameter to the thermal fatigue life of exhaust manifold.(3)A simplified model is built to compare the influence of two kinds of different thermal boundary setting method—the experience value method and the analysis of the steady state flow field method—to the calculated temperature field, it is founded that the difference of the results is very small. Using the body mapping method, build a thermal FSI model for the exhaust manifold, and get and analyze the heat load of the exhaust manifold within a work cycle.