Low Cycle Thermal Fatigue Life Prediction And Structure Improvement Of A Catalytic Converter Assembly For Passenger Vehicle

Catalytic converter assembly,an important components of engine exhaust system,bears the impact of vibration and thermal load continuously under different engine working conditions.Meanwhile,the average exhaust pressure and temperature of high-power engine is much higher than that of ordinary engine as it develops,which results the thermal load of catalytic converter assembly approaches to the fatigue limit of parts.The occurrence of durability failure in an exhaust hot end,leading to a consequent engine failure,brings about high cost on maintenance and replacement.Therefore,the thermal fatigue life research for catalytic converter assembly is of great significance.The main research contents of this paper are as follows:Simulated the temperature field of the catalytic converter assembly,analyzed and verified the results.Used CATIA to establish a three-dimensional model of catalytic converter assembly according to some passenger car model.Used ANSA to preprocess the geometry of the model and to divide surface grids.Used STAR-CCM+to divide the polyhedron grids,and set the boundary conditions of the catalytic converter assembly for calculation.Conducted a temperature bench test on the catalytic converter assembly under a full engine load.A comparison between the experimental results and simulated data found the maximum error of 8.2%occurred in bracket（Point G）,within the tolerance of 10%conformed to the standard of the enterprise.Thus,this simulation model was proved to be highly reliable,which made it credible to provide temperature boundary for future study on low cycle thermal fatigue.Conducted a simulation analysis of thermal cycle on the catalytic converter assembly.Mapped the simulation results of temperature field to the finite element model of the catalytic converter assembly;Established a complete thermal cycle simulation model of catalytic converter assembly,with the T_maxax calculated through simulation and T_minin assigned20℃;The PEEQ and?PEEQ variations of the maximum equivalent plastic strain were obtained in the calculation process of four thermal cycles.The maximum equivalent plastic strain was located at the hole opening edge of the heat shield on the catalytic converter housing,and the maximum value after four thermal cycles was 3.374%,providing important parameters for the prediction of low cycle thermal fatigue life of the catalytic converter assembly.Used the low cycle thermal fatigue life prediction formula of SUS441 catalytic converter assembly to obtain parameters and compared them with the test results.Conducted tensile test under high temperature to obtain the test data of SUS441 material under different test temperatures.The material yield ratio was concluded to be approximate to 0.7 at temperatures below 700℃according to varied stretching temperatures yield ratio,showing that the material deformation resistance to changes in the process of repeated plastic deformation is not obvious.Thus,we got the life prediction formula with adjusted parameters.Suggested by the thermal shock test of the catalytic converter assembly,the modified formula（2013 cycles）is more practical than the empirical general formula（857cycles）,with 2400 cycles being the CAE standard of the catalytic converter assembly,providing theoretical guidance for the sample improvement.Carried out structural improvement and CAE verification on catalytic converter assembly.The sensitivity analysis suggested that the low cycle thermal fatigue life of catalytic converter assembly was the most sensitive to the thickness of catalytic converter shell,with its relative sensitivity up to 87.3%.Therefore,an improvement scheme to change the thickness of catalytic converter shell to 2mm is proposed.The constrained thermal mode analysis on the catalytic converter assembly showed that the natural frequency value of the constrained thermal mode in order 1⁴ in the improved scheme is greater than the operating frequency of the engine,which meant that there is no resonance hazard between the improved scheme and the engine.The simulation analysis of the temperature field and thermal cycle were carried out for the improved scheme,and the low-cycle thermal fatigue life of the improved scheme predicted based on the research results of life prediction.The results showed that the life prediction of the improved scheme was 3579 cycles,higher than the standard of 2400 cycles discussed in the study.Therefore,the improved scheme was verified by CAE simulation.