Study On Thermal Stress Alleviating Technology Of Diesel Exhaust Manifold

As the diesel engine becomes more intensified, the thermal load of the keycomponents becomes higher. Directly contacting the high speed and high temperatureexhaust, working in a toughing environment, the exhaust manifold is a main part bearingthe thermal load. Traditional approach to design the exhaust manifold is hard to satisfy thereliability requirement of high-intensified diesel exhaust manifold. This paper aims atsolving the local stress concentration problem emerged at the heat intensity analysis ofexhaust manifold. This paper starts from relieving the restriction of distortion of high-stressarea, and researches on the relief of thermal stress at the exhaust manifold. This paperprovides a guideline and theoretic basis of designing high-intensified diesel exhaustmanifold.The main contents and conclusions are:1.To simulate the heat-transfer and structure of the exhaust system more accurate, thispaper builds a FEM model of exhaust system, and sets the boundary conditions accordingto empirical formula. This paper also adjusts and modifies the model according to thetemperature field obtained by experiment, and calculates the stress field under thermal loadcondition and thermal-mechanical coupled condition using the modified temperature fieldas thermal load. This paper studies the influence on the calculating result of different modelincluding elastic model at ordinary temperature, plastic model at ordinary temperature,andelastic/plastic model at varying temperature. The result shows that the bolt hole and thebend area at the exit of exhaust manifold are the risk areas, and special measurement forrelieving the stress shall be taken.2.This paper builds the local flange-bolt model and optimizes the thermal structuredesign of flange. This paper proposes two new structures of the flange, and comparesbetween the two structures by analyzing the stress distribution. The result shows that thesecond structure is better to satisfy the requirement, which reduces the maximum stress by9.8%, and reduces the maximum stress at the bolt hole by26.7%. Stress in the axil directionof the bolt hole is relieved by11%. The hollow structure, helpful to relieve the thermal stress at the flange, is useful in practise.3.To relieve the stress at the bend area of exhaust manifold, this paper studies the mainfactors influencing the thermal stress of exhaust manifold and other relevant trends, andproposes technical measurement to relieve such thermal stress. The result shows that thethermal stress may be relieved by changing the curvature radius, wall thickness, flangestructure or rigidity of cylinder head, or by setting stress dispersion slots. This paperprovides theoretic basis for designing the exhaust manifold.