Exhaust Manifold

Thermal Stress for a Liquid-Cooled Exhaust Manifold


THE PROBLEM

Diagram of the geometry of the manifold.

Liquid-cooled exhaust manifolds are widely used in turbocharged diesel engines. The exhaust manifold which is mounted on the cylinder head of an engine collects the exhausted gas and then sends it to a turbocharger. The manifold suffers from relatively high operating temperature which can lead to significant thermal expansion. Therefore, expansion causes significant thermal stress which may result in crack damage. On the other hand, heat loss from the exhaust gas should be minimized in order to provide the maximum amount of energy to the turbocharger and the compressor.


THE PROJECT

Diagram of the transient temperature distribution.The overall goal of the project is to determine if any thermal stress issues can be prevented in order to increase the durability of exhaust manifold. To achieve the objective, the combination between Computational Fluid Dynamics (CFD) with Finite Element (FE) is introduced. First, CFD analysis is conducted to obtain temperature distribution, providing conditions of the thermo-mechanical loading on the FE mesh. Next, FE analysis is carried out to determine the thermal stress. To accurately quantify the thermal stress, nonlinear material behavior is considered. Based on stresses and strains, the fatigue life can be estimated.


THE OUTCOME

Diagram of the thermal stress distribution.Increasing the exhaust pipe thickness makes maximum stress levels decrease and this will lead a longer life span to the manifold. However, it does not mean the thicker the better. The optimum thickness would be around 4.5mm from the material cost and life span point of view. By using 4.5mm exhaust wall thickness instead of 3mm, the life span of the modified design is expected to extend to 150% of the original design.

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 Exhaust Manifold
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