It is a common practice to mount a catalytic converter directly at the outlet of the exhaust manifold in order to reduce the cold start emissions from the automotive engines by improving the light-off time. The so called Close-Coupled Converter, because of its nearness to the engine, is more exposed to non-uniform fluid flows comingfrom the individual manifold runners, what frequently yields a stream of gases flowing mainly through a section of the monolith causing the aging of the catalyst and low conversion efficiency, among other negative effects. Computational Fluid Dynamics (CFD) has became a very useful and widely used tool to analyze and optimize this kind of exhaust after-treatment systems in a relatively fast and accurate way for design purposes. There are several designs of exhaust manifolds whose shape and dimensions are mainly restricted not only by the engine characteristics but by the space constraints of the particular vehicles they are designedfor. In this work the commercial CFD code ANSYS FLUENT was used to evaluate and compare the effect of three shapes of exhaust manifolds on the fluid flow uniformity at the entrance of the Close-Coupled Converter. Pressure drop in the entire Manifold-Converter device is also an important parameter considered for the design evaluation. The manifolds investigated are of type cast, 4-2-1 and L-shaped, which are commonly used in the automotive industry.
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