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This paper gives an overview of the development work for a diesel after-treatment system, used in heavy duty trucks to fulfill the new US emissions limits. The paper starts with the description of design evaluation and optimization studies on heavy duty diesel exhaust after-treatment system using numerical simulation. The studies involve initial conceptual design evaluation of the entire after-treatment system for fluid flow, temperature distribution, and subsequent structural loads. Computer modeling, as complementary approach to prototyping and experimental investigations, helps to make basic design decisions and therefore to shorten the overall development process. The numerical simulation involves computational fluid dynamics (CFD) analysis for fluid flow and temperature distribution and finite element analysis (FEA) for subsequent structural analysis. The first part of the paper involves computational fluid dynamic optimization study related to diesel exhaust system.

Some design optimization studies of automotive exhaust systems are carried out using numerical simulation. The numerical simulation involves computational fluid dynamics (CFD) for fluid flow and temperature distribution and finite element analysis (FEA) for subsequent structural analysis. The emphasis is given to optimization related to exhaust system design parameters such as shape and profile of manifold, catalyst inlet tube, inlet cone, exit cone, and exit tube under a given exhaust gas conditions. Several examples of optimization involving study of design parameters on the index of flow uniformity and backpressure are illustrated. Some studies in the past have shown that angular inflow in to catalyst substrate would give high flow uniformity index and flow out let profiles may not significantly affect the uniformity flow index near the inlet of catalyst. The present study shows that this is not always the case and some examples are illustrated to highlight these aspects.

The ability to accurately predict skin temperatures of catalytic converter and manifold is very important for a robust/durable design of the vehicle exhaust system, especially in the development of close coupled converter system. In this paper, Computational Fluid Dynamics (CFD) is used to calculate the skin temperature of complicated components in vehicle exhaust system such as catalytic converter. Generally, a catalytic converter consists of substrate, mat, outer shell, inner cone, cone insulation, and outer cone. 3-D compressible turbulent fluid flow with heat transfer involved in force and natural convections, heat conduction and radiation is numerically simulated. First, both numerical calculation and experimental tests are conducted for a catalytic converter under the same operation conditions to evaluate the accuracy of current numerical method. Good agreement is found between CFD prediction and experimental tests.

Stricter emission standards are forcing automakers to attach catalytic converters directly to the exhaust manifold. Mounting catalytic converter at or near the exhaust manifold helps to reduce the increase in emissions that occurs during the first few minutes after a cold engine is started. The spike occurs because cold engines require a richer air-fuel mixture to run smoothly. The emission standards can be met only by new designs of exhaust system with the catalyst being as close as possible to the engine, and with the thin walled exhaust manifolds. With concept-to-customer timing continuously shrinking in the automotive industry, the need to quickly validate the engineering designs is becoming ever more critical. It is no longer acceptable to design a component, produce soft tooling, build and test a prototype, analyze what failed, and then redesign.

In order to meet the mandated EPA2010 emissions for heavy duty commercial vehicle regulations, most applications require very large, complex, yet compact exhaust after-treatment systems. These systems not only contain the necessary substrates and filters to perform the proper emissions conversion, they also typically will consist of mixing pipes and internal reversing chambers all within very tight space proximity. Some of these systems are able to accomplish the complete emissions reduction and conversion within a single, large packaging unit. While there are advantages in fuel efficiency and perhaps overall packaging with these “single box” units, the disadvantage of these types of designs is that it prohibits many internal components from cooling down by the outside environment, which can pose thermal mechanical durability challenges.