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The function of the inlet header of a catalytic converter is to diffuse the inlet exhaust flow, decreasing its velocity and increasing its static pressure with as little loss in total pressure as possible. In practice, very little diffusion takes place in most catalytic converter inlet headers because the flow separates at the interface of the pipe and the tapered section leading to the substrate. This leads to increased converter pressure loss and flow maldistribution. An improved inlet-header design called the Enhanced Diffusion Header (EDH) was developed which combines a short, shallow-angle diffuser with a more abrupt expansion to the substrate cross section. Tests conducted in room air (cold flow) and engine exhaust showed that improved inlet-jet diffusion leads to substantial reductions in converter restriction. EDH performance was not compromised by the presence of a right-angle bend upstream of the converter.

Temperatures were measured in the exhaust manifolds, takedown pipes, and post-converter components (tailpipe section) of several passenger cars under equilibrium conditions on a chassis dynamometer. Cast and fabricated exhaust manifolds of several designs were instrumented, as were single-wall and double-wall takedown pipes. Steady-state heat fluxes in exhaust components correlated with interior Reynolds numbers regardless of the engine operating condition. Nusselt numbers on component interior and exterior wall surfaces also correlated with Reynolds number. The interior Nusselt number was greater than that calculated for ideal flow with fully developed boundary layers. The ratio of measured to ideal-flow Nusselt numbers was relatively constant for a given component. These parameters are required for analytical modeling of the heat transfer performance of exhaust components, e.g., for the study of underhood and underbody heat loads.

The steady flow-field inside a monolith catalytic converter was examined by means of water-flow visualization. These tests, conducted with transparent, full-scale converter models with several different header geometries, showed that flow invariably separated from the inlet-header diffuser walls. A constant-diameter jet proceeded to the front monolith face, where it impacted and expanded to cover the substrate frontal area. For some visualization tests, the jet was constrained within a transparent tube which was translated toward the front monolith face, simulating shorter and shorter headers. The monolith internal flow field and pressure loss were found to be unaffected until the tube was within a few centimeters of the substrate. A converter with very short inlet and outlet headers is termed a truncated converter.

Pressure-loss characteristics of a variety of single- and double-substrate metal-foil and ceramic-substrate converters with tapered and truncated inlet and outlet headers were measured in room-air flow, hot-gas flow, and engine-exhaust tests. Test data in the three different media correlated with the inlet-pipe Reynolds number when expressed as a loss coefficient, i.e., pressure loss normalized by the inlet-pipe dynamic head. Because restriction measurements made in different media correlate well as a Reynolds number-dependent loss coefficient, inexpensive room-air test data can be used to estimate converter pressure losses in the engine environment. The normalized losses in the substrate varied inversely with inlet-pipe Reynolds number, ranging from, e.g., 6 at Re = 30 000 to 2 at Re = 200 000. The remainder of the losses occurred in the inlet and outlet headers and in the section between the substrates.

The peak power of some vehicles can be limited by exhaust system backpressure contributed by a catalytic converter. A computer model was developed for flow and pressure drop in a generalized single-bed bead-bed catalytic converter to determine the sources of converter pressure drop and suggest improved designs with lower restriction. In an accompanying experimental study, pressure losses in the components of two different converters were determined in an engine-dynamometer test cell over a wide range of engine operating conditions. The measured and predicted pressure drops were in good agreement. The experience gained in this study was used to develop a low-restriction converter for truck applications.

Catalytic converters will be required in some future truck exhaust systems because of recently enacted emission standards. A computer model was developed to evaluate means to cool the exhaust to protect the converters to improve their durability. The model showed that changing the pipe diameter or wall thickness had little effect on exhaust cooling. Adding internal ribbing or exterior fins improved the exhaust cooling by only a few degrees C. Counterflow air-cooling was not found to be effective, but counterflow water-cooling was able to cool the muffler-out exhaust gas below the target temperature £or all test conditions.

Internal flow details of a prototype dual-bed monolith converter were determined in water-flow visualization tests run on a full-scale transparent acrylic model. Using steadily flowing water seeded with a small quantity of tracer particles, fluid motion within transparent sections of the flow model was deduced from particle pathlines illuminated with a thin plane of laser light. Flow in the inlet transition separated from the diffuser walls and impinged as a constant-diameter jet on the leading face of the first monolith. Velocity profiles from streak photographs showed that the level of flow maldistribution in the first monolith was a function of Reynolds number. Secondary air injected between the monoliths was uniformly distributed along the major axis of the converter under all flow conditions. At dilution ratios of 16% or more the jet penetration was adequate to provide a uniform, well-mixed diluent distribution.