Influence of Spacing on the Thermal Efficiency of a Dual-Monolithic Catalytic Converter During Warmup

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A one-dimensional monolithic catalyst model was used to evaluate the effect of space between two bricks on the thermal efficiency of a dual-monolithic catalytic converter with a Palladium-only (Pd-only) catalyst and a Palladium/Rhodium (Pd/Rh) catalyst butted together, and which are, in turn, mounted on a commercial vehicle equipped with a 2-L, four-cylinder spark ignition (SI) engine. Prior to the numerical investigation of the converter, tuning of the preexponential factor and activation energy of each reaction for each catalyst was performed to achieve acceptable agreement with experimental data under typical operating condition of automobile application. Two higher cell density 600 cpsi/4 mil substrates were used for faster light-off and improved warm-up performance of the catalytic converter, and the two monoliths have been connected without a space between them. As a result of the constriction of the monoliths, a slight temperature decrease occurred at the substrate interface due to thermal contact resistance. Therefore, to examine the heat transfer mechanism throughout the conforming rough surfaces, the thermal joint conductance between adjacent monoliths was determined using existing theory and correlation. The adequacy of the theory and correlation for thermal joint conductance was elucidated by analyzing heat transfer across the joint. Additionally, parametric investigations were performed to examine how the apparent contact pressure and mass flow rate of the exhaust gas affects the overall temperature drop across the joint. Over a wide range of operating conditions, temperature drop across the interface in the dual-catalyst converter with an air-gap is smaller than that without an air-gap. The relatively large temperature drop across the interface occurs especially during the early warmup period with a lower mass flow rate, that is, under conditions of cold-start and warmup at idle.