Thermal Modelling of Aerogel-Based Washcoats for Three-Way Catalytic Conversion: Effects of Washcoat Composition and Thickness on Light-Off Time


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Open Access


Mechanical Engineering


Three-way catalytic converters typically consist of an encased cordierite honeycomb structure, coated with an alumina washcoat impregnated with platinum group metals (PGMS). While effective under normal operating conditions, there is typically a significant warm-up time during which the TWC is ineffective. Aerogels are nanoporous materials that have a large surface area, low density, and low thermal conductivity. The use of aerogel in place of the more dense and thermally conductive alumina washcoat might reduce the time needed to heat up the TWC and decrease overall pollutant emissions. This idea was investigated using a one-dimensional model to simulate heat transfer in a cordierite wall with three different coatings: silica aerogel washcoat, a catalytically active copper-alumina (CuAl) aerogel washcoat and a traditional alumina washcoat. Simulations were performed using a transient finite difference model in MATLAB and confirmed in Abaqus. The exhaust gas was assumed to flow over the surface at a temperature of 600 K with a heat transfer coefficient of 35 W/m2K, which is typical for a catalytic converter. Two different honeycomb structures were analyzed, 75-µm-thick and 150-µm-thick cordierite walls, to simulate 400 and 300 cells per square inch (CPSI) honeycombs. A nominal washcoat thickness of 20 µm was modelled, and the surface temperature of the washcoat in direct contact with the exhaust was analyzed over time. A number of scenarios were examined including: (a) the effect of washcoat properties; (b) the effect of the percent (0-100%) of the washcoat thickness in the total wall composition; (c) the effect of washcoat thickness (10-100 µm) for fixed cordierite thickness; and (d) the effect of a transient exhaust temperature. The results show that there is a large initial increase in surface temperature for the aerogel-based washcoats (compared to that of the alumina) which then levels off as the heat penetrates into the cordierite layer. The aerogel-based washcoat reaches light-off temperature (the temperature at which the TWC converts 50% of the pollutants) 2-3 sec faster than the alumina washcoat for the 20-µm layer. For a 100-µm layer, the aerogel washcoats reach light off 13-14 sec faster. Increasing the percentage of aerogel in the total wall composition significantly reduces the time to light-off by 29 sec (300 CPSI honeycomb) and by 16 sec (400 CPSI); however, light-off time increases with washcoat layer thickness. Therefore, using a thinner coating is better under all conditions modelled. When using a more realistic model with an exhaust temperature that increases with time, similar trends are observed. Although there are a number of challenges associated with using an aerogel-based washcoat, these results indicate that their use could reduce TWC light-off time. Finding a way to maintain the initial surface temperature rise would allow even shorter light-off times to be achieved.

Thermal Modelling of Aerogel-Based Washcoats for Three-Way Catalytic Conversion: Effects of Washcoat Composition and Thickness on Light-Off Time


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