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The heat and mass transfer characteristics of a monolith reactor influence both its light-off performance and the steady-state temperature and concentration profiles. The transient behavior of an adiabatic monolith reactor is bounded by two limiting channel geometries: (a) cylindrical passages and (b) slits confined between parallel plates. These geometries represent extremes in heat/mass transfer characteristics for monolith channels and, therefore, allow the analysis of light-off behavior for the practical range of geometries and process conditions in automobile exhausts. Also, these two geometries allow an analytical solution for the velocity, concentration and temperature profiles in the gas phase. This study analyzes the light-off performance of monolith reactors with comparable voidage and surface area using automotive exhaust oxidation kinetics. The energy balance for the solid walls includes all three modes of heat transport: convection, conduction and radiation.

Further stringent emission legislation requires advanced technologies, such as sophisticated engine management and advanced catalyst and substrate to achieve high catalytic performance, especially during the light-off phase. This paper presents the results of calculations and measurements of hydrocarbon and carbon monoxide light-off performance for substrates of different wall thickness, cell density and cell shapes. The experimental data from catalyst light-off testing on an engine dynamometer are compared with theoretical results of computer modeling under different temperature ramps and flow rates. The reaction kinetics in the computer modeling are derived from the best fit for the performance of conventional ceramic substrate (6mil/400cpsi), by comparing the theoretical and experimental results on both HC and CO emissions. The calibrated computer model predicts the effects of different wall thickness, cell density and cell shape.

Growing world vehicle populations and persistent air quality problems require further reductions in the emissions from engines. Future tailpipe emission limits mandate 98%+ reductions in hydrocarbons, 95%+ reductions in Carbon Monoxide and 95%+ reductions in Nitrogen Oxides. Further complexities are introduced by new, demanding driving norms according to which these standards are applied, as well as the on-board diagnostics to ensure in-use compliance. Customized and optimized emission control system designs will be needed for each specific engine family. The new emission control systems will incorporate integrated heat management, advanced catalyst formulations, optimized substrates and substrate packaging as well as the use of pollutant storage materials and supplemental heating devices.

New ultra-low vehicle emission legislation requires advanced catalyst systems to achieve high conversion requirements. Manufacturers have to improve both the washcoat formulations and the catalyst substrate technology to meet these new regulations. This paper will present the results of a computer modeling study on the effects of ultra-thinwall catalysts on hydrocarbon and carbon monoxide light-off performance improvement. The experimental data from catalyst light-off testing on an engine dynamometer are compared with theoretical results of advanced substrate modeling for ultra-thin wall ceramic substrates. Results show that thermal mass has the greatest effect on light-off performance. Decreases in wall thickness offer the greatest benefit to light-off performance by lowering the thermal mass of the substrate, thus allowing it to reach light-off temperature faster.