Advanced Three-Way Converter System for High Temperature Exhaust Aftertreatment 970265

An advanced three-way converter system with significant improvements in light-off performance, conversion efficiency, thermal stability and physical durability at high operating temperature is described. The converter system is comprised of a light-weight ceramic substrate with high surface area triangular cell structure, a new catalyst formulation with enhanced thermal stability and good substrate compatibility, and a durable packaging design which together lead to consistent improvements in high temperature performance and durability.

Experimental data including FTP performance, canning trials, and high temperature vibration and thermal shock tests for both the advanced and standard three-way converter systems are presented.

THE TIGHTER emissions standards, and severe Federal Test Procedure over the entire cycle, combined with extended durability requirements are posing a considerable challenge to substrate manufacturers, washcoat and catalyst developers, and canners to come up with advanced converter systems capable of meeting both performance and durability requirements over a wide range of temperatures^{[1, 2, 3, 4, 5, 6, 7, 8, 9 and 10]}. Although alternate approaches, including preconverter, electrically heated catalyst and in-line adsorber have demonstrated their feasibility for meeting the cold-start emissions regulations, they add to complexity and/or cost of emissions control system^{[11, 12, 13, 14, 15 and 16]}. Obviously, the automakers are seeking simple, passive and cost effective systems that require minimal modifications in space and/or ancillary equipment like batteries and air pumps.

This paper focuses on an advanced three-way converter system comprised of a light-weight ceramic substrate, a high temperature catalyst with enhanced thermal stability, and a durable packaging design which together provide the required conversion efficiency and extended durability over the operating temperature range. In particular, relative to current substrate, the new substrate enjoys 26% lower weight and 22% higher geometric surface area both of which promote light-off performance; its 18% higher open frontal area helps reduce gas velocity and back pressure; and its triangular cell geometry together with 7% lower wall porosity ensure mechanical durability over the required lifetime. Similarly, relative to current three-way catalyst, the new catalyst formulation and processing technique result in significantly better thermal over the peak temperature range of 900°-1050°C, characteristic of close-coupled applications ^[1]. The new three-way catalyst formulation demonstrates consistent improvements over the current catalyst during vehicle FTP test following high temperature aging. The use of current clamshell can with multiple stiffener ribs, together with an improved intumescent mat, ensures a durable packaging design capable of with-standing mounting, vibrational and thermal stresses over the operating temperature range. Such a packaging design helps meet the durability requirement by ensuring a positive mounting pressure and good thermal insulation thereby minimizing both the substrate movement and clamshell deformation notably at the peak operating temperature ^[17].

The paper emphasizes the use of systems approach in optimizing the converter system wherein the substrate manufacturer, the catalyzer, the mat supplier and the canner work together with the automaker, review the tradeoffs associated with their individual technologies, and arrive at the most cost-effective system capable of meeting performance and durability requirements ^[18]. In particular, the substrate/washcoat compatibility in terms of promoting both physical and catalytic durabilities is most critical as is the selection of appropriate mat and mount density for meeting the performance and durability requirements.