EHC Design Options and Performance 960341

Engine-aged EHC integrated cascades with equivalent overall volumes and several different design features were evaluated for FTP emission performance on a late-model V6 test vehicle. Design options evaluated included low and high cell densities (160 cpsi vs. 400 cpsi, a non-straight flow channel geometry (160 cpsi), and several low-power, zoned heating strategies (all with 160 cpsi). Cold-start hydrocarbon emission performance for the aged low cell density, high cell density, and non-straight channel designs (all with full face heating strategies) were found to be equivalent in the under-floor location used on the test vehicle in this program. Zoned heating strategies offered significant power reductions (0.75-1.0 kW compared to 2.0-2.5 kW) at the expense of reduced cold-start hydrocarbon emission performance versus full face heating strategies (ca a 50% NMHC cold-start emission reduction for zoned heating options vs. a 65% NMHC reduction for full face heating, both relative to unheated operation without secondary air injection). Electrical and mechanical durability of integrated EHC cascades were found acceptable in a severe combined hot vibration/thermal cycling protocol.
Tighter tailpipe emission standards (in-place and proposed) for both the U.S. and Europe for the late 1990s and beyond continue to focus attention on cold-start emission events especially with respect to hydrocarbon emissions. In the past five years a variety of technologies have evolved or been developed aimed at reducing cold-start hydrocarbon emissions by accelerating the heat-up of catalytic converters after engine start. A significant fraction of this technology development has been associated with electrically heated catalytic converters (EHCs). The SAE literature on EHC development/performance during this decade is substantial, as are the resources dedicated to both the evaluation of the technology by automobile manufacturers and the EHC development effort by suppliers. Recent EHC activities have focused efforts on electrical energy reductions and product/system durability (1-4). This work addresses both of these EHC technology themes.
With respect to EHC energy requirements, the performance of several metal foil-based Cam-E-Lite™ EHC/light-off converter cascades (5) were evaluated in this study for FTP emission performance. This comparative study includes EHC design options offering several different heating patterns (covering a range of EHC power levels from 0.75 kW to 2.5 kW) for the front heated zone of these cascade converters. On the high power end of the spectrum are EHC designs that utilize a full frontal inlet face heating strategy, while the low end of the power range utilizes designs offering several “zoned” heating options. Zoned heating strategies (first introduced by Toyota as their “front face heated EHC” (6) and later explored by W.R. Grace (7)) reduce EHC energy consumption by concentrating heated elements in small regions of the inlet face. This type of heating strategy attempts to provide sufficient “reaction ignition zones” distributed on the inlet cascade converter face which provide local oxidation catalysis and subsequent release and transfer of chemical energy. Three different zoned heating options have been evaluated as a part of this program including a “hot spot” design that makes use of nodal-type resistively heated features. Among the full heated face designs are Cam-E-Lites™ configured with low and high cell density straight flow channels and a low cell density design incorporating a more tortuous herringbone corrugation pattern. These last three design options allow for observations on the impact of cascade geometric area, core weight, and flow geometry on cold-start emission performance. Finally the electrical and mechanical durability of these EHC design options is reviewed.


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