Electrically heated converters for automotive emission control: determination of the best size regime for the heated element

In view of the significant cold-start hydrocarbon emission reduction potential of the electrically heated converter (EHC) technology demonstrated in recent studies, there is considerable interest in better understanding the behavior and design aspects of an EHC during the cold-start portion of actual vehicle emission tests. Computer simulations based on the EHC model developed previously (Oh et al., 1993, Industrial and Engineering Chemistry Research 32, 1560-1567) show that although decreasing the volume of an electric heater generally improves the emission performance of the EHC system, there is a critical heater volume below which no significant emission benefit is obtained. This paper examines the EHC sizing issue in more detail by analyzing a simplified version of the EHC model to investigate why the emission benefit of decreasing electric heater volume eventually disappears and how the critical heater volume where this occurs is related to system parameters. Analysis of the simplified EHC model identifies this critical volume, V-h, which is proportional to the exhaust flow rate through the EHC and inversely proportional to the product of the interphase (gas-solid) heat transfer coefficient within the heated element and its geometric surface area per unit volume. It is shown that the recommended heater size regime must be where the physical heater volume, V, and V-h are comparable in magnitude. When V much greater than V-h the electric heater is clearly too large because the emission performance suffers from the slow heat-up rate caused by the large thermal mass. Also, decreasing the heater volume to values much smaller than V-h (i.e., V much less than V-h) is not desirable because it would simply lead to unnecessarily higher (potentially damaging) solid-phase temperatures without producing additional emission reductions. In this smallest size regime, the emission performance of the EHC system becomes insensitive to heater size variation because the higher solid-phase temperatures within the smaller heater tend to be compensated by its poor interphase (solid-to-gas) heal transfer arising from the insufficient heat transfer area available. (C) 1999 Elsevier Science Ltd. All rights reserved.