Alternative EHC Heating Patterns and Their Impact on Cold-Start Emissions Performance 941996
EHC heating patterns which utilize zones covering less than the available inlet face cross-sectional area have been evaluated for cold-start FTP performance. Both NMHC and CO cold-start emission performance were found to be significantly reduced relative to an EHC-inactive basecase for heating patterns that covered as little as 44% of the cross-section. In low-mileage tests, NMHC and CO cold-start emission dependencies on heating patterns were found to be relatively constant for patterns with heating coverages of 44% or more of the inlet face cross-sectional area. In these low mileage tests, reductions in Bag 1 FTP NMHC and CO emissions averaged about 30% lower with the preferred zoned heating patterns relative to the EHC-inactive basecase. FTP tests run on a similar engine-aged EHC showed less asymptotic dependence on EHC zoned heating strategies. Improvements in cold-start NMHC and CO performance were observed for heating patterns as zoned heating coverages increased from 44% to 89% of the available inlet cross-sectional area.
Electrically heated catalytic converter (EHC) technology received attention in the late 1980s in response to initiatives by the EPA and CARB aimed at reducing cold-start vehicle emissions. First generation EHC prototypes demonstrated significant reductions in cold-start hydrocarbon and CO emissions in low-mileage FTP evaluations, but were characterized by large power demands of 5 kW or more (1, 2 and 3). More recently EHC electrical energy demands have been significantly reduced by reductions in EHC mass (4, 5 and 6). These low-mass EHCs have also been combined with small volume, unheated metal or ceramic-based converter functions in either cascade or fully integrated single-core designs to provide additional cold-start emission reduction benefits (5, 6, 7, 8 and 9). Low-power EHC designs have allowed for substantial simplification of EHC system characteristics (and in-turn system costs) including operation with a single battery or direct connection to a vehicle alternator (6, 9, 10). Cascade arrangements that combine heated and unheated core functions take advantage the “ignitor” characteristics of the EHC. In this scheme the EHC quickly reaches catalytically active temperatures and serves to initiate the oxidation reactions in the exhaust stream shortly after engine start. The combustion energy released from these oxidation reactions is then efficiently transferred to the unheated core function in order to accelerate its warm-up. This cascade arrangement quickly provides for sufficient active catalyst area to cope with the initial high exhaust flows encountered during the first acceleration of the FTP driving cycle.
In a recent paper authored by Toyota (11), this EHC ignitor function was taken one step further by considering designs in which only portions of the EHC cross-section were electrically heated. Reductions in EHC energy demand were realized with this approach while still maintaining high cold-start emission reduction efficiencies. Here again the success of this concept relied on the initiation of combustion reactions within the heated zones of the EHC and the transfer of the combustion energy to both unheated regions of the EHC and a downstream, unheated light-off converter. In the work presented here, a closer look is given to the concept of partial cross-sectional heating of the EHC as a means of reducing EHC energy demands. A specially designed EHC prototype that includes provisions for electrically heating various extents of the EHC cross-section has been evaluated in both low-mileage and simulated high-mileage conditions using the FTP driving cycle. The impact of various “zoned” EHC heating patterns on cold-start emission performance is presented and discussed below.