EHC Impact on Extended Hot Soak Periods

Emission performance of a late model vehicle equipped with an electrically-heated catalytic converter (EHC) system was evaluated after extended vehicle soak periods that ranged from 30 to 180 minutes. As soak periods lengthened, NMHC and CO emissions measured in hot transient driving cycles increased by 125 percent and 345 percent, respectively. These tests were baseline operations which had no resistance heating or secondary air injection to the converter system. Sources of increased NMHC and CO emissions as a function of vehicle soak time were both the converter system cool-down characteristics and engine restart calibration strategy. For soak periods of 30 and 60 minutes, EHC resistance heating without secondary air injection resulted in large improvements in NMHC and CO emission performance (i.e., 74 percent and 54 percent lower NMHC emissions versus no heat, no air operation after a 30- and 60-minute period, respectively). For the longer 180-minute soak period, EHC operation with air injection resulted in the lowest NMHC and CO emissions (i.e., 55 percent lower NMHC than the no heat, no air injection operation). The use of secondary air after soak times of 60 minutes or less resulted in higher NMHC and CO emissions due to converter temperatures being lowered by the additional flow of air. Following a 180-minute soak period, air injection improved emission performance because the additional oxygen in the injected air compensates for the enriched engine restart calibration strategy employed at this soak condition. Secondary air injection generally increased NO_x emissions likely due to leaner inlet converter stoichiometries at vehicle restart.