Experiences with Electrically Heated Catalysts

Increasing pressure on lowering vehicle emissions to meet stringent California and Federal 1993/1994 emission standards (TLEV) of 0.125 gpm NMOG, 3.4 gpm CO and 0.4 gpm NO_x and future ULEV emission standards of 0.04 gpm NMOG, 1.7 gpm CO, and 0.2 gpm NO_x has focused specific attention on the cold start characteristics of the vehicle's emission system, especially that of the catalyst. From test data it is evident that the major portion of the total HC and CO emissions occur within the first two minutes of the driving cycle. The use of an electrically heated catalyst (EHC) is shown to be advantageous in lowering cold-start emissions during this portion of the drive cycle. This paper addresses the effect on emissions from the standpoint of EHC location, catalyst volume, and engine calibration in an overall emission system approach. Results indicate that locating the EHC downstream of the front TWC provides a level of protection for the mini-EHC without a loss in exhaust gas aftertreatment capability. Further, engine calibration and catalyst volume coupled with an EHC show that these optimization factors can contribute to meeting ULEV standards. However, these emission improvements have been at a premium, i.e. lower fuel economy, additional vehicle weight, power consumption, and yet to be totally accessed EHC durability.