Vehicle evaluations and model calculations were conducted on a vacuum-insulated catalytic converter (VICC). This converter uses vacuum and a eutectic PCM (phase-change material) to prolong the temperature cool-down time and hence, may keep the converter above catalyst light-off between starts. Tailpipe emissions from a 1992 Tier 0 5.2L van were evaluated after 3hr, 12hr, and 24hr soak periods. After a 12hr soak the HC emissions were reduced by about 55% over the baseline HC emissions; after a 24hr soak the device did not exhibit any benefit in light-off compared to a conventional converter. Cool-down characteristics of this VICC indicated that the catalyst mid-bed temperature was about 180°C after 24hrs.
Model calculations of the temperature warm-up were conducted on a VICC converter. Different warm-up profiles within the converter were predicted depending on the initial temperature of the device. If the device was at 227°C, the initial exotherm occurred downstream of the inlet and eventually spread over the entire converter as a result of convective heat transfer. If the device was at ambient (27°C), the initial exotherm occurred at the inlet and the temperature profile decreased monotonically across the catalyst, typical of a conventional converter. The model simulations show that there is little cold-start benefit unless the converter temperature after the soak exceeds 200°C.
To be considered for widespread automotive usage, either overall VICC design must be improved to meet a 36hr catalyst light-off temperature retention period as specified in US emissions regulations or the cold soak period must be shortened to about 16-18hrs to match the characteristics of a VICC-type catalyst. In addition, other challenges for a VICC-type device include reduced packaging size, improved real world durability and reduced amount of driving preparation necessary to fully melt its PCM material so as to allow it to retain heat for the longer period of time.