Substrate Effects on Light-Off - Part I Thermal Energy Requirements

The light-off segment of the automotive Federal Test Procedure (FTP) cycle is receiving considerable attention because of the contribution this portion of the test makes to the overall emissions of the automobile, now that the emissions during the remainder of the cycle have been virtually eliminated. In order for the precious metal catalyst to react quickly to the temperature of the exhaust gas and reach conversion temperature as quickly as possible, the effects of all other heat sinks in the system need to be minimized. With the overall heat capacity of the system in mind, there have been several papers published in the past 10 years which analyze the heat capacity of parts of the catalyst system. Unfortunately, there are differences among the values for both specific and substrate heat capacity when all of these references are compared.

The objective of this paper is to establish a single valid set of data which can be used to evaluate the relative contributions of substrate and washcoat to the absorption of heat from the automotive engine exhaust. To this end the specific heat capacity of the substrate and washcoat materials are identified by consulting the materials literature and also through experiment. Using these data, the uncoated and coated substrate heat capacities as well as their respective energy requirements are calculated and translated into terms used in the industry. In addition, the amounts of energy required to heat uncoated and coated substrates to a particular temperature are established through laboratory experiments for confirmation, for the two most common products available in the market.