Previous papers have considered the role of the substrate in the catalyst system. It has been shown that the total catalyzed surface area of the substrate (defined as the substrate geometric surface area multiplied by the substrate volume) can act as a surrogate for the catalyst performance. The substrate affects the back pressure of the exhaust system and therefore, the available power. Relationships have been developed between the substrate physical characteristics, and both the pressure drop and total surface area of the substrate. The substrate pressure drop has also been related to power loss. What has been lacking is a means of quantitatively relating the substrate properties to the conversion efficiency.
This paper proposes a simple relationship between the substrate total surface area and the emissions of the vehicle as measured on the FTP cycle. This relationship allows the prediction of catalyst system behavior over the total range of converter volumes but is limited at the present time in that it does not explicitly consider any other features of the catalyst system. In spite of the limitations of the present analysis, the concept of a simple mathematical representation of catalyst performance opens the possibility of more design freedom for the auto maker. The design of the total catalyst system earlier in the overall design of a new automobile will provide much more flexibility and allow for a more cost efficient new product development process.
Previous papers (1-2)* have reported experiments performed over the past several years which speak to the design of an automotive catalyst support. The intention of all of this work has been to understand the contributions the support makes to the total system performance. As a result of these activities, new catalyst substrates have been designed which take into account the major substrate contributions to catalyst system performance (3). From this process two new substrates have resulted: the first has minimum back pressure at the same surface area as the current ceramic product; and the second has maximum surface area at the back pressure of the current product. Combined with a stronger material, these two designs allow the performance properties to be optimized with respect to the current ceramics without changing the operating characteristics of the substrate.
As a result of additional experimental work, the reduction in back pressure has been correlated with an increase in power available from the engine (4). By so doing the way is now clear to calculating the horsepower gain for any combination of engine and exhaust system, given a couple of fairly simple experiments.
These studies and other literature (5-6) have both identified a qualitative relationship between the substrate property of total surface area and the system conversion efficiency; as the total surface area is increased, the conversion efficiency increases as well. Models have been developed (7, 8 and 9) which allow the calculation of the conversion efficiency under particular conditions but, at best, only a general outline of the mathematical treatment has been given. What has been lacking is a relatively simple quantitative engineering tool which offers a means of predicting conversion efficiency given certain measured properties of the catalyst system. Specifically, the question has been: is there a simple mathematical relationship between the conversion efficiency of the converter system and substrate, catalyst, and exhaust system parameters?
Recent analysis of experimental data has begun to clarify the form of an equation and has specifically allowed the identification of the relationship between the catalyst system performance and the substrate total catalyzed surface area. The analysis and results are the topics of the present paper.