The 290,000 vehicle-mile durability requirement for diesel/natural gas oxidation catalysts calls for robust packaging systems which ensure a positive mounting pressure on the ceramic flow-through converter under all operating conditions. New data for substrate/washcoat interaction, intumescent mat performance in dry and wet states, and high temperature strength and oxidation resistance of stainless steels, and canning techniques insensitive to tolerance stack-up are reviewed which help optimize packaging durability. Factors contributing to robustness of converter components are identified and methods to quantify their impact on design optimization are described.
CERAMIC FLOW-THROUGH catalysts for diesel exhaust aftertreatment have met with much success since their introduction in 1993. Both performance and durability requirements have been met via optimization of engine management, substrate design, washcoat formulation, catalyst selection, and packaging robustness[1, 2, 3 and 4]* Further tightening of emissions regulation, notably particulate matter and NOx, and continued growth of natural gas technology will impose additional demands on converter durability due to more severe operating conditions. For example, the exhaust temperature and relative humidity will be3 significantly higher for natural gas engines and result in higher stresses and fatigue than those for diesel engines.
The key objective of this paper is to review new advances in packaging design which have a great bearing on converter durability. Indeed, such advances have been instrumental in meeting the 100K mile durability requirement for automotive catalysts [5,6], while at the same time they have helped enhance the durability of motorcycle catalysts [7,8]. More recently, robust packaging designs have proven their effectiveness in close-coupled light-off applications with much more demanding operating conditions [9, 10 and 11].
Specifically, the paper will focus on substrate/washcoat interaction as it impacts the mechanical and thermal properties which, in turn, bear heavily on converter durability. Next, the variation of mount density and mounting pressure on ceramic substrate is examined as a function of canning technique. Such variation can have an adverse effect on both the mat integrity and substrate durability. Factors responsible for variation of mount density and ways to minimize it are discussed. Performance trade-offs associated with the cell geometry of diesel oxidation catalyst support are also addressed. Finally, the value of the axial push-out test on fresh and aged converters, as a quality control test, is demonstrated by examining changes in mounting pressure with aging. In this manner, packaging durability may be ascertained by requiring the push-out force to meet a certain minimum value even after aging.