Validation Methods for Lean NOx Trap Mount Designs 2006-01-3567
A unique validation method is proposed for mount designs of Lean NOx Traps (LNT's), in which characteristic curves of failure points as functions of thermal cycles and vibration amplitudes are generated. LNT's are one of the several new types of emissions control devices applied to Diesel Exhaust Systems, and they reduce the amount of NOx through chemical adsorption. Desulfation must occur nearly every hour, which involves raising the inlet gas temperature of the LNT to around 700°C to “burn off” sulfur from the catalyst, which otherwise would decrease its catalytic activity. This temperature is held for several minutes, and its cyclic occurrence has a negative effect on the long-term performance of the support mat, a major component of its mount design. As substrate temperatures increase, shell temperatures do as well, and thermal growth differences between the ceramic substrate and metallic shell cause the gap between them, which is filled by the support mat, to increase. As the gap expands and contracts due to the thermal cycles, the pressure exerted by the support mat, which holds the fragile and costly LNT substrate in place, decays as a result of fatigue. This nature of the LNT increases the number of thermal cycles applied to the mount design by five to ten times more than traditional Diesel Oxidation Catalysts (DOC's) or Diesel Particulate Filters (DPF's). Therefore, LNT mount design validation methods deserve special attention, quantifying the degrading effect of thermal cycles on the support mat's holding pressure on the substrate. A validation method is proposed, such that thermal cycling and hot vibration stair case loads are applied to generate failure points at maximum g-loads as a function of thermal cycles. This produces a characteristic curve for specific LNT mount designs and allows A-B comparisons of various support mats, gaps, mount densities, and even effects of external heat shields.