NOx-Trap System Development and Characterization for Diesel Engines Emission Control 2000-01-2910
Laboratory and vehicle tests were carried out to investigate behaviour and potentiality of NOx-trap catalytic system in Diesel conditions. Three main aspects were studied. The first one deals with the NOx storage capacity of adsorber under laboratory and vehicle conditions, especially regarding the influence of driving conditions. The second one focuses on the regenerability of different materials. At length, special attention is devoted to the sulphur poisoning rate.
A representative laboratory test method was built up, to evaluate NOx storage capacity under Diesel conditions. It is shown that NOx adsorption occurs from 100 to 400°C. Low temperature activity (100 to 250°C) is conditioned by low NOx flow emission, mainly due to the use of high EGR rate. Higher temperatures lead to an increase in the intrinsic NOx Storage capacity of the material, but are also accompanied by high NOx concentration and space velocity.
Very good correlation is obtained between laboratory test method and vehicle tests. Promising results were obtained over NMVEG test cycle, with EURO III tuning engine. Under passive conditions, without any specific regeneration strategy, high NOx depletion were achieved, ranging from 40 to 85 %, depending on the catalyst. Dealing with the NOx-Trap technology influence, same ranking has been observed between laboratory evaluation and vehicle test results.
Engine NOx Regeneration strategy has been built up to achieve, for a significant period of time, exhaust rich conditions. It has been developed using a combination of high EGR rate, throttling and fuel post-injection. An intensive laboratory program has been set in motion in order to investigate NOx regeneration behaviour under rich conditions. It is clearly shown that Diesel NOx-Trap formulation suffers from a lack in term of low temperature regeneration activity.
Unfortunately, NOx-Traps are drastically poisoned by SO2 emitted from the engine. Impact of fuel sulphur content is tremendously highlighted within this study. The higher sulphur dioxide concentration is, the faster poisoning occurs. Nevertheless, very low Diesel sulphur fuel (< 5 ppm) does not prevent from sulphur poisoning as sulphur content of the lubricant becomes very important and must also be reduced to get to the lowest poisoning.
On the other hand, desulphation operation does not allow to recover initial NOx efficiency, meaning that a part of NOx storage capacity is irreversibly damaged. It has been evaluated to 30 % of the initial NOx storage capacity.