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Automotive Industry

Considering diesel catalysts

Automotive Design & Production, Jan, 2005 by William Kimberley

In the exhaust after-treatment business, Emitec GmbH (Lohmar, Germany; www.emitec.com) is a relatively small player when compared to the likes of ArvinMeritor and Faurecia, but it nevertheless punches above its weight by being innovative and pushing the technology boundaries. Recently, MAN, a German truck manufacturer, announced that it was going into series production with Emitec's PM-Filter-Catalyst. It's thought that the system may be used by an automobile manufacturer for a diesel car program in the near future. "The system is made up of an oxidation catalyst which eradicates the hydrocarbons and carbon monoxide whilst at the same time oxidizing up the NO to N[O.sub.2]," says Wolfgang Maus, Emitec CEO. "This is essential for the conversion of the particles in the PM-Filter-Catalyst."

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The PM-Filter-Catalyst is a filtration system that directs an exhaust gas by-pass stream against the sintered metal fleece and through it into the neighboring channels using the MX-blade structure. The sintered metal fleece layers store a high quantity of the particles that accumulate in this way. These are then continuously converted at relatively low temperatures--from approximately 200[degrees]C--under the influence of the N[O.sub.2] gained from the upstream oxidization catalyst. Here the carbon and hydrocarbon react with the N[O.sub.2] which gives off oxygen to become NO again. The final components are C[O.sub.2] and nitrogen. Using this reaction, which is constantly repeating within the PM-Filer-Catalyst, the carbon that is stored along the entire length of the PM-Filter-Catalyst can be converted at a high level of efficiency. These advantages, says Maus, are especially noticeable when the PM-Filter-Catalyst is itself deployed as a coated oxidization catalyst. Not only are CO and HC oxidized to air components, but NO also becomes N[O.sub.2].

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"Particulate reduction using N[O.sub.2] is therefore even more intensive. This is due to the fact that the N[O.sub.2]--in contrast to the ceramic-coated filter--is still available even after passing through the filter fleece in the next channel and in the bypass process. In addition, the mixer structure in conjunction with the porous fleece layer is particularly effective because the channels that have been opened due to the 'blades' effectively 'communicate' with each other. In this way they guarantee an internal balance of flow and concentration." At a temperature of about just over 350[degrees]C, the particles stored in the metal fleece burn very effectively with the oxygen present in the exhaust gas--also a continuous process.

"The PM-Filter-Catalyst system always works in equilibrium between particle storage and particle conversion via oxidation," says Maus. "When the metal fiber fleece becomes blocked by the particles, no more particles can be stored. As soon as they have been burnt off by N[O.sub.2], new particles are collected again. The filter efficiency is therefore dependent on the fact that particles are stored in the 'puffer' and sufficient N[O.sub.2] or oxygen is available at temperatures above 180[degrees]C to provide oxidation potential. Diameter and length of the PM-Filter-Catalyst elements are adapted to suit the engine size. Depending on the space available, it is possible to use one substrate with a large diameter or several substrates of a smaller diameter arranged in parallel may also be used. There is a proportional increase in the efficiency of the PM-Filter-Catalysts with length provided if there is sufficient oxidation potential. At a length of around 300 mm, an efficiency of over 80% can be achieved in a truck. Therefore, the efficiency of the PM-Filter-Catalyst is in the region of that seen in wall flow systems, without having their disadvantages such as clogging and increases in back-pressure and consumption."

According to Maus, the PM-Filter-Catalyst can also be used with the conventional particle filter. "If a coated PM-Filter-Catalyst is inserted upstream of a conventional filter, it converts HC and CO and also reduces the particulate mass as a pre-filter using N[O.sub.2] and the super-fine particles. The conventional particle filter is therefore relieved of load and can be kept more compact if required. This also leads to a lengthening of the operational period until regeneration of the particle filter is next required, which in turn reduces operational costs."

In order to meet Euro 5--2.0 g/kWh and a 20-30% reduction in NOx--which comes into force in 2008, many engine manufacturers see Selective Catalytic Reduction (SCR) as the best solution, but there are no clear answers, as Maus explains. "Future requirements are posing enormous problems for engine developers and exhaust gas suppliers because of the conflicting demands that have to be fulfilled simultaneously. While internal engine measures can reduce particulate mass, they often simultaneously increase the levels of NOx emissions, but both must be reduced at the same time. Whoever optimizes engines for low NOx tends to have more particulate emission in the exhaust gas. The use of the only available ceramic particle filter brings with it the danger that unburned particles can clog the filter channels. There is also the danger that above the required lifetime performance of several hundred thousand kilometers, the filter surface can become blocked through the accumulation of oil incineration ash. This would not only increase exhaust gas back-pressure and therefore fuel consumption, but would also result in considerable costs caused by filter servicing or exchange."

Automotive Industry

Considering diesel catalysts

Automotive Design & Production, Jan, 2005 by William Kimberley

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