Introduction to the technology for emissions control
Catalytic converters
The term covers the stainless steel box mounted in the exhaust system. Inside is the autocatalyst - a ceramic or metallic substrate with an active coating incorporating chemical compounds (the washcoat)to support a combination of catalytical materials or minerals selected for their effectiveness in the required emissions reductions. It can also be an homogeneous honeycomb ceramics in which only active compounds are extruded simultaneously. The autocatalyst is mounted in a can and is protected from vibration and shock by a resilient 'mat'. The catalytic converter then looks similar to an exhaust muffler. The different types of autocatalysts are described in more detail on the catalyst page.
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Autocatalysts in protective 'mat' inside strong steel can |
Metallic manifold converter |
Particulate filters
Based on engine technology and application specificities, different filter technologies may be used to reduce particles emissions. The 'wall-flow' filter is a ceramic substrate in which the gas is forced to flow through the walls, thus filtering out particulate matter with more than 99% efficiency. It is aslo mounted in a stainless steel can and protected by a resilient mat. These filters can also have an active coating similar to that used in autocatalysts to ensure their regeneration. Fuel-borne catalysts or engine calibration measures can also be used for the necessary regeneration.
The 'partial-flow' filter is a device which typically separates 30 to 60% of the partuclates from the exhaust gas. These filters are available in various materials, from metallic to fiber-based.
Further details are given on the particulate filters page.
Particulate filter
Traps and adsorbers
Traps and adsorbers are related to autocatalysts but are used to control the emissions of specific pollutants - usually NOx or HC - where some operating conditions or exhaust composition are unsuitable for continuous catalytic action. They store the pollutant for a period of time but then release it when conditions are right for it to react over the catalytic materials, reducing the emissions. The page on Adsorbers gives further details.
NOx storage catalyst
Substrates
The substrates on which the active catalyst is supported can be ceramic or metallic, with each offering particular advantages for specific applications and positions in the exhaust system. The development of strong ultra-thin wall substrates with cell densities of up to 1200 cells per square inch (186 cells/square cm) has been a major factor in the increasing efficiency of autocatalysts.
1) Ceramic substrates formed in a wide range of shapes and sizes to suit the application
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2) Wall-flow particulate filter substrates |
3) Metallic substrates |
The technology of the substrates on which the active catalyst is supported has seen great progress. In 1974, ceramic substrates had a density of 200 cells per square inch (cpsi) of cross section (31 cells/cm2) and a wall thickness of 0.012 inch or 12 mil (0.305 mm). By the end of the 1970s, the cell density had increased from 300 to 400 cpsi and wall thickness had been reduced by 50% to 6 mil. Now 400, 600 and 900 and even 1200 cpsi substrates are available and wall thickness can be reduced to 2 mil - almost 0.05 mm. In parallel, in the late 1970s, substrates derived from ultra-thin foils of corrosion-resistant steels came on to the market. In the beginning, the foils could be made from material only 0.05 mm thick allowing high cell densities to be achieved. Complex internal structures can now be developed; 800 and 1000 cpsi metallic substrates are available and their wall thickness is down to 0.025 mm. This progress in ceramic and metal substrate technology has major benefits. A larger catalyst surface area can be incorporated into a given converter volume and this allows better conversion efficiency and durability. The thin walls reduce thermal capacity and limit pressure losses. Alternatively, the same performance can be incorporated into a smaller converter volume, making the catalyst easier to fit close to the engine as cars are made more compact.
Evolution of ceramic and metallic substrates with thinner walls and increased catalyst surface.
Catalytic coatings
Coatings systems have been developed which allow the maximum efficiency with optimum use of the precious metals platinum (Pt), palladium (Pd) and rhodium (Rh). The nanotechnology used in catalytic coatings involves stabilised crystallites, washcoat materials that maintain high surface area at temperatures around 1000°C, improved oxygen storage components and novel coating processes to optimise the distribution of the coatings. All play a part in the high efficiencies of autocatalysts.
Homogeneous catalysts
The homogeneous catalyst is an alternative to the coated substrate. in that case, only active compounds are extruded simultaneously. today, this technology can be used for Selective Catalytic Reduction catalysts.
