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Support mat
The main components of a catalytic converter or a diesel particulate filter are:
• a ceramic substrate or monolith (typically cordierite or silicon carbide)
• a support mat or autowrap
• An outer metal shell or can - the visible part of the exhaust
The substrate has a honeycomb structure made up of mostly square channels, to which the exhaust gas can flow through and over the precious metal catalyst. Ceramic substrates are fragile structures and they need to be protected from the surrounding environment using a support mat.
The main purpose of the support mat is to hold and protect the substrate within the exhaust system and to thermally insulate the converter to ensure the shell temperature meets the desired values. The support mat maintains a positive pressure between the ceramic substrate and the metal shell, this is also known as the holding force. The action of the support mat is to accommodate the effects of thermal expansion and engineering tolerances which change the gap between the substrate and the shell. This ensures support of the filter or catalyst during engine operation. Mats are available typically in fibre only form, like the ECOFLEX® series from Saffil Automotive, or in an intumescent form like the products from YESO or Unifrax.
The key mat properties are:
• Resiliency
Due to the difference in thermal expansion between the substrate and the metal shell, the gap will generally expand when the exhaust system heats up and contract when it cools down. The support mat will therefore experience a certain level of cyclic damage during its application lifetime resulting in a reduction of its holding force. This decay in performance must be taken into account to ensure long term durability.
• Friction
The friction is an important parameter when calculating the mat holding force: the higher the friction, the higher the holding force.
• Erosion resistance
Mat erosion, caused by the exhaust gas flow eroding the integrity of the mat could potentially lead to converter failure. To eliminate this risk, intumescent support mat are generally assembled with protective edge seals. Due to their superior erosion resistance this feature is not necessary when dealing with fibre mats.
• Thermal conductivity
Low thermal conductivity means lower shell temperature, smaller gap expansion and therefore less cyclic damage. In Some applications where a low shell temperature is required, a support mat with excellent thermal insulation properties can eliminate the need of additional heat shields.
• Maximum pressure
The application trend in ceramic substrates is to increase the efficiency of the converter by manufacturing the internal walls thinner and increasing the number of channels / cells (cell density). However, the systematic reduction in wall thickness results in much more fragile substrates so there is an increased risk of damage during manufacture and operation making the selection of the substrate support system critically important. The maximum pressure coming from the mat must not exceed the isostatic strength of the substrate.
References
edithttp://yeso.en.alibaba.com/product/252320985-200296105/Catalytic_Converter_Support_Mat.html http://www.saffil.com/newwebsite.nsf/0/5EEDDD4C9BA6833980257316004A3653?OpenDocument&Expand=F1FC41891FB0DF828025731000492883
External links
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