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Feeling the Heat (4)

exhaustsIn the previous RET Monitor article on the subject of exhaust insulation, we looked at plasma sprayed thermal barrier coatings for exhaust systems. In preceding articles, we have examined the reasons why thermal insulation of exhaust systems is necessary in certain applications, and advantageous in many others. An alternative to coating the exhaust system are the various types of thermally insulating wrappings, which may be in the form of a metallic 'bag' or 'blanket' or alternatively a refractory cloth, such as asbestos.

There is an alternative philosophy besides insulating the exhaust and this is to insulate the components which are at risk of heat damage, or to insulate parts of the engine where excessive heat pick-up would cause performance to be impaired, or where excessive heat transfer to the cooling and lubricating fluids of the engine would render the cooling provisions of the engine insufficient.

Heat shields are a common sight on may racing cars, not least in Formula One, where these are often polymers reinforced with either carbon fibre, 'Kevlar' or a mixture of the two. The advantages of these types of materials, besides their relatively low thermal conductivity, are their ability to be formed into complex shapes which, for example, are an offset of the engine castings or other components. The heat shields are often covered, on the side closest to the heat source, with a reflective covering, and these have been the subject of extensive trials in the past to determine which proves to be the best at reducing radiant heat transfer. There are alternative fibre composite materials which may prove fundamentally better than carbon or Kevlar as insulating heat shields.

For those without access to 3D CAD geometry of their engines to produce models of such parts, or the funds to make them, a new solution is available which promises to tackle heat transfer on a number of fronts. Wayne Ward spoke recently to Dr Andy McCabe, Technical Director of Zircotec about this new development. The supplier realises that it isn't the correct solution for everyone to have their exhaust systems coated, and that for many the best solution to the problem of heat transfer from exhausts is to locally insulate vulnerable components. Their new product is within reach of everyone but the most cash-strapped of racers. It is a ceramic backed aluminium foil which, as can be seen from the accompanying picture, has the ceramic applied in a regular pattern. Whilst the incomplete ceramic coating lends the foil some compliance, there is another important reason for this feature. As the ceramic is quite thick, the gaps in between it form a thin layer of still air which, as we are probably aware is a very effective insulator in its own right. The ceramic offers a high thermal resistance owing not only to low conductivity, but also by virtue of the low contact area. The final method by which radiant heat transfer is mitigated is due to the foil itself. Whilst not highly polished, it is more reflective than most cast surfaces and will therefore absorb less radiant heat energy.

Fig. 1 - Zircoflex, a ceramic backed aluminium foil.

Written by Wayne Ward

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