Page 65 - Automotive Engineering

CH AP TER 3 .1 Emissions control

Fig. 3.1-5 Comparison between ceramic and metallic monoliths. The thermal stresses in monoliths comprising alternate strips of plain
and corrugated metal foil in looped S-form are considerably lower than those that are simply spirally wound.

directed at producing acceptable metallic matrices. Emicat is an Fe, 20% Cr, 5% A1, 0.05% Y alloy.
These offer advantages of compactness, minimum back- Yttrium, chemical symbol Y, has a melting point of

pressure in the exhaust system, rapid warm-up to the 1250 C. It is a metal but with a strong chemical re-
minimum effective operating temperature (widely semblance to rare earths, with which it therefore is
termed the light-off temperature) which, for this type of usually classified. Its oxide, Y 2 O 3 , forms on the surface of

monolith, is claimed to be about 250 C. the foil and protects the substrate from further oxida-
Two obstacles had to be overcome. First was the dif- tion. At a content of 0.05% yttrium is very effective in
ficulty of obtaining adequate corrosion resistance with enabling the alloy to withstand not only temperatures of

the very thin sections needed for both compactness and up to 1100 C over long periods, but also the higher peaks
acceptably low back-pressure. Secondly, it was difficult that can be attained in catalytic converters in the event of
to join the very thin sections while retaining the robust- a malfunction of the ignition system. Even better pro-
ness necessary to withstand the severe thermal loading tection, however, can be had by increasing it up to 0.3%,
and fatigue. though at higher cost.
By 1989, these problems had been solved by Emitec, The advantages obtained with the monoliths made of
a GKN-Unicardan company in Germany. They had de- Emicat include: rapid warm-up; resistance to both ther-
veloped a special stainless steel alloy, called Emicat, which mal shock and rapid cyclical temperature changes up to

they used in foil strips only 0.04 mm thick to construct the well over 1300 C (both due to the good thermal con-
catalyst carriers in monolithic form. These are now made ductivity of the material and low heat capacity of the
up into matrices comprising alternate plain and corru- assembly); minimal back-pressure, by virtue of the thin
gated strips, wound in an S-form, as shown in Fig. 3.1-5. sections of the catalyst carrier foil (Fig. 3.1-5); com-
The matrices are inserted into steel casings and the whole pactness due to thinness of the sections and the absence
assembly joined by a patented high-temperature brazing of the mat needed around a ceramic monolith (to absorb
process. S-form matrices proved to be more durable than its thermal expansion); large area of the catalyst exposed
spirally wound cylindrical units. to the gas flow (owing to the high surface:volume ratio of

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