Page 60 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Emissions control CHAPTER 3.1

Fig. 3.1-3 The AC-Delco stainless steel housing for monolithic catalyst carriers is enclosed in an aluminised steel outer casing.
Sandwiched between top halves of the outer and inner shells is heat insulation material. The perforations in the lower half of the outer shell,
termed the grass shield, facilitate local cooling.

Either platinum (Pt) alone or platinum and palladium 25 mm from its leading edge, accentuating thermal stress
(P1) are used as catalysts. The cost of this noble metal problems. Packaging for pellets, on the other hand, is
content is of the order of 15 to 20 times that of the more complex, so both assembly and servicing of the
stainless steel shell that houses them, so other catalysts monolithic type are easier.
such as copper and chromium have been tried, with some
success, but have not come into general use because they
are prone to deterioration owing to attack by the 3.1.7 Metallic monoliths
sulphuric acid formed by combustion of impurities in the for catalytic converters
fuel. A typical two-way converter for an American car
contains about 1.6 g of noble metals in the Pt:P1 ratio Another important aim is of course durability at both
of 5:2. very high and rapidly changing temperatures. Ceramics
do not satisfy all these requirements, so efforts were
3.1.6 Catalyst support

Considerable development effort has been devoted to
monolithic catalyst supports in the form of one-piece
extruded ceramic honeycomb structures having large
surface areas on to which the noble metal catalysts are
deposited. Gas flow paths through them are well defined
and their mass is smaller than that of the pellet type,
warming up more rapidly to their working temperature

of about 550 C. In some applications, for ease of man-
ufacture two such monoliths are installed in tandem in
a single chamber (Fig. 3.1-4).
Pellet systems are, nevertheless, widely employed in
the USA for trucks, where compactness is not an over-
riding requirement but durability under extremely ad-
verse conditions is. The pellets are relatively insensitive
to thermal stress because they can move to relieve it.
Moreover, the hottest part of such a bed is near its
Fig. 3.1-4 Two monolithic catalyst carriers being assembled in
centre, whereas that of a ceramic monolith is about series into their casing in the AC-Delco factory at Southampton.

57