Page 62 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Emissions control CHAPTER 3.1

carbon monoxide, but also hydrogen. The hydrogen gas is
Table 3.1-2 Metal and ceramic monolith materials compared
highly inflammable, and therefore is utilised to light up
Property Metal Ceramic the other constituents. Once alight, the mixture will
continue to burn and generate the heat needed to bring
Wall thickness, mm 0.04 0.15–0.2 the temperature of the catalyst up to its light-off value.
Cell density, cell/in 2 400 400 Hydrogen is one of the products of combustion of rich
mixtures at high temperatures. The chemical processes,
Clear cross section, % 91.6 67.1 which are similar to those occurring during the pro-
2
Specific surface area, m /l 3.2 2.4 duction of water gas, are as follows:
Thermal conductivity, W/m K 14–22 1–1.08 CO þ H 2 O ¼ CO 2 þ H 2
Heat capacity, kJ/kg K 0.5 1.05 CO 2 þ H 2 ¼ H 2 O þ CO
Density, g/cm 3 7.4 2.2–2.7
These two reactions alternate in the combustion cham-
Thermal expansion, DL/L,10 6 K 15 1 ber which is why, with the rich mixture, there is always
some hydrogen in the exhaust, especially since some of
Note: Thicknesses and cross sections are of metals uncoated with catalyst. the products of combustion are frozen by the cold walls
of the chamber.
Alternative methods of expediting warm-up can be
less satisfactory. For example, placing the catalytic con-
the foil); and avoidance of local overheating, by virtue of verter close to the engine entails a risk of degradation of
both the compactness of the unit and the good thermal the catalyst due to overheating when the car is driven at
conductivity of the metal as compared with that of ce- high speed and load. The alternative of electric heating
ramic; and, finally, because the complete unit is directly requires a current of about 500 A at 12 V, and therefore
welded into the exhaust system, the costs of assembling calls for a significant and costly uprating of the battery
ceramic monoliths and their wire mesh or fibre mat charging system.
elastic supports into their cans are avoided. The
properties of the two types of converter are set out in
Table 3.1-2. 3.1.9 Three-way conversion

By 1978 GM had developed a three-way converter, the
3.1.8 Ford Exhaust Gas Ignition
term implying the conversion of a third component,
system for preheating catalysts namely the NO x . Whereas two-way conversion is done in
a single stage, three-way conversion calls for two stages.
Generally, catalysts on ceramic monoliths do not become By 1980 it became necessary for meeting the stringent
reasonably effective until they have attained a tempera- requirements for the control of NO x in California and, by

ture of approximately 350 C, and are not fully so until 1981, in the rest of the USA.

a temperature of 450 C is attained. On average, two- The additional catalytic bed contains rhodium (Rh)
thirds of all car journeys undertaken are less than 5 miles for reduction of the oxides of nitrogen. An outcome was
in length. Indeed, on a short journey, as much as 80% of an increase to about 3 g in the total noble metal content.
the total emissions after starting with a cold engine are In practice, with a 0.1% rich mixture, about 95% of the
produced during the first 2 min, and the situation is even NO x can be removed by such a catalytic converter. A
worse in cold climates and in urban conditions. reducing atmosphere is essential so the mixture must not
Ford have obtained catalyst light-off consistently in be lean, and therefore the conversion of NO x has to
a few seconds by briefly burning a measured mixture of precede the oxidation of the HC and CO.
fuel and air in an afterburner just upstream of the catalyst. Oxygen released in the initial reduction process, in the
They have termed their system exhaust gas ignition (EGI). Rh bed, immediately starts the second stage of the
Immediately after start-up from cold, three actions overall process while the exhaust gas is still in the first
are initiated by the electronic control unit (ECU): the stage. The oxygen that remains unused then passes on
engine is run on a rich mixture; air is delivered to the into the Pt or Pt-P1 second stage of the converter, in
afterburner by a pump which is electrically driven, so a separate housing downstream of the first. Here extra air
that it too can be controlled by the ECU; and sparks are is supplied for completion of the oxidation. On the other
continuously fired across the points of a plug situated in hand, if what is termed a dual-bed converter is used, both
the afterburner. Because the mixture is rich, the exhaust stages are in a single housing, though in separate com-
gases contain not only unburned hydrocarbons and partments between which is sandwiched a third chamber

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