Page 59 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CH AP TER 3 .1 Emissions control

controversy over the alleged toxic effects of lead salts in
Table 3.1-1
the environment was neatly side-stepped.
HC, CO and NO x
Year respectively gm/mile
3.1.4 Two-way catalytic conversion
1975 1.5, 15.0 and 3.1

1977–79 1.5, 15.0 and 2.0 The emissions regulations for the 1975 model year re-
quired reductions of 87% in HC, 82% in CO and 24% in
1980 0.41, 7.0 and 2.0
NO x by comparison with 1960 levels. GM concluded
1981 and beyond 0.41, 3.4 and 1.0 that to meet these requirements while simultaneously
improving not only economy but also driveability, both of
which had deteriorated severely as a result of emission
control by engine modifications, two-way catalytic con-
temperatures of combustion in cars for California in verters were needed. The term two-way conversion im-
1972, and extended nationwide in 1973, when legal plies oxidation of the two constituents in the exhaust,
limits, at 3.1 gm/mile, were first imposed on NO x HC and CO, to form CO 2 and H 2 O. Such a converter
emissions. Subsequently, the overall requirements were therefore contains only oxidation catalysts and, more-
progressively tightened as shown in Table 3.1-1. The over, without oxygen in the exhaust cannot function.
1981 regulations were so tight that, for diesel engines and Consequently, the air–fuel mixture supplied to the
innovative power units, a delay of 4 years had to be engine must be at least stoichiometric or, better still,
allowed for NO x and up to 2 years on CO. Since then the lean. Incidentally, the earlier practice of feeding air into
regulations have been tightened periodically and clearly the exhaust was intended primarily for burning the
this process will continue. excess hydrocarbons during the first five cycles of the test
after a cold start with engines equipped with carburet-
tors. It is unnecessary with the accurate regulation of
3.1.3 Catalytic conversion air:fuel ratio by computer-controlled injection.
If a spark plug were to fail, air–fuel mixture would
At this point while some other maunfacturers were enter the two-way catalytic converter and burn, seriously
promoting the lean-burn concept as the way forward, overheating the unit. Consequently, high-energy ignition
GM engineers, accepting the penalty of low Octane systems became a necessary adjunct for the 1975 models,
Number, opted for unleaded fuel and catalytic conver- and copper-cored spark plugs were fitted to obviate cold
sion for meeting regulations on both emissions and fuel fouling. The overall result of all these measures on the
economy, while avoiding adverse effects on engine GM models was a reduction in fuel consumption of 28%
durability. As a first step, all their car engines for 1971 by comparison with that of their 1974 cars. By 1977 this
were designed for fuel rated at 91 Motor Octane No., figure had been further improved by 48% and, by 1982,
mainly by reducing compression ratios and modifying the owing to the stimulus of the Corporate Average Fuel
valves and their seats. Economy (CAFE) legislation, by 103%. Incidentally,
They argued that unleaded fuel offers several benefits: under the CAFE legislation the average fuel consumption
first, the major source of particulate emissions, lead of all cars marketed by each corporation in the USA had
oxyhalide salts, is eliminated; secondly, there is a conse- to improve in stages, from 18 mpg in 1978, by 1 mpg
quent reduction in combustion chamber deposits, which each year to 1980, then 2 mpg annually to 1983 and again
have the effect of thickening the boundary layers in the by 1 mpg for 1984, and then to 0.5 mpg, to 27.5 mpg, for
gas in the combustion chambers and this, by quenching 1985.
them, encourages the formation of HC; thirdly, a further
reduction in HC is obtained because of the additional
oxidation that occurs in the exhaust system owing to the 3.1.5 The converter
absence of lead additives and also because the lead salt
deposits tend to cause deterioration of the NO x control Two-way catalytic converters comprise a container, usu-
system by adversely affecting the flow characteristics of ally of chromium stainless steel, and the catalysts and
the EGR orifices; fourthly, maintenance of spark plugs, their supports, all enclosed in an aluminised steel heat
exhaust systems and the frequency of changing lubri- shield (Fig. 3.1-3). Initially, the alumina pellet type of
cating oil are all reduced by the elimination of the lead support for the catalyst was the most favoured because it
salts, as also of course is the generation of acids by the had been developed to an advanced stage in other in-
halide scavengers that have to be used with them; finally, dustries. The monolithic type (one-piece) did not go into
because catalytic converters call for unleaded fuels regular production until 1977.

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