Page 56 - Automotive Engineering Powertrain Chassis System and Vehicle Body

3.1
Chapter 3.1

Emissions control

T.K. Garrett, K. Newton and W. Steels

Attention was first directed to atmospheric pollution in the oxygen necessary for sustaining life. The toxicity of
Los Angeles in 1947. Subsequently, in 1952, Dr Arie hydrocarbons and oxides of nitrogen, on the other hand,
J. Haagen-Smit asserted on the basis of his research that, arises indirectly as a result of photochemical reactions
at least locally, it was due mainly to automotive exhaust between the two in sunlight, leading to the production of
emissions. It was subsequently said, however, that it other chemicals.
would have cost the USA less to have moved Los Angeles There are two main oxides of nitrogen: nitric acid and
than to have converted all their vehicles to reduce the nitrogen dioxide, NO and NO 2 , of which the latter is of
obnoxious emissions to the levels now required by law! greatest significance as regards toxic photochemical ef-
Japan was close behind the USA with emission control fects. Under the influence of solar radiation, the NO 2
laws, and Europe has practically caught up. breaks down into NO þ O, the highly reactive oxygen
Given complete combustion, each kg of hydrocarbon atom then combining with O 2 to make O 3 , which of
fuel when completely burnt produces mainly 3.1 kg of course is ozone. Normally, this would then rapidly
CO 2 and 1.3 kg of H 2 O. Most of the undesirable ex- recombine with the NO to form NO 2 again, but the
haust emissions are produced in minute quantities presence of hydrocarbons inhibits this reaction and
(parts per million), and these are: oxides of nitrogen, causes the concentration of ozone to rise. The ozone then
generally termed NO x , unburnt hydrocarbons (HCs), goes on, in a complex manner, to combine with the other
carbon monoxide (CO), carbon dioxide (CO 2 ), lead substances present to form chemicals which, in combi-
salts, polyaromatics, soots, aldehydes ketones and nitro- nation with moisture in an atmospheric haze, produce
olefins. Of these, only the first three are of major what has been described as the obnoxious smoky fog now
significance in the quantities produced. However, con- known as smog.
centrations in general could become heavier as in- Unburnt hydrocarbons can come from evaporation
creasing numbers of vehicles come onto our roads. By from the carburettor float chamber and fuel tank vent
the end of the 1980s, CO 2 was beginning to cause as well as from inefficient combustion due in different
concern, not because it is toxic but because it was instances to faulty ignition, inadequate turbulence,
suspected of facilitating the penetration of our atmo- poor carburation, an over-rich mixture, or long flame
sphere by ultraviolet rays emitted by the sun. Contro- paths from the point of ignition. The relationships
versy has raged over lead salts, but no proof has been between emissions and the air:fuel ratio are illustrated
found that, in the quantities in which they are present in in Fig. 3.1-1. Other factors are overcooling, large
the atmosphere, they are harmful. For many years, quench areas in the combustion chamber, the un-
manufacturers of catalytic converters pressed for un- avoidable presence of a quench layer of gas a few
leaded petrol because lead deposits rapidly rendered hundredths of a millimetre thick clinging to the walls
their converters ineffective. of the combustion chamber, and quenching in crevices
Carbon monoxide is toxic because it is absorbed by such as the clearance between the top land of the
the red corpuscles of the blood, inhibiting absorption of piston and the cylinder bore.
The Motor Vehicle, 13th edn; ISBN: 9780750644495
Copyright Ó 2000 Elsevier Ltd. All rights of reproduction, in any form, reserved.