701MOBILE SOURCE POLLUTION
Mobile sources, alternatively called transportation or vehicular
sources, include cars, trucks, buses, ships and various aircraft.
Air pollutants emitted will vary, depending on the fuel being
combusted or reacted (in the case of fuel cells or batteries) and
the engine design of each vehicle.THE AUTOMOBILEThe automobile’s discovery appears to satisfactorily com-
bine a human desire for rapid transportation with the desire
for independence and fl exibility. However, the increasing
vehicle population poses a series of threats to continued
physical and psychological well-being and to the future of
our environment.
As the automotive industry expands, other auxiliary
industries such as petroleum production and concrete and
tire manufacturing also grow, with additional potential for
pollution problems. Additional roads also have to be built,
with a negative impact on both ecology and landscape. A bal-
ance must be made between the right of an individual to use
his own car when and where he drives and the harm brought
upon society as a whole by his doing so. If it is accepted that
society needs to be protected, a number of legislative and
economic measures can be initiated to discourage automo-
tive usage.
Legislation has been enacted to limit the emission of HC
(hydrocarbons), NO x (oxides of nitrogen), CO (carbon mon-
oxide) and particulate matter including lead compounds.
However, other waste materials such as tires and the auto-
mobile itself (including the repugnant abandoned cars) must
be disposed of.
The broad approach to automotive pollution control is to
encourage alternative means of transportation. This would
include improvements in mass transit such as high-speed
trains, moving sidewalks, increased links and modernization.
In the city, bicycle riding and walking are low pollution, high
exercise alternatives; in rural areas car pools might be formed.
Cars have been built using different energy concepts
(ex. battery, turbine engine, sterling engine) and to run on
different fuels (ex. natural gas, alcohol). Special control
devices (ex. catalytic reactors, afterburners) placed at the
exhaust of internal combustion engines are the primary
means of reducing emissions of various materials. The total
solution to the problem will probably combine technical
and strategic methods.The US EPA and US DOE produce the Fuel Economy
Guide to help car buyers choose from the best fuel gallons
per mile (mpg) ratings for both city and highway traffi c
modes. A pdf version may be found at http://www.epa.gov/
cgibin/epaprintonly.cgi^1
In the present discussion, we shall concentrate on the
major moving source of pollutants, the internal combus-
tion engine. In the automobile, evaporative losses of pol-
lutants occur from the fuel tank and carburetor (ca. 5%),
fumes from the crankcase (ca. 20%) and the exhaust system
(ca. 75%). The major offenders are unburned hydrocarbons,
carbon monoxide, nitrogen oxides and HC oxidants. Positive
Crankcase Ventilation (PCV) on all modern cars reduces the
emissions by pulling air and fumes into the engine by main-
taining a vacuum at the engine. Of course a highly effi cient
combustion process will eliminate the partially oxidized
substances. One reason for incomplete combustion is that as
the “fl ame front” generated from the spark moves toward the
relatively cool cylinder walls, a quenching action takes place
preventing further reaction, the kinetics of which are very
temperature sensitive. Other factors infl uencing incomplete
combustion are improper dilution by poor cycle timing and
less than the proper excess of oxygen admitted at the carbu-
retor. Standard engine cycles have been developed so that
pollutant guidelines might be drawn. The Federal Driving
Cycle initiated in 1972 in considered the standard of vehi-
cle testing. A federal short cycle Table 1 has been used for
convenience in some instances. Measured emission values
averaged for various test sites in representative US cities are
presented in Table 2 for steady state at various miles per hour
(mph) and for the short cycle. Carbon monoxide and NO x
levels increase as the mph level increases. The federal 1975
proposed standards for CO, hydrocarbons and NO x are 11,
0.5 and 0.9 grams per mile, respectively, and 0.5%, 40 ppm
and 225 ppm, respectively. Their extent of enforcement is
increasing as of the early 1990s.
The CO standard is based primarily on tests which have
shown that the capacity of blood to carry oxygen is hampered
by CO absorption.^2 Brain function is retarded after an expo-
sure of 10 to 15 ppm CO for several hours. The HC and NO x
levels are mainly based on their role in photochemical smog
using ambient levels. Shy et al.^3 found in their Chattanooga
study that “NO 2 alone and exposure to suspended particulate
matter alone appear to be the most probable explanation for
the observed excess in respiratory illness rates.” The data
are not completely convincing as pointed out by automobileC013_005_r03.indd 701C013_005_r03.indd 701 11/18/2005 10:42:26 AM11/18/2005 10:42:26 AM