Automobile Emissions
Pollution from automobile emissions has become
over the past few decades an
issue of great concern. With a growing number of
motor vehicles on our roads
great concern has been attributed to the effects
of these emissions to our
health and to the environment. Several of the gases
emitted, which when present
in certain concentrations in our atmosphere can
be toxic, therefor these
ultimate concentrations must never be achieved.
Strict legislation as well as
sophisticated control technology has been
implemented in the automotive industry
in order to limit the pollution
caused. These aspects of automotive pollution
shall be further discussed in
this paper. KEYWORDS: Pollution, Car Pollution,
Automotive emissions,
Emission gases, Catalysts 1. INTRODUCTION The relationship
between air
pollution and automobile exhaust emissions has been established
largely due
to studies done in California. At first the problem was believed to
be a
combination of smoke and fog, which was similar to problems faced in
London
since the middle ages. In Los Angeles the severity of air pollution
has caused
vegetation damage, eye and throat irritation, a decrease in
visibility as well
as several other effects. Automobile and truck exhausts
contain substances which
can adversely affect human health when exposed to
concentrations above ambient
level. Emissions from automobiles usually
consist of carbon monoxides, oxides
from sulfur and nitrogen, unburned
hydrocarbons, smog, and particulate matter,
which includes smoke. Pollutant
concentration and time of exposure are the two
main factors which affect
human health. Air emissions from automobiles can also
have an overall effect
on the environmental quality in several ways. Emissions
from nitrogen oxides
(NOx) can contribute to the acid deposition problem,
combinations of NOx and
hydrocarbons can help produce ozone and photochemical
oxidants and lastly
pollutants from automobiles and ozone formation can
contribute to the ambient
air pollution problem in urban areas. As a result of
increasing concern about
the role of the motor vehicle in contributing to these
health and
environmental problems as well as the possibility of these problems
to
increase due to a growing number of cars worldwide, strict legislation
has
caused engine emission control technology to quickly develop. As
legislations
become more severe, emission control technology is constantly
changed or
modified in order to meet the new requirements and reduce the
emissions
produced. This report shall focus on the health effects that
automotive
emissions such as gases and particulates may have as well as
discuss the control
of these emissions via legislation and technology. The
technology discussed is
primarily the present technology implemented to
control automotive emissions,
namely catalysts. 2. HEALTH EFFECTS OF
AUTOMOTIVE EMISSIONS 2.1 EFFECTS OF
GASEOUS EMISSIONS 2.1.1 Carbon
Monoxide Carbon monoxide (CO) is found in high
levels in the exhausts of
diesel and petrol powered automobiles. CO is a
colorless and odorless gas and
can be toxic at certain levels. The effects of
carbon monoxide is felt when
inhaled, it enters the blood stream and binds to
hemoglobin (which the CO has
a higher affinity than oxygen by 240 to 1). The
resulting compound formed is
carboxlhemoglobin. The blood is then unable to
supply oxygen to the cells.
And depending the level of exposure, death may be
the ultimate consequence.
The formation of carboxlhemoglobin lowers the
available hemoglobin. Normal
individuals will not feel any effects until 5% to
10% of hemoglobin is
transformed. As carboxlhemoglobin increases, symptoms such
as headaches,
visual disturbances, nausea and vomiting and coma may occur. Death
may occur
if levels of carboxlhemoglobin reach the vicinity of 70%. Usually
levels of
carbon monoxide are low except in enclosed areas. On average
most
carboxlhemoglobin levels are under 5%. Since low level exposure to
carbon
monoxide is not well understood, it is believed that it might
contribute to
cardiovascular disease. The heaviest exposures to motorist
occur in heavy (stop
and go) traffic. When considering the effects of carbon
monoxide, it is usually
easily overlooked. Barometric pressure has a direct
influence of the amount of
oxygen available in the body (especially if there
is a drop). But in general
people who live in high altitudes have higher
levels of hemoglobin in their
bodies (hence compensates for lower levels of
oxygen). For cities at high
elevations with pollution problems such as Mexico
the same CO concentrations at
sea level may have no effect to the population
but may have impact with those
with health problems. 2.1.2 Nitrogen Oxides
There are several species of
nitrogen oxides. But for our discussion we will
consider N2O since the others
have relatively no toxic effects. Nitric oxide
is produced in the greatest
quantity during combustion. It has no direct
effects on health because it has a
tendency to rapidly disappear into the
atmosphere. In the atmosphere in the
presence of sunlight and other reactive
hydrocarbons is transformed into N2O and
other photochemical oxidants.
Nitrogendioxide (a brownish gas) is a visible
component of smog, which
directly affects human health. The following figure
illustrates this cycle
Figure 1. Figure1 Long term studies were done on animals
to determine the
overall effects of nitrogendioxide. There were changes observed
such as
ciliary loss in upper respiratory tract in rats and mice,
emphysematous
changes in dogs, and edema in squirrel monkeys. Also scientists
observed that NO
reduces resistance to bacterial and viral infections.
Research on humans, based
on exposure levels of 4-5 ppm. Researchers noticed
an increase in expiratory
flow resistance. High occupational exposure has
lead researchers to record
exposure levels of unto 250 ppm. In some cases
weeks apart, there were rapid
onset of fever, chills and difficulty
breathing. But there were no definite
effects of nitrogen dioxide at ambient
levels. 2.1.3 Volatile Organic Compounds
These volatile organic compounds
(VOCs) make up the lower boiling fractions of
fuels and lubricants, and
partially combusted fuels. These VOCs are emitted
during refueling, leakage
in the engine, and tailpipe. VOCs are complex
compounds of aliphatics,
olefins, aldehydes, hetones and aromatics. Many these
compounds are known to
be potentially hazardous to human health. But in general
these compounds are
found in such low quantities there are no fears of having
direct effects on
human health. Rather these compounds have a direct effect on
photochemical
smog. 2.1.3.1 Effects of Benzene Prolonged exposure to benzene
especially in
the respiratory tract or cutaneous contact can result in aplastic
anemia or
acute myelogenous leukemia. Bone marrow is also affected. When the
bone
marrow is affected it decreases circulation in the erythrocyte, platelets
and
leukocytes. Benzene related leukemia usually affects workers exposed to
it
for periods of forty years. 2.1.3.2 Effects of Aromatics Aromatics have
been
added in modern day fuels which contain high levels of benzene. The
total
benzene emission increase is directly proportional to the amount of
aromatics
found in fuels. For about every 1% of aromatics there is 4% of
benzene. It was
also found that the amount of non-benzene aromatics in fuels
also results in a n
increase in tailpipe emissions of benzene. 2.1.3.3
Effects of Hydro Carbons
Aliphatic hydrocarbons upon inhalation may be
harmful, because in high
concentrations, they depress the central nervous
system causing dizziness and
incoordination. It is generally accepted that
low level exposures have no or
little effects on the human body. But they do
play an important role in
photochemical smog. 2.1.3.4 Effects of Alcohol With
the additions of methanol
and ethanol as fuel additives was implemented to
reducing emissions. But the
problem is that these additives are very volatile
hence they will contribute to
the overall VOC load. The problem with
additives such as methanol tends to emit
formaldehyde. And formaldehyde is a
carcinogen and a key component to
photochemical smog. 2.2 PHOTOCHEMICAL SMOG
There are two types of smog. The
first, which has been known for a long time,
is when there is an incomplete
combustion of coal. This phenomena produces
sulfur dioxide and smoke and in
combination with fog forms smog. The second
type is when automobiles exhaust
produces oxidative pollutants, which leads
to photochemical smog. Photochemical
smog results from the atmospheric
reaction between certain hydrocarbons and
oxides of nitrogen in the presence
of sunlight. The most common effects on the
human body by photochemical smog
are eye irritation, potential effects on the
respiratory system, reduced
visibility and plant damage. During intense smog
periods, ozone levels tend
to reach hazardous levels. Hence these levels will
also have an adverse
effect on human health. Studies have been done in
determining the effects of
ozone on animals and humans. Exposures to 6 ppm of
ozone for a period of four
hours will have about a 50% mortality rate among rats
and mice. At levels of
(ozone) about 1 ppm will have adverse effects (permanent
damage) on the
respiratory tracts of small animals. Some animals also developed
some form of
immunity to low levels of ozone. Studies done on humans were done
using low
levels of ozone for relatively short periods of time. Hence long term
effects
are unknown. For short-term effects to ozone exposure humans
expressed
similar patterns to those of animals. It was found that humans
obtain some form
of immunization. Other research showed that asthmatics did
not suffer more
effects from ozone exposure than did other individuals with
or without light
exercise, there was irritation at 0.12 ppm with high
exercise levels and the
effect at high exercise levels was a product of ozone
concentration, ventilation
rate and exposure time. 2.3 PARTICULATE EMISSIONS
2.3.1 Lead Because of high
compression ratios built automobiles (generally
American built cars), these
automobiles use to require high-octane (90-100)
octane gasoline for high
performance. To obtain such levels at the time
either tetraethyl lead or other
organometallic compounds, or by increasing
the aromatic content of the gasoline.
But through environmental awareness
advanced countries have reduced or cut out
lead in gasoline products. The
removal of lead was also necessary for catalyst
equipped cars to function
properly. The effects of lead were very important for
the removal from
gasoline powered automobiles. High lead concentrations have
adverse effects
on human heath such as neurotic, renal, and reproductive
effects. At lower
levels of lead exposure it may cause hyperactivity, auditory
deficiencies,
reduction in intelligence, and reduced nerve conduction. Also by
measuring
blood lead levels in humans it was found by lowering the lead emission
lower
the lead blood levels. 2.3.2 Diesel Emissions Diesel engine
powered
automobiles are very similar to powered by petrol with the exception
that diesel
engines produce a lot more particulate emissions. As discussed
earlier
particulate emissions are believed to be carcinogenic. High exposures
to diesel
particulate resulted in lung inflammation, accumulations of soot
and chronic
lung disease in rats. Lung tumors also increased at high
concentrations but none
were found at low levels. 2.3.3 Manganese
Methylcyclopentadienyl manganese
tricarbon (MMT) is another metal containing
anti lock additive. This additive
has been used in petrol cars since the
phase out of leaded fuels to increase
compression. The concentration of MMT
is very low in petrol fuels. Hence there
has been little or no effect in the
rise of manganese emissions. Chronic
exposure to high levels of manganese (in
occupational settings) has resulted in
maganism. Maganism is a disease, which
produces psychotic behavior with
hallucinations, delusions and compulsions.
Also it may result in a condition
resembling Parkinson and eventually death
may occur in a severe case. 3.
EMISSION CONTROL 3.1 EXHAUST EMISSIONS
CONTROL LEGISLATION Legislation requiring
the control of emissions from motor
vehicles was first introduced in America in
the 1600's and has been
progressively revised by incorporating reduced emissions
requirements. An
important step in emission control was taken in the 1970
amendment to the
United States Clean Air Act which required a 90 % reduction in
carbon
monoxide, hydrocarbon, and nitrogen oxide emissions. Figure 3.1
illustrates
the percentage of these pollutant resulting from automobile
emissions.
POLLUTANT TOTAL AMOUNT VEHICLE EMISSIONS Amount Percentage
NITROGEN
OXIDES 36 019 17 012 47 HYDROCARBONS 33 869 13 239 39 CARBON
MONOXIDE 119 148 78
227 66 Table 3-1 Pollution Accounted by Automobile
Emissions in 1989 (1000 tons)
The 1970 amendment requirements were so
stringent for that period that they
could not be met with available engine
technology. New technology has since been
developed and the requirements have
been met. However, more rigid standards are
continuously being proposed to
improve emissions. While significant improvements
to fuel economy, power
output, and emissions have been made in recent years by
modification and
control, none of them have resulted in an engine capable of
meeting current
American standards while maintaining satisfactory driveability,
power output,
and fuel economy without the use of catalyst units in the exhaust
system. 3.2
THE USE OF CATALYSTS FOR EMISSION CONTROL The concept of using a
catalyst to
convert carbon monoxide, hydrocarbons, and nitrogen oxides to
less
environmentally threatening compounds such as nitrogen, water and carbon
dioxide
was a well established practice prior to the need arising from motor
vehicle
emissions. However, rapid changes in exhaust gas temperature, volume
and
composition were features not previously encountered in chemical and
petroleum
industry applications. Other unique requirements were the control
of emissions
such as ammonia, hydrogen sulfide and nitrous oxide which could
result from
secondary catalytic reactions and for the catalyst system to
maintain its
performance after high temperature excursions up to 1000°C and
in the presence
of trace catalyst poisons such as lead and phosphorous.7 The
principal reactions
on automobile exhaust Catalysts are as follows: Oxidation
Reactions: 2CO + O2 Þ
2CO2 4HC + 5O2 Þ 4CO2 + 2H2O Reduction Reactions:
2CO + 2NO Þ 2CO2 + N2 4HC +
10NO Þ 4CO2 + 2H2O + 5N2 By the nature of the
oxidation and reduction reactions
which are involved in the removal of carbon
monoxide, hydrocarbons and nitrogen
oxides and the operating characteristics
of the preferred catalyst, several
combinations of engine/catalyst systems
have been used since catalysts were
introduced on American cars in 1975.
3.2.1 The Carbon Monoxide/Hydrocarbon
Oxidation Catalyst Concept When
emission control is primarily concerned with
carbon monoxide and hydrocarbons
and not with nitrogen oxide, such as is the
case in the European "Euronorms"
standards, oxidation catalysts are
used. Key features of this system are the
use of a secondary air supply to the
exhaust gas stream to ensure oxidizing
conditions under all engine operating
loads and the use of exhaust gas
recirculation (EGR) to limit nitrogen oxide
emissions from the engine. A
schematic of this system is shown in Figure 3.1.
Figure 3-1 The Oxidation
Catalyst This System was used initially in America to
meet interim emission
standards and is likely to be adopted to meet similar
standards on medium and
smaller engine cars (less than 2 litter engines) in
Europe. 3.2.2 Dual
Bed and Threeway Catalyst Concepts In order to overcome the
limitations
imposed by the use of EGR and to meet more rigid nitrogen oxide
standards,
catalysts capable of reducing nitrogen oxide emissions are
necessary.
Initially, as a result of the difficulty of controlling
air/fuel ratios to the
tolerances required by a single catalyst unit, a dual
catalyst bed was used. In
order to ensure reducing conditions in the first
catalyst bed, where nitrogen
oxides were reacted, the engine was tuned
slightly rich of the stoichiometric
ratio. Secondary air was then injected
into the exhaust stream ahead of the
second catalyst bed (oxidation bed) to
complete the removal of carbon monoxide
and hydrocarbons. With developments
in engine control and catalyst technology
involving widening the air/fuel
operating window for 90 % removal of
hydrocarbons, carbon monoxide and
nitrogen oxides, the dual bed system has been
replaced with a single threeway
catalyst unit. A schematic of this system is
shown in Figure 3.2. Figure 3-2
The Three-way Catalyst Key features of this
system, in addition to the
catalyst unit, are an electronically controlled
air/fuel management system
incorporating in its most advanced form, the use of
an oxygen sensor to
monitor and control exhaust gas combustion. Systems such as
this are now
universal on American and Japanese cars and in those countries that
have
adopted similar emission standards. The performance of the
Threeway
Catalyst system is summarized in Table 3.2 and Table 3.3. Cold
ECE 15 HC + NOX
NOX CO cycle, g/test Without Catalyst With Catalyst
Without Catalyst With
Catalyst Without Catalyst With Catalyst PEUGEOT 205
18.3 8.5 7.8 5.8 26.3 8.8
FIAT UNO 45 15.2 4.1 6.2 2.7 26.7 9.8 VW GOLF C
16.1 6.4 5.7 2.0 50.5 42.7 ROVER
213 12.3 5.2 3.6 1.4 46.7 27.5 Table 3-2
Emission Levels from small vehicles
Polycyclic Aromatic Emissions,
mg/mile Hydrocarbon Without Catalyst With
Catalyst phenanthrene 1.85 0.16
anthracene 0.61 0.04 fluoranthrene 2.27 0.23
pyrene 2.91 1.50 perylene 1.21
0.40 benzo(a)pyrene 0.94 0.17 benzo(e)pyrene 2.76
0.41 dibenzopyrenes
0.28 0.23 coronene 0.41 0.27 Table 3-3 Polycyclic Aromatic
Hydrocarbon
Emissions from a Programmed Combustion Engine 3.2.3 Lean Burn
Catalyst
Systems Engine operations with air/fuel ratios of 20:1 is a good way
of
reducing nitrogen emissions and improving fuel economy. However, with
current
engine technology, in order to achieve nitrogen emissions consistent
with US
legislation, the engine must operate in a very lean region where, as
shown in
Figure 3.3, hydrocarbon emissions that increase to levels which
may exceed
current American standards. In these situations an oxidation
catalyst is
incorporated into the exhaust system to control hydrocarbon
emissions. Figure
3-3 The Effect of Air/Fuel Ratio on Engine Operation A
feature of the ECE15
European test cycle was its low average speed as it
is intended to be
representative of city driving. The emissions that result
are therefore typical
of low speed, low acceleration conditions. A more
representative cycle
incorporating higher speeds and accelerations has been
introduced so as to
assess emissions under other conditions including urban
and highway driving. In
order to develop and maintain a higher speed more
power is required from the
engine which, in the case of the lean burn system,
means decreasing the air/fuel
ratio. This in turn increases nitrogen oxide
emissions to levels where current
engine technology is likely to exceed
standards (See Figure 3.3). It is
therefore desirable that catalysts used on
lean burn engines should in addition
to having a hydrocarbon oxidation
capability also have a nitrogen oxide
reduction capability when fuel
enrichment occurs for increased engine power. The
effect on the reduction of
hydrocarbons and nitrogen oxide emissions which can
be achieved on a lean
burn engine using a catalyst with oxidation and reduction
capabilities is
shown in Table 3.4 for a Volkswagen Jetta Series 1, powered by a
1.4
litter Ricardo High Ratio Compact Chamber lean burn engine. ECE 15
Cold
Start Cycle g/test Hydrocarbons Carbon Monoxide Nitrogen Oxides
Without Catalyst
11.7 15.9 5.9 With Catalyst 1.7 12.4 4.2 Table 3-4 Lean
Burn Engine Emissions
3.2.4 Diesel Exhaust Emission Control Although
Diesel engines emit relatively
low concentrations of carbon monoxide and
hydrocarbons and have a better fuel
economy compared to gasoline powered
vehicles, particulate emissions are of
concern. Along with the carbon
particulates which are produced during the
combustion process are a range of
aromatic hydrocarbons, which was one of the
main reasons that the EPA
established standards to limit particulate emissions.8
The carbon and the
associated organics produced during combustion may be
collected on a filter
and removed by oxidation so that the filter regenerates
and is effective for
the life of the vehicle. As the particulates are not
oxidized at a
significant rate below 600°C which occurs in the exhaust system
only when the
engine is running at or near full power, catalysts are introduced
into the
filter which reduces the oxidation temperature to approximately
300°C.
Table 3.5 compares emissions from an exhaust system with a
catalyst to that of a
system without.9 g/mile HC CO NOX Particulate Without
catalyst 0.24 1.01 0.90
0.23 With catalyst 0.05 0.16 0.79 0.11 Table 3-5
Catalytic Control of Diesel
Exhaust Emissions 3.2.5 Catalytic Combustion
Nitrogen oxide emissions result
mainly from the reaction between oxygen and
nitrogen at temperatures arising
from the combustion of fuel whether it is
initiated by spark, as in the gasoline
engine, or compression as in the
diesel engine. Leanburn operation of a gasoline
engine, as described earlier,
offers a partial solution to the problem but is
limited by hydrocarbon
emissions as the non-flammability limit for spark
ignition is approached.
While the diesel engine does not have these advantages
it is limited by high
particulate emissions. A solution to this problem is to
use a catalyst to
ignite the air/fuel mixture thus overcoming the constraining
factors of the
gasoline and diesel engines. Having removed this constraint, the
engine is
able to operate at a compression ratio of 12 to 1. Combustion
efficiency and
mechanical energy is thus optimized which results in a maximized
fuel
economy.10 The principle of the catalytic engine is that during the
engine
operating cycle, the fuel is injected into the combustion chamber just
before
the start of combustion is required. This fuel is then mixed with the
air
already in the cylinder and then passed through the catalyst, where heat
release
occurs. Since the charge is passed through a catalyst, oxidation can
occur at
low temperatures and very lean mixtures. This results in complete
fuel oxidation
which enables the engine to run unthrottled and therefore
lean, which provides
good fuel economy. The formation of nitrogen oxides and
carbon monoxide in the
combustion chamber is also strongly dependent on the
air/fuel ratio and lean
operation results in reduced emissions of these
pollutants in the exhaust. The
catalyst enables oxidation of hydrocarbons at
much lower temperatures than
normally possible, so the emission is also
reduced. 4. CONCLUSION Since the
introduction of legislation in America in
1970 requiring substantial reductions
in emissions from motor vehicles,
catalyst technology has played a major part in
maintaining air quality. With
the introduction of similar standards in other
countries, the automobile
industry represents the largest single use for
catalyst systems. However, it
must be noted that the internal combustion engine
will soon approach its
development limit as far as emission technology is
concerned. The need for
significant reduction in carbon dioxide, hydrocarbon,
and nitrogen oxide
emissions will ultimately require the use of an alternative
energy source to
power vehicles. Developments are being pursued in the use of
"clean fuels"
such as reformulating gasoline and diesel fuel as well
as methanol and
natural gas in advanced engine design. Ultimately however, we
can expect
severe environmental legislation which will be met only by a
completely new
power source. Efforts are being undertaken by the automotive
industry to
replace the current power source for automobiles. Electric powered
cars,
solar powered cars and vehicles which utilize several power
sources
concurrently (hybrid) are all being intensively researched. While the
emission
standards for cars set by the 1970 Clean Air Act Amendments were
considered
adequate at the time, air quality has not significantly improved
as projected
due to the expanding car population in industrialized countries.
By observing
the possible ill effects to human health and well being
mentioned earlier, it
can only be concluded that for the eventual "cleaning"
of our
atmosphere, a power source with 0 emission will one day need to be
implemented
in our main means of transportation, the
automobile.
Bibliography
K.C. Taylor, Chem Tech., London, New
York: Chapman and Hall, 1990; pp 525-60
8. H Klingenberg & H.
Winneke, Total Environment, Houston: Gulf publishing,
1990; pp 95-106. 9.
B.E. Enga, Platinum Metals Review, New York: Chapman and
Hall,
1982;pp26-32 10. Ibid., pp
45-54