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