Automobile Emissions 8513 - Essay Example

Pollution from automobile emissions has become over the past few decades an

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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 ?

2CO2 4HC + 5O2 ? 4CO2 + 2H2O Reduction Reactions: 2CO + 2NO ? 2CO2 + N2 4HC +

10NO ? 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