Automobile Emissions Essay, Research Paper
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
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.
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.
18.104.22.168 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.
22.214.171.124 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.
126.96.36.199 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.
188.8.131.52 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 PHOHEMICAL 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
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.
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
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:
2CO + O2 ? 2CO2
4HC + 5O2 ? 4CO2 + 2H2O
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
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