Automobile Emission 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 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 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 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 x 2CO2 4HC + 5O2 x 4CO2 + 2H2O Reduction Reactions: 2CO + 2NO x 2CO2 + N2 4HC + 10NO x 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. REFERENCES 7. 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
Bibliography
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.