What is Air Pollution?
Pollution is now a common place term, that our ears are attuned to. We hear about the various forms of pollution and read about it through the mass media. Air pollution is one such form that refers to the contamination of the air, irrespective of indoors or outside. A physical, biological or chemical alteration to the air in the atmosphere can be termed as pollution. It occurs when any harmful gases, dust, smoke enters into the atmosphere and makes it difficult for plants, animals and humans to survive as the air becomes dirty.
Air pollution can further be classified into two sections- Visible air pollution and invisible air pollution. Another way of looking at Air pollution could be any substance that holds the potential to hinder the atmosphere or the well being of the living beings surviving in it. The sustainment of all things living is due to a combination of gases that collectively form the atmosphere; the imbalance caused by the increase or decrease of the percentage of these gases can be harmful for survival.
Read here more about 40 facts of air pollution.
The Ozone layer considered crucial for the existence of the ecosystems on the planet is depleting due to increased pollution. Global warming, a direct result of the increased imbalance of gases in the atmosphere has come to be known as the biggest threat and challenge that the contemporary world has to overcome in a bid for survival.
Types of Pollutants
In order to understand the causes of Air pollution, several divisions can be made. Primarily air pollutants can be caused by primary sources or secondary sources. The pollutants that are a direct result of the process can be called primary pollutants. A classic example of a primary pollutant would be the sulfur-dioxide emitted from factories
Secondary pollutants are the ones that are caused by the inter mingling and reactions of primary pollutants. Smog created by the interactions of several primary pollutants is known to be as secondary pollutant.
Causes of Air pollution
1. Burning of Fossil Fuels: Sulfur dioxide emitted from the combustion of fossil fuels like coal, petroleum and other factory combustibles is one the major cause of air pollution. Pollution emitting from vehicles including trucks, jeeps, cars, trains, airplanes cause immense amount of pollution. We rely on them to fulfill our daily basic needs of transportation. But, there overuse is killing our environment as dangerous gases are polluting the environment. Carbon Monooxide caused by improper or incomplete combustion and generally emitted from vehicles is another major pollutant along with Nitrogen Oxides, that is produced from both natural and man made processes.
2. Agricultural activities: Ammonia is a very common by product from agriculture related activities and is one of the most hazardous gases in the atmosphere. Use of insecticides, pesticides and fertilizers in agricultural activities has grown quite a lot. They emit harmful chemicals into the air and can also cause water pollution.
3. Exhaust from factories and industries: Manufacturing industries release large amount of carbon monoxide, hydrocarbons, organic compounds, and chemicals into the air thereby depleting the quality of air. Manufacturing industries can be found at every corner of the earth and there is no area that has not been affected by it. Petroleum refineries also release hydrocarbons and various other chemicals that pollute the air and also cause land pollution.
4. Mining operations: Mining is a process wherein minerals below the earth are extracted using large equipments. During the process dust and chemicals are released in the air causing massive air pollution. This is one of the reason which is responsible for the deteriorating health conditions of workers and nearby residents.
5. Indoor air pollution: Household cleaning products, painting supplies emit toxic chemicals in the air and cause air pollution. Have you ever noticed that once you paint walls of your house, it creates some sort of smell which makes it literally impossible for you to breathe.
Suspended particulate matter popular by its acronym SPM, is another cause of pollution. Referring to the particles afloat in the air, SPM is usually caused by dust, combustion etc.
Effects of Air pollution
1. Respiratory and heart problems: The effects of Air pollution are alarming. They are known to create several respiratory and heart conditions along with Cancer, among other threats to the body. Several millions are known to have died due to direct or indirect effects of Air pollution. Children in areas exposed to air pollutants are said to commonly suffer from pneumonia and asthma.
2. Global warming: Another direct effect is the immediate alterations that the world is witnessing due to Global warming. With increased temperatures world wide, increase in sea levels and melting of ice from colder regions and icebergs, displacement and loss of habitat have already signaled an impending disaster if actions for preservation and normalization aren’t undertaken soon.
3. Acid Rain: Harmful gases like nitrogen oxides and sulfur oxides are released into the atmosphere during the burning of fossil fuels. When it rains, the water droplets combines with these air pollutants, becomes acidic and then falls on the ground in the form of acid rain. Acid rain can cause great damage to human, animals and crops.
4. Eutrophication: Eutrophication is a condition where high amount of nitrogen present in some pollutants gets developed on sea’s surface and turns itself into algae and and adversely affect fish, plants and animal species. The green colored algae that is present on lakes and ponds is due to presence of this chemical only.
5. Effect on Wildlife: Just like humans, animals also face some devastating affects of air pollution. Toxic chemicals present in the air can force wildlife species to move to new place and change their habitat. The toxic pollutants deposit over the surface of the water and can also affect sea animals.
6. Depletion of Ozone layer: Ozone exists in earth’s stratosphere and is responsible for protecting humans from harmful ultraviolet (UV) rays. Earth’s ozone layer is depleting due to the presence of chlorofluorocarbons, hydro chlorofluorocarbons in the atmosphere. As ozone layer will go thin, it will emit harmful rays back on earth and can cause skin and eye related problems. UV rays also have the capability to affect crops.
When you try to study the sources of Air pollution, you enlist a series of activities and interactions that create these pollutants. There are two types of sources that we will take a look at: Natural sources and Man-made sources.
Natural sources of pollution include dust carried by the wind from locations with very little or no green cover, gases released from the body processes of living beings (Carbon dioxide from humans during respiration, Methane from cattle during digestion, Oxygen from plants during Photosynthesis). Smoke from the combustion of various inflammable objects, volcanic eruptions etc along with the emission of polluted gases also make it to the list of Natural sources of Pollution.
While looking at the man-made contributions towards air pollution, smoke again features as a prominent component. The smoke emitted from various forms of combustion like in bio mass, factories, vehicles, furnaces etc. Waste used to create landfills generate methane, that is harmful in several ways. The reactions of certain gases and chemicals also form harmful fumes that can be dangerous to the well being of living creatures.
Solutions for Air Pollution
1. Use public mode of transportation: Encourage people to use more and more public modes of transportation to reduce pollution. Also, try to make use of car pooling. If you and your colleagues come from the same locality and have same timings you can explore this option to save energy and money.
2. Conserve energy: Switch off fans and lights when you are going out. Large amount of fossil fuels are burnt to produce electricity. You can save the environment from degradation by reducing the amount of fossil fuels to be burned.
3. Understand the concept of Reduce, Reuse and Recycle: Do not throw away items that are of no use to you. In-fact reuse them for some other purpose. For e.g. you can use old jars to store cereals or pulses.
4. Emphasis on clean energy resources:Clean energy technologies like solar, wind and geothermal are on high these days. Governments of various countries have been providing grants to consumers who are interested in installing solar panels for their home. This will go a long way to curb air pollution.
5. Use energy efficient devices: CFL lights consume less electricity as against their counterparts. They live longer, consume less electricity, lower electricity bills and also help you to reduce pollution by consuming less energy.
Several attempts are being made world wide on a personal, industrial and governmental levels to curb the intensity at which Air Pollution is rising and regain a balance as far as the proportions of the foundation gases are concerned. This is a direct attempt at slacking Global warming. We are seeing a series of innovations and experiments aimed at alternate and unconventional options to reduce pollutants. Air Pollution is one of the larger mirrors of man’s follies, and a challenge we need to overcome to see a tomorrow.
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Rinkesh is passionate about clean and green energy. He is running this site since 2009 and writes on various environmental and renewable energy related topics. He lives a green lifestyle and is often looking for ways to improve the environment around him.
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The automobiles play an important role in the transport system. With an increase in population and living standard, the transport vehicles as well as car population is increasing day by day. In addition to this there is steep increase in the number of two wheelers during the last two decades. All these are increasing exhaust pollution and particularly in metros as density of these vehicles in metros are very high. The main pollutants contributed by I.C. engines are CO, NOX unburned hydro-carbons (HC) and other particulate emissions. Other sources such as Electric power stations industrial and domestic fuel consumers also add pollution like NOX, SO2 and particulate matters. In addition to this, all fuel burning systems emit CO2in large quantities and this is more concerned with the Green House Effect which is going to decide the health of earth. Lot of efforts are made to reduce the air pollution from petrol and diesel engines and regulations for emission limits are also imposed in USA and in a few cities of India. An extensive analysis of energy usage and pollution shows that alternative power systems are still a long way behind the conventional ones. Further developments in petrol and diesel engines, combined with improvements in the vehicles, will make fuel consumption reduction of 40% or more in the future cars. This, in turn, will reduce the CO2 emissions, a gas which is responsible for greenhouse effect.
Chapter 1 Introduction
Chapter 2 Environmental Pollution from Internal Combustion Engine Vehicles.
2.3 Health Hazards
Chapter 3 Formation of Pollutants and Their Estimation.
3.1 Internal Combustion Engines.
3.2 Formation of Pollutants.
Chapter 4 Emission Control Methods
4.1 Exhaust Gas After-Treatment
4.2 Three-Way Catalytic Converter
4.3 Exhaust Gas Recirculation (EGR)
4.4 Variable Compression Ratio (VCR)
Chapter 5 Conclusions
Report on plagiarism
LIST OF FIGURES:
Figure 1: Percentage of Emission
Figure 2: Air Pollution by a Car.
Figure 3: Three-Way Catalytic Converter.
Figure 4: EGR System with One-Stage Cooling.
Figure 5: Schematic Representation of a High-Speed Passenger Car EGR/Intake Throttle System.
Internal combustion (IC) engines are used in a variety of stationary applications ranging from power generation to inert gas production. Both spark ignition and compression ignition engines can be found. Depending on the application, stationary IC engines range in size from relatively small (~50 hp) for agricultural irrigation purposes to thousands of horsepower for power generation. Often when used for power generation, several large engines will be used in parallel to meet the load requirements. A variety of fuels can be used for IC engines including diesel and gasoline among others. The actual fuel used depends on the owners/operators preference but can be application dependent as well.
The operation of IC engines results in the emission of hydrocarbons (NMHC or VOC), carbon monoxide (CO), nitrogen oxides (NOx), and particulate matter (PM). The actual concentration of these criteria pollutants varies from engine to engine, mode of operation, and is strongly related to the type of fuel used.
Various emission control technologies exist for IC engines which can afford substantial reductions in all four criteria pollutants listed above. However depending on whether the engine is being run rich, lean, or stoichiometrically and the emission control technology used, the targeted emissions vary as do the levels of control. For example, an oxidation catalyst can be used to control NMHC, CO, and PM emissions from diesel engines which inherently operate in a lean environment, whereas selective catalytic reduction (SCR) could be used to additionally control NOx emissions. More recently, lean-NOx catalysts have been demonstrated to provide greater than a 80 percent reduction in NOx emissions from a stationary diesel engine, while providing significant CO, NMHC, and PM control as well.
PM emissions from stationary diesel engines are more of a concern than those for IC engines using other fuels. Several emission control technologies exist for diesel engine PM control. Oxidation or lean-NOx catalyst can be used to not only reduce the gaseous emissions associated with the use of diesel engines but further provide significant PM control. Likewise, diesel particulate filter systems can be used to achieve up to and greater than 90 percent PM control while in some instances, also providing reductions in the gaseous emissions. Additionally, special ceramic coatings applied to the combustion zone surfaces of the piston crown, valve faces, and head have shown the ability to significantly reduce NOx and PM emissions in diesel engines. These ceramic coatings can be used by themselves or combined with an oxidation catalyst to give even greater reduction of PM. Ceramic engine coatings change the combustion characteristics such that less dry, carbon soot, is produced. Also, when combined with an oxidation catalyst, ceramic coatings allow retarding of the engine to reduce NOx, whilen CO and particulates are maintained at low levels.
In the case of gaseous fuels, ceramic coatings have shown the ability to allow the user to operate their engines with timing significantly advanced generating higher power levels. Also, wider ranges of fuel composition and ambient air temperature fluctuations are tolerated without the deleterious effects of pre-combustion. Tests are currently underway to evaluate the effects of the coatings on specific emissions from gaseous fueled engines.
Emission control technology for stationary IC engines is currently available and can be used to provide substantial reductions in the CO, NMHC, NOx, and PM emissions from these sources in a cost-effective manner.
CHAPTER – 2
2.ENVIRONMENTAL POLLUTION FROM INTERNAL COMBUSTION ENGINE VEHICLES.
Emissions from mobile sources constitute a major negative anthropogenic influence on the environment. In 1997, they were roughly 30, 4, and 67 per cent of the generated in the USA CO2, CH4, and N2O. Total emissions of nitrogen oxides (NOx) from mobile sources represented 49 per cent of national emissions, while CO, NMVOC and SO2 emissions contributed approximately 81, 40 and 7 per cent, respectively. Emissions of particulate matter in the USA for the same year are reported as particulate matter smaller than 10 microns (PM10) – 2.569 Gg and total particulate matter (PT) – 5.489
Table 1 illustrates the relative participation of major vehicles groups to the total air pollution from mobile sources in the US in 1997. The above data show that even in countries, leading in pollution control and efficiency, vehicles with ICE contribute a significant portion to air pollution. The contribution of the particular vehicle types can also be seen. Leaded gasoline was banned in the US in 1995. Still, one source cited more than 1270 tons of lead emissions from vehicles in that country in 1994. Brazil and Japan eliminated leaded gasoline in the 1980s. The first European country, which prohibited leaded gasoline (in 1993), was Austria. Most of the countries in Central and Eastern
Gasoline Highway, Gg 44 225 4629 4528 211 191
Passenger Cars, % 55.1 56.1 54.5 47.4 56.0
Light Duty Trucks, % 37.6 37.2 39.4 43.1 41.9
Heavy Duty Vehicles, % 6.9 6.5 5.4 7.6 2.1
Motorcycles, % 0.4 0.2 0.7 1.9 +
Diesel Highway, Gg 1 368 1 753 217 12 7
Passenger Cars, % 2.0 1.8 5.1 + +
Light Duty Trucks, % 0.7 0.6 2.3 + +
Heavy Duty Vehicles, % 97.3 97.6 92.6 91.7 100
Non Highway, Gg 15 201 4 137 2 205 20 9
Ships and Boats, % 11.2 6.6 21.2 15.0 11.1
Locomotives, % 0.7 20.8 2.0 10.0 11.1
Farm Equipment, % 2.0 23.2 5.3 30.0 11.1
% 7.1 27.2 9.9 5.0 +
Aircraft a, % 6.0 3.9 7.8 35.0 66.7
Other b , % 73.0 18.3 53.8 5.0 +
Total, Gg 60 794 10 519 6 949 242 207
FIGURE 1:-PERCENTAGE OF EMISSION
2.3. HEALTH HAZARDS
Cardiopulmonary disease linked to breathing fine particle air pollution
Pneumonia related deaths
CHAPTER – 3
3. FORMATION OF POLLUTANTS AND THEIR ESTIMATION.
The relative proportions of the major pollutants in exhaust emissions depend mainly on the specific organization and parameters of the ignition and combustion processes in the ICE. Fuels are usually matched to engines although the full utilization of the advantages of some alternative fuels requires modification of the engine or the development of a new type of engine.
FIGURE 2: – AIR POLLUTION BY A CAR.
3.1. INTERNAL COMBUSTION ENGINES.
ICE may be classified by different criteria. For instance, -according to fuel type, to power, to ignition type and so on. An appropriate classification from the point of view of combustion chemistry and air pollution will divide them into two major groups. The first group includes engines in which combustion is performed periodically in a chamber of changing volume (i. e. reciprocating piston engines). In the second group combustion takes place continuously (steady flow) in a chamber of constant volume.
The first group may be further divided into spark ignition (SI) and compression ignition (CI) engines, although there are engines combining both principles. SI engines may be classified as two stroke and four stroke engines, CI engines –as direct and indirect injection engines.
The second group includes the jet engines, which may use a gas turbine, liquid fuel, air as oxidation agent and a turbo compressor (aircraft jet engines), and the rocket jet engines, which have chemical agents as fuels and oxidizers.
Another useful distinction between internal combustion engines is the fact that only in SI engines the fuel is evaporated and mixed with the oxidizing agent before the ignition takes place. In other designs, the fuel is sprayed in the combustion chamber, in the form of drops of different size.
Modern SI engines used in passenger and freight vehicles are four-stroke, although two-stroke passenger cars are still running in some European countries. Two-stroke engines have the important advantages of lower weight and cost per unit of power output. They are widely used in small motorcycles, as outboard motors and other small power equipment.
The main pollutants from four-stroke gasoline engines are hydrocarbons, CO and nitrogen oxides. They are contained in exhaust emissions, but hydrocarbons are contributed both with the exhaust, and with the evaporative emissions. Particulate matter is usually negligible and is produced mainly from oil components brought in the combustion chamber by the piston. Sulfur oxides are typically low and for some time were not considered a problem.
New regulations restrict drastically sulfur in gasoline, because of reconsidering its influence on catalysts. Two-stroke engines emit 20 to 50 per cent of their fuel unburned in the exhaust, but also a considerable amount of oil, which is a component of the air-fuel mixture by design.
Air pollution from them is higher and includes a considerable amount of particulate matter. Two-stroke engine designs with advanced fuel injection, lubrication and combustion systems are being developed. They are expected to achieve lower emissions and higher fuel efficiency, while keeping their traditional other advantages.
In direct injection CI engines, the fuel is sprayed directly into compressed heated air. It evaporates, and ignites. These engines are typical for medium and larger vehicles. They provide higher power output and better efficiency than engines with indirect ignition, but are noisier. For passenger cars, indirect injection engines are more common, because of lower noise and high performance characteristics. Ignition takes place in a pre-chamber, often by a glow spark, and combustion then spreads into the main chamber. Direct ignition engines with improved noise and efficiency controls are being introduced lately in passenger vehicles as well.
Diesel engines can be also classified as naturally aspired, supercharged or turbo charged, depending on the organization of the airflow and its pressure before spraying the fuel. If properly controlled, emissions from turbo charged engines are smaller than from the other two types. Compared to the typical gasoline SI engines, both light and heavy duty diesel engines have considerably higher compression ratios and better fuel efficiency, which leads to lower CO and hydrocarbon emissions. Light duty vehicles emit also less nitrogen oxides than comparable gasoline engines, but nitrogen oxides from heavy-duty diesel engines are higher. Particulate matter and the poly-cyclic carcinogenic hydrocarbons adsorbed on it are eight to ten times as much as in gasoline counterparts and pose a serious problem or diesel engines. They also emit a considerable amount of sulfur compounds due to the presence of more sulfur still allowed in diesel, than in gasoline fuels. Stratified charge engines are a hybrid between typical gasoline and diesel engines. Indirect injection engines relyon spark for ignition of fuel rich mixture. They utilize non-uniform fuel distribution to improve turbulence mixing,and more efficiently avoid fuel rich zones in which the amount of the fuel is more than the stoichiometric extent. Direct injection engines use a high-pressure injector to create a swirl tangential motion of the fuel towards the spark plug. Stratified charge engines have high fuel efficiency, but use fuels with volatility closer to gasoline, and their emission performance is not very impressive.
Rotary car and motorcycle engines have a triangular rotor, which replaces the piston. They are smaller, lighter and simpler, but their CO and hydrocarbon emissions are higher than from conventional engines, while the NOx emissions are the same. Stirling engines are capable of higher fuel efficiency and lower emissions, but are impractical for common automotive use, because of high cost, poor transient response and power to weight ratio. Gas turbine engines are widely used in aircraft, high speed trains, marine vessels and stationary applications, and have been tested without a great success in road vehicles. They use jet fuel, which in volatility is intermediate between gasoline and diesel fuels.
The combustion environment and emissions from aircraft gas turbines vary widely with load. At low loads, temperature and NOx emissions are low, but CO and hydrocarbon emissions are higher. At high loads, CO and hydrocarbon emissions are low, but temperatures and NOx emissions are high. Soot is also formed at high loads, because of the imperfect mixing and fuel rich zones. The rapid quenching of combustion products in gas turbines promotes the emission of significant quantities of NO2, which might be comparable to those of nitric oxide, NO. As a whole, the emissions from gas turbine engines per power output are smaller than from the other engines.
Rocket internal combustion engines may use petroleum fractions, alcohols, ammonia, hydrazin, liquid hydrogen, etc. as fuels. The oxidation agents include nitric acid, tetranitromethane, hydrogen peroxide, liquid oxygen, etc. The emissions from rockets, especially those formed from nitrogen compounds are highly toxic.
3.2. FORMATION OF POLLUTANTS.
Chemical elements under ideal reaction conditions should burn to the corresponding oxides, so CO2 and water are the most likely products to be expected from oxidation of hydrocarbons. However, conditions in engineering applications of combustion reactions are not ideal. The stoichiometric quantity of air needed to burn completely 1 g of gasoline is roughly considered to be around 14.7 g, approximately eleven grams of this being nitrogen. If the air quantity is higher than the stoichiometric proportion the flames are called “lean”, if it is lower – “rich”. In a real combustion chamber flames may be both, depending on the particular chamber type. Other important factors include flame temperatures, gas residence time at high temperatures and combustion chamber turbulence. None of these factors is at its most favorable values throughout the whole chamber space. The fuels are in fact mixtures of compounds with different molecular masses, volatility, diffusion coefficients, etc. Under the non-ideal conditions in the chamber, they undergo pyrolysis and oxidation pyrolysis, prior to reaching the flame propagation zone. The mode of ignition of the fuel, and the intermediate products through which it passes in combustion reactions are very important for the type and the relative quantity of pollutants formed.
CHAPTER – 4
4. EMISSION CONTROL METHODS
Modification of current IC engine design and development of effective after-treatment systems are required to fulfill the requirements of the future emission norms. Emissions from IC engines have already been controlled to a certain extent by precise fuel metering, quality of air supply, better fuel-air mixing, using homogeneous mixtures, lower combustion temperatures, precise ignition timing, and fully computerized engine management. These techniques are just sufficient to meet the current emission regulations. However, better technologies are needed to meet future severe emission norms. Along with these, quality of fuel plays an important role in reducing emissions and enhancing performance of IC engines.
4.1. EXHAUST GAS AFTER-TREATMENT
Exhaust gas after-treatment system is usually used to chemically treat the exhaust gases produced from IC engines. Hazardous exhaust gases like CO, HC, and NOx are chemically treated with oxidization and reduction processes, outside the engine before letting it to the atmosphere.
4.2. THREE-WAY CATALYTIC CONVERTER
The three-way catalytic converter is used to reduce NOx along with the oxidization of CO and HC. Oxidation and reduction are simultaneously possible only when the engine runs using near stoichiometric mixture. The converter consists of a ceramic matrix or corrugated metal,bundled with a wash coat to provide a large surface area for chemical reactions. The wash coat is composed of actual catalyst materials, which consist of noble metals like platinum (Pt), rhodium (Rh) and palladium (Pd). Pt and Rh are used as reduction catalyst, while Pt and Pd are used as oxidization catalyst (Twigg,2011).Normally three-way catalytic converters achieved good conversion rates (~ 80%) and have a long service life in range of 400oCto 800oC temperature.
FIGURE 3:-THREE-WAY CATALYTIC CONVERTER.
They are in effective at exhaust gas temperatures below 250oC, and will be destroyed at exhaust gas temperatures above 1000oC.Leadand lead compounds reduce the effectiveness of catalysts,so this technology works better with the introduction of unleaded gasoline (Vora, 2000). Three-way catalytic converters are suitable for gasoline engines which normally run with near stoichiometric mixture. Diesel engines always run on an overall lean mixture, hence benefit of NOx reduction by this catalyst is very less.
4.3. EXHAUST GAS RECIRCULATION (EGR).
In internal combustion engines, exhaust gas recirculation (EGR) is a nitrogen oxide (NOx) emissions reduction technique used in petrol/gasoline and diesel engines. EGR works by recirculating a portion of an engine’s exhaust gas back to the engine cylinders. This dilutes the O2 in the incoming air stream and provides gases inert to combustion to act as absorbents of combustion heat to reduce peak in-cylinder temperatures. NOx is produced in a narrow band of high cylinder temperatures and pressures.
FIGURE 4:- EGR SYSTEM WITH ONE-STAGE COOLING
In a gasoline engine, this inert exhaust displaces the amount of combustible matter in the cylinder. In a diesel engine, the exhaust gas replaces some of the excess oxygen in the pre-combustion mixture. Because NOx forms primarily when a mixture of nitrogen and oxygen is subjected to high temperature, the lower combustion chamber temperatures caused by EGR reduces the amount of NOx the combustion generates (though at some loss of engine efficiency).Gasses re-introduced from EGR systems will also contain near equilibrium concentrations of NOx and CO; the small fraction initially within the combustion chamber inhibits the total net production of these and other pollutants when sampled on a time average. Most modern engines now require exhaust gas recirculation to meet emissions standards.
The exhaust gas, added to the fuel, oxygen, and combustion products, increases the specific heat capacity of the cylinder contents, which lowers the adiabatic flame temperature.
In a typical automotive spark-ignited (SI) engine, 5% to 15% of the exhaust gas is routed back to the intake as EGR. The maximum quantity is limited by the need of the mixture to sustain a continuous flame front during the combustion event; excessive EGR in poorly set up applications can cause misfires and partial burns. Although EGR does measurably slow combustion, this can largely be compensated for by advancing spark timing. The impact of EGR on engine efficiency largely depends on the specific engine design, and sometimes leads to a compromise between efficiency and NOx emissions. A properly operating EGR can theoretically increase the efficiency of gasoline engines via several mechanisms:
• Reduced throttling losses: – The addition of inert exhaust gas into the intake system means that for a given power output, the throttle plate must be opened further, resulting in increased inlet manifold pressure and reduced throttling losses.
• Reduced heat rejection: – Lowered peak combustion temperatures not only reduce NOx formation, it also reduces the loss of thermal energy to combustion chamber surfaces, leaving more available for conversion to mechanical work during the expansion stroke.
• Reduced chemical dissociation: – The lower peak temperatures result in more of the released energy remaining as sensible energy near TDC (Top Dead-Center), rather than being bound up (early in the expansion stroke) in the dissociation of combustion products. This effect is minor compared to the first two.
EGR is typically not employed at high loads because it would reduce peak power output. This is because it reduces the intake charge density. EGR is also omitted at idle (low-speed, zero load) because it would cause unstable combustion, resulting in rough idle. The EGR valve also cools the exhaust valves and makes them last far longer (a very important benefit under light cruise conditions).
Since the EGR system recirculates a portion of exhaust gases, over time the valve can become clogged with carbon deposits that prevent it from operating properly. Clogged EGR valves can sometimes be cleaned, but replacement is necessary if the valve is faulty.
FIGURE 5:- SCHEMATIC REPRESENTATION OF A HIGH-SPEED PASSENGER CAR EGR/INTAKE THROTTLE SYSTEM
In modern diesel engines, the EGR gas is cooled with a heat exchanger to allow the introduction of a greater mass of recirculated gas. Unlike SI engines, diesels are not limited by the need for a contiguous flamefront; furthermore, since diesels always operate with excess air, they benefit from EGR rates as high as 50% (at idle, when there is otherwise a large excess of air) in controlling NOx emissions. Exhaust recirculated back into the cylinder can increase engine wear as carbon particulate wash past the rings and into the oil.
Since diesel engines are unthrottled, EGR does not lower throttling losses in the way that it does for SI engines. Exhaust gas—largely carbon dioxide and water vapor—has a higher specific heat than air, so it still serves to lower peak combustion temperatures. However, adding EGR to a diesel reduces the specific heat ratio of the combustion gases in the power stroke. This reduces the amount of power that can be extracted by the piston. EGR also tends to reduce the amount of fuel burned in the power stroke. This is evident by the increase in particulate emissions that corresponds to an increase in EGR.
Particulate matter (mainly carbon) that is not burned in the power stroke is wasted energy. Stricter regulations on particulate matter (PM) call for further emission controls to be introduced to compensate for the PM emissions increase caused by EGR. The most common is a diesel particulate filter in the exhaust system which cleans the exhaust but causes a constant minor reduction in fuel efficiency due to the back pressure created. The nitrogen dioxide component of NOx emissions is the primary oxidizer of the soot caught in the DPF at normal operating temperatures. This process is known as passive regeneration. Increasing EGR rates cause passive regeneration to be less effective at managing the PM loading in the DPF. This necessitates periodic active regeneration of the DPF by burning diesel fuel in the oxidation catalyst in order to significantly increase exhaust gas temperatures through the DPF to the point where PM is quickly burned by the residual oxygen in the exhaust.
By feeding the lower oxygen exhaust gas into the intake, diesel EGR systems lower combustion temperature, reducing emissions of NOx. This makes combustion less efficient, compromising economy and power. The normally “dry” intake system of a diesel engine is now subject to fouling from soot, unburned fuel and oil in the EGR bleed, which has little effect on airflow. However, when combined with oil vapor from a PCV system, can cause buildup of sticky tar in the intake manifold and valves. It can also cause problems with components such as swirl flaps, where fitted. Diesel EGR also increases soot production, though this was masked in the US by the simultaneous introduction of diesel particulate filters. EGR systems can also add abrasive contaminants and increase engine oil acidity, which in turn can reduce engine longevity.
Though engine manufacturers have refused to release details of the effect of EGR on fuel economy, the EPA regulations of 2002 that led to the introduction of cooled EGR were associated with a 3% drop in engine efficiency, bucking a trend of a .5% a year increase.
4.4. VARIABLE COMPRESSION RATIO (VCR)
Variable compression ratio is a technology to adjust the compression ratio of an internal combustion engine while the engine is in operation. This is done to increase fuel efficiency while under varying loads. Higher loads require lower ratios to be more efficient and vice versa. Variable compression engines allow for the volume above the piston at ‘Top dead centre’ to be changed. For automotive use this needs to be done dynamically in response to the load and driving demands.
Higher compression ratio (CR) is always desired to get better thermal efficiency but it increases NOx emissions. Research works were carried out to optimize CR and effective CR according to load on engine so as to get better overall efficiency as well as low engine-out NOx.
Christensen et al. (1999) did experiments with variable compression ratio engine to demonstrate the multi fuel capability of HCCI engine. Secondary piston was placed in the cylinder head to achieve VCR from 10 to 28 by replacing one of the exhaust valves. They used different kind of fuel mixtures (mixture of iso-octane and n-heptane as well as gasoline and diesel) for experimental purpose. All tests were carried out with an equivalence ratio of 0.33. Test results showed that pure n-heptane (diesel) and iso-octane (gasoline) required a CR of about 11 and 22 respectively to get auto-ignition at TDC. Because of the poor atomization and vaporization of diesel fuel at low CR and at low inlet temperature (below 90oC), combustion quality became very poor. For all operating conditions, NOx emissions observed were very low, while soot was generated only with diesel fuel. Studies had shown that thermal efficiency was increased with increased CR but combustion efficiency was decreased leading to a minor variation in gross indicated efficiency. Higher amount of charge might be trapped in crevice volume which reduces the combustion efficiency with increased CR.
In general, compression ratio was varied either by having a secondary piston or by adopting variable valve timing. Late intake valve closure reduced the effective CR and hence reduced the peak temperature leading to reduced NOx. However, HC and CO were increased at lower CR.
CHAPTER – 5
Compared to gasoline engines, carbon dioxide, carbon monoxide and hydrocarbons produced in diesel engines are much lower. The issue of simultaneous control of NOx and PM becomes more complex in diesel engines. Both after-treatment and in-cylinder technologies to reduce emissions in CI engines have been reviewed. It is understood that various technologies to reduce emissions can just meet the present emission regulations. Better in-cylinder and/or after-treatment technologies have to be developed to meet future stringent emission norms.
EGR seems to be a simple and most effective way of reducing NOx emissions, but suitable measures need to be taken to reduce soot. There is a need to modify the EGR system along with some in-cylinder solutions viz. various injection techniques, charge conditions, late intake valve closing to improve the combustion phenomena and to reduce the emissions However, achieving wider operating range and controlling HC and CO emissions are the major challenges with this technique. An optimized combination of all in-cylinder solutions is required to overcome these issues.
1) Wikipedia-the free encyclopedia.
2) Internal Combustion Engines: V. Ganesan.
3) Bolshakov, G. F. (1987). 209 pp., Physicochemical Fundamentals of the Application of Fuels and Oils, Nauka, Novosibirsk, USSR (in Russian).[Covering theoretic and practical aspects concerning all engines and fuels.]
4) Flagan, R. C. and Seinfeld, J. H. (1988), Fundamentals of Air Pollution Engineering, 542 pp., Prentice Hall, New Jersey, USA [Engineering fundamentals concerning combustion processes and pollution control technologies].
6) Handbook of Air Pollution from Internal Combustion Engines: Pollutant formation and Control (1998), 663 pp., Ed. E. Sher, Academic Press, Boston, London, 1998. [Latest handbook, on pollution ICE].