The earth's atmosphere, at or near sea level, consists approximately of 79 percent nitrogen, 20 percent oxygen and 1 percent other gases. If it were possible to remain in this state, 100 percent clean air would result. There are many natural and man made sources that affect air quality. Volcanic eruptions, forest fires and pine trees all contribute to the levels of pollution in our atmosphere.
Some of these pollutants are visible while others are invisible, each having the capability to irritate the eyes, ears, throat, and skin and cause respiratory distress. Concentrated pollutants under certain conditions in a specific area could result in death due to the displacement or chemical change of the oxygen content in the air. These pollutants can also cause great damage to the environment and to the many man made objects that are exposed to the elements. To better understand the causes of air pollution, the pollutants can be categorized into three separate types, natural, industrial and automotive.
Natural Pollutants
Natural pollution has been present on earth since before man appeared and continues to be a factor when discussing air pollution. Natural pollution comprises the largest quantity of pollutants in the atmosphere. It is the direct result of decaying organic matter, wind born smoke and particulates. Natural pollution occurs from natural events as forest fires (ignited by heat or lightning), volcanic eruptions, sand, dust and pine trees; the largest contributor to NOx emissions.
Industrial Pollutants
Industrial pollution is caused primarily by industrial processes; the burning of coal, oil and natural gas, which in turn produce smoke and fumes. Because the burning fuels contain large amounts of sulfur, the principal ingredients of smoke and fumes are sulfur dioxide and particulate matter. This type of pollutant occurs most severely during still, damp and cool weather, such as at night. Even in its less severe form, this pollutant is not confined to just cities. Because of air movements, the pollutants move for miles over the surrounding countryside, leaving in its path a barren and unhealthy environment for all living things.
Automotive Pollutants
The third major source of air pollution is automotive emissions. The emissions from the internal combustion engines were not an appreciable problem years ago because of the small number of registered vehicles and the nation's small highway system. During the early 1950's, the trend was to move from the cities to the surrounding suburbs. The problem remained that while people were leaving the cities for the suburbs, the infrastructure for mass transit did not exist. The absence of mass transit alternatives created an attractive market for automobile manufacturers. The end result was an increase in the number of vehicles produced and sold. There was also a marked increase in highway construction between cities and suburban areas. Multi-vehicle families emerged with a growing emphasis placed on multiple vehicles per family. Pollutant levels increased along with vehicle ownership in and around cities due a daily commute from home to work and back to home each day.
It was noted that a smoke and fog type haze was being formed and at times, remained in suspension over the cities, taking time to dissipate. At first this "smog,'' derived from the words "smoke'' and "fog,'' were thought to result solely from industrial sources but it was determined that automobile emissions shared the blame. It was discovered that when normal automobile emissions were exposed to sunlight for a period of time, complex chemical reactions take place.
These chemical reactions form smog which is a photo-chemical layer that develops when certain oxides of nitrogen (NOx) and unburned hydrocarbons (HC) from are exposed to sunlight. Pollution was more severe when smog would become stagnant over an area in which a warm layer of air settled over the top of a cooler air mass, trapping and holding the cooler mass at ground level. This inversion layer (or temperature inversion) would trap cooler air at a lower altitude and prevent emissions from being dispersed through normal air flow.
Temperature Inversion
In normal weather situations, surface air is warmed by heat radiating from the earth's surface and the sun's rays. This causes it to rise upward, into the atmosphere. Upon rising it will cool through a convection type heat exchange with the cooler air in the upper atmosphere. As warm air rises, the surface pollutants are carried upward and dissipated into the atmosphere.
When a temperature inversion occurs, we find the upper atmosphere air is no longer cooler, but is warmer than the surface air, causing the cooler surface air to become trapped. This warm air blanket can extend from above ground level to a few hundred or even a few thousand feet into the air. As the surface air is trapped, so are the pollutants, causing a severe smog condition. Should this stagnant air mass extend to a few thousand feet high, enough air movement with the inversion takes place to allow the smog layer to rise above ground level but the pollutants still cannot dissipate. This inversion can remain for days over an area, with the smog level only rising or lowering from ground level to a few hundred feet. Meanwhile, the pollutant levels increase, causing eye irritation, respiratory problems, reduced visibility, plant damage and in some cases, even disease.
This inversion phenomenon was first noted in the Los Angeles, California area. The city lies in terrain resembling a basin and with certain weather conditions a cold air mass is held in the basin while a warmer air mass covers it like a lid.
Because this type of condition was first documented as prevalent in the Los Angeles area this type of trapped pollution was named Los Angeles Smog, although it occurs in other areas where a large concentration of automobiles are used and the air remains stagnant for any length of time.
Heat Transfer
Consider the internal combustion engine as a machine in which raw materials must be placed so a finished product comes out. As in any machine operation, a certain amount of wasted material is formed. When we relate this to the internal combustion engine, we find that through the input of air and gasoline, we obtain power from the combustion process to drive the vehicle. The by-product or waste of this power is both a mix of desirable and undesirable emissions. Water vapor (H2O), carbon dioxide (CO2) and nitrogen dioxide (N2) are desirable since they are absorbed by plants and trees and converted to oxygen and harmless gases. Hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx) are undesirable emissions but are still a byproduct of combustion.
Heat generated during combustion rise to between 900 and 1300°F (482-705°C). To help control combustion temperature the combustion chamber and cylinder are surrounded by coolant. This coolant provides a convection of heat that helps to control combustion temperatures. The coolant is pumped through a radiator where the fluid is cooled by radiant cooling. The rush of air through the radiator, or pulled through by the cooling fan maintains a median temperature to protect engine components from failure due to excessive heat.
Cooling or controlling combustion temperature is an important part in the control of exhaust emissions. To understand the behavior of the combustion and transfer of its heat, consider the air/fuel charge. It is ignited and the flame front burns progressively across the combustion chamber until the burning charge reaches the cylinder walls. As fuel vapor comes in contact with cooler areas of the combustion chamber is becomes larger droplets and does not completely burn in the combustion cycle. This leaves unburned fuel in the combustion chamber.
Some of the fuel in contact with the walls is not hot enough to burn, thereby snuffing out or quenching the combustion process. This leaves unburned fuel in the combustion chamber. During the normal combustion cycle, air and fuel are drawn into the combustion chamber when the intake valve opens and the piston travels to the bottom of the cylinder. The intake valve closes and the piston is forced upward to compress the air/fuel mixture. At the top of the cycle the ignition system ignites the spark plug causing the volatile mixture to burn. The majority of the mixture burns in the cylinder. There is a small quantity of unburned fuel that is forced out of the cylinder and into the exhaust system along with the exhaust gases. An engine running at 4000rpm does this 67 times per second so ensuring that all of the fuel is burned before exiting the cylinder head is a difficult task. If the combustion temperature is too high or too low harmful emissions form that are expelled to the exhaust.
Many attempts have been made to minimize quenching, by increasing the coolant temperature and lessening the contact area of the coolant around the combustion area. Typical coolant temperature is around 200°F (93.33°C). Design limitations within the combustion chamber prevent the complete burning of the air/fuel charge so a certain amount of the unburned fuel is still expelled into the exhaust system, regardless of modifications to the engine.
Automotive Emissions
Sources of engine pollutants were known before emission controls were mandated on internal combustion engines. It was determined that pollution from early vehicles originated from three sources. The engine’s exhaust emissions accounted for 60 percent of the total vehicle emissions. The other 40 percent originated from the crankcase as a result of combustion blow-by and from fuel evaporation from fuel tank and carburetor vents.
Over time, enhancements have accounted for a reduction of total vehicle emissions. Tailpipe emissions have been reduced through combustion control enhancements. Crankcase emissions have been reduced through engine modifications. Evaporative emissions have been addressed with modification to the control and containment of hydrocarbon vapors. When the total "emissions pie" is considered, tailpipe and crankcase emissions have become a smaller slice, or percentage of the total. This puts more emphasis on evaporative emissions. Evaporative emissions accounts for a greater percentage of total vehicle emissions. In recent years with the advent and implementation of OBD II engine management systems the control and diagnosis of the evaporate system has increased.
Exhaust Gases
Exhaust gases include a combination of burned and unburned fuel. To understand exhaust emissions and its composition requires some basic chemistry.
When air and gasoline are introduced into the engine we are mixing air (oxygen, nitrogen and trace gases) with gasoline, which is 100 percent hydrocarbons (HC), in a semi-controlled ratio.
Combustion is the catalyst that produces power to move the vehicle down the road. The exhaust gases are then composed of nitrogen (N2), a diatomic gas, the same as was introduced in the engine, carbon dioxide (CO2), the same gas that is used in beverage carbonation, and water vapor (H2O).
Hydrocarbons
Hydrocarbons (HC) emissions originate from fuel that did not burn during combustion or from fuel evaporation. The main sources of incomplete combustion are rich air/fuel mixtures, low engine temperatures and improper spark timing. The main source of hydrocarbon emission through fuel evaporation on most vehicles used to be the vehicle's fuel tank and carburetor float bowl. Today’s vehicles are equipped with an elaborate evaporative emissions system that monitors, stores, and releases fuel vapor at precise times to prevent contact with the atmosphere.
To reduce combustion hydrocarbon emissions, engine modifications were made to minimize "dead space" in the combustion chamber to reduce quenching. The ECM now makes finite adjustments in milliseconds to adjust the air/fuel ratio thereby maintaining a fine balance between lean and rich to reduce combustion temperatures and increase fuel mileage and performance. Additionally the use of catalytic converters as a post combustion emissions control device aids in the reduction of hydrocarbons. Air injection reactors also help to reduce hydrocarbon emissions by injecting fresh air into the exhaust manifold. To control hydrocarbon emissions through fuel evaporation, modifications were made to the fuel tank to allow storage of fuel vapors during periods of engine shut-down. Modifications were also made to the air intake system so that at specific times during engine operation, these vapors may be purged and burned by blending them with the air/fuel mixture.
Carbon Monoxide
Carbon monoxide is formed when not enough oxygen or too much gasoline is present during the combustion process to convert carbon (C) to carbon dioxide (CO2). An increase in carbon monoxide (CO) emissions is normally accompanied by an increase in hydrocarbon (HC) emissions due to the lack of oxygen to completely burn all of the fuel mixture.
Carbon monoxide (CO) also increases the rate at which the photo chemical smog is formed by speeding up the conversion of nitric oxide (NO) to nitrogen dioxide (NO2). To accomplish this, carbon monoxide (CO) combines with oxygen (O2) and nitric oxide (NO) to produce carbon dioxide (CO2) and nitrogen dioxide (NO2). (CO + O2 + NO = CO2 + NO2). Photochemical smog is not a tailpipe emission.
Nitrogen
Normally, nitrogen is an inert gas. When heated to approximately 1300°F (704.5°C) through the combustion process, this gas becomes active and causes an increase in the nitric oxide (NO) emission. The higher the combustion temperature and the availability of oxygen determine the varying compounds of NOx. Nitrogen (N2) remains inert throughout the combustion event and exits the tailpipe as a harmless gas.
Oxides of nitrogen (NOx) are composed of approximately 97-98 percent nitric oxide (NO). Nitric oxide is a colorless gas but when passed into the atmosphere, it can combine with oxygen and form nitrogen dioxide (NO2). The nitrogen dioxide then can combine with chemically active hydrocarbons (HC) and when in the presence of sunlight, forms photo-chemical smog.
Ozone
To further complicate matters some of nitrogen dioxide (NO2) is broken apart by the sunlight to form nitric oxide and oxygen. (NO2 + sunlight = NO + O). This single atom of oxygen then combines with diatomic (meaning 2 atoms) oxygen (O2) to form ozone (O3). Ozone is one of the smells associated with smog. It has a pungent and offensive odor, irritates the eyes and lung tissues, affects the growth of plant life and causes rapid deterioration of rubber products. Ozone can be formed by sunlight as well as electrical discharge into the air.
The most common discharge area on the automobile engine is the secondary ignition electrical system, especially when inferior quality spark plug cables are used. As the surge of high voltage is routed through the secondary cable, the circuit builds up an electrical field around the wire, which acts upon the oxygen in the surrounding air to form the ozone. The faint glow along the cable with the engine running that may be visible on a dark night, is called the "corona discharge.'' It is the result of an electrical field passing from a high along the cable, to a low in the surrounding air, which forms the ozone gas. The combination of corona and ozone has been a major cause of cable deterioration. Recently, better quality insulating materials have lengthened the life of the electrical cables.
Although ozone at ground level can be harmful, ozone is beneficial to the earth's inhabitants. By having a concentrated ozone layer called the "ozonosphere,'' at approximately 20 miles (32 km) up in the atmosphere, much of the ultra violet radiation from the sun's rays are absorbed and screened. If the ozone layer were not present, much of the earth's surface would be burned, and unfit for human life.
Oxides of Sulfur
Oxides of sulfur (SOx) were initially not addressed in the exhaust system emissions, since the sulfur content of gasoline is less than 1/10 of 1 percent. Because of this small amount, it was felt that it contributed little to the overall pollution problem. Since light duty trucks are now being equipped with diesel engines the focus on SOx emissions are on the EPA’s radar. Oxides of sulphur are more common in diesel emissions that gasoline emissions. The automobile exhaust system, when equipped with a catalytic converter, changes the sulfur dioxide (SO2) into sulfur trioxide (SO3).
When this combines with water vapors (H2O) a sulfuric acid mist (H2SO4) is formed. It is a very difficult pollutant to handle since it is extremely corrosive. This sulfuric acid mist, is the same mist that rises from the vents of an automobile battery when an active chemical reaction takes place within the battery cells. Modern day batteries are sealed so the levels of SOx emissions from batteries are negligible. When a large concentration of vehicles equipped with catalytic converters are operating in one area, this acid mist may rise and be distributed over a large ground area causing land, plant, crop, paint and building damage.
Particulate Matter
A certain amount of particulate matter is present in the burning of any fuel, with carbon constituting the largest percentage of particulates. In gasoline, the remaining particulates are the burned remains of the various other compounds used in its manufacture. When an engine is in good condition the particulate emissions are low. As the engine wears and it develops oil seepage from worn rings or valve guides the particulate emissions increase. By visually inspecting the tail pipe emissions, a determination can be made as to where an engine defect may exist. There are a number of reasons that cause discolored smoke from exiting the tailpipe. An engine with light gray or blue smoke emitting from the tail pipe normally indicates an increase in oil consumption through burning due to internal engine wear. Black smoke indicates a rich condition in the fuel delivery system. Regardless of the smoke's color, the internal parts of the engine or the fuel delivery system should be repaired to prevent excessive particulate emissions.
Diesel and turbine engines emit a darkened plume of smoke from the exhaust system when the mixture is rich. This normally occurs under wide-open-throttle conditions. Emission control regulations are mandated for this type of emission and more stringent measures are being used to prevent excess emission of the particulate matter.
Electronic components are being introduced to apply the correct quantity of fuel to a given quantity of air to meet the requirements of the operating conditions. To help reduce particulate emissions, good grades of petroleum based engine oils and synthetic based engine oils should be used that meet the manufacturers’ specification. Low priced oils often have a high sulphur content and low flash point contributing to particulate emissions.