How to Control Air Pollution from Motor Vehicle Emissions?

Everything you need to learn about controlling air pollution from motor vehicle emissions.

Introduction:

Transportation in India, is the major emission source of three primary pollutants- hydrocarbons (HCs), carbon monoxide (CO) and nitrogen oxides (NO_x). Most of these emissions are associated with light- duty cars and trucks. Heavy-duty trucks, motorcycles and off-road vehicles are also significant sources. In urban areas, motor vehicles are estimated to be responsible for 40 to 50 per cent of HC, 50 per cent of NO_x and 80 to 90 per cent of CO emissions.

Similar patterns have been reported in USA, Europe and Japan. Motor vehicle pollutants such as non-methane hydrocarbons (NMHCs) and NO_x serve as precursor molecules for the production of elevated tropospheric ozone (O₃) levels. Within the context of primary pollutants and subsequent formation of secondary pollutants such as O₃ and other smog and Haze constituents, it is not surprising that motor vehicle emission control has been a major regulatory priority in the US for more than three decades and an increasing priority in other developed and developing countries.

Regulatory efforts to control motor vehicle emissions have a history dating back to the early 1960s. Emission reduction requirements over that time have become more stringent, requiring motor vehicle manufacturers to continuously develop and employ new emission control techniques.

As a consequence of regulatory and technological efforts, aggregate CO and HC emissions from motor vehicles were reduced by ∼40 per cent and NO_x by 20 per cent, in the period from 1999 to 2005. This reduction occurred despite a substantial increase in vehicle miles driven.

Motor Vehicle Engines:

Two types of engine systems are used in motor vehicles:

(i) Otto cycle spark ignition (SI) reciprocating internal combustion engines (ICEs); and

(ii) Diesel cycle compression ignition (CI) ICEs.

The former are widely used in cars and light-duty trucks, motorcycles, boats and other small-horsepower consumer products. The latter are used in utility vehicles, e.g., trucks, earth-moving and mining equipment, farm tractors and ships, where high-torque, low-speed performance is required. Because of their higher fuel economy, CI engines are widely used in light-duty vehicles in Europe and other developing countries.

Otto Cycle Spark Ignition Engines:

Engine Characteristics:

Otto cycle SI engines may be four-stroke or two-stroke systems. The former are used in cars and light- duty trucks; the latter, in motorcycles and lower-horsepower-requiring vehicles.

Operation of a four-stroke SI engine is illustrated in Fig. 14.1. The cycle begins as a mixture of air and fuel is drawn or injected into the combustion chamber (i.e., the volume described by the top of the piston at the beginning of the compression stroke, cylinder walls and intake valves); this is the intake stroke.

In the compression stroke, the valves close and the piston moves up through the cylinder to top dead center where ignition takes place. The piston returns to its original position (power stroke) before it moves upward again to purge exhaust gases (exhaust stroke). As a consequence, there is one power stroke per two piston revolutions.

Ignition of the fuel-air mixture drives the piston downward, providing power to the drive shaft that propels the vehicle. Residence time (the time it takes to complete the power stroke) in a 3000 rpm engine is on the order of 10 msec.

In a two-stroke engine, the basic processes (intake, compression, heat induction, expansion, heat exhaust, gas exhaust) are essentially the same. They occur in half the cycle period of a four-stroke engine. This has the advantage of increasing the power-to-weight ratio and the disadvantage of allowing a shorter time period for each process. This results in a substantial fuel penalty (up to 25 per cent) and higher HC emissions.

Emissions:

Pollutant emissions from motor vehicles powered by SI four-stroke ICEs include combustion by-products (exhaust gases), ‘blow-by’ gases and evaporative fuel losses.

Exhaust emissions consist mainly of unburned or partially burned HCs, CO and NO_x. They comprise 90 to 92 per cent of emissions from uncontrolled vehicles.

Some unburned HCs ‘blow by’ piston rings and accumulate in the crankcase. In an uncontrolled vehicle, they would be vented to the atmosphere through a small tubular crankcase exhaust port.

Gasoline evaporative emissions from motor vehicle fuel systems are (in decreasing order of importance) diurnal, hot soak and operating losses. Diurnal emissions occur when fuel tanks of parked vehicles cool at night, drawing air in and ‘exhaling’ vapour-containing air on heating during the day. This fuel tank ‘breathing’ may produce evaporative HC emissions of 50 g/car on hot days. Hot soak emissions occur just after the engine is turned off and residual fuel evaporates from the warm vehicle.

Operating, or running, losses occur as vapours are driven from the fuel tank when the vehicle is being operated and the fuel warmed. Operating losses may be significant at elevated ambient temperatures or when the fuel system becomes hot during vehicle operation. Evaporative losses are also associated with vehicle refueling and may result in HC emissions comparable to those associated with the exhaust system.

Formation of Combustion Pollutants:

A variety of by-products are produced in ICE gasoline combustion. Because of significant emission concentrations and effects on the environment. CO, HCs and NO, are of particular importance.

Gasoline consists of a variety of HC species. It has an average composition that can be described by the formula (CH_m)_n, where m has a value of ∼1.85 to 2.0 and n is ∼7.0. An HC molecule with m = 1.85 and n = 7 (C₇H₁₃) would require 10.24 molecules of oxygen (O₂) to be fully oxidised to carbon dioxide (CO₂) and water (H₂O) vapour. This concentration of gasoline HCs to atmospheric O₂ is the stoichiometric ratio and corresponds to a mass-based air-fuel (A:F) ratio of approximately 14.5:1.

The A:F ratio is often expressed as an equivalence ratio, ϕ:

Note that the equivalence ratio is calculated from a fuel:air rather than an air: fuel ratio.

When an HC fuel is oxidised. CO is initially formed and then oxidised to CO by O₂. If O₂ is present in less than stoichiometric proportions, combustion will be incomplete, with some CO remaining unoxidised. The amount of CO produced in exhaust gas is a function of the A:F or equivalence ratios.

This relationship can be seen in Fig. 14.2. As the A:F ratio increases (ϕ decreases), CO production significantly declines. At a slightly lean A:F ratio (i.e., less fuel, more air), with a ϕ of ∼0.9, very little CO is produced. As the A:F mixture becomes richer (i.e., more fuel, less air), CO production rapidly increases.

As can be seen in Fig. 14.2, the A:F ratio also affects emissions of HCs. However, significant emissions occur even when sufficient air is present due to quenching (cooling) phenomena within the combustion chamber. A severe temperature gradient forms as a result of the large quantity of heat that is transferred from combustion gases to cylinder walls and then to the engine’s cooling system.

Near cylinder walls, temperatures may be less than required to maintain a flame. The flame is ‘quenched’ and thus goes out, leaving a small zone of uncombusted fuel. These ‘quench’ zones exist in many locations in combustion chambers. A quench zone of 125 μm would represent 1.5 per cent of chamber volume.

‘Crevice’ zones of unburned HCs are also formed. These are spaces that are confined on several sides. They include the upper ring between the piston and wall, behind the rings, between the upper rings, within the spark plug screw threads, where the gasket has a small mismatch with the engine block and head, and where the different shapes of valves and seats form a small gap. Crevice zones can contain as much as 8 per cent gasoline by mass and 3 per cent of the cylinder clearance volume.

Another major source of unburned HCs is sorption on lubricating oils during intake and desorption on exhaust. It has been estimated that ∼25 per cent of unburned HCs come from lubricating oil sorption under stoichiometric conditions, and up to 50 per cent under lean A:F ratios (ϕ = 0.95).

Major engine operating parameters that affect HC emissions include equivalence and compression ratios (chamber volume remaining at the end of the compression stroke compared to that at the end of the intake stroke), engine speed, and spark timing. Unburned HC emissions increase as the compression ratio increases due to the fact that crevice regions, being of constant volume, make up a larger portion of the compressed volume.

Emissions of unburned HCs decrease with increased engine speed as the associated increase in turbulence enhances combustion and post-combustion mixing and oxidation. They also decrease as the spark is retarded as more combustion occurs and exhaust gases are hotter.

Formation of NO_x, which includes nitric oxide (NO), nitrogen dioxide (NO₂) and nitrous oxide (N₂O), is highly dependent on combustion chamber temperatures, which are directly related to A:F ratios. As can be seen in Fig. 14.2, peak NO_x emissions occur under stoichiometric conditions when combustion chamber temperatures are high (2000 to 3000 K).

Compression Ignition Engines:

Emissions reduction concerns associated with CI engines vary across the developed and developing world. In the US they are primarily associated with light and heavy-duty trucks and commercial vehicles such as buses. Due to poor consumer acceptance, use of CI engines in automobiles in the US is negligible.

However, use of CI engines in automobiles in a number of European countries is significant. In Europe, diesel cars have a reputation as fuel-efficient, durable vehicles. Lack of consumer acceptance in the US has been due to the poor performance and reliability of early models built by US manufacturers. CI vehicles are noisier and produce malodours and soot particles.

CI engines are naturally aspirated (air is inducted), turbocharged (inlet air is compressed by an exhaust-driven turbine-compressor combination) or supercharged (air is compressed by a mechanically driven pump or blower). Both turbocharging and supercharging increase engine power output by increasing air-flow and therefore fuel flow. Like SI engines, CI engines utilise either four or two-stroke cycles.

Fuel is injected under high pressure into a combustion chamber or bowl in the top of the piston toward the end of the compression stroke. The atomised fuel vapourises, mixes with high temperature air and because of high compression ratios used (12 to 24:1, depending on engine type), spontaneously ignites. Once combustion is initiated, additional fuel mixes with air and is combusted.

Emissions of HC and CO are low since combustion is nearly 100 per cent complete and the engine operates with excess air. Because temperatures produced are high, NO_x emissions are also high. In addition, the fuel-mixing process produces carbon particles in fuel-rich regions where fuel is sprayed. Some of this elemental carbon passes through the combustion process unburned and sorbs high-molecular-weight HC and sulphur from the fuel and lubricating oil.

Because of lean engine operation, NO_x emissions cannot be controlled using three-way catalysts. Reduction of NO_x has been achieved by careful control of inlet air temperatures and injection retardation to delay most of the combustion process to the early expansion stroke. Nitrogen oxide emission reductions have been on the order of 50 to 65 per cent.

Elemental carbon or soot, formation has been reduced by 65 to 80 per cent as a result of modifications to the combustion process. These have included use of fuel injection equipment with very high injection pressures and carefully matching the geometry of the bowl-in-piston combustion chamber, air motion and spray.

Lubricant control has reduced the concentration of high-molecular-weight substances sorbed onto elemental carbon particles formed during combustion. Oxidation catalysts may also be used to reduce soluble organic components of particulate-phase emissions. Oxidation catalysts reduce emissions of both HC and CO by SO to 90 per cent and particulate matter (PM) by 50 to 60 per cent. These catalysts are fitted to all European diesel light-duty motor vehicles to meet European stage 2 emission standards.

Particulate-phase emissions can be reduced by filtration systems. A number of design approaches have been utilised in the last decade and a half. Soot filters are located on the tailpipe. They are subject to high cost and technical problems such as filter regeneration and avoidance of excessive backpressure. Soot filters typically work by removing and storing PM until a certain level of resistance is detected. Filters are then regenerated in situ.

Automotive Fuels:

Different fuel types are used in SI and CI engines. In the former, HC mixtures called gasoline are used; in the latter, diesel fuel.

Gasolines:

Gasoline or gasoline products are produced in crude oil refining. Due to different oil sources and refining procedures, gasoline products are complex mixtures of HC species that include paraffins, cycloparaffins, olefins, and aromatic HCs. Consequently, the composition of HC constituents varies from one refiner to another. It also varies as gasolines are formulated to provide optimum engine performance under different driving and climatic conditions.

Individual constituents vary in ignition temperature and other combustion characteristics. Gasolines may contain small quantities of non-HC substances. These include normal constituents of petroleum such as P and S or contaminants introduced in the gasoline production process (such as H₂O).

Phosphorous and sulphur reduce the effectiveness of catalytic converters; as such, additional refining steps are needed to reduce their levels. Water negatively affects engine performance.

Gasolines may contain additives such as Pb, Mn or oxy-genates. The use of Pb in gasolines has been restricted in many countries to minimise damage to catalytic converters and safeguard public health. Nevertheless, it is still used in a number of developing countries.

A Mn-obtaining compound, methyl- cyclo-pentadienyl manganese tricarbonyl (MMT), is used in some countries as an octane booster. Oxygenates are used in the US to boost gasoline octane ratings and decrease emissions of CO.

Octane Rating:

Gasolines are formulated to prevent knock, the noise transmitted through the engine structure when spontaneous ignition of the end-gas (fuel, air, residual gas) mixture occurs before the propagating flame. Such ignition produces high local pressures and pressure waves with substantial amplitude. Although knock can be severe enough to cause major engine damage, in most cases it is just an objectionable source of noise.

The presence (or absence) of knock is a function of the antiknock quality of gasoline. A gasoline’s resistance to knock is defined by its octane number. The higher the octane number, the greater the knock resistance. The octane scale is based on two HCs, heptane and iso-octane. The former has an octane rating of zero; the latter, 100. Blends of these define the knock resistance of octane numbers between 0 and 100.

The antiknock quality of gasolines is determined by two different methods. These produce what are called motor octane numbers (MONs) and research octane numbers (RONs). The antiknock quality of a gasoline is determined by averaging its MON and RON values.

Earlier until the advent of catalyst-equipped motor vehicles, Pb alkyls were added to gasoline to increase octane numbers. Their use allowed an increase in antiknock quality to be achieved at lower cost than modifying fuel composition by additional refining steps. Lead additives used were tetraethyl lead ((C₂H₅)₄Pb) and tetramethyl lead ((CH₃)₄Pb). As previously indicated, a Mn antiknock agent. MMT, has also been used.

Octane ratings can be increased by adding oxy-genates such as ethanol (C₂H₅OH), methanol (CH₃OH), tertiary butyl alcohol and methyl-tertiary-butyl-ether (MTBE). Octane ratings can also be increased by increasing the aromatic HC content of gasolines.

Aromatic HCs such as benzene, toluene, ethylbenzene and xylene (when combined, commonly referred to as BTEX) significantly enhance antiknock performance. The composition of aromatic HCs in unleaded gasolines has averaged about 30 per cent, but has been as high as 40+ per cent. Because of health concerns (it is regulated as a hazardous pollutant), benzene concentrations in gasoline have recently been limited to 1 per cent.

Gasoline Composition and Emissions:

Motor vehicle emissions are significantly affected by fuel constituents and formulation. A prime example of this is Pb. Its use was the major source of Pb in the environment and human exposure to it. Emissions are also affected by the presence of low-molecular-weight HCs such as butane that have a high Reid vapour pressure (RVP), or volatility.

Fuels with high RVPs have increased evaporative, running and refuelling emissions even in evaporative emissions-controlled systems. Increases in what is described as midrange volatility lead to reduced HC and CO emissions and increased NO_x emissions.

The olefin content of gasoline is an important emissions concern because of the reactivity of olefins and their role in producing elevated O₃ levels in urban-suburban areas. High olefin contents are therefore undesirable.

The aromatic HC content of gasolines affects emissions of CO, HCs, benzene, polycyclic aromatic hydrocarbons (PAHs) and NO_x. As aromatic HC concentrations in fuels increase, emissions of CO, HCs, benzene and PAHs also increase: NO_x emissions, on the other hand, decrease.

Reformulated Gasolines and Oxygenated Additives:

Gasolines described as reformulated have had their compositions changed to reduce the reactivity of exhaust products and decrease emissions of NMHC, CO and NO_x. Reformulated gasolines can be used in conventional ICEs with no modification of the propulsion system. They are produced by refining modifications and adding oxygenated compounds. They have lower olefinic and aromatic HC contents and lower RVPs.

Oxygenated additives enhance octane ratings decreased when the aromatic and olefinic HC content is reduced. They improve the efficiency of combustion and reduce CO emissions. Commonly used oxygenated HC additives include MTBE, ethyl-t-butyl ether (ETBE), tertiary-amyl-methyl ether (TAME), CH₃OH and C₂H₅OH. Though C₂H₅OH is widely used in the Midwestern US, gasoline producers favour the use of ethers.

Alcohols increase the vapour pressure of gasoline and thus increase evaporative emissions. Use of oxygenates in gasoline (which may vary from 2 to 15 per cent) results in increased emission of formaldehyde (HCOH), a potent mucous membrane irritant. Reformulated gasolines have been introduced into fuel markets where significant O₃ or CO problems exist.

Oxygenated compounds such as MTBE, ETBE, TAME and C₂H₅OH have been added to gasoline for wintertime use in dozens of American cities to achieve emission reductions necessary to comply with the CO air quality standard. The typical oxygenate content is 15 per cent.

As a consequence, a number of epidemiological studies were conducted to determine whether use of MTBE in gasoline may have contributed to adverse health effects in exposed population. These studies failed to show any significant relationship between MTBE use in gasoline and health complaints. Nevertheless, use of MTBE has been controversial in some areas of the US.

MTBE use has also been associated with a new environmental issue: contamination of groundwater by leaking underground fuel storage tanks. Storage tank leakage has been a long-standing environmental problem.

Since gasoline is insoluble in H₂O, gasoline and H₂O don’t mix. As a consequence, gasoline- ground H₂O contamination is not a major ‘consumer’ issue. Since MTBE is H₂O soluble and has a strong ethery odour and taste, contamination of ground H₂O by gasoline containing MTBE is a significant consumer and environmental concern in areas subject to such problems.

Alternative Fuels:

A variety of fuels have been evaluated or are being used as lower-emission alternatives to conventional gasolines. These include alcohol-gasoline blends, C₂H₅OH, CH₃OH, liquefied natural gas (LNG) and liquefied petroleum gas (LPG). Such fuels have the potential to improve air quality, particularly in urban areas.

Reduction in emissions and improvement in air quality depend on the type of fuel used and other factors. Alternative fuels may improve air quality by reducing mass emission rates from motor vehicles or reducing emissions of photo-chemically reactive HC compounds.

Alcohol Fuels:

There has been considerable interest in using CH₃OH, C₂H₅OH and alcohol-gasoline blends as motor vehicle fuels. Methanol is a colourless liquid with a low vapour pressure and high heat of vapourisation. Both properties contribute to lower emissions under warm conditions, but tend to make pure CH₃OH- operated vehicles difficult to start when cold (thus resulting in increased emissions).

Additives such as gasoline (5 to 15 per cent by volume) are blended with CH₃OH to increase vapour pressure and improve starting under colder weather conditions. The most common blend, containing 85 per cent CH₃OH by volume, is described as M85. Flexibly fuelled and dedicated CH₃OH-fuelled vehicles have been developed.

The major emission products (on a mass basis) from CH₃OH-fuelled vehicles are CH₃OH, HCOH, gasoline-like NMHC components and NO_x. Of greatest concern is HCOH. Even after catalytic reduction, M85 vehicles may emit ∼48 to 64.5 mg HCOH/km (30 to 40 mg/mile), about three to six times more than conventional vehicles. Formaldehyde has high photochemical reactivity.

Limited studies of flexibly fuelled vehicles indicate that NO_x may be slightly reduced. However, dedicated CH₃OH-fuelled vehicles use higher compression ratios and thus produce higher NO_x emissions. Evaporative emissions are lower and CH₃OH is less reactive than many constituents of gasoline.

Flexibly fuelled vehicles can also run on C₂H₅OH. Air quality benefits associated with C₂H₅OH are less than those of CH₃OH. Ethanol is more reactive, producing 15 per cent more O₃ on a carbon atom basis. Little information is available on emission characteristics of well-controlled C₂H₅OH vehicles; limited tests have shown high emissions.

Use of C₂H₅OH as a gasoline additive, while simultaneously allowing an increase in vapour pressure, may not have significant air quality benefits. Increased evaporative emissions and increased emissions of C₂H₅OH and acetaldehyde would increase atmospheric levels of per-oxyacyl nitrate (PAN).

Compressed and Liquified Gases:

Both natural gas (which is >90 per cent methane, CH₄) and propane (C₃H₈) can be used to power motor vehicles. Natural gas is compressed and stored at pressures of 4500 PSI (pounds per square inch) (2.32 × 10⁵ mmHg) or liquefied for use as an automotive fuel. Because LNG must be cryogenically cooled and stored, its use has been relatively limited compared to compressed natural gas (CNG).

Natural gas vehicles (NGVs) emit primarily CH₄. Because of its low photochemical reactivity, CH₄ has low O₃-forming potential. This benefit is reduced by CNG impurities such as ethane (C₂H₆) and C₃H₆ that may produce up to 25 times more O₃ than CH₄. The presence of olefins may further reduce the benefits of CNG use. Though combustion by-products such as aldehydes are quite reactive, mass emission rates are relatively small.

Carbon monoxide emissions from the lean-burn operating condition of NGVs are very low—90 per cent less than gasoline-powered vehicles. Because of lean-burning conditions, CNG vehicles produce lower engine-out (released prior to the catalytic converter) emissions of NMHCs, NO_x and CO, with higher fuel economy. Evaporative emissions from CNG vehicles are limited and contribute little to O₃ formation.

LPG consists primarily of C₃H₈, a by-product of petroleum refining. It has many of the attributes of CNG but several disadvantages. These include limited supply and higher exhaust reactivity than CNG. However, it has a higher energy output per unit volume and thus can be stored in smaller tanks.

Low Emission and Zero Emission Vehicles:

The Spark ignition (SI) Internal combustion engine (ICE) has been the dominant propulsion system used in light-duty motor vehicles for a century. It is reliable, economical and gives excellent performance.

Because of pollutant emissions associated with SI engines, a number of alternative propulsion systems have been evaluated by motor vehicle manufacturers and others. These have included the stratified charge, gas turbine, Wankel and Rankine cycle engines and electric-powered and hybrid electric vehicles. Most have been shown to have significant limitations both from technical and economic standpoints.

Low Emission Vehicles:

In the 1990, light-duty motor vehicles were to meet more stringent emission limits that were to be phased in starting in 1994. These are referred to as Tier I standards. California was given authority to develop stricter requirements to address the more serious problems that it faces in urban areas. As a consequence, the USEPA approved California’s low emission vehicle (LEV) programme in 1993.

California’s LEV programme is intended to require development of LEVs that would be phased into the vehicle population over a period of time. These would include transitional LEVs (TLEVs), LEVs, ultra LEVs (ULEVs) and zero emission vehicles (ZEVs). These would reduce, respectively, HC emissions from Tier 1 levels by 50, 70, 85 and 100 per cent; CO emissions by 0, 0, 50 and 100 per cent; and NO_x emissions by 0, 50, 50 and 100 per cent.

Hybrid Electric Vehicles:

Hybrid electric vehicles (HEVs), as implied, combine features of the ICE with the battery and motor of an electric car. This combination approximately doubles the fuel economy of SI engine systems and decreases emissions by a comparable amount. Hybrid systems were developed to compensate for the limitations of battery technology (primarily limited driving range).

HEVs combine a power unit, a vehicle propulsion system and an energy storage unit (Fig. 14.8). Power units may include SI engines, CI direct injection engines, gas turbines or fuel cells. Propulsion can result entirely from an electric motor in a series configuration or the SI engine and electric motor in a parallel configuration. Energy storage may be achieved by use of batteries, ultra capacitors or flywheels.

In a series configuration, an SI engine drives a generator that charges a battery that powers an electric motor. In a parallel configuration, the drive shaft is turned by either the SI engine or electric motor. The series hybrid is generally more efficient but less powerful and has a shorter driving range than a parallel HEV. The parallel configuration provides performance that is relatively similar to that of a conventional light-duty vehicle.

An HEV is generally configured with the gasoline engine, electric motor and electronic circuitry in the front and battery in the rear. As of 2003, three HEVs were marketed, the Toyota Prius, Honda Insight and Honda Civic. These vehicles have been well accepted in Japan. In the US, federal and state incentives have been applied to offset some HEV purchase costs.

HEV technology can be applied to a variety of vehicles, including sport utility vehicles (SUVs) and small trucks. The HEV has the potential to receive widespread acceptance and use because it meets stringent emission limits for HCs, CO and NO_x and has high fuel efficiency. High fuel efficiency would result in a significant decrease in CO₂ emissions, a major greenhouse gas. By increasing fuel economy and thus reducing CO₂ emissions, HEVs have the potential to significantly reduce global emissions of CO₂ in the coming decades.

Electric Vehicles:

Electric vehicles have been used for years in a variety of utility applications, ranging from golf carts and forklifts to small automobiles. Unfortunately, they provide only modest acceleration and power and a limited driving range before recharging. The electric car is considered a zero emissions vehicle. Even if power plant emissions are considered, an electric vehicle produces only ∼0.5 per cent of the emissions of the cleanest Si-equipped gasoline-powered vehicle.

Future Control Technologies:

Future emission control systems are likely to utilise dual catalytic or three-way catalytic systems. In the dual catalytic system the HC-CO oxidation catalyst is located downstream of nitrogen oxide reduction catalyst. The two catalysts may be housed in a single or separate containers. The oxidation catalyst would utilise platinum and or palladium or base metals promoted with these noble metals.

The NO_x reduction catalysts would use platinum, palladium, ruthenium, base metals or base metals promoted with the above noble metals. The development of reduction catalysts lags significantly behind that of oxidation ones. As of today, their durability has not been proven. Three-way catalyst differs from the dual catalyst in that it utilises a single catalytic bed.

Development of auto emission control technology has led to certain problems. Most control technologies are associated with undesirable performance-characteristics including decreased power, drivability and fuel economy.

The most noticeable side effects of control measures have been the loss of vehicle power and decreased driveability. This decrease in performance has been attributed to the use of lean A/F mixture, retarded spark timing, exhaust gas recirculation and lower compression ratio.

In pre-emission control motor vehicle operation, ignition takes place at about 15 to 20 degrees before the piston reaches top dead center. Such spark timing provided excellent performance. Retardation of spark to decrease NO_x resulted in both decreased vehicle power and driveability.

The use of exhaust gas recirculation to cool combustion temperature for NO_x control also results in decrease vehicle power and driveability. Under full load the combination of exhaust gas recirculation and lean A/F mixtures may reduce engine power by 25 per cent.

High compression ratio 10 to 11:1 and high octane fuels maximise power output. However reduced compression ratio and NO_x control significantly reduces the vehicle power.

To sum up motor vehicle emissions can be controlled as below:

1. Control of Crankcase Emissions:

A control device called ‘positive crankcase ventilation, PCV’ prevents blow-by from the crankcase by returning gases to the cylinders to be burned inside the engine instead of being vented into the atmosphere. To function properly positive crankcase ventilation or smog value must be clean and should be inspected at every oil change and be replaced every 20000 kms or one year.

2. Control of Exhaust Emissions:

Exhaust emissions are reduced in various ways. One approach is based on the principle of reducing HC and CO emissions by adding fresh air to the hot exhaust gas to supply necessary oxygen needed for more complete burning as the mixture moves through the exhaust system. This system also includes engine modifications to increase the effectiveness of burning within the combustion chamber itself.

Modifications include an engine-driven air pump, air control and distribution equipment to deliver air to each exhaust port, carburetor modifications, distributor and vacuum advance modification and manufacturer’s recommended maintenance.

Another approach is the controlled combustion system which utilises engine design parameters to achieve emission control through combustion. This system includes an air-fuel mixture control, modified ignition timing and recommended maintenance. Exhaust emission device systems require periodic servicing of ignition and carburetor systems, especially engine idle speeds, spark timing and air-fuel ratio adjustments.

3. Control of Evaporation Emissions:

Fuel evaporation emissions can be reduced by the installation of a fuel tank with a built-in chamber to provide an assured thermal expansion volume for the fuel. The tank is vented into a vapour-liquid separator which returns liquid to the tank and passes vapour through a pressure-vacuum relief valve to an activated carbon canister.

Vapour is stored in the canister until purged by a vacuum created when engine operation empties the vapour into the intake manifold and then into the combustion chamber, where it is burned. Some vapour control systems have no carbon canister, but instead use the engine crankcase as a storage container. Evaporative emissions are low for diesel engines because they use a closed injection fuel system and because diesel is less volatile than petrol.

4. Alternatives to the Gasoline Engine:

Many alternatives to the gasoline engine have been suggested including the steam engine, the gas turbine engine of air that reduces HC and CO, the stratified charge-fuel injection engine, the free-piston diesel engine, the electric car (using zinc-air battery/lithium-nickel-halide battery/sodium-sulphur battery).

Of these, the one appearing to have best possibility is an electric car using a fuel cell, which uses hydrazine, ammonia or alcohol linked with an auxiliary fast-discharge battery for peak acceleration. Using improved fuels is another approach. These fuels include diesel fuel, liquid petroleum gas (LPG), compressed natural gas (CNG), liquid natural gas (LNG), solar energy, nuclear power and the fuel produced from liquid hydrogen.

5. Strict Implementation of Legislations:

Immediate control measures need to be introduced to curtail the growing degradation of the urban environment by implementing the legislations strictly.

The Indian Standards for vehicular emission are:

(a) For every motor vehicle powered by compression ignition (diesel engine) smoke density shall not exceed 65 Hartridge smoke units as measured by the free acceleration method.

(b) Vehicles powered by spark ignition engine (Otto engine) shall comply with the emission standards for carbon monoxide not exceeding 3 per cent by volume of exhaust gases during idling. Vehicles which have five years of life shall comply with emission standards of CO not exceeding 4.5 per cent by volume of exhaust gases during idling.

Automobile emissions can be minimised by the formation of more stringent emission standards for new as well as used vehicles. Also, emission levels on new vehicles should be evaluated to meet the long-term emission control objectives. Reductions of lead in gasoline, can, by a great extent, reduce the lead levels in the environment.

Further, the catalytic converters in automobiles should be encouraged to reduce the emission of other pollutants, such as CO and hydrocarbons. Regular monitoring of air quality at several locations in urban areas with high-traffic density and regular collection of epidemiological data in the metropolitan cities also will help reduce the automobile emissions. Finally, mass transportation systems appear to be the ultimate solution to traffic problems in the metropolis, where traffic and pollution are becoming extremely critical problems.

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