1.
INTRODUCTION
2.2 Types of Emission and Pollutants
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| Inamul Hoque, Assam Union, Meerut |
Transportation
sector alone utilizes most of the fossil fuels such as petrol, diesel, kerosene
and methanol. Considering all major anthropogenic source categories, with the exception of agriculture, the transportation sector of our economy releases
about one-third of the total emissions of Volatile Organic Compounds (VOCs),
nitrogen oxides (NOx), and lead (Pb) and more than two-thirds of the carbon
monoxide (CO).
The
problems of traffic pollution are particularly acute in a number of major
cities, especially in cities where a traffic jam is a common experience as a considerable amount of fuel is used while cars are trapped in traffic
congestion. As congestion in towns and cities increases and traffic slows down,
emissions are increasing much faster than the actual growth in vehicle numbers.
Also, research carried out by Bull (1991) on emissions from different types of
motor vehicle and fuel type reveals that older vehicles and those that are
incorrectly maintained contribute a disproportionate amount to aerial
pollution.
This
paper presents a review of the emissions of pollutants from vehicles and their
transformation and impact on the environment. The options for control and
management of vehicular emissions have also been discussed.
2.
VEHICLE EMISSIONS
Since
the 1950‟s it has been recognized that transportation engines in developed
countries are the major source of air pollution (Milton, 1995), while it is
apparent that the proportions to be attributed to various causes vary both in
time and from place to place, typical USA, figures are shown in Table 1. It can
be seen that transportation is responsible for the biggest share of CO, HC, and
NOx in the atmosphere as well as a large proportion of the particulate matter
Table
1: Proportion of atmospheric pollutants from various sources:-
2.1
Sources of Pollutants (Emissions) from Vehicles
Petrol
and diesel engines, both internal combustion engines, are the only engines in
wide use in the world‟s automotive transportation systems. And they are the
major source of urban air pollution (John, 1998). Table 2 gives a list of
common engine types, fuels, and associated emissions. Petrol is a readily
volatilized fuel and in the fuel tank, the pressure build-up which would result
from this evaporation is obviated by the introduction of a „breather‟ vent or pipe
into the tank. This still permits evaporation of the fuel.
Table
2: Vehicle types and pollutant for emissions for common engines and fuel
combinations:-
Evaporation
of the raw fuel also occurs in the carburetor at all times except when running
at high speed. Some unburned fuel (mixed with air) plus escaping exhaust gases
from around ill-fitting pistons leaves the car as crankcase blow-by which is a
further hydrocarbon loss. Exhaust emissions are more variable in nature and
hence more difficult to control. The composition depends on several variables
e.g. air/fuel ratio, speed and engine condition. Driving conditions play a
major role with exhaust emissions high in CO and HC at low and idling speeds,
and NOx high at high engine speeds. At low speeds, especially when cold and the
fuel mixture is fuel-rich, incomplete combustion is common resulting in the
formulation of more carbon monoxide. Similarly, unburned hydrocarbons can be
part of the exhaust. Table 3 shows emissions from petrol engine under various
operating conditions
Table
3: Emissions from petrol engine under various operating conditions :-
2.2 Types of Emission and Pollutants
2.2.1
Hydrocarbons
Emissions
of hydrocarbons indicate low combustion efficiency in internal combustion
engines and they arise when vaporised unburned fuel or partially burned fuel
products, leave the combustion region and are emitted with the exhaust.
Unburned hydrocarbon emissions are independent of air/fuel ratio.
2.2.2
Oxides of nitrogen (NOx)
Motor
vehicles are the principal source of NO and of its oxidation product NO2.
Nitric Oxide (NO) and Nitrogen dioxide (NO2) are usually grouped together as
Nitrogen oxides (NOx) emissions. Nitric oxide is the predominant oxide of
nitrogen produced inside the engine cylinder. The principal source of NO is the
oxidation of atmospheric (molecular) nitrogen. However, if the fuel contains
significant nitrogen the oxidation of the fuel nitrogen-containing compounds
is an additional source of NO.
2.2.3
Carbon Monoxide (CO)
Most
of the CO in the ambient air comes from vehicle exhaust. Internal combustion
engines do not burn the fuel completely to CO2 and water; some unburned fuel will
always be exhausted, with CO as a component. For rich air/fuel mixtures, CO
concentration in the exhaust is high, since the amount of excess fuel (unburned
fuel) will be high. While for weak air/fuel mixtures, CO emissions are very
low, therefore, they are not considered as important. According to John (1998),
the levels of CO observed in spark-ignition engine exhaust gases are lower than
the value measured within the combustion chamber. Therefore, some of the CO
that formed in the combustion process is oxidized to CO2 before they are
discharged into the atmosphere.
2.2.4
Carbon dioxide (CO2)
Combustion
of petrol takes place in the internal part of the engine of a vehicle. The fuel consists of organic molecules, which
are mostly hydrocarbon. When such compounds are burnt in automobile engines
they yield carbon dioxide and water. Carbon dioxide also contributes to the
acidity of rainfall, but more important, CO2 is transparent to short-
wavelength radiation from the sun but opaque to longer wavelengths radiated
back to space from the earth. Therefore, increased concentrations of CO2 may
result in a heating of the earth‟s atmosphere and global warming.
2.2.5
Photochemical smog
The
Components of automobile exhaust are particularly important in the formation of
atmospheric ozone and are primary contributors to smog. Smog is a mixture of
ozone, aldehydes, oxides of nitrogen and hydrocarbon. It results from the reaction
of these compounds in the atmosphere through a complex chain mechanism
requiring photolysis due to the action of sunlight. The amount of smog depends
on the concentration of reactants, their reactivity, and the temperature and
light intensity. Photochemical smog causes severe irritation of the eyes,
throat and respiratory system. In addition to this, it causes damage to some
materials and is, therefore, a major problem especially when it is held down in
the local atmosphere of major cities by a temperature inversion.
2.2.6
Lead emissions
The
combustion of gasoline containing lead (Pb) additives gives rise to large
amounts of lead particulates, which are deposited downwind of highways. High
lead levels from automobile exhaust are controlled by elimination or phasing
out of lead from gasoline i.e by using unleaded gasoline.
3.
DISTURBANCE OF NATURAL ENVIRONMENT
3.1
Thermal Air Pollution
This
type of pollution is applied generally to the discharge of heat into the air
environment from the combustion of fuels. The increase in the temperature of
any place at a given time above its normal ambient air temperature is evidence
that thermal air pollution has occurred in that place. The mean temperature of
our planet is fixed by a steady-state balance between the energy received from
the sun and the quantity of heat energy radiated back into space by the earth.
Disturbance in either incoming or outgoing energy would upset this balance, and
the average temperature of the earth‟s surface would drift off to a different
steady-state value.
3.2
Greenhouse Effect
The
greenhouse effect takes its name from the warmth of greenhouses, a warmth
stemming in part from the ease with which warming sunlight enters through the
glass panes, and the difficulty encountered by infrared radiation in escaping
off through the same panes with the greenhouse heat. This means that the glass
panes act in much the same way as the atmosphere, which allows the free passage
of incoming radiation but interferes with outgoing radiant energy. The outgoing
radiant energy is absorbed by H2O and CO2. With energy escape hindered, the
earth becomes warmer than it otherwise would be. Any addition of H2O and CO2
would cause an additional greenhouse effect.
3.3
Effects on Agriculture
Optimum
plant growth requires adequate light, heat, moisture, nutrients and appropriate
soil conditions. An imbalance in any of these results in stress to the plant,
which may result in restricted growth or foliage markings. Pollution provides extra undesirable stress. If this stress is too high, then the plant will
die, despite the relatively complex biological defense mechanisms (e.g. the rebuilding of damaged tissue).
3.4 Health Effects of Air Pollution
Air
pollution has serious economic repercussions. Human health itself has an
economic component by virtue of medical costs and work-days lost. Damage to
health occurs as pollutant molecules interact unfavorably with the intricate
molecules and fluids of the human body. Living systems are so complex that the
detailed chemistry of these interactions is unclear in all but a few
exceptional cases
3.5.1
Effects of Carbon (II) Oxides, CO
The
one activity of CO in the human body that gives it notoriety is its strong
inclination to combine with the haemoglobin of blood. Research has shown that
CO has a much greater affinity for Hb than O2. So when both are present in an equal concentration, CO ties up about 220 times more Hb than does O2, leading
to almost complete O2 starvation and sure death. With 220-fold advantage, low
level of CO in the lungs can immobilize enough Hb to cause a dangerous shortage
of oxygen. As much as 100 mg/l CO can kill quickly and 250 mg/l CO will cause
loss of consciousness (Henderson-Sellers, 1984). The upper limit for industrial
exposure to health workers is 100 mg/l. At this level, many people experience
dizziness, headache, and lassitude (Ademoroti, 1996)
3.5.2
Effects of NO and NO2
The
compound NO is moderately toxic. Like CO, NO can combine with haemoglobin (Hb)
forming methemoglobin and thus reducing oxygen transport. In contrast to NO,
NO2 is set at 5 ppm, rabbits exposed to concentrations as low as 1 ppm over a
period of one hour have suffered protein changes (Henderson-Sellers, 1984).
3.5.3
Effects Particulate Matter
Inhaled
particles greater than 10 μm are lodged in the nostril no protective mucous
blanket exists. Some of these particles may be retained in the lungs. It has
been suggested that particles enhance the damage to lungs caused by SO2 because
they carry SO2 to deep regions of the lung that are not otherwise reached
(Waller, 1983).
4.
CONTROL OF VEHICLE EMISSIONS
The
carbon monoxide (CO) and volatile organic compounds (VOCs) are products of
inefficient combustion, which would be eliminated by burning the fuel to carbon
dioxide (CO2) and water (H2O) in the engine of the vehicle to produce power if
possible. Most of the VOC emissions are from the tailpipe. These are controlled
using catalytic reactors and by injecting air at the exhaust ports of the
engine to burn emitted hydrocarbons in this high-temperature zone. Neither
process recovers useful energy, so efforts to modify engine design have been
intense. However, more than 20% of the uncontrolled vehicle engine VOC
emissions are from the crankcase vent (blow-by and evaporating oil) and from
the carburetor vent to the atmosphere. These emissions are controlled using a
crankcase vent pipe to the engine intake duct (requiring a pollution control
value or PCV) and a “carbon canister” absorption unit for evaporative losses.
Fuel injection systems, with their advantage of providing much more precise
metering of fuel to the cylinders significantly reduces pollutant emissions,
including further reduction of evaporative losses.
5.
CONCLUSION
The dramatic increase in public awareness and concern about the state of the global
and local environments, which has occurred in recent decades, has been
accompanied and partly prompted by an ever-growing body of evidence on the extent to which pollution has caused severe environmental degradation.
Considering all major anthropogenic source categories, with the exception of
agriculture, the transportation sector of our economy accounts for the major
part of atmospheric pollution.
The
vehicle exhaust emits volatile organic compounds, nitrogen oxides, leads, and
carbon monoxides into the atmosphere. These emissions are discussed in this
paper. The introduction of vehicle pollutants into the environment has been
shown to have many adverse effects on human health, agricultural productivity, and natural ecosystems. It has also been shown that the highest emissions occur in
vehicle deceleration on a volumetric basis, which is due to low air-fuel ratio
and low exhaust flow. Thermal air pollution, greenhouse effect, ozone depletion
and other pollution resulting in disturbance of natural environment are
discussed. Several methods of effective control of automobile emissions to meet
current and future environmental demands are also analysed.
