Vehicle Emissions and Their Effects on Natural Environment

1. INTRODUCTION

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.