Characteristics of Pollutants from Car Exhaust and suggested solution:

Characteristics of Pollutants from Car Exhaust and suggested solution:

The internal combustion engine is an engine in which the combustion of fuel and an oxidizer (typically air) occurs, and generates gases at high temperature and pressure, which are permitted to expand to perform useful work. Internal combustion engines are most commonly used in automobiles such as trucks, cars, motorcycles etc.; in a wide variety of aircraft and locomotives; running of various equipments, and they appear mostly in the form of turbines where a very high power is required, such as in jet aircraft, helicopters, and large ships.

Motor fuel, by which almost all internal combustion engine runs, is obtained from crude oil. Its major constituents are the elements carbon (C), hydrogen (H), oxygen (O) and nitrogen (N), along with some amounts of sulfur (S). In other words, motor fuel contains hydrocarbons and organic compounds containing nitrogen and sulfur. When these are burned in air the products are water (H2O), carbon dioxide (CO2), carbon monoxide (CO) and oxides of nitrogen (NOx). Nitrogen gas in the atmosphere may also react with oxygen at the high temperatures in the combustion chamber to form oxides.

Characteristics of Pollutants from Car Exhaust:

a. Carbon dioxide (CO2) - This gas is naturally present in the atmosphere at low concentration (approximately 0.035%). It absorbs infrared energy and is thus a greenhouse gas, a contributor to global warming. Concentrations of CO2 in the earth's atmosphere appear to be increasing. This is a great concern as it has a substantial effect on the climate. The internal combustion engine contributes to the increased concentrations of CO2 in the atmosphere.

b. Carbon monoxide (CO) - The main source of CO in cities is the internal combustion engine, where it is produced by incomplete combustion. Anthropogenic sources account for approximately 6% of the 0.1 ppm concentration of CO in the earth's atmosphere globally. In an urban area, the concentration can be much higher. CO is highly toxic. It binds to haemoglobin more strongly than oxygen does, thus reducing the capacity of the haemoglobin to carry oxygen to the cells of the body. CO also has the nasty habit of sticking to haemoglobin and not coming off. This means that a fairly small amount of it can do a lot of damage. However, CO can be oxidized to the far less harmful CO2, if there is enough O₂ available. At higher air-fuel ratios the level of CO emission goes down. CO can also be oxidized to CO2 in a catalytic converter.

c. Oxides of nitrogen (NOx) - While some nitrogen may be present in the fuel, most oxides of nitrogen are produced when elemental nitrogen (N2) in the air is broken down and oxidized at high temperatures (approximately at 700 degree Celsius or greater) and pressures within the internal combustion engine. Nitrogen monoxide (NO) is produced in higher concentration than nitrogen dioxide (NO2) but the two species are in any case inter-convertible by means of photochemical interactions. Other oxides of nitrogen, such as N₂O₄, may occur; but chances are rare. NO and NO₂ are toxic species. Oxides of nitrogen also play a major role in the formation of photochemical smog.

d. Hydrocarbons (HC) - Hydrocarbon fuel, sometime, passes through the process unconsumed and is expelled into the atmosphere along with other exhaust fumes. Fuel close to the wall of the combustion chamber may be quenched by the relative coolness of that area and not be burned. Also, if the engine is poorly designed or is not in proper working order the proportion of unburned fuel rises. Some hydrocarbon fuels are also released to the atmosphere by direct evaporation from fuel tanks. It may be noted; hydrocarbons can be dangerous to human health and are also part of the makeup and cause of photochemical smog.

e. Benzene and its derivatives (C6H6) - Benzene is, of course, a hydrocarbon, but is sufficiently different from straight-chain hydrocarbons. The six carbons (C) in benzene form a regular hexagon, with one hydrogen (H) attached to each carbon and sticking out. All 12 atoms lie on one plane. This structure of benzene is quite stable — stable enough for a large proportion of the benzene in fuel to pass unchanged through the combustion process. There is quite a lot of benzene in fuel. It acts as an anti-knock agent, making cars run more smoothly. Since the abolition of lead additives as anti-knock agents, the levels of benzene and benzene-related compounds (modified form where one or more hydrogen have been removed to form a phenyl ring and other things have been attached in their places) in car fuel have increased. Benzene (C6H6), and also many of its derivatives such as toluene (PhCH3) and phenol (PhOH), is carcinogenic (the level of toxicity varies). Benzene vapors are therefore quite dangerous. It has been suggested that benzene is more dangerous to filling station attendants than to the general public in the streets as the concentration of benzene will be higher in the raw fuel than in the combustion products.

f. Sulfur dioxide (SO2) - Fossil fuels are derived from once-living organisms. Some sulfur occurs in protein and will still be present in the fuel. Under combustion this sulfur reacts with oxygen to form sulfur di- and trioxide. Sulfur dioxide emission does occur from cars. SO2 and SO3 are acidic pollutants which dissolve in moisture in the atmosphere to form sulfurous and sulfuric acids (H2SO3 and H2SO4), which are components of 'acid rain'. These corrode metal surfaces and weather limestone buildings.

Acid rain also mobilizes toxic aluminum ions in the soil, washing them out into streams and ponds. This causes sticky mucus to accumulate in the gills of fish and eventually kills them. Trees and other plants which absorb aluminum ions will be damaged. In humans, sulfur dioxide irritates the eyes, the mucous membranes and the respiratory tract, along with the skin in general. SO2 also has the effect of slowing down the movements of the cilia (the hairs in the trachea which act to prevent dust entering into lungs), thus exacerbating the irritation caused by allowing more pollutant to access the respiratory system.

g. Particles micro-particulate, 10 microns (Particulate matters – PM10) - These are ultra-fine particles which are less than one-hundredth of a millimeter across. Thus they are too small to settle or be dispersed by rain. These particles absorb acidic gases which are also present in exhaust fumes and, when inhaled, penetrate into the microscopic air sacs of the lungs (alveoli). Scavenging white blood cells are overwhelmed by these particles, and release a stream of chemicals that trigger an inflammatory reaction in the lungs, and increase the stickiness of red blood cells, thus increasing the likelihood of blood clots. The main victims of this type of pollution are the elderly, smokers, and those suffering from chest complaints, heart conditions and asthma. It is considered that PM10s may be the most important and dangerous component of vehicle pollution. These particles can drift for miles, and accumulate inside buildings. The major source of PM10s in urban air is motor vehicles, particularly diesel engines.

h. Photochemical Smog - Reactive pollutant hydrocarbons in the presence of NOx and under certain atmospheric conditions can produce a brown haze known as photochemical smog. It is formed by photochemical reactions (that is, reactions catalyzed by light) between NOx and hydrocarbons (HC). Photochemical smog is most common on windless sunny days when the ingredients are not dispersed and there is plenty of light energy available to power the reaction. Photochemical smog is characterized by the presence of particulate matter (which creates a sort of haze), oxidants such as ozone, and noxious organic species such as aldehydes.

Suggested Solutions to the vehicle exhaust problems:

(i) Catalytic Converters - Most modern cars contain catalytic converters. In these, exhaust fumes and added air pass over a catalyst where they are broken down to less harmful products.

(ii) Drive less.

(iii) Use cleaner engines – Use cars that are designed to be more fuel-efficient and less pollutant.

(iv) Drive hybrid vehicles - Whether a hybrid engine is more environmentally sound than a normal engine depends on how the car is used. For city stop-start driving, they're usually better.

(v) Keep your car in good working order

(vi) Use smaller cars.

(vii) Drive intelligently - The way a car is driven can have a huge effect on its fuel-consumption and hence on its effect on the environment.