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3.9 Vehicle Engines

3.9 Vehicle Engines. The major source of NOx and hydrocarbons are vehicle engines Those are formed by some of the hydrocarbons in fuel remain uncombusted and some are incompletely oxidized to CO

diana-schultz
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3.9 Vehicle Engines

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  1. 3.9 Vehicle Engines • The major source of NOx and hydrocarbons are vehicle engines • Those are formed by some of the hydrocarbons in fuel remain uncombusted and some are incompletely oxidized to CO • Tuning-adjustments to the fuel supply air/fuel ratio and spark timing of the engine. Reduces pollutants. The problem with tuning is that the composition of the exhaust changes based on driving conditions. • The air/fuel ratio is the mass of air divided by the mass of fuel. • Rich mixture- high in CO, high in hydrocarbons, and low NOx. It results from less air in the ratio. • Lean mixture- lower CO and hydrocarbons, but has high NOx. Results with excess air. • Changing the air/fuel ratio does not get rid of pollutants, simply changes from one to the other.

  2. 3.9 Vehicle Engines • Knocking- when the fuel vapor ignites before the ignition spark. Characterized by sudden increase in pressure. • Iso-octane: high resistance to knocking, octane number=100. • Heptane: low resistance to knocking, octane number=0 • TEL-tetraethyllead Pb(C2H5)4 added to fuel in amounts between 0.5-1.0 g/L reduces knocking. • Problem is that lead compounds are toxic. • TEL is not used very much today because most fuel is unleaded • Lean Burn Engines are another way to reduce emissions • Use an air/fuel ratio of 18:1 • Produces less NOx and has better fuel economy but tends to misfire. • Thermal Exhaust Reactor • Engine exhaust is combined with more air and reacts due to the heat of the gases. CO is oxidized to CO2 and unburned hydrocarbons are combusted. • 2CO + O2 2CO2

  3. Catalytic converters oxidize CO and hydrocarbons to make them harmless. • Catalytic converter- Pipe that has Platinum or Rhodium catalyst • Oxidizes COCO2 • Oxidizes hydrocarbons to CO2 and water • NOx cannot be oxidized • Using leaded fuel causes the catalytic converter to be ineffective • Three-way catalytic system- oxidizes CO and hydrocarbons as well as reduces NOx. This is done by passing the exhaust over a reduction catalyst such as platinum, palladium, cobalt, or nickel. It is then passed over an oxidation catalyst such as palladium, platinum, or rhodium. • Oxidation of CO: 2NO(g) + 2CO(g) N2(g) + 2CO2(g) • Reduction of octane: 2C8H18(g) + 25O2(g) 16CO2(g) + 18H2O(g) 2CO(g) + O2(g) 2CO2(g) • It is not possible to put a three-way catalytic system unless the car is manufactured with it.

  4. Primary Air Pollutants

  5. 3.10 Alternative Fuels • They are substances mixed with gas to increase the octane number. The substances are referred to oxygenates. They cause less pollution. • Ethanol- when mixed with gas is called gasohol. • Made from the fermentation of sugar and starches. • Methanol- it is inexpensive and must be mixed with a co-solvent. • Disadvantages- the fuel is corrosive, conducts electricity, and produces only 60%of the energy of an equal volume of gas. It is also toxic and exhaust gases contain methanal-lachrymatory and carcinogenic. • MTBE-methyl tertiary butyl ether (CH3)3COCH3 • Made from methanol and 2-methlypropene • CH3OH(g) + (CH3)2C=CH2(g) (CH3)3COCH3(g) • Disadvantages-more than 15%by volume of MTBE makes the fuel mixture too volatile. Also more expensive than gasoline. • Hydrogen • Clean fuel, forms water vapor only when combusted. Internal combustion engines can be modified to run on hydrogen. • Disadvantages- impure hydrogen is very explosive.

  6. 3.11 Ozone in the Stratosphere • Chlorine- is more effective than NOx at removing ozone • Chloromethane is the source of 25% of the chlorine in the stratosphere, mostly biological. • HCl is formed by hydrolysis of chloromethane and gives Cl·

  7. Chlorofluorocarbons- chemically stable, low toxicity and are volatile. • Used as refrigerants, blowing agents, and plastic foams. • Three most common CFCs; CFCl3, CF2Cl2, and CF2ClCFCl2 • They are photolysed to form Cl· • CCl2F2 + hv  ·CClF2 + Cl· • CCl3F + hv  ·CCl2F + Cl· • Residence time of 60 years: no natural source • Ozone Depletion • Evidence • Large decrease in the ozone over Antarctica during the spring. This is known as the ozone hole. It recovers during the summer. • Clouds form in the spring in that area and on the surface of the water droplets reactions occur that increase the rate of the decomposition of chlorine reservoirs. When the clouds warm up, they release ClO· • The air mass over the Antarctic rotates in isolation from the rest of the global air mass. • Similar effect is observed in the Arctic.

  8. Environmental Effects of Ozone Depletion • Ultraviolet light has sufficient energy to damage biological molecules, such as amino acids, proteins, and nucleic acids. Because of ozone depletion more UV light has been reaching the Earth’s surface.

  9. 3.12 Ozone in the Troposphere • When found in the troposphere, ozone is a pollutant that damages plants and has negative effects on human health. • Nitrogen dioxide can be photolysed to produce free radicals which then react with oxygen to form ozone. • NO2· + hv  NO· + O· • O· + O2 + M  O3 + M* • NO· + O3 NO2 + O2 • The 2nd reaction produces ozone, while the third reaction destroys it. This causes an equilibrium to be reached. • Ozone can react to form hydroxyl radicals which can abstract H from hydrocarbons to form alkyl radicals which can add molecular oxygen to form peroxy radicals which can then react with NO to form NO2. The NO2 then can absorb radiation to form more ozone. • O3 + hv  O· + O2 • O· + H2O  2 ·OH • ·OH + RCH3  H2O + ·RCH2 • RCH2O2· + NO·  RCH2O· + NO2· • Ozone levels in the troposphere vary according to time of day, season, and location.

  10. Effects on human health • At about 100-300 ppb, ozone causes breathing difficulty. • Ozone, in tandem with sulfur dioxide, has a synergistic effect, meaning that their combined effects are worse than their separate effects. • Ozone can react with some of the compounds in smog to form carcinogenic and mutagenic compounds. • Effects on plants • Plants are damaged by ozone levels as low as 40 ppb. • Ozone is believed to cause the most damage to vegetation of all air pollutants. • Reduces yields of crops as well as causes unsightly blemishes. • Effects on materials • Tires crack. • Plastics lose pliability. • Greenhouse effect • Absorbs radiation and contributes to the greenhouse effect. • Control of ozone • Is a secondary pollutant. • To reduce ozone pollution, primary pollutants must be reduced, such as CO, hydrocarbons and NOx from cars and combustion of fossil fuels.

  11. 3.13 Global Warming • Greenhouse effect • Natural atmospheric phenomenon that allows the passage of incoming shortwave radiation but absorbs reflected IR radiation and reradiates it back to the Earth’s surface. The main greenhouse gas is water. • Greenhouse gases • CO2, methane, NOx, ozone, CFCs, and other gases. • An increase in the concentration of greenhouses gases causes global warming. • Earth’s average temperature has increased 1º C. • Ice core samples show that there have been large changes in global temperature in the past. • Effects of global warming • Changes in agriculture and biodistribution as the climate changes. • Rising sea levels as glaciers/polar ice caps melt.

  12. 3.14 Radon • Radon • Naturally occurring radioactive gas. • Has been shown to cause leukemia. • Formed by decay of uranium in granite and other rocks. • It can be found in high concentrations in buildings made of granite. • Methods of removing radon levels in buildings with high radon concentration • Maintain air circulation with fans and open windows. • Radon levels are measured by means of an α-track detector • α-particles make tracks in plastic which can be seen through a microscope from which the radioactivity can be calculated.

  13. 3.15 Accidental Pollution • An example of accidental pollution was the leak of poisonous gas from a Union Carbide factory producing the pesticide Sevin. • An estimated 2,500 people died and 200,000 were injured. • Plants for miles around the factory were destroyed. • A tank of methyl isocyanate rose to a temperature of 38º C causing the liquid to vaporize and increase the pressure. The automatic relief valves and refrigeration plant were not working. A valve ruptured, allowing gas to escape the tank. The gas entered the scrubber tanks, but only one of the tanks worked and it was overwhelmed. All 3,840 gallons of methyl isocyanate escaped, creating a cloud of poisonous gas that swept across the land. Company medical officers told doctors that the gas was methyl isocyanate and that it was non-poisonous. Doctors tried to treat the victims, but did not know what was the cause of the poisonings. • The actual gas that caused the poison was a mixture of phosgene, a substance used in the formation of methyl isocyanate, and HCN, was formed from the decomposition of methyl isocyanate. • COCl2 + CH3NH2 CH3NCO + 2HCl • Phosgene was used during WWI as chemical warfare, causes mild eye irritation (can lead to blindness), pulmonary oedema: lungs swollen with water. • Union Carbide gave little assistance to the town of Bhopal and the local authorities were not prepared to handle a situation like this.

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