Millions of cars are running in the world by petrol engines. In petrol engine, a mixture of petrol vapor and air is drawn into the cylinder in downward stroke. The fuel mixture is then compressed in upward stroke. Just before the completion of compression the spark plug ignites the petrol air mixture and smooth combustion takes place whereby the automobiles run smoothly. If poor quality fuels are used then combustion takes place during compression before the spark plug fires. In such a case the piston is pushed violently, and irregular force is transmitted to crankshaft whereby metallic noise is produced. Such action in automobile engine is known as knocking of the engine. Knocking of the engine shortens the life and efficiency of engine.
Different fuels have different knocking tendencies. The quality of fuel is judged from the antiknock property of the fuel and is indicated in terms of octane number. It is found that straight chain alkanes are more prone to induce engine knock than the branched chain alkanes. Heptanes are a particularly bad fuel and are assigned arbitrarily an octane number of 0 and iso-octane (2, 2, 4-trimethyl pentane) which is excellent fuel is assigned an octane number of 100.
Remember to consider the following:
N-heptane (octane number =0) iso-octane (octane number 100)
The octane number to any fuel is given by comparing its antiknock property with the mixture of iso-octane and n-heptanes. If the antiknock property of a given fuel is identical with a mixture of 75% iso-octane and 25% n-heptane then its octane number is given as 75. The greater the octane number the better is the fuel. The gasoline obtained by fractional distillation of petroleum contains more straight chain alkanes and is a poor fuel. Owing to this the petroleum chemists carry out different processes to increase the percentage of branched chain alkanes to increase the octane number of fuel.
One way of increasing octane number of fuel is increase the percentage of branched chain alkanes or to add aromatic hydrocarbons. Another way of increasing combustion of fuel in engine is to add small quantity of tetra alkyl lead specially tetra ethyl lead(TEL) or tetra methyl lead. Gasoline containing tetra alkyl lead is known as leaded gasoline or leaded petrol. Tetra-ethyl lead reduces the knocking property of un-branched alkanes and is known as gasoline additive. A layer of lead oxide is formed when leaded petrol is used. To prevent this 1,2 bromoethane is added.This will convert waste lead to volatile lead bromide, which passes out with exhaust gases.About 6 gram of tetra ethyl lead is added per gallon of gasoline in UK. Because of large number of vehicles running in the city, the levels of lead emitted to the atmosphere by the use of leaded petrol have reached to hazardous level.Therefore many developed countries have decided to use only those vehicles which run in lead free petrol.
The petroleum fraction containing C1 to C12 are in great demand . So, scientists have developed a technique of converting hydrocarbons with bigger molecules to more useful smaller molecules. This can be achieved by passing the vapor of higher alkanes over Al2O3 or SiO2 catalyst at about 450 degree celsius. The industry calls this type of reaction as cracking.
Straight alkanes containing 6 or more carbon atoms can be converted to aromatic compound by the process known as reforming or aromatization.In reforming the straight chain alkanes are heated to about 400 degree celsius in presence of catalyst such as platinum, palladium, nickel or cobaltby aromatic compounds are produced.
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