OCTANE NUMBERGasoline which is commonly known as petrol is a mixture of n-hexane and n-heptane. During the process of fractional distillation, it is obtained in the upper most portion of the tower.Octane number is a standard which determines the knocking ability and quality of gasoline. Higher is the octane number of a gasoline, lower is the knocking it produces.Octane number can be increased by the following two methods:

Reforming  By adding TEL (tetraethyl lead)

Knocking is a sharp metallic sound produced in the internal combustion engine. Knocking is caused by the low octane number of gasoline.

REFORMING OF PETROLEUM

The conversion of straight chain hydrocarbon into branched chain hydrocarbon is called reforming of petroleum. By the process of reforming, the octane number of a gasoline is improved. The process of reforming is carried out in the presence of catalyst n-alkanes burn in internal combustion engine with explosion and produce knocking but branched chain hydrocarbons burn smoothly. Reforming is a process similar to cracking, which converts n-alkanes into branched alkanes. 

 The octane number of gasoline may also be improved by adding tetraethyl lead (TEL) Pb(C2H5)4 which is a knock inhibitor. But the use of compounds containing lead cause environment pollution.

Petroleum

Knocking :

In an internal combustion engine, the fuel mixture of gasoline vapours and air burns in cylinder. The mixture is fired by means of a spark in the compressed state. For better and maximum efficiency, the fuel mixture is highly compressed before firing in cylinder. A good fuel on sparking burns steadily, completely in time at a definite compression ratio. However, if compression ratio exceeds a limit, the sparking to a fuel gives rise to knocking i.e., all the fuel does not burn smoothly and a part of fuel burns suddenly with explosive nature giving rise to metallic rattle or knocking. This results in loss of power. The knocking however falls with nature of product fuel.

Knocking order falls with branching, unsaturation, aromatization etc. The knocking order is:

Straight chain paraffins > branched chain paraffins > olefins > naphthenes > Arenes.

The knocking may be prevented by adding compounds like TEL or tetra ethyl lead. The gasoline from petrol pump which we get contain anti knock compounds (TEL).

Octane no. of gasoline is the no. for rating anti knock qualities of gasoline. The higher the octane no., lesser the tendency for knocking and better being the fuel. Octane no. is derived by assuming 100 arbitrary value for isooctane and zero for n-heptane. Thus no. is the % of

isooctane in a mixture of isooctane and n-heptane whose knocking resembles with the knocking of fuel used.

The octane no. of fuel is increased by two to three units by adding 2 to 4 ml of TEL in one Gallon petrol. Tetra ethyl lead is supposed to dissociate as

                        (C2H5)4Pb  →   4C2H5         +         Pb

                                  Ethyl free radical

The ethyl free radical converts some of the straight chain hydrocarbons into branched chain hydrocarbons and thus increases the octane number. In addition to TEL, a little ethylene dibromide is also added which converts Pb to PbBr2, which comes out through exhaust and prevents its deposition in engine due to decomposition of TEL.

                        Pb + CH2BrCH2Br → PbBr2 + CH2=CH2

                                                  Volatile

A gasoline fuel with octane no. 30 or above is good fuel. The efficiency of fuel increases as the octane no increases.

Addition of TEL in petro, no doubt improves the octane number of fuel but it also causes lead pollution in atmosphere. The attempts are therefore been made to improve octane number of fuel by some other means. Unleaded or lead free gasoline marketed today is the gasoline obtained by isomerisation and alkylation blended with BTX (Benzene, toluene, xylene) or methyl-t. butyl ether. Its octane number is 90. Thus following processes are used for getting lead free gasoline with better octane number.

(i)             Isomerisation (Reforming) : By passing gasoline vapours over aluminium chloride (AICI3) at 200oC.

          

(ii)           Alkylation : Isobutylene formed during cracking on alkylation with isobutene forms iso-octane in presence of concentrated H2SO4.