Gasoline is the fraction of oil boiling in a temperature interval from 40 to 200˚C. It is considered one of the most valuable oil products as serves as fuel for internal combustion engines. For assessment of quality of gasoline octane numbers are used.
What processes happen in cylinders of the petrol engine
Gasoline – the main motor fuel. Previously compressed mix of vapors of gasoline and air which is set on fire in the engine by an electric spark burns with release of energy which part by means of the piston turns into mechanical. Mix burns down quickly, and at the same time carbon dioxide, water and products of incomplete oxidation are formed (including carbon monoxide).
As the octane number characterizes properties of fuel
Different types of fuel for petrol engines can have different properties. The motor works with some of them well, and with others knocks. It means that combustion happens too quickly, and instead of uniform burning there was a detonation that leads to uneven distribution of energy in the compressed space. For example, CH3(CH2)5CH3 heptane is unsuitable fuel, and 2,2,4-trimetilpentan ("isooctane"), on the contrary, has unique properties in this plan. On the basis of two of these connections the scale of octane numbers is built: the zero value, and to "isooctane" – 100 is appropriated to heptane. Properties of the gasoline having octane number 90 on this scale are similar to mix in which 90% of "isooctane" and 10% of heptane. The fuel higher than an octane number (some connections can have it more than 100), the it is more qualitative.
The gasoline received by simple distillation from oil and having octane number 50-55 is unsuitable for use in engines. More quality fuel, with octane number from 70 to 80, is received when cracking. For receiving fuel with octane number higher than 90, demanded for modern internal combustion engines, apply reforming and alkylation.
What is cracking of hydrocarbons
Cracking is a gomolitichesky rupture of carbon-carbon communications in molecules of hydrocarbons. It consists in heating of the highest alkanes to high temperatures without air access. It leads to their splitting on alkenes and the lowest alkanes. For example, when cracking N hexane of C6H14 butane and eten, ethane and butene, methane and pentene, hydrogen and hexene can be formed. The gap can be thermal and catalytic.
What occurs during the reforming and alkylation
Reforming is a catalytic isomerization of unbranched or low-branched alkanes. More branched alkanes received by isomerization have big octane numbers. Alkylation represents association of alkenes and the lowest alkanes in the highest branched. This ionic reaction proceeds when heating and is catalyzed by inorganic acids – for example, a chamois.