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What processes occur in the cylinders of a gasoline engine

Gasoline is the main motor fuel. A pre-compressed mixture of gasoline and air vapors, ignited in an electric spark by the engine, burns with the release of energy, part of which is converted into a mechanical piston by means of a piston. The mixture burns quickly, and at the same time carbon dioxide, water and products of incomplete oxidation (including carbon monoxide) are formed.

How octane number characterizes the properties of fuel

Different types of fuel for gasoline enginescan have different properties. With one of them the motor works well, but with the others it knocks. This means that combustion takes place too quickly, and instead of uniform combustion, detonation occurs, which leads to an uneven distribution of energy in the compressed space. For example, heptane CH3 (CH2) 5CH3 is an unsuitable fuel, and 2,2,4-trimethylpentane ("isooctane"), on the other hand, has unique properties in this respect. On the basis of these two compounds, a scale of octane numbers is built up: heptane is assigned a zero value, and "isooctane" is 100. The properties of gasoline having this octane rating of 90 are similar to a mixture in which 90% of "isooctane" and 10% of heptane. The higher the octane number of fuel (in some compounds it may be more than 100), the better it is. The benzene, obtained by simple distillation from oil and having an octane number of 50-55, is unsuitable for use in engines. Fuel of higher quality, with octane number from 70 to 80, is obtained with cracking. Reforming and alkylation are used to produce fuels with an octane number above 90, which is required for modern internal combustion engines.

What is the cracking of hydrocarbons?

Cracking is a homolytic breakcarbon-carbon bonds in hydrocarbon molecules. It consists in heating the higher alkanes to high temperatures without access to air. This leads to their splitting into alkenes and lower alkanes. For example, when cracking n-hexane C6H14, butane and ethane, ethane and butene, methane and pentene, hydrogen and hexene may be formed. The rupture can be thermal and catalytic.

What happens when reforming and alkylating

Reforming is a catalytic isomerizationunbranched or slightly branched alkanes. More branched alkanes obtained by isomerization have large octane numbers. Alkylation is the combination of alkenes and lower alkanes into higher branched ones. This ionic reaction proceeds with heating and is catalyzed by inorganic acids - for example, sulfuric acid.