What processes take place in the cylinders of a gasoline engine

Petrol - the main motor fuel.The pre-compressed mixture of gasoline vapor and air is ignited by an electric spark in the engine, burning to release energy, some of which with the help of the piston is converted into mechanical energy.The mixture burns quickly, and thus form carbon dioxide, water, and the products of incomplete oxidation (including carbon monoxide).

As the octane number characterizes the properties of the fuel

different fuels for gasoline engines may have different properties.With some of them, the motor works well, but other knocks.This means that combustion takes place too rapidly, and instead a uniform combustion knock has occurred, resulting in an uneven distribution
of energy in the compressed space.For example, heptane, CH3 (CH2) 5CH3 - degraded fuel, and 2,2,4-trimethylpentane ("isooctane"), by contrast, has unique properties in this regard.On the basis of these two compounds is built scale octane: heptane values ​​of zero, and "isooctane" - 100. Properties of gasoline having on this scale octane number 90 are similar to the mixture in which 90% "isooctane," and 10% heptane.The higher the octane number of the fuel (some compounds it may be greater than 100), so it is better.

Gasoline obtained by simple distillation of crude oil and having an octane number of 50-55, not suitable for use in engines.Fuel higher quality octane from 70 to 80, obtained from the cracking.For fuel with the octane number greater than 90, required for modern internal combustion engines used reforming and alkylation.

What is the cracking of hydrocarbons

Cracking - a homolytic cleavage of carbon-carbon bonds in the molecules of hydrocarbons.It is higher alkanes heated to high temperatures without air.This leads to splitting into alkenes and lower alkanes.For example, in the cracking of n-hexane, butane C6H14 can be formed and ethene, ethane, and butene, pentene and methane, hydrogen and hexene.The gap may be thermal and catalytic.

What happens when reforming and alkylation

Reforming - a catalytic isomerization of linear or low-branched alkanes.More branched alkanes obtained by isomerization have large octanes.

Alkylation is an association of alkenes and lower alkanes to higher branched.This ionic reaction occurs under heating, and is catalyzed by mineral acid - eg, sulfuric acid.