Gasoline or petrol is a petroleum-derived liquid mixture consisting mostly
of aliphatic hydrocarbons and enhanced with aromatic hydrocarbons toluene, benzene or iso-octane
to increase octane ratings, primarily used as fuel in internal combustion engines. Most Commonwealth
countries or former Commonwealth countries, with the exception of Canada, use the term "petrol"
(abbreviated from petroleum spirit). The term "gasoline" is commonly used in North America where it is
often shortened in colloquial usage to "gas". This should be distinguished in usage from genuinely gaseous
fuels used in internal combustion engines such as liquified petroleum gas (which is stored pressurised
as a liquid but is allowed to return naturally to a gaseous state before combustion). The term mogas,
short for motor gasoline, distinguished automobile fuel from aviation gasoline, or avgas. The word
"gasoline" can also be used in British English to refer to a different petroleum derivative historically
used in lamps; however, this use is now uncommon.
Chemical Analysis
Gasoline is produced in oil refineries. Material that is separated from crude oil via distillation,
called virgin or straight-run gasoline, does not meet the required specifications for modern engines
(in particular octane rating; see below), but will form part of the blend.
The bulk of a typical gasoline consists of hydrocarbons with between 5 and 12 carbon atoms per molecule.
Many of these hydrocarbons are considered hazardous substances and are regulated in the United States by OSHA.
The Material Safety Data Sheet for unleaded gasoline shows at least fifteen hazardous chemicals occurring in various amounts.
These include benzene (up to 5% by volume), toluene (up to 35% by volume), naphthalene (up to 1% by volume),
trimethylbenzene (up to 7% by volume), MTBE (up to 18% by volume) and about 10 others.
The various refinery streams blended together to make gasoline all have different characteristics. Some important streams are:
Reformate, produced in a catalytic reformer with a high octane rating and high aromatic content, and very low olefins (alkenes).
Cat Cracked Gasoline or Cat Cracked Naphtha, produced from a catalytic cracker, with a moderate octane rating, high olefins (alkene)
content, and moderate aromatics level. Here, "cat" is short for "catalyst".
Hydrocrackate (Heavy, Mid, and Light), produced from a hydrocracker, with medium to low octane rating and moderate aromatic levels.
Virgin or Straight-run Naphtha (has many names), directly from crude oil with low octane rating, low aromatics (depending on the
crude oil), some naphthenes (cycloalkanes) and no olefins (alkenes).
Alkylate, produced in an alkylation unit, with a high octane rating and which is pure paraffin (alkane), mainly branched chains.
Isomerate (various names) which is obtained by isomerising the pentane and hexane in light virgin naphthas to yield their higher
octane isomers.
(The terms used here are not always the correct chemical terms. They are the jargon normally used in the oil industry.
The exact terminology for these streams varies by refinery and by country.)
Overall a typical gasoline is predominantly a mixture of paraffins (alkanes), naphthenes (cycloalkanes),
aromatics and olefins (alkenes). The exact ratios can depend on
the oil refinery that makes the gasoline, as not all refineries have the same set of processing units.
the crude oil used by the refinery on a particular day.
the grade of gasoline, in particular the octane rating.
Currently many countries set tight limits on gasoline aromatics in general, benzene in particular, and olefins (alkene) content.
This is increasing the demand for high octane pure paraffin (alkane) components, such as alkylate, and is forcing refineries to add
processing units to reduce the benzene content.
Gasoline can also contain some other organic compounds: such as organic ethers (deliberately added), plus small levels of contaminants,
in particular sulfur compounds such as disulfides and thiophenes. Some contaminants, in particular thiols and hydrogen sulfide, must be
removed because they cause corrosion in engines.
My Opinion
I, personally agree with opinions that un-lead fuels and gasoline is "safer" for nature, though both aren't
totally 100% enviromentalistic compatible.
