Car and Truck Gasoline

Gasoline ("gas", mogas=motor gasoline, petrol, benzene) is a liquid petroleum derivative consisting mostly of hydrocarbons enhanced with benzene or iso-octane to increase octane ratings.   It is primarily used as fuel in automobile internal combustion engines.   Before internal-combustion engines were invented in the mid-19th century, gasoline was sold in small bottles as a treatment against lice and their eggs.   In the U.S. gasoline was also sold as a cleaning fluid to remove grease stains from clothing.   Before dedicated gasoline filling stations were established, early motorists would buy gasoline in cans to fill their tanks.   Gasoline was also used in kitchen ranges and for lighting, and it is still available in a highly purified form, known as camping fuel or white gas (naphtha), for use in lanterns and portable stoves. ^{Wiki n.p.}

Gasoline is produced in oil refineries.   Hydrocarbons separated from crude oil via distillation, called virgin or straight-run gasoline, do not meet the required specifications for modern engines, but it will form part of the blend.   Gasoline consists mostly of hydrocarbons with between 5 to 12 carbon atoms per molecule (C₅H₁₂ through C₁₂H₂₆).   For example, n-octane, C₈H₁₈ and iso-octane, its isomer, and n-heptane, C₇H₁₆.   Other gasoline components are 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 (Methyl tertiary-butyl ether; up to 18% by volume) and about 10 others. ^{Wiki n.p.}

The various refinery streams blended together to make gasoline with different characteristics.   Some important streams are the following: ^{Wiki n.p.}

• 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, 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.

Overall a typical gasoline is predominantly a mixture of alkanes, naphthenes (cycloalkanes), aromatics, and 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.   Because of the harmful effects of benzene, government regulations requires high octane pure alkane components, such as alkylate, which forces 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. ^{Wiki n.p.}

The octane rating is a measure of the autoignition resistance of gasoline and other fuels used in spark-ignition internal combustion engines.   It is a measure of anti-detonation of a gasoline or fuel. Octane number is the number which gives the percentage by volume of iso-octane, which has a high auto-ignition temperature, in a mixture of iso-octane and normal heptane (n-heptane), which has a low auto-ignition temperature, that would have the same anti-knocking capacity as the fuel which is under consideration.   For example, gasoline with the same knocking characteristics as a mixture of 90% iso-octane and 10% heptane would have an octane rating of 90.   The octane ratings of n-heptane and iso-octane are respectively exactly 0 and 100, by definition.   This does not mean that the gasoline actually should contain these chemicals in these proportions.   It simply means that it has the same autoignition resistance as the described mixture.   A high tendency to autoignite, or low octane rating, is undesirable in a gasoline engine, but desirable in a diesel engine.   The standard for the combustion quality of diesel fuel is the cetane number.   A diesel fuel with a high cetane number has a high tendency to autoignite, which is preferred. ^{Wiki n.p.}

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