Gasoline fuel

Petroleum or crude oil is taken out of the ground as a mix of impurities and highly flammable compounds of hydrogen and carbon, called hydrocarbons.

It is processed into many products, including petrol/gasoline and diesel fuel.

Gasoline is used in different forms in different engines.

It is very volatile, mixing easily with air to form gas or vapor. The more effectively liquid gasoline is changed into vapor, the more efficiently it burns in the engine, so high volatility is desirable.

And if liquid gasoline is heated, it is even more volatile.

However, if it vaporizes in the fuel line, bubbles of vapor can block the flow of fuel and stop the engine. This is called vapor lock.

Chemical analysis and production

Gasoline is produced in oil refineries. These days, material that is simply separated from crude oil via distillation, called natural gasoline, will not meet the required specifications (in particular octane rating; see below) for modern engines, but these streams will form part of the blend.

The bulk of a typical gasoline consists of hydrocarbons with between 5 and 12 carbon atoms per molecule.

The various refinery streams that are blended together to make gasoline all have different characteristics. Some important streams are:

• Reformate, produced in a catalytic reformer with a high octane and high aromatics content, and very low olefins (alkenes).
• Cat Cracked Gasoline or Cat Cracked Naphtha, produced from a catalytic cracker, with a moderate octane, 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 and moderate aromatic levels.
• Natural Gasoline (has very many names), directly from crude oil with low octane, low aromatics (depending on the crude oil), some naphthenes (cycloalkanes) and zero olefins (alkenes).
• Alkylate, produced in an Alkylation unit, with a high octane and which is pure paraffin (alkane), mainly branched chains.
• Isomerate (various names) which is made by isomerising Natural Gasoline to increase its octane and is very low in aromatics and benzene content.

(The terms used here are not always the correct chemical terms. Typically they are old fashioned, but they are the terms normally used in the oil industry. The exact terminology for these streams varies by oil company 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.

These days, gasoline in many countries has tight limits on 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 mercaptans and hydrogen sulfide must be removed because they cause corrosion in engines.

Source: CDX Global & Wikipedia - en.wikipedia.org