With the recent rise of both crude oil and gasoline (petrol) prices, there has been an increasing amount of interest in how crude oil is used to make gasoline. In this article, I will explain the basic refining process and point you to other sources where you can find more detailed information on this topic.
Crude oil, otherwise known as petroleum, is a fossil fuel that is extracted from the ground and refined to make a wide variety of products ranging from liquids like cleaning solvents and motor fuels to solids such as asphalt and plastics. The proportion of crude oil that is ultimately converted into gasoline varies according to region and the type of refinery. In the United States, where demand for gasoline is remarkably high, this proportion is around 50 percent while in other areas of the world such as Europe and Asia, this amount is somewhat less. However, refineries in these regions produce proportionately higher percentages of diesel oil, heating oil, and heavy fuel oil. A good estimate of the exact breakdown of petroleum products by percentage of crude oil in the United States can be found at the California Energy Commission.
In order to make gasoline (and all of the other petroleum products), crude oil is sent to a refinery and put through a process called fractional distillation, which is the same basic method that is used to make stronger alcoholic beverages such as vodka, whiskey, and rum. The crude oil is first heated in a boiler by superheated steam, which effectively vaporizes most of it before being transferred to a distillation column. This column is a tall, cylindrical tower that contains many trays and plates on which the vaporized oil is allowed to condense as it rises and cools. The heavier components of oil which contain longer hydrocarbon chains and have higher boiling points will condense first near the bottom of the column, while the lighter components will rise further and condense into liquids near the top.
Most of these separated components are processed further before being made into finished products and shipped to market. For example, some of the heavier hydrocarbons can be “cracked” by intense heat, high pressures, and various chemical catalysts to produce additional gasoline as well as other fuels such as kerosene and diesel. Other oil byproducts serve as precursors to plastics or other solid goods, and some are also made into lubricants such as Vaseline. After the secondary cracking processes are completed, one of the final products that is left over as a residue is called coke (not to be confused with the beverage Coca-Cola), a substance that is almost pure carbon and is used in steel production and for other metallurgical purposes. In the 19th century, coke was also used as a fuel for locomotive engines.
A more detailed explanation of the process of refining crude oil into gasoline can be found in the Nontechnical Guide to Petroleum Geology. Another notable content-rich resource that I ran across is called Oilfield Processing: Crude Oil. Although it is a bit heavy on text and detail, it contains a wealth of information about crude oil and how it is processed into gasoline and many other useful products. A somewhat related article on the benefits of synthetic oil can be found in a later post.