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Gasoline, petrol (British English) or gas (American English) is a colorless petroleum-derived flammable liquid that is used primarily as a fuel in spark-ignited internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. On average, a 42-U.S.-gallon (160-liter) barrel of crude oil yields about 19 U.S. gallons (72 liters) of gasoline after processing in an oil refinery, though this varies based on the crude oil assay.

Gasoline used in internal combustion engines can have significant effects on the local environment, and is also a contributor to global human carbon dioxide emissions. Gasoline can also enter the environment uncombusted, both as liquid and as vapor, from leakage and handling during production, transport and delivery (e.g., from storage tanks, from spills, etc.). As an example of efforts to control such leakage, many underground storage tanks are required to have extensive measures in place to detect and prevent such leaks. Gasoline contains benzene and other known carcinogens.

The use of the word gasoline instead of petrol outside North America can be confusing, particularly given the common shortening of gasoline to gas, because various forms of gaseous products are also used as automotive fuel, such as compressed natural gas (CNG), liquefied natural gas (LNG) and liquefied petroleum gas (LPG). In many languages, the name of the product is derived from benzene, such as Benzin in German or benzina in Italian. Argentina, Uruguay and Paraguay use the colloquial name nafta derived from that of the chemical naphtha.

On 2 August 1917, the United States Bureau of Mines arranged to study fuels for aircraft in cooperation with the Aviation Section of the U.S. Army Signal Corps and a general survey concluded that no reliable data existed for the proper fuels for aircraft. As a result, flight tests began at Langley, McCook and Wright fields to determine how different gasolines performed under different conditions. These tests showed that in certain aircraft, motor vehicle gasolines performed as well as "High-Test" but in other types resulted in hot-running engines. It was also found that gasolines from aromatic and naphthenic base crude oils from California, South Texas and Venezuela resulted in smooth-running engines. These tests resulted in the first government specifications for motor gasolines (aviation gasolines used the same specifications as motor gasolines) in late 1917.

Between 1917 and 1919, the amount of thermally cracked gasoline utilized almost doubled. Also, the use of natural gasoline increased greatly. During this period, many U.S. states established specifications for motor gasoline but none of these agreed and were unsatisfactory from one standpoint or another. Larger oil refiners began to specify unsaturated material percentage (thermally cracked products caused gumming in both use and storage and unsaturated hydrocarbons are more reactive and tend to combine with impurities leading to gumming). In 1922, the U.S. government published the first specifications for aviation gasolines (two grades were designated as "Fighting" and "Domestic" and were governed by boiling points, color, sulphur content and a gum formation test) along with one "Motor" grade for automobiles. The gum test essentially eliminated thermally cracked gasoline from aviation usage and thus aviation gasolines reverted to fractionating straight-run naphthas or blending straight-run and highly treated thermally cracked naphthas. This situation persisted until 1929.

In the five-year period prior to 1929, a great amount of experimentation was conducted on different testing methods for determining fuel resistance to abnormal combustion. It appeared engine knocking was dependent on a wide variety of parameters including compression, cylinder temperature, air-cooled or water-cooled engines, chamber shapes, intake temperatures, lean or rich mixtures and others. This led to a confusing variety of test engines that gave conflicting results, and no standard rating scale existed. By 1929, it was recognized by most aviation gasoline manufacturers and users that some kind of antiknock rating must be included in government specifications. In 1929, the octane rating scale was adopted, and in 1930 the first octane specification for aviation fuels was established. In the same year, the U.S. Army Air Force specified fuels rated at 87 octane for its aircraft as a result of studies it conducted.

Even after the Nazis conquered the vast territories of Europe, this did not help the gasoline shortage. This area had never been self-sufficient in oil before the war. In 1938, the area that would become Nazi-occupied would produce 575,000 barrels per day. In 1940, total production under German control amounted to only 234,550 barrels—a shortfall of 59 percent. By the spring of 1941 and the depletion of German gasoline reserves, Adolf Hitler saw the invasion of Russia to seize the Polish oil fields and the Russian oil in the Caucasus as the solution to the German gasoline shortage. As early as July 1941, following the 22 June start of Operation Barbarossa, certain Luftwaffe squadrons were forced to curtail ground support missions due to shortages of aviation gasoline. On 9 October, the German quartermaster general estimated that army vehicles were 24,000 barrels short of gasoline requirements.

With the war in Europe in 1939 a reality, all predictions of 100-octane consumption were outrunning all possible production. Neither the Army nor the Navy could contract more than six months in advance for fuel and they could not supply the funds for plant expansion. Without a long term guaranteed market the petroleum industry would not risk its capital to expand production for a product that only the government would buy. The solution to the expansion of storage, transportation, finances and production was the creation of the Defense Supplies Corporation on 19 September 1940. The Defense Supplies Corporation would buy, transport and store all aviation gasoline for the Army and Navy at cost plus a carrying fee.

In the 1950s oil refineries started to focus on high octane fuels, and then detergents were added to gasoline to clean the jets in carburetors. The 1970s witnessed greater attention to the environmental consequences of burning gasoline. These considerations led to the phasing out of TEL and its replacement by other antiknock compounds. Subsequently, low-sulfur gasoline was introduced, in part to preserve the catalysts in modern exhaust systems.

The lower energy content of LPG by liquid volume in comparison to gasoline is due mainly to its lower density. This lower density is a property of the lower molecular weight of propane (LPG's chief component) compared to gasoline's blend of various hydrocarbon compounds with heavier molecular weights than propane. Conversely, LPG's energy content by weight is higher than gasoline's due to a higher hydrogen-to-carbon ratio.

Gasoline, when used in high-compression internal combustion engines, tends to autoignite or "detonate" causing damaging engine knocking (also called "pinging" or "pinking"). To address this problem, tetraethyllead (TEL) was widely adopted as an additive for gasoline in the 1920s. With the discovery of the seriousness of the extent of environmental and health damage caused by lead compounds, however, and the incompatibility of lead with catalytic converters, leaded gasoline was phased out in the United States beginning in 1973. By 1995, leaded fuel accounted for only 0.6 percent of total gasoline sales and under 2000 short tons (1814 t) of lead per year. From 1 January 1996, the U.S. Clean Air Act banned the sale of leaded fuel for use in on-road vehicles in the U.S. The use of TEL also necessitated other additives, such as dibromoethane.