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A Shell gasoline station in Hiroshima, Japan

Gasoline (American English), or petrol (British English), is a transparent, petroleum-derived 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-gallon barrel of crude oil (159 liters) yields about 19 gallons (72 liters) of gasoline when processed in an oil refinery, though this varies based on the crude oil assay.

The characteristic of a particular gasoline blend to resist igniting too early (which causes knocking and reduces efficiency in reciprocating engines) is measured by its octane rating. Gasoline is produced in several grades of octane rating. Tetraethyllead and other lead compounds are no longer used in most areas to increase octane-rating. Other chemicals are added to gasoline to improve chemical stability and performance characteristics, control corrosiveness and provide fuel system cleaning. Gasoline can contain oxygen containing chemicals such as ethanol, MTBE or ETBE to improve combustion.

Gasoline used in internal combustion engines has a significant effect on the environment, both in local effects (e.g., smog) and in global effects (e.g., effect on the climate). Gasoline may also enter the environment uncombusted, as liquid and as vapors, from leakage and handling during production, transport and delivery, 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.[1] Gasoline contains benzene and other known carcinogens.[2][3][4]


"Gasoline" is a North America word that refers to fuel for automobiles. The Oxford English Dictionary dates its first recorded use to 1863 when it was spelled "gasolene". The term "gasoline" was first used in North America in 1864.[5] The words is a derivation from the word "gas" and the chemical suffixes "-ol" and "-ine" or "-ene".[6]

However, the term may also have been influenced by the trademark "Cazeline" or "Gazeline". On 27 November 1862, the British publisher, coffee merchant, and social campaigner John Cassell placed an advertisement in The Times of London:

The Patent Cazeline Oil, safe, economical, and brilliant … possesses all the requisites which have so long been desired as a means of powerful artificial light.[7]

This is the earliest occurrence of the word to have been found. Cassell discovered that a shopkeeper in Dublin named Samuel Boyd was selling counterfeit cazeline and wrote to him to ask him to stop. Boyd did not reply and changed every ‘C’ into a ‘G’, thus coining the word "gazeline".[7]

"Petrol" is used in most Commonwealth countries. "Petrol" was first used as the name of a refined petroleum product around 1870 by British wholesaler Carless, Capel & Leonard, who marketed it as a solvent.[8] When the product later found a new use as a motor fuel, Frederick Simms, an associate of Gottlieb Daimler, suggested to Carless that they register the trade mark "petrol",[9] but by this time the word was already in general use, possibly inspired by the French pétrole,[6] and the registration was not allowed. Carless registered a number of alternative names for the product, but "petrol" became the common term for the fuel in the British Commonwealth.[10][11]

British refiners originally used "motor spirit" as a generic name for the automotive fuel and "aviation spirit" for aviation gasoline. When Carless was denied a trademark on "petrol" in the 1930s, its competitors switched to the more popular name "petrol". However, "motor spirit" had already made its way into laws and regulations, so the term remains in use as a formal name for petrol.[12][13] The term is used most widely in Nigeria, where the largest petroleum companies call their product "premium motor spirit".[14] Although "petrol" has made inroads into Nigerian English, "premium motor spirit" remains the formal name that is used in scientific publications, government reports, and newspapers.[15]

The use of the word gasoline instead of petrol outside North America can often be confusing. Shortening gasoline to gas, which happens often, causes confusion with various forms of gas used as car fuel (compressed natural gas (CNG), liquefied natural gas (LNG) and liquefied petroleum gas (LPG)). In many countries, gasoline has a colloquial name derived from that of the chemical benzene (e.g., German Benzin, Czech benzín, Dutch benzine, Italian benzina, Russian бензин benzin, Polish benzyna, Chilean Spanish bencina, Thai เบนซิน bensin, Greek βενζίνη venzini, Romanian benzină, Hebrew בנזין benzin, Swedish bensin, Arabic بنزين binzīn, Catalan benzina). Argentina, Uruguay and Paraguay use the colloquial name nafta derived from that of the chemical naphtha.[16]


The first automotive combustion engines, so-called Otto engines, were developed in the last quarter of the 19th century in Germany. The fuel was a relatively volatile hydrocarbon obtained from coal gas. With a boiling point near 85 °C (octanes boil about 40 °C higher), it was well suited for early carburetors (evaporators). The development of a "spray nozzle" carburetor enabled the use of less volatile fuels. Further improvements in engine efficiency were attempted at higher compression ratios, but early attempts were blocked by knocking (premature explosion of fuel).

United States, 1903 to 1917[edit]

During the early period of gasoline engine development aircraft were forced to use motor vehicle gasoline since aviation gasoline did not exist. These early fuels were termed straight run gasolines and were byproducts from the distillation of a single crude oil to produce kerosene which was the principal product sought for lighting in kerosene lamps. Gasoline production would not surpass kerosene production until 1916. The earliest straight run gasolines were the result of distilling eastern crude oils and there was no mixing of distillates from different crudes. The composition of these early fuels was unknown and the quality varied greatly as crude oils from different oil fields created different mixtures of hydrocarbons in different ratios. The engine effects produced by abnormal combustion (engine knocking and pre-ignition) due to inferior fuels had not yet been identified and as a result there was no rating of gasoline in terms of its resistance to abnormal combustion. The general specification of early gasolines was that of specific gravity via the Baumé scale and later the volatility (ability to vaporize) specified in terms of boiling points which would be the primary focus of the producers. These early eastern crude oil gasolines had relatively high Baumé test results (65 to 80 degrees Baumé) and were called Pennsylvania "High-Test" or simply "High-Test" gasolines and these would often be used in aircraft engines.

By 1910 increased automobile production and the resultant increased gasoline consumption combined with the growing electrification of lighting producing a drop in kerosene demand created a supply problem. It appeared that the oil industry would be trapped into over producing kerosene and under producing gasoline since simple distillation could not alter the ratio of the two products from any given crude. The solution appeared in 1911 when the Burton process created thermal cracking of crude oils which increased the percent yield of gasoline from the heavier hydrocarbons and this was combined with expansion of foreign markets for the export of surplus kerosene which the domestic market no longer needed. These new thermally "cracked" gasolines were believed to have no harmful effects and would be added to straight run gasolines. There also was the practice of mixing heavy and light distillates to achieve a desired Baumé reading and collectively these were called "blended" gasolines.[17] Gradually volatility gained favor over the Baumé test though both would be used in combination to specify a gasoline. As late as June, 1917 Standard Oil (the largest refiner of crude oil in the United States at this time) would state that the most important property of a gasoline was its volatility.[18] It is estimated that the rating equivalent of these straight run gasolines varied from 40 to 60 octane and that the "High-Test" (sometimes referred to as "fighting grade") probably averaged 50 to 65 octane.[19]

World War I[edit]

Prior to the American entry into World War I the European Allies were using fuels derived from crude oils from Borneo, Java and Sumatra which gave satisfactory performance in their military aircraft. With the United States entry in April, 1917, the U.S. became the principal supplier of aviation gasoline to the Allies and a decrease in engine performance was noted.[20] Soon it was realized that motor vehicle fuels were unsatisfactory for aviation and after the loss of a number of combat aircraft attention turned to the quality of the gasolines being used. Later flight tests conducted in 1937 showed that an octane reduction of 13 points (from 100 down to 87 octane) decreased engine performance by 20% and take-off distance was increased 45 percent.[21] If abnormal combustion were to occur the engine could lose enough power to make getting airborne impossible and a take-off roll became a threat to the pilot and aircraft. On August 2, 1917, the Bureau of Mines arranged to study fuels for aircraft in cooperation with the Aviation Section of the 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. Also, 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. [22]

United States, 1918 to 1929[edit]

Engine designers knew that according to the Otto cycle power and efficiency increased with compression ratio but experience with these early gasolines during WW I showed that higher compression ratios increased the risk of abnormal combustion producing lower power, lower efficiency, hot running engines, and could lead to severe engine damage. To compensate for these poor fuels early engines used low compression ratios and this required relatively large, heavy engines to produce limited power and efficiency. The Wright Brothers first engine used a compression ratio as low as 4.7 to one and developed only 12 horsepower from 201 cubic inches and weighed 180 pounds. [23] [24]. This was a major concern for aircraft designers and the needs of the aviation industry led the search for fuels that could be used in higher compression engines.

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 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 gummming in both use and storage. See Saturated and unsaturated compounds and unsaturated hydrocarbons are more reactive and tend to combine with impurities leading to gumming). In 1922 the 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 and thus aviation gasolines reverted back to fractionating straight-run naphthas or blending straight-run and highly treated thermally cracked naphthas. This situation persisted until 1929. [25]

The automobile industry reacted to the increase in thermally cracked gasoline with alarm. Thermal cracking produced large amounts of both mono- and diolefins (unsaturated hydrocarbons) which increased the risk of gumming.[26] Also the volatility was decreasing to the point that fuel did not vaporize and was sticking to spark plugs and fouling them, creating hard starting and rough running in winter and sticking to cylinder walls, bypassing the pistons and rings and going into the crankcase oil.[27] One journal stated, "...on a multi-cylinder engine in a high-priced car we are diluting the oil in the crankcase as much as 40 percent in a 200-mile run, as the analysis of the oil in the oil-pan shows."[28] Being very unhappy with the consequent reduction in overall gasoline quality the automobile manufacturers suggested imposing a quality standard on the oil suppliers. The oil industry accused the automakers of not doing enough to improve vehicle economy and this became known within the two industries as ‘The Fuel Problem’. Animosity grew between the industries, each accusing the other of not doing anything to resolve matters and relationships deteriorated. The situation was resolved when the American Petroleum Institute (API) initiated a conference to address ‘The Fuel Problem’ and a Cooperative Fuel Research (CFR) Committee was established in 1920 to oversee joint investigative programs and solutions. Apart from representatives of the two industries the Society of Automotive Engineers (SAE) also played an instrumental role with the American Bureau of Standards being chosen, as an impartial research organization, to carry out many of the studies. Initially all the programs were related to volatility and fuel consumption, ease of starting, crankcase oil dilution and acceleration.[29]

As early as 1917-1918, researchers such as Gibson, Ricardo, Midgely and Boyd began to investigate abnormal combustion and this led to the discovery in the 1920s of antiknock compounds, the most important being that of Thomas Midgley Jr. and Boyd, specifically tetraethyllead (TEL). This innovation started a cycle of improvements in fuel efficiency that coincided with the large-scale development of oil refining to provide more products in the boiling range of gasoline.

The Leaded Gasoline Controversy, 1924-1925[edit]

With the increased use of thermally cracked gasolines came an increased concern over its effects on abnormal combustion and this led to research for antiknock additives. Beginning in 1916 Charles F. Kettering began investigating additives based on two paths, the "high percentage" solution where large quantities of ethanol were added and the "low percentage" solution which led to the discovery of tetraethyllead (TEL) in December, 1921 where only 2-4 grams per gallon were needed. Ethanol could not be patented but TEL could so Kettering secured a patent for TEL and began promoting it instead of other options. The dangers of lead were well established by then and Kettering was directly warned by Robert Wilson of MIT, Reid Hunt of Harvard, Yandell Henderson of Yale, and Charles Kraus of the University of Pottsdam in Germany about its use. Kraus had worked on tetraethyl lead for many years and called it “a creeping and malicious poison” that had killed a member of his dissertation committee.[30] [31] On October 27, 1924 newspaper articles around the nation told of the workers at the Standard Oil refinery near Elizabeth, New Jersey who were producing TEL and were suffering from lead poisoning. By October 30, 1924 the death toll had reached five.[32] In November the New Jersey Labor Commission closed the Bayway refinery and a grand jury investigation was started which resulted in no charges by February, 1925. Leaded gasoline sales were banned in New York City, Philadelphia, and New Jersey. GM, DuPont, and Standard Oil who were parteners in Ethyl, the company created to produce TEL, began to argue that there were no alternatives to leaded gasoline. After flawed studies determined that TEL treated gasoline was not a public health issue, the controversy subsided.[33]

United States, 1930-1941[edit]

In the 5-year period prior to 1929, a great amount of experimentation was being conducted on different testing methods to determine fuel resistance to abnormal combustion. 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 more. 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 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 Army Air Force specified 87 octane for its aircraft as a result of studies it conducted.[34]

During this period research showed that hydrocarbon structure was extremely important to antiknocking properties of fuel. Straight-chain paraffins in the boiling range of gasoline had low antiknock qualities while ring-shaped molecules such as aromatic hydrocarbons (an example is Benzene) had higher resistance to knocking.[35] This development led to the search for processes that would produce more of these compounds from crude oils than achieved under straight distillation or thermal cracking. Research by the major refiners into conversion processes yielded isomerization, dehydration, and alkylation that would change the cheap and abundant butane into isooctane, which became an important component in aviation fuel blending. To further complicate the situation, as engine performance increased, the altitude that aircraft could reach increased which resulted in concerns about fuel freezing. The average temperature decrease is 3.6 degrees Fahrenheit per 1,000-ft increase in altitude and at 40,000-feet the temperature can approach -70 degrees. The presence of additives like benzene with a freezing point of 42 degrees would freeze in the gasoline and plug fuel lines. Substitute aromatics such as toluene, xylene, or cumene combined with limited benzene would solve the problem.[36]

By 1935 there were seven different aviation grades based on octane rating, two Army grades, four Navy grades and three commercial grades. By 1937 and the introduction of 100 octane gasoline, the confusion increased to 14 different grades in addition to 11 in foreign countries. With some companies required to stock 14 grades of aviation fuel, none of which could be interchanged, the effect on the refiners was negative. The refining industry could not concentrate on large capacity conversion processes for so many different grades and a solution had to be found. By 1941, principally through the efforts of the Cooperative Fuel Research Committee, the number of grades was reduced to three; 73, 91, and 100 octane. [37]

In 1937 Eugene Houdry developed the Houdry process of catalytic cracking which produced a high octane base stock of gasoline which was superior to the thermally cracked product since it did not contain the high concentration of olefins.[38] In 1940 there were only 14 Houdry units in operation in the U.S. By 1943 this had increased to 77, either of the Houdry process or of the Thermofor Catalytic or Fluid Catalyst type.[39]

WW II[edit]

- Germany[edit]

Oil and its byproducts, especially high octane aviation gasoline would be a driving concern for how Germany conducted the war. As a result of the lessons of WW I, Germany had stockpiled oil and gasoline for its Blitzkrieg offensive and had annexed Austria adding 18,000 barrels per day of oil production but this was not sufficient to sustain the planned conquest of Europe. Because captured supplies and oil fields would be necessary to fuel the campaign, the German high command had created a special squad of oil-field experts drawn from the ranks of domestic oil industries. They would be sent in to put out oil-field fires and get production going again as soon as possible. But capturing oil fields would be an obstacle throughout the war. In the Invasion of Poland German estimates of gasoline consumption turned out to be vastly underestimated. Heinz Guderian and his Panzer divisions consumed 1,000 gallons of gasoline per mile on the drive to Vienna. But when they were engaged in combat across open country, gasoline consumption almost doubled. On the second day of battle, a unit of the XIX Corps was forced to halt when it ran out of gasoline.[40] One of the major objectives of the Polish invasion was their oil fields but the Soviets invaded and captured 70% of the Polish poduction before the Germans could reach it. Through the German-Soviet Commercial Agreement (1940) Stalin agreed in vague terms to supply Germany with additional oil equal to that produced by now Soviet-occupied Polish oil fields at Drohobych and Boryslav in exchange for hard coal and steel tubing.

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 a day. In 1940, total production under German control amounted to only 234,550 barrels - a shortfall of 59%. [41] By the Spring of 1941 and the depletion of German gasoline reserves, 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, after the June 22, 1941 start of Operation Barbarossa, certain Luftwaffe squadrons were forced to curtail ground support missions due to shortages of aviation gasoline. On October 9, the German quartermaster general estimated that army vehicles were 24,000 barrels short of gasoline requirements.[42]

- United States[edit]

United States, 1946 to present[edit]

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.[43]

Octane rating[edit]

Spark ignition engines are designed to burn gasoline in a controlled process called deflagration. However, the unburned mixture may autoignite by detonating from pressure and heat alone, rather than ignite from the spark plug at exactly the right time. This causes a rapid pressure rise which can damage the engine. This is often referred to as engine knocking or end-gas knock. Knocking can be reduced by increasing the gasoline's resistance to autoignition, which is expressed by its octane rating.

Octane rating is measured relative to a mixture of 2,2,4-trimethylpentane (an isomer of octane) and n-heptane. There are different conventions for expressing octane ratings, so the same physical fuel may have several different octane ratings based on the measure used. One of the best known is the research octane number (RON).

The octane rating of typical commercially available gasoline varies by country. In Finland, Sweden, and Norway, 95 RON is the standard for regular unleaded gasoline and 98 RON is also available as a more expensive option. In the UK, ordinary regular unleaded gasoline is 95 RON (commonly available), premium unleaded gasoline is always 97 RON, and super unleaded is usually 97–98 RON.[citation needed] However, both Shell and BP produce fuel at 102 RON for cars with high-performance engines and in 2006 the supermarket chain Tesco began to sell super unleaded gasoline rated at 99 RON. In the US, octane ratings in unleaded fuels can vary between 85[44] and 87 AKI (91–92 RON) for regular, through 89–90 AKI (94–95 RON) for mid-grade (equivalent to European regular), up to 90–94 AKI (95–99 RON) for premium (European premium).

As South Africa's largest city, Johannesburg, is located on the Highveld at 1,753 metres (5,751 ft) above sea level, the South African AA recommends 95 octane gasoline (petrol) at low altitude and 93 octane for use in Johannesburg because "The higher the altitude the lower the air pressure, and the lower the need for a high octane fuel as there is no real performance gain".[45]

The octane rating became important as the military sought higher output for aircraft engines in the late 1930s and the 1940s. A higher octane rating allows a higher compression ratio or supercharger boost, and thus higher temperatures and pressures, which translate to higher power output. Some scientists even predicted that a nation with a good supply of high octane gasoline would have the advantage in air power. In 1943, the Rolls-Royce Merlin aero engine produced 1,320 horsepower (984 kW) using 100 RON fuel from a modest 27 liter displacement. By the time of Operation Overlord during World War II both the RAF and USAAF were conducting some operations in Europe using 150 RON fuel (100/150 avgas), obtained by adding 2.5% aniline to 100 octane avgas.[46] By this time the Rolls-Royce Merlin 66 was developing 2,000 hp using this fuel.


Quality gasoline should be stable for six months if stored properly but gasoline will break down slowly over time due to the separation of the components. Gasoline stored for a year will most likely be able to be burned in an internal combustion engine without too much trouble but the effects of long term storage will become more noticeable with each passing month until a time comes when the gasoline should be diluted with ever-increasing amounts of freshly made fuel so that the older gasoline may be used up. If left undiluted, improper operation will occur and this may include engine damage from misfiring and/or the lack of proper action of the fuel within a fuel injection system and from an onboard computer attempting to compensate (if applicable to the vehicle). Storage should be in an airtight container (to prevent oxidation or water vapors mixing in with the gas) that can withstand the vapor pressure of the gasoline without venting (to prevent the loss of the more volatile fractions) at a stable cool temperature (to reduce the excess pressure from liquid expansion, and to reduce the rate of any decomposition reactions). When gasoline is not stored correctly, gums and solids may be created, which can corrode system components and accumulate on wetted surfaces, resulting in a condition called “stale fuel”. Gasoline containing ethanol is especially subject to absorbing atmospheric moisture, then forming gums, solids, or two phases (a hydrocarbon phase floating on top of a water-alcohol phase).

The presence of these degradation products in the fuel tank, fuel lines plus a carburetor or fuel injection components makes it harder to start the engine or causes reduced engine performance. On resumption of regular engine use, the buildup may or may not be eventually cleaned out by the flow of fresh gasoline. The addition of a fuel stabilizer to gasoline can extend the life of fuel that is not or cannot be stored properly though removal of all fuel from a fuel system is the only real solution to the problem of long term storage of an engine or a machine or vehicle. Some typical fuel stabilizers are proprietary mixtures containing mineral spirits, isopropyl alcohol, 1,2,4-trimethylbenzene, or other additives. Fuel stabilizer is commonly used for small engines, such as lawnmower and tractor engines, especially when their use is seasonal (low to no use for one or more seasons of the year). Users have been advised to keep gasoline containers more than half full and properly capped to reduce air exposure, to avoid storage at high temperatures, to run an engine for ten minutes to circulate the stabilizer through all components prior to storage, and to run the engine at intervals to purge stale fuel from the carburetor.[43]

Gasoline stability requirements are set in standard ASTM D4814. The standard describes the various characteristics and requirements of automotive fuels for use over a wide range of operating conditions in ground vehicles equipped with spark-ignition engines.

Energy content[edit]

A gasoline-fueled internal combustion engine obtains energy from combustion of gasoline's various hydrocarbons with oxygen from the ambient air, yielding carbon dioxide and water exhaust. The combustion of octane, a representative species, performs the chemical reaction:

Gasoline contains about 46.7 MJ/kg (127 MJ/US gal, 35.3 kWh/US gal, 13.0 kWh/kg, 120,405 BTU/US gal), quoting the lower heating value.[47] Gasoline blends differ, and therefore actual energy content varies according to the season and producer by up to 1.75% more or less than the average.[48] On average, about 74 L of gasoline (19.5 US gal, 16.3 imp gal) are available from a barrel of crude oil (about 46% by volume), varying due to quality of crude and grade of gasoline. The remainder are products ranging from tar to naphtha.[49]

A high-octane-rated fuel, such as liquefied petroleum gas (LPG) has an overall lower power output at the typical 10:1 compression ratio of an engine design optimized for gasoline fuel. An engine tuned for LPG fuel via higher compression ratios (typically 12:1) improves the power output. This is because higher-octane fuels allow for a higher compression ratio without knocking, resulting in a higher cylinder temperature, which improves efficiency. Also, increased mechanical efficiency is created by a higher compression ratio through the concomitant higher expansion ratio on the power stroke, which is by far the greater effect. The higher expansion ratio extracts more work from the high-pressure gas created by the combustion process. An Atkinson cycle engine uses the timing of the valve events to produce the benefits of a high expansion ratio without the disadvantages, chiefly detonation, of a high compression ratio. A high expansion ratio is also one of the two key reasons for the efficiency of diesel engines, along with the elimination of pumping losses due to throttling of the intake air flow.

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 energy content by weight is higher than gasoline due to a higher hydrogen to carbon ratio.

Molecular weights of the representative octane combustion are C8H18 114, O2 32, CO2 44, H2O 18; therefore 1 kg of fuel reacts with 3.51 kg of oxygen to produce 3.09 kg of carbon dioxide and 1.42 kg of water.


The density of gasoline ranges from 0.71–0.77 kg/L (719.7 kg/m3 ; 0.026 lb/in3; 6.073 lb/US gal; 7.29 lb/imp gal), higher densities having a greater volume of aromatics.[50] Since gasoline floats on water, water cannot generally be used to extinguish a gasoline fire unless used in a fine mist. Finished marketable gasoline is traded with a standard reference of 0.755 kg/L, and its price is escalated/de-escalated according to its actual density.

Chemical analysis and production[edit]

Some of the main components of gasoline: isooctane, butane, 3-ethyltoluene, and the octane enhancer MTBE.
A pumpjack in the United States
An oil rig in the Gulf of Mexico

Gasoline is produced in oil refineries. Roughly 19 US gallons (72 L) of gasoline is derived from a 42-gallon (159 L) barrel of crude oil.[51] Material separated from crude oil via distillation, called virgin or straight-run gasoline, does not meet specifications for modern engines (particularly the octane rating, see below), but can be pooled to the gasoline blend.

The bulk of a typical gasoline consists of hydrocarbons with between 4 and 12 carbon atoms per molecule (commonly referred to as C4-C12).[43] It is a mixture of paraffins (alkanes), cycloalkanes (naphthenes), and olefins (alkenes), where the usage of the terms paraffin and olefin is particular to the oil industry. The actual ratio depends on:

  • the oil refinery that makes the gasoline, as not all refineries have the same set of processing units;
  • the crude oil feed used by the refinery;
  • the grade of gasoline, in particular, the octane rating.

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

  • straight-run gasoline, commonly referred to as naphtha, is distilled directly from crude oil. Once the leading source of fuel, its low octane rating required lead additives. It is low in aromatics (depending on the grade of crude oil), containing some cycloalkanes (naphthenes) and no olefins (alkenes). Between 0 and 20% of this stream is pooled into the finished gasoline, because the supply of this fraction is insufficient[clarification needed] and its RON is too low.[citation needed] The chemical properties (namely octane and RVP) of the straight-run gasoline can be improved through reforming and isomerisation. However, before feeding those units, the naphtha needs to be split in light and heavy naphtha. Straight-run gasoline can be also used as a feedstock into steam-crackers to produce olefins.
  • reformate, produced in a catalytic reformer has a high octane rating with high aromatic content, and relatively low olefins (alkenes). Most of the benzene, toluene, and xylene (the so-called BTX) are more valuable as chemical feedstocks and are thus removed to some extent.
  • catalytic cracked gasoline or catalytic cracked naphtha, produced from a catalytic cracker, with a moderate octane rating, high olefins (alkene) content, and moderate aromatics level.
  • hydrocrackate (heavy, mid, and light) produced from a hydrocracker, with medium to low octane rating and moderate aromatic levels.
  • alkylate is produced in an alkylation unit, using as feedstocks isobutane and alkenes. Alkylate contains no aromatics and alkenes and has high MON.
  • isomerate is obtained by isomerizing low octane straight run gasoline to iso-paraffins (non-chain alkanes, like isooctane). Isomerate has medium RON and MON, but nil aromatics and olefins.
  • butane is usually blended in the gasoline pool, although the quantity of this stream is limited by the RVP specification.

The terms above are the jargon used in the oil industry and terminology varies.

Currently, many countries set limits on gasoline aromatics in general, benzene in particular, and olefin (alkene) content. Such regulations led to increasing preference for high octane pure paraffin (alkane) components, such as alkylate, and is forcing refineries to add processing units to reduce benzene content. In the EU the benzene limit is set at 1% volume for all grade of automotive gasoline.

Gasoline can also contain other organic compounds, such as organic ethers (deliberately added), plus small levels of contaminants, in particular organosulfur compounds, but these are usually removed at the refinery.


Antiknock additives[edit]

A plastic container for storing gasoline used in Germany

Almost all countries in the world have phased out automotive leaded fuel. In 2011 six countries[52] were still using leaded gasoline: Afghanistan, Myanmar, North Korea, Algeria, Iraq and Yemen. It was expected that by the end of 2013 those countries would ban leaded gasoline,[53] but it has not occurred. Algeria will replace leaded with unleaded automotive fuel only in 2015.[clarification needed] Different additives have replaced the lead compounds. The most popular additives include aromatic hydrocarbons, ethers and alcohol (usually ethanol or methanol). For technical reasons the use of leaded additives is still permitted worldwide for the formulation of some grades of aviation gasoline such as 100LL, because the required octane rating would be technically infeasible to reach without the use of leaded additives.

A gas can


Gasoline, when used in high-compression internal combustion engines, tends to autoignite (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 extent of environmental and health damage caused by the lead, however, and the incompatibility of lead with catalytic converters, leaded gasoline was phased out in the USA beginning in 1973. By 1995, leaded fuel accounted for only 0.6% of total gasoline sales and under 2000 short tons (1814 t) of lead per year in the USA. From 1 January 1996, the U.S. Clean Air Act banned the sale of leaded fuel for use in on-road vehicles in the USA. The use of TEL also necessitated other additives, such as dibromoethane. First European countries started replacing lead by the end of the 1980s and by the end of the 1990s leaded gasoline was banned within the entire European Union. Reduction in the average blood lead level is believed to have been a major cause for falling violent crime rates in the United States[54] and South Africa.[55] A statistically significant correlation has been found between the usage rate of leaded gasoline and violent crime: taking into account a 22-year time lag, the violent crime curve virtually tracks the lead exposure curve.[56][57]

Lead replacement petrol (Gasoline)[edit]

Lead replacement petrol (LRP) was developed for vehicles designed to run on leaded fuel and incompatible with unleaded. Rather than tetraethyl lead it contains other metals such as potassium compounds or methylcyclopentadienyl manganese tricarbonyl (MMT); these are purported to buffer soft exhaust valves and seats so that they do not suffer recession due to the use of unleaded fuel.

LRP was marketed during and after the phaseout of leaded motor fuels in the United Kingdom, Australia, South Africa and some other countries.[vague] Consumer confusion led to widespread mistaken preference for LRP rather than unleaded,[58] and LRP was phased out 8 to 10 years after the introduction of unleaded.[59]

Leaded gasoline was withdrawn from sale in Britain after 31 December 1999, seven years after EEC regulations signalled the end of production for cars using leaded gasoline in member states. At this stage, a large percentage of cars from the 1980s and early 1990s which ran on leaded gasoline were still in use, along with cars which could run on unleaded fuel. However, the declining number of such cars on British roads saw many gasoline stations withdrawing LRP from sale by 2003.[60]


Methylcyclopentadienyl manganese tricarbonyl (MMT) is used in Canada and in Australia to boost octane.[61] It also helps old cars designed for leaded fuel run on unleaded fuel without need for additives to prevent valve problems.[citation needed] Its use in the US has been restricted by regulations.[62] Its use in the EU is restricted by Article 8a of the Fuel Quality Directive[63] following its testing under the Protocol for the evaluation of effects of metallic fuel-additives on the emissions performance of vehicles.[64]

Fuel stabilizers (antioxidants and metal deactivators)[edit]

Substituted phenols and derivatives of phenylenediamine are common antioxidants used to inhibit gum formation in gasoline (gasoline).

Gummy, sticky resin deposits result from oxidative degradation of gasoline upon long term storage. These harmful deposits arise from the oxidation of alkenes and other minor components in gasoline (see drying oils). Improvements in refinery techniques have generally reduced the susceptibility of gasolines to these problems. Previously, catalytically or thermally cracked gasolines are most susceptible to oxidation. The formation of these gums is accelerated by copper salts, which can be neutralized by additives called metal deactivators.

This degradation can be prevented through the addition of 5–100 ppm of antioxidants, such as phenylenediamines and other amines.[43] Hydrocarbons with a bromine number of 10 or above can be protected with the combination of unhindered or partially hindered phenols and oil-soluble strong amine bases, such as hindered phenols. "Stale" gasoline can be detected by a colorimetric enzymatic test for organic peroxides produced by oxidation of the gasoline.[65]

Gasolines are also treated with metal deactivators, which are compounds that sequester (deactivate) metal salts that otherwise accelerate the formation of gummy residues. The metal impurities might arise from the engine itself or as contaminants in the fuel.


Gasoline, as delivered at the pump, also contains additives to reduce internal engine carbon buildups, improve combustion, and to allow easier starting in cold climates. High levels of detergent can be found in Top Tier Detergent Gasolines. The specification for Top Tier Detergent gasolines was developed by four automakers: GM, Honda, Toyota and BMW. According to the bulletin, the minimal EPA requirement is not sufficient to keep engines clean.[66] Typical detergents include alkylamines and alkyl phosphates at the level of 50–100 ppm.[43]


European Union[edit]

In the EU, 5% ethanol can be added within the common gasoline spec (EN 228). Discussions are ongoing to allow 10% blending of ethanol (available in Finnish, French and German gas stations). In Finland most gasoline stations sell 95E10, which is 10% of ethanol; and 98E5, which is 5% ethanol. Most gasoline sold in Sweden has 5–15% ethanol added.


In Brazil, the Brazilian National Agency of Petroleum, Natural Gas and Biofuels (ANP) requires gasoline for automobile use to have 27.5% of ethanol added to its composition.[67] Pure hydrated ethanol is also available as a fuel.


Legislation requires retailers to label fuels containing ethanol on the dispenser, and limits ethanol use to 10% of gasoline in Australia. Such gasoline is commonly called E10 by major brands, and it is cheaper than regular unleaded gasoline.

United States[edit]

The federal Renewable Fuel Standard (RFS) effectively requires refiners and blenders to blend renewable biofuels (mostly ethanol) with gasoline, sufficient to meet a growing annual target of total gallons blended. Although the mandate does not require a specific percentage of ethanol, annual increases in the target combined with declining gasoline consumption has caused the typical ethanol content in gasoline to approach 10%. Most fuel pumps display a sticker that states that the fuel may contain up to 10% ethanol, an intentional disparity that reflects the varying actual percentage. Until late 2010, fuels retailers were only authorized to sell fuel containing up to 10 percent ethanol (E10), and most vehicle warranties (except for flexible fuel vehicles) authorize fuels that contain no more than 10 percent ethanol.[citation needed] In parts of the United States, ethanol is sometimes added to gasoline without an indication that it is a component.


The Government of India in October 2007 decided to make 5% ethanol blending (with gasoline) mandatory. Currently, 10% Ethanol blended product (E10) is being sold in various parts of the country.[68][69]

Ethanol has been found in at least one study to damage catalytic converters.[70]


In Australia, the lowest grade of gasoline (RON 91) is dyed a light shade of red/orange and the medium grade (RON 95) is dyed yellow.[71]

In the United States, aviation gasoline (avgas) is dyed to identify its octane rating and to distinguish it from kerosene-based jet fuel, which is clear.[72]

In Canada the gasoline for marine and farm use is dyed red and is not subject to sales tax. [73]

Oxygenate blending[edit]

Oxygenate blending adds oxygen-bearing compounds such as MTBE, ETBE, ethanol, and biobutanol. The presence of these oxygenates reduces the amount of carbon monoxide and unburned fuel in the exhaust gas. In many areas throughout the US, oxygenate blending is mandated by EPA regulations to reduce smog and other airborne pollutants. For example, in Southern California, fuel must contain 2% oxygen by weight, resulting in a mixture of 5.6% ethanol in gasoline. The resulting fuel is often known as reformulated gasoline (RFG) or oxygenated gasoline, or in the case of California, California reformulated gasoline. The federal requirement that RFG contain oxygen was dropped on 6 May 2006 because the industry had developed VOC-controlled RFG that did not need additional oxygen.[74]

MTBE was phased out in the US due to ground water contamination and the resulting regulations and lawsuits. Ethanol and, to a lesser extent, the ethanol-derived ETBE are common replacements. A common ethanol-gasoline mix of 10% ethanol mixed with gasoline is called gasohol or E10, and an ethanol-gasoline mix of 85% ethanol mixed with gasoline is called E85. The most extensive use of ethanol takes place in Brazil, where the ethanol is derived from sugarcane. In 2004, over 3.4 billion US gallons (2.8 billion imp gal/13 million m³) of ethanol was produced in the United States for fuel use, mostly from corn, and E85 is slowly becoming available in much of the United States, though many of the relatively few stations vending E85 are not open to the general public.[75]

The use of bioethanol, either directly or indirectly by conversion of such ethanol to bio-ETBE, is encouraged by the European Union Directive on the Promotion of the use of biofuels and other renewable fuels for transport. Since producing bioethanol from fermented sugars and starches involves distillation, though, ordinary people in much of Europe cannot legally ferment and distill their own bioethanol at present (unlike in the US, where getting a BATF distillation permit has been easy since the 1973 oil crisis).


HAZMAT class 3 gasoline

Environmental considerations[edit]

Combustion of 1 U.S. gallon (3.8 L) of gasoline produces 8.74 kilograms (19.3 lb) of carbon dioxide (2.3 kg/l), a greenhouse gas.[76][77]

The main concern with gasoline on the environment, aside from the complications of its extraction and refining, is the effect on the climate through the production of carbon dioxide [78]. Unburnt gasoline and evaporation from the tank, when in the atmosphere, reacts in sunlight to produce photochemical smog. Vapor pressure initially rises with some addition of ethanol to gasoline, but the increase is greatest at 10% by volume.[citation needed] At higher concentrations of ethanol above 10%, the vapor pressure of the blend starts to decrease. At a 10% ethanol by volume, the rise in vapor pressure may potentially increase the problem of photochemical smog. This rise in vapor pressure could be mitigated by increasing or decreasing the percentage of ethanol in the gasoline mixture.

The chief risks of such leaks come not from vehicles, but from gasoline delivery truck accidents and leaks from storage tanks. Because of this risk, most (underground) storage tanks now have extensive measures in place to detect and prevent any such leaks, such as monitoring systems (Veeder-Root, Franklin Fueling).

Production of gasoline consumes 0.63 gallons of water per mile driven.[79]


The safety data sheet for unleaded gasoline shows at least 15 hazardous chemicals occurring in various amounts, including benzene (up to 5% by volume), toluene (up to 35% by volume), naphthalene (up to 1% by volume), trimethylbenzene (up to 7% by volume), methyl tert-butyl ether (MTBE) (up to 18% by volume, in some states) and about ten others.[80] Hydrocarbons in gasoline generally exhibit low acute toxicities, with LD50 of 700–2700 mg/kg for simple aromatic compounds.[81] Benzene and many antiknocking additives are carcinogenic.

People can be exposed to gasoline in the workplace by swallowing it, breathing in vapors, skin contact, and eye contact. The National Institute for Occupational Safety and Health (NIOSH) has designated gasoline as a carcinogen.[82]

Inhalation for intoxication[edit]

Inhaled (huffed) gasoline vapor is a common intoxicant. Users concentrate and inhale gasoline vapour in a manner not intended by the manufacturer to produce euphoria and intoxication. Gasoline inhalation has become epidemic in some poorer communities and indigenous groups in Australia, Canada, New Zealand, and some Pacific Islands.[83] The practice is thought to cause severe organ damage, including mental retardation.[84][85][86]

In Canada, Native children in the isolated Northern Labrador community of Davis Inlet were the focus of national concern in 1993, when many were found to be sniffing gasoline. The Canadian and provincial Newfoundland and Labrador governments intervened on a number of occasions, sending many children away for treatment. Despite being moved to the new community of Natuashish in 2002, serious inhalant abuse problems have continued. Similar problems were reported in Sheshatshiu in 2000 and also in Pikangikum First Nation.[87] In 2012, the issue once again made the news media in Canada.[88]

Australia has long faced a petrol (gasoline) sniffing problem in isolated and impoverished aboriginal communities. Although some sources argue that sniffing was introduced by United States servicemen stationed in the nation's Top End during World War II[89] or through experimentation by 1940s-era Cobourg Peninsula sawmill workers,[90] other sources claim that inhalant abuse (such as glue inhalation) emerged in Australia in the late 1960s.[citation needed] Chronic, heavy petrol sniffing appears to occur among remote, impoverished indigenous communities, where the ready accessibility of petrol has helped to make it a common substance for abuse.

In Australia, petrol sniffing now occurs widely throughout remote Aboriginal communities in the Northern Territory, Western Australia, northern parts of South Australia and Queensland. The number of people sniffing petrol goes up and down over time as young people experiment or sniff occasionally. "Boss", or chronic, sniffers may move in and out of communities; they are often responsible for encouraging young people to take it up.[91] In 2005, the Government of Australia and BP Australia began the usage of opal fuel in remote areas prone to petrol sniffing.[92] Opal is a non-sniffable fuel (which is much less likely to cause a high) and has made a difference in some indigenous communities.


Uncontrolled burning of gasoline produces large quantities of soot and carbon monoxide.

Like other hydrocarbons, gasoline burns in a limited range of its vapor phase and, coupled with its volatility, this makes leaks highly dangerous when sources of ignition are present. Gasoline has a lower explosive limit of 1.4% by volume and an upper explosive limit of 7.6%. If the concentration is below 1.4%, the air-gasoline mixture is too lean and does not ignite. If the concentration is above 7.6%, the mixture is too rich and also does not ignite. However, gasoline vapor rapidly mixes and spreads with air, making unconstrained gasoline quickly flammable.

Use and pricing[edit]

The United States accounts for about 44% of the world’s gasoline consumption.[93] In 2003, the United States consumed 476 gigaliters (126 billion U.S. gallons; 105 billion imperial gallons),[94] which equates to 1.3 gigaliters (340 million U.S. gallons; 290 million imperial gallons) of gasoline each day. The United States used about 510 gigaliters (130 billion U.S. gallons; 110 billion imperial gallons) of gasoline in 2006, of which 5.6% was mid-grade and 9.5% was premium grade.[95]


Countries in Europe impose substantially higher taxes on fuels such as gasoline, when compared to the USA. The price of gasoline in Europe is typically higher than that in the US due to this difference.

United States[edit]

From 1998 to 2004, the price of gasoline fluctuated between US$1 and US$2 per U.S. gallon.[96] After 2004, the price increased until the average gas price reached a high of $4.11 per U.S. gallon in mid-2008, but receded to approximately $2.60 per U.S. gallon by September 2009.[96] More recently, the U.S. experienced an upswing in gasoline prices through 2011,[97] and by 1 March 2012, the national average was $3.74 per gallon.

In the United States, most consumer goods bear pre-tax prices, but gasoline prices are posted with taxes included. Taxes are added by federal, state, and local governments. As of 2009, the federal tax is 18.4¢ per gallon for gasoline and 24.4¢ per gallon for diesel (excluding red diesel).[98] Among states, the highest gasoline tax rates, including the federal taxes as of 2005, are New York (62.9¢/gal), Hawaii (60.1¢/gal), and California (60¢/gal).[97]

About 9% of all gasoline sold in the US in May 2009 was premium grade, according to the Energy Information Administration. Consumer Reports magazine says, "If [your owner’s manual] says to use regular fuel, do so—there's no advantage to a higher grade."[99] The Associated Press said premium gas—which is a higher octane and costs more per gallon than regular unleaded—should be used only if the manufacturer says it is "required".[100] Cars with turbocharged engines and high compression ratios often specify premium gas because higher octane fuels reduce the incidence of "knock", or fuel pre-detonation.[101] The price of gas varies during the summer and winter months.[102]

CO2 production[edit]

About 19.64 pounds (8.91 kg) of carbon dioxide (CO2) are produced from burning a (US) gallon (3.78l) of gasoline that does not contain ethanol (2.36 kg/l). About 22.38 pounds (10.15 kg) of CO2 are produced from burning a (US) gallon (3.78l) of diesel fuel (2.69 kg/l).[77]

The US EIA estimates that U.S. motor gasoline and diesel (distillate) fuel consumption for transportation in 2015 resulted in the emission of about 1,105 million metric tons of CO2 and 440 million metric tons of CO2, respectively, for a total of 1,545 million metric tons of CO2.[77] This total was equivalent to 83% of total U.S. transportation sector CO2 emissions and equivalent to 29% of total U.S. energy-related CO2 emissions in 2015.[77]

Most of the retail gasoline now sold in the United States contains about 10% fuel ethanol (or E10) by volume.[77] Burning a gallon of E10 produces about 17.68 pounds of CO2 that is emitted from the fossil fuel content. If the CO2 emissions from ethanol combustion are considered, then about 18.95 pounds of CO2 are produced when a gallon of E10 is combusted.[77] About 12.73 pounds of CO2 are produced when a gallon of pure ethanol is combusted.[77]

Comparison with other fuels[edit]

Volumetric and mass energy density of some fuels compared with gasoline (in the rows with gross and net, they are from[103]):

Fuel type[clarification needed] Gross MJ/l      MJ/kg Gross BTU/gal
Gross BTU/gal
Net BTU/gal (U.S.)     RON
Conventional gasoline 34.8 44.4[104] 150,100 125,000 115,400 91–92
Autogas (LPG) (Consisting mostly of C3 and C4 hydrocarbons) 26.8 46 95,640 108
Ethanol 21.2[104] 26.8[104] 101,600 84,600 75,700 108.7[105]
Methanol 17.9 19.9[104] 77,600 64,600 56,600 123
Butanol[2] 29.2 36.6 125,819 104,766 91–99[clarification needed]
Gasohol 31.2 145,200 120,900 112,400 93/94[clarification needed]
Diesel(*) 38.6 45.4 166,600 138,700 128,700 25
Biodiesel 33.3–35.7[106][clarification needed] 126,200 117,100
Avgas (high octane gasoline) 33.5 46.8 144,400 120,200 112,000
Jet fuel (kerosene based) 35.1 43.8 151,242 125,935
Jet fuel (naphtha) 127,500 118,700
Liquefied natural gas 25.3 ~55 109,000 90,800
Liquefied petroleum gas 46.1 91,300 83,500
Hydrogen 10.1 (at 20 kelvin) 142 130[107]

(*) Diesel fuel is not used in a gasoline engine, so its low octane rating is not an issue; the relevant metric for diesel engines is the cetane number

See also[edit]



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