A jar of gasoline

Gasoline or petrol  is a clear but slightly yellowish petroleum-derived liquid mixture which is primarily used as a fuel in internal combustion engines. It is also used as a solvent, mainly known for its ability to dilute paints. It consists mostly of aliphatic hydrocarbons obtained by the fractional distillation ofpetroleum, enhanced with iso-octane or the aromatic hydrocarbons toluene and benzene to increase its octane rating. Small quantities of various additives are common, for purposes such as tuning engine performance or reducing harmful exhaust emissions. Some mixtures also contain significant quantities of ethanol as a partialalternative fuel. Most current or former Commonwealth countries (excluding Canada) use the term petrol. In North America, the substance is called gasoline, a term often shortened in colloquial usage to gas. It is not a genuinely gaseous fuel (unlike, for example, liquefied petroleum gas, which is stored under pressure as a liquid, but returned to a gaseous state before combustion). The term petrogasoline is also used.

Old gasoline pumps, Norway

The term mogas, short for motor gasoline, is used to distinguish automobile fuel from aviation gasoline, or avgas. In British English, gasoline can refer to a different petroleum derivative historically used in lamps, but this usage is relatively uncommon.

Etymology

A gasoline can (which are typically red) from Midwest Can Company

"Gasoline" is cited (under the spelling "gasolene") from 1865 in the Oxford English Dictionary. The trademark Gasoline was never registered, and eventually became generic in North America and the Philippines.

The word "petrol" has been used in English to refer to raw petroleum since the sixteenth century. However, it was first used to refer to the refined fuel in 1892, when it was registered as a trade name by British wholesaler Carless, Capel & Leonard at the suggestion of Frederick Richard Simms, as a contraction of 'St. Peter's Oil.'  Carless's competitors used the term "motor spirit" until the 1930s. The Oxford English Dictionary suggests that this usage may have been inspired by the French pétrole.

In many countries, gasoline has a colloquial name derived from that of the chemical benzene (e.g., German Benzin, Dutch Benzine). In other countries, especially in those portions of Latin America where Spanish predominates (i.e., most of the region except Brazil), it has a colloquial name derived from that of the chemical naphtha (e.g., Argentine/Uruguaian/Paraguaian nafta). However the standard Spanish word is 'gasolina'.

Chemical analysis and production

Refinery Minatitlán, Mexico

A United States pumpjack

An oil rig in the Gulf of Mexico

Gasoline is produced in oil refineries. Material that is separated from crude oil via distillation, called virgin or straight-run gasoline, does not meet the required specifications for modern engines (in particular octane rating; see below), but will form part of the 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).

Many of the hydrocarbons are considered hazardous substances and are regulated in the United States by the Occupational Safety and Health Administration. The material safety data sheet for unleaded gasoline shows at least fifteen 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.

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

• 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.
• 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 grade of 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 isomerizing the pentane and hexane in light virgin naphthas to yield their higher octane isomers.

The terms above are the jargons used in the oil industry. The exact terminology for these streams varies by refinery and by country.

Overall, a typical gasoline is predominantly a mixture of paraffins (alkanes), naphthenes (cycloalkanes), and olefins (alkenes). The actual ratio depends on:

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

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.

Gasoline can also contain other organic compounds such as organic ethers (deliberately added), plus small levels of contaminants, in particular sulfur compounds such asdisulfides and thiophenes. Some contaminants, in particular thiols and hydrogen sulfide, must be removed because they cause corrosion in engines. Sulfur compounds are usually removed by hydrotreating, yielding hydrogen sulfide, which can then be transformed into elemental sulfur via the Claus process.

Density

The specific gravity (or relative density) of gasoline ranges from 0.71–0.77 (0.026 lb/in³; 719.7 kg/m³; 6.073 lb/US gal; 7.29 lb/imp gal), higher densities having a greater volume of aromatics. Gasoline floats on water; water cannot generally be used to extinguish a gasoline fire, unless used in a fine mist.

Volatility

A plastic container for storing gasoline used in Germany

Gasoline is more volatile than diesel oil, Jet-A or kerosene, not only because of the base constituents, but because of the additives that are put into it. The final control of volatilityis often achieved by blending with butane. The Reid Vapor Pressure (RVP) test is used to measure the volatility of gasoline. The desired volatility depends on the ambient temperature. In hot weather, gasoline components of higher molecular weight and thus lower volatility are used. In cold weather, too little volatility results in cars failing to start.

In hot weather, excessive volatility results in what is known as "vapor lock", where combustion fails to occur, because the liquid fuel has changed to a gaseous fuel in the fuel lines, rendering the fuel pump ineffective and starving the engine of fuel. This effect mainly applies to camshaft-driven (engine mounted) fuel pumps which lack a fuel return line. Vehicles with fuel injection require the fuel to be pressurized, to within a set range. Because camshaft speed is nearly zero before the engine is started, an electric pump is used. It is located in the fuel tank so that the fuel may also cool the high-pressure pump. Pressure regulation is achieved by returning unused fuel to the tank. Therefore, vapor lock is almost never a problem in a vehicle with fuel injection.

In the United States, volatility is regulated in large cities to reduce the emission of unburned hydrocarbons. In large cities, so-called reformulated gasoline that is less prone to evaporation, among other properties, is required. In Australia, summer petrol volatility limits are set by state governments and vary among states. Most countries simply have a summer, winter, and perhaps intermediate limit.

Volatility standards may be relaxed (allowing more gasoline components into the atmosphere) during gasoline shortages. For example, on 31 August 2005, in response to Hurricane Katrina, the United States permitted the sale of non-reformulated gasoline in some urban areas, effectively permitting an early switch from summer to winter-grade gasoline. As mandated by EPA administrator Stephen L. Johnson, this "fuel waiver" was made effective until 15 September 2005.

Modern automobiles are also equipped with an evaporative emissions control system (called an EVAP system in automotive jargon), which collects evaporated fuel from the fuel tank in a charcoal-filled canister while the engine is stopped and then releases the collected vapors into the engine intake for burning when the engine is running (usually only after it has reached normal operating temperature.) The evaporative emissions control system also includes a sealed gas cap to prevent vapors from escaping via the fuel filler tube. Modern vehicles with OBD-II emissions control systems will illuminate the malfunction indicator light, (MIL) "check engine" or “Service Engine Soon” light if the leak detection pump (LDP) detects a leak in the EVAP system. If the Electronic Control Unit (ECU) or Powertrain Control Module (PCM) detects a leak, it will store an OBD-II code representing either a small or large leak, thus illuminating the MIL to indicate a failure. Some vehicles can detect whether the gas cap is incorrectly fitted, and will indicate this by illuminating a gas cap symbol on the dash.

Octane rating

Internal combustion engines are designed to burn gasoline in a controlled process called deflagration. But in some cases, gasoline can also combust abnormally by detonation, which wastes energy and can damage the engine. One way to reduce detonation is to increase 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 a fuel may have several different octane ratings based on the measure used.

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, and thus higher temperatures and pressures, which translate to higher power output.

World War II and octane ratings

During World War II, Germany received much of its oil from Romania. From 2.8 million barrels (450×10³ m³) in 1938, Romania’s exports to Germany increased to 13 million barrels (2.1×10⁶ m³) by 1941, a level that was essentially maintained through 1942 and 1943, before dropping by half, due to Allied bombing and mining of the Danube. Although these exports were almost half of Romania’s total production, they were considerably less than the Germans had expected. Even with the addition of the Romanian deliveries, oil imports over land after 1939 could not make up for the loss of overseas shipments. To become less dependent on outside sources, the Germans undertook a sizable expansion program of their own meager domestic oil pumping. After 1938, they had access to the Austrian oil fields, and the expansion of Nazi crude oil output was chiefly concentrated there. Primarily as a result of this expansion, the Reich's domestic output of crude oil increased from approximately 3.8 million barrels (600×10³ m³) in 1938 to almost 12 million barrels (1.9×10⁶ m³) in 1944, but even that output was not sufficient to meet all the needs of the Nazi military.

Instead, Germany had developed a synthetic fuel capacity that was intended to replace imported or captured oil. Fuel was generated from coal, using either the Bergius process or the Fischer-Tropsch process. Between 1938 and 1943, synthetic fuel output underwent a respectable growth from 10,000,000 barrels (1,600,000 m³) to 36,000,000 barrels (5,700,000 m³). The percentage of synthetic fuels compared with the yield from all sources grew from 22% to more than 50% by 1943. The total oil supplies available from all sources for the same period rose from 45 million barrels (7.2×10⁶ m³) in 1938 to 71 million barrels (11.3×10⁶ m³) in 1943.

By the early 1930s, automobile gasoline had an octane rating of 40 and aviation gasoline a rating of 75-80. Aviation gasoline with such high octane numbers could only be refined through a process of distillation of high-grade petroleum. Germany’s domestic oil was not of this quality. Only the additive tetra-ethyl lead could raise the octane to a maximum of 87. The license for the production of this additive was acquired in 1935 from the American holder of the patents, but without high-grade Romanian oil even this additive was not very effective. 100 octane fuel, designated either 'C-2' (natural) or 'C-3' (synthethic) was introduced in late 1939 with the Daimler-Benz DB 601N engine, used in certain of the Luftwaffe's Bf 109E and Bf 109F single-engined fighters, Bf 110C twin-engined fighters, and several bomber types. Some later combat types, most notably the BMW 801D-powered Fw 190A, F and G series, and later war Bf 109G and K models, used C-3 as well. The nominally 87 octane aviation fuel designated 'B-4' was produced in parallel during the war.

In the United States the oil was not "as good", and the oil industry had to invest heavily in various expensive boosting systems. This turned out to have benefits: the US industry started delivering fuels of increasing octane ratings by adding more of the boosting agents, and the infrastructure was in place for a post-war octane-agents additive industry. Good crude oil was no longer a factor during wartime, and by war's end American aviation fuel was commonly 130 octane, and 150 octane was available in limited quantities for fighters from mid-1944. This high octane could easily be used in existing engines to deliver much more power by increasing the pressure delivered by the superchargers.

In late 1942, the Germans increased the octane rating of their high-grade 'C-3' aviation fuel to 150 octane. The relative volumes of production of the two grades B-4 and C-3 cannot be accurately given, but in the last war years perhaps two-thirds of the total was C-3. Every effort was being made toward the end of the war to increase isoparaffin production; more isoparaffin meant more C-3 available for fighter plane use.

A common misconception exists concerning wartime fuel octane numbers. There are two octane numbers for each fuel, one for lean mix and one for rich mix, rich being greater. The misunderstanding that German fuels had a lower octane number (and thus a poorer quality) arose because the Germans quoted the lean mix octane number for their fuels while the Allies quoted the rich mix number. Standard German high-grade 'C-3' aviation fuel used in the later part of the war had lean/rich octane numbers of 100/130. The Germans listed this as a 100 octane fuel, the Allies as 130 octane.

After the war, the US Navy sent a technical mission to Germany to interview German petrochemists and examine German fuel quality. Its report entitled “Technical Report 145-45 Manufacture of Aviation Gasoline in Germany” chemically analyzed the different fuels, and concluded that “Toward the end of the war the quality of fuel being used by the German fighter planes was quite similar to that being used by the Allies.”

Energy content (High and low heating value)

A plastic container used widely for storing gasoline

Gasoline contains about 35 MJ/L (9.7 kW·h/L, 132 MJ/US gal, 36.6 kWh/US gal) (higher heating value) or 13 kWh/kg. This is an average; gasoline blends differ, and therefore actual energy content varies from season to season and from batch to batch, by up to 4% more or less than the average, according to the US EPA. On average, about 19.5 US gallons (16.2 imp gal; 74 L) of gasoline are available from a 42-US-gallon (35 imp gal; 160 L) barrel of crude oil (about 46% by volume), varying due to quality of crude and grade of gasoline. The remaining residue comes off as products ranging from tar to naptha.

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

Fuel type	Gross MJ/L	MJ/kg	GrossBTU/gal (imp)	Gross BTU/gal (U.S.)	Net BTU/gal (U.S.)	RON
Conventional gasoline	34.8	44.4	150,100	125,000	115,400	91-92
Autogas (LPG) (60% Propane + 40%Butane)	26.8	46				108
Ethanol	21.2	26.8	101,600	84,600	75,700	108.7
Methanol	17.9	19.9	77,600	64,600	56,600	123
Butanol	29.2	36.6				91-99
Gasohol	31.2		145,200	120,900	112,400	93/94
Diesel	38.6	45.4	166,600	138,700	128,700	25
Biodiesel	33.3-35.7			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				91,300	83,500
Hydrogen	10.1 (at 20 kelvin)	142			130

 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

A high octane fuel such as liquefied petroleum gas (LPG) has a lower energy content than lower octane gasoline, resulting in an overall lower power output at the regular compression ratio an engine ran at on gasoline. However, with an engine tuned to the use of LPG (i.e. via higher compression ratios such as 12:1 instead of 8:1), this lower power output can be overcome. This is because higher-octane fuels allow for a higher compression ratio—this means less space in a cylinder on its combustion stroke, hence a higher cylinder temperature which improves efficiency according to Carnot's theorem, along with fewer wasted hydrocarbons (therefore less pollution and wasted energy), bringing higher power levels coupled with less pollution overall because of the greater combustion efficiency. Also, increased mechanical efficiency is created by a higher compression ratio through the concommitant 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. The applicable formula is PV=nRT. 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 throtttling of the intake air flow. A high compression ratio can be viewed as a necessary evil in order to have a high expansion ratio.

The lower energy content (per litre) of LPG in comparison to gasoline is due mainly to its lower density. Energy content per kilogram is higher than for gasoline (higher hydrogen to carbon ratio). The weight-density of gasoline is about 740 kg/m³ (6.175 lb/US gal; 7.416 lb/imp gal).

Different countries have some variation in what RON (research octane number) is standard for gasoline, or petrol. In Finland, Sweden and Norway, 95 RON is the standard for regular unleaded petrol and 98 RON is also available as a more expensive option. In the UK, ordinary regular unleaded petrol is 91 RON (not commonly available), premium unleaded petrol is always 95 RON, and super unleaded is usually 97-98 RON. However both Shell and BP produce fuel at 102 RON for cars with hi-performance engines, and the supermarket chain Tesco began in 2006 to sell super unleaded petrol rated at 99 RON. In the US, octane ratings in unleaded fuels can vary between 86-87 AKI (91-92 RON) for regular, through 89-90 AKI (94-95 RON) for mid-grade (European Premium), up to 90-94 AKI (95-99 RON) for premium (European Super).