Gasoline

Gasoline or petrol is a petrochemical product characterized as a transparent, yellowish and flammable liquid normally used as a fuel for spark-ignited internal combustion engines. When formulated as a fuel for engines, gasoline is chemically composed of organic compounds derived from the fractional distillation of petroleum and later chemically enhanced with gasoline additives. It is a high-volume profitable product produced in crude oil refineries.
The ability of a particular gasoline blend to resist premature ignition is measured by its octane rating or performance number. Tetraethyl lead was once widely used to increase the octane rating but is not used in modern automotive gasoline due to the health hazard. Aviation, off-road motor vehicles, and racing car engines still use leaded gasolines. Other substances are frequently added to gasoline to improve chemical stability and performance characteristics, control corrosion, and provide fuel system cleaning. Gasoline may contain oxygen-containing chemicals such as ethanol, MTBE, or ETBE to improve combustion.

History and etymology

English dictionaries show that the term gasoline originates from gas plus the chemical suffixes -ole and -ine. Petrol derives from the Medieval Latin word petroleum.
Interest in gasoline-like fuels started with the invention of internal combustion engines suitable for use in transportation applications. The so-called Otto engines were developed in Germany during the last quarter of the 19th century. The fuel for these early engines was a relatively volatile hydrocarbon obtained from coal gas. With a boiling point near , it was well suited for early carburettors. The development of a "spray nozzle" carburettor enabled the use of less volatile fuels. Further improvements in engine efficiency were attempted at higher compression ratios, but early attempts were blocked by the premature explosion of fuel, known as knocking. In 1891, the Shukhov cracking process became the world's first commercial method to break down heavier hydrocarbons in crude oil to increase the percentage of lighter products compared to simple distillation.

Chemical analysis and production

Commercial gasoline, as well as other liquid transportation fuels, are complex mixtures of hydrocarbons. The performance specification also varies with season, requiring less volatile blends during summer, in order to minimize evaporative losses.
Gasoline is produced in oil refineries. Roughly of gasoline is derived from a barrel of crude oil. Material separated from crude oil via distillation, called virgin or straight-run gasoline, does not meet specifications for modern engines, but can be pooled to the gasoline blend.
The bulk of a typical gasoline consists of a homogeneous mixture of hydrocarbons with between four and twelve carbon atoms per molecule. It is a mixture of paraffins, olefins, naphthenes, and aromatics. The use of the term paraffin in place of the standard chemical nomenclature alkane is particular to the oil industry. The composition of a gasoline depends upon:

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 sought.

The various refinery streams blended to make gasoline have different characteristics. Some important streams include the following:

Straight-run gasoline, sometimes referred to as naphtha , is distilled directly from crude oil. Once the leading source of fuel, naphtha's low octane rating required organometallic fuel additives prior to their phaseout from the gasoline pool which started in 1975 in the United States. Straight run naphtha is typically low in aromatics and contains some cycloalkanes and no olefins. Between 0 and 20 percent of this stream is pooled into the finished gasoline because the quantity of this fraction in the crude is less than fuel demand and the fraction's Research Octane Number is too low. The chemical properties of the straight-run gasoline can be improved through reforming and isomerization. However, before feeding those units, the naphtha needs to be split into light and heavy naphtha. Straight-run gasoline can also be used as a feedstock for steam crackers to produce olefins.
Reformate, produced from straight run gasoline in a catalytic reformer, has a high octane rating with high aromatic content and relatively low olefin content. Most of the benzene, toluene, and xylene are more valuable as chemical feedstocks and are thus removed to some extent. Also the BTX content is regulated.
Catalytic cracked gasoline, or catalytic cracked naphtha, produced with a catalytic cracker, has a moderate octane rating, high olefin content, and moderate aromatic content.
Hydrocrackate, produced with a hydrocracker, has a medium to low octane rating and moderate aromatic levels.
Alkylate is produced in an alkylation unit, using isobutane and C3-/C4-olefins as feedstocks. Finished alkylate contains no aromatics or olefins and has a high MON. Alkylate was used during World WarII in aviation fuel. Since the late 1980s, it is sold as a specialty fuel for gardening and forestry tools with a combustion engine.
Isomerate is obtained by isomerizing low-octane straight-run gasoline into iso-paraffins. Isomerate has a medium RON and MON, but no aromatics or olefins.
Butane is usually blended in the gasoline pool, although the quantity of this stream is limited by the RVP specification.
Oxygenates are mostly blended into the pool in the US as ethanol. In Europe and other countries, the blends can contain ethanol in addition to Methyl tertiary-butyl ether and Ethyl tert-butyl ether. MTBE in the United States was banned by most states in the early-to-mid-2000s. A few countries still allow methanol as well to be blended directly into gasoline, especially in China. More about oxygenates and blending is covered further in this article.

The terms above are the jargon used in the oil industry, and the terminology varies.
Currently, many countries set limits on gasoline aromatics in general, benzene in particular, and olefin content. Such regulations have led to an increasing preference for alkane isomers, such as isomerate or alkylate, as their octane rating is higher than n-alkanes. In the European Union, the benzene limit is set at one percent by volume for all grades of automotive gasoline. This is usually achieved by avoiding feeding C6, in particular cyclohexane, to the reformer unit, where it would be converted to benzene. Therefore, only heavy virgin naphtha is fed to the reformer unit.
Gasoline can also contain other organic compounds, such as organic ethers, plus small levels of contaminants, in particular organosulfur compounds.
On average, U.S. petroleum refineries produce about 19 to 20 gallons of gasoline, 11 to 13 gallons of distillate fuel diesel fuel and 3 to 4 gallons of jet fuel from each barrel of crude oil. The product ratio depends upon the processing in an oil refinery and the crude oil assay.

Physical properties

Density

The specific gravity of gasoline ranges from 0.71 to 0.77, with higher densities having a greater volume fraction of aromatics. Finished marketable gasoline is traded with a standard reference of,. Its price is escalated or de-escalated according to its actual density. Because of its low density, gasoline floats on water, and therefore water cannot generally be used to extinguish a gasoline fire unless applied in a fine mist.

Stability

Quality gasoline should be stable for six months if stored properly, but can degrade over time. Gasoline stored for a year will most likely be able to be burned in an internal combustion engine without too much trouble. Gasoline should ideally be stored in an airtight container that can withstand the vapor pressure of the gasoline without venting at a stable cool temperature. When gasoline is not stored correctly, gums and solids may result, which can corrode system components and accumulate on wet 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.
The presence of these degradation products in the fuel tank or fuel lines plus a carburettor 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. Typical fuel stabilizers are proprietary mixtures containing mineral spirits, isopropyl alcohol, 1,2,4-trimethylbenzene or other additives. Fuel stabilizers are commonly used for small engines, such as lawnmower and tractor engines, especially when their use is sporadic or seasonal. 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 carburettor.
Gasoline stability requirements are set by the standard ASTM D4814. This 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.

Combustion energy content

A gasoline-fueled internal combustion engine obtains energy from the combustion of gasoline's various hydrocarbons with oxygen from the ambient air, yielding carbon dioxide and water as exhaust. The combustion of octane, a representative species, performs the chemical reaction:
By weight, combustion of gasoline releases about or by volume, quoting the lower heating value. Gasoline blends differ, and therefore actual energy content varies according to the season and producer by up to 1.75 percent more or less than the average. On average, about of gasoline are available from a barrel of crude oil, varying with the quality of the crude and the grade of the gasoline. The remainder is products ranging from tar to naphtha.
A high-octane-rated fuel, such as liquefied petroleum gas, 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 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 airflow.
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 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.
Molecular weights of the species in the representative octane combustion are 114, 32, 44, and 18 for C₈H₁₈, O₂, CO₂, and H₂O, respectively; therefore of fuel reacts with of oxygen to produce of carbon dioxide and of water.