Editing Gasoline (section)

==Physical properties==
[[File:GasStationHiroshima.jpg|thumb|A [[Royal Dutch Shell|Shell]] [[Gasoline station|station]] in [[Hiroshima]], [[Japan]]]]

===Density===
The [[specific gravity]] of gasoline ranges from 0.71 to 0.77,<ref>{{cite web |author=Bell Fuels |title=Lead-Free gasoline Material Safety Data Sheet |url=http://www.sefsc.noaa.gov/HTMLdocs/Gasoline.htm |url-status=dead |archive-url=https://web.archive.org/web/20020820074636/http://www.sefsc.noaa.gov/HTMLdocs/Gasoline.htm |archive-date=20 August 2002 |publisher=[[NOAA]]}}</ref> with higher densities having a greater volume fraction of aromatics.<ref>{{cite book |last=Demirel |first=Yaşar |url=https://books.google.com/books?id=TsY8gJP7b58C&pg=PA33 |title=Energy: Production, Conversion, Storage, Conservation, and Coupling |date=26 January 2012 |publisher=Springer Science & Business Media |isbn=978-1-4471-2371-2 |page=33 |access-date=31 March 2020 |archive-url=https://web.archive.org/web/20200728070621/https://books.google.com/books?id=TsY8gJP7b58C&pg=PA33 |archive-date=28 July 2020 |url-status=live}}</ref> Finished marketable gasoline is traded (in Europe) with a standard reference of {{Convert|0.755|kg/L|lb/U.S.gal|abbr=|sp=us}}, (7,5668 lb/ imp gal) its price is escalated or de-escalated according to its actual density.{{clarify|reason=Denser gasoline is more expensive or less expensive?|date=May 2019}} 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 <span class="anchor" id="Stability"></span>===
{{More citations needed|section|date=November 2022}}

Quality gasoline should be [[Shelf life|stable]] for six months if stored properly, but can degrade over time.<ref name=Stable>{{cite journal |title= It’s True: Gasoline Has an Expiration Date |first= Matt |last= Crisara |date= March 6, 2023 |journal= [[Popular Mechanics]] |url= https://www.popularmechanics.com/cars/a43168378/gas-has-an-expiration-date/ |access-date= March 14, 2025 |archive-url= https://web.archive.org/web/20250115025634/https://www.popularmechanics.com/cars/a43168378/gas-has-an-expiration-date/ |archive-date= January 15, 2025 |url-status= live |url-access= subscription}}</ref> Gasoline stored for a year will most likely be able to be burned in an internal combustion engine without too much trouble.<ref name=Stable/> <!-- UNSOURCED SINCE NOV 2022 However, 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 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).--> Gasoline should ideally be stored in an airtight container (to prevent [[oxidation]] or water vapor 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 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 (a hydrocarbon phase floating on top of a water-alcohol phase).<ref name=Stable/>

The presence of these degradation products in the fuel tank or fuel lines plus a carburetor or fuel injection components makes it harder to start the engine or causes reduced engine performance <ref>{{cite journal |first1=Florian |last1=Pradelle |first2=Sergio L. |last2=Braga |first3=Ana Rosa F. A. |last3=Martins |first4=Franck |last4=Turkovics |first5=Renata N. C. |last5=Pradelle |date=November 3, 2015 |title=Gum Formation in Gasoline and Its Blends: A Review |journal=Energy & Fuels |volume=29 |issue=12 |pages=7753–7770 |doi=10.1021/acs.energyfuels.5b01894 }}</ref> 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 [[Gasoline additive|other additives]]. Fuel stabilizers are commonly used for small engines, such as lawnmower and tractor engines, especially when their use is sporadic or seasonal (little 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.<ref name="Ullmann2" />

Gasoline stability requirements are set by the standard [[ASTM International|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:

: {{chem2|2 C8H18 + 25 O2 -> 16 CO2 + 18 H2O}}

By weight, combustion of gasoline releases about {{convert|46.7|MJ/kg|kWh/kg MJ/lb|lk=on}} or by volume {{convert|33.6|MJ/L|kWh/L MJ/U.S.gal BTU/U.S.gal|lk=in|sp=us}}, quoting the [[lower heating value]].<ref>{{cite web |title=Energy Information Administration |url=http://www.eia.gov/Energyexplained/?page=about_energy_units |url-status=live |archive-url=https://web.archive.org/web/20151215012732/http://www.eia.gov/Energyexplained/?page=about_energy_units |archive-date=15 December 2015 |website=www.eia.gov |df=dmy-all}}</ref> 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.<ref>{{cite web |title=Fuel Properties Comparison |url=http://www.afdc.energy.gov/fuels/fuel_comparison_chart.pdf |url-status=dead |archive-url=https://web.archive.org/web/20161031034323/http://www.afdc.energy.gov/fuels/fuel_comparison_chart.pdf |archive-date=31 October 2016 |access-date=31 October 2016 |website=Alternative Fuels Data Center |df=dmy-all}}</ref> On average, about {{Convert|74|l|USgal|sp=us}} of gasoline are available from a barrel of crude oil (about 46 percent by volume), varying with the quality of the crude and the grade of the gasoline. The remainder is products ranging from tar to [[naphtha]].<ref>{{cite web |title=Oil Industry Statistics from Gibson Consulting |url=http://www.gravmag.com/oil.html |url-status=live |archive-url=https://web.archive.org/web/20080912232920/http://www.gravmag.com/oil.html |archive-date=12 September 2008 |access-date=31 July 2008 |df=dmy-all}}</ref>

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 [[Engine tuning|tuned]] for [[Autogas|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 [[Heat engine|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 engine]]s, 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]] (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.

Molecular weights of the species in the representative octane combustion are 114, 32, 44, and 18 for C<sub>8</sub>H<sub>18</sub>, O<sub>2</sub>, CO<sub>2</sub>, and H<sub>2</sub>O, respectively; therefore {{Convert|1|kg|lb|spell=in}} of fuel reacts with {{Convert|3.51|kg|lb}} of oxygen to produce {{Convert|3.09|kg|lb}} of carbon dioxide and {{Convert|1.42|kg|lb}} of water.