Editing Gasoline (section)

===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.