Editing Gasoline (section)

==Octane rating==
{{main|Octane rating}}

[[Spark-ignition engine]]s are designed to burn gasoline in a controlled process called [[deflagration]]. However, the unburned mixture may autoignite by pressure and heat alone, rather than igniting from the [[spark plug]] at exactly the right time, causing a rapid pressure rise that 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 temperature|autoignition]], which is expressed by its octane rating. A detailed analysis further explores the conditions where premium fuel provides actual performance benefits versus when it is unnecessary.<ref>{{cite web |last=Gomez |first=Aly |title=Premium vs. Regular Gas: Are You Getting What You Pay For? |url=https://open.substack.com/pub/alygomez/p/premium-vs-regular-gas-are-you-getting |website=Substack |date=February 6, 2025 |access-date=February 8, 2025}}</ref>

Octane rating is measured relative to a mixture of [[2,2,4-Trimethylpentane|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 United Kingdom, over 95 percent of gasoline sold has 95 RON and is marketed as Unleaded or Premium Unleaded. Super Unleaded, with 97/98 RON and branded high-performance fuels (e.g., Shell V-Power, BP Ultimate) with 99 RON make up the balance. Gasoline with 102 RON may rarely be available for racing purposes.<ref>{{Cite web |title=Quality of petrol and diesel fuel used for road transport in the European Union (Reporting year 2013) |url=https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:52015DC0070&rid=1 |url-status=live |archive-url=https://web.archive.org/web/20210422161209/https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:52015DC0070&rid=1 |archive-date=22 April 2021 |access-date=July 31, 2020 |publisher=European Commission}}</ref><ref>{{Cite web |title=Types Of Car Fuel |url=https://www.simplemotoring.co.uk/car-fuel-types/ |url-status=live |archive-url=https://web.archive.org/web/20200925104546/https://www.simplemotoring.co.uk/car-fuel-types/ |archive-date=25 September 2020 |access-date=July 31, 2020}}</ref><ref>{{Cite web |title=Sunoco CFR Racing Fuel |url=https://aaoil.co.uk/product/sunoco-cfr-102-ron-90mon-2-2-oxygen/ |url-status=live |archive-url=https://web.archive.org/web/20201021192041/https://aaoil.co.uk/product/sunoco-cfr-102-ron-90mon-2-2-oxygen/ |archive-date=21 October 2020 |access-date=July 31, 2020}}</ref>

In the U.S., octane ratings in unleaded fuels vary between 85<ref>{{cite web |author=Ryan Lengerich Journal staff |title=85-octane warning labels not posted at many gasoline stations |url=http://rapidcityjournal.com/news/local/octane-warning-labels-not-posted-at-many-gas-stations/article_681e07bc-3cd3-5e0c-a3c7-c06fcc4d319c.html |url-status=live |archive-url=https://web.archive.org/web/20150615025518/http://rapidcityjournal.com/news/local/octane-warning-labels-not-posted-at-many-gas-stations/article_681e07bc-3cd3-5e0c-a3c7-c06fcc4d319c.html |archive-date=15 June 2015 |work=Rapid City Journal |date=17 July 2012 |df=dmy-all}}</ref> and 87 AKI (91–92 RON) for regular, 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).
{| class="wikitable" style="text-align:center;"
|
|91
|92
|93
|94
|95
|96
|97
|98
|99
|100
|101
|102
|-
|Scandinavian
| colspan="4" scope="row" |
|Regular
|
|
|Premium
|
|-
|UK
| colspan="4" scope="row" |
|Regular
|
| colspan="2" scope="row" |Premium
|Super
|
|
|High-performance
|-
|USA
| colspan="2" scope="row" |Regular
|
| colspan="2" scope="row" |Mid-grade
| colspan="4" scope="row" |Premium
|}
As South Africa's largest city, [[Johannesburg]], is located on the [[Highveld]] at {{convert|1753|m|ft|sp=us}} above sea level, the [[Automobile Association of South Africa]] recommends 95-octane gasoline 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".<ref>{{cite web |title=95/93 – What is the Difference, Really? |url=http://www.aa.co.za/about/press-room/press-releases/9593-what-is-the-difference-reallyij.html |url-status=dead |archive-url=https://web.archive.org/web/20161229112643/https://www.aa.co.za/about/press-room/press-releases/9593-what-is-the-difference-reallyij.html |archive-date=29 December 2016 |access-date=26 January 2017 |publisher=Automobile Association of South Africa (AA) |df=dmy-all }}</ref>

Octane rating became important as the military sought higher output for [[aircraft engine]]s in the late 1920s 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{{who|date=August 2018}} 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 {{Convert|1320|hp|kW|order=flip}} using 100 RON fuel from a modest {{Convert|27|l|cuin|sp=us}} displacement. By the time of [[Operation Overlord]], both the RAF and USAAF were conducting some operations in Europe using 150 RON fuel (100/150 [[avgas]]), obtained by adding 2.5 percent [[aniline]] to 100-octane avgas.<ref name="Magazines19362">{{cite journal |author=Hearst Magazines |date=April 1936 |title=Popular Mechanics |url=https://books.google.com/books?id=lNsDAAAAMBAJ&pg=PA524 |url-status=live |journal=Popular Mechanics |publisher=Hearst Magazines |pages=524– |issn=0032-4558 |archive-url=https://web.archive.org/web/20130619054026/http://books.google.com/books?id=lNsDAAAAMBAJ&pg=PA524 |archive-date=19 June 2013 |df=dmy-all}}</ref> By this time, the Rolls-Royce Merlin 66 was developing {{Convert|2000|hp|kW|order=flip}} using this fuel.