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Compression ratio
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===Petrol engines=== In [[petrol]] (gasoline) engines used in passenger cars for the past 20 years, compression ratios have typically been between 8:1 and 12:1. Several production engines have used higher compression ratios, including: * Cars built from 1955 to 1972 which were designed for [[octane rating|high-octane]] [[tetraethyllead|leaded gasoline]], which allowed compression ratios up to 13:1. * Some Mazda [[Skyactiv#Skyactiv-X|SkyActiv]] engines released since 2012 have compression ratios up to 16:1.<ref>{{cite magazine |title=2012 Mazda 3 gets 40-mpg SkyActiv engine option; diesel expected in 2014 |magazine=Autoweek |date=2011-04-22 |url=http://www.autoweek.com/article/20110422/NEWYORK/110429942 |access-date=2012-05-29 |url-status=dead |archive-url=https://web.archive.org/web/20120229064513/http://www.autoweek.com/article/20110422/NEWYORK/110429942 |archive-date=2012-02-29}}</ref><ref>[https://www.cnet.com/roadshow/news/mazdas-new-direct-injection-engine-family-arrives-in-2011/] {{webarchive|url=https://web.archive.org/web/20120312043501/http://reviews.cnet.com/8301-13746_7-20015698-48.html|date=March 12, 2012}}</ref><ref>{{cite web |date=August 2010 |last=Vanderwerp |first=Dave |title=Mazda Engine News: Mazda Sky Gas and Diesel Details |work=Car and Driver |url=https://www.caranddriver.com/news/a15129125/mazda-skyactiv-g-and-skyactiv-d-engines-news/ |access-date=2012-05-29}}</ref> The SkyActiv engine achieves this compression ratio with ordinary unleaded gasoline (95 RON in the United Kingdom) through improved scavenging of exhaust gases (which ensures cylinder temperature is as low as possible before the intake stroke), in addition to direct injection. * [[Toyota Dynamic Force engine]] has a compression ratio up to 14:1. * The 2014 [[Ferrari 458 Speciale]] also has a compression ratio of 14:1. When [[forced induction]] (e.g. a [[turbocharger]] or [[supercharger]]) is used, the compression ratio is often lower than [[naturally aspirated engine]]s. This is due to the turbocharger or supercharger already having compressed the air before it enters the cylinders. Engines using [[fuel injection#Multi-point injection|port fuel-injection]] typically run lower boost pressures and/or compression ratios than [[fuel injection#Direct injection systems|direct injected]] engines because port fuel injection causes the airโfuel mixture to be heated together, leading to detonation. Conversely, directly injected engines can run higher boost because heated air will not detonate without a fuel being present. Higher compression ratios can make gasoline (petrol) engines subject to [[engine knocking]] (also known as "detonation", "pre-ignition", or "pinging") if lower octane-rated fuel is used.<ref>{{cite journal |title=High Compression! |journal=Popular Science |date=January 1949 |volume=154 |pages=166โ172 |publisher=Bonnier Corporation |language=en |issn=0161-7370 |url=https://books.google.com/books?id=YyQDAAAAMBAJ&q=1949+Popular+Science+%22Popular+Science%22+first+flat+top+ever+designed&pg=PA166 |access-date=14 July 2019}}</ref> This can reduce efficiency or damage the engine if knock sensors are not present to modify the ignition timing.
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