Editing Compression ratio (section)

==Effect and typical ratios==
A high compression ratio is desirable <!-- Please explain, if it is desirable, why not all engines are designed to maximize it. Cost of premium fuel is one reason; what others, as those bearing on engine loads and the expense of higher-grade components to take higher compression ratios. --> because it allows an engine to extract more mechanical energy from a given mass of air–fuel mixture due to its higher [[thermal efficiency]]. This occurs because internal combustion engines are [[heat engine]]s, and higher compression ratios permit the same combustion temperature to be reached with less fuel, while giving a longer expansion cycle, creating more mechanical power output and lowering the exhaust temperature.

===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 air–fuel 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=166–172 |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.

===Diesel engines===
[[Diesel engine]]s use higher compression ratios than petrol engines, because the lack of a spark plug means that the compression ratio must increase the temperature of the air in the cylinder sufficiently to ignite the diesel using [[compression ignition]]. Compression ratios are often between 14:1 and 23:1 for direct injection diesel engines, and between 18:1 and 23:1 for [[indirect injection]] diesel engines.

At the lower end of 14:1, NOx emissions are reduced at a cost of more difficult cold-start.<ref>{{cite journal |last1=Pacaud |first1=P. |last2=Perrin |first2=H. |last3=Laget |first3=O. |date=2009 |title=Cold Start on Diesel Engine: Is Low Compression Ratio Compatible with Cold Start Requirements? |journal=SAE International Journal of Engines |volume=1 |issue=1 |pages=831–849 |doi=10.4271/2008-01-1310 |issn=1946-3936 |jstor=26308324}}</ref> Mazda's [[Skyactiv#Skyactiv-D|Skyactiv-D]], the first such commercial engine from 2013, used adaptive fuel injectors among other techniques to ease cold start.<ref name="Difference Engine: Born again">{{cite news |title=Difference Engine: Born again |date=2013-07-08 |newspaper=The Economist |issn=0013-0613 |url=https://www.economist.com/babbage/2013/07/08/difference-engine-born-again |access-date=2019-05-02}}</ref>

===Other fuels===
The compression ratio may be higher in engines running exclusively on [[liquefied petroleum gas]] (LPG or "propane autogas") or [[compressed natural gas]], due to the higher octane rating of these fuels.

[[Kerosene]] engines typically use a compression ratio of 6.5 or lower. The [[petrol-paraffin engine]] version of the [[Ferguson TE20]] tractor had a compression ratio of 4.5:1 for operation on [[tractor vaporising oil]] with an [[octane rating]] between 55 and 70.<ref>{{cite web |title=Tractor Vaporising Oil |date=2005-04-18 |url=http://tractorbits.com/infofiles/TVO.asp |access-date=2014-08-10 |url-status=usurped |archive-url=https://web.archive.org/web/20071012012756/http://tractorbits.com/infofiles/TVO.asp |archive-date=October 12, 2007}}</ref>

===Motorsport engines===
[[Motorsport]] engines often run on high-octane petrol and can therefore use higher compression ratios. For example, motorcycle racing engines can use compression ratios as high as 14.7:1, and it is common to find motorcycles with compression ratios above 12.0:1 designed for 95 or higher octane fuel.

Ethanol and methanol can take significantly higher compression ratios than gasoline. Racing engines burning [[methanol]] and [[ethanol fuel]] often have a compression ratio of 14:1 to 16:1.