Compression ratio: Difference between revisions

[[File:4StrokeEngine Ortho 3D Small.gif|thumb|right|225px|In piston engines, static compression ratio is determined using the cylinder volume when the piston is at the top and bottom of its travel.]]

The '''compression ratio''' is the ratio between the maximum and minimum volume during the compression stage of the power cycle in a [[reciprocating engine|piston]] or [[Wankel engine]]. A fundamental specification for such engines, it can be measured in two different ways. The simpler way is the '''static compression ratio''': in a [[reciprocating engine]], this is the ratio of the volume of the cylinder when the piston is at the [[bottom dead center|bottom of its stroke]] to that volume when the piston is at the [[top dead center|top of its stroke]].<ref>{{citation

</ref> The '''dynamic compression ratio''' is a more advanced calculation which also takes into account gases entering and exiting the cylinder during the compression phase.<ref>{{Cite book |title=Marks' standard handbook for mechanical engineers |date=2018 |publisher=McGraw-Hill Education |isbn=978-1-259-58850-1 |editor-last=Sadegh |editor-first=Ali M. |edition=Twelfth |location=New York, NY |chapter=Internal Combustion Engines: General Features |editor-last2=Worek |editor-first2=William M.}}</ref>  

A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given amount of fuel due to its higher [[thermal efficiency]].<ref>{{cite journal |last1=Caton |first1=Jerald A. |date=2018 |title=Maximum efficiencies for internal combustion engines: Thermodynamic limitations |journal=International Journal of Engine Research |volume=19 |issue=10 |pages=1005-1023 |doi=10.1177/1468087417737700}}</ref> This occurs because internal combustion engines are heat engines, and higher compression ratios allow more energy to be converted into useful work from the same amount of fuel, while giving a longer expansion cycle, creating more mechanical power output and potentially lowering exhaust gas temperatures.<ref>{{cite conference |last1=Caton |first1=Jerald A. |date=2007 |title=The Effects of Compression Ratio and Expansion Ratio on Engine Performance Including the Second Law of Thermodynamics: Results From a Cycle Simulation |conference=ASME 2007 Internal Combustion Engine Division Fall Technical Conference |pages=139-154 |doi=10.1115/ICEF2007-1647}}</ref>

However, several engineering constraints limit the practical implementation of very high compression ratios. Higher compression ratios increase peak cylinder pressures and temperatures, requiring stronger engine components and materials with higher strength and heat resistance to withstand the additional mechanical and thermal stresses.<ref>{{cite journal |last1=Xing |first1=Kongzhao |last2=Yang |first2=Jianguo |last3=Fan |first3=Bolan |last4=Wang |first4=Zhenfeng |last5=Du |first5=Yanyan |date=2023 |title=Potential of high compression ratio combined with knock suppression strategy for improving thermal efficiency of spark ignition stoichiometric natural gas engine |journal=Fuel |volume=331 |doi=10.1016/j.fuel.2022.125765}}</ref> Additionally, high compression ratios make engines more susceptible to knock and [[Detonation#In engines and firearms|detonation]], particularly when using lower-octane fuels, which can damage engine components and reduce efficiency.<ref>{{cite journal |last1=Li |first1=Yunlong |last2=Pei |first2=Yiqiang |last3=Qin |first3=Jing |last4=Zhang |first4=Shaozhe |date=2014 |title=Exhaust Gas Recirculation, Late Intake Valve Closure and High Compression Ratio for Fuel Economy Improvement in a MPI Gasoline Engine |journal=SAE Technical Paper 2014-01-1197 |doi=10.4271/2014-01-1197}}</ref> The thermal efficiency gains from increasing compression ratio also diminish beyond approximately 10:1, as increased friction and heat losses begin to offset the thermodynamic benefits.<ref>{{cite conference |last1=Caton |first1=Jerald A. |date=2007 |title=The Effects of Compression Ratio and Expansion Ratio on Engine Performance Including the Second Law of Thermodynamics: Results From a Cycle Simulation |conference=ASME 2007 Internal Combustion Engine Division Fall Technical Conference |pages=139-154 |doi=10.1115/ICEF2007-1647}}</ref>

When [[forced induction]] (e.g. a [[turbocharger]] or [[supercharger]]) is used, the compression ratio is often lower than [[naturally aspirated engine]]s.<ref>{{Cite book |title=Marks' standard handbook for mechanical engineers |date=2018 |publisher=McGraw-Hill Education |isbn=978-1-259-58850-1 |editor-last=Sadegh |editor-first=Ali M. |edition=Twelfth |location=New York, NY |chapter=Internal Combustion Engines: Gas Exchange Processes: Supercharging |editor-last2=Worek |editor-first2=William M.}}</ref> 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.<ref>{{Cite report |url=https://www.sae.org/publications/technical-papers/content/910451/ |title=A Design and Experimental Study of an Otto Atkinson Cycle Engine Using Late Intake Valve Closing |last=Blakey |first=S. C. |last2=Saunders |first2=R. J. |last3=Ma |first3=T. H. |last4=Chopra |first4=A. |date=1991-02-01 |publisher=SAE Technical Paper |issue=910451 |location=Warrendale, PA |language=English}}</ref>

[[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>

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.<ref>{{Cite web |title=Alternative Fuels Data Center: Fuel Properties Comparison |url=https://afdc.energy.gov/fuels/properties?fuels=GS,LPG,CNG&properties=energy_comparison,energy_content_per_gallon,energy_content_higher_value,octane_number |access-date=2025-06-20 |website=afdc.energy.gov |language=en}}</ref> [[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 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.

Most engines use a fixed compression ratio, however a [[variable compression ratio]] engine is able to adjust the compression ratio while the engine is in operation. The first production engine with a variable compression ratio was introduced in 2019. Variable compression ratio is a technology to adjust the compression ratio of an internal combustion engine while the engine is in operation. This is done to increase [[fuel efficiency]] while under varying loads. Variable compression engines allow the volume above the piston at top dead centre to be changed.<ref>{{cite web |title=Variable Compression Engine |website=fs.isy.liu.se |url=https://www.fs.isy.liu.se/Lab/EngineLab/SVC/ |url-status=dead |archive-url=https://web.archive.org/web/20050311210702/http://www.fs.isy.liu.se/Lab/SVC/ |archive-date=11 March 2005}}</ref> Higher loads require lower ratios to increase power, while lower loads need higher ratios to increase efficiency, i.e. to lower fuel consumption. For automotive use this needs to be done as the engine is running in response to the load and driving demands. The 2019 [[Infiniti QX50#Second generation (P71A)|Infiniti QX50]] is the first commercially available car that uses a variable compression ratio engine.

The dynamic compression ratio is higher with more conservative intake [[camshaft#Timing|camshaft timing]] (i.e. soon after BDC), and lower with more radical intake camshaft timing (i.e. later after BDC).<ref>{{cite web |title=Cam Timing vs. Compression Analysis |website=victorylibrary.com |url=https://victorylibrary.com/mopar/cam-tech-c.htm |access-date=14 July 2019}}</ref> Regardless, the dynamic compression ratio is always lower than the static compression ratio. Absolute cylinder pressure is used to calculate the dynamic compression ratio, using the following formula:

Under ideal (adiabatic) conditions, the ratio of specific heats would be 1.4, but a lower value, generally between 1.2 and 1.3 is used, since the amount of heat lost will vary among engines based on design, size and materials used. For example, if the static compression ratio is 10:1, and the dynamic compression ratio is 7.5:1, a useful value for cylinder pressure would be 7.5^1.3 × atmospheric pressure, or 13.7&nbsp;[[bar (unit)|bar]] (relative to atmospheric pressure). The two corrections for dynamic compression ratio affect cylinder pressure in opposite directions, but not in equal strength. An engine with high static compression ratio and late intake valve closure will have a dynamic compression ratio similar to an engine with lower compression but earlier intake valve closure.<ref>{{Cite journal |last=Lanzanova |first=Thompson |last2=Nora |first2=Macklini Dalla |last3=Zhao |first3=Hua |date=2017-03-28 |title=Investigation of Early and Late Intake Valve Closure Strategies for Load Control in a Spark Ignition Ethanol Engine |url=https://www.sae.org/publications/technical-papers/content/2017-01-0643/ |journal=SAE International Journal of Engines |language=English |volume=10 |issue=3 |pages=858–872 |doi=10.4271/2017-01-0643 |issn=1946-3936|url-access=subscription }}</ref>