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{{short description|Form of thermal efficiency}}
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'''Fuel efficiency''' (or '''fuel economy''') is a form of [[thermal efficiency]], meaning the [[ratio]] of effort to result of a process that converts [[chemical energy|chemical]] [[potential energy]] contained in a carrier ([[fuel]]) into kinetic energy or [[Mechanical work|work]]. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance is often illustrated as a continuous [[energy profile]]. Non-transportation applications, such as [[Industrial sector|industry]], benefit from increased fuel efficiency, especially [[fossil fuel power plant]]s or industries dealing with [[combustion]], such as [[ammonia]] production during the [[Haber process]].

In the context of [[transport]], fuel economy is the [[energy efficiency in transportation|energy efficiency]] of a particular vehicle,  given as a [[ratio]] of distance traveled per unit of [[Motor fuel|fuel]] consumed. It is dependent on several factors  including [[engine efficiency]], [[transmission (mechanics)|transmission]] design, and [[tire]] design. In most countries, using the [[metric system]], fuel economy is stated as "fuel consumption" in [[liter]]s per 100 [[kilometers]] (L/100&nbsp;km) or kilometers per liter (km/L or kmpl). In a number of countries still using other systems, fuel economy is expressed in [[mile]]s per [[gallon]] (mpg), for example in the US and usually also in the UK ([[Imperial units|imperial]] gallon); there is sometimes confusion as the imperial gallon is 20% larger than the US gallon so that mpg values are not directly comparable.  Traditionally, litres per [[Scandinavian mile|mil]] were used in [[Norway]] and [[Sweden]], but both have aligned to the EU standard of L/100&nbsp;km.<ref>{{cite web|url=http://www.bilsweden.se/miljo-sakerhet/miljo/information-gallande-bransleforbrukning-for-nya-bilar|access-date=7 November 2019|title=Information on the fuel consumption of new cars|archive-date=8 September 2019|archive-url=https://web.archive.org/web/20190908124154/http://www.bilsweden.se/miljo-sakerhet/miljo/information-gallande-bransleforbrukning-for-nya-bilar|url-status=dead}}</ref>

Fuel consumption is a more accurate measure of a vehicle's performance because it is a linear relationship while fuel economy leads to distortions in efficiency improvements.<ref>{{cite web |url= http://www.fueleconomy.gov/feg/label/learn-more-gasoline-label.shtml#fuel-consumption-rate |title= Learn More About the Fuel Economy Label for Gasoline Vehicles |url-status= live |archive-url= https://web.archive.org/web/20130705143242/http://www.fueleconomy.gov/feg/label/learn-more-gasoline-label.shtml#fuel-consumption-rate |archive-date= 2013-07-05 }}</ref> Weight-specific efficiency (efficiency per unit weight) may be stated for [[freight]], and passenger-specific efficiency (vehicle efficiency per passenger) for passenger vehicles.

==Vehicle design==
Fuel efficiency is dependent on many parameters of a vehicle, including its [[engine]] parameters, [[Drag (physics)|aerodynamic drag]], weight, AC usage, fuel and [[rolling resistance]]. There have been advances in all areas of vehicle design in recent decades. Fuel efficiency of vehicles can also be improved by careful maintenance and driving habits.<ref>{{Cite news|url=https://www.carsangrah.com/blog/simple-tips-tricks-enhance-fuel-efficiency-car|title=Simple tips and tricks to increase fuel efficiency of your car {{!}} CarSangrah|date=2018-06-07|work=CarSangrah|access-date=2018-07-24|language=en-US}}</ref>

[[Hybrid vehicles]] use two or more power sources for propulsion. In many designs, a small combustion engine is combined with electric motors.  Kinetic energy which would otherwise be lost to heat during braking is recaptured as electrical power to improve fuel efficiency. The larger batteries in these vehicles power the [[Automotive electronics|car's electronics]], allowing the engine to shut off and avoid prolonged [[Start-stop system|idling]].<ref>{{cite web|title=How Hybrids Work|url=http://www.fueleconomy.gov|publisher=[[U.S. Department of Energy]]|access-date=2014-01-16|url-status=live|archive-url=https://web.archive.org/web/20150708093450/http://www.fueleconomy.gov/|archive-date=2015-07-08}}</ref>

==Fleet efficiency==
[[File:1975- US vehicle production share, by vehicle type.svg |thumb |Trucks' share of US vehicles produced, has tripled since 1975. Though vehicle fuel efficiency has increased within each category, the overall trend toward less efficient types of vehicles has offset some of the benefits of greater fuel economy and reduction in carbon dioxide emissions.<ref name=EPA_AutomotiveTrends_202212>{{cite web |title=Highlights of the Automotive Trends Report |url=https://www.epa.gov/automotive-trends/highlights-automotive-trends-report |website=EPA.gov |publisher=U.S. Environmental Protection Agency (EPA) |archive-url=https://web.archive.org/web/20230902145941/https://www.epa.gov/automotive-trends/highlights-automotive-trends-report |archive-date=2 September 2023 |date=12 December 2022 |url-status=live}}</ref> Without the shift towards SUVs, energy use per unit distance could have fallen 30% more than it did from 2010 to 2022.<ref name=GlobalFuelEfficInit_202311>{{cite web |last1=Cazzola |first1=Pierpaolo |last2=Paoli |first2=Leonardo |last3=Teter |first3=Jacob |title=Trends in the Global Vehicle Fleet 2023 / Managing the SUV Shift and the EV Transition |url=https://www.globalfueleconomy.org/media/792523/gfei-trends-in-the-global-vehicle-fleet-2023-spreads.pdf |publisher=Global Fuel Economy Initiative (GFEI) |archive-url=https://web.archive.org/web/20231126092826/https://www.globalfueleconomy.org/media/792523/gfei-trends-in-the-global-vehicle-fleet-2023-spreads.pdf |archive-date=26 November 2023 |page=3 |doi=10.7922/G2HM56SV |date=November 2023 |url-status=live }}</ref>]]
Fleet efficiency describes the average efficiency of a population of vehicles.  Technological advances in efficiency may be offset by a change in buying habits with a propensity to heavier vehicles that are less fuel-efficient.<ref name=EPA_AutomotiveTrends_202212/>

==Energy efficiency terminology==
[[Energy efficiency (physics)|Energy efficiency]] is similar to fuel efficiency but the input is usually in units of energy such as [[megajoules]] (MJ), [[kilowatt-hours]] (kW·h), kilocalories (kcal) or [[British thermal units]] (BTU). The inverse of "energy efficiency" is "[[energy intensity]]", or the amount of input energy required for a unit of output such as MJ/passenger-km (of passenger transport), BTU/ton-mile or kJ/t-km (of freight transport), GJ/t (for production of steel and other materials), BTU/(kW·h) (for electricity generation), or litres/100&nbsp;km (of vehicle travel). Litres per 100&nbsp;km is also a measure of "energy intensity" where the input is measured by the amount of fuel and the output is measured by the [[distance]] travelled.  For example: [[Fuel economy in automobiles]].

Given a heat value of a fuel, it would be trivial to convert from fuel units (such as litres of gasoline) to energy units (such as MJ) and conversely. But there are two problems with comparisons made using energy units:
* There are two different heat values for any hydrogen-containing fuel which can differ by several percent (see below).
* When comparing transportation energy costs, a [[kilowatt hour]] of electric energy may require an amount of fuel with heating value of 2 or 3 kilowatt hours to produce it.

==Energy content of fuel==

The specific [[energy content]] of a fuel is the heat energy obtained when a certain quantity is burned (such as a gallon, litre, kilogram).  It is sometimes called the [[heat of combustion]].  There exists two different values of [[specific heat]] energy for the same batch of fuel.  One is the high (or gross) heat of combustion and the other is the low (or net) heat of combustion.  The high value is obtained when, after the combustion, the water in the exhaust is in liquid form.  For the low value, the exhaust has all the water in vapor form (steam).  Since water vapor gives up heat energy when it changes from vapor to liquid, the liquid water value is larger since it includes the [[latent heat]] of vaporization of water.  The difference between the high and low values is significant, about 8 or 9%.  This accounts for most of the apparent discrepancy in the heat value of gasoline. In the U.S. (and the table) the high heat values have traditionally been used, but in many other countries, the low heat values are commonly used.

<!--This table originally contained MJ/L values that were too low compared to the BTU/gal figures, with a reference to an ''Automotive Handbook''.<ref>''Automotive Handbook, 4th Edition'', Robert Bosch GmbH, 1996. ISBN 0-8376-0333-1</ref> These have now been replaced with values from the ''Transportation Energy Data Book'',<ref name=TEDB>[http://www-cta.ornl.gov/data/Appendix_B.html Appendix B, Transportation Energy Data Book] from the [[Center for Transportation Analysis]] of the [[Oak Ridge National Laboratory]]</ref>
but which does not give the MJ/kg or the densities.)

Note: I modified this table because the values in SI units did not agree with the values in British or U.S. units. So I used another source, but it did not have MJ/kg, and I did not have the time to try to find accurate densities in order to convert to MJ/kg. If someone can fill in the blanks using good data, it would be useful.-->
{| class="wikitable sortable"
! align = "left"|Fuel type
! align ="right"|MJ/L
! align ="right"|MJ/kg
! align ="right"|[[British thermal unit|BTU]]/[[gallon|imp gal]]
! align ="right"|BTU/[[US gallon|US gal]]
! align ="right"|[[octane rating|Research octane<br /> number (RON)]]
|-
| Regular [[gasoline]]/petrol
| align ="right"|34.8
| align ="right"|{{Sort|47|~47}}
| align ="right"|150,100
| align ="right"|125,000
| align ="right"|Min. 91
|-
| Premium [[gasoline]]/petrol
| align ="right"|
| align ="right"|{{Sort|46|~46}}
| align ="right"|
| align ="right"|
| align ="right"|Min. 95
|-
| [[Autogas]] ([[Liquefied petroleum gas|LPG]]) (60% [[propane]] and 40% [[butane]])
| align ="right"|25.5–28.7
| align ="right"|{{Sort|51|~51}}
| align ="right"|
| align ="right"|
| align ="right"|108–110
|-
|[[ethanol fuel|Ethanol]]
| align ="right"|23.5
| align ="right"|31.1<ref>Calculated from heats of formation. Does not correspond exactly to the figure for MJ/L divided by density.</ref>
| align ="right"|101,600
| align ="right"|84,600
| align ="right"|129
|-
| [[Methanol]]
| align ="right"|17.9
| align ="right"|19.9
| align ="right"|77,600
| align ="right"|64,600
| align ="right"|123
|-
| [[Alcohol fuel|Gasohol]] (10% ethanol and 90% gasoline)
| align ="right"|33.7
| align ="right"|{{Sort|45|~45}}
| align ="right"|145,200
| align ="right"|121,000
| align ="right"|93/94
|-
| [[E85]] (85% ethanol and 15% gasoline)
| align ="right"|25.2
| align ="right"|{{Sort|33|~33}}
| align ="right"|108,878
| align ="right"|90,660
| align ="right"|100–105
|-
| [[Diesel fuel|Diesel]]
| align ="right"|38.6
| align ="right"|{{Sort|48|~48}}
| align ="right"|166,600
| align ="right"|138,700
| align ="right"|[[cetane number|N/A (see cetane)]]
|-
| [[Biodiesel]]
| align ="right"|35.1
| align ="right"|39.9
| align ="right"|151,600
| align ="right"|126,200
| align ="right"|[[cetane number|N/A (see cetane)]]
|-
| [[WVO|Vegetable oil]] (using 9.00&nbsp;kcal/g)
| align ="right"|34.3
| align ="right"|37.7
| align ="right"|147,894
| align ="right"|123,143
| align ="right"|
|-
| [[Aviation gasoline]]
| align ="right"|33.5
| align ="right"|46.8
| align ="right"|144,400
| align ="right"|120,200
| align ="right"|80-145
|-
| [[Jet fuel]], naphtha
| align ="right"|35.5
| align ="right"|46.6
| align ="right"|153,100
| align ="right"|127,500
| align ="right"|N/A to turbine engines
|-
| [[Jet fuel]], kerosene
| align ="right"|37.6
| align ="right"|{{Sort|47|~47}}
| align ="right"|162,100
| align ="right"|135,000
| align ="right"|N/A to turbine engines
|-
| [[Liquefied natural gas]]
| align ="right"|25.3
| align ="right"|{{Sort|55|~55}}
| align ="right"|109,000
| align ="right"|90,800
| align ="right"|
|-
| [[Liquid hydrogen]]
| align ="right"|{{0}}9.3
| align ="right"|{{Sort|99|~130}}
| align ="right"|40,467
| align ="right"|33,696
| align ="right"|
|}<ref name=TEDB>[http://www-cta.ornl.gov/data/Appendix_B.html Appendix B, Transportation Energy Data Book] from the [[Center for Transportation Analysis]] of the [[Oak Ridge National Laboratory]]</ref>

Neither the gross heat of combustion nor the net heat of combustion gives the theoretical amount of mechanical energy (work) that can be obtained from the reaction. (This is given by the change in [[Gibbs free energy]], and is around 45.7&nbsp;MJ/kg for gasoline.) The actual amount of mechanical work obtained from fuel (the inverse of the [[Brake-specific fuel consumption|specific fuel consumption]]) depends on the engine. A figure of 17.6&nbsp;MJ/kg is possible with a gasoline engine, and 19.1&nbsp;MJ/kg for a diesel engine. See [[Brake-specific fuel consumption]] for more information.{{clarify|Why is RON listed in the above table (does not reflect the energy content of fuel)|date=July 2019}}

== Transportation ==
{{excerpt|Energy efficiency in transport|templates=-transport}}

=== Fuel efficiency of motor vehicles ===
{{excerpt|Fuel economy in automobiles}}

===Driving technique===
{{excerpt|Energy-efficient driving|templates=-condensed}}

==Advanced technology==
The most efficient machines for converting energy to rotary motion are electric motors, as used in [[electric vehicles]]. However, electricity is not a primary energy source so the efficiency of the electricity production has also to be taken into account. [[Railway]] trains can be powered using electricity, delivered through an additional running rail, overhead [[overhead lines|catenary]] system or by on-board generators used in [[Diesel-electric transmission|diesel-electric]] locomotives as common on the US and UK rail networks. Pollution produced from centralised generation of electricity is emitted at a distant power station, rather than "on site". Pollution can be reduced by using more railway electrification and [[low carbon power]] for electricity. Some railways, such as the French SNCF and Swiss federal railways derive most, if not 100% of their power, from hydroelectric or nuclear power stations, therefore atmospheric pollution from their rail networks is very low. This was reflected in a study by AEA Technology between a [[Eurostar]] train and airline journeys between London and Paris, which showed the trains on average emitting 10 times less CO<sub>2</sub>, per passenger, than planes, helped in part by French nuclear generation.<ref>{{cite web|url=http://www.transportenvironment.org/Article267.html|title=Rail 10 times better than air in London-Paris CO2 comparison - Transport & Environment|url-status=live|archive-url=https://web.archive.org/web/20070928024427/http://www.transportenvironment.org/Article267.html|archive-date=2007-09-28}}</ref>

=== Hydrogen fuel cells ===
In the future, [[hydrogen vehicle|hydrogen cars]] may be commercially available. Toyota is test-marketing vehicles powered by hydrogen fuel cells in southern California, where a series of hydrogen fueling stations has been established. Powered either through chemical reactions in a [[fuel cell]] that create electricity to drive very efficient electrical motors or by directly burning hydrogen in a combustion engine (near identically to a [[natural gas vehicle]], and similarly compatible with both natural gas and gasoline); these vehicles promise to have near-zero pollution from the tailpipe (exhaust pipe). Potentially the atmospheric pollution could be minimal, provided the hydrogen is made by [[electrolysis]] using electricity from non-polluting sources such as solar, wind or [[hydroelectricity]] or nuclear. Commercial [[hydrogen production]] uses fossil fuels and produces more carbon dioxide than hydrogen.

Because there are pollutants involved in the manufacture and destruction of a car and the production, transmission and storage of electricity and hydrogen, the label "zero pollution" applies only to the car's conversion of stored energy into movement.

In 2004, a consortium of major auto-makers — [[BMW]], [[General Motors]], [[Honda]], [[Toyota]] and [[Volkswagen]]/[[Audi]] — came up with ''"Top Tier Detergent Gasoline Standard"'' to [[gasoline]] brands in the US and Canada that meet their minimum standards for [[detergent]] content<ref name="Top Tier Gasoline">[http://www.toptiergas.com/retailers.html Top Tier Gasoline] {{webarchive|url=https://web.archive.org/web/20130815031514/http://www.toptiergas.com/retailers.html |date=2013-08-15 }}</ref> and do not contain metallic additives. Top Tier gasoline contains higher levels of detergent additives in order to prevent the build-up of deposits (typically, on [[fuel injector]] and [[intake valve]]) known to reduce fuel economy and engine performance.<ref>{{Cite web
| url = http://www.toptiergas.com/deposit_control.html
| title = Deposit Control Standards
| access-date = 2012-10-19
| archive-url = https://web.archive.org/web/20040806012056/http://www.toptiergas.com/deposit_control.html
| url-status = dead
| archive-date = 2004-08-06
}}</ref>

===In microgravity===
How fuel combusts affects how much energy is produced. The [[National Aeronautics and Space Administration]] (NASA) has investigated fuel consumption in [[microgravity]].

The common distribution of a flame under normal gravity conditions depends on [[convection]], because soot tends to rise to the top of a flame, such as in a candle, making the flame yellow. In microgravity or [[zero gravity]], such as an environment in [[outer space]], convection no longer occurs, and the flame becomes [[sphere|spherical]], with a tendency to become more blue and more efficient. There are several possible explanations for this difference, of which the most likely one given is the hypothesis that the temperature is evenly distributed enough that soot is not formed and complete combustion occurs., National Aeronautics and Space Administration, April 2005. Experiments by NASA in microgravity reveal that [[diffusion flame]]s in microgravity allow more soot to be completely oxidised after they are produced than diffusion flames on Earth, because of a series of mechanisms that behaved differently in microgravity when compared to normal gravity conditions.[https://web.archive.org/web/20070312020123/http://microgravity.grc.nasa.gov/combustion/lsp/lsp1_results.htm LSP-1 experiment results], National Aeronautics and Space Administration, April 2005. [[Premixed flame]]s in microgravity burn at a much slower rate and more efficiently than even a candle on Earth, and last much longer.<ref>[http://microgravity.grc.nasa.gov/combustion/lsp/lsp1_results.htm SOFBAL-2 experiment results] {{webarchive|url=https://web.archive.org/web/20070312020123/http://microgravity.grc.nasa.gov/combustion/lsp/lsp1_results.htm |date=2007-03-12 }}, National Aeronautics and Space Administration, April 2005.</ref>

==See also==
{{Portal|Energy}}
{{columns-list|colwidth=18em|
* [[Annual fuel utilization efficiency]] (AFUE)
* [[ACEA agreement]]
* [[Alternative propulsion]]
* [[Camless piston engine]]
* [[Carbon dioxide equivalent]]
* [[Corporate Average Fuel Economy]] (CAFE)
* [[EcoAuto]] (in Canada)
* [[Efficient energy use]]
* [[Emission standard]]
* [[Energy content of Biofuel]]
* [[Energy conservation]]
* [[Energy conversion efficiency]]
* [[Energy density]]
* [[FF layout]]
* [[Front-wheel drive]]
* [[Fuel economy in aircraft]]
* [[Fuel economy in automobiles]]
* [[Fuel economy-maximizing behaviors|Fuel economy maximising behaviors]]
* [[Fuel efficiency in transportation]]
* [[Gas-guzzler]]
* [[Heating value]]
* [[Jevons paradox]]
* [[Life cycle assessment]]
* [[Low-rolling resistance tires]]
* [[Miles per gallon gasoline equivalent]]
* [[Marine fuel management]]
* [[Twinjet]]
* [[Variable valve timing]]
* [[Vehicle frame#Unibody|Unibody]]
* [[Automobile costs]]
* [[Vehicle metrics]]
}}

==References==
{{Reflist}}

==External links==
{{external cleanup|date=November 2023}}
* [http://www.fueleconomy.gov/ US Government website on fuel economy]
* [http://www.dft.gov.uk/stellent/groups/dft_railways/documents/page/dft_railways_611287.pdf UK DfT comparisons on road and rail]
* [http://apps1.eere.energy.gov/news/news_detail.cfm/news_id=12694 NASA Offers a $1.5 Million Prize for a Fast and Fuel-Efficient Aircraft] {{Webarchive|url=https://web.archive.org/web/20160303200449/http://apps1.eere.energy.gov/news/news_detail.cfm/news_id=12694 |date=2016-03-03 }}
* [http://www.carfuelconsumption.com Car Fuel Consumption Official Figures]
* [http://www.spritmonitor.de/de/die_sparsamsten_autos.html Spritmonitor.de "the most fuel efficient cars" - Database of thousands of (mostly German) car owners' actual fuel consumption figures] (cf. [[Spritmonitor]])
* {{usurped|1=[https://web.archive.org/web/20131029195658/http://fueleconomy.carconsumption.com/ Searchable fuel economy data from the EPA - United States Environmental Protection Agency]}}
* [http://www.xpower-ionizer.com/ penghemat bbm - Alat penghemat bbm]
* [https://www.nytimes.com/2014/11/18/science/earth/hydrogen-cars-join-electric-models-in-showrooms.html Ny Times: A Road Test of Alternative Fuel Visions]

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