Editing Fuel efficiency (section)

==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}}