Editing Stoichiometry (section)

== Stoichiometric air-to-fuel ratios of common fuels ==
{{See also|Air–fuel ratio|Combustion}}

In the [[combustion]] reaction, oxygen reacts with the fuel, and the point where exactly all oxygen is consumed and all fuel burned is defined as the stoichiometric point. With more oxygen (overstoichiometric combustion), some of it stays unreacted. Likewise, if the combustion is incomplete due to lack of sufficient oxygen, fuel remains unreacted. (Unreacted fuel may also remain because of slow combustion or insufficient mixing of fuel and oxygen – this is not due to stoichiometry.) Different hydrocarbon fuels have different contents of carbon, hydrogen and other elements, thus their stoichiometry varies.

Oxygen makes up only 20.95% of the volume of air, and only 23.20% of its mass.<ref>{{cite web | url=http://www.uigi.com/air.html | title=Universal Industrial Gases, Inc: Composition of Air - Components & Properties of Air - Answers to "What is air?" - "What is air made up of?" -" What are air products and what are they used for?" }}</ref> The air-fuel ratios listed below are much higher than the equivalent oxygen-fuel ratios, due to the high proportion of inert gasses in the air.

{| class="wikitable sortable" style="text-align:right;"
|-
! Fuel
! data-sort-type="number" | Ratio by mass <ref>John B. Heywood: "Internal Combustion Engine Fundamentals page 915", 1988</ref>
! data-sort-type="number" | Ratio by volume <ref>North American Mfg. Co.: "North American Combustion Handbook", 1952</ref>{{Full citation needed|date=December 2024}}
! data-sort-type="number" | Percent fuel by mass
! Main reaction
|-
| [[Gasoline]]
| 14.7 : 1
| {{sdash}}
| 6.9%
| {{chem2|2 C8H18 + 25 O2 → 16 CO2 + 18 H2O}}
|-
| [[Natural gas]]
| 14.5 : 1
| 9.7  : 1
| 6.9%
| {{chem2|CH4 + 2 O2 → CO2 + 2 H2O}}
|-
| [[Propane]] ([[Liquid propane|LP]])
| 15.67 : 1
| 23.9 : 1
| 6.45%
| {{chem2|C3H8 + 5 O2 → 3 CO2 + 4 H2O}}
|-
| [[Ethanol]]
| 9 : 1 
| {{sdash}}
| 11.1%
| {{chem2|C2H6O + 3 O2 → 2 CO2 + 3 H2O}}
|-
| [[Methanol]]
| 6.47 : 1
| {{sdash}}
| 15.6%
| {{chem2|2 CH4O + 3 O2 → 2 CO2 + 4 H2O}}
|-
| [[N-Butanol|''n''-Butanol]]
| 11.2 : 1 
| {{sdash}}
| 8.2%
| {{chem2|C4H10O + 6 O2 → 4 CO2 + 5 H2O}}
|-
| [[Hydrogen]]
| 34.3 : 1
| 2.39 : 1
| 2.9%
| {{chem2|2 H2 + O2 → 2 H2O}}
|-
| [[Diesel fuel|Diesel]]
| 14.5 : 1 {{Citation needed|date=December 2024|reason=This was not listed in Heywood (Citation 14)}}
| {{sdash}}
| 6.8%
| {{chem2|2 C12H26 + 37 O2 → 24 CO2 + 26 H2O}}
|-
| [[Methane]]
| 17.23 : 1
| 9.52 : 1
| 5.5%
| {{chem2|CH4 + 2 O2 → CO2 + 2 H2O}}
|-
| [[Acetylene]]
| 13.26 : 1 {{Citation needed|date=December 2024|reason=This was not listed in Heywood (Citation 14)}}
| 11.92 : 1
| 7.0%
| {{chem2|2 C2H2 + 5 O2 → 4 CO2 + 2 H2O}}
|-
| [[Ethane]]
| 16.07 : 1 {{Citation needed|date=December 2024|reason=This was not listed in Heywood (Citation 14)}}
| 16.68 : 1
| 5.9%
| {{chem2|2 C2H6 + 7 O2 → 4 CO2 + 6 H2O}}
|-
| [[Butane]]
| 15.44 : 1 {{Citation needed|date=December 2024|reason=This was not listed in Heywood (Citation 14)}}
| 30.98 : 1
| 6.1%
| {{chem2|2 C4H10 + 13 O2 → 8 CO2 + 10 H2O}}
|-
| [[Pentane]]
| 15.31 : 1 {{Citation needed|date=December 2024|reason=This was not listed in Heywood (Citation 14)}}
| 38.13 : 1
| 6.1%
| {{chem2|C5H12 + 8 O2 → 5 CO2 + 6 H2O}}
|}

Gasoline engines can run at stoichiometric air-to-fuel ratio, because gasoline is quite volatile and is mixed (sprayed or carburetted) with the air prior to ignition. Diesel engines, in contrast, run lean, with more air available than simple stoichiometry would require. Diesel fuel is less volatile and is effectively burned as it is injected.<ref>{{Cite web|url=https://x-engineer.org/automotive-engineering/internal-combustion-engines/performance/air-fuel-ratio-lambda-engine-performance/|title=Air-fuel ratio, lambda and engine performance|language=en-US|access-date=2019-05-31}}</ref>