Equivalence Ratio
Definition
The ratio of actual fuel-to-air ratio to the stoichiometric fuel-to-air ratio. An equivalence ratio above 1 (fuel-rich) produces CO, soot, and unburned hydrocarbons; below 1 (fuel-lean) produces complete combustion products.
- Symbol
- phi
- Formula
- Actual fuel/air ratio divided by stoichiometric fuel/air ratio
- Above 1
- Fuel-rich, produces CO, soot, unburned hydrocarbons
- Below 1
- Fuel-lean, complete combustion
Common questions
Why does fire debris from a fuel-rich burn often show heavier soot deposition?+
A fuel-rich mixture has more fuel than the available oxygen can fully oxidise, so carbon particles and unburned hydrocarbons form instead of being converted completely to carbon dioxide and water.
How does equivalence ratio help distinguish a well-ventilated fire from a confined one?+
A fire in a well-ventilated space tends toward a lean, fuel-controlled ratio, while one in a confined or oxygen-starved compartment shifts toward a rich, ventilation-controlled ratio, which investigators use alongside burn patterns to reconstruct fire behaviour.
Related terms
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- Flashover
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- Incomplete Combustion
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- Levoglucosan
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- NFPA 921
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