Air/Fuel Ratio and Lambda - EngineKnowHow

Air / Fuel Ratio

In an engine the air / fuel ratio, or AFR is the ratio between the mass flowrate of air to the mass flowrate of fuel to the cylinder and is an important parameter for performance, fuel consumption, emissions and hardware protection.  Petrol engines typically run between 12-18kg of air per kg of fuel whilst diesel engines operate over a greater range, typically 18 – 70kg of air per kg of fuel.


The air / fuel ratio is calculated via:





When and engine is said to be operating at “stoichiometric”, then there is the perfect amount of air available to completely combust all of the fuel and convert it to the final combustion products of CO2, H2O and N2.  For the stoichiometric combustion of petrol there is ≈14.7kg of air per kg of fuel.



Lambda, also known as the relative air / fuel ratio is a term that indicates whether the engine is running under stoichiometric conditions, at an excess of air or below the required amount of air for complete combustion of the fuel.


Lambda is calculated via:






When an engine is running at λ = 1 it is running at stoichiometric.  When an engine is running at an excess of air to fuel, then the engine is running lean.  When there is not enough air available to combust all of the fuel then the engine is running rich.


λ = 1: Stoichiometric

λ > 1: Lean

λ < 1: Rich


Most spark ignition engines operate predominantly at λ = 1 to ensure maximum conversion efficiency of the catalytic converter and due to emissions legislation.  Variations from λ = 1 occur during transient periods, eg. under heavy acceleration or for component protection, eg. at high exhaust gas temperatures which compromise a turbocharger turbine temperature limit.


Diesel engines always operate under lean conditions.


Equivalence Ratio

Diesel engines typically operate under the term of equivalence ratio as opposed to lambda which is calculated as:




Diesel engines typically operate at an equivalence ratio between 0.2 – 0.8.