Question

how do you relate the air fuel ratio to the equivalence ratio, and the ‘lambda’ ratio

Answer 1

Air–fuel equivalence ratio or lambda (λ) is the actual ratio of actual air–fuel ratio (AFR) to stoichiometric air–fuel ratio (AFR). For a given mixture, λ=1.0 is at stoichiometric air–fuel ratio (AFR), rich air–fuel mixtures has λ<1.0, and lean mixtures has λ>1.0. There is a direct relationship between lambda (λ) and air–fuel ratio (AFR). To calculate the air–fuel ratio (AFR) from a given lambda (λ) value, multiply the measured lambda (λ) by the stoichiometric air–fuel ratio (AFR) for that fuel. Alternatively, to recover lambda (λ) value from an air–fuel ratio (AFR), divide air fuel ratio (AFR) by the stoichiometric air–fuel ratio (AFR) for that fuel.The defnition of lambda (λ)

λ = AFR/AFRstoich

where AFR=actual AFR, AFRstoich=stoichiometric AFR Because of common fuels composition varies seasonally and many modern cars can handle diferent fuels when operating, it is making more sense to talk about lambda (λ) values rather than air–fuel ratio (AFR). Most practical measurement air–fuel ratio (AFR) devices in fact measure the amount of residual oxygen (O2) for lean mixes or un-combusted hydrocarbons (HC) for rich mixtures in the vehicles exhaust emissions