From a nonchemist. A search on AFR lambda and stoich may yield more.
AFR is short for air-fuel ratio. It is the ratio of the mass of air in combustion and the mass of fue in combustion. Lambda is a measure of stoichiometry.
Chemical reactions have reactants and bybroducts - when the reactants fully convert to only the end by products the relationship of the mass of the reactants are at stoichiometry or stoich.
Hydrocarbons and oxygen in the chemical reaction of combustion will result in only water and CO2 as its byproducts. When air is involved there are other byproducts. Ignoring the other byproducts when gasoline and the oxygen in the air are mixed in the proper mass relationship they will be stoich or completely combusted with no other byproducts. This occurs with gasoline at around 14.7 air to 1 gasoline (by mass). This point of stoichiometry is lambda of 1.0 - for any fuel at stoich lambda is 1.0 no matter what the mass ratio of the air to fuel is. Lower than 1.0 is richer (less air than stoich to fuel), higher than 1.0 is leaner (more air than stoich to fuel). Some also use 1/lambda which just serves to confuse me personally.
Chemically inclined people please clarify any mistatements above.
Shade tree mechanics like myself rarely knew all this but without releasing it were using it when tuning even before fuel injection was on mass produced cars. It was always there in the background affecting us we just didn't have a way to monitor it and worse no way to manage it on such refined basis as we do now with EFI.
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