The term Stoichiometric ratio describes the chemically correct air-fuel ratio necessary to achieve complete combustion of the fuel. It is represented by the Greek letter, lambda.
For gasoline fuel, or petrol, the value of the ratio is 14.7 parts of air, to 1 part of fuel. By mass, that's 14.7 kilograms or pounds of air to each kilogram or pound of fuel. So if lambda equals 1, the air-fuel mixture is at the Stoichiometric ratio, of 14.7 to 1.
If an air-fuel mixture has a higher figure, say, 1.03, there is more air in proportion to the fuel than 14.7 to 1, and the mixture is slightly lean.
A mixture with a lower lambda value, say 0.97, has proportionately less air than fuel, and the mixture is slightly rich.
The exhaust gas oxygen sensor is also called the lambda sensor, since it can be used to maintain the air-fuel ratio at lambda equal to 1, within very close limits. It can be installed in the exhaust manifold, where it measures the percentage of oxygen in the exhaust gases.
A high percentage of oxygen may mean too little fuel is entering the engine, the mixture is too lean, and lambda is greater than 1. The sensor delivers this information to the ECU, which adjusts the mixture accordingly.
Similarly, a low percentage of oxygen may indicate too much fuel is entering the engine, the mixture is too rich, and lambda is less than 1.
Different fuels have a different Stoichiometric ratio. For instance, with Methanol the air-to-fuel ration is 6.4:1, and with Ethanol it is 9:1.