The term stoichiometric ratio describes the chemically correct air-fuel ratio necessary to achieve complete combustion of the fuel.
In an internal combustion engine the fuel is a mixture of different hydrocarbons, and for gasoline/petrol the average Hydrocarbon Ratio,is approximately 1.78:1 This corresponds to a mass Air to Fuel Ratio (AFR) of between 14.6 and 14:7:1. This is known as the "stoichiometric ratio".
In operation the difference between the engine's actual "air fuel ratio"(AFR) to the "stoichiometric air fuel ratio" (SAFR) is known (and is represented by the Greek letter), lambda.
If the actual mixture fed to the engine contains more air (a leaner ratio), then some oxygen is left in the exhaust. On the other hand, if the fuel is in excess, (a rich mixture ratio), then complete combustion achieved. In reality this produces a mixture of CO and CO2 in the exhaust.
To combat this modern petrol engines are fitted with catalytic converters in the exhaust that remove this CO along with any hydrocarbons left over due to poor combustion, and in addition any other polutants such as nitrogen oxides (NOx). These devices, however, only work correctly when the mixture is very close to the stoichiometric ratio. In practice the system must regulated so the average value of "lambda" is be between about 0.996 and 1.003 at all times.
Lambda is normally given a value of "1" and the stoichiometric air fuel ratio is sometimes refered to as "lambda" for simplicity where in reality lambda is the ratio between AFR and SAFR.
For simple and practical purposes it 's worked out in the following way:
For a gasoline engine the optimal operational value of the fuel /air ratio is 14.7 parts of air, to 1 part of fuel. By mass, that’s 14.7 kilograms of air to each kilogram of fuel.
So if "lambda" is given a value that equals "1", and the the air-fuel mixture is at the stoichiometric ratio, of 14.7 to 1 then "lambda" equals 14.7:1 the same as the stoichiometric ratio.
If a gasoline 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 has less air, proportionately, than fuel, and the mixture is slightly rich.
Engines are often run slightly on one side or the other of this perfect mix for a variety of reasons.
Other common fuels include:
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 one.
Etymology
Stoichiometry (/stoi-kE-'a-m&-trE/) (from Greek stoicheion meaning element or principle, and metron measure). The Stoichiometria of Nicephorus gave line counts of the canonical books of the New Testament and some of the Apocrypha.
Sources : CDX Global and Wikipedia - en.wikipedia.org
