For a fuel to oxidize, or burn, it must have a reagent or oxidizer with which it can react. In the case of most engines, the oxidizer is the Oxygen in the air that they take in. An oxidizing agent is a substance that oxidizes another substance in electrochemistry or redox chemical reactions in general. ...

For best efficiency, an engine should admit only as much air as is necessary to burn the amount of fuel it will use to produce power, when doing so the engine is said to have a stoichiometric mixture. In this case, assuming a complete combustion, each molecule of fuel will be completely burned into carbon dioxide and water. If the mixture of fuel and air is not stoichiometric then one of the following is true: In chemistry, stoichiometry is the study of the combination of elements in chemical reactions. ...

If LESS than the stoichiometric amount of air needed to burn a certain amount fuel is admitted to the engine, the fuel will not combust completely and in this case the engine is said to be running RICH.

If MORE than the stoichiometric amount of air needed to burn a certain amount fuel is admitted to the engine, the fuel will combust completely and some oxygen will be left over in the exhaust gases and in this case the engine is said to be running LEAN.

It is possible, depending on the fuel used, to produce mixtures that will not burn because they are either too lean or too rich, the points at which this happens are referred to as the lower and upper explosive limits and these vary from fuel to fuel. An engine with a mixture too rich to ignite is said to be a flooded engine The explosive limit of a gas or a vapour, is the limiting concentration (in air) that is needed for the gas to ignite and explode. ...