Stoichiometry

When an engine runs with a fuel-air mixture for which the combustion chemistry can just go to completion the mixture is said to be stoichiometric. If there is insufficient oxygen present the combustion will not be complete and the combustion products may contain unburned hydrocarbons and carbon monoxide. If more oxidizer is present than required to produce a stoichiometric reaction the temperature of the reaction will be reduced. Under both the fuel rich and lean conditions the fuel air mixture may reach a "flammability limit" and the combustion reaction will not take place. This limit is dependent on other parameters such as the energy and size of the ignition spark.

The engine performance is dependent on the air/fuel ratio and the following graphs show how the fuel consumption, Carbon Monoxide and NO_x emmisions depend upon this ratio.