Stoichiometric Ratio
The ideal air-to-fuel ratio required for complete combustion, where all fuel is burned with minimal emissions.
The stoichiometric ratio is the precise chemical balance at which an internal combustion engine achieves complete combustion of fuel. For standard gasoline engines, this ratio is approximately 14.7 parts of air to 1 part of fuel by weight (14.7:1). For other fuels, such as diesel (14.5:1) or ethanol (9.0:1), the ratio differs depending on the fuel's chemical structure.
Operating at the stoichiometric ratio allows the catalytic converter to operate at its highest conversion efficiency, reducing carbon monoxide, hydrocarbons, and nitrogen oxide emissions to their lowest possible levels. The engine control unit uses oxygen sensors to monitor exhaust chemistry and adjust the fuel injection system continuously to maintain this target ratio.
When an engine runs with more air than the stoichiometric ratio, it is operating 'lean,' which can cause rough idling, engine hesitation, increased cylinder temperatures, and nitrogen oxide emissions. Running with more fuel than needed is 'rich,' leading to wasted fuel, black exhaust smoke, carbon buildup, and potential damage to the catalytic converter.