Air-fuel ratio

Air-fuel ratio, often abbreviated AFR, is the mass ratio of air to fuel present in a combustion process, whether in a controlled setting such as an internal combustion engine or industrial furnace, or an uncontrolled one such as a dust explosion. The stoichiometric ratio is the exact mixture that burns all the available fuel completely; for gasoline engines this works out to roughly 14.7 parts air to 1 part fuel by mass. A mixture with less air than the stoichiometric ratio is described as rich, meaning excess fuel, while a mixture with more air is lean, meaning excess air. The ratio strongly shapes engine performance and emissions: rich mixtures tend to produce more power but burn less efficiently and cooler, while lean mixtures improve efficiency but can push combustion temperatures higher and increase nitrogen oxide formation. Engine management systems use oxygen sensors to monitor air-fuel ratio continuously and keep it near the target for the desired balance of power, efficiency and emissions.