Air-Fuel Ratio

The volume of fuel relative to the volume of air in the combustion mixture. If the ratio is just right (stoichiometric), the mixture burns most efficiently and produces lower exhaust emissions. If there is too much fuel (rich), not all of the fuel will be burned completely and high carbon monoxide emissions will result. If there is not enough fuel (lean), the combustion may stop prematurely and high hydrocarbon emissions will result.