The ratio by weight of air and fuel in the mixture being sent into the cylinders of a reciprocating engine. Ideally, the ratio should be 15:1. Besides the ideal (stoichiometric) ratio, the air-fuel mixture may be rich or lean depending on the quantity of fuel in the mixture. With more fuel, the mixture is rich, and with less fuel, it is a weak or lean mixture.
Central to that benchmark is a closed-loop fuel system that automatically optimizes the air-fuel mixture ratio. Working with a three-way catalytic muffler, the system minimizes hydrocarbon, oxides of nitrogen and carbon monoxide emissions.