What does an equivalence ratio value less than 1 signify?

A value for the equivalence ratio that is less than 1 indicates that there is less fuel present in the mixture compared to the amount of air required for complete combustion. This condition leads to a mixture being categorized as "lean." In combustion processes, the equivalence ratio is defined as the actual fuel-to-air ratio divided by the stoichiometric fuel-to-air ratio. Therefore, when the equivalence ratio is less than 1, it implies that the fuel is limiting and there is an excess of air. In practical terms, a lean mixture typically results in more efficient combustion, lower emissions, and higher thermal efficiency, although it can also lead to difficulties in ignition and combustion stability under certain conditions.