What is meant by "leaning" the fuel mixture?

Leaning the fuel mixture refers to the process of reducing the amount of fuel in the fuel-air mixture used in an engine, which enhances the combustion efficiency. When the fuel mixture is leaned, there is a decreased fuel flow relative to the amount of air entering the engine. This adjustment is vital for optimizing engine performance, particularly at higher altitudes or lower power settings, where the air becomes less dense. Leaning helps to improve fuel economy and reduce exhaust emissions while ensuring that combustion remains efficient and effective, allowing the engine to operate cleanly without sacrificing power.

The other options pertain to different concepts in fuel management. Increasing fuel flow can lead to a richer mixture, which is the opposite of leaning. Adding more air to the mixture does complement the lean adjustment but is not the primary definition. Lastly, maintaining a constant fuel-to-air ratio is not the essence of leaning, as leaning specifically implies a decrease in fuel volume relative to air.