What does an increase in excess air typically do to combustion efficiency?

An increase in excess air during combustion generally decreases combustion efficiency. This occurs because as more air is introduced into the combustion process, it dilutes the fuel-air mixture. When there is too much air, not all of the fuel can burn completely before all the oxygen is consumed. This leads to a higher proportion of unburned fuel and energy being wasted in the flue gas, which means that less energy is extracted from the fuel.

Furthermore, additional excess air can lead to higher temperatures in the flue gases, indicating that energy is being lost from the system. Moreover, excess air can also result in a cooler flame and incomplete combustion, which increases the production of carbon monoxide and other pollutants, further compromising efficiency.

In summary, while some excess air is necessary to ensure complete combustion and avoid issues like soot formation, too much excess air is counterproductive as it reduces the overall efficiency of the combustion process.