Excess air is required in combustion because it:

Excess air in combustion is primarily required to ensure complete combustion of the fuel being burned. Complete combustion occurs when there is sufficient oxygen to react with all of the fuel, converting it entirely into carbon dioxide, water, and other products without leaving unburned fuel behind. When there is inadequate air, some of the fuel can remain unburned, leading to incomplete combustion, which produces carbon monoxide and other pollutants.

By introducing excess air, the combustion process ensures that all of the available fuel can oxidize completely, maximizing efficiency and minimizing the emission of harmful byproducts. This is particularly important in applications where fuel efficiency and environmental regulations are critical.

While other considerations such as reducing air pollution and ensuring appropriate energy release are important, the fundamental reason for requiring excess air is to achieve the complete combustion of the fuel efficiently.