What is Secondary Air (SA) in relation to fuel contact?

Secondary Air (SA) refers to the air introduced into a combustion system after the initial burning of the fuel has begun. Its primary role is to promote complete combustion of the fuel and help control emissions by supporting the combustion process. In the context of the combustion process, primary air is often mixed with the fuel before ignition, while secondary air is introduced subsequently to optimize combustion.

The correct option highlights that Secondary Air is the second introduction of air in relation to the fuel, which typically comes into play after primary air has already been utilized. This allows for more thorough combustion by providing the necessary oxygen to consistently combust unburned fuel particles that may remain after the initial reaction facilitated by primary air.

In comparison, the other options misidentify the role or timing of air interactions with the fuel. The first air that comes into contact with the fuel is considered primary air, and the idea of air used solely for cooling does not accurately define secondary air's role in combustion processes.