What term describes the additional air that is not required for combustion but enters the system?

The term that accurately describes the additional air entering the system, which is not required for combustion, is referred to as "excess air." In combustion processes, the exact amount of air needed for the complete burning of fuel is known as stoichiometric air. Excess air is any additional air supplied beyond this required amount.

In practical applications, excess air can help ensure more complete combustion by providing a buffer against variations in fuel quality or combustion conditions, but it can also lead to inefficiencies. This excess air can cool the combustion process, potentially reducing system efficiency and leading to increased emissions if not managed properly.

The other terms listed do not reflect the same concept. For instance, re-entry air typically refers to ambient air that may be pulled back into a system due to pressure differences. Fuel-air mixture describes the combination of fuel and combustion air that is needed to achieve efficient combustion. Combustion air defines the air specifically required for the combustion process itself, excluding any additional air that enters the system beyond what is necessary for combustion.