What percentage of the combustible air supply is used for combustion?

• A. Only about 66%
• B. All air is used for combustion
• C. 50%
• D. Only about 33%

Answer: (D)

In a turboshaft engine’s combustor, only a portion of the incoming compressor air actually participates in the combustion process. The air is split into primary air (used for burning fuel) and secondary air (used for cooling and dilution). Typically, about one third of the total air is the primary air that supports combustion, while the remaining roughly two thirds pass through for cooling the combustor and diluting the hot gases before they reach the turbine. That’s why the correct answer is about 33%—a small portion is used for the chemical reaction, with the rest serving cooling and aerodynamic needs to protect the engine and control temperatures. The other options imply much more or all of the air participates in combustion, which isn’t the case because cooling and flame stabilization require substantial secondary air.