What part of a jet engine has the most potential energy?

• A. Immediately after the combustion chamber
• B. Just before the combustion chamber
• C. Immediately after the HP compressor
• D. In the exhaust

Answer: (B)

Potential energy in a jet engine is the pressure energy stored in the working fluid. The air is repeatedly compressed by the stages of the compressor, raising its pressure to the highest level in the cycle. The point just before entering the combustion chamber is where this compressed air reaches its peak pressure, so it holds the most potential energy to be converted into heat and later into kinetic energy as the engine operates.

The combustor then burns fuel, adding heat and raising the temperature, but the pressure doesn’t rise to higher levels than the compressor discharge; after combustion the gas expands through turbines and the nozzle, so the pressure falls, reducing the stored pressure (potential) energy. The exhaust is at a much lower pressure as the gas exits to ambient, so it has less potential energy.

So, the location just before the combustion chamber is where the highest pressure (and thus the most potential energy) resides in the engine cycle.