There are a few points I will make but first off a jet engine is NOT self sustaining. It takes the input of fuel to keep running. The fuel is the source of energy that keeps it running. Now lets look at the mechanics a bit, to simplify things I will use a Ramjet instead of a turbine driven jet since the compression is taken care of by the simple shape of the engine, no moving parts. Lets assume the jet is moving (ram jets dont work at zero speed).
- When the engine is thrust forward air enters the front
- The shape of the nozzle compresses (squeezes) the air
- Fuel is mixed into the air (same as in a carb)
- The air/fuel mixture enters the combustion chamber
- The air/fuel mixture is ignited
- The ignited mixture expands and is directed out of the back
- The engine (as a result of newtons 3rd law) moves forward (opposite reaction)
7b. The air/fuel ignition contained enough energy to not only move the engine but the craft as well. Total Energy = Energy to Drive Engine + Energy to Move Object
- The forward moving force causes air to enter the front of the engine
- Back to step 1.
To make this all work the output energy has to be greater than the energy needed to compress the air. This energy comes from the fuel that is added to the mixture. Jet fuel has about 46 MJ/Kg.
In the case of a turbine engine we simply introduce a compressor mechanism. Instead of relying on the shape of the engine to compress the air, some of the output energy is sacrificed to spin (and operate) the compressor mechanism. It should be noted that shape does play a roll in turbine jets as well.