How is energy wasted in a jet exhaust leaving the jet engine?

Question

More is the momentum imparted to the jet exhaust leaving out back, more is the thrust that can be generated. How is the energy wasted?

Answer 1

First you have to know where the energy is comming from.  As the Air enters the intake of a jet engine it is compressed in the compressor section of the engine.  The compression of the air causes its velocity to decrease to a point where the air entering the engine is slower than the air exiting the compressor and entering the combustion chamber.  At this point fuel is added to the air and the mixture is ignited.  The ignition process creates heat as well as forces the combustive gases to expand.  These gases exit the eombustion section and enter the turbine section.  Here the exhaust gases are used spin the turbines which in turn spin the compressor section.  (i.e. the faster the turbins spin the faster the compressor spins and draws in more air thus making the engine to speed up to a higher RPM)  As the turbins spin they increase the velocity of the exhaust gases but decrease the temperature of the gasses to do so until the gases exit out into the exhaust section at which point the exhaust gases are given their final velocity increase and temperatuer and pressure decrease so that they can impart the actual thurst on the engine.

Answer 2

If you measure the temperature at each turbine stage, you will find that the gas temperature decreases as energy is taken out to turn each rotor group. For example, in a typical three stage turbine engine, the first turbine may be subjected to a gas temperature (turbine inlet temperature) exceeding 1900 deg F. As the energy from the flow is used to turn the first turbine, that temperature is reduced. The second turbine may only see 1600 deg F, and the third turbine perhaps 1400 deg F. The exhaust temperature out the back (after the third stage) may be "down" in the 1200 deg F range. But ideally, if all the energy were efficiently removed from the gas path, the exhaust temperature would be the same as the surrounding air. That cannot occur because of issues like entropy and irreversible processes, but what's left over is wasted energy that cannot be recovered - like the heat from your car engine.

ADDED:  As mentioned above, there are other areas where waste occurs; noise, vibration, etc. but the largest by far is heat.

Answer 3

If the exhaust is hotter than ambient air after leaving the engine, energy is being wasted.  Modern designs attempt to extract as much energy as possible before the exhaust leaves, which translates to lower exhaust temperatures.

Answer 4

As the primary function of a jet engine is to produce thrust, then energy that goes to any other purpose can be said to be "wasted."

So all sound, heat, and light produced by a jet engine is considered waste energy, in engineering terms.