Jets Thermal Efficiency
This is an excerpt from a book on Jet engines.
In comparing the two engines,it is interesting to note that the reciprocating engine obtains its work output by employing very high pressures(as much as 1000 psi) in the cylinder during combustion.With these high pressures, a large amount of work can be obtained from a given quantity of fuel,thus raising the thermal efficiency(the relationship between the potential heat energy in the fuel and the actual energy output of the engine) of this type of engine.On the other hand, a jet engine's thermal efficiency is limited by the ability of the compressor to build up high pressure without an excessive temperature rise.But increasing the compression ratio would also increase the compressed air temperature.Since most gas turbine engines are already operating at maximum temperature limits,this increased air temperature would result in a mandatory decrease in fuel flow,thus making it extremely difficult to increase compression ratios without designing more efficient compressors,i.e compressors able to pump air with a minimum temperature rise.Ideally we would like to burn as much fuel as possible in the jet engine inorder to raise the gas temperature and increase the useful output.
Could someone please explain /elaborate on what the lines in bold are trying to convey.Why are gas turbines already opeating at max temp limits?Why would the increased air temp result in mandatory decrease in fuel flow?Why would it be ideal to burn more fuel ?
Thanx