Turbojet - Cycle Improvements

Cycle Improvements

Thermodynamics of a typical air-breathing jet engine are modeled approximately by a Brayton Cycle.

Increasing the overall pressure ratio of the compression system also raises the combustor entry temperature. Therefore, at fixed fuel and air flows, there is an increase in turbine inlet temperature. Although the temperature rise across the compression system will also cause the turbine temperature to increase, the nozzle temperature remains unaffected because the same amount of heat is being added to the system. There is, however, a rise in nozzle pressure, because overall pressure ratio increases faster than the turbine expansion ratio. Consequently, net thrust increases, while specific fuel consumption (fuel flow/net thrust) decreases.

Thus turbojets can be made more fuel efficient by raising overall pressure ratio and turbine inlet temperature in union. However, better turbine materials and/or improved vane/blade cooling are required to cope with increases in both turbine inlet temperature and compressor delivery temperature. Increasing the latter requires better compressor materials.

Minimizing heat losses and optimizing the inlet temperature ratio will increase the system's useful work and the thermal efficiency of the turbo jet engine.