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a b
Fig. 4.8 – An open-cicle a and a closed-cycle b gas-turbine engine
these plants, the exhaust gases of the gas turbine serve as the heat source for the steam.
The gas-turbine cycle can also be executed as a closed cycle for use in nuclear
power plants. This time the working fluid is not limited to air, and a gas with more
desirable characteristics (such as helium) can be used.
The majority of the Western world’s naval fleets already use gas-turbine engines for
propulsion and electric power generation. The General Electric LM2500 gas turbines
used to power ships have a simple-cycle thermal efficiency of 37 percent. The General
ElectricWR-21 gas turbines equipped with intercooling and regeneration have a thermal
efficiency of 43 percent and produce 21.6 MW. The regeneration also reduces the
exhaust temperature from 600°C to 350°C. Air is compressed to 3 atm before it enters
the intercooler. Compared to steam-turbine and diesel-propulsion systems, the gas
turbine offers greater power for a given size and weight, high reliability, long life, and
more convenient operation. The engine start-up time has been reduced from 4h required
for a typical steam-propulsion system to less than 2 min for a gas turbine. Many modern
marine propulsion systems use gas turbines together with diesel engines because of the
high fuel consumption of simple-cycle gas-turbine engines. In combined diesel and gas-
turbine systems, diesel is used to provide for efficient low-power and cruise operation,
and gas turbine is used when high speeds are needed.
The open gas-turbine cycle described above (Fig. 16a) can be modeled as a closed
cycle (Fig. 4.8b) by utilizing the air-standard assumptions. Here the compression and
expansion processes remain the same, but the combustion process is replaced by a
constant-pressure heat-addition process from an external source, and the exhaust
process is replaced by a constant-pressure heat-rejection process to the ambient air.
The ideal cycle that the working fluid undergoes in this closed loop is the Brayton
cycle, which is made up of four internally reversible processes:
1-2 Isentropic compression (in a compressor)
2-3 Constant-pressure heat addition
3-4 Isentropic expansion (in a turbine)

4-1 Constant-pressure heat rejection
The T-s and P-v diagrams of an ideal Brayton cycle are shown in Fig. 4.9. Large
amount of work is consumed in process 4-1 for a gas turbine cycle as the working

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