---
title: "Modifications of the basic gas turbine cycle"
subject: "Turbo Machines"
chapter: 6
key: "semester_7/turbo_machines:variations-basic-gas-turbine-cycle"
source: https://www.bctengineer.com/q/semester_7/turbo_machines/variations-basic-gas-turbine-cycle
site: BCT Engineering
---

# Modifications of the basic gas turbine cycle

Turbo Machines · Chapter 6: Gas Turbine Cycles of Aircraft Propulsion

- Asked in: year not given, 5 marks
- Video: [Lec 2 modifications of gas turbine systems](https://www.youtube.com/watch?v=1w89MXRDDHg) — Ahmed sharaf
- Page: https://www.bctengineer.com/q/semester_7/turbo_machines/variations-basic-gas-turbine-cycle
- Whole chapter: https://www.bctengineer.com/md/semester_7/turbo_machines/chapter-6

## Question

Explain, with T-s diagrams, how regeneration, intercooling, reheating and afterburning modify the basic gas turbine cycle. State their effects on the net work, thermal efficiency and thrust.

## Short answer

Regeneration uses exhaust heat to preheat compressor air, raising efficiency without changing net work. Intercooling reduces compressor work, and reheating increases turbine work; both raise net work but lower efficiency unless combined with a regenerator. Afterburning burns fuel in the exhaust to boost thrust, sharply increasing fuel consumption.

## Answer

The simple Brayton cycle has a low work ratio and, at moderate pressure ratio, a high exhaust temperature. These modifications address that.

#### 1. Regeneration (heat exchanger)
Hot turbine exhaust pre-heats compressor delivery air before the combustor, which reduces the fuel needed.
- Work output is unchanged; heat supplied falls, so **efficiency rises**.
- Useful only when the turbine exhaust is hotter than the compressor exit, i.e. at low pressure ratio. Effectiveness $\epsilon = (T_x - T_2)/(T_4 - T_2)$ is 0.6 to 0.85.
- For aircraft it adds too much weight; used in stationary and marine plants.

#### 2. Intercooling
Compression is done in two stages with cooling in between to near the inlet temperature.
- Compressor work falls (work per kg $\propto$ absolute inlet temperature), so **net work and work ratio rise**.
- Efficiency alone may fall because more fuel is needed to heat the cooler air, unless a regenerator is also used.

#### 3. Reheating
The gas is expanded in two turbine stages with an additional combustor between them.
- Turbine work rises, so **net work increases** (and the exhaust temperature too).
- Efficiency without regeneration usually falls slightly; with regeneration it improves.

```text
 Arrangement with all three modifications:

 air->[LPC]->[Intercooler]->[HPC]->[Regenerator]
                                      |
        [Comb 1]->[HPT]->[Comb 2 (reheat)]->[LPT]
                                      |
                       exhaust <------+ (heat to regenerator)
```

#### 4. Afterburning (thrust augmentation)
Extra fuel is burnt in the turbine exhaust, which still contains much unburnt oxygen, before the propelling nozzle.
- Jet velocity rises; thrust increases by 40 to 70%.
- Fuel consumption rises sharply (TSFC two to three times higher), so used only for take-off, climb and combat.
- A variable-area nozzle is needed.

#### Combined arrangement
Intercooling + reheating + regeneration approach the Ericsson cycle (isothermal compression and expansion) and so give the highest efficiency of all, at the price of weight, cost and complexity.

| Modification | Net work | Efficiency | Mainly used in |
|---|---|---|---|
| Regeneration | Same | Increases | Power plant |
| Intercooling | Increases | Decreases (alone) | Power plant |
| Reheat | Increases | Decreases (alone) | Power plant |
| Afterburner | Thrust increases | Decreases | Military aircraft |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Regeneration in a gas turbine cycle
   A. increases net work and lowers efficiency
   B. keeps work output the same, lowers heat supplied, so efficiency rises
   C. reduces turbine work
   D. raises thrust of jets
   Answer: B. Exhaust heat pre-heats compressor delivery air, reducing fuel, while net work is unchanged. It works when turbine exhaust is hotter than compressor exit. (Key point: Regeneration effect)

2. Why does intercooling increase net work?
   A. It adds heat to the turbine
   B. It raises combustion temperature
   C. Compressor work per kg is proportional to inlet temperature, so cooling between stages reduces it
   D. It increases mass flow
   Answer: C. Cooling the air in between lowers compressor work; efficiency alone may fall because more fuel is needed to heat cooler air. (Key point: Intercooling effect)

3. Reheating
   A. lowers turbine work
   B. removes the combustor
   C. reduces exhaust temperature
   D. increases turbine work and net work, with efficiency falling slightly unless combined with regeneration
   Answer: D. Extra combustion between turbine stages raises turbine work and exhaust temperature. (Key point: Reheating effect)

4. Afterburning in a jet engine
   A. burns fuel in turbine exhaust oxygen, raising thrust by 40 to 70% but with much higher TSFC
   B. lowers jet velocity
   C. is used for continuous cruise
   D. reduces fuel use
   Answer: A. The unburnt oxygen in the exhaust allows more fuel to burn before the nozzle, so it is only for take-off, climb and combat. (Key point: Afterburning)

5. Intercooling + reheating + regeneration approximate which cycle?
   A. Otto cycle
   B. Ericsson cycle (isothermal compression and expansion), giving the highest efficiency
   C. Rankine cycle
   D. Humphrey cycle
   Answer: B. Many stages of intercooling and reheating approach isothermal processes, at the price of weight, cost and complexity. (Key point: Combined arrangement)

## Related in Turbo Machines

- [Actual gas turbine cycle with component efficiencies](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Aactual-brayton-cycle-numerical) — Turbo Machines, chapter 3
- [Ideal Brayton cycle work and efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Aideal-brayton-numerical) — Turbo Machines, chapter 3
- [Gas turbine power from isentropic efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Agas-turbine-power-isentropic-efficiency) — Turbo Machines, chapter 1
- [Impulse turbine blade work and diagram efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Aimpulse-turbine-diagram-efficiency) — Turbo Machines, chapter 2
- [Turbojet cycle, thrust and propulsive efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Aturbojet-cycle-thrust-propulsive-efficiency) — Turbo Machines, chapter 6
- [Brayton cycle efficiency and optimum pressure ratio](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Abrayton-cycle-efficiency-optimum-pressure-ratio) — Turbo Machines, chapter 3

## Related in other subjects

- [Gas turbine with intercooling, reheat and regeneration](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-intercool-reheat-regen) — Power Plant Equipment, chapter 6
- [Open cycle gas turbine with intercooling](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-intercooling-sketch) — Power Plant Equipment, chapter 6
- [Closed gas turbine cycle components; thermal efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Aclosed-gas-turbine-cycle-efficiency) — Fundamental of Thermodynamics & Heat Transfer, chapter 6
- [Ideal gas turbine 100 kW: pressure ratio, work, efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Aideal-gas-turbine-100kw-pressure-ratio) — Fundamental of Thermodynamics & Heat Transfer, chapter 6