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The gas turbine works as an open cyclic process. Typical for an open cycle is: the working medium is taken from the environment and fed back to it after the process.
 
- Adiabatic compression of the cold gas with compressor (A) from ambient pressure p₁ to pressure p₂, associated with temperature rising from T₁ to T₂.
 
- Isobaric heating of gas from T₂ to T₃ because of heat input. Heat input by burning fuel with oxygen of the air in combustion chamber (B).

- Adiabatic expansion of hot gas in a turbine (C) from pressure p₂ to p₁, associated with temperature decreasing from T₃ to T₄.
 
One part of the mechanical power generated by the turbine is used for driving the compressor. The rest is available as effective power for driving a generator (D) etc.

T-s diagram: Illustration of the ideal gas turbine process
q₁ heat input,  q₂ heat output,  w effective work,  T temperature,  s entropy

Fields of application
 
Gas turbines are used when high power and lightweight are required. Contrary to piston engines, turbomachines operate at high flow rates with compact dimensions.
 
- Driving aircraft with propeller or jet engine
- Driving fast ships, locomotives or heavy motor vehicles
- Driving generators in power plants
- Driving compressors and pumps in the petroleum and natural gas industries

Schematic of a simple gas turbine system:
A compressor, B combustion chamber, C turbine, D generator;
Arrows: air (blue), fuel (red), exhaust (orange)

Quick starting is another advantage of gas turbines. They run up quickly to full load and therefore are often used as backup drives and for peak loads. In comparison to diesel engines, the higher fuel consumption is a disadvantage.

Principle of a two-shaft gas turbine 
A two-shaft gas turbine consists of two independent turbines. The first turbine (the high-pressure turbine) is coupled tightly to the compressor and drives the compressor. The second turbine (the power turbine) is not mechanically coupled with the high-pressure turbine and generates the effective power of the system. A vehicle, a propeller or a generator can be driven.
The advantage of the two-shaft gas turbine is that compressor and high-pressure turbine are driven at optimal speed for the respective power. Speed or torque at the power turbine can be adapted optimised to the respective drive function. Vehicles require a variable speed, a synchronous generator needs constant speed.

Process schematic of two-shaft gas turbine ET 794 with independent power turbine and generator

The turbine is operated with combustion gas. An electrically driven auxiliary compressor (starter fan) starts the turbine. At a certain minimum speed fuel gas is blown into the combustion chamber and is electrically ignited. After reaching idle speed, the auxiliary compressor is turned off and the turbine runs on its own.
An oil circuit with thermostatically controlled oil-to-water cooler, pump and filter lubricates and cools the turbine bearings. The turbine shuts down if the oil temperature is too high or the oil pressure is too low.

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