Main types of gas turbines

Main Types of Gas Turbines

Turbojet
The turbojet, the simplest and earliest type of gas turbine, is used principally in high-speed aircraft where its relatively small frontal area and high jet velocity are advantageous. The turbine extracts only sufficient energy from the gas stream to drive the compressor, leaving the remaining energy to provide the thrust.

Examples of the turbojet are the OLYMPUS 593 in the Concorde supersonic airliner and the VIPER which is used in a variety of military aircraft.

Turbofan
The turbofan is the most common type of gas turbine used for aircraft propulsion today. Part of the air entering the engine is compressed fully and passed into the combustion chamber, while the remainder, compressed to a lesser extent, bypasses the combustion section, to provide cold thrust. This bypass flow rejoins the hot flow downstream of the turbine, as in the ADOUR engine. In both cases, the overall jet velocity is reduced to give better propulsive efficiency, lower noise levels and improved specific fuel consumption, which make the turbofan ideal for both civil and military aircraft.

Examples of the turbofan are the RB211 in the Boeing 747, the 535 in the Boeing 757, the TAY in the Gulfstream IV and Fokker 100, the ADOUR in the Jaguar and Hawk, and the RB199 in the Tornado. The PEGASUS in the Harrier is a variation of the turbofan engine.

Turboprop
The turboprop is a turbojet with an additional turbine which uses the energy remaining in the gas stream, after sufficient has been absorbed to drive the compressor, to drive a propeller. The additional turbine, called the power turbine, drives the propeller through a shaft and a reduction gear. A small amount of residual thrust remains in the exhaust gases during normal operation.
The turboprop is a very efficient powerplant for relatively low-speed, low-altitude aircraft, (eg, 400 mph/30000 ft), although developments in propeller technology have demonstrated the feasibility of a new generation of high-speed propeller-driven aircraft.

Examples of the turboprop are the DART in the British Aerospace 748 and the Fokker F27, and the TYNE in the Transall C-160 and Dassault-Breguet Atlantic.
Turboprop power is measured in total equivalent horsepower (tehp) or kilowatts (kW), ie: the shaft horsepower plus the residual thrust.

Turboshaft
The turboshaft is effectively a turboprop without a propeller, the power turbine in this case being coupled to a reduction gearbox or directly to an output shaft. In the same way as the turboprop, the power turbine absorbs as much of the remaining gas energy as possible and the residual thrust is very low.

Turboshaft power is normally measured in shaft horsepower (shp) or kilowatts (kW).

The most common application of the turboshaft is the helicopter, in which the engine drives both the main and tail rotors. Turboshafts are also widely used for industrial and marine installations, including power and pumping stations, hovercraft and ships.

Examples of the turboshaft are the GEM in the Westland Lynx and the GNOME in the Westland Sea King helicopters.