Gas-turbine engine


A gas turbine engine, or, informally, a gas turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part and are, in the direction of flow:
Additional components have to be added to the gas generator to suit its application. Common to all is an air inlet but with different configurations to suit the requirements of marine use, land use or flight at speeds varying from stationary to supersonic. A propelling nozzle is added to produce thrust for flight. An extra turbine is added to drive a propeller or ducted fan to reduce fuel consumption at subsonic flight speeds. An extra turbine is also required to drive a helicopter rotor or land-vehicle transmission, marine propeller or electrical generator. Greater thrust-to-weight ratio for flight is achieved with the addition of an afterburner.
The basic operation of the gas turbine is a Brayton cycle with air as the working fluid: atmospheric air flows through the compressor that brings it to higher pressure; energy is then added by spraying fuel into the air and igniting it so that the combustion generates a high-temperature flow; this high-temperature pressurized gas enters a turbine, producing a shaft work output in the process, used to drive the compressor; the unused energy comes out in the exhaust gases that can be repurposed for external work, such as directly producing thrust in a turbojet engine, or rotating a second, independent turbine that can be connected to a fan, propeller, or electrical generator. The purpose of the gas turbine determines the design so that the most desirable split of energy between the thrust and the shaft work is achieved. The fourth step of the Brayton cycle is omitted, as gas turbines are open systems that do not reuse the same air.
Gas turbines are used to power among others aircraft, trains, ships, electric generators, pumps, gas compressors, and tanks.

Timeline of development

  • 50: Earliest records of Hero's engine. It most likely served no practical purpose, and was rather more of a curiosity; nonetheless, it demonstrated an important principle of physics that all modern turbine engines rely on.
  • 1000: The "Trotting Horse Lamp" was used by the Chinese at lantern fairs as early as the Northern Song dynasty. When the lamp is lit, the heated airflow rises and drives an impeller with horse-riding figures attached on it, whose shadows are then projected onto the outer screen of the lantern.
  • 1500: The Smoke jack was drawn by Leonardo da Vinci: Hot air from a fire rises through a single-stage axial turbine rotor mounted in the exhaust duct of the fireplace and turns the roasting spit by gear-chain connection.
  • 1791: A patent was given to John Barber, an Englishman, for the first true gas turbine. His invention had most of the elements present in the modern day gas turbines. The turbine was designed to power a horseless carriage.
  • 1894: Sir Charles Parsons patented the idea of propelling a ship with a steam turbine, and built a demonstration vessel, the Turbinia, easily the fastest vessel afloat at the time.
  • 1899: Charles Gordon Curtis patented the first gas turbine engine in the US.
  • 1900: Sanford Alexander Moss submitted a thesis on gas turbines. In 1903, Moss became an engineer for General Electric's Steam Turbine Department in Lynn, Massachusetts. While there, he applied some of his concepts in the development of the turbocharger.
  • 1903: A Norwegian, Ægidius Elling, built the first gas turbine that was able to produce more power than needed to run its own components, which was considered an achievement in a time when knowledge about aerodynamics was limited. Using rotary compressors and turbines it produced.
  • 1904: A gas turbine engine designed by, based on his earlier 1873 patent application, is built and tested in Berlin. The Stolze gas turbine was too inefficient to sustain its own operation.
  • 1906: The Armengaud-Lemale gas turbine tested in France. This was a relatively large machine which included a 25-stage centrifugal compressor designed by Auguste Rateau and built by the Brown Boveri Company. The gas turbine could sustain its own air compression but was too inefficient to produce useful work.
  • 1910: The first operational Holzwarth gas turbine achieves an output of. Planned output of the machine was and its efficiency is below that of contemporary reciprocating engines.
  • 1920s The practical theory of gas flow through passages was developed into the more formal theory of gas flow past airfoils by A. A. Griffith resulting in the publishing in 1926 of An Aerodynamic Theory of Turbine Design. Working testbed designs of axial turbines suitable for driving a propeller were developed by the Royal Aeronautical Establishment.
  • 1930: Having found no interest from the RAF for his idea, Frank Whittle patented the design for a centrifugal gas turbine for jet propulsion. The first successful test run of his engine occurred in England in April 1937.
  • 1932: The Brown Boveri Company of Switzerland starts selling axial compressor and turbine turbosets as part of the turbocharged steam generating Velox boiler. Following the gas turbine principle, the steam evaporation tubes are arranged within the gas turbine combustion chamber; the first Velox plant is erected at a French Steel mill in Mondeville, Calvados.
  • 1936: The first constant flow industrial gas turbine is commissioned by the Brown Boveri Company and goes into service at Sun Oil's Marcus Hook refinery in Pennsylvania, US.
  • 1937: Working proof-of-concept prototype turbojet engine runs in UK and Germany. Henry Tizard secures UK government funding for further development of Power Jets engine.
  • 1939: The First 4 MW utility power generation gas turbine is built by the Brown Boveri Company for an emergency power station in Neuchâtel, Switzerland. The turbojet powered Heinkel He 178, the world's first jet aircraft, makes its first flight.
  • 1940: Jendrassik Cs-1, a turboprop engine, made its first bench run. The Cs-1 was designed by Hungarian engineer György Jendrassik, and was intended to power a Hungarian twin-engine heavy fighter, the RMI-1. Work on the Cs-1 stopped in 1941 without the type having powered any aircraft.
  • 1944: The Junkers Jumo 004 engine enters full production, powering the first German military jets such as the Messerschmitt Me 262. This marks the beginning of the reign of gas turbines in the sky.
  • 1946: National Gas Turbine Establishment formed from Power Jets and the RAE turbine division to bring together Whittle and Hayne Constant's work. In Beznau, Switzerland the first commercial reheated/recuperated unit generating 27 MW was commissioned.
  • 1947: A Metropolitan Vickers G1 becomes the first marine gas turbine when it completes sea trials on the Royal Navy's M.G.B 2009 vessel. The Gatric was an aeroderivative gas turbine based on the Metropolitan Vickers F2 jet engine.
  • 1995: Siemens becomes the first manufacturer of large electricity producing gas turbines to incorporate single crystal turbine blade technology into their production models, allowing higher operating temperatures and greater efficiency.
  • 2011: Mitsubishi Heavy Industries tests the first >60% efficiency combined cycle gas turbine at its Takasago, Hyōgo, works.

    Theory of operation

In an ideal gas turbine, gases undergo four thermodynamic processes: an isentropic compression, an isobaric combustion, an isentropic expansion and isobaric heat rejection. Together, these make up the Brayton cycle, also known as the "constant pressure cycle". It is distinguished from the Otto cycle, in that all the processes, occur at the same time, continuously.
In a real gas turbine, mechanical energy is changed irreversibly into pressure and thermal energy when the gas is compressed. Heat is added in the combustion chamber and the specific volume of the gas increases, accompanied by a slight loss in pressure. During expansion through the stator and rotor passages in the turbine, irreversible energy transformation once again occurs. Fresh air is taken in, in place of the heat rejection.
Air is taken in by a compressor, called a gas generator, with either an axial or centrifugal design, or a combination of the two. This air is then ducted into the combustor section which can be of a annular, can, or can-annular design. In the combustor section, roughly 70% of the air from the compressor is ducted around the combustor itself for cooling purposes. The remaining roughly 30% the air is mixed with fuel and ignited by the already burning air-fuel mixture, which then expands producing power across the turbine. This expansion of the mixture then leaves the combustor section and has its velocity increased across the turbine section to strike the turbine blades, spinning the disc they are attached to, thus creating useful power. Of the power produced, 60-70% is solely used to power the gas generator. The remaining power is used to power what the engine is being used for, typically an aviation application, being thrust in a turbojet, driving the fan of a turbofan, rotor or accessory of a turboshaft, and gear reduction and propeller of a turboprop.
If the engine has a power turbine added to drive an industrial generator or a helicopter rotor, the exit pressure will be as close to the entry pressure as possible with only enough energy left to overcome the pressure losses in the exhaust ducting and expel the exhaust. For a turboprop engine there will be a particular balance between propeller power and jet thrust which gives the most economical operation. In a turbojet engine only enough pressure and energy is extracted from the flow to drive the compressor and other components. The remaining high-pressure gases are accelerated through a nozzle to provide a jet to propel an aircraft.
The smaller the engine, the higher the rotation rate of the shaft must be to attain the required blade tip speed. Blade-tip speed determines the maximum pressure ratios that can be obtained by the turbine and the compressor. This, in turn, limits the maximum power and efficiency that can be obtained by the engine. In order for tip speed to remain constant, if the diameter of a rotor is reduced by half, the rotational speed must double. For example, large jet engines operate around 10,000–25,000 rpm, while micro turbines spin as fast as 500,000 rpm.
Mechanically, gas turbines can be considerably less complex than Reciprocating engines. Simple turbines might have one main moving part, the compressor/shaft/turbine rotor assembly, with other moving parts in the fuel system. This, in turn, can translate into price. For instance, costing for materials, the Jumo 004 proved cheaper than the Junkers 213 piston engine, which was, and needed only 375 hours of lower-skill labor to complete, compared to 1,400 for the BMW 801. This, however, also translated into poor efficiency and reliability. More advanced gas turbines may have 2 or 3 shafts, hundreds of compressor and turbine blades, movable stator blades, and extensive external tubing for fuel, oil and air systems; they use temperature resistant alloys, and are made with tight specifications requiring precision manufacture. All this often makes the construction of a simple gas turbine more complicated than a piston engine.
Moreover, to reach optimum performance in modern gas turbine power plants the gas needs to be prepared to exact fuel specifications. Fuel gas conditioning systems treat the natural gas to reach the exact fuel specification prior to entering the turbine in terms of pressure, temperature, gas composition, and the related Wobbe index.
The primary advantage of a gas turbine engine is its power to weight ratio.
Since significant useful work can be generated by a relatively lightweight engine, gas turbines are perfectly suited for aircraft propulsion.
Thrust bearings and journal bearings are a critical part of a design. They are hydrodynamic oil bearings or oil-cooled rolling-element bearings. Foil bearings are used in some small machines such as micro turbines and also have strong potential for use in small gas turbines/auxiliary power units