gas-turbine engine
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What is a gas-turbine engine?
A gas-turbine engine is an internal combustion engine that uses gas as the working fluid to turn a turbine. It consists of a compressor, a combustion chamber, and a turbine. Air from the atmosphere is compressed and fed into a combustion chamber where fuel is burned at a constant pressure. Additional compressed air is mixed with the hot combustion gases to lower the combustion chamber exit temperature enough for the turbine to operate continuously. Gas turbines are used in aviation for jet propulsion, electric power generation, and to drive compressors for pumping natural gas through pipelines. The first successful gas turbine was built in 1903 in Paris.
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How does a gas-turbine engine produce power?
A gas-turbine engine produces power by taking in air, compressing it, and mixing it with fuel in a combustion chamber. The burning of the fuel and air mixture generates hot combustion gases, which then expand through the turbine, causing it to rotate. Most of the turbine's output is used to drive the compressor, while the remaining output supplies shaft work to a generator, pump, or other device. In a jet engine, the turbine is designed to provide just enough output to drive the compressor and auxiliary devices. The stream of gas then leaves the turbine at an intermediate pressure and is fed through a nozzle to produce thrust. Gas turbines are typically smaller and lighter than reciprocating internal-combustion engines for the same power output.
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What are the main parts of a gas-turbine engine?
A gas-turbine engine is an internal combustion engine that uses gas as the working fluid to turn a turbine. The main parts of a gas turbine engine are:
* Compressor Gas turbines can have either a centrifugal or axial-flow compressor to bring air from the atmosphere into the engine and compress it.
* Combustion chamber The compressed air from the compressor is then fed into a combustion chamber, where fuel is added and burned.
* Turbine The combustion products are expanded in the turbine. Most of the turbine output is required to operate the compressor.
* Exhaust nozzle The terminal portion of the tail pipe of a jet engine.2 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
Where are gas-turbine engines commonly used?
Gas-turbine engines have diverse applications across various sectors. They are most notably used in aviation for jet propulsion. Industrial gas-turbine engines, typically ranging from 1,000 to 50,000 horsepower, drive compressors for pumping natural gas through pipelines and can be incorporated into oil refining processes.
In electric power generation, gas turbines serve as medium-sized "peak load" plants due to their quick start-up times. They have also been tested in marine applications and were selected to power U.S. naval vessels because they are lightweight and compact. Though proposed for automobiles in the 1960s, high costs and low efficiency have made gas-turbine cars uneconomical. Gas-turbine locomotives have seen moderate success for long runs but have not replaced diesel locomotives.
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How do gas-turbine engines compare to piston engines?
Gas-turbine engines and piston engines represent different approaches to internal combustion. Gas-turbine engines use a gas as the working fluid to turn a turbine, incorporating a compressor and combustion chamber. They are smaller and lighter for the same power output compared to reciprocating internal-combustion engines, which rely on the back-and-forth motion of a piston converted to rotary motion.
Gas turbines deliver rotary shaft power directly, making them conceptually simpler, but they require careful design and costly materials due to high temperatures and stresses. While gas turbines excel in aviation for jet propulsion, their adoption has been limited by the high cost of efficient components and the need for moderate turbine-inlet temperatures. Piston engines are often used in smaller planes.
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What are some challenges and advantages of using gas-turbine engines?
Gas-turbine engines offer a compact, lightweight design, making them suitable for applications where power-to-weight ratio is critical, such as in aviation and marine propulsion. They can also start quickly, making them useful for peak power generation.
However, gas turbines face challenges including high manufacturing costs and lower thermal efficiency compared to alternatives like diesel engines or steam turbines. They also require high turbine-inlet temperatures, and their part-load and idling performance can be uneconomical.
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gas-turbine engine, any internal-combustion engine employing a gas as the working fluid used to turn a turbine. The term also is conventionally used to describe a complete internal-combustion engine consisting of at least a compressor, a combustion chamber, and a turbine.
General characteristics
Useful work or propulsive thrust can be obtained from a gas-turbine engine. It may drive a generator, pump, or propeller or, in the case of a pure jet aircraft engine, develop thrust by accelerating the turbine exhaust flow through a nozzle. Large amounts of power can be produced by such an engine that, for the same output, is much smaller and lighter than a reciprocating internal-combustion engine. Reciprocating engines depend on the up-and-down motion of a piston, which must then be converted to rotary motion by a crankshaft arrangement, whereas a gas turbine delivers rotary shaft power directly. Although conceptually the gas-turbine engine is a simple device, the components for an efficient unit must be carefully designed and manufactured from costly materials because of the high temperatures and stresses encountered during operation. Thus, gas-turbine engine installations are usually limited to large units where they become cost-effective.
Gas-turbine engine cycles
Idealized simple open-cycle gas-turbine engine
Most gas turbines operate on an open cycle in which air is taken from the atmosphere, compressed in a centrifugal or axial-flow compressor, and then fed into a combustion chamber. Here, fuel is added and burned at an essentially constant pressure with a portion of the air. Additional compressed air, which is bypassed around the burning section and then mixed with the very hot combustion gases, is required to keep the combustion chamber exit (in effect, the turbine inlet) temperature low enough to allow the turbine to operate continuously. If the unit is to produce shaft power, the combustion products (mostly air) are expanded in the turbine to atmospheric pressure. Most of the turbine output is required to operate the compressor; only the remainder is available to supply shaft work to a generator, pump, or other device. In a jet engine the turbine is designed to provide just enough output to drive the compressor and auxiliary devices. The stream of gas then leaves the turbine at an intermediate pressure (above local atmospheric pressure) and is fed through a nozzle to produce thrust.
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An idealized gas-turbine engine operating without any losses on this simple Brayton cycle is considered first. If, for example, air enters the compressor at 15° C and atmospheric pressure and is compressed to one megapascal, it then absorbs heat from the fuel at a constant pressure until the temperature reaches 1,100° C prior to expansion through the turbine back to atmospheric pressure. This idealized unit would require a turbine output of 1.68 kilowatts for each kilowatt of useful power with 0.68 kilowatt absorbed to drive the compressor. The thermal efficiency of the unit (net work produced divided by energy added through the fuel) would be 48 percent.
Actual simple open-cycle performance
If for a unit operating between the same pressure and temperature limits the compressor and the turbine are only 80 percent efficient (i.e., the work of an ideal compressor equals 0.8 times the actual work, while the actual turbine output is 0.8 times the ideal output), the situation changes drastically even if all other components remain ideal. For every kilowatt of net power produced, the turbine must now produce 2.71 kilowatts while the compressor work becomes 1.71 kilowatts. The thermal efficiency drops to 25.9 percent. This illustrates the importance of highly efficient compressors and turbines. Historically it was the difficulty of designing efficient compressors, even more than efficient turbines, that delayed the development of the gas-turbine engine. Modern units can have compressor efficiencies of 86–88 percent and turbine efficiencies of 88–90 percent at design conditions.
Efficiency and power output can be increased by raising the turbine-inlet temperature. All materials lose strength at very high temperatures, however, and since turbine blades travel at high speeds and are subject to severe centrifugal stresses, turbine-inlet temperatures above 1,100° C require special blade cooling. It can be shown that for every maximum turbine-inlet temperature there is also an optimum pressure ratio. Modern aircraft gas turbines with blade cooling operate at turbine-inlet temperatures above 1,370° C and at pressure ratios of about 30:1.
Intercooling, reheating, and regeneration
In aircraft gas-turbine engines attention must be paid to weight and diameter size. This does not permit the addition of more equipment to improve performance. Accordingly, commercial aircraft engines operate on the simple Brayton cycle idealized above. These limitations do not apply to stationary gas turbines where components may be added to increase efficiency. Improvements could include (1) decreasing compression work by intermediate cooling, (2) increasing turbine output by reheating after partial expansion, or (3) decreasing fuel consumption by regeneration.
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The first improvement would involve compressing air at nearly constant temperature. Although this cannot be achieved in practice, it can be approximated by intercooling (i.e., by compressing the air in two or more steps and water-cooling it between steps back to its initial temperature). Cooling decreases the volume of air to be handled and, with it, the compression work required.
The second improvement involves reheating the air after partial expansion through a high-pressure turbine in a second set of combustion chambers before feeding it into a low-pressure turbine for final expansion. This process is similar to the reheating used in a steam turbine.
Both approaches require considerable additional equipment and are used less frequently than the third improvement. Here, the hot exhaust gases from the turbine are passed through a heat exchanger, or regenerator, to increase the temperature of the air leaving the compressor prior to combustion. This reduces the amount of fuel needed to reach the desired turbine-inlet temperature. The increase in efficiency is, however, tied to a large increase in initial cost and will be economical only for units that are run almost continuously.