The Brayton cycle, also known as the Joule cycle, is a

thermodynamic cycle A thermodynamic cycle consists of linked sequences of thermodynamic processes that involve heat transfer, transfer of heat and work (physics), work into and out of the system, while varying pressure, temperature, and other state variables within t ...

that describes the operation of certain

heat engine A heat engine is a system that transfers thermal energy to do mechanical or electrical work. While originally conceived in the context of mechanical energy, the concept of the heat engine has been applied to various other kinds of energy, pa ...

s that have air or some other gas as their

working fluid For fluid power, a working fluid is a gas or liquid that primarily transfers force, motion, or mechanical energy. In hydraulics, water or hydraulic fluid transfers force between hydraulic components such as hydraulic pumps, hydraulic cylinders, a ...

. It is characterized by

isentropic An isentropic process is an idealized thermodynamic process that is both adiabatic and reversible. The work transfers of the system are frictionless, and there is no net transfer of heat or matter. Such an idealized process is useful in eng ...

compression and expansion, and

isobaric Isobar may refer to: * Isobar (meteorology), a line connecting points of equal atmospheric pressure reduced to sea level on the maps. * Isobaric process, a process taking place at constant pressure * Isobar (nuclide), one of multiple nuclides with ...

heat addition and rejection, though practical engines have adiabatic rather than isentropic steps. The most common current application is in

airbreathing jet engine An airbreathing jet engine (or ducted jet engine) is a jet engine in which the exhaust gas which supplies jet propulsion is atmospheric air, which is taken in, compressed, heated, and expanded back to atmospheric pressure through a propelling noz ...

s and

gas turbine A gas turbine or gas turbine engine is a type of Internal combustion engine#Continuous combustion, continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas gene ...

engines. The engine cycle is named after

George Brayton George Bailey Brayton (1830–1892) was an American mechanical engineer and inventor. He was noted for introducing the constant pressure engine that is the basis for the gas turbine, and which is now referred to as the Brayton cycle. Brayton' ...

(1830–1892), the American

engineer Engineers, as practitioners of engineering, are professionals who Invention, invent, design, build, maintain and test machines, complex systems, structures, gadgets and materials. They aim to fulfill functional objectives and requirements while ...

, who developed the Brayton Ready Motor in 1872, using a piston compressor and piston expander. An engine using the cycle was originally proposed and patented by Englishman John Barber in 1791, using a reciprocating compressor and a turbine expander. There are two main types of Brayton cycles: closed and open. In a closed cycle, the working gas stays inside the engine. Heat is introduced with a

heat exchanger A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contac ...

external combustion An external combustion engine (EC engine) is a reciprocating heat engine where a working fluid, contained internally, is heated by combustion in an external source, through the engine wall or a heat exchanger. The fluid then, by expanding and ...

and expelled with a heat exchanger. With the open cycle, air from the atmosphere is drawn in, goes through three steps of the cycle, and is expelled again to the atmosphere. Open cycles allow for

internal combustion An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal comb ...

. Although the cycle is open, it is conventionally assumed for the purposes of

thermodynamic Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of th ...

analysis that the exhaust gases are reused in the intake, enabling analysis as a closed cycle.

History

In 1872, George Brayton applied for a patent for his "Ready Motor", a reciprocating heat engine operating on a gas power cycle. The engine was a two-stroke and produced power on every revolution. Brayton engines used a separate piston compressor and piston expander, with compressed air heated by internal fire as it entered the expander cylinder. The first versions of the Brayton engine were vapor engines which mixed fuel with air as it entered the compressor; town gas was used or a surface carburetor was also used for mobile operation . The fuel / air was contained in a reservoir / tank and then it was admitted to the expansion cylinder and burned. As the fuel/air mixture entered the expansion cylinder, it was ignited by a pilot flame. A screen was used to prevent the fire from entering or returning to the reservoir. In early versions of the engine, this screen sometimes failed and an explosion would occur. In 1874, Brayton solved the explosion problem by adding the fuel just prior to the expander cylinder. The engine now used heavier fuels such as kerosene and fuel oil. Ignition remained a pilot flame. Brayton produced and sold "Ready Motors" to perform a variety of tasks like water pumping, mill operation, running generators, and marine propulsion. The "Ready Motors" were produced from 1872 to sometime in the 1880s; several hundred such motors were likely produced during this time period. Brayton licensed the design to Simone in the UK. Many variations of the layout were used; some were single-acting and some were double-acting. Some had under walking beams; others had overhead walking beams. Both horizontal and vertical models were built. Sizes ranged from less than one to over 40 horsepower. Critics of the time claimed the engines ran smoothly and had a reasonable efficiency. Brayton-cycle engines were some of the first internal combustion engines used for motive power. In 1875, John Holland used a Brayton engine to power the world's first self-propelled submarine (Holland boat #1). In 1879, a Brayton engine was used to power a second submarine, the '' Fenian Ram''.

John Philip Holland John Philip Holland (; February 24, 1841August 12, 1914) was an Irish marine engineer who developed the first submarine to be formally commissioned by the US Navy, USS Holland (SS-1) and the first Royal Navy submarine, ''Holland 1''. Early lif ...

's submarines are preserved in the

Paterson Museum Paterson Museum is a museum in Paterson, in Passaic County, New Jersey, in the United States. Founded in 1925, it is owned and run by the city of Paterson and its mission is to preserve and display the industrial history of Paterson. It is lo ...

in the

Old Great Falls Historic District The Great Falls of the Passaic River is a prominent waterfall, high, on the Passaic River in the city of Paterson, New Jersey, Paterson in Passaic County, New Jersey, Passaic County, New Jersey. One of the List of waterfalls of the United Sta ...

Paterson, New Jersey Paterson ( ) is the largest City (New Jersey), city in and the county seat of Passaic County, New Jersey, Passaic County, in the U.S. state of New Jersey. In 1878,

George B. Selden George Baldwin Selden (September 14, 1846 – January 17, 1922) was an American patent lawyer and inventor from New York who was granted a U.S. patent for an automobile in 1895.Flink, p. 51 ''Probably the most absurd action in the history of pa ...

patented the first internal combustion automobile. Inspired by the

internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal comb ...

invented by Brayton displayed at the

Centennial Exposition The Centennial International Exhibition, officially the International Exhibition of Arts, Manufactures, and Products of the Soil and Mine, was held in Philadelphia, Pennsylvania, from May 10 to November 10, 1876. It was the first official wo ...

in Philadelphia in 1876, Selden patented a four-wheel car working on a smaller, lighter, multicylinder version. He then filed a series of amendments to his application which stretched out the legal process, resulting in a delay of 16 years before the patent was granted on November 5, 1895. In 1903, Selden sued Ford for patent infringement and

Henry Ford Henry Ford (July 30, 1863 – April 7, 1947) was an American Technological and industrial history of the United States, industrialist and business magnate. As the founder of the Ford Motor Company, he is credited as a pioneer in making automob ...

fought the Selden patent until 1911. Selden had never actually produced a working car, so during the trial, two machines were constructed according to the patent drawings. Ford argued his cars used the four-stroke

Alphonse Beau de Rochas Alphonse Eugène Beau de Rochas (9 April 1815, Digne-les-Bains, Alpes-de-Haute-Provence – 27 March 1893, Vincennes) was a French engineer. He was the first to patent the four-stroke engine in 1862, but he did not build one and the idea was sub ...

cycle or

Otto cycle An Otto cycle is an idealized thermodynamic cycle that describes the functioning of a typical spark ignition piston engine. It is the thermodynamic cycle most commonly found in automobile engines. The Otto cycle is a description of what happ ...

and not the Brayton-cycle engine used in the Selden auto. Ford won the appeal of the original case. In 1887, Brayton developed and patented a four-stroke direct-injection oil engine. The fuel system used a variable-quantity pump and liquid-fuel, high-pressure, spray-type injection. The liquid was forced through a spring-loaded, relief-type valve (injector) which caused the fuel to become divided into small droplets. Injection was timed to occur at or near the peak of the compression stroke. A platinum igniter provided the source of ignition. Brayton describes the invention as: “I have discovered that heavy oils can be mechanically converted into a finely divided condition within a firing portion of the cylinder, or in a communicating firing chamber.” Another part reads, “I have for the first time, so far as my knowledge extends, regulated speed by variably controlling the direct discharge of liquid fuel into the combustion chamber or cylinder into a finely divided condition highly favorable to immediate combustion.” This was likely the first engine to use a lean-burn system to regulate engine speed and output. In this manner, the engine fired on every power stroke and speed and output were controlled solely by the quantity of fuel injected. In 1890, Brayton developed and patented a four-stroke, air-blast oil engine. The fuel system delivered a variable quantity of vaporized fuel to the center of the cylinder under pressure at or near the peak of the compression stroke. The ignition source was an igniter made from platinum wire. A variable-quantity injection pump provided the fuel to an injector where it was mixed with air as it entered the cylinder. A small crank-driven compressor provided the source for air. This engine also used the lean-burn system.

Rudolf Diesel Rudolf Christian Karl Diesel (, ; 18 March 1858 – 29 September 1913) was a German inventor and mechanical engineer who invented the Diesel engine, which burns Diesel fuel; both are named after him. Early life and education Diesel was born on 1 ...

originally proposed a very high compression, constant-temperature cycle where the heat of compression would exceed the

heat of combustion The heating value (or energy value or calorific value) of a substance, usually a fuel or food (see food energy), is the amount of heat released during the combustion of a specified amount of it. The ''calorific value'' is the total energy relea ...

, but after several years of experiments, he realized that the constant-temperature cycle would not work in a piston engine. Early Diesel engines use an air blast system which was pioneered by Brayton in 1890. Consequently, these early engines use the constant-pressure cycle. File:Brayton Gas engine 1872.jpg, Brayton gas engine 1872 File:Brayton walking beam engine 1872.jpg, Brayton walking beam engine 1872 File:Brayton engine 1875.jpg, Brayton engine 1875 File:Brayton double acting constant pressure engine cut away 1877.jpg, Brayton double-acting constant-pressure engine cut away 1877 File:Brayton 4 stroke air blast engine 1889.jpg, Brayton four-stroke air blast engine 1889 File:Brayton 4 stroke air blast engine 1890.jpg, Brayton four-stroke air blast engine 1890

Early gas turbine history

* 1791 First patent for a gas turbine (John Barber, United Kingdom) * 1904 Unsuccessful gas turbine project by Franz Stolze in Berlin (first axial compressor) * 1906

Armengaud-Lemale gas turbine The Armengaud-Lemale gas turbine was an early experimental turbine engine built by the ''Société Anonyme des Turbomoteurs'' at their facility in Saint-Denis, Paris during 1906. The machine is named after the society's founders, Rene Armengaud ...

in France (centrifugal compressor, no useful power) * 1910 First gas turbine featuring intermittent combustion ( Holzwarth 150 kW, constant volume combustion) * 1923 First exhaust-gas turbocharger to increase the power of diesel engines * 1939 Turbojet powered Heinkel He 178, world's first jet aircraft, makes first flight. World's first gas turbine for power generation by

Brown-Boveri Brown, Boveri & Cie. (Brown, Boveri & Company; BBC) was a Swiss group of electrical engineering companies. It was founded in Baden bei Zürich, in 1891 by Charles Eugene Lancelot Brown and Walter Boveri who worked at the Maschinenfabrik Oerlik ...

, Neuchâtel, Switzerland (velox burner, aerodynamics by Stodola)

Models

A Brayton-type

engine An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power ge ...

consists of three components: a

compressor A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. An air compressor is a specific type of gas compressor. Many compressors can be staged, that is, the gas is compressed several times in steps o ...

, a mixing chamber, and an

expander Expander may refer to: *Dynamic range compression operated in reverse *Part of the process of signal compression *Part of the process of companding *A component used to connect SCSI computer data storage, devices together *Turboexpander, a turbin ...

. Modern Brayton engines are almost always a turbine type, although Brayton only made piston engines. In the original 19th-century Brayton engine, ambient air is drawn into a piston compressor, where it is compressed; ideally an

isentropic process An isentropic process is an idealized thermodynamic process that is both Adiabatic process, adiabatic and Reversible process (thermodynamics), reversible. The work (physics), work transfers of the system are friction, frictionless, and there is ...

. The compressed air then passes through a mixing chamber where fuel is added, an

isobaric process In thermodynamics, an isobaric process is a type of thermodynamic process in which the pressure of the Thermodynamic system, system stays constant: Δ''P'' = 0. The heat transferred to the system does work (thermodynamics), work, but a ...

. The pressurized air and fuel mixture is then ignited in an expansion cylinder and energy is released, causing the heated air and combustion products to expand through a piston/cylinder, another ideally isentropic process. Some of the work extracted by the piston/cylinder is used to drive the compressor through a crankshaft arrangement. Gas turbine engines are also Brayton engines, with three components: an air compressor, a

combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...

chamber, and a gas turbine. Ideal Brayton cycle: #

– ambient air is drawn into the compressor, where it is pressurized. #

– the compressed air then passes through a combustion chamber, where fuel is burned, heating that air—a constant-pressure process, since the chamber is open to flow in and out. # isentropic process – the heated, pressurized air then gives up its energy, expanding through a turbine (or series of turbines). Some of the work extracted by the turbine is used to drive the compressor. # isobaric process – heat rejection (in the atmosphere). Actual Brayton cycle: #

adiabatic process An adiabatic process (''adiabatic'' ) is a type of thermodynamic process that occurs without transferring heat between the thermodynamic system and its Environment (systems), environment. Unlike an isothermal process, an adiabatic process transf ...

– compression # isobaric process – heat addition # adiabatic process – expansion # isobaric process – heat rejection Since neither the compression nor the expansion can be truly isentropic, losses through the compressor and the expander represent sources of inescapable working inefficiencies. In general, increasing the

compression ratio The compression ratio is the ratio between the maximum and minimum volume during the compression stage of the power cycle in a piston or Wankel engine. A fundamental specification for such engines, it can be measured in two different ways. Th ...

is the most direct way to increase the overall

power Power may refer to: Common meanings * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power, a type of energy * Power (social and political), the ability to influence people or events Math ...

output of a Brayton system. The efficiency of the ideal Brayton cycle is

\eta = 1 - \frac  = 1 - \left(\frac\right)^

, where

\gamma

is the

heat capacity ratio In thermal physics and thermodynamics, the heat capacity ratio, also known as the adiabatic index, the ratio of specific heats, or Laplace's coefficient, is the ratio of the heat capacity at constant pressure () to heat capacity at constant vol ...

. Figure 1 indicates how the cycle efficiency changes with an increase in pressure ratio. Figure 2 indicates how the specific power output changes with an increase in the gas turbine inlet temperature for two different pressure ratio values. GFImg7.png, Figure 1: Brayton-cycle efficiency GFImg8.png, Figure 2: Brayton-cycle specific power output The highest gas temperature in the cycle occurs where work transfer to the high pressure turbine (rotor inlet) takes place. This is lower than the highest gas temperature in the engine (combustion zone). The maximum cycle temperature is limited by the turbine materials and required turbine life. This also limits the pressure ratios that can be used in the cycle. For a fixed-turbine inlet temperature, the net work output per cycle increases with the pressure ratio (thus the

thermal efficiency In thermodynamics, the thermal efficiency (\eta_) is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc. For ...

) and the net work output. With less work output per cycle, a larger mass flow rate (thus a larger system) is needed to maintain the same power output, which may not be economical. In most common designs, the pressure ratio of a gas turbine ranges from about 11 to 16.

Methods to increase power

The power output of a Brayton engine can be improved by: * Reheat, wherein the

—in most cases air—expands through a series of turbines, then is passed through a second combustion chamber before expanding to ambient pressure through a final set of turbines, has the advantage of increasing the power output possible for a given compression ratio without exceeding any

metallurgical Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys. Metallurgy encompasses both the ...

constraints (typically about 1000 °C). The use of an

afterburner An afterburner (or reheat in British English) is an additional combustion component used on some jet engines, mostly those on military supersonic aircraft. Its purpose is to increase thrust, usually for supersonic flight, takeoff, and combat ...

for jet aircraft engines can also be referred to as "reheat"; it is a different process in that the reheated air is expanded through a thrust nozzle rather than a turbine. The metallurgical constraints are somewhat alleviated, enabling much higher reheat temperatures (about 2000 °C). Reheat is most often used to improve the specific power, and is usually associated with a drop in efficiency; this effect is especially pronounced in afterburners due to the extreme amounts of extra fuel used. * In overspray, after the first compressor stage, water is injected into the compressor, thus increasing the mass-flow inside the compressor, increasing the turbine output power significantly and reducing compressor outlet temperatures. In the second compressor stage, the water is completely converted to a gas form, offering some intercooling via its latent heat of vaporization.

Methods to improve efficiency

The efficiency of a Brayton engine can be improved by: * Increasing pressure ratio, as Figure 1 above shows, increasing the pressure ratio increases the efficiency of the Brayton cycle. This is analogous to the increase of efficiency seen in the

when the

is increased. However, practical limits occur when it comes to increasing the pressure ratio. First of all, increasing the pressure ratio increases the compressor discharge temperature. Since the turbine temperature has a limit determined by metallurgical and life constraints the allowable rise in temperature (fuel added) in the combustor becomes smaller. Also, because the length of the compressor blades becomes progressively smaller in the higher pressure stages a constant running gap, through the compressor, between the blade tips and the engine casing becomes a bigger percentage of the compressor blade height increasing air leakage past the tips. This causes a drop in compressor efficiency, and is most likely to occur in smaller gas turbines (since blades are inherently smaller to begin with). Finally, as can be seen in Figure 1, the efficiency levels off as pressure ratio increases. Hence, little gain is expected by increasing the pressure ratio further if it is already at a high level. * Recuperator – If the Brayton cycle is run at a low pressure ratio and a high temperature increase in the combustion chamber, the exhaust gas (after the last turbine stage) might still be hotter than the compressed inlet gas (after the last compression stage but before the combustor). In that case, a heat exchanger can be used to transfer thermal energy from the exhaust to the already compressed gas, before it enters the combustion chamber. The thermal energy transferred is effectively reused, thus increasing efficiency. However, this form of heat recycling is only possible if the engine is run in a low-efficiency mode with low pressure ratio in the first place. Transferring heat from the outlet (after the last turbine) to the inlet (before the first compressor stage) would reduce efficiency, as hotter inlet air means more volume, thus more work for the compressor. For engines with liquid cryogenic fuels, namely

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

, it might be feasible, though, to use the fuel to cool the inlet air before compression to increase efficiency. This concept is extensively studied for the

SABRE A sabre or saber ( ) is a type of backsword with a curved blade associated with the light cavalry of the Early Modern warfare, early modern and Napoleonic period, Napoleonic periods. Originally associated with Central European cavalry such a ...

engine. * A Brayton engine also forms half of the

combined cycle A combined cycle power plant is an assembly of heat engines that work in tandem from the same source of heat, converting it into mechanical energy. On land, when used to make electricity the most common type is called a combined cycle gas turb ...

system, which combines with a Rankine engine to further increase overall efficiency. However, although this increases overall efficiency, it does not actually increase the efficiency of the Brayton cycle itself. *

Cogeneration Cogeneration or combined heat and power (CHP) is the use of a heat engine or power station to generate electricity and useful heat at the same time. Cogeneration is a more efficient use of fuel or heat, because otherwise- wasted heat from elec ...

systems make use of the waste heat from Brayton engines, typically for hot water production or space heating.

Variants

Closed Brayton cycle

A closed Brayton cycle recirculates the

; the air expelled from the turbine is reintroduced into the compressor, this cycle uses a

to heat the working fluid instead of an internal combustion chamber. The closed Brayton cycle is used, for example, in

closed-cycle gas turbine A closed-cycle gas turbine is a turbine that uses a gas (e.g. air, nitrogen, helium, argon, etc.) for the working fluid as part of a closed thermodynamic system. Heat is supplied from an external source. Note: front matter (including preface a ...

s for power generation in space.

Solar Brayton cycle

In 2002, a hybrid open solar Brayton cycle was operated for the first time consistently and effectively with relevant papers published, in the frame of the EU SOLGATE program. The air was heated from 570 to over 1000K into the combustor chamber. Further hybridization was achieved during the EU Solhyco project running a hybridized Brayton cycle with solar energy and biodiesel only. This technology was scaled up to 4.6 MW within the project Solugas located near Seville, where it is currently demonstrated at precommercial scale.

Reverse Brayton cycle

A Brayton cycle that is driven in reverse uses work to move heat. This makes it a form of gas refrigeration cycle. When air is the working fluid, it is known as the Bell Coleman cycle. It is also used in the

LNG Liquefied natural gas (LNG) is natural gas (predominantly methane, CH4, with some mixture of ethane, C2H6) that has been cooled to liquid form for ease and safety of non-pressurized storage or transport. It takes up about 1/600th the volume o ...

industry for subcooling LNG using power from a gas turbine to drive the compressor.

Inverted Brayton cycle

This is an open Brayton cycle which also generates work from heat, but with a different order of the stages. Incoming air is first heated at atmospheric pressure, and then passes through the turbine, generating work. The gas, now at a pressure lower than atmospheric, is cooled in a heat exchanger. The compressor raises the pressure again so the gas can be expelled to the atmosphere.

References

External links

Today in Science article on Brayton Engine

Solar Gas Turbine Systems: Design, Cost and Perspectives
* {{DEFAULTSORT:Brayton Cycle Thermodynamic cycles