 | This articlemay be in need of reorganization to comply with Wikipedia'slayout guidelines. Please help byediting the article to make improvements to the overall structure.(September 2024) (Learn how and when to remove this message) |
| Thermodynamics | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
 The classicalCarnot heat engine | ||||||||||||
| ||||||||||||
TheBrayton cycle, also known as the Joule cycle, is athermodynamic cycle that describes the operation of certainheat engines that have air or some other gas as theirworking fluid.It is characterized byisentropic compression and expansion, andisobaric heat addition and rejection, though practical engines haveadiabatic rather than isentropic steps.
The most common current application is inairbreathing jet engines andgas turbine engines.
The engine cycle is named afterGeorge Brayton (1830–1892), the Americanengineer, who developed the Brayton Ready Motor in 1872, using a piston compressor and piston expander.[1]An engine using the cycle was originally proposed and patented by EnglishmanJohn Barber in 1791, using a reciprocating compressor and a turbine expander.[2]
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 aheat exchanger orexternal combustion 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 forinternal combustion.Although the cycle is open, it is conventionally assumed for the purposes ofthermodynamic analysis that the exhaust gases are reused in the intake, enabling analysis as a closed cycle.
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 asurface carburetor was also used for mobile operation .[3] 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.[4] 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.[4]
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, theFenian Ram.John Philip Holland's submarines are preserved in thePaterson Museum in theOld Great Falls Historic District ofPaterson, New Jersey.[5]
In 1878,George B. Selden patented the first internal combustion automobile.[6] Inspired by theinternal combustion engine invented by Brayton displayed at theCentennial Exposition 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[6] was granted on November 5, 1895. In 1903, Selden sued Ford for patent infringement andHenry Ford 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-strokeAlphonse Beau de Rochas cycle orOtto cycle and not the Brayton-cycle engine used in the Selden auto. Ford won the appeal of the original case.[7]
In 1887, Brayton developed and patented a four-stroke direct-injection oil engine.[8] 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.[9] 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 originally proposed a very high compression, constant-temperature cycle where the heat of compression would exceed theheat of combustion, 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.[10]
(velox burner, aerodynamics by Stodola)
A Brayton-typeengine consists of three components: acompressor, a mixing chamber, and anexpander.
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 iscompressed; ideally anisentropic process. The compressed air then passes through a mixing chamber where fuel is added, anisobaric process. 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, acombustion chamber, and a gas turbine.
Ideal Brayton cycle:
Actual Brayton cycle:
 |
Since neither the compression nor the expansion can be truly isentropic, losses through the compressor and the expander represent sources of inescapable workinginefficiencies. In general, increasing thecompression ratio is the most direct way to increase the overallpower output of a Brayton system.[12]
The efficiency of the ideal Brayton cycle is, where is theheat capacity ratio.[13] 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.
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 thethermal efficiency) 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.[14]
The power output of a Brayton engine can be improved by:
The efficiency of a Brayton engine can be improved by:
A closed Brayton cycle recirculates theworking fluid; the air expelled from the turbine is reintroduced into the compressor, this cycle uses aheat exchanger to heat the working fluid instead of an internal combustion chamber. The closed Brayton cycle is used, for example, inclosed-cycle gas turbines for power generation in space.[17]
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.[18]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.[19]This technology was scaled up to 4.6 MW within the project Solugas located near Seville, where it is currently demonstrated at precommercial scale.[20]
A Brayton cycle that is driven in reverse uses work to move heat.This makes it a form ofgas refrigeration cycle.When air is the working fluid, it is known as the Bell Coleman cycle.[21]
It is also used in theLNG industry for subcooling LNG using power from a gas turbine to drive the compressor.[dubious –discuss][citation needed]
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.
