The Brayton cycle is a thermodynamic cycle that describes the operation of certain heat engines that have air or some other gas as their working fluid. The original Brayton engines used a piston compressor and piston expander, but modern gas turbine engines and airbreathing jet engines also follow the Brayton cycle. Although the cycle is usually run as an open system (and indeed must be run as such if internal combustion is used), it is conventionally assumed for the purposes of thermodynamic analysis that the exhaust gases are reused in the intake, enabling analysis as a closed system.
The engine cycle is named after George Brayton (1830–1892), the American engineer who developed it originally for use in piston engines, although it was originally proposed and patented by Englishman John Barber in 1791. It is also sometimes known as the Joule cycle. The reversed Joule cycle uses an external heat source and incorporates the use of a regenerator. One type of Brayton cycle is open to the atmosphere and uses an internal combustion chamber; and another type is closed and uses a heat exchanger.
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 by means of a heated-surface carburetor. 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's submarines are preserved in the Paterson Museum in the Old Great Falls Historic District of Paterson, New Jersey.
In 1878, George B. Selden patented the first internal combustion automobile. Inspired by the internal combustion engine invented by Brayton displayed at the Centennial 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 was granted on November 5, 1895. In 1903, Selden sued Ford for patent infringement and Henry 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-stroke Alphonse Beau de Rochas cycle or Otto cycle 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 originally proposed a very high compression, constant-temperature cycle where the heat of compression would exceed the heat 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.
Just as steam turbines were an adaptation of steam piston engines, gas turbines were an adaptation of early piston constant-pressure engines.
(velox burner, aerodynamics by Stodola)
A Brayton-type engine consists of three components: a compressor, a mixing chamber, and an expander.
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. The compressed air then runs through a mixing chamber where fuel is added, an isobaric 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 turbines are also Brayton engines, with three components: a gas compressor, a burner (or combustion chamber), and an expansion 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 working inefficiencies. In general, increasing the compression ratio is the most direct way to increase the overall power output of a Brayton system.
The efficiency of the ideal Brayton cycle is , where is the heat capacity ratio. 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 temperature in the cycle occurs at the end of the combustion process, and it is limited by the maximum temperature that the turbine blades can withstand. 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) 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.
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 the working fluid; the air expelled from the turbine is reintroduced into the compressor, this cycle uses a heat exchanger to heat the working fluid instead of an internal combustion chamber. The closed Brayton cycle is used, for example, in closed-cycle gas turbine and space power generation.
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.
A Brayton cycle that is driven in reverse, via net work input, and when air is the working fluid, is the gas refrigeration cycle or Bell Coleman cycle. Its purpose is to move heat, rather than produce work. This air-cooling technique is used widely in jet aircraft for air conditioning systems using bleed air tapped from the engine compressors. It is also used in the LNG industry where the largest reverse Brayton cycle is for subcooling LNG using 86 MW of power from a gas turbine-driven compressor and nitrogen refrigerant.
Main article: Inverted Brayton cycle
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