Brayton cycle

The Brayton cycle is a thermodynamic cycle named after George Bailey Brayton that describes the workings of a constant pressure heat engine. The original Brayton engines used a piston compressor and piston expander, but more 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 Ericsson cycle is similar to the Brayton cycle but uses external heat and incorporates the use of a regenerator. There are two types of Brayton cycles, open to the atmosphere and using internal combustion chamber or closed and using a heat exchanger.

In 1872, George Brayton applied for a patent for his "Ready Motor," a reciprocating constant-pressure engine. The engine was a 2-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 / 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 kerosine and fuel oil. Ignition remained 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. How many of these engines were produced is not known: probably there were several hundred; possibly a thousand. Brayton licensed the design to Simone in the UK. There were many variations of the layout; some were single-acting and some were double-acting. Some had under walking beams; others had overhead walking beams. There were both horizontal and vertical models. Sizes ranged from less than one horsepower to over 40 horsepower. Critics of the time claimed the engines ran smoothly and had a reasonable efficiency.

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