The Brayton cycle is the thermodynamic process that powers every jet engine and most natural gas power plants. Named after American engineer George Brayton, who developed it in 1872, it describes how a gas turbine converts fuel into useful work through four continuous steps: compression, heating, expansion, and exhaust. If you’ve ever watched a jet take off or flipped on a light powered by natural gas, you’ve seen the Brayton cycle at work.
The Four Steps of the Cycle
The Brayton cycle breaks down into four distinct stages, each happening in a different part of the engine. Understanding them in order is the easiest way to grasp how the whole system works.
Compression. Air enters the compressor, where spinning blades squeeze it into a smaller volume. This raises both its pressure and temperature without adding any heat from fuel. The compression happens so quickly that very little energy escapes as heat to the surroundings, a condition engineers call “isentropic” (meaning constant entropy, or roughly, no wasted heat).
Combustion. The compressed air flows into a combustion chamber where fuel is injected and ignited. This is where the energy input happens. The burning fuel heats the air dramatically, but the pressure stays essentially constant because the chamber is open at both ends, allowing the gas to expand freely forward.
Expansion. The hot, high-pressure gas slams into the blades of a turbine, spinning it and doing useful work. In a jet engine, some of this work pushes the airplane forward. In a power plant, it spins a generator. The gas loses pressure and temperature as it gives up energy to the turbine.
Exhaust. The spent gas exits the system. In a jet engine, a nozzle accelerates the exhaust backward to produce thrust, bringing it back down to the surrounding air pressure. In a stationary power plant, the hot exhaust is simply vented or, more often, routed to a secondary system to squeeze out additional energy.
Open Cycle vs. Closed Cycle
Most real-world Brayton cycle engines are “open cycle” systems. Fresh air enters the front, fuel burns inside, and exhaust leaves out the back. Every jet engine and most industrial gas turbines work this way. The air makes a single pass through the machine and never returns.
A “closed cycle” system, by contrast, seals the working gas inside a loop. Instead of burning fuel directly in the gas stream, heat is added from an external source, like a nuclear reactor or a concentrated solar array. The gas circulates continuously, getting heated, expanded through a turbine, cooled, and compressed again. Closed cycles are less common but important in applications where you can’t burn fuel directly in the working fluid or where you want to use a gas other than air.
What Determines Efficiency
The thermal efficiency of an ideal Brayton cycle depends on a single variable: the pressure ratio, which is how much the compressor increases the air pressure. A higher pressure ratio means the air gets squeezed more before combustion, and the cycle extracts a larger fraction of the fuel’s energy as useful work. In formula terms, ideal efficiency equals 1 minus the inverse of the temperature ratio across the compressor, which itself is set by the pressure ratio.
In practice, though, real gas turbines fall short of this ideal. One major reason is the “back work ratio,” the share of the turbine’s output that gets consumed just to run the compressor. In a typical gas turbine, the compressor eats up 40 to 80 percent of the turbine’s total power output. Only the remainder is available as net useful work. This is a much bigger penalty than in, say, a steam turbine, where pumping water back to high pressure takes very little energy. It’s one of the fundamental constraints of gas turbine design.
Material limits also cap efficiency. The hotter you can run the gas entering the turbine, the more work you can extract. But turbine blades have to survive those temperatures for thousands of hours. Advances in blade cooling and heat-resistant alloys have steadily pushed allowable temperatures higher over the decades, improving real-world efficiency along the way.
Combined Cycle Power Plants
One of the most important applications of the Brayton cycle today is in combined cycle gas turbine (CCGT) power plants. These pair a gas turbine running the Brayton cycle with a steam turbine running a separate cycle (the Rankine cycle). The exhaust from the gas turbine, still very hot, boils water to drive the steam turbine. This two-stage approach captures energy that a standalone gas turbine would waste.
The results are impressive. CCGT plants built between 2010 and 2022 achieved average heat rates of about 6,960 BTU per kilowatt-hour, according to the U.S. Energy Information Administration. That translates to roughly 49 percent thermal efficiency, far better than a simple gas turbine alone (typically in the mid-30s) and better than most coal plants. These combined cycle plants now generate a large share of electricity in the United States and many other countries, largely because they pair high efficiency with the relatively low emissions of natural gas.
Aviation and the Brayton Cycle
Every turbine-powered aircraft, from small turboprops to massive widebody jets, runs on the Brayton cycle. The core of a jet engine contains exactly the three components the cycle requires: a compressor, a combustion section, and a turbine. Air enters the front, gets compressed, mixed with jet fuel and ignited, then expands through the turbine. The remaining high-pressure exhaust accelerates through a nozzle at the back, producing thrust.
In a modern turbofan engine, the turbine does double duty. Part of its energy drives the compressor, as in any Brayton cycle machine. But it also drives a large fan at the front of the engine that pushes a much bigger volume of air around the outside of the core. This “bypass” air generates most of the thrust in a modern airliner engine while keeping noise and fuel consumption lower than older turbojet designs. The thermodynamic cycle inside the core, though, is still the same Brayton process.
Newer Variations
Researchers have been experimenting with running the Brayton cycle using supercritical carbon dioxide instead of air. When CO₂ is heated above about 500°C and pressurized above 7.6 megapascals, it enters a “supercritical” state where it behaves as something between a liquid and a gas. In this state it’s extremely dense, which means the turbines and compressors can be dramatically smaller. Sandia National Laboratories has estimated that a supercritical CO₂ Brayton system could produce 20 megawatts of electricity from equipment with a volume as small as four cubic meters.
The efficiency gains are significant too. These systems could reach thermal efficiencies around 50 percent, representing a 40 percent improvement over simple gas turbines. They could also achieve performance comparable to helium-based systems but at considerably lower temperatures, around 250 to 300°C. That makes them attractive for pairing with nuclear reactors, concentrated solar plants, and waste heat recovery systems where extremely high temperatures aren’t available. The technology is still in development, but it represents one of the most promising directions for squeezing more electricity out of every unit of heat.