What Is the Brayton Cycle?

Operations managers do not need advanced thermodynamics training to benefit from knowing how the Brayton cycle works. A practical grasp of this concept provides a clearer picture of what happens inside a gas turbine engine, from the moment air enters the compressor to the point where exhaust gases leave the turbine.

Natural gas was the largest individual source of U.S. electricity generation in 2024, underscoring the importance of gas turbine technology in power generation. This article offers a plain-language overview of the Brayton cycle, including its four practical stages, key components, and real-world applications.

How Does the Brayton Cycle Work?

The Brayton cycle describes how gas turbines add energy to a working fluid, typically some kind of gas, and convert that energy into mechanical work. Key characteristics include:

• System types: Open-cycle systems draw in atmospheric air and exhaust combustion gases, while closed-cycle systems recirculate the working fluid. Open-cycle gas turbines are common in power generation and are an important topic in power plant operator training.
• Open-cycle operation: In an open-cycle gas turbine, air enters from the atmosphere. Then it’s compressed, heated through combustion, and expanded through a turbine before the exhaust leaves the system.
• Practical vs. ideal processes: The practical stages of gas turbine operation differ from the four idealized thermodynamic processes of the Brayton cycle. 

4 Stages of Open-Cycle Gas Turbine Operation

In an open-cycle system, a gas turbine’s operation can be explained through four practical stages — air intake, compression, combustion, and expansion. Understanding the relationship between the Brayton cycle compressor and turbine helps explain how the system pressurizes atmospheric air, adds heat through combustion of fuel, and converts expansion into useful work.

1. Air Intake

Atmospheric air enters the gas turbine through the intake system. Key factors include:

• Air source: In an open-cycle turbine, air comes directly from the atmosphere and becomes the working fluid that moves through the compressor, combustion system, and turbine.
• Incoming conditions: Temperature, atmospheric pressure, humidity, and air density influence how much air the compressor processes, which in turn affects the turbine’s output and efficiency.

2. Air Compression

The compressor raises the pressure of the incoming air before it enters the combustion system. This stage involves several key processes:

• Pressure and temperature increase: Compressing the air also increases its temperature, creating the conditions needed for efficient heat addition during combustion.
• Pressure ratio: This metric compares the pressure of the air leaving the compressor with the pressure of the air entering it, which is a factor in Brayton cycle efficiency.
• Real-world performance factors: Gas turbine performance depends on component efficiency, turbine inlet temperature, operating conditions, and other design factors beyond pressure ratio alone.
• Typical pressure ratios: Heavy-frame gas turbine engines generally use lower pressure ratios than aeroderivative engines.

3. Combustion and Heat Addition

Compressed air enters the combustion system, where fuel is added and ignited with a spark. Critical aspects of this stage include:

• Heat energy addition: The resulting combustion adds a large amount of heat energy to the working fluid, which was atmospheric air and is now the hot combustion gases. This heat substantially increases its temperature before it enters the turbine.
• Ideal vs. real conditions: In the ideal Brayton cycle, heat is added at constant pressure as a thermodynamic model, though real combustion systems experience pressure losses.
• Performance factors: Combustion conditions, including fuel flow, airflow, and turbine inlet temperature, influence the amount of energy available for expansion through the turbine and affect gas turbine performance.

4. Expansion and Exhaust

The hot, high-pressure working fluid expands as it passes through the turbine. This stage produces the useful output:

• Energy transfer: Expansion transfers energy to the turbine blades and produces shaft work that can be used to drive a generator or other equipment.
• Work distribution: Most of the turbine’s work drives the compressor, while the remaining shaft work provides useful mechanical output.
• Exhaust in open systems: After expansion, the exhaust gases leave the turbine in an open-cycle system.
• Combined-cycle operation: Some facilities recover energy from the hot exhaust gases from an open-cycle turbine and use it to produce additional useful power through a steam cycle. Teams operating these systems may benefit from combined-cycle plant training to connect thermodynamic theory with facility operations.

Real-World Brayton Cycle Applications

The Brayton cycle is not an abstract theory but rather the operating framework for much of the country’s power generation infrastructure. Practical applications include:

• Simple-cycle power plants: These facilities represent the most direct application of the open Brayton cycle, converting fuel into electricity using the four stages. Modern aeroderivative open-cycle gas turbines operate at 33%-43% efficiency at maximum load.
• Combined-cycle plants: These systems use Brayton cycle exhaust heat to feed a Rankine steam cycle, improving overall efficiency to 50%-63%. Combined-cycle natural gas heat rate was 7,548 Btu/kWh as of 2024. 
• District energy and industrial facilities: Campuses, manufacturing plants, and process industries also rely on gas turbines operating on the Brayton cycle for their energy needs.

How the Brayton Cycle Influences Gas Turbine Efficiency

Knowing how the Brayton cycle works helps explain why gas turbine output, fuel consumption, and heat rate change as operating conditions and component performance change. Several factors influence efficiency.

Pressure Ratio

Compressor pressure ratio connects directly to Brayton cycle efficiency. Higher pressure ratios increase the theoretical thermodynamic efficiency of the cycle. This is why operations teams monitor compressor performance. Changes in pressure ratio can signal shifts in turbine efficiency.

Turbine Inlet Temperature

Higher turbine inlet temperatures can increase the energy available for expansion. This improves performance. Real-world limits exist due to materials, cooling requirements, and emissions standards.  

Advanced turbines have achieved up to 2,600° Fahrenheit through advances in materials, coatings, and cooling technology.

Ambient Conditions

Inlet-air conditions affect gas turbine output and efficiency. Changes in temperature, pressure, and humidity alter the amount of air entering the compressor. This influences the entire cycle. 

For an open-cycle system, variations in ambient temperature can result in efficiency changes of approximately 5.8%, demonstrating how external conditions affect performance.

Heat Recovery

Typical simple-cycle gas turbines can achieve approximately 20%-35% energy-conversion efficiency. Advanced turbine technology has achieved efficiencies as high as 60%. Where waste heat is captured for useful heating or industrial purposes, overall energy-cycle efficiency can approach 80%. Electrical efficiency measures power output, while energy-cycle efficiency accounts for all useful energy recovered from the system.

Gas turbine operators must also consider emissions requirements alongside efficiency and output. The Environmental Protection Agency maintains combustion turbine emission standards that influence how facilities balance performance with regulatory compliance.

Take the Next Step With FCS Gas Turbine Training

Knowing how the Brayton cycle works is one step toward gaining a deeper understanding of gas turbine operation and performance. FCS provides gas turbine training courses designed around each team’s equipment, operational needs, and budget. Instructors bring practical plant experience to the training. 

We offer a range of gas turbine short courses that address the specific challenges operations and plant personnel face. 

Teams interested in deepening their knowledge of gas turbine systems can contact us to learn more or request a proposal tailored to their training goals and budget.