Simple cycle

A simple cycle is the basic gas turbine power cycle in Thermodynamics II: air is compressed, fuel is burned, and the hot gases expand through a turbine to produce work. It is the starting point for analyzing gas turbine power plants.

What is simple cycle?

A simple cycle in Thermodynamics II is the most basic gas turbine power cycle, usually modeled as the Brayton Cycle with no regeneration, reheat, or heat recovery. You take air into a compressor, raise its pressure, add fuel in the combustor, and then let the hot gas expand through a turbine to produce shaft work.

The “simple” part means the plant has a clean, direct energy path. Compressor work goes in, combustion adds energy, and turbine work comes out. In the idealized version, the compression and expansion are treated as isentropic processes, while heat is added at roughly constant pressure during combustion and rejected with the exhaust.

That structure matters because it gives you a clear way to track energy flow. The compressor needs work from the turbine output, so the net power output is the turbine work minus the compressor work. If the turbine produces a lot of power but the compressor also takes a lot, the useful output drops fast. That is why pressure ratio, component efficiency, and turbine inlet temperature matter so much in this cycle.

In real gas turbines, the cycle is not perfectly ideal. The compressor and turbine have losses, pressure drops occur in the combustor, and the exhaust leaves with a lot of unused thermal energy. A simple cycle is still the first model engineers use because it shows the core tradeoff clearly, how to get more work out of the turbine without making the compressor penalty too large.

You will also see why it is called the base configuration. More advanced setups, like a regenerative cycle, try to recover some exhaust heat before it leaves the plant. The simple cycle does not do that, so it is easier to analyze and cheaper to build, but it usually has lower thermal efficiency than upgraded gas turbine layouts.

Why simple cycle matters in Thermodynamics II

Simple cycle shows the baseline performance of a gas turbine, so it is the reference point for almost every comparison in this unit. When you change pressure ratio, compressor efficiency, turbine efficiency, or turbine inlet temperature, you are usually measuring the effect relative to the simple cycle first.

It also connects the textbook thermodynamics to real power plants. A simple cycle plant can start quickly and respond fast to load changes, which is why it shows up in utility peaking applications. That makes it a useful example when your class talks about power generation choices, not just ideal cycle math.

This term also sets up later topics in Thermodynamics II. Once you know where the exhaust energy goes in a simple cycle, it becomes much easier to see why combined cycles and regenerative cycles exist. They are basically attempts to fix the biggest weakness of the simple layout, wasted exhaust heat.

In problem solving, this cycle gives you a clean framework for energy balance diagrams, T-s diagrams, and component work calculations. If you can trace the compressor, combustor, and turbine correctly, you can handle most introductory gas turbine questions without getting lost in the details.

How simple cycle connects across the course

Brayton Cycle

The simple cycle is the practical gas turbine version of the Brayton Cycle. In ideal problems, the two are often treated almost the same way, with compression, constant-pressure heat addition, expansion, and heat rejection. If your instructor uses the word Brayton Cycle, they are usually pointing to the same basic process model behind a simple gas turbine loop.

Compressor

The compressor is the first big work consumer in a simple cycle. It raises the pressure of the inlet air before combustion, and the amount of work it takes directly affects the net power output. If compressor performance drops, the whole cycle gets less efficient because the turbine has to cover a bigger internal load.

Turbine

The turbine is where the cycle produces useful shaft work. Hot combustion gases expand through it, and that expansion drives both the compressor and the external load. In simple-cycle analysis, turbine efficiency and turbine inlet temperature are major drivers of output power and thermal efficiency.

thermal efficiency

Thermal efficiency tells you how much of the fuel’s input energy becomes useful work. For a simple cycle, this is often limited by exhaust losses, because a lot of heat leaves with the turbine exhaust. When you compare the simple cycle to a regenerative or combined cycle, thermal efficiency is usually the first number you check.

Is simple cycle on the Thermodynamics II exam?

A quiz question or problem set may give you a simple gas turbine schematic and ask you to identify the compressor, combustor, turbine, and the direction of energy flow. You might also be asked to draw the T-s or h-s behavior, then calculate net work as turbine work minus compressor work. If the problem includes pressure ratio or component efficiencies, you use the simple cycle as the baseline model and see how those values change power output and thermal efficiency.

In a written response, you may need to explain why the simple cycle is attractive for fast-start power generation but wastes exhaust heat. If a case study compares cycle types, the simple cycle is usually the simplest setup to describe before you move on to regenerative or combined systems.

Simple cycle vs regenerative cycle

A simple cycle and a regenerative cycle both use the same basic gas turbine loop, but a regenerative cycle adds heat recovery from the exhaust before it leaves the plant. The simple cycle sends exhaust out directly, which makes it easier to analyze but usually less efficient. If the question mentions a regenerator or exhaust heat reuse, it is no longer a simple cycle.

Key things to remember about simple cycle

• A simple cycle in Thermodynamics II is the basic gas turbine loop with compression, combustion, and expansion.

• The cycle is usually modeled as the Brayton Cycle, with an idealized compressor and turbine and constant-pressure heat addition.

• Net power output equals turbine work minus compressor work, so the compressor load matters a lot.

• Simple cycles are easy to analyze and quick to respond, but they leave a lot of energy in the exhaust.

• If a system recovers exhaust heat, adds reheat, or changes the basic loop, it is moving beyond the simple cycle.

Frequently asked questions about simple cycle

What is simple cycle in Thermodynamics II?

It is the basic gas turbine power cycle where air is compressed, fuel is burned, and the hot gases expand through a turbine to make work. In Thermodynamics II, it is the starting point for studying gas turbine configurations and performance. You usually compare it with improved cycles that recover waste heat or boost efficiency.

Is simple cycle the same as the Brayton Cycle?

They are closely related, and in many class problems they refer to the same ideal gas turbine process. The simple cycle is the practical layout, while the Brayton Cycle is the idealized thermodynamic model. If your problem mentions compression, constant-pressure heat addition, and expansion, you are working in that same framework.

Why is a simple cycle less efficient than a combined cycle?

Because a simple cycle sends its hot exhaust out without using that heat again. A combined cycle uses the exhaust to produce more power, so less energy is wasted. That difference is why simple cycles are often easier and cheaper to build, but not as efficient overall.

How do you identify a simple cycle on a problem set?

Look for the compressor, combustor, and turbine arranged in one loop with no exhaust heat recovery component. If the question asks for net work, pressure ratio, or thermal efficiency using only those three main components, it is probably a simple cycle problem. A regenerator or bottoming cycle means the setup has moved beyond the simple case.