Thermodynamic cycle
A thermodynamic cycle is a series of processes that change the state of a system, such as a gas, until it returns to the state it started in. Engineers use cycles like this to understand how engines work.[1] Once the working substance is back where it began, the same sequence starts again.[2] Cycles are studied in thermodynamics, the branch of physics that deals with the energy and work of a system.[3]
Energy in a cycle
Because a system ends a cycle in exactly the same state as it began, its internal energy (the energy stored inside it) is the same at the end as at the start. The first law of thermodynamics therefore says that, over a whole cycle, the change in internal energy is zero.[4] When a cycle is drawn on a graph of pressure against volume, it makes a closed loop, and the total work done by the gas is equal to the area inside the loop.[5]
Heat engines
A heat engine works by taking a working substance through a cycle. In a steam power plant, for example, the working substance is water: it starts as a liquid, is turned into steam, is used to drive a turbine, and is then condensed back into a liquid.[6] Heat engines come in many designs, including internal combustion engines like those in most cars and external combustion engines like the steam engines of old trains.[7] Different engines are described by different ideal cycles. NASA's educational guide, for example, uses the Brayton cycle to describe gas turbine engines, the most widely used type of engine for modern aircraft.[8]
The Carnot cycle
In 1824 the French engineer Sadi Carnot, who wanted to make practical heat engines more efficient, proposed an imaginary cycle with the highest possible efficiency for an engine working between a hot and a cold reservoir. It is now known as the Carnot cycle.[9] For a gas, it has four steps: the gas expands while kept at the hot temperature, expands further while insulated so that it cools, is squeezed while kept at the cold temperature, and is then squeezed while insulated until it returns to its starting state.[10] The Carnot cycle is a model of how a steam power plant, a refrigerator or a heat pump would work without any losses, and it played a major part in the development of the second law of thermodynamics.[11]
References
- ↑ "Turbine Engine Thermodynamic Cycle - Brayton Cycle". Beginner's Guide to Aeronautics. NASA Glenn Research Center. Retrieved 20 October 2026.
It is possible to perform a series of processes, in which the state is changed during each process, but the gas eventually returns to its original state. Such a series of processes is called a cycle and forms the basis for understanding engine operation.
- ↑ "4.2 Heat Engines". University Physics Volume 2. OpenStax. Retrieved 20 October 2026.
As is the case for all working substances in cyclic processes, once the water returns to its initial state, it repeats the same sequence.
- ↑ "Thermodynamics". Beginner's Guide to Aeronautics. NASA Glenn Research Center. Retrieved 20 October 2026.
Thermodynamics is a branch of physics which deals with the energy and work of a system.
- ↑ "3.3 First Law of Thermodynamics". University Physics Volume 2. OpenStax. Retrieved 20 October 2026.
When a closed cycle is considered for the first law of thermodynamics, the change in internal energy around the whole path is equal to zero.
- ↑ "4.5 The Carnot Cycle". University Physics Volume 2. OpenStax. Retrieved 20 October 2026.
This work is equal to the area enclosed by the loop shown in the pV diagram of Figure 4.12 .
- ↑ "4.2 Heat Engines". University Physics Volume 2. OpenStax. Retrieved 20 October 2026.
Heat engines operate by carrying a working substance through a cycle. In a steam power plant, the working substance is water, which starts as a liquid, becomes vaporized, is then used to drive a turbine, and is finally condensed back into the liquid state.
- ↑ "4.2 Heat Engines". University Physics Volume 2. OpenStax. Retrieved 20 October 2026.
Actual heat engines have many different designs. Examples include internal combustion engines, such as those used in most cars today, and external combustion engines, such as the steam engines used in old steam-engine trains.
- ↑ "Turbine Engine Thermodynamic Cycle - Brayton Cycle". Beginner's Guide to Aeronautics. NASA Glenn Research Center. Retrieved 20 October 2026.
On this page we discuss the Brayton Thermodynamic Cycle which is used in all gas turbine engines. ... The most widely used form of propulsion system for modern aircraft is the gas turbine engine.
- ↑ "4.5 The Carnot Cycle". University Physics Volume 2. OpenStax. Retrieved 20 October 2026.
In the early 1820s, Sadi Carnot (1786−1832), a French engineer, became interested in improving the efficiencies of practical heat engines. In 1824, his studies led him to propose a hypothetical working cycle with the highest possible efficiency between the same two reservoirs, known now as the Carnot cycle .
- ↑ "4.5 The Carnot Cycle". University Physics Volume 2. OpenStax. Retrieved 20 October 2026.
Isothermal expansion. The gas is placed in thermal contact with a heat reservoir ... Adiabatic expansion . The gas is thermally isolated and allowed to expand further ... Because this expansion is adiabatic, the temperature of the gas falls ... Isothermal compression . The gas is placed in thermal contact with a cold reservoir ... The gas is thermally isolated and returned to its initial state by compression.
- ↑ "4.5 The Carnot Cycle". University Physics Volume 2. OpenStax. Retrieved 20 October 2026.
At a practical level, this cycle represents a reversible model for the steam power plant and the refrigerator or heat pump. Yet, it is also very important theoretically, for it plays a major role in the development of another important statement of the second law of thermodynamics.