Short Answer:

A thermodynamic cycle is a series of processes that involve the transfer of heat and work into and out of a system, in such a way that the system returns to its original state at the end of the cycle. During the cycle, energy enters the system as heat, and some of it is converted into useful work.

Thermodynamic cycles are the basis of all heat engines, power plants, refrigerators, and air conditioners. Since the system returns to its initial condition, the internal energy change over one complete cycle is zero, but work and heat are exchanged with the surroundings.

Detailed Explanation:

Thermodynamic cycle

A thermodynamic cycle is a combination of two or more thermodynamic processes (such as compression, expansion, heating, or cooling) that occur in a specific sequence and eventually return the system to its initial condition. This means all properties like pressure, volume, and temperature come back to the starting point after completing one full cycle.

These cycles are used to model how machines work, especially those that convert heat into work (like engines) or use work to move heat (like refrigerators).

Basic Working Principle

In every thermodynamic cycle:

The system receives heat input (Q_in).
Part of this heat is converted into work output (W_out).
The remaining heat is rejected to the surroundings as waste heat (Q_out).
The net work done in the cycle is the difference between heat in and heat out.

Mathematically:

W_net = Q_in – Q_out

Because the system returns to its initial state, the net change in internal energy is zero (ΔU = 0), according to the first law of thermodynamics.

Types of Thermodynamic Cycles

Thermodynamic cycles are mainly divided into two categories:

Power cycles (Work-producing cycles):
- These cycles convert heat into mechanical work.
- Examples: Carnot cycle, Otto cycle, Diesel cycle, Brayton cycle, Rankine cycle
Refrigeration cycles (Work-consuming cycles):
- These cycles use mechanical work to transfer heat from low temperature to high temperature.
- Examples: Vapor compression cycle, Absorption refrigeration cycle, Reverse Brayton cycle

Examples of Common Thermodynamic Cycles

Carnot Cycle:
- Theoretical, most efficient cycle.
- Used for ideal performance comparison.
Otto Cycle:
- Represents petrol engines.
- Has constant volume heat addition and rejection.
Diesel Cycle:
- Used in diesel engines.
- Has constant pressure heat addition.
Brayton Cycle:
- Used in gas turbines and jet engines.
- Has constant pressure heat addition and rejection.
Rankine Cycle:
- Used in steam power plants.
- Involves phase change of water (liquid to vapor and back).

Importance of Thermodynamic Cycles

Helps design and improve engines and machines.
Used to calculate thermal efficiency — how much heat is converted to work.
Explains energy flow in power generation and refrigeration.
Acts as a reference to compare real systems with ideal performance.
Reduces energy loss by identifying inefficiencies in each part of the cycle.

Efficiency of Thermodynamic Cycles

Thermal efficiency of a power cycle is given by:

η = (W_net / Q_in) × 100%

Higher the efficiency, better the cycle performs. The goal of engineers is to design cycles that:

Use more heat for work,
Waste less energy,
And operate reliably and safely.

Real vs Ideal Cycles

Ideal cycles assume no friction, perfect insulation, and reversible processes.
Real cycles include losses due to heat transfer, friction, and pressure drops.
Ideal cycles help understand the best possible performance. Real cycles show what can actually be achieved.

Conclusion

A thermodynamic cycle is a series of heat and work interactions where the system returns to its original state after completing all processes. These cycles are the foundation of machines like engines and refrigerators. They help us understand, design, and improve energy systems by showing how heat is converted to work or how heat is moved using work. Studying these cycles is essential in mechanical and thermal engineering for creating efficient and powerful machines.