Thermodynamic Cycle (engineering)
A thermodynamic cycle is a series of thermodynamic processes that return a system to its initial state, enabling continuous conversion of heat into work or vice versa. Engineering thermodynamic cycles such as the Rankine, Brayton, and Otto cycles form the basis of power plants, jet engines, and internal combustion engines respectively. The thermal efficiency of a cycle quantifies the fraction of heat input that is converted into net work output.
Key Formula
η_th = 1 - Q_out/Q_in = W_net/Q_in
LaTeX: \eta_{th} = 1 - \frac{Q_{out}}{Q_{in}} = \frac{W_{net}}{Q_{in}}
| Symbol | Meaning | Unit |
|---|---|---|
| \eta_{th} | Thermal efficiency | dimensionless |
| Q_{in} | Heat input to the cycle | J |
| Q_{out} | Heat rejected from the cycle | J |
| W_{net} | Net work output | J |
Worked Example
Problem
A steam power plant operating on the Rankine cycle receives 3000 kJ/kg of heat and rejects 1900 kJ/kg. Calculate the thermal efficiency and net work output per kg of steam.
Solution
Step 1: Identify given values — Q_in = 3000 kJ/kg, Q_out = 1900 kJ/kg. Step 2: Net work: W_net = Q_in − Q_out = 3000 − 1900 = 1100 kJ/kg. Step 3: Thermal efficiency: η = W_net / Q_in = 1100 / 3000 = 0.3667.
Answer
η_th = 36.7%, W_net = 1100 kJ/kg
Major Engineering Thermodynamic Cycles
| Cycle | Application | Working Fluid | Typical Efficiency |
|---|---|---|---|
| Rankine | Steam power plants | Water/steam | 30–45% |
| Brayton | Gas turbines, jet engines | Air/combustion gas | 25–40% |
| Otto | Petrol engines | Air-fuel mixture | 25–35% |
| Diesel | Diesel engines | Air-fuel mixture | 35–45% |
| Refrigeration (vapour) | Refrigerators, AC | Refrigerant | COP 2–5 |
Interactive Tools
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Related Terms
Compressor (engineering)
A compressor is a mechanical device that increases the pressure of a gas by reducing its volume, thereby increasing its energy content. Compressors are fundamental components in refrigeration systems, gas turbines, pneumatic tools, and industrial processes where pressurised gas is required. The work input to a compressor is governed by thermodynamic principles, and the efficiency of the compression process determines overall system performance.
Heat Exchanger
A heat exchanger is a device designed to transfer thermal energy between two or more fluids at different temperatures, without the fluids mixing. Heat exchangers are ubiquitous in engineering applications including power generation, chemical processing, HVAC systems, and automotive radiators. The rate of heat transfer depends on the temperature difference, the overall heat transfer coefficient, and the heat transfer area.
Engineering Nozzle
An engineering nozzle is a shaped duct or orifice designed to accelerate a fluid by converting pressure energy into kinetic energy, thereby increasing the fluid's velocity. Nozzles are critical components in rocket engines, gas turbines, steam turbines, and fluid spray systems. In a converging nozzle, the cross-sectional area decreases in the flow direction, while a converging-diverging (de Laval) nozzle is used to achieve supersonic flow.
From Greek "thermē" (heat) and "dynamikos" (powerful, active). The term "thermodynamic" was introduced by Lord Kelvin (William Thomson) around 1849. "Cycle" comes from Greek "kyklos" (circle), denoting the return to initial state.