Real-world thermodynamic cycles

Ideal cycles and real machines

Real thermodynamic devices are complicated. They involve combustion, turbulence, friction, heat exchangers, valves, phase changes, and finite-time operation. To understand them, physicists and engineers use idealized cycles that capture the main energy transfers.

An ideal cycle is not exact reality. It is a model that helps organize compression, heating, expansion, and cooling.

Otto cycle

The Otto cycle is an idealized model for spark-ignition gasoline engines. It consists of adiabatic compression, constant-volume heat addition, adiabatic expansion, and constant-volume heat rejection.

Its ideal efficiency depends on compression ratio $r$ and heat capacity ratio $gamma$:

e_{Otto}=1-rac{1}{r^{gamma-1}}

Higher compression ratio improves efficiency, but real engines are limited by knocking, materials, emissions, and heat losses.

Diesel cycle

The Diesel cycle models compression-ignition engines. It includes adiabatic compression, constant-pressure heat addition, adiabatic expansion, and constant-volume heat rejection.

Diesel engines often use higher compression ratios than gasoline engines, contributing to high efficiency. Real diesel performance also depends on fuel injection, combustion timing, turbocharging, and heat losses.

Brayton cycle

The Brayton cycle models gas turbines and jet engines. It includes adiabatic compression, constant-pressure heat addition, adiabatic expansion, and constant-pressure heat rejection.

In a jet engine, air is compressed, fuel is burned, hot gases expand through turbines and nozzles, and thrust is produced. In power plants, gas turbines produce shaft work to drive generators.

The ideal Brayton cycle efficiency improves with pressure ratio.

Rankine cycle

The Rankine cycle models steam power plants. Water is pumped to high pressure, heated and vaporized in a boiler, expanded through a turbine, condensed, and pumped again.

A key advantage is that pumping liquid water requires much less work than compressing a gas. Rankine cycles are used in fossil fuel, nuclear, biomass, and some solar thermal power plants.

The basic components are pump, boiler, turbine, and condenser.

Refrigeration cycle

The vapor-compression refrigeration cycle uses a refrigerant that evaporates at low temperature and condenses at higher temperature. Its main components are evaporator, compressor, condenser, and expansion valve.

This cycle is used in refrigerators, air conditioners, and heat pumps. Performance depends on refrigerant properties, compressor efficiency, heat exchanger design, and operating temperatures.

Irreversibilities

Real cycles fall short of ideal cycles because of entropy production. Common irreversibilities include friction, turbulence, heat transfer across finite temperature differences, shock waves, throttling, electrical resistance, and non-ideal compression or expansion.

These effects reduce work output in engines and increase work input in refrigerators.

Regeneration and improvement

Engineers improve cycles using regeneration, reheating, intercooling, combined cycles, improved materials, and better heat exchangers. A combined-cycle power plant uses a gas turbine and then uses hot exhaust to run a steam Rankine cycle, increasing overall efficiency.

The big idea

Real thermodynamic cycles are modeled by ideal cycles such as Otto, Diesel, Brayton, Rankine, and vapor-compression refrigeration cycles. These models reveal the main energy flows and efficiency limits, while real-world performance depends on irreversibilities, materials, design, and operating conditions.