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Heat Engines and Efficiency
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Brayton Cycle (Jet Engine)
The efficiency of the Brayton cycle is given by , where is the pressure ratio across the compressor and turbine, and is the heat capacity ratio.
Steam Turbine
The efficiency of a steam turbine is calculated by the ratio of the useful work output divided by the heat input, but specific formulas vary based on the cycle (e.g., Rankine cycle) used.
Co-generation (Combined Heat and Power, CHP)
CHP efficiency cannot be defined by a simple formula, but it's the sum of the electrical and heat efficiency in utilizing the fuel energy, typically much higher than conventional power plants.
Electric Motor
The efficiency of an electric motor is typically given by the ratio of the mechanical power output to the electrical power input, often exceeding 90% efficiency.
Refrigerator (Coefficient of Performance, COP)
The COP of a refrigerator is given by , where is the heat removed from the cold space and is the work input. For ideal refrigerators, COP can also be based on temperatures, .
Diesel Engine
Diesel engine efficiency is given by , where is the compression ratio, is the heat capacity ratio, and is the cut-off ratio (volume before combustion/volume at the end of compression).
Atkinson Cycle
The efficiency of an Atkinson cycle engine is increased by a longer expansion stroke compared to its compression stroke, which allows for better thermal efficiency but does not have a simple formula.
Magnetic Refrigeration
The efficiency of magnetic refrigeration can be described by the magnetic Carnot cycle and is dependent on the magneto-caloric effect, but does not have a specific formula for efficiency.
Stirling Engine
Efficiency for a Stirling engine can be approximated by the same formula as a Carnot engine: , because it operates on a closed regenerative thermodynamic cycle.
Rankine Cycle
Efficiency for a simple Rankine cycle is given by , however, real cycle efficiency is reduced by factors such as pump work and irreversibilities in the actual components.
Otto Engine
Otto engine efficiency is given by where is the volume at the beginning of compression, is the volume at the end of compression, and is the heat capacity ratio ().
Carnot Engine
The efficiency of a Carnot engine is given by , where is the cold reservoir temperature and is the hot reservoir temperature.
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