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Heat Engines and Efficiency

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Brayton Cycle (Jet Engine)

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The efficiency of the Brayton cycle is given by 11rγ11 - \frac{1}{r^{\gamma-1}}, where rr is the pressure ratio across the compressor and turbine, and γ\gamma is the heat capacity ratio.

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Steam Turbine

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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.

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Co-generation (Combined Heat and Power, CHP)

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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.

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Electric Motor

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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.

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Refrigerator (Coefficient of Performance, COP)

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The COP of a refrigerator is given by QCW\frac{Q_C}{W}, where QCQ_C is the heat removed from the cold space and WW is the work input. For ideal refrigerators, COP can also be based on temperatures, TCTHTC\frac{T_C}{T_H - T_C}.

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Diesel Engine

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Diesel engine efficiency is given by 1[1rγ1(ργ1γ(ρ1))]1 - \left[\frac{1}{r^{\gamma-1}}\left(\frac{\rho^\gamma - 1}{\gamma(\rho - 1)}\right)\right], where rr is the compression ratio, γ\gamma is the heat capacity ratio, and ρ\rho is the cut-off ratio (volume before combustion/volume at the end of compression).

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Atkinson Cycle

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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.

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Magnetic Refrigeration

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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.

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Stirling Engine

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Efficiency for a Stirling engine can be approximated by the same formula as a Carnot engine: 1TCTH1 - \frac{T_C}{T_H}, because it operates on a closed regenerative thermodynamic cycle.

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Rankine Cycle

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Efficiency for a simple Rankine cycle is given by 1TCTH1 - \frac{T_C}{T_H}, however, real cycle efficiency is reduced by factors such as pump work and irreversibilities in the actual components.

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Otto Engine

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Otto engine efficiency is given by 1(V1V2)γ11 - \left(\frac{V_1}{V_2}\right)^{\gamma-1} where V1V_1 is the volume at the beginning of compression, V2V_2 is the volume at the end of compression, and γ\gamma is the heat capacity ratio (cp/cvc_p/c_v).

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Carnot Engine

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The efficiency of a Carnot engine is given by 1TCTH1 - \frac{T_C}{T_H}, where TCT_C is the cold reservoir temperature and THT_H is the hot reservoir temperature.

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