Air-Standard Power Cycles Reference Page
Cycle Wars

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Intro to Air-Standard Power Cycles

Air-standard power cycles are simplified thermodynamic models used to analyze many common heat engines.
In these models the working fluid is assumed to be air behaving as an ideal gas with constant heat capacity.
Although real engines use combustion gases and complex chemistry, the air-standard assumption allows the fundamental thermodynamic behavior of power cycles to be analyzed clearly.

Cycle Wars: The Power Awakens

This video analyzes the air-standard Carnot cycle, and derives the thermal efficiency in terms of temperature, compression ratio, and pressure ratio.

Visuals

Cycle Wars: The Rise of Otto Cycles

Tis video derives the thermal efficiency in terms of temperature, compression ratio, and/or pressure ratio for the Otto cycle, the Diesel cycle, the Brayton cycle, the Turbojet cycle, the Ericsson cycle, and the Stirling cycle.

Visuals

Examples and Definitions

Definitions

Air-Standard Cycle
A simplified thermodynamic model of a power cycle in which the working fluid is assumed to be air behaving as an ideal diatomic gas with constant heat capacity, typically approximated as
\(C_p = \frac{7}{2}R\).
Air-Standard Carnot Cycle
A model of a Carnot heat engine using air as the working fluid. The steps in the cycle are:
  1. Isentropic compression
  2. Isothermal expansion (heat addition)
  3. Isentropic expansion
  4. Isothermal compression (heat removal)
Air-Standard Otto Cycle
An air-standard cycle consisting of:
  1. Isentropic compression
  2. Isochoric heat addition
  3. Isentropic expansion
  4. Isochoric heat removal

Automobile spark-ignition engines are commonly modeled using the Otto cycle.

Air-Standard Diesel Cycle
An air-standard cycle consisting of:
  1. Isentropic compression
  2. Isobaric heat addition
  3. Isentropic expansion
  4. Isochoric heat removal

Compression-ignition engines are modeled using the Diesel cycle.

Air-Standard Brayton Cycle
An air-standard cycle consisting of:
  1. Isentropic compression
  2. Isobaric heat addition
  3. Isentropic expansion
  4. Isobaric heat removal

Gas-turbine engines are modeled using the Brayton cycle.

Air-Standard Turbojet Cycle
A Brayton-type air-standard cycle in which the high-velocity exhaust jet produces thrust. The modeled steps are:
  1. Isentropic compression
  2. Isobaric heat addition
  3. Isentropic expansion through a turbine
  4. Isentropic expansion through a nozzle
  5. Heat rejection to the surroundings
Air-Standard Ericsson Cycle
A cycle consisting of:
  1. Isothermal compression
  2. Isobaric heat addition
  3. Isothermal expansion
  4. Isobaric heat removal

The Ericsson cycle is typically implemented with external combustion and a regenerator.

Air-Standard Stirling Cycle
A cycle consisting of:
  1. Isothermal compression
  2. Isochoric heat addition
  3. Isothermal expansion
  4. Isochoric heat removal

The Stirling cycle is also an external-combustion engine that uses regeneration.

Heat-Engine or Thermal Efficiency, \(\eta\)
The ratio of the work produced by a heat engine to the heat extracted from the high-temperature reservoir:

\[ \eta \equiv \frac{|W|}{|Q_\mathrm{hot}|} \]

Compression Ratio, \(r\)
The ratio of the initial volume to the final volume during the compression step:

\[ r \equiv \frac{V_\mathrm{initial}}{V_\mathrm{final}} \]

Expansion Ratio
The ratio of the final volume to the initial volume during the expansion step:

\[ r_e \equiv \frac{V_\mathrm{final}}{V_\mathrm{initial}} \]

Pressure Ratio
The ratio of the final pressure to the initial pressure during compression:

\[ r_p \equiv \frac{P_\mathrm{final}}{P_\mathrm{initial}} \]