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Gas power cycles produce power. Analysis is complex and confusing. Thus assumptions have been made to make the analysis simple. These assumptions help in better understanding. But more are the assumptions, more will be the deviations from actuality.


A gas power cycle is one where the working substance is gas. In the analysis of gas power cycles, we take the working substance as air  for simplicity. Thus these cycles are also called Air Standard Cycles.

List of Air Standard Cycles

  1. Otto cycle: It is used in petrol engines.

  2. Diesel cycle: It is used in diesel engines.

  3. Dual Cycle: It is used in diesel engines.

  4. Brayton Cycle: It is used in gas turbines.

All these cycles are thermodynamic cycles in which one form of energy is changed into another form. In a thermodynamic cycle, normally four processes are carried out in series. The cycle is repeated time and again to get motion, power or electricity or all of these.


In actuality, the working substance is different in different cycles. However for simplicity of analysis, it is assumed that the working substance is air. Consequently these cycles have been called Air Standard Cycles. In all these cycles, there are pressure, temperature, volume, entropy, enthalpy and internal energy variations. Therefore, real cycles will be highly complex and confusing.

Steps for simplicity

In order to have simplicity in cycle analysis, some processes are assumed to have one or more aspect constant. Thus, one or more  of the constant temperature, constant pressure, constant volume, constant enthalpy, constant entropy and constant internal energy processes will be found in each cycle. Further, source of heat supply and heat rejected (sink) are considered as external to the cycle.

Assumptions used in air standard cycles

  1. The working substance is air and it is a perfect gas.

  2. The chemical composition of air does not change in the entire cycle.

  3. The air obeys all the gas laws in Toto.

  4. The specific heat of air is constant in the entire range of temperatures and pressures in the cycle.

  5. The compression and expansion processes are truly adiabatic i.e. no heat is gained or lost during the processes.

  6. The cycle is assumed to be a closed cycle i.e. a constant quantity of air is used repeatedly.

  7. All the processes are internally reversible i.e. there is no friction or any other type of losses in the cycle.