Heat engines follow the Carnot cycle:

  1. Reversible isothermal expansion: In this process the system absorbs heat from the heat source. This makes the gas, whose temperature remains constant during all the process, expand, producing work on the surroundings.

 

  1. Reversible adiabatic gas expansion: In this phase, no heat is transferred in or out the system. The gas continues to expand ad do work on surroundings, what makes the system to cool to a lower temperature.

 

  1. Reversible isothermal gas compression: In this process, the surroundings do work on the gas by compressing it, causing a loss of heat.

 

  1. Reversible adiabatic gas compression: Like in the second phase, in this process no heat is transferred in or out the system. The surroundings continue to do work on the gas, compressing it and causing the gas temperature to rise.

 

Carnot cycle picture