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mvcdc eNotes educator| Certified Educator

In brief, a Carnot heat engine is a type of engine that follows the reversible Carnot cycle. A heat engine is a machine that converts heat or thermal energy, and chemical energy, into mechanical energy. Work is done by moving from a state of higher temperature into a lower temperature (heat is transferred into a heat sink) and work is generated (attached is a generic diagram for this). There is also a heat source (can be an exothermic chemical reaction) that brings up the temperature to start the process. A typical example of this type of engine is the steam engine.

A Carnot heat engine is one that operates using the Carnot cycle, and is the most efficient heat engine empirically possible -- it makes use of isothermal and adiabatic processes. There are four steps to a Carnot cycle:

  1. Reversible isothermal expansion of a gas (high temperature). This is the process roughly equivalent to increasing the temperature of the system. Increasing temperature, according to the gas laws, would expand the gas (2nd step). The expansion needs to be reversible because this will allow for maximum energy.
  2. Adiabatic expansion of the gas. This performs the work as the gas expands. It is adiabatic so that the system is thermally isolated and no heat is lost. Temperature starts to go down (heat sink) to the initial lower temperature.
  3. Reversible isothermal compression of the gas (low temperature).
  4. Adiabatic compression of the gas at the lower temperature -- and then cycles back to step 1.

The laws of thermodynamics dictate that this is the most efficient heat engine. (A visualization of the process can be seen in reference 2.)

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