Keywords:  Heat engines, carnot cycle, internal, external, combustion engines , ...
 
 The 'gas' is an unchanging bunch of molecules.
 
 carnot cycle  http://en.wikipedia.org/wiki/Carnot_cycle
 
 All steps reversible.  The 'gas' is an unchanging bunch of molecules.
 
 1 -> 2: Isothermal expansion of the gas at the "hot" temperature. Work output & heat adition. Pressure drops.
 2 -> 3: Adiabatic (no heat transfer) expansion of the cooling gas Work output. Pressure drops.
 3 -> 4: Isothermal compression of the gas at the "cold" temperature. Work input & heat removal. Pressure rises.
 4 -> 1: Adiabatic compression of the warming gas. Work input. Pressure rises.



 
 Process 3 -> 4: Isothermal expansion.
 Process 4 -> 1: Isobaric heat removal. 
 Process 1 -> 2: Isothermal compression.
 Process 2 -> 3: Isobaric heat addition.


These potentially-implementable cycles (the internal combustion ones) use a new bunch of molecules for each cycle:

 brayton cycle
 otto cycle
 ericsson cycle
 stirling cycle
 atkinson cycle
 
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