Hi I am going through an extreme headache trying to udnerstand the carnot engine and the second law of thermodynamics. To prove that the carnot engine (refreigerator engine) is the most efficient, my text included a prototype engine with a carnot refrigetrator together. So, the author says that the work done in the heat engine drives the carnot engine and we assume that the prototype engine is mroe efficient than the carnot engine. Then he says that because the heat engine is more efficient than the carnot one, heat is expected to flow from the cold to the hot reservoir. ( I agree so far). Then because of thee second law of thermodynamics, this cannot be the case because the net work done is zero. Therefore, the carnot engine must be mroe rfficient than the protoype one. This is where i got lost. What is so special about the carnot engine that we cannot place a prototype refrigerator in its place? Second, a carnot engine has alternate adiabatic and isothermal compressions/expansions and is a reversible engine. What is the significance of this in the example above? What is the connection between these adiabatic and isother compresisons expansions that makes the engine reversible?