Thoughts on the topic

 To enable the conversion of heat energy into mechanical energy with a closed cycle of working substance must have a temperature difference between the hot and the cold part of the unit. Method by creating a temperature difference between the hot and cold part by heating the hot part will not discuss, because if we create cold in cold part "cost cheap" and is significant. In order to use the heat of the environment to transform into mechanical energy, we will need cold in the cold part of the unit that we create.
 If the unit is not heat insulated, environment will warm the cold part, the difference between hot and cold part will decrease and there will reduce the net power. Will need to be continually "spend" for a cold. When heat insulated unit after the initial creation of the cold part (with the help of an external force), we can maintain  the temperature on the cold part - the system for redistributing heat and a system  for liquefaction of working substance will do the job. When working substance had transmitted a large amount of heat (energy) into mechanical energy to send a "residual" heat from the cold in the warm part of the unit will require less power than we have already received. So we are left "profit" - mechanical energy.

Temperature difference between the hot and cold part is necessary in order to allow the transformation of heat into mechanical energy, so we need to monitor the temperature in the evaporators and tune the elements of the systems to maintain such a difference in every moment of the work of the unit.

Svetozar the Cold