These diagrams represent practical applications of the Carnot cycle. The top is the Woolf steam engine and the bottom is a simplified diagram represents the steam cycle used by fossil fuel and nuclear power plants. However, these are not ideal heat engines; their efficiency will be less than e= 1 - (Thot / Tcold) due to energy lost as friction, noise, and heat that isn't used to boil water. The Wolf steam engine is the heat engine that Carnot had in mind to improve when he developed his ideal cycle. He stated in Reflections on the Motive Power of Fire that in order to make a practical engine more like the ideal heat engine, all its parts and stages should function continuously in very small steps.

The internal core of the earth has a temperature that is thought to be around 7000 degrees C. At 7 or 8 kilometers, at depth that may be possible to drill, the temperature is about 300 degrees C. This represents a vast, untapped heat reservoir. The diagram below is simplified; the water would contain salt and methane, which would need to be separated out, but the underlying principle is the same. There are currently no geothermal generators of this type in commercial use.