Ideal Heat Engines
Heat engines convert internal energy to mechanical energy.
The operation of a reversible heat engine can be described on a PV diagram.
The efficiency of a reversible heat engine depends upon the temperatures between which it operates.
We will describe a heat engine with a diagram like this:
Qh = Qc + W
The efficiency of a heat engine describes how efficiently it turns heat into work.
Carnot's Principle: An irreversible heat engine operating between two heat reservoirs at constant temperatures cannot have an efficiency greater than that of a reversible heat engine operating between the same two temperatures.
Corollary: All reversible heat engines operating between the same temperatures have the same efficiency.
Carnot's reversible "engine" uses isothermal and adiabatic processes between two heat reserviors at temperatures Th (hot) and Tc (cold).
A Carnot cycle can also be represented on a PV diagram.
A refrigerator is a heat engine, run in reverse.
W + Qc = Qh
Another form of the Second Law of Thermodynamics is that
It is not possible to make a heat engine whose only effect is to absorb heat from a high-temperature region and turn all that heat into work.
That is, it is not possible to design a heat engine that does not exhaust heat to the environment.
Or, it is not possible to design a heat engine that has an efficiency of 1.00 or 100%.
If we could design such a 100% efficient heat engine, we could then use that heat engine to power a refrigerator. And the net result of that combination would be to cause heat to flow from a cold temperature to a high temperature.
And that gets us back to our original statement of the Second Law of Thermodynamics:
(c) 2002, Doug Davis; all rights reserved
Second Law Real Heat Engines Return to ToC, Heat Transfer