1. Entropy in the Real World: Engines
A heat engine, or more simply, an engine, is a device that extracts energy from its environment in the form of heat and does useful work.
At the heart of every engine is a working substance.
Steam engine Water, in both its vapor and its liquid form.
Automobile engine  Gasoline–Air mixture.
A Carnot Engine or Ideal Engine:
In an ideal engine, all processes are reversible and no wasteful energy transfers occur due to, say, friction and turbulence.

2. Efficiency of a Carnot Engine

3. Entropy in the Real World: Refrigerators
A refrigerator is a device that uses work to transfer energy from a low-temperature reservoir to a high-temperature reservoir as the device continuously repeats a set series of thermodynamic processes. In a household refrigerator, for example, work is done by an electrical compressor to transfer energy from the food storage compartment (a low-temperature reservoir) to the room (a high-temperature reservoir).

4. Problem 29, Page 557
Figure shows a reversible cycle through which 1.00 mol of a monatomic ideal gas is taken. Assume that p = 2p0, V = 2V0, p0 = 1.01 × 105 Pa, and V0 =  m3. Calculate (a) the work done during the cycle, (b) the energy added as heat during stroke abc, and (c) the efficiency of the cycle. (d) What is the efficiency of a Carnot engine operating between the highest and lowest temperatures that occur in the cycle? (e) Is this greater than or less than the efficiency calculated in (c)?