Image 3 Caption: A annotated, colorized version of Sadi Carnot's (1796--1832) 1824 schematic diagram of the Carnot engine.

Features Extended:

1. The Carnot engine is the most efficient heat engine/refrigerator possible.

2. Heat engines and refrigerators operate between hot baths and cold baths and using a working fluid.

3. A heat engine takes heat energy from the hot bath, turns some of it into work (in the physics) using the working fluid and rejects some of it to the cold bath.

   A refrigerator does the reverse.

4. The illustrated version of the Carnot engine is a piston-cylinder heat engine.

   The piston-cylinder heat engine is one of the basic heat-engine designs.

   The first practical heat engine, the Newcomen engine, was a piston-cylinder heat engine.

   The overwhelminly the most common design for internal combustion engines is the piston-cylinder heat engine---as in your car.

5. It is impossible to convert heat energy entirely into work in any cyclic process.

   It is impossible to move heat energy from a cold to a hot bath without doing any work. (This would be heat energy spontaneously flowing from cold to hot which is never seen.)

6. The two impossibilities are fundamental and follow from the 2nd law of thermodynamics.

7. The short definition of thermodynamics is that it is the science of heat energy and temperature.

   Thermodynamics developed, at least in part, from the need to understand and improve heat engines.

8. The efficiency of a heat energy is defined by the following formula:

         η = W/Q_H = 1 - Q_C/Q_H ,

   where η is the efficiency, W is work done in a cycle, Q_H is heat energy absorbed from the hot bath, and Q_C is heat energy rejected to cold bath.

   An IMPOSSIBLE perfect heat engine would have η = 1.

   In fact, it can be demonstrated by irrefutable theoretical argument that

         η_max = 1 - T_C/T_H ,

   where η_max is the maximum possible efficiency, T_H is the Kelvin temperature of the hot bath, T_C is the Kelvin temperature of the cold bath, and where we have used modern terminology and the Kelvin scale which were unknown to Sadi Carnot (see Wikipedia: Heat Engine: Efficiency).

   So the temperature ratio T_C/T_H sets the maximum efficiency.

   Only if T_C = 0 could the IMPOSSIBLE perfect heat energy exist. But T_C = 0 is virtually impossible for a practicable heat engine.

   In actual practical heat engines, T_C is usually the temperature of the ambient medium: often just the surrounding air.

9. The maximum efficiency is achieved by the Carnot engine.

   The Carnot engine run in reverse is also the maximum efficient refrigerator.

   In fact, it is part of the proof of the maximum efficiencies that only a reversible heat engine can obtain them.

10. A perfect Carnot engine CANNOT be built---it is an ideal limit.

    However, very-close-to-perfect ones can built and have their special uses---mainly yours truly thinks in some kinds of experimentation.

11. Why is the most efficient heat engine the Carnot engine NOT widely used? It's extremely low power.

    Actually, to obtain it's ideal maximum efficiency requires running at zero power.

12. However, the Carnot engine is very useful as setting the practical design limit for heat engines and refrigerators.

    Also, the Carnot engine suggests the true general rule that heat engine generally increases as the ratio T_C/T_H decreases.

    Since T_C is usually fixed by the ambient medium, this means that a key path to making more efficient heat engines is by making T_H higher---but there are limits in practice to making very high T_H's to contend with too.

13. Sadi Carnot, by the by, was the son of Lazare Carnot (1753--1823), one of the leaders of the French Revolution. One of the lucky ones who were NOT executed during the Reign of Terror (c.1793--1794). Lazare Carnot was called the "Organizer of Victory".

14. For a semi-proof of the Carnot engine properties, see incomplete Thermodynamics file: heat_engine_schematic_1bbb.html.