MEPX
Chapter 4 of 10All chapters
  1. 1Heat and temperature
  2. 2The first law
  3. 3The second law
  4. 4Engines and efficiency
  5. 5How heat moves
  6. 6Gases
  7. 7Phase changes
  8. 8Thermodynamics in the world
  9. 9The statistical view
  10. 10Time and the universe

Chapter 4 of 10

Engines and efficiency

Why perfect is impossible.

The Carnot limit

A heat engine takes heat from a hot source, does work, and must dump some heat to a cold sink. Maximum efficiency depends only on the two temperatures, not on the design.

  • A car engine sits near 30 percent, a power station near 40.
  • Raising the hot temperature or lowering the cold one is the only route to more.

Heat pumps

Run an engine backwards and it moves heat from cold to hot using work. That is a fridge, an air conditioner and a heat pump, all the same machine pointed differently.