The Heat Engine

A heat engine converts heat to work. Example: a gasoline car engine.

To be useful, the engine must operate cyclically, with a certain amount of work being done every cycle.

Critical feature of cyclic operation: two temperatures---the higher temperature causes the gas inside a car cylinder system to expand, doing work, and the lower temperature re-sets the engine for another cycle.

In a full cycle of a heat engine, three processes occur:

  1. Heat Qh is added at a high temperature Th.
  2. Some of the energy from the input heat is used to do work W.
  3. The rest of the energy is removed as heat Qc at a lower temperature Tc.

Schematics of a heat engine:

Energy conservation: Qh = Qc + W


Efficiency:   η ≡ W/Qh = 1 -(Qc/Qh) < 1
W = useful output
Qh = input energy