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 Q_h is added at a high temperature T_h.
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 Q_c at a lower temperature T_c.

Schematics of a heat engine:

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