A heat engine is a device that uses heat to do work. The gasoline-powered car engine is a good example. In the car engine there are several cylinders. In each cylinder a gas is confined by a piston.

To be useful, the engine must go through cycles, with a certain amount of work being done every cycle. A critical component of any heat engine is that two temperatures are involved. The higher temperature causes the system to expand, doing work, and the lower temperature re-sets the engine so another cycle can begin.

In a full cycle of a heat engine, three things happen:

1. Heat Q_H is added at a relatively 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_L at a relatively low temperature T_L.

This is really a statement of the First Law of Thermodynamics, for a cycle:

|Q_H| = W + |Q_L|

Efficiency

The efficiency of an engine tells you how much of the input energy ends up doing useful work. The efficiency can be stated as a fraction or as a percentage:

e

=

W |QH|

=

|QH| - |QL| |QH|

= 1 -

|QL| |QH|

This is the maximum possible efficiency of an engine. In practice losses from friction and other sources reduce the efficiency.