A Heat Engine

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 QH is added at a relatively 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 QL at a relatively low temperature TL.

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

|QH| = W + |QL|

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.