# Heat engine

A heat engine is any mechanical device that takes advantage of a difference in temperatures between two systems to do work. Examples of heat engines are: the steam engine,the diesel engine, the gas engine in your car, and an imaginary one called the Carnot heat engine.

From the laws of thermodynamics, we can conclude that:

H = C - W

where H is the energy exchanged with the high temperature system, C is the energy exchanged with the cold system, and W is the work done by the engine.

The efficiency of a heat engine is defined by:

e = W / H = (C / H) - 1

The efficiency of any real engine can not be 1. In fact, the most efficient a heat engine operating between two temperatures (Th [h for hot] and Tc [c for cold]) can possibly be is determined by how efficiently a Carnot Engine would work; given by:

ecarnot = 1 - Tc / Th

The reasoning behind this relates to the laws of thermodynamics and the fact that if one were to hook a Carnot engine (also works in reverse to force heat from cold to hot) to a more efficient engine, one could theoretically make entropy go backwards.