Peoples Physics Book Version-2

19.4. Heat Engines http://www.ck12.org

19.4 Heat Engines

Heat engines transform input heat into work in accordance with the laws of thermodynamics. For instance, as we
learned in the previous chapter, increasing the temperature of a gas at constant volume will increase its pressure.
This pressure can be transformed into a force that moves a piston.

The mechanics of various heat engines differ but their fundamentals are quite similar and involve the following steps:

1. Heat is supplied to the engine from some source at a higher temperature(Th).


2. Some of this heat is transferred into mechanical energy through work done(W).


3. The rest of the input heat is transferred to some source at a lower temperature(Tc)until the system is in its
   original state.

A single cycle of such an engine can be illustrated as follows:

In effect, such an engine allows us to ’siphon off’ part of the heat flow between the heat source and the heat sink.
The efficiency of such an engine is define as the ratio of net work performed to input heat; this is the fraction of heat
energy converted to mechanical energy by the engine:

e=

W

Qi

[5] Efficiency of a heat engine

If the engine does not lose energy to its surroundings (of course, all real engines do), then this efficiency can be
rewritten as

e=

Qi−Qo

Qi

[6] Efficiency of a lossless heat engine

ACarnot Engine, the most efficient heat engine possible, has an efficiency equal to

ec= 1 −

Tc

Th

[7] Efficiency of a Carnot (ideal) heat engine

whereTcandThare the temperatures of the hot and cold reservoirs, respectively.