Peoples Physics Concepts

http://www.ck12.org Chapter 18. Thermodynamics and Heat Engines

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

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

Efficiency of a lossless heat engine

An ideal engine, the most efficienttheoreticallypossible, is called a Carnot Engine. Its efficiency is given by the

following formula, where the temperatures are, respectively, the temperature of the exhaust environment and the

temperature of the heat input, in Kelvins. In a Carnot engine heat is input and exhausted in isothermal cycles, and

the efficiency isη= 1 −TcoldT

hot.Inallrealenginesheatislosttotheenvironment,thustheideale f f iciencyisneverevenclosetobeingobtained.

The Stirling engine is a real life heat engine that has a cycle similar to the theoretical Carnot cycle. The Stirling

engine is very efficient compared to a gasoline engine and could become an important player in today’s world

where green energy and efficiency will reign supreme.

Some Important Points

• In a practical heat engine, the change in internal energy must be zero over a complete cycle. Therefore, over
  a complete cycleW=∆Q.


• The work done by a gas during a portion of a cycle =P∆V, note∆Vcan be positive or negative.