Energy Efficiency

NEW SCREEN 3.5 Heat Engines

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3.5 Heat Engines

Energy conversions occurring in an automobile are illustrated below:

Chemical Energy

Thermal Energy

Mechanical Energy

Energy Conversions in an Automobile
Any device that converts Thermal energy into mechanical energy - such as automobiles or power plants - is called a heat engine. In these devices, high temperature heat (thermal energy) produced by burning a fuel is partly converted to mechanical energy to do work and the rest is rejected into the atmosphere, typically as a low temperature exhaust.

Animate this – An animated version is on Athena in animations folder – but needs learner controls

Need to emphasize that the objective is to maximize Work to increase efficiency.
If High T increases OR Low T decreases, the efficiency increases

Energy Flow in a Heat Engine


A general expression for the efficiency of a heat engine can be written as

We know that all the energy that is put into the engine has to come out either as work or waste heat. So work is equal to Heat at High temperature minus Heat rejected at Low temperature. Therefore, this expression becomes

Where, QHot = Heat input at high temperature and Qcold= Heat rejected at low temperature. The symbol is often (Greek letter eta) used for efficiency this expression can be rewritten as

The above equation is multiplied by 100 to express the efficiency as percent.

French Engineer Sadi Carnot showed that the ratio of QHighT to QLowT must be the same as the ratio of temperatures of high temperature heat and the rejected low temperature heat. So this equation, also called “Carnot Efficiency,” can be simplified as:

This is only equation in blackboard format

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