Energy Efficiency
NEW SCREEN 3.5 Heat Engines
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- Energy Flow in a Heat Engine NEW SCREEN
NEW SCREEN 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 NEW SCREEN 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: 
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