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Big Ideas Of Science


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This simulation shows the energy flow in a heat engine, such as a gasoline-powered car engine. For every 100 J (QH) of heat generated by burning fuel at a higher temperature, only a fraction can be used to do useful work (W). The rest is wasted (QL) as the heat engine is re-set at a lower temperature, ready for another cycle.

The fact that a significant fraction of the energy must be wasted is a consequence of the second law of thermodynamics. The Carnot efficiency is the maximum possible efficiency the heat engine can have. Sadi Carnot showed that this maximum efficiency depends on the temperatures between which the engine operates, and is given by:
e = 1 - TL/TH

Real engines have losses because of friction, etc., and thus operate with an efficiency lower than the Carnot efficiency. The slider at the bottom allows you to adjust the efficiency to be more realistic than the Carnot value.

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