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| We see examples of heat engines all the time in everyday life. The steam engine on a vintage locomotive, the internal combustion engine in our automobiles, and diesel engines are all examples of heat engines. An interesting application of the heat engine though, is the heat pump which essentially works in reverse. | |
Heat Pumps: Refrigerators and Air Conditioners A heat pump, any device that transfers energy from the cold reservoir to the hot reservoir, is put to work in the form of air conditioners and refrigerators. Since this is not the direction that energy normally moves in a heat engine, work must be done on the engine. Of course, the goal is to transfer a maximum amount of energy with a minimum amount of work. If this transfer could be done without doing any work the heat engine would be considered perfect. However this would be in complete violation of the Second Law of Thermodynamics. This idea was first expressed by Rudolph Clausius who said: |
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“It is impossible to construct a cyclical machine whose sole effect is to transfer energy continuously by heat from one object to another object at a higher temperature without the input of energy by work” 1 |
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Efficiency of a Heat Engine Different heat engines achieve different efficiencies. A good automobile engine has an efficiency of around 20%. This means that only about 20% (2) of the energy is converted to work and the other 80% is expelled to the cold reservoir (in this case the atmosphere). In the case of the Bugatti Veyron W16 automobile engine, pictured at right, the efficiency is probably much lower. It is sport tuned for maximum power, response, and performance. Generally the most efficient automobile engines are four-cylinder, naturally aspirated engines that are conservatively tuned. |
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| The thermal efficiency of a heat engine is best thought of as the quotient of what one gains (the work) to what one has to give (the energy transfer). Because something must always be given up, the efficiency must always be less than 100%. In order to achieve 100% efficiency would have to convert all of the input energy into work and expel none to the cold reservoir. Why is this impossible? Because, like with heat pumps, it would violate the Second Law of Thermodynamics. The Kevin-Planck form of this law states that: “It is impossible to construct a heat engine that, operating in a cycle, produces no effect other than the input of energy by heat from a reservoir and the performance of an equal amount of work.” 3 |
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Use the navigation frame to the left to learn more about heat engines. |
