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The Carnot cycle is an idealized thermodynamic cycle that describes a perfect heat engine. Heat engines produce work from temperature differences between two tanks and all work on temperature gradients. The Carnot cycle can be done in reverse to create a refrigerator.
The Carnot cycle is an idealized thermodynamic cycle that describes a perfect heat engine. All true heat engines are imperfect approximations of the theoretical perfection embodied by the Carnot cycle. In the Carnot cycle, not all of the thermal energy is converted into mechanical work, but a large part is—the largest part permitted by physical law.
A heat engine produces work from the temperature difference between two tanks. In a combustion engine, one reservoir is the heat created within the engine (source) and the other is the external environment (sink). The heat generated by the source causes the gas inside the cylinder to expand, driving a piston that works. The thermodynamic state of the gas — expanding but at a constant temperature — is called isothermal.
Eventually the heat source is removed and the gas stops expanding so rapidly. If the heat were kept on continuously, the cylinder would explode. The gas begins to lose temperature when it reaches its maximum volume and no longer acts on the piston. This is called adiabatic expansion of the gas. Then the piston reverses course, compressing the gas, until it begins to reach the maximum temperature and physical resistance due to the heat it contains, bringing the system back to its starting state. Then the cycle begins again.
There are many different types of heat engines. All work on the familiar temperature gradient between a spring and a sink. To maximize the efficiency of heat engines, they must be well insulated. In most engines used today, the gas remains a gas throughout the cycle, but in steam engines there is a phase change between the liquid and the gas.
In a perfect Carnot cycle, all four steps occur very slowly, to minimize the entropy, or thermodynamic irreversibility, created by the process. In reality, the steps proceed rapidly and entropy is generated, which means that the cycle cannot go on forever. The cylinder walls degrade, heat from inside the engine is lost to the outside environment and so on. The Carnot cycle can be done in reverse to create a refrigerator.
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