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Understanding the Carnot cycle

  1. Jul 3, 2015 #1
    My textbook explains the Carnot cycle as follows:

    1. Heat is added to the boiler, where the steam inside expands isothermally at high temperature (T(High))w(both valves closed).
    2. The intake valve is opened (exhaust valve closed) and the steam expands adiabatically. The expanding steam does work by pushing the piston out.
    3. As a result of the expansion, the gas cools to the lower temperature (T(Low)).
    4. The exhaust valve opens (intake valve closed). The piston compresses the cool gas and pushes it out to the exhaust (isothermal compression). The condenser removes heat from the steam to the air outside, and steam condenses to lower-temperature liquid.
    5. A pump moves water to the boiler (adiabatic compression).

    I understand step 1, 2, and 3; however, step 4 and 5 I don't grasp. I know that The first steps entail expanding a gas while keeping it at the same temperature, opening the intake valve in order to use the expanding gas to move a piston and do work, and I understand that, according to the first law of thermodynamics, the internal energy of the gas decreases as a result of this work. However, what is the point of step 4 and 5? Why does the gas need to be compressed? Why can't it just go back to the boiler as is? In addition to answering, could someone, in plain language, explain what steps 4 and 5 are for? For example, I am confused about the point that step 4 says that the gas is already cool because it lost energy from doing work. If so, then why is there a lower-temperature reservoir at all. Also, what does "compress" mean in the contexts of step 4 and 5? What is the purpose of this "compression?"
  2. jcsd
  3. Jul 3, 2015 #2


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    The cycle needs to end up where it started so that a second identical cycle can begin. Unless the process ends up where it started (completes the cycle), there is no way to know what the efficiency of a continuously running machine is.
  4. Jul 3, 2015 #3
    Why does the temperature of the gas need to be lowered in order to restart the cycle?
  5. Jul 3, 2015 #4


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    It is describing a typical steam piston machine and indicating how the phases of the Carnot cycle match up. The machine doesn't have to be designed this way, but this is one machine design. The matching parts of the Carnot cycle are not perfect or clear cut. Clearly, the piston must return to the top and push the gas out the exhaust. Because the volume is decreasing during this, and the gas is being pushed out, all at the lower temperature, this matches the isothermal compression stage of the Carnot cycle. The steam could just be exhausted to the atmosphere rather than being condensed and returned to the tank.
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