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I What moves the piston in a Carnot heat engine?

  1. Jun 4, 2016 #1
    Hi everyone,

    I have been looking at Carnot heat engines in a bid to better understand entropy, and I can't figure out how it actually does work. Why does the piston move?

    In some diagrams I have seen weights being removed from the piston, reducing the pressure at constant temperature and therefore causing expansion. In other diagrams however, the piston is attached to a wheel and the expansion seems to just happen.

    Can anybody help me understand why the piston is actually moving? And if it is due to external control of the piston, then surely this would require work- does this not defeat the object?

  2. jcsd
  3. Jun 4, 2016 #2
    Expanding gas.
  4. Jun 4, 2016 #3


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    In the high pressure phase, the expanding gas moves the piston. In the low pressure phase, something else must move the piston back. That is often done with inertia from a flywheel or with a crank shaft connection to another piston that is in the high pressure phase. The greater the pressure difference between the high and low pressure phases, the more work can be extracted from the machine.
    Last edited: Jun 4, 2016
  5. Jun 4, 2016 #4
    you should realise that a Carnot cycle is a theoretical, ideal cycle. There are no cycles that achieve the Carnot predictions.
  6. Jun 4, 2016 #5
    The ideal Carnot cycle consists of reversible expansions and compressions, both isothermal and adiabatic. To do work reversibly during the expansion phases, the force applied by the piston has to be decreased very gradually. Your example of removing tiny weights from the piston is one way of doing this. In the case of removing tiny weights, the piston is rising, and the weights are thus being lifted and delivered to a series of higher elevations. This work on the weights increases their potential energy. But adding and removing tiny weights is not the only way that reversible work can be done by the working fluid. In the case of a piston attached to a wheel, the force applied by the wheel must also be decreasing gradually in order for the work being done on it to be reversible. So the examples you have seen in books are just different ways of making the work done in the cycle to be reversible. Remember that a reversible process is one in which the system is only slightly removed from being in thermodynamic equilibrium throughout the change. So, a reversible process can be regarded as a continuous sequence of thermodynamic equilibrium states of the system. This can be achieved by making sure that the force applied by the system is never more than slightly different from that of the entity that it is doing work on or receiving work from.
  7. Jun 4, 2016 #6
    Thank you very much Chestermiller, a very helpful response.

    So would you say that the weights lifted can in theory just be put back on to the piston to do the work necessary for the compression phases, and this means that you are offsetting the work required to lift the weights in the first place? You are still, therefore, doing overall useful work during the expansion.
  8. Jun 4, 2016 #7
    The amounts that you put on at the various elevations during the compression phases do not match the amounts you take off during the expansion phases. We know this because a net amount of work is done.
  9. Jun 5, 2016 #8
    Thank you, this alleviates my confusion. I'm glad I asked.
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