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Why are there heat engines?

  1. Apr 18, 2015 #1
    The main result of thermodynamics is that if you have a place that is hot and another place that is colder, you can operate a heat engine by absorbing heat from the hot place and dumping it in the cold place, extracting some useful work in the process. It gives you a way to calculate the maximum possible useful work per unit heat absorbed from the hot source.

    Thermodynamics doesn't seem to care whether you actually do extract some useful work from the tendency of heat to flow from hot to cold. It allows for maximally irreversible processes, such as the free expansion of a perfect gas. If you start with a gas initially confined to one half of a container, and then remove the partition, it fills the whole container without doing any work. That is a squandered opportunity to extract work that the universe will never get back.

    So how is it that the universe actually does use temperature differences, voltages, chemical potential gradients, and so on to do work--to create structures? What stops the universe from being maximally wasteful, letting all differences simply smear out, without generating any structures along the way?
  2. jcsd
  3. Apr 18, 2015 #2


    Staff: Mentor

    You're assuming that creating structures requires doing work. In the presences of gravity, that's not actually the case. The structure in our current universe--i.e., the fact that the matter is not all spread out evenly, but clumped into planets, stars, etc.--happened without requiring any work to be done, just as a result of gravitational clumping.
  4. Apr 19, 2015 #3


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    This 'smeared out' state existed in the very early universe. It turns out that this is actually a high potential energy situation, and that the formation of structures is more favorable than no structures since it is a lower energy state.
  5. Apr 19, 2015 #4


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    One word, 'gravity'.

    The effect of an attractive gravitational force is not used in your example of removing the partition in the half-filled container.

  6. Apr 19, 2015 #5

    Are these responses in agreement? Also I'm confused by the statement that large scale structure due to gravitational clumping, is created without work being done?
  7. Apr 19, 2015 #6
  8. Apr 19, 2015 #7
    What's even more bizzare, if you narrowly define "heat engine" as a literal engine, like a reciprocating combustion engine, or combustion turbine, or a boiler+condensing steam turbine, you are talking about dissipative structures that have been intentionally created by other dissipative structures.o_O
    Last edited: Apr 19, 2015
  9. Apr 19, 2015 #8


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    If the matter was evenly distributed at the beginning, then our universe would be a cloud of dust.
    It's the fact that the matter densities showed some perturbations that led to the creation of structure. The more dense areas became gravitational sources, and your initial soup started collapsing here and there.
    Those perturbations are actually measured in the CMB spectrum. One reasonable answer for their existence is these perturbations is the quantum fluctuations that were stretched to large distances during inflation.

    Then every "heating machine" is due to gravity.
  10. Apr 19, 2015 #9
    I agree, I think with other statements, that the existence of the energy flow that "dissipative structures" dissipate is due to the low probability even mass distribution that characterizes early space-time, a situation far from equilibrium w/respect to gravity.
  11. Apr 19, 2015 #10
    Thanks for all the replies :)

    Yes, I'm taking for granted the equivalence of work and structure. Or at least their inter-convertibility. Where there's smoke, there's fire. In fact I think I want to use the word "structure" very broadly, even for the temperature differences and potential differences themselves. In a reversible universe, the total amount of structure in the universe is conserved. When there are temperature differences, heat energy stored at the higher temperature has more structure than the same amount of energy at the lower temperature. So when you move energy from high temperature to low temperature in a reversible way, destroying some structure, you must be creating it somewhere else, say by lifting a weight. And then that weight can be used in turn to drive a refrigerator that pumps the heat at low temperature back up to a higher temperature. The conversion factor between the amounts of energy (heat vs work) is the Carnot efficiency.

    But the universe isn't reversible. Structure is being frittered away constantly. I'm guessing this is fundamentally because the number of particles in the universe is not constant. By certain processes that I don't completely understand yet, a yellow photon can get converted into several infrared photons (without doing any work?), but the reverse doesn't happen spontaneously.

    So that's why I was saying that it's not obvious why everything doesn't just smear out and wind down uniformly with nothing interesting along the way, since irreversibility doesn't seem to have any limits. If a process can generate work or structure but doesn't have to, why should it?

    But yeah, you guys are right gravity has got to be a key ingredient. That makes sense now. It pulls matter together, the kinetic energy gets converted into thermal energy, the thermal energy gets radiated away into outer space and the matter settles into a clump. Structure, in the form of gravitational potential energy, has been converted into heat and radiation. But in the process matter becomes concentrated into local clumps.

    I'm still not 100% convinced that having matter in clumps already represents structure. It 'looks' more structured because I'm used to thinking in terms of diffusion, smearing out, uniformity as a lack of structure. But with the attractive force of gravity it's the other way. This is what Drakkith and others are getting at.

    But real structure is generated in the process of this clumping. You lose the gravitational potential but gain some temperature differences in return. Then somehow these temperature differences can be tapped into to generate other structures. Maybe I'm starting to get it, talking it out.
  12. Apr 19, 2015 #11
    That is exactly what I'm interested in. I am interested in organization at all levels. It is as if humans have domesticated the heat engine in much the same way that they have domesticated plants and animals. And domestication generally works both ways. It is mutual adaptation in disguise. Sheep, orange trees, and car engines have cleverly outsourced most or all of their maintenance to us by being useful. Sneaky.

    Thanks for the Prigogine link, too. The library has some of his books. They have been added to the "need to read" list.
  13. Apr 19, 2015 #12


    Staff: Mentor

    This looks to me like a very non-standard usage of the term "structure". What exactly do you mean by "structure"?
  14. Apr 19, 2015 #13
    You might be interested in this also.
    Last edited: Apr 19, 2015
  15. Apr 19, 2015 #14
    I agree with PeterDonis that terms need to be precise. He is pretty expert in General Relativity, the technical theory of Gravity... So throwing around technical terms like Gravitational Potential Energy along with vagaries like "structure", can get you educated in a hurry on this forum! That said, I took your loose association to be referencing the the link below, as well as other posts.

  16. Apr 19, 2015 #15
    Fair enough. I'm sure it isn't standard. I needed a term that encompasses mechanical energy and everything that can be converted into mechanical energy. Gravitational potential would be structure, the way I'm using the term. So structure is in units of energy. But I wanted to take account of the fact that only part of 1 joule worth thermal energy at some temperature is convertible into work unless you have a sink at absolute zero. So that 1 joule of thermal energy is, in some sense, not a full joule of structure. You always have to deal with the 3K cosmic background, at the very least. Usually your cold sink is much hotter than that.

    I am trying to stretch the heat engine concept as far as possible without it breaking. I have this impression that Darwin's natural selection mechanism is a sort of very generalized heat engine. Mutation and recombination vaguely resembles the power stroke, while natural selection resembles the exhaust stroke. Instead of turning a shaft, the engine sifts out populations of adapted genomes. These genomes and their associated organisms would be examples of what I'm calling structure. As Jimster41 pointed out, these organisms are themselves dissipative structures, which seems to be a generalization of the heat engine concept. So there are heat engines within heat engines. Admittedly this is all pretty vague, I get a kick out of thinking about it.

    Does anyone know of a treatment of Benard convection cells that takes account of the thermodynamics of it? Surely each little parcel of water in the pan is acting as a tiny heat engine, doing work on its neighbors. The whole convection cell seems to be a heat engine, all of whose work output goes back into maintaining the cell. This is much different than the case of a man-made engine. We would never put up with an engine that only did enough work to keep itself together and running :).
  17. Apr 19, 2015 #16
    My fridge does that and I put up with it.
  18. Apr 19, 2015 #17


    Staff: Mentor

    But that isn't the way you were using "structure" in your OP. There you used it to denote things that (you claimed) required work to be done to create them. Now you're using "structure" to denote the source of the work (mechanical energy) instead of the product (according to your claim) of the work. Which is it?

    As I noted in post #2, your original assumption, that the things you were calling "structure" in your OP required work to produce them, is false. Gravitational clumping is not a "heat engine" in any useful sense that I can see.

    I think there is a sense in which it is, but discussing that is getting way off topic for this forum and you should open a separate thread in the appropriate forum if you want to talk about it.

    Now you're reverting to the meaning of "structure" that you used in your OP--"structure" is the product of some "heat engine-like" process. But that's certainly not the same as "mechanical energy and everything that can be converted into mechanical energy".

    Same comment here as above--this is off topic for this forum and you should open a separate thread in the appropriate forum if you want to talk about it.
  19. Apr 20, 2015 #18
  20. Apr 20, 2015 #19

    My understanding of the proposed cosmological theories being discussed is that Gravitational clumping, releases energy through fusion, in clumps called stars, providing the free energy that drives a whole variety of systems away from thermodynamic equilibrium, causing them to self organize to states of lower entropy, more complex structure: Life on this planet, and the planets themselves being examples.

    @techmologist we can take this offline if you want, if it is considered inappropriate content... somehow.
    Last edited: Apr 20, 2015
  21. Apr 20, 2015 #20
    In support of the mission of this forum, which seems to be to make the conversations as serious as possible. I've been searching for specific research into the detailed theoretical understanding of the principles invoked in the broader, more holistic, descriptions (that I'm familiar with), referred to here in this thread in very general and vague terms. I think it's arguably a stretch to consider this about cosmology, but I would lobby that it is, in that it is research into the theory of thermodynamics in the non-equilibrium case - relevant to the vague phenomenon of "self organization" and "dissipative structure" - which are main tenets of "Evolutionary" Cosmology.

    I believe the journal is a reputable one. It's a pretty hard core paper and not very accessible to the lay person. But it's worth exposing oneself to the introduction and conclusion if nothing else, since they provide tantalizing hints of just what the heck the hard part is saying. I hope that it is not a breach of copyright protocol to link to the paper this way, showing only the abstract.

    Stochastic thermodynamics, fluctuation theorems and molecular machines
    Udo Seifert

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    Udo Seifert 2012 Rep. Prog. Phys. 75 126001

    © 2012 IOP Publishing Ltd
    Received 18 May 2012, in final form 6 August 2012
    Published 20 November 2012

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      This article was invited by Erwin Frey.

      Stochastic thermodynamics as reviewed here systematically provides a framework for extending the notions of classical thermodynamics such as work, heat and entropy production to the level of individual trajectories of well-defined non-equilibrium ensembles. It applies whenever a non-equilibrium process is still coupled to one (or several) heat bath(s) of constant temperature. Paradigmatic systems are single colloidal particles in time-dependent laser traps, polymers in external flow, enzymes and molecular motors in single molecule assays, small biochemical networks and thermoelectric devices involving single electron transport. For such systems, a first-law like energy balance can be identified along fluctuating trajectories. For a basic Markovian dynamics implemented either on the continuum level with Langevin equations or on a discrete set of states as a master equation, thermodynamic consistency imposes a local-detailed balance constraint on noise and rates, respectively. Various integral and detailed fluctuation theorems, which are derived here in a unifying approach from one master theorem, constrain the probability distributions for work, heat and entropy production depending on the nature of the system and the choice of non-equilibrium conditions. For non-equilibrium steady states, particularly strong results hold like a generalized fluctuation–dissipation theorem involving entropy production. Ramifications and applications of these concepts include optimal driving between specified states in finite time, the role of measurement-based feedback processes and the relation between dissipation and irreversibility. Efficiency and, in particular, efficiency at maximum power can be discussed systematically beyond the linear response regime for two classes of molecular machines, isothermal ones such as molecular motors, and heat engines such as thermoelectric devices, using a common framework based on a cycle decomposition of entropy production.
    Last edited: Apr 20, 2015
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