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Thread: When is a Heat Engine Not a Heat Engine?

  1. #1 When is a Heat Engine Not a Heat Engine? 
    Join Date
    May 2013
    Some crank on another forum came up with a conundrum that got me thinking.

    He proposes an engine driven by detonation of an explosive that turns from solid to gas when detonated. Supposing the explosion reaction were one that did not cause any temperature increase, one would instantly form a volume of high pressure gas, at room temperature, equal to the volume of the solid explosive. This could be allowed to expand and do work against a piston or turbine.

    The question is whether this should be treated as a heat engine and whether it would be subject to Carnot Cycle efficiency limitations.

    On the one hand the gas does PV work and will cool adiabatically as it expands, so there will be a T1 and T2 - it would exhaust cold gas to the environment - but on the other hand, the pressure is initially developed not from heating but from a chemical reaction causing a phase change. So to be honest I am not sure how one would set about analysing this thermodynamically, in order to determine what would limit the theoretically achievable efficiency of such an engine.

    Any help appreciated.

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  3. #2  
    Bullshit Intolerant PhDemon's Avatar
    Join Date
    Feb 2013
    Newcastle-upon-Tyne, UK
    Off the top of my head but...

    The Carnot cycle is all about entropy (dS = dQ/T at constant pressure) but in the case where you have changing pressures I think you would have to work with internal energy

    dU = TdS + SdT - pdV - Vdp + dG

    Rearrange to give an expression for efficiency in terms of the change in entropy of the "hot" and "cold" parts of the system

    It's been about 10 years since I taught undergraduate thermodynamics though!

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