Lower and Higher Heat Values (LHV or HHV) seem to have multiple definitions based on things I can't figure out.

What I want to know is, which one is appropriate to use for calculating the thrust from a jet engine? I'm thinking of large commercial turbofan engines like on the 747 or A380.

Also, since water vapor seems to have something to do with it, what about jet engines that run on hydrogen like the experimental Tu-155? Should we use LHV or HHV to calculate thrust in those engines?

  • $\begingroup$ Use for what? Those are just numbers that describe the fuel. $\endgroup$ – Jan Hudec Jul 28 '15 at 6:29
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    $\begingroup$ I'm voting to close this question as off-topic because it is more about thermodynamic calculations than aviation and as such is better suited for physics.stackexchange.com. After clarification what you want to use the specific heat of combustion for. $\endgroup$ – Jan Hudec Jul 28 '15 at 6:32
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    $\begingroup$ On a side note, fuel suppliers usually use the lower heat values. It then leads to paradoxes like natural gas boilers with 109% efficiency. $\endgroup$ – Jan Hudec Jul 28 '15 at 6:34
  • $\begingroup$ @JanHudec I'm not sure what is unclear. I want to use it for jet engines, for thrust. $\endgroup$ – DrZ214 Jul 28 '15 at 7:19
  • $\begingroup$ Well, the thrust just is. It is generated by burning the fuel, not by chewing up numbers! You don't need to use numbers for it. Now if you want to calculate thrust, that's another matter, but you didn't say that. $\endgroup$ – Jan Hudec Jul 28 '15 at 7:31

The difference between both values is the energy set free when the water created by the combustion process condenses. Use the lower heat value for jet engine calculations. The water in a jet engine leaves it as vapor, and the vaporisation heat is lost for propulsion.

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