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Re: [Small4-strokeEngines] Digest Number 695

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  • George Bearden
    ... Is this a universally accepted truism? Is this a sort of intuitive thing, or do you have a basis for saying this? A part of the waste heat issue depends
    Message 1 of 8 , Dec 1, 2006
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      > When you burn twice the fuel to get
      > twice the power, you must reject twice
      > the waste heat

      Is this a universally accepted truism? Is this a sort
      of intuitive thing, or do you have a basis for saying
      this?

      A part of the waste heat issue depends on the
      relationship between volume and exposed surface area.
      Which remains the same as you pump up the hp.

      And I wonder what happens to the insulating effect of
      the boundary layer of compressed gases adjacent to the
      exposed metal. Does it insulate more or less as hp
      (and pressure) goes up?

      And does the (I assume) increased turbulence counter
      these factors?

      Then I think of the fact that heat transfer increases
      in some relation to the Delta-T, which I think would
      be greater as you increase hp.

      I don't know any answers, it just seems a wild
      coincidence for it to be true.

      I mean this in a nice way.. I am interested in the
      convoluted and esoteric. And when I was a kid I liked
      to poke sticks into ppls spokes.
    • Scott Perkins
      I understand the concept.. conceivably at higher hp and higher rpms a greater percentage of the waste heat could be dispursed through the exhause gasses
      Message 2 of 8 , Dec 1, 2006
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        I understand the concept.. conceivably at higher hp and higher
        rpms a greater percentage of the waste heat could be
        dispursed through the exhause gasses instead of through
        the engine. We know most of the waste heat goes out the
        exhaust but do the percentages remain the same at all
        levels of hp production ?
        That would be a hard question to answer unless some
        laboratory has done the experiments.
        Scott

        George Bearden wrote:
        >
        > > When you burn twice the fuel to get
        > > twice the power, you must reject twice
        > > the waste heat
        >
        > Is this a universally accepted truism? Is this a sort
        > of intuitive thing, or do you have a basis for saying
        > this?
        >
        > A part of the waste heat issue depends on the
        > relationship between volume and exposed surface area.
        > Which remains the same as you pump up the hp.
        >
        > And I wonder what happens to the insulating effect of
        > the boundary layer of compressed gases adjacent to the
        > exposed metal. Does it insulate more or less as hp
        > (and pressure) goes up?
        >
        > And does the (I assume) increased turbulence counter
        > these factors?
        >
        > Then I think of the fact that heat transfer increases
        > in some relation to the Delta-T, which I think would
        > be greater as you increase hp.
        >
        > I don't know any answers, it just seems a wild
        > coincidence for it to be true.
        >
        > I mean this in a nice way.. I am interested in the
        > convoluted and esoteric. And when I was a kid I liked
        > to poke sticks into ppls spokes.
        >
        > ### Lets use this list to build a knowledge base for those who are
        > interested in flying with these engines. Don't forget you can search the
        > messages archive for past discussions. Please keep comments on subject.
        > Sorry, no attachments allowed.
        >
        > For access to the home page:
        > >http://groups.yahoo.com/group/Small4-strokeEngines/<
        >
        > For direct access to the files area:
        > >http://groups.yahoo.com/group/Small4-strokeEngines/files/<
        > Yahoo! Groups Links
        >
        >
        >
      • jeremy_harris_uk
        ... Most of the waste heat goes out of the exhaust anyway, so it s a bit of a moot point. You can change the percentage of heat absorbed by the piston and
        Message 3 of 8 , Dec 1, 2006
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          --- In Small4-strokeEngines@yahoogroups.com, George Bearden
          <gab16@...> wrote:
          >
          > > When you burn twice the fuel to get
          > > twice the power, you must reject twice
          > > the waste heat
          >
          > Is this a universally accepted truism? Is this a sort
          > of intuitive thing, or do you have a basis for saying
          > this?
          >
          > A part of the waste heat issue depends on the
          > relationship between volume and exposed surface area.
          > Which remains the same as you pump up the hp.
          >
          > And I wonder what happens to the insulating effect of
          > the boundary layer of compressed gases adjacent to the
          > exposed metal. Does it insulate more or less as hp
          > (and pressure) goes up?
          >
          > And does the (I assume) increased turbulence counter
          > these factors?
          >
          > Then I think of the fact that heat transfer increases
          > in some relation to the Delta-T, which I think would
          > be greater as you increase hp.
          >
          > I don't know any answers, it just seems a wild
          > coincidence for it to be true.
          >
          > I mean this in a nice way.. I am interested in the
          > convoluted and esoteric. And when I was a kid I liked
          > to poke sticks into ppls spokes.
          >


          Most of the waste heat goes out of the exhaust anyway, so it's a bit
          of a moot point. You can change the percentage of heat absorbed by
          the piston and combustion chamber by using hi-tech coatings (ceramics
          and the like) with the net result that a greater proportion of the
          waste heat will go out the exhaust. This also holds true for other
          mods, like high efficiency exhaust systems. The quicker you get the
          hot gases out, the less heat gets absorbed into the piston, cylinder
          and head.

          Jeremy
          SALISBURY
          UK
        • Jim
          This question of yours has elicited responses from folks that do NOT seem to have a fundamental understanding of the thermodynamics of the OTTO cycle. This is
          Message 4 of 8 , Dec 1, 2006
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            This question of yours has elicited responses from folks that do NOT
            seem to have a fundamental understanding of the thermodynamics of the
            OTTO cycle. This is the heat cycle that spark ignition engines
            follow. see this link.
            http://www.grc.nasa.gov/WWW/K-12/airplane/otto.html

            Coatings and other changes may affect temperature, but not the total
            heat flow, And the thermodynamic cycle defines exactly how much heat
            must be rejected for a given set of engine parameters. You must
            understand the pV diagrams to grasp the heat/work output of the cycle.

            To your question, is it a universal statement. The answer is yes. If
            the two engines being compared have the same OTTO cycle efficiency,
            the same proportion of the heat available from the fuel is turned into
            work and the same proportion must be rejected by the engine. Overall,
            the changes suggested by others do not have a great affect on the
            cycle efficiency. And, if there is any difference at all, the slower
            engine is more efficient.

            I would suggest that it is pure uniformed speculation to suggest that
            the faster the engine runs, the more heat that goes out the exhaust.
            The OTTO cycle does not consider rpm, per se, in determining the cycle
            efficiency and therefore, the heat rejection. If this theory had any
            merit, we could just run our engines faster and faster and not need a
            radiator. Just a bit silly.

            If the link above is of interest, Google 'Diesel' for the Diesel cycle
            that controls how a diesel engine works, You will see from a
            comparison of the PV diagrams that the Diesel cycle is fundamentally
            more efficient. That is why the diesel engine gets better fuel
            efficiency, period.

            Jim

            - In Small4-strokeEngines@yahoogroups.com, George Bearden <gab16@...>
            wrote:
            >
            > > When you burn twice the fuel to get
            > > twice the power, you must reject twice
            > > the waste heat
            >
            > Is this a universally accepted truism? Is this a sort
            > of intuitive thing, or do you have a basis for saying
            > this?
            >
            > A part of the waste heat issue depends on the
            > relationship between volume and exposed surface area.
            > Which remains the same as you pump up the hp.
            >
          • Kevin Whisler
            When you burn twice the fuel to get ... Is this a universally accepted truism? Is this a sort of intuitive thing, or do you have a basis for saying this? I
            Message 5 of 8 , Dec 1, 2006
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              "> When you burn twice the fuel to get
              > twice the power, you must reject twice
              > the waste heat

              Is this a universally accepted truism? Is this a sort
              of intuitive thing, or do you have a basis for saying
              this?"

              I would say that is an accurate statement. The energy present in the fuel goes two places: (1) mechanical energy and (2) waste heat, and as Howard has stated, most of it goes to waste heat.

              If you are burning twice the fuel to make twice the power, that tells me that the percentage of energy going to mechanical energy is the same in each case, and therefore so is the percentage of energy going to waste heat. So, twice the fuel burn means twice the waste heat.

              As far as your several questions about cooling differences, yes those may have some effect and may help offset the situation. But you are still initially facing the need to remove twice the waste heat. How (or whether) this can be accomplished is a whole nuther topic.

              Kevin



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            • Kevin Whisler
              I agree Jim, most people do not understand thermodynamics, or the OTTO cycle, or even know what a pV diagram is. But thats OK. Afterall, this is a group for
              Message 6 of 8 , Dec 1, 2006
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                I agree Jim, most people do not understand thermodynamics, or the OTTO cycle, or even know what a pV diagram is. But thats OK. Afterall, this is a group for flying enthusiasts, not engineers or scientists.

                Let me just say (without an ounce of ego) that I am an engineer, and I agree with what you have posted. Anyone still questioning this topic or wanting a more thorough understanding would be wise to visit the websites you provided. Your post was very helpful, just please don't be too tough on people without your technical background.

                Respectfully,
                Kevin

                Jim <captain_jims@...> wrote:
                This question of yours has elicited responses from folks that do NOT
                seem to have a fundamental understanding of the thermodynamics of the
                OTTO cycle. This is the heat cycle that spark ignition engines
                follow. see this link.
                http://www.grc.nasa.gov/WWW/K-12/airplane/otto.html

                Coatings and other changes may affect temperature, but not the total
                heat flow, And the thermodynamic cycle defines exactly how much heat
                must be rejected for a given set of engine parameters. You must
                understand the pV diagrams to grasp the heat/work output of the cycle.

                To your question, is it a universal statement. The answer is yes. If
                the two engines being compared have the same OTTO cycle efficiency,
                the same proportion of the heat available from the fuel is turned into
                work and the same proportion must be rejected by the engine. Overall,
                the changes suggested by others do not have a great affect on the
                cycle efficiency. And, if there is any difference at all, the slower
                engine is more efficient.

                I would suggest that it is pure uniformed speculation to suggest that
                the faster the engine runs, the more heat that goes out the exhaust.
                The OTTO cycle does not consider rpm, per se, in determining the cycle
                efficiency and therefore, the heat rejection. If this theory had any
                merit, we could just run our engines faster and faster and not need a
                radiator. Just a bit silly.

                If the link above is of interest, Google 'Diesel' for the Diesel cycle
                that controls how a diesel engine works, You will see from a
                comparison of the PV diagrams that the Diesel cycle is fundamentally
                more efficient. That is why the diesel engine gets better fuel
                efficiency, period.

                Jim

                - In Small4-strokeEngines@yahoogroups.com, George Bearden <gab16@...>
                wrote:
                >
                > > When you burn twice the fuel to get
                > > twice the power, you must reject twice
                > > the waste heat
                >
                > Is this a universally accepted truism? Is this a sort
                > of intuitive thing, or do you have a basis for saying
                > this?
                >
                > A part of the waste heat issue depends on the
                > relationship between volume and exposed surface area.
                > Which remains the same as you pump up the hp.
                >






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              • John R. Lewis
                ... Twice the power, twice the waste heat is a first approximation. Several factors: First, the efficiency increases with the peak temperature. This goes up
                Message 7 of 8 , Dec 1, 2006
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                  George Bearden wrote:

                  > > When you burn twice the fuel to get
                  > > twice the power, you must reject twice
                  > > the waste heat
                  >
                  > Is this a universally accepted truism? Is this a sort
                  > of intuitive thing, or do you have a basis for saying
                  > this?
                  >
                  > A part of the waste heat issue depends on the
                  > relationship between volume and exposed surface area.
                  > Which remains the same as you pump up the hp.
                  >
                  > And I wonder what happens to the insulating effect of
                  > the boundary layer of compressed gases adjacent to the
                  > exposed metal. Does it insulate more or less as hp
                  > (and pressure) goes up?
                  >
                  > And does the (I assume) increased turbulence counter
                  > these factors?
                  >
                  > Then I think of the fact that heat transfer increases
                  > in some relation to the Delta-T, which I think would
                  > be greater as you increase hp.
                  >
                  > I don't know any answers, it just seems a wild
                  > coincidence for it to be true.
                  >
                  > I mean this in a nice way.. I am interested in the
                  > convoluted and esoteric. And when I was a kid I liked
                  > to poke sticks into ppls spokes.
                  >
                  >
                  >
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                  >
                  >No virus found in this incoming message.
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                  >
                  >
                  Twice the power, twice the waste heat is a first approximation. Several
                  factors: First, the efficiency increases with the peak temperature.
                  This goes up with compression ratio, manifold pressure, and A/F ratio.
                  More efficient combustion means less waste heat. Heat also goes away by
                  different means at higher powers. The waste heat in the exhaust is a
                  big chunk, usually more than half Much of the heat rejection in a nitro
                  fueled funny car is in vaporizing the (alcohol/nitro) fuel. Also
                  consider the heat in the oil. Bottom line is more power = hotter engine.


                  [Non-text portions of this message have been removed]
                • Noel Loveys
                  I look at the infernal combustion engine as simply a big air pump. The more air you can shoe horn through the thing in one unit of time, the more power you
                  Message 8 of 8 , Dec 2, 2006
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                    I look at the infernal combustion engine as simply a big air pump. The more
                    air you can shoe horn through the thing in one unit of time, the more power
                    you can leech from it. Of course there is a scale of diminishing returns or
                    a Swiss watch could power the space shuttle.

                    that is why I would describe the fuel statement as wrong.... It doesn't
                    address increases in frictions or other heat generating parasitics.



                    Noel

                    -----Original Message-----
                    From: Small4-strokeEngines@yahoogroups.com
                    [mailto:Small4-strokeEngines@yahoogroups.com] On Behalf Of Kevin Whisler
                    Sent: Friday, December 01, 2006 8:09 PM
                    To: Small4-strokeEngines@yahoogroups.com
                    Subject: Re: [Small4-strokeEngines] Digest Number 695



                    "> When you burn twice the fuel to get
                    > twice the power, you must reject twice
                    > the waste heat

                    Is this a universally accepted truism? Is this a sort
                    of intuitive thing, or do you have a basis for saying
                    this?"

                    I would say that is an accurate statement. The energy present in the fuel
                    goes two places: (1) mechanical energy and (2) waste heat, and as Howard has
                    stated, most of it goes to waste heat.

                    If you are burning twice the fuel to make twice the power, that tells me
                    that the percentage of energy going to mechanical energy is the same in each
                    case, and therefore so is the percentage of energy going to waste heat. So,
                    twice the fuel burn means twice the waste heat.

                    As far as your several questions about cooling differences, yes those may
                    have some effect and may help offset the situation. But you are still
                    initially facing the need to remove twice the waste heat. How (or whether)
                    this can be accomplished is a whole nuther topic.

                    Kevin

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