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Re: [toyota-prius] Mileage Report

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  • David Kelly
    ... Burn time is not important. Peak pressure is important. Study the basics of the Carnot cycle to understand the efficiency of a heat engine depends on the
    Message 1 of 35 , Jul 4, 2013
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      On Jul 3, 2013, at 11:01 PM, Chris Wells <radioactive55man@...> wrote:

      > If you can't burn the gas as long due to the low octane rating then the
      > efficiency of pulling the energy out of the gas goes down.
      >
      > The low octane knock causes the engine control module to reduce the spark
      > advance and reduce the burn time.

      "Burn time" is not important. Peak pressure is important. Study the basics of the Carnot cycle to understand the efficiency of a heat engine depends on the difference between the high pressure and the low pressure points in the cycle. The greater the difference the more work is extracted. Higher pressures result in greater efficiencies. The longer the air/fuel mixture burns the higher the pressure and often one can have higher pressures at TDC by advancing the spark timing.

      But thats the theory. Its worth knowing to have avenues to explore. In practice promising theories don't always work especially when things are as complex as with a Prius. Remember the Prius has a 13:1 mechanical compression ratio to start with when 10.5:1 is the max usually found in conventional engines. Prius controls combustion pressures not only with spark advance but with valve timing limiting the charge in the combustion chamber to control peak pressures.

      Higher combustion pressures produce greater amounts of NOx, a regulated emission. This problem is so great in diesel engines that a urea solution (DEF) is injected in the exhaust which combines in a selective catalytic reactor (SCR) to break NOx into N2 and H2O.

      > That all translates to an effect on mpg which I believe is worth looking into.
      >
      > Of course there are many variables here that the group is aptly spinning and
      > sharing.

      Of course its worth looking in to but its not so readily explainable. Most do not get better MPG from higher octane fuels because modern engines are already getting what they can from the fuel. But I do find one station's fuel often produces better MPG than another's so its quite possible the different octane ratings at a station will produce different MPGs. I don't think the difference is explainable in octane but if 89 from a particular station gets better MPG then by all means take advantage of it.

      --
      David Kelly N4HHE, dkelly@...
      ============================================================
      Whom computers would destroy, they must first drive mad.
    • Walter Lee
      Driving conditions make a big difference in fuel efficency - especially with the Prius and especially if you know some hypermiling tricks. Sofar in over three
      Message 35 of 35 , Jul 11, 2013
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        Driving conditions make a big difference in fuel efficency - especially with the Prius and especially if you know some hypermiling tricks.
        Sofar in over three years of driving my overall fuel efficiency calculated has been from 44 to 71 mpg depending on the overall driving conditions.
        My (Scangauge2)  fuel efficiency estimates per trip has vary even wider from about 11 mpg to 132  mpg depending on the driving conditions.
        The Prius' HSI display  fuel efficiency display estimates more optimistic .

        [Non-text portions of this message have been removed]
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