RPM as related to 15-18kw output

Discussion in 'Gen 2 Prius Fuel Economy' started by rustystew, May 2, 2011.

  1. ystasino

    ystasino Active Member

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    Will test, are we confident that these scangauge II equations are showing kWs the same way this hypothesis has them calculated?


     
  2. ystasino

    ystasino Active Member

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    And how does that relate to scanguage II kw?
     
  3. SageBrush

    SageBrush Senior Member

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    No idea.
    I suggest using MAF instead if enough sig figures are reported, since the Prius seems to do a fine job of keeping burn stoichiometric.

    If I remember right, petrol has an average of 5 carbon chains,
    So each chain will contain 5 carbons and 12 hydrogen.
    Carbon is oxidized to CO2, and hydrogen to H2O.
    Carbon's atomic mass is 12, and hydrogen's is 1. Oxygen is 16
    A bit of molar arithmetic later ... 72 grams of petrol requires 256 grams of O2 based on
    5 O2s to oxidize five C to five CO2;
    3 O2s to oxidize 12 H to six H2Os

    O2 is about 21% of air, so you just need air density to make your conversion (assuming the MAF is reporting volume/time)

    addendum: A quick google says average chain length is 8, so the combustion will be
    8C + (6*2+6) H
     
  4. BAllanJ

    BAllanJ Active Member

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    You've got to allow for the water vapour that'll come in with the air, don't you? So air is less than 21% oxygen. And I thought the maf would report mass, not volume... isn't that what the m is?
     
  5. 2009Prius

    2009Prius A Wimpy DIYer

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    I don't think terrain matters since we will be doing some averaging over several trips.

    As I posted before the ScanGauge reports the highest kW number among the three (2 CAN messages and ScanGauge). Not sure which one should be used in the calculation.
     
  6. ystasino

    ystasino Active Member

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    Okay this what I figured today

    At 15 kw, iMPG=0.8xMPH
    At 12 kw, iMPG=MPH
    At 8 kw, iMPG=1.5xMPH

    This is at 50-55 MPH and the kW output is calculated using Hobbit's xgauge math.

    For 50 mph speed trips I prefer prolonged acceleration at 15 kW rather than short pulses of 20 kW when possible. That's more of a practical observation than a scientific result. For higher speeds these numbers are shifted upwards. I use WS as much as possible in steep or prolonged downhill stretches.
     
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