Why premium is wasted in a Prius

Discussion in 'Gen 3 Prius Fuel Economy' started by bwilson4web, Nov 30, 2013.

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  1. wjtracy

    wjtracy Senior Member

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    This web report gives historic EPA correlations for gasoline energy content vs. density and MPG vs. density. It's old (from when EPA first started measuring MPG in 1978) but indicates density is a correction factor up to 7% MPG.
    See pages 31-33.

    Document Display | NSCEP | US EPA

    The implication to me is, whereas I assume EPA RFG is lower density (need to check), we are talking up to 10% MPG penalty for E10 RFG: 3% for ethanol and maybe 7% for gaso density compared to a non-RFG gasoline sample in a non-regulated area. So up to 5 MPG debit for those of us on E10 RFG, in other words, up to 10% less MPG than EPA MPG test which uses non-RFG with no ethanol. (RFG = EPA Reformulated Gasoline)
     
    #21 wjtracy, Dec 1, 2013
  2. bwilson4web

    bwilson4web BMW i3 and Model 3

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    Thank you!
    I was not aware of this paper and that energy content is proportional to density makes sense. So I'm at a loss to explain why I did not see a similar relationship with my measurements.

    Now one thing I saw with an Exxon sample were the butane bubbles form on the hydrometer, a gasoline version, during one of the measurements. So I shook the sample until bubble stopped forming. So now I'm wondering if I should of put each sample under a vacuum to remove the butane. Sad to say that was pre-E10 days in this area. Today I would have to remove the butane and the ethanol and that is more involved than I have time and resources to pursue.

    So I'm back to using the car, OBD data, to come up with the energy content of the fuel. In effect, a 'live' BSFC measurement that reads out what the engine is producing per unit of fuel regardless of the mix.

    Once again, thanks for the paper, I had not seen it before but suspected there was a density-energy relationship.

    LATE NOTE:

    The paper goes on to discuss heptane, octane 0, and iso-octane, octane 100. Heptane has lower density and lower heat of combustion yet it is the lower octane hydrocarbon. My field study showed a different result with some lower octane gasolines out performing higher octane fuels. Looks like I may be repeating my study even with the exact ethanol ratio unknown.

    Bob Wilson
     
    #22 bwilson4web, Dec 2, 2013
  3. wjtracy

    wjtracy Senior Member

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    • The butane bubbles might be explained by hydrometer being warmer than the sample. One approach would be refrigerate sample and hydrometer prior to measuring density. Of course we would have to correct for temp.
    • I would say remove ethanol impact by math (if we know the amount of ethanol)
    • Low density straight chain paraffins (heptane and so on) are near zero octane and near 100 cetane (diesel). High density aromatics are >100 octane and near zero cetane. This means that higher density makes a good gasoline and lower density makes a good diesel. Premium gasoline and high cetane diesel could conceivably have the same energy content, in other words, what is happening in a refinery is they have to increase the energy content of raw gasoline (to make the octane they take the hydrogen out of the gasoline literally makes H2) and then they have to reduce the energy content of raw diesel (to make higher cetane they inject the H2 back into diesel) so basically the refining process is working to equalize energy content of gasoline and diesel (just because that's what the physics of gasoline and diesel engines require).
    • Can we get O2% in the exhaust from the SCANgauge?
     
    #23 wjtracy, Dec 2, 2013
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  4. bwilson4web

    bwilson4web BMW i3 and Model 3

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    So this is what has me scratching my head:
    Column 1 Column 2
    0 [tr][th]fuel[th]Gasoline Gallon Equ[th]BTU/Gal[th]density kg/l
    1 [tr][td]regular unleaded[td2]1.00[td2]114 100[td2]0.71-0.77
    2 [tr][td]diesel #2[td2]0.88[td2]129 500[td2]~0.832
    Source: Gasoline gallon equivalent - Wikipedia, the free encyclopedia

    Diesel is denser so the energy content per gallon should be higher. I'm OK with that. It is the regular vs premium where I'd like to see some metrics showing premium is denser.

    FYI, the O{2} sensors are binary, ON/OFF. However, we do the mass airflow in grams/second. This is what I used in the past for fuel consumption when I did not have injector timing and engine rpm.

    Bob Wilson
     
    #24 bwilson4web, Dec 2, 2013
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  5. wjtracy

    wjtracy Senior Member

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    Thanks re: O2 sensor read-out

    There is no guarantee that Premium is more dense. It would be very possible to formulate a low density Premium using 100% isooctane for example 0.69 density at 100 octane. However, in general, one would probably expect Premium to be more dense than Regular. But I do not know for sure the trends today with all the EPA regs etc.
     
    #25 wjtracy, Dec 2, 2013
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