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Prius Fuel measurement systems and displays?

Discussion in 'Gen 3 Prius Fuel Economy' started by Robert Holt, Jul 22, 2016.

  1. Robert Holt

    Robert Holt Senior Member

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    Am I correct that the fuel blips and the low fuel warning beep and flashing light are all based on a the position of a float immersed in fuel in the gas tank?
    Am I correct that the instantaneous 0-100 mpg display and the cumulative mpg estimate on the Multi Function Display are based on the metered fuel being injected by the Engine Control Unit on a millisecond by millisecond basis?
    I ask this because when I shift from reformulated E10 fuel to non-rfg E0 fuel, I seem to observe discrepancies with the indicators from these two systems. I suspect either the higher density of the non-rfg E0 is making the fuel float sit higher than it would with E10 and thus shifting the calibration of that system, or that the ECU's adjustment to injecting less of the higher-energy E0 fuel relative to the E10 fuel is changing the calibration of that fuel consumption measuring system. The goal is to accurately calibrate the "Tank To Empty" display of fuel remaining on a Scanguage II, and that calibration seems to shift with E0 versus E10 fuels. Any help is appreciated.
     
    #1 Robert Holt, Jul 22, 2016
  2. Redpoint5

    Redpoint5 Senior Member

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    Your assumptions sound correct to me. I'm not sure exactly how the fuel gauge is implemented, but it is based on volume. I don't think the density of the fuel would vary much between E0 and E10, so I don't think volume measurements would vary significantly. Regardless, I don't think 3rd party gauges can read the fuel gauge. I have to tell Torque when I fill up, as it isn't able to read that info from the ECU.

    E0 contains significantly more energy, so less volume of fuel is needed to produce the same amount of power as E10. The fuel injectors will have a shorter pulse duration with E0 compared to E10. This is how instantaneous fuel consumption is known by the Scangauge and the ECU. It isn't measured, but instead instructed by the ECU.

    I'm not sure why Scangauge and UltraGauge require calibration to accurately display distance to empty and fuel remaining. I would think that just knowing how much fuel is being commanded by the ECU would be enough info to very accurately estimate remaining fuel, but my TSX was off by 10% on my UltraGauge. Perhaps someone more knowledgeable can chime in.

    Perhaps fuel pressure and injector flow rates vary from vehicle to vehicle too much to provide an accurate estimate, and that's why calibration is necessary.

    What percentage values do you use to calibrate the SG for both E0 and E10? I'm curious if the SG is estimating more fuel consumption, or less fuel consumption with E0 compared to E10?
     
    #2 Redpoint5, Aug 1, 2016
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  3. Robert Holt

    Robert Holt Senior Member

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    Thanks for input!
    Consuming fuel to beep and flashing last pip takes me almost 600 miles with E0 summer blend fuel, so I have only driven enough for 3 runs so far. The % correction setting, which I adjust at each refueling is currently at 6.3%. Previously , a 4% correction setting resulted in a slight but noticeable underestimate of fuel consumption and a 11% correction setting resulted in a marked overestimate of fuel consumption .
    First noticed the different calibration for summer blend E10 vs E0 on trip to Alaska where USA Midwest and lower Canadian provinces had cheap E10 and the correction setting had to be 10-11%, and then used the E0 fuel in the Yukon and Alaska, where I ended up resetting the correction to about 6-8% if I recall correctly. Trying to get systematic estimates this summer as getting the very best estimate of the exact "Tank to Empty" gallons is very useful. Using the pure-gas.org website to find E0 stations on this trip, and if I am confident in that estimate I will switch to E10 to find whether that setting is the same or noticeably different, as I suspect it might be.
    Based on Alaska trip experience, when correctly set the Scangauge estimates less fuel consumption for the E0 compared to E10, which estimates then match the actual refueling data.
     
    #3 Robert Holt, Aug 1, 2016
  4. wjtracy

    wjtracy Senior Member

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    Ethanol density is about 0.78 g/cc and RFG E10 is around 0.74 (lower if we remove the 10% ethanol).
    If you had 0.78 g/cc in E0 that would be about 0.78/0.73 or about 7% more energy just in the gasoline portion of the fuel not mention +3% for not having E10, so potentially 10% more miles/tank. But I would consider 0.78 density pretty juicy energy E0 (hard to find? do not know - we don't have any, so I cannot measure it)
     
    #4 wjtracy, Aug 1, 2016
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  5. Robert Holt

    Robert Holt Senior Member

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    I'm using the WAWA station just off I95 in Fredricksburg, which has separate, blue-colored signs on the E0 pumps. It is the WAWA station right next to the shopping center, not the one further out, and hard to find. In upstate New York I used a Stewart Sweet Shop, many of which sell gasoline and a reasonable percentage of those sell E0 fuel. Trying to drive consistently under similar climactic and road conditions to get comparable E0 and E10 data, but so far your estimate of a 7-10% difference in mpg appears reasonable.
     
    #5 Robert Holt, Aug 1, 2016
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