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Expansion of gasoline at higher temperatures

Discussion in 'Gen 2 Prius Main Forum' started by DougSlug, Jun 19, 2007.

  1. DougSlug

    DougSlug Member

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    I read an article today about the fact that gasoline expands at higher temperatures (no surprise there), resulting in less energy per dispensed gallon, and that gasoline pumps aren't designed to compensate for this. The overall effect is that we are paying more per "standard" gallon when it's hot, so make sure you fill up when it's cooler (e.g., at night) to minimize the effect.

    OK, this has always been going on, but my question is: how will this little fact of physics affect the Prius closed fuel system and mileage calculations (manual and computer), and is there a correction factor that could be used for better accuracy during the summer months?

    - Doug
     
    #1 DougSlug, Jun 19, 2007
  2. Marlin

    Marlin New Member

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    One gallon of gas will expand from 231 cubic inches at 60 degrees to 235 cubic inches at 90 degrees. That's a difference of 1.7%.

    At 50 MPG, that would be a difference of 0.85 MPG.

    I wouldn't worry about it if I were you. It's just part of the noise in your calculations.
     
    #2 Marlin, Jun 19, 2007
  3. qbee42

    qbee42 My other car is a boat

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    <div class='quotetop'>QUOTE(DougSlug @ Jun 19 2007, 08:27 AM) [snapback]464570[/snapback]</div>
    There is not much short term variation in fuel temperature at the pump. Fuel is stored under ground in large tanks, which buffers it against temperature change. Maybe if we buy our fuel only in the winter. :)

    Tom
     
    #3 qbee42, Jun 19, 2007
  4. priusincc

    priusincc Member

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    I read the same article, and had the same questions. On the other hand, the gas tank bladder can expand more with the warmer temps, so that may be a partial compensation. (You can get more gas in the tank.) :blink:

    The gas expansion thing has always been around, we may be more aware now because we are tracking mpg closer with the Prii. Just another of many factors which effects mileage.
     
    #4 priusincc, Jun 19, 2007
  5. David Beale

    David Beale Senior Member

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    The fuel bladder will allow the fuel to expand with no problem. If you fill to the top of the filler tube, then replace the fuel cap (and actually have a full tank and expanded fuel bladder), then as the just pumped cool fuel expands it can force its' way into the vapour recovery system, and possibly past that and into the space between the bladder and steel tank. This will eventually result in a destroyed tank and large repair cost. Search this site for pictures and further details.

    Do not overfill.
     
    #5 David Beale, Jun 19, 2007
  6. DougSlug

    DougSlug Member

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    <div class='quotetop'>QUOTE(Marlin @ Jun 19 2007, 08:39 AM) [snapback]464576[/snapback]</div>
    Thanks. I was hoping someone would do the calculation. I didn't have time (or, frankly, the desire) to look up the relevant info. I agree that the difference is essentially noise. It seems like the effects of temperature on the bladder would have a greater influence on the accuracy of the manual MPG calculations than the effects on gasoline volume would.

    I was also wondering if the Prius computer factored in the temperature when calculating MPG, but this may be a difficult question to answer.

    Also, point taken from Tom about the short term variations in temperature of the underground storage tank. That may be the most important factor leading us to the conclusion that this temperature effect is negligible.

    - Doug
     
    #6 DougSlug, Jun 19, 2007
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