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Prius Hates Being Below Sea Level ! (Extreme Prius Driving Part 2)

Discussion in 'Gen 2 Prius Main Forum' started by dianeinreno, Aug 10, 2012.

  1. dianeinreno

    dianeinreno Member

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    No I did not run my 07 Prius off the Santa Monica Pier. But I did take a long drive through the California Salton Sea area with sea level elevations (as reported by GPS) from -100 to -200 feet BELOW sea level. Average outside temperature was about 109.

    The MPG went from my normal 52 down to 38 (driving at 65 mph) and stayed there for about a hundred miles or so until the sea level elevation got about 200 feet above sea level when it returned to about 48 mph.
     
  2. Jason dinAlt

    Jason dinAlt Member

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    Probably the temperature, don't you think?
     
  3. xs650

    xs650 Senior Member

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    Yup, the miniscule change in atmospheric pressure from being 200 feet below sea level make almost no difference.
     
  4. dianeinreno

    dianeinreno Member

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    All I can tell you is that the temp remained in the 106-109 range all the time - so the temp was constant . What changed was the altitude. I went down to a constant 35-38 mpg for the 100 -150 miles I was at 100 below sea level and when I got above 200 feet above sea level the mpg went back to about 45-48. I'm just reporting what I saw on the gauges a week ago.
     
  5. ETC(SS)

    ETC(SS) The OTHER One Percenter.....

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    I can't speak for Priuses....but during my 10 patrols, I've spent about two years of my life below sea level. I wouldn't take a million dollars for the experience, but I wouldn't give a penny to get to do it again!!!!

    My efficiency dropped off somewhat as well, but mostly I think it was the lack of sleep. I'm thinking that your G2 was less happy about the 109-degree temperature than it was about the barometric pressure.
     
  6. rcf@eventide.com

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    I'm skeptical of the data. The air is a tiny bit denser at 200 ft below sea level, just as it is a bit less dense at 200 ft above, yet there is no mysterious increase in MPG when driving on a hill or plateau. The temperature is routinely 100 to 120 in the Southwest (e.g., Phoenix,) and you don't see such MPG variations there. And all of Florida is essentially at sea level, where small calibration errors in the Prius sensors probably make some ECUs believe they are 100-200 ft below sea level with no reported discrepancies of this magnitude.

    In other words, there is neither a physical nor software-bug explanation for a discontinuous MPG curve slightly below sea level. That leaves the usual fuel bladder and human error (sorry, OP) possibilities.

    (I'm not dogmatic - I'd be curious to hear a logical explanation that excludes these minor altitude and temperature variations.)

    Richard
     
  7. xs650

    xs650 Senior Member

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    Correlation [​IMG] Causation
     
  8. RobH

    RobH Senior Member

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    One of the sensors the Prius uses is for absolute atmospheric pressure. It's entirely possible that some computation breaks when that sensor goes outside the design range.
     
  9. dianeinreno

    dianeinreno Member

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    I'm thinking along those lines myself i.e. a software bug of some sort. The only variation was in altitude - the gas remained the same as did the temperature and speed of the vehicle (I was using cruise control) . For over an hour the mpg went down to the 38 range but rose 10 mpg back to to 48 when I got above 200 feet above sea level. I've been driving the vehicle since March and have put about 6K miles on it so far. This is the first time I've noticed such a pronounced mileage anomaly.
     
  10. uart

    uart Senior Member

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    The problem with that theory is that a 200 ft change in elevation only changes barometric pressure by about 6 mmHg. Normal weather patterns (high and low pressure systems) change the barometric pressure by far more than this (at least 5 to 10 times more).
     
  11. uart

    uart Senior Member

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    The change in air density for a 200 ft change in elevation is about 0.8%

    At the speed that the OP is traveling we could guesstimate about 80% of the external are losses due to air resistance, so we would expect a loss of about 0.8*0.008*52 which equals about 0.3 MPG.
     
  12. ETC(SS)

    ETC(SS) The OTHER One Percenter.....

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    ^ Besides....unlike the Redneck Riviera, Nevada is a geographically diverse area.
    If there's a fuel efficiency delta when the vehicle goes from "sea level"...however you define this, to "sea level plus several hundred feet" why no corresponding delta? Why do you only see this shift when you go below "sea level?"
    Let's stipulate for a moment that the code slingers made a mathematical boo-boo when dealing with "sea level" in the Rube Goldberg formula that they probably used for barometric pressure compensation.
    What "sea level" are we talking about????
    North America Left coast???
    Tokyo Bay??? They're different, and you would think that IF the folks in Aichi slung the code, and IF they used "sea level" as some kind of magic number, they'd use MSL closer to where they live.
    Now...I'm not a cartographer, and I don't play one on TV, but I would presume that if "sea" level were a factor in the barometric pressure readings for the motor, then they'd probably use some "reference ellipsoid sea level" value, which would mean that you'd never REALLY know if you were above MSL or below without a GPS input, which would be absent in cars (like mine) that are not thusly equipped, since you'd have to know where you are in (on) the Earth.
    The Earth isn't "round." It's an oblated spheroid according to all of the inertial navigation schools that I slept through.

    So.
    I'm thinking that the decrease was terrain, temperature, HVAC related or perhaps throttle input....or fuel.....or wind....or lots and lots of bugs...or....hey waitaminute!!!!
    I GOT IT!!!

    I think I just noodled this whole thing out!!!
    Two Words....

    Area 51.

    :)
     
    jabecker and GrumpyCabbie like this.
  13. roflwaffle

    roflwaffle Member

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    Probably a headwind.
     
  14. Patrick Wong

    Patrick Wong DIY Enthusiast

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    That seems to be the most likely, certainly more so than the minor altitude differences.

    I have noticed that California-formula fuel costs ~20% more, and seems to result in around 15%-20% worsened fuel economy compared to the fuel available in Arizona. Hence I fill up before entering CA and minimize fuel purchases while there.

    If the OP was using fuel from different sources while making these mpg observations, perhaps a portion of the mpg changes might be attributable to the fuel.
     
  15. roflwaffle

    roflwaffle Member

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    I've noticed something similar within CA, and it depends on the time of the year as well as where someone fills up. It's mostly due to smog regs IIRC. For example the gas I buy that's outside of the SCAQMD seems to have slightly more energy per gallon than the SCAQMD stuff, and summer blend (more energy per gallon) is used longer. I've seen a good 5-10% difference between summer blend gas in the boonies and winter blend gas I purchased in Orange County.
     
  16. dianeinreno

    dianeinreno Member

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    Just to answer a few questions:

    1. If there was any headwind there was no noticeable buffeting that you always experience when it is windy outside. It certainly didn't appear to be windy outside in the least.

    2. Regarding CA gas - I wondered about that at first but using the same tank with no fill up the mpg changed during the 100 miles I watched it. It did not recover to the 48 mpg range until I went over 200 feet above sea level (according to the gps). While the mpgs were bad the terrain ranged from -100 to -200 below sea level and was very flat - not hilly.
     
  17. fuzzy1

    fuzzy1 Senior Member

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    Actually there is an MPG increase on plateaus, and it is very noticeable in non-hybrids at multi-thousand foot elevations (Montana, Wyoming, Colorado, etc.). But the change is less pronounced in the Prius, and should be very difficult to detect in any car at a mere 200 feet.

    Absent multiple observations, I'll also be skeptical. Air density alone should not cause anywhere near that much difference, so there must be some other unnoticed factor (e.g. wind, road surface, slope) or something unexpected out of left field (e.g. software/firmware bug) involved.
     
  18. R-P

    R-P Active Member

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    Aaahhh, so that's why 50mpg are unattainable for me... (I live at sealevel and do all my driving within 50feet below and above sealevel.

    ;)
     
  19. ursle

    ursle Gas miser

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    So, this April on my NH to S Fl drive in a new to me 11' my mileage which at 60-63 was 62 all the way to just below Jacksonville where it immediately dropped to 42-43 and stayed there for the 8 days I was in state, well, it came up some because of 30mph average around town speeds but after getting north of Fl it went right back to 62, I'd assumed the Traction Battery had been besotten with heat but have to admit that a lot of the trip is below sea level on 95 in Fl, I find this info interesting as I didn't like the discrepancy .
     
  20. roflwaffle

    roflwaffle Member

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    What were your route/day/time? You don't need to be going into a strong headwind to incur significant decreases in mileage. A 15mph breeze/headwind in a car at 60mph will increase the power needed to overcome aerodynamic drag by 56%.