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First five minutes are the worst!

Discussion in 'Gen 2 Prius Technical Discussion' started by john_dough, Sep 15, 2008.

  1. john_dough

    john_dough New Member

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    As others have noted--the MPG displayed on the MFD for the first five minutes is much less than subsequent five minute intervals. I figured it was because the engine is cold during this period (even though I live in Florida). So I did a test---ran the car for twenty minutes at highway speeds--pulled over into a rest area--shut the car down and immediately restarted and got back onto the highway (took about 2 minutes to do this). Got the usual lousy MPG for the first five minute bar. I'm now thinking that no matter what the engine temp is, a five minute timer keeps the ICE running during this time period. Any thoughts?
     
    #1 john_dough, Sep 15, 2008
  2. qbee42

    qbee42 My other car is a boat

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    A cold engine makes it worse, but startup does put the system through a starting cycle. Some drivers keep their Prius running for short stops, just to avoid this problem.

    Tom
     
    #2 qbee42, Sep 15, 2008
  3. archae86

    archae86 Member

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    I think:

    1. cold engine is definitely harmfull-we all see that

    2. "No" on the 5 minute timer penalizing you by running the ICE even when warm

    3. Anyway, if the land was nearly flat, on the freeway your ICE would be running non-stop regardless.

    Consider another possibility: Just accelerating from full stop to full freeway speed uses substantial fuel--enough to pull down the average for the 5-minute segment containing it to appreciably below other segments.

    If you did a similar test, but on a much lower speed road, the excess acceleration fuel would be much less compared to 5 minutes of cruise fuel (even though the cruise mpg would be better). If I'm right, you should see a smaller percentage effect. Of course, maybe you don't have rural roads allowing a steady 25 mph nearby.
     
    #3 archae86, Sep 15, 2008
  4. Neicy

    Neicy Member

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    #4 Neicy, Sep 15, 2008
  5. 9G-man

    9G-man Senior Member

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    There is no doubt that the most efficient modes of operation are inhibited when the engine is cold.
    Your test when the car was warm didn't allow you to take advantage of those modes by pulling off of, and returning onto the highway.

    Get an engine block heater if you want to mitigate the inefficiencys of the first 5 min.
     
    #5 9G-man, Sep 15, 2008
  6. usbseawolf2000

    usbseawolf2000 HSD PhD

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    The first 5 mins includes "get up to speed" penalty. Therefore, you should really average up with the last 5 mins (rolling off the highway ramp with ICE off) before you shut down.
     
    #6 usbseawolf2000, Sep 15, 2008
  7. TonyPSchaefer

    TonyPSchaefer Your Friendly Moderator
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    You need to listen to this guy; he knows what he's talking about and summed up exactly what was witnessed in one sentence, citing the experience and solutions of other drivers in the second. A very accurate and succinct post.
     
    #7 TonyPSchaefer, Sep 16, 2008
  8. qbee42

    qbee42 My other car is a boat

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    Wow, Tony, how much do I owe you for this glowing recommendation. Your coffee must have been extra good today. :)

    Tom
     
    #8 qbee42, Sep 16, 2008
  9. Sheepdog

    Sheepdog C'Mere Sheepie!

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    heh heh. He aint saying yer omipotent Tom, Just that you nailed it on this issue!

    I have to admit that both of you are Prius potentates to me!
     
    #9 Sheepdog, Sep 18, 2008
  10. qbee42

    qbee42 My other car is a boat

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    I get lucky once in a while. Even a blind squirrel gets a nut...

    Tom
     
    #10 qbee42, Sep 18, 2008
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