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Why larger MPG range when engine spins?

Discussion in 'Gen 3 Prius Technical Discussion' started by wfolta, Jul 20, 2009.

  1. wfolta

    wfolta Active Member

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    I accidentally posted this in the General forum instead of the Technical Discussion forum...

    I've been watching the instantaneous MPG meter to the right of the speedometer and noticed that there is a big difference between speeds less than ~45, and speeds greater, when the ICE is warmed up:

    * At lower speeds, I either get 10-35 MPG (depending on acceleration) or 100 MPG (coasting/gliding), with seemingly nothing in between.

    * At higher speeds, I can get any MPG indicated. For example, I can cruise at an indicated 70 MPG for quite some time on the level or a slight downhill, maintaining 60-65 MPH.

    The ~45 MPH cutoff leads me to suspect that it has something to do with the ICE spinning: at lower speeds, the engine is off (100 MPG) or on (35 MPG or less), while at higher speeds it is always spinning but may not always be firing.

    Has anyone else noticed the same thing, and does anyone have an explanation?

    I could imagine that at higher speeds, I might get 1 second of engine burn followed by 1 second of no burn (but still rotating), followed by 1 second of burn, etc, which would yield about 70 MPG if the ICE's base MPG under current conditions was 35 MPG. But does the 2010 Prius turn gas flow off and on that quickly?

    I'm puzzled...
     
    #1 wfolta, Jul 20, 2009
  2. qbee42

    qbee42 My other car is a boat

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    At higher speeds the engine can run at any needed power level. At lower speeds you reach a limit to the lowest efficient output of the engine, so the control system tends to run it for awhile, then run off electricity, and repeat as necessary.

    Tom
     
    #2 qbee42, Jul 20, 2009
  3. HTMLSpinnr

    HTMLSpinnr Super Moderator
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    The ICE must spin above 46mph to prevent over-spinning the generator (MG1). It can spin with modified intake valve timing that makes it "near" freewheeling.

    As for gas usage, it may depend on how steady your foot is, and whether the ECU determines that MG1 (via ICE) needs to charge the battery, provide motive power to MG2 via MG1, or drive the wheels directly.
     
    #3 HTMLSpinnr, Jul 21, 2009
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  4. FireEngineer

    FireEngineer Active Member

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    wfolta, you really need to get a ScanGage. The Prius dash will not give you enough accurate feed back to let you know what is happening. With the ScanGage you can see RPM's and IGN timing, this will solve your puzzle as to why your MPG's seem lower at lower speeds.

    Wayne
     
    #4 FireEngineer, Jul 21, 2009
  5. MSantos

    MSantos EcoAccelerometry

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    +1 :thumb:
     
    #5 MSantos, Jul 21, 2009
  6. wfolta

    wfolta Active Member

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    Between the iPhone, Prius and a new laptop I need to get, I'll have spent both of my gadget budgets ($$ and wife-goodwill) for quite some time. :rolleyes:

    Not sure that I really want to cross the line into wires-hanging-down car tech at this point, either.
     
    #6 wfolta, Jul 21, 2009
  7. bwilson4web

    bwilson4web BMW i3 and Model 3

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    Below the threshold speed, 46 mph, the warmed-up, hybrid runs in one of these modes:

    • engine running - excess power charging battery, typically lower instantaneous MPG
    • engine off - using excess power from battery, typically 100+ MPG
    The vehicle cycles between these modes depending upon speed with longer engine off cycles at lower speeds. Use the instantaneous MPG as a rough indicator of the mode (there are special cases.)

    A better approach to understanding Prius mileage is to simply use a trip meter to record the mileage at a given, constant speed for a non-trivial distance, 10 miles is a good minimum. Then run the tests in both directions over the same route to equalize altitude changes and minimize wind effects.

    GOOD LUCK!
    Bob Wilson
     
    #7 bwilson4web, Jul 21, 2009
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  8. wfolta

    wfolta Active Member

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    Good info, thanks!

    The way I phrased my question may have also been confusing. I'm not doubting actual, long-term MPG at lower and higher speeds. I'm just interested in how very different the car is run at the two speed ranges, and was wondering how the engine was working at the higher speeds. I was just using the instantaneous MPG gauge as a poor-man's indicator of when the ICE was running and when it was not. (At low speeds it's easy to tell because it's somewhat binary, as you note.)
     
    #8 wfolta, Jul 21, 2009
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