Short Trip Math and Idle from Cold-soak

Discussion in 'Gen 3 Prius Fuel Economy' started by bwilson4web, Jul 29, 2013.

  1. bwilson4web

    bwilson4web BMW i3 and Model 3

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    So this afternoon, I decided to gather some metrics from our 2010 Prius:
    Column 1
    0 [tr][th]mi[th]gal[th]MPG[th]tot mi[th]tot gal[th]tot MPG
    1 [tr][td2]0.0[td2]0.0000[td2]0.0[td2]0.0[td2]0.0000[td2]0.0
    2 [tr][td2]0.0[td2]0.0090[td2]0.0[td2]0.0[td2]0.0090[td2]0.0
    3 [tr][td2]0.7[td2]0.0223[td2]31.3[td2]0.7[td2]0.0313[td2]22.4
    4 [tr][td2]0.6[td2]0.0136[td2]44.1[td2]1.3[td2]0.0449[td2]29.0
    5 [tr][td2]1.3[td2]0.0230[td2]56.5[td2]2.6[td2]0.0679[td2]38.3

    This data is gathered from the following screen shots that any Prius owner can replicate:
    [​IMG]
    [​IMG]
    These two photos show the fuel consumed in an initial, startup that ends when the catalytic converter reaches operating temperature.

    [​IMG]
    This is the second segment at neighborhood speeds, 25 mph, to 60C engine coolant.

    [​IMG]
    Here is the third segment at neighborhood speeds to 70C engine coolant.

    [​IMG]
    This is the fourth segment when the engine coolant reached 85C and began to 'auto-stop.' The car was already in the 40 mph speed range.

    The duration of trip is the sum of the segments. In this case, the car was sitting on the driveway, cold-soak. The engine ran for two minutes and burned 0.009 gallons. Since it did not move, 0 MPG. This is the extreme limit of a short trip. But as the engine warms up, the efficiency significantly increases:
    • 31.3 MPG - miles 0-0.7
    • 44.1 MPG - miles 0.7-1.3
    • 56.5 MPG - miles 1.3-2.6 hb
    The end of trip is when the car sits long enough to lose operating heat:
    • catalytic converter - a couple of minutes
    • engine coolant - a couple of hours
    The short trip penalty is the warm-up cost. So the rule of thumb still works:
    1. order errands with the longest first for maximum warm-up
    For those of us who actually pay for our gas and have a Prius, knowing how short trips and cold-soak works can lead to fuel savings . . . often with little or no impact on our lives. Just efficient scheduling and driving to maximize our return on investment.

    Bob Wilson
     
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  2. SageBrush

    SageBrush Senior Member

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    The conclusions seem sound, but I keep wondering just how much the warm-up penalty actually is. As your pictures demonstrate, the battery icon increases two bars -- presumably during the warm-up.

    If I wildly guess that the battery icon increase of 2 bars is 130 Wh of energy, that will be about 0.01 gallons of fuel. So then the combined MPG at 2.6 miles =
    2.6/0.0579 = 45 MPG.
     
  3. bwilson4web

    bwilson4web BMW i3 and Model 3

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    Good points!

    I wanted to also want to lay out a basic methodology, a way, we can approach getting some hard numbers for Prius warm-up. I've been and remain an advocate of:
    • start car
    • in first 45 seconds accelerate in EV to speed
    • shift into "N"
    But with the warm weather, 81F yesterday, it isn't clear this is as effective as it seemed in colder weather 30-40F.

    Bob Wilson
     
  4. SageBrush

    SageBrush Senior Member

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    I might be a pretty good guinea pig for some data.

    First, I have a SG II so I think I can capture SOC change. Others have presented SOC -> capacity translations that take into account hysteresis.

    Second, I live on a hill that IIRC is about 150 meters. I can choose a route that is first flat for about 0.5 miles and then down, or a route that is about 1.5 miles and then down.

    My neighborhood has no traffic to speak of, and the three stop signs can be gently rolled through.
     
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  5. g4_power

    g4_power Junior Member

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    Hi there. I hope you could answer my question which is related to warming up the car. I'm thinking of getting an engine heater before the arrival of winter. Where am I suppose to put this heater? It would be a pain if I had to pop the hood every morning to attach the heater to the engine.
     
  6. ksstathead

    ksstathead Active Member

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    The EBH is inserted in a slot in the block designed for that purpose. The plug is routed around the engine bay and out the front grill.
     
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  7. fotomoto

    fotomoto Senior Member

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    I'd suggest doing the same thing in January and avg. the two.
     
  8. kgall

    kgall Active Member

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    Bob and all,
    Here's something I've noticed in my car. When looking at mpg by 5 minute chunks (on the MFD), the first 5 minutes is the lowest, the second 5 minutes is higher, and the third 5 minutes gets you up to where you hope a Prius will be.
    But, it seems to me the official "warm up period" as described by you and the various other engineers on PC is a lot shorter than that. I frequently have my ICE turn off when I stop at the top of my drive to drop off trash or pick up mail. That's only about 2 minutes after I turn the Prius on, sometimes less. I've always thought that meant that the 5 stage warm up cycle is over.
    Yet the evidence is that the car keeps putting energy into warming up.
    Am I wrong?
     
  9. SageBrush

    SageBrush Senior Member

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  10. kgall

    kgall Active Member

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    Thanks. I've seen that, and that's what I'm asking about.
     
  11. bwilson4web

    bwilson4web BMW i3 and Model 3

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    You might try setting the histogram display to 1 minute chunks, higher resolution. But what I'm seeing in my data:
    • ~45 seconds (call it first minute) - catalytic converter light-off, engine runs 'open loop' using the stored 'trim' for mixture control. If you are gentle on the throttle, you can accelerate up to ~40 mph on traction battery energy. Any heavier and the engine sucks fuel.
    • ~2-3 minutes - to get the engine coolant warm enough, ~60-70C, to allow engine off or hybrid mode
    • ~20 minutes - to get the transmission fully warmed and reduce viscosity losses
    I captured some more data this morning and will add it to the original to compare and contrast.

    Bob Wilson
     
  12. kgall

    kgall Active Member

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    Thanks.
    When I do the 1-minute histogram, it varies too wildly, probably because I have lots of uphills and downs going to work and traffic lights coming back, though the first two or three minutes are almost always low.
    But you say something that might explain part of it--that it takes 20 minutes to warm up the transmission fully. Any idea how much of a difference a cold tranmission makes?
     
  13. bwilson4web

    bwilson4web BMW i3 and Model 3

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    It is not easy to quantify in our vehicles because of the other variables. However, there is a good paper, "ORNL/TM-2004/247" that shows the effect of temperature on transmission drag. Every BTU of transmission loss is three BTUs of gasoline energy.

    Bob Wilson
     
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