Prime in cold weather

Discussion in 'Prius Prime' started by bwilson4web, Jan 3, 2018.

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  1. bwilson4web

    bwilson4web Well-Known Member Subscriber


    We are investigating cold weather performance of the Prime. Living in Dixie, we only have a couple of weeks to test cold soaking.

    BELOW 50F (10C)

    We found undocumented:
    • cruise control and SOC 90% - setting cruise control when the battery SOC is above 90% causes an undocumented "engine ON" event. The car remains in "EV mode" but the engine starts a 3-4 minute engine warm-up cycle. For a 10 mile commute this can kill mileage even with plenty of energy in the battery.
    • ascending some grades - under investigation, we are using iPhone GPS and six-axis accelerometer to get better grade metrics.
    • descending some grades - also under investigation, not previously documented in the Prime.
    Restart car if any of these "engine ON" events occur, Stop at the next safe place and power cycle the car. When it comes on, the engine will not be running and be in the default "EV mode." Also, it is important to recognize when it occurs:

    Use "," and "." to step back and forward single frames. When the EV car icon changes from solid to an outline, the engine is running. The engine is too quiet to depend on hearing it.

    BELOW 14F (-10C)

    Well documented, the heat pump cabin heater can not provide enough heat. So the engine runs to provide the heat needed. However, keeping the car in "EV mode" allows the car to frequently enter EV mode:
    • 10-14F (-12 to -10C), 20 mile (32 km), loop
    • 80 MPG, 51% EV mode

    We found video recording is critical to documenting the problem. Humans can notice the problem but their descriptions often include 'noise' that has to be filtered out. Video recording is key.

    We tried to use Goggle Earth to get grade information on routes for hill climbs. Unfortunately Google Earth wanders +/- 5m from the graphical route. This leads to amusing profiles where the car apparently does not use the bridge but appears to drive down the bank to cross a creek and up the other side. It is worse on a hill side route where Goggle Earth reports impossible grades, 78%, as it shows the car driving up the hill side. It is significantly worse than GPS wander which have their problems but the error is much less. I also have a six-axis accelerometer, 400 Hz sample rate, that can provide even better grade resolution.


    We don't find this cold weather performance to be a serious problem as much as it is poorly or undocumented including how to minimize the problem. We can still get full battery performance in the 14F to 50F (-10C to 10C) range provided we know the risks and restart the car. On the gas engine, the car still has +600 mile range.

    Bob Wilson
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  3. Pushmi-Pullyu

    Pushmi-Pullyu Well-Known Member

    Bob, did you write that post yourself, or is it a copy-and-paste from elsewhere?

    It seems to be a professional writing job, with a lot more care and organization than is normally seen in the usual internet forum post.

    But regardless of who originally wrote it, thanks for the detailed info!
  4. bwilson4web

    bwilson4web Well-Known Member Subscriber

    A retired engineer, this is what I do for fun. Still if you can find someone who is doing similar work, I would like to contact them.

    My work was originally posted in PriusChat and shared on Facebook, the Prius Prime group, and the alt.prius Goggle group. I try to collaborate with folks in any forum.

    Bob Wilson
    Domenick likes this.
  5. Pushmi-Pullyu

    Pushmi-Pullyu Well-Known Member

    It's quite rare to see someone with expertise in both the fields of engineering and in writing documentation that is so well organized, clearly written, and professionally formatted and styled. Would that every forum had contributors like you!

    Okay, I'm probably embarrassing you, so I'll stop now. :cool:
  6. bwilson4web

    bwilson4web Well-Known Member Subscriber


    Hill testing in 60F (16C) has shown:
    • -10% grade - frequently causes the 'engine ON' to occur. It appears to be more frequent on dynamic cruise control but it also happens with manual control.
    • +10% grade at highway speeds - there is one spot where it is reproducible at will. This happens both on cruise control and manual.
    In either case, coming to a stop and restarting the car puts it back in EV mode. However, the Prime will revert to EV mode with the engine off within ~2 miles. This is where the bimodal BMW i3-REx excels.

    The Toyota Prime is a hybrid with an improved EV mode but ultimately software controlled that tries to protect the battery. In contrast, the BMW i3-REx is an electric car with a manually enabled or low threshold REx. I've driven the same hill route with the BMW and it never approached automatic REx enable levels although I could have manually started it on the last hill climb.

    We also take driving, cross country trips, and have no interest in today's BEV only vehicles. The 65-70 mph 39-40 MPG BMW i3-REx fully meets our cross country requirements. The Prius Prime, backup to the BMW that has had one too many mechanical failures, excels at 70-75 mph 55-58 MPG but has cold weather, EV issues.

    Bob Wilson
    Domenick likes this.
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