Why does ICE come on going downhill at <50 °F?

Discussion in 'Prime Technical Discussion' started by FuzzyFNY, Dec 9, 2018.

  1. FuzzyFNY

    FuzzyFNY New Member

    Joined:
    Dec 9, 2018
    2
    1
    0
    Location:
    Waterford, NY
    Vehicle:
    2017 Prius Prime
    Model:
    Prime Plus
    Why does the engine come on when going downhill with the outside temperature < ~50 °F? I do not have the heater or the seat warmer running; I use breaking mode. During my morning commute after 3 miles on the flat I go down a steep hill which kicks on the engine, then have another ~2 miles on the flat during which the engine stays on (I read elsewhere on this forum that ICE runs for 2-3 minutes after coming on). I want to drive my whole commute in EV so is there a way to stop ICE starting? I have experimented with speed and sometimes at low speed the engine stays off, but not always, even though my speed and the outside temperature are the same as the day before. I drive 40-50 mph on the flat and 20-35 mph down the hill.
    And I think somewhat related, If I turn on cruise control at 50 mph in the cold the ICE comes on.
     
    priuscatprimeguy likes this.
  2. schja01

    schja01 One of very few in Chicagoland

    Joined:
    Apr 22, 2005
    1,067
    718
    0
    Location:
    Chicagoland
    Vehicle:
    2018 Prius
    Model:
    Prime Advanced
    If the battery is near full this is normal. Don’t fully charge the battery before departing on your trip.
     
    priuscatprimeguy likes this.
  3. FuelMiser

    FuelMiser Senior Member

    Joined:
    Jun 29, 2008
    780
    526
    0
    Location:
    Monument, CO
    Vehicle:
    2016 Prius
    Model:
    Touring
    Selecting B on the shift lever engages the engine to slow the car. That's the purpose of that position. It's designed to save the brake pads when descending long grades. If you don't want the engine to come on, leave it in D and use the brake pedal to maintain safe speed. By the way, never select N because that will disengage the entire hybrid drive and you'll be relying entirely on your brake pads to control the car.
     
    priuscatprimeguy and Merkey like this.
  4. fuzzy1

    fuzzy1 Senior Member

    Joined:
    Feb 26, 2009
    13,119
    6,653
    90
    Location:
    Western Washington
    Vehicle:
    2012 Prius
    Model:
    Three
    This sounds more like a regular Prius answer. But OP's Prime is a bit more complex than this, and can use B in some situations where the gasoline engine is not intended to spin up.

    I'd want to know the level of charge on his traction battery, if too full to take the regeneration then it will spin the engine. But I don't yet know the other issues that may come into play, such as ambient temperature.
     
  5. m8547

    m8547 Active Member

    Joined:
    Aug 8, 2018
    596
    350
    0
    Location:
    Colorado
    Vehicle:
    2018 Prius Prime
    Model:
    Prime Plus
    B mode + EV mode will turn on the engine sometimes.
    • If the battery is full
    • If the hill you're going down is very long and at relatively high speeds. There seems to be a limit to how much charge the battery can accept, since regen is the equivalent of super fast charging.
    • If it's cold you will hit the limit of the battery sooner.
    B mode + HV mode will almost always turn on the engine, so you're better off using the brakes for more regen to hopefully get enough charge to get into EV mode. In EV mode + B mode, using the brakes might not turn on the engine if it's a short hill, not too step, and not too fast. But for a longer hill the engine will likely come on because you'll be putting too much charge into the battery too fast with regen braking.

    Cruise control in cold weather will also turn on the engine sometimes, but it's not clear when or why. Once the engine comes on, it will stay on until it's warmed up.

    Some things you could do that might help:
    • Use the charge timer so the battery is warm and freshly charged when you leave in the morning
    • Set the charge timer to finish about 1 hour after you normally leave, so the battery isn't completely full
    A couple anecdotes:
    The road down from Mt. Evans is a winding narrow road with a speed limit of something like 30mph. It descends from 14,000 feet and is relatively gradual. In the summer I was able to get to almost 80% charge (from nearly empty) by the time I got to Idaho Springs, using a combination of B mode and the brakes.

    The road down from Freemont Pass descends steeply to Copper Mountain, with a speed limit of 55 or 65. I was barely able to get any regenerative braking whether I used B mode or the brakes. Temperature was around 40, but the battery was probably toasty from the drive up to the top of the pass.
     
    bisco and fuzzy1 like this.
  6. CraftyCoder

    CraftyCoder Member

    Joined:
    Mar 24, 2018
    48
    64
    0
    Location:
    Washougal, WA
    Vehicle:
    2018 Prius Prime
    Model:
    Prime Premium
    I have seen the ICE turn on with a full battery on a downhill, but this winter I am seeing it come on when going down a steep hill. Normally, I get about 10% charge increase going down the hill, but lately the ICE has been turning on. I suspect that there is a rate of charge constraint in place during cold weather.
     
  7. FuzzyFNY

    FuzzyFNY New Member

    Joined:
    Dec 9, 2018
    2
    1
    0
    Location:
    Waterford, NY
    Vehicle:
    2017 Prius Prime
    Model:
    Prime Plus
    Thanks everyone for the advice.
    This morning I was excited to see the post about not having a full charge when leaving. I thought this could have explained my previous observations of the ICE coming on at the same temperature and speed, then not the next day - there were certainly days when I didn't have a full charge, but I'd not correlated that to ICE coming on. This morning at 30 °F I had 18 miles of charge at the start of the hill and the ICE kicked on - very disappointing! Last week I had a day at about this temperature when the ICE didn't come on. My commute is short enough that I can drive to work and back home on less than a full charge so I will continue to experiment with less than a full charge.

    Previously I tried B and D modes going down the hill and I never noticed a connection between the mode and ICE coming on so I settled on using B mode.
     
  8. m8547

    m8547 Active Member

    Joined:
    Aug 8, 2018
    596
    350
    0
    Location:
    Colorado
    Vehicle:
    2018 Prius Prime
    Model:
    Prime Plus
    Someone above mentioned:
    Maybe you could try driving down the hill in neutral if it's safe. Since the car won't be able to charge the battery, it would seem like there's no reason for it to turn on the engine. Of course it will waste the energy you could recover going downhill, and it will put more wear on the brakes. Also be aware that it might not be legal to drive in neutral.
     
  9. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    85,462
    37,675
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    what is 'breaking mode'?

    is there any way to go a different direction and avoid the steep downhill?
     
    priuscatprimeguy likes this.
  10. priuscatprimeguy

    priuscatprimeguy Senior Member

    Joined:
    Nov 1, 2016
    2,730
    2,625
    0
    Location:
    Farmington Hills, Michigan
    Vehicle:
    2017 Prius Prime
    Model:
    Prime Advanced
    up?:confused:
     
  11. CraftyCoder

    CraftyCoder Member

    Joined:
    Mar 24, 2018
    48
    64
    0
    Location:
    Washougal, WA
    Vehicle:
    2018 Prius Prime
    Model:
    Prime Premium
    I would rather burn a few ounces of gasoline than wear down my brake pads. Gasoline is much easier to replace and cheaper than the pads. Using neutral mode is emotionally satisfying but economically unwise.
     
  12. Roy2001

    Roy2001 Active Member

    Joined:
    Apr 1, 2009
    369
    152
    0
    Location:
    Sacramento, CA
    Vehicle:
    2017 Prius Prime
    Model:
    Prime Premium
    That is part of the reason. With lower temperature, charging is slower. So when I drive downhill, even if the battery is not full, ICE could still be turned on.

    For the same 0.7 mile downhill, I used to get charged by 1.2-1.3 miles, but now with 40F outside, it can only get charged by 0.7 mile. And ICE just kicks in almost every morning.
     
  13. LabDad

    LabDad Junior Member

    Joined:
    Apr 5, 2016
    5
    1
    0
    Location:
    Central NY State
    Vehicle:
    2018 Prius Prime
    Model:
    Prime Plus
    Thanks to all for this interesting thread. My experience is similar to OP, except (1) I do not use B mode going down the hill, (2) when the ICE kicks in the traction battery is only around 75%, and (3) at bottom of hill (with ICE now running), it's somewhere around 80-85%. This is on my morning commute, and it was plugged in to household AC to charge the previous evening with no timer, so fully charged several hours before I start in the AM.

    A few comments from above I don't understand-- a couple of folks suggested starting with battery at less than full charge could prevent the ICE coming on, and there was some related talk about "too much regen" going down the hill. I don't see why any of this is relevant-- in my '08 Prius, if I went down a long hill and the battery came up to 100%, it did not start the ICE (I've always wondered what became of the excess regen in those cases). Why would the Prime be different?
     
  14. fuzzy1

    fuzzy1 Senior Member

    Joined:
    Feb 26, 2009
    13,119
    6,653
    90
    Location:
    Western Washington
    Vehicle:
    2012 Prius
    Model:
    Three
    How much vertical drop were you experiencing on those hills?

    Both my 2010 and 2012 Prii spin up the ICE to screaming banshee levels for engine braking (not gas burning) on our 7%, 2000 vertical foot mountain descents. I don't know why your 2008 wouldn't do the same.

    Note that these are at highway speed, above the auto-off threshold speed, so the ICE must already be spinning ~1000 RPM when first starting downhill.
     
  15. LabDad

    LabDad Junior Member

    Joined:
    Apr 5, 2016
    5
    1
    0
    Location:
    Central NY State
    Vehicle:
    2018 Prius Prime
    Model:
    Prime Plus
    Based on measurements of elevation and distance in Google Earth, the hill I had in mind when referencing the '08 Prius is 138 m (453 ft) drop over 2600 m (1.6 miles), a grade of 5.3%. That's a state highway, so typically going around 60 mph, either coasting or light touch on the brake pedal, no engine braking. The hill I had in mind re my 2018 Prime ICE kicking on is shorter (1026 m), steeper (7.6% grade) and lower-speed (40-45 mph), and again no B mode.

    I have experienced, as you describe, the ICE "wailing like a banshee" in B mode in the '08, which is one reason I rarely use it. It just sounds like that can't really be good for the car (but I acknowledge that impression is uninformed by any deep technical understanding of what's going on). Is it really OK to have the engine sounding like it's protesting so much?
     
  16. m8547

    m8547 Active Member

    Joined:
    Aug 8, 2018
    596
    350
    0
    Location:
    Colorado
    Vehicle:
    2018 Prius Prime
    Model:
    Prime Plus
    Toyotas are really reliable, especially the engines. You have to trust that their cars are designed to be essentially idiot proof. There's not much you can do in normal use that will damage them.

    If the engine is cold, B mode is probably not great for it. But it probably still won't damage anything. The oil is synthetic so it flows at cold temperatures. And the car will warm up the engine if it needs to.

    The engine on the Prime gets so little use for most people that it will probably last forever. For me about 2/3 of my miles are in EV mode, and then in HV mode the engine is probably only on half of the time.
     
  17. fuzzy1

    fuzzy1 Senior Member

    Joined:
    Feb 26, 2009
    13,119
    6,653
    90
    Location:
    Western Washington
    Vehicle:
    2012 Prius
    Model:
    Three
    On a 25 mph hill with a similar drop (400 feet), my Gen3 will sometimes barely fill the battery, sometimes not, in D mode with pure regen braking. On the days it does fill, it barely starts winding up the ICE before the road flattens out at the bottom.

    At 60 mph, you are getting significantly more air drag than I get at 25, so the battery would be filling less. And at least on my Gen3, the battery gauge displays 8 bars before the battery is really filled. I.e. it seems that an internal reading of 7.5 gets rounded up to 8 displayed bars, and regen continues for a short time longer before quitting and winding up the engine.

    So I'm guessing that your battery wasn't quite fillled.
    I'm figuring that as only 256 vertical feet, so less regenerated energy is pushed into the battery.

    For those of us facing hills with a greater risk of crashing from overheated brakes, this loud engine noise is definitely a better alternative. But even for most of our hills, this risk is mostly limited to commercial trucks, not passenger cars. I don't recall anyone here burning their Prius brakes with less 3000 vertical feet.

    On your hills, the brakes will not overheat to dangerous levels, so B is not necessary. And since it would generally prevent the battery from filling, it is even counterproductive compared to leaving the car in D and harvesting more energy.
     
  18. QT Luong

    QT Luong New Member

    Joined:
    Dec 7, 2019
    2
    0
    0
    Location:
    San Jose, CA
    Vehicle:
    2020 Prius Prime
    Model:
    LE
    Can someone explain why having a full battery would force the car to switch from EV to ICE?
     
  19. Prodigyplace

    Prodigyplace Senior Member

    Joined:
    Nov 1, 2016
    10,683
    10,181
    0
    Location:
    Central Virginia
    Vehicle:
    2017 Prius
    Model:
    Two
    The ICE does not actually start but the MG1 motor spins the ICE to use excess electricity from braking to avoid overcharging the battery.
     
    schja01, fuzzy1 and jerrymildred like this.
  20. jerrymildred

    jerrymildred Senior Member

    Joined:
    Oct 28, 2016
    6,965
    8,569
    0
    Location:
    Tampa, FL
    Vehicle:
    2017 Prius Prime
    Model:
    Prime Premium
    Where's it going to put the electrons from the regenerative braking? The battery is full. It has to spin the ICE to soak up the energy..
     
    fuzzy1 and Prodigyplace like this.
Loading...