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Engine started when I activated cruise control?

Discussion in 'Prime Main Forum (2017-2022)' started by sylvaing, Apr 29, 2024.

  1. sylvaing

    sylvaing Senior Member

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    Might just be a coincidence but today, after merging onto an highway and stabilized my speed at 100 km/h (ie, no longer accelerating), I turned on the cruise control and pushed the stick down to set it to my current speed. At that same time, the engine started. What? As to be a coincidence, right? Why would the engine turn on? The battery was still at 98% as that highway is just one km from my place and it was 9°C outside so really not cold enough to limit the battery power draw. I had enough battery power to do the round trip. Sucks that the engine decided to start.
     
  2. dbstoo

    dbstoo Senior Member

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    There are a couple of threads that talk about the engine running at various times to keep the gas fresh and the engine lubed, etc.

    It happened to me the first time as I entered a drive through car wash. I thought that something was broken at first! :) All was fine when the engine shut down a few minutes later.
     
  3. sylvaing

    sylvaing Senior Member

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    That could just be it. I've had unexplained engine starting in the past. Would make sense, thanks.
     
  4. Trollbait

    Trollbait It's a D&D thing

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    Volt would give a message if start up was for maintenance. Don't know if any past Toyota PHEVs did.
     
  5. sylvaing

    sylvaing Senior Member

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    I didn't see any message on screen.
     
  6. m8547

    m8547 Senior Member

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    As far as I know, no one has ever proven it's that.

    There are lots of reasons the engine starts. Some of them are logical, but the cruise control one is likely a bug.

    Off the top of my head:
    • Using cruise control with >80-90% battery
    • Pressing the front defrost button
    • If the available power from the battery is limited (due to cold weather or low charge), particularly if you demand acceleration that's close to or exceeds the available power
    • Too much regen into the battery, for example
      • Using B mode with a nearly fully charged battery.
      • Extended braking with a cold and/or nearly full battery
      • Driving down a long mountain grade, regardless of SOC.
    • Using the heat below 12F
    • Driving above a certain speed (84mph?) I don't think I've ever driven the Prius that fast.
    You can use the Hybrid Assistant app to watch the DCL and CCL. They are basically the charge and discharge limits the car determines to protect the battery. If either starts dropping rapidly, the engine is likely to start soon if the condition causing the drop persists.

    If the battery temperature is around or below freezing it's extremely difficult to drive in EV mode. If it's plugged in the battery heaters will keep it warm for a few days, but after a few days or if it's unplugged for an extended time the battery will get cold. I think I've seen down into the 20's F after leaving it parked a few days ona ski trip.

    And if the outside temperature is below 12F it's extremely hard to drive in EV mode. My personal best is around 9F with the heat off (seat heater on of course). I had to open the windows to keep the humidity down. Not particularly pleasant.
     
  7. schja01

    schja01 One of very few in Chicagoland

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    The Toyota engineers are just screwing with us.
    This happened to me (again) last week. No message, no LED, no nothing, The engine just came on.
    Personally I stopped loosing sleep over it.
     
    Tightwad77 likes this.
  8. sylvaing

    sylvaing Senior Member

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    Wait, what? That would explain, but doesn't explain why it was never fixed!

    I guess I'll wait until the battery has drained a bit before enabling cruise control now...

    Being in Canada, that few days has been extended to 30 days. The only time the engine ran in winter, beside running out of battery, was when it was below -10°C (14°F) and the climate was on.
     
  9. Doug McC

    Doug McC Senior Member

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    As your owners manual explains, there a multitude of reasons the ICE will run and it lists SOME of them. The vehicle monitors so many parameters and protects itself. One of the very crucial parameters is the hybrid battery temperature and it anticipates what may happen, then takes action to prevent it. The hybrid battery temperature will increase due to current flowing in or out. It could be the system is anticipating a temperature increase (or a multitude of other parameters that are trending in a direction it feels it needs to mitigate) and it just so happens that it is occurring at the time you are engaging the cruise control.
     
  10. m8547

    m8547 Senior Member

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    Another one I have heard of, which I haven't verified, is coming to a full stop in B mode (could cause the engine to start). I always switch back to D before coming to a stop to avoid that situation. I think I have accidentally stopped in B mode once or twice without the engine coming on, but under some conditions it might.

    The cruise control one is surely a bug, which is why it isn't documented. They haven't fixed it because they aren't Tesla. They will only fix things that are a major problem (if there's a recall or enough people complain). It's pretty harmless as far as bugs go, but it also gives me concern about their software quality. This car was developed around the same time as the whole unintended acceleration fiasco.

    Using the Hybrid Assistant app, the temperature, at least reported, doesn't change quickly. That makes sense because it's a big (250 pound?) mass. But the DCL and CCL limits do change, for example too much regen for too long will eventually cause the CCL to drop, and if it drops close to how much regen you're demanding then the engine will start. I assume they have some kind of thermal/electrochemical model that models what's happening at a microscopic scale inside the battery using known parameters like current over time, battery temperature, etc. It's interesting to watch Hybrid Assistant, and what the car is doing makes more sense when you have a little more info. I assume the battery protection algorithm is extremely proprietary, so we will probably never know exactly how it determines the safe limits.

    Finally, I will add an observation that the charge and discharge limits seem to depend on the "freshness" of the charge. If you let the car sit for a few days without charging (even in mild weather), it will be easier to reach a condition where the engine starts next time you drive it. That is a reason to try to charge it as close to when you plan to drive it (in addition to being better for battery longevity and allowing the battery heater to work outside of Canada in the winter).
     
  11. The Big Sleaze

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    I've had a few random ICE starts. Been told if you are in EV mode and smash the throttle, it will sometimes start ICE a few seconds later even after no longer smashing. Maybe it thinks the smash and current hwy speeds means it thinks you will need full ICE+EV power soon. I wouldn't worry, seems semi-common and harmless.