Cruise Control While Going Down Steep Hills Produces Terrifying Noise

Discussion in 'Gen 3 Prius Technical Discussion' started by Paradise Tech, Feb 15, 2020.

  1. fuzzy1

    fuzzy1 Senior Member

    Joined:
    Feb 26, 2009
    13,312
    6,864
    90
    Location:
    Western Washington
    Vehicle:
    2012 Prius
    Model:
    Three
    If the hill is really steep, then cruise control is simply the wrong setting. Use B mode instead, plus the brakes.

    Braking from cruise control cancels cruise and its engine braking, putting the car in regular D mode.

    Braking from B mode keeps B mode and its engine braking engaged, both while braking, and after releasing the brake pedal.
    I've driven only around on the coastal route, not over this top.

    But 900 feet in 2.5 miles? That is an average of just under 7% grade, i.e. 'normal' on many of my travels. And the total elevation change is small, I commonly do ~2000 vertical foot grades, and elsewhere have experienced far more, and not just on Pikes Peak.

    Elsewhere I'm seeing 1168 maximum elevation, and a 3.5 mile descent on the north side. But the online maps are being finicky right now, not showing me how long the main south side descent is.
     
    #21 fuzzy1, Feb 17, 2020
    Last edited: Feb 17, 2020
  2. sam spade 2

    sam spade 2 Senior Member

    Joined:
    Mar 31, 2018
    2,747
    1,233
    0
    Location:
    USA
    Vehicle:
    2017 Prius c
    Model:
    Four
    Have you ever tried it ?
    In my experience, the cruise control did quite a good job all by itself.......since it automatically uses "B mode".
    But I usually start down big grades at about 10 below the posted limit too.
     
  3. ChapmanF

    ChapmanF Senior Member

    Joined:
    Mar 30, 2008
    8,501
    6,136
    0
    Location:
    Indiana, USA
    Vehicle:
    2010 Prius
    Model:
    IV
    Cruise control will automatically apply engine braking in some circumstances. (So will just plain using the brake pedal in D now and then, if the battery is near full.)

    "B mode" involves more than just using the engine as an energy dump; it's a package of settings including a lower regen rate to the battery overall, and an increased setpoint for the "drag" feeling when foot is lifted from go pedal.

    It's probably better to just say that cruise can use the engine for dumping energy, similarly to one aspect of B mode.
     
  4. fuzzy1

    fuzzy1 Senior Member

    Joined:
    Feb 26, 2009
    13,312
    6,864
    90
    Location:
    Western Washington
    Vehicle:
    2012 Prius
    Model:
    Three
    Yes, many times. Touching the brake pedal disengages cruise control, leaving the car rolling in D once the brake pedal is released. Cruise control cannot be used with real B selected.

    My regular route to help dad has a hill that is steeper than B mode can hold down to a safe speed for the whole distance. Starting at 10 mph below speed limit (50 mph, posted 60), B can almost hold the speed almost half way down, rising just a few mph, to the inflection point where the grade steepens a bit. From there the speed slowly goes into runaway, requiring three or four brake applications to scrub down to 50 each time, for the remaining thousand vertical feet.

    I've been on numerous hills elsewhere on which starting at even half the speed limit, B mode gradually fails to hold the speed down to a safe level for the particular combination of slope and twisty turns. Occasional manual braking applications are required, though far less than hazardous "riding the brake".
     
    #24 fuzzy1, Feb 17, 2020
    Last edited: Feb 17, 2020
  5. jerrymildred

    jerrymildred Senior Member

    Joined:
    Oct 28, 2016
    7,190
    8,848
    0
    Location:
    Tampa, FL
    Vehicle:
    2017 Prius Prime
    Model:
    Prime Premium
    I'm going from some pretty old memories aided by Google Earth, but I think some places were well over 7% and some a little under.

    BTW, I wonder if @Paradise Tech has gotten bored with us yet. :LOL:
     
    fuzzy1 likes this.
  6. dig4dirt

    dig4dirt MoonGlow

    Joined:
    Apr 6, 2012
    819
    490
    0
    Location:
    Lancaster Co PA
    Vehicle:
    2014 Prius c
    Model:
    Two
    Yeah you guys are too funny!

    I got some chuckles out of this one
     
  7. sam spade 2

    sam spade 2 Senior Member

    Joined:
    Mar 31, 2018
    2,747
    1,233
    0
    Location:
    USA
    Vehicle:
    2017 Prius c
    Model:
    Four
    OK, so once again the explanation is more complicated than the problem is.

    If and when you decide that the drag applied by the cruise is not enough, you apply a semi-firm brake AND switch it to B before removing your foot from the brake.

    Easy.

    Pretty much the same that you would do with a conventional car only there you downshift a gear or two.
     
  8. DimitrisTS

    DimitrisTS Junior Member

    Joined:
    Jan 24, 2018
    18
    38
    0
    Location:
    Thessaloniki
    Vehicle:
    2010 Prius
    Model:
    III
    Is there any change that use of engine braking will cause some engine damage?

    I mean what if the engine is not yet at operational temperature or the temp has just dropped because of little usage.

    Revving up to 5000+ rpm with water/oil temperature lower than optimal wouldn't cause some extra wear?
     
  9. jerrymildred

    jerrymildred Senior Member

    Joined:
    Oct 28, 2016
    7,190
    8,848
    0
    Location:
    Tampa, FL
    Vehicle:
    2017 Prius Prime
    Model:
    Prime Premium
    Nope. In that situation, the "engine" is simply an air compressor. And modern multi viscosity oils work at just about all temperatures.
     
  10. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

    Joined:
    May 12, 2018
    3,200
    3,067
    1
    Location:
    Pennsylvania
    Vehicle:
    2018 Prius c
    Model:
    Two
    You are correct in that there is no free lunch, the operation you describe will cause some wear. But have a sense of proportion. You are not trying to harness 77hp worth of exploding gasoline during this procedure. However, you are trying to do exactly that every time you do a sustained acceleration up to freeway speed.

    You are also correct in that it's always better to warm up an engine before asking a lot of it. Cold engine braking is likely to produce more wear than hot. But again, it's far less stressful than hard acceleration when cold, and you're sometimes doing that already.

    The bottom line is that the wear from engine braking is trivial, not even worth thinking about. It's mostly an annoying noise that could save your life if used correctly.
     
    DimitrisTS likes this.
  11. Grit

    Grit Senior Member

    Joined:
    Aug 22, 2017
    3,242
    2,647
    1
    Location:
    NorCal (Non Bay Area)
    Vehicle:
    2012 Prius
    Model:
    Four
    People do that every car in general. Prius car temps will drop with ICE off when not very hot climate. But they’ll smash the pedal to get up to speed, car doesn’t have thermometer so poor prius has no chance to even warm up before acceleration rage begin
     
    #31 Grit, Feb 25, 2020
    Last edited: Feb 25, 2020
    DimitrisTS likes this.
  12. sam spade 2

    sam spade 2 Senior Member

    Joined:
    Mar 31, 2018
    2,747
    1,233
    0
    Location:
    USA
    Vehicle:
    2017 Prius c
    Model:
    Four
    WHAT ? REALLY ??
    First time I've heard that.
     
  13. Grit

    Grit Senior Member

    Joined:
    Aug 22, 2017
    3,242
    2,647
    1
    Location:
    NorCal (Non Bay Area)
    Vehicle:
    2012 Prius
    Model:
    Four
    Yikes, thermometer is what I meant.
     
    RMB and Leadfoot J. McCoalroller like this.
  14. fuzzy1

    fuzzy1 Senior Member

    Joined:
    Feb 26, 2009
    13,312
    6,864
    90
    Location:
    Western Washington
    Vehicle:
    2012 Prius
    Model:
    Three
    Mine appears to stay warm, or continue warming up, during this engine compression braking. Whether it is from the discarded gravitational energy, or a bit of intentional fuel burned purely for warmup, I don't know.
    I've never noticed mine go above about 4600 RPM in this mode, even when compression braking has reached its limit and vehicle speed starts climbing in a slow run-a-way. The ECUs are not allowing the ICE to overspeed.
     
    #34 fuzzy1, Feb 25, 2020
    Last edited: Feb 25, 2020
    DimitrisTS and jerrymildred like this.
  15. sam spade 2

    sam spade 2 Senior Member

    Joined:
    Mar 31, 2018
    2,747
    1,233
    0
    Location:
    USA
    Vehicle:
    2017 Prius c
    Model:
    Four
    I don't quite understand that either, in the sentence where it was used.
    Are you trying to say "temperature gauge" ?
    In vehicles WITH a temp gauge, the engine actually gets quite warm before the needle on the gauge comes up off of the peg......usually.
     
Loading...