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. Paradise Tech

    Paradise Tech New Member

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    I'm constantly driving up and down very long and steep hills (back and forth over the Pali Highway).

    I love to use Cruise Control. Just set it and forget it.

    When going down the steep hills, the car will try to hold the speed that I have it set at, and the traction motor starts producing a very loud humming sound. If I have any passengers, they get concerned and they always say something about it.

    But, the hills are too steep and the car starts going faster and faster and the noise gets higher in pitch and sounds more abrasive.

    I have to disable Cruise Control because it just sounds too awful when that starts happening.

    Similarly, if I am not using Cruise Control and I am going down the steep hills, and I switch the car from the normal [ D ] Drive to [ B ] Braking (?) function (by pulling downwards on the joystick when driving), to help slow the car down during the steep decent, it will make the same loud noises.

    Am I correct to be concerned?
     
  2. Pluggo

    Pluggo Active Member

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    There are a couple of things to consider. First, the hill is too steep for cruise control. Second, the sound you describe is the engine revving at a high rate while it tries to hold your speed down by engine-braking. I think you confirmed that when you shifted to B and heard the same sound. It's the sound of your engine spinning very fast. Since you continue to accelerate, it seems you are falling into a dangerous runaway condition. Use your foot brake. Maybe the engine is not technically spinning faster than allowed, but without a tachometer, you cannot know for sure. So again, use your brake pedal to slow down. It would be a very good idea to shift to B at the top of the hill before you pick up too much speed.
     
  3. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    The sound you are describing is engine braking.

    Engine braking is a safety technique- you bleed off energy by using the engine as an air compressor. It can be sustained all day long without harm. It shouldn't concern you in the slightest. It has the benefit of slowing the car without heating up your brakes. This keeps the brakes cold enough to fully stop the car on demand further down the hill. If you were to let your brakes get too hot, they may not have enough stopping power to keep you safe.

    The use of engine braking sounds annoying in cars with small 4-cylinder engines. That's just the way it is. You would still need to use engine braking in a car with a large engine, but the sound would be somewhat lower and less bothersome.
     
  4. ChapmanF

    ChapmanF Senior Member

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    It is a soothing and comforting sound once you know what it is. It is your car doing exactly what it is designed to do to keep your speed controlled on the descent, and the computer in the car knows exactly how fast it is safe to spin the engine, and it will go up to that RPM and no higher.

    As you have noticed, on a hill that is steep enough, you can reach the maximum engine RPM that the computer will use, and the engine sound will level off at that (rather whiny but perfectly safe) pitch, but the car may continue to pick up speed because that maximum engine braking level still works out to be less than your energy gain from gravity.

    In that situation, I'll just wait until the road speed is as far above my preference as I'm comfortable with, and then use one steady firm application of the brakes to snub it down 10 or 20 MPH below my preference, let off the brakes again so they can cool, and rely again on engine braking until the road speed winds up higher than I want again.

    There is one descent on the way to my sister's where I'll end up doing that three times in a row. It's pretty predictable (the hill doesn't change much between trips, and neither does gravity).

    If you're letting cruise control handle the engine braking, you have to re-engage it after using the foot brake to snub the speed.

    If you don't want to do the wind-up/snub, wind-up/snub business, and there are no impatient drivers behind you, you can find a speed for your descent slow enough that the engine braking is sufficient to hold that speed. Depending on the grade, what speed that is may be substantially below the posted limit. But if nobody's behind you honking, it can be a relaxing way to take in some scenery.

    Suggest to your alarmed passengers that they remember the last time they came in for a landing on a jet and the engines reversed thrust to slow them down on the runway. It's not exactly the same mechanism (the jets burn fuel to do that; your engine is just being a dead well-oiled spinny thing that uses up work to spin around), but it's designed-in, intentional, and safe the same way.
     
  5. Paradise Tech

    Paradise Tech New Member

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    Wow. I had no idea. I'm just learning how the whole car works, so that's good to know.

    BUWAHAHA yes! It is super-smart technology, after all.

    Thank you for easing my mind!
     
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  6. jerrymildred

    jerrymildred Senior Member

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    I've been down that hill a couple times (20 years ago, oh my!!). But I still remember it. It's really something!

    @Leadfoot J. McCoalroller and @ChapmanF combined to describe extremely well what is happening and how to deal with it. The Prius is incredibly well thought out and, as a result, sometimes handles stuff in ways that sound quite different than older designs that were more profligate with gasoline use.
     
  7. Mendel Leisk

    Mendel Leisk Senior Member

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    Ah geez Jerry, my head hurts just seeing that. Guess I'll have to go look it up.
     
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  8. sam spade 2

    sam spade 2 Senior Member

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    Kind of sounds like a prop plane engine.
    Tell your passengers you are about to take off !!! :)

    Seriously, perfectly normal.
    And if the "hill" is really steep, it is NOT a good idea to disengage the cruise and use the brakes instead........or let your speed increase a lot.
     
  9. Mendel Leisk

    Mendel Leisk Senior Member

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    I don't see why. I'd be inclined to not use cruise on a steep downhill, one scenario I'd much prefer to be in control of the speed. And if it's a protracted downhill, switch the "gear" to B, especially in the steeper sections. And brake as needed.
     
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  10. ChapmanF

    ChapmanF Senior Member

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    When I'm on a route of many hills (Midland Trail, anyone?), I will typically set the cruise—for 25 MPH, its lowest allowed setting. Then I just drive faster using my foot. It gives a nice approximation of one-pedal driving, because any time I back off the go pedal, the cruise will aggressively harvest speed.

    Another happy discovery is what happens when you use the brake in that scenario. We all know that if cruise is on, touching the brake pedal cancels it. The other thing is that the brake ECU always considers that an "urgent" brake action (no matter how gently you go on the pedal). If you're in cruise and want to brake non-"urgent"ly, just pull the cruise stalk first to cancel it.

    What's special about the brake ECU classifying the use as "urgent" is that it skips right over regen and goes straight to the friction brakes at all four wheels, ready to deploy VSC or ABS as needed with no delay.

    Which is pretty much exactly what I would like it to do, when I go on the brake on a twisty road.
     
  11. jerrymildred

    jerrymildred Senior Member

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    For those who haven't driven this hill, it's a treat. I can't find anything about the grade percentage, but on Google Earth it looks like it drops about 900 feet in about two to 2-1/2 miles with a nice hairpin tossed in for good measure.

    It's absolutely a good place for the "wind up and snub" method described by @ChapmanF.
     
  12. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    To be fair, deceleration fuel cut-off (DFCO) has been a feature of just about every fuel injection system made in the last 4 decades.

    That allows the engine to be spun for engine braking with absolutely zero fuel usage. In many of those cars, the driver would have to select a lower gear in order to take advantage of it.

    Cruise control is another twist- the most modern gas burners will behave exactly like a Prius. If you head down a hill with cruise enabled, the car will automatically downshift to make use of engine braking. Older cars didn't/couldn't do this; the cruise control had no authority over the transmission. I've driven a 2007 Jeep Grand Cherokee with this feature; it's the oldest one I can think of where the cruise is allowed to shift gears but there are probably others out there.
     
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  13. ChapmanF

    ChapmanF Senior Member

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    Prius Gen 1 was that way too. I think its cruise control logic was literally copy-pasted from one of their other cars' ECUs; physically, it lived in the HV ECU, but had its own distinct address on the bus, and just acted like any other old car cruise control, acting on the go-pedal signal but nothing else.

    As a sort of compensation, Gen 1 would let you be in B with cruise on, so you could have sort of the same effect. Gen 3 (and, I think, 2?) only allow cruise in D, but then, the cruise control has B-like engine braking authority.
     
  14. Maarten28

    Maarten28 Active Member

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    I did notice that too when using hybrid assistant. Never knew it was deliberate. Always thought it was a bug.
    Still think it's a bug since using regen would give that little bit extra decelleration force, wouldn't it?
     
  15. ChapmanF

    ChapmanF Senior Member

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    Thing is, regen works through the powertrain, so it only affects the two front wheels. Braking a car using only two of the four wheels can be a little dicey if you have any concern about traction. Plus, when the car is using regen, there is a tiny delay if it then decides it has to use the friction brakes instead (that's the little instant of "whoopee!" feeling that always freaks new owners out if they drive over a manhole cover, etc.). You don't need the regen to give 'extra' deceleration force, because the friction brakes are perfectly capable by themselves of more deceleration force than the tires on the road can sustain.

    So it seems like they just decided if you're ever in a situation where you have cruise on and for some reason decide to brake without taking the time to cancel cruise first, you might really mean it, so they take no chances or delays and go right to the friction brakes, which also means the computer can instantly escalate from there to VSC or ABS if needed.

    The other way they classify a braking as "urgent" is by the pedal stroke sensor; if it sees the pedal being pushed faster than a certain speed, again they skip regen and go straight to the brakes, even without cruise. The interesting thing about the braking-from-cruise rule is that it's always classified as urgent even if you go on the pedal gently. If you don't want it "urgent", you just have to cancel cruise first.
     
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  16. jerrymildred

    jerrymildred Senior Member

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    I see why they do that. Safety first! But apparently there's some kind of reset that will let you turn off the cruise with the brake pedal, then release the brake pedal, and then re-apply it with regeneration. For me, though, it's just easier to cancel it with the stalk since my fingers are only an inch or two away most of the time.
     
  17. sam spade 2

    sam spade 2 Senior Member

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    If you just LEAVE the cruise ON, it will do all of that FOR you.
    Most notably, it automatically switches to B mode.

    Most humans are not qualified judges of how much brake usage is too much.
     
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  18. Mendel Leisk

    Mendel Leisk Senior Member

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    Yeah, but steep/twisty downhills never strike me as "cruising".
     
  19. sam spade 2

    sam spade 2 Senior Member

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    Through 18 posts, you are the first one to use the "twisty" word.
    Maybe you should re-label your "cruise control" with a word that you would find more acceptable. :)

    I don't think that flying down a freeway at 80 MPH while letting the electronics maintain a following distance that is WAY to close qualifies as "cruising" either.......but it seems a lot of people are doing that.
     
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  20. ChapmanF

    ChapmanF Senior Member

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    Rolling along the ground on rubber things never strikes me as "flying".
     
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