Windy road, do frequent switches between B and D hurt anything?

Discussion in 'Gen 4 Prius Main Forum' started by mudworm, Jul 16, 2018.

  1. mudworm

    mudworm Member

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    Every trip of mine pretty much involves 13 miles of windy road in the mountains with first half going up and second half going down (going either direction). There are usually no other cars around me so I can drive however I like. Currently, I'm playing with B position quite a bit when I go down hill. Not that B eliminates my braking, but when I shift to B, I can feel the braking action kick in. So far, I've been shifting in and out of B (from D) quite often to make use of its braking action and quick charging. I could make such shift a dozen times on one down hill session.

    Do such frequent shifts hurt anything?
     
  2. The Professor

    The Professor Senior Member

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    B mode doesn't charge the battery quicker than D.

    In short, B mode converts kinetic energy, which otherwise might have been used to charge the battery in D mode, to heat. It charges your battery less.

    In D mode, when you brake gently, kinetic energy is converted into electricity which is then used to charge the battery. However, if you're going downhill on average for an extended period, there will come a point where regenerative braking will cease to be effective due to the battery being full. You can't store any more energy in it. If the charging circuit didn't step in to protect the battery, the battery would start to produce lots of heat at this point, and would eventually overheat. So, instead, the car stops using regenerative braking, and switches to use the traditional friction brakes for all braking. When you brake now the friction brakes are doing all of the work (work = heating up and wearing down your brakes). This is more work/wear than in a traditional car, because in a traditional car all braking is usually shared between the friction brakes and engine braking.

    This is where B mode comes in. What it does is pretty much turn your Hybrid into something much closer to a traditional system. Instead of all braking being achieved by sending energy to the battery, or converting it to heat via the friction of the brakes, B mode instead sends some of that energy to the combustion engine where it's converted to heat via adiabatic heating within the engine, just like it would in a traditional car.

    To answer you question though, switching to or from B mode often won't hurt anything. My rule of thumb is that if the battery is showing as either full, or nearly full (with only 1 bar or 2 empty bars), and I am going to be going downhill a lot from that point, it's probably worth switching to B mode.
     
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  3. bisco

    bisco cookie crumbler

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    most people don't bother with d unless they are going down a mountain.
     
  4. mudworm

    mudworm Member

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    Thanks for sharing.

    I thought B mode was more like down shifting (like in my regular vehicle), so it would save the brake pads. That said, with my previous car, I never downshifted on my commute because the down is never very steep for long. But mostly, I thought the B mode would quickly charge the battery (more than just using my foot braking). Maybe I misunderstood this video I watched the other day.


    Will study more...
     
  5. The Professor

    The Professor Senior Member

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    You mean B, right?
     
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  6. alanclarkeau

    alanclarkeau Senior Member

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    Do you mean "B"?
     
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  7. The Professor

    The Professor Senior Member

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    Yeah that's pretty close to how it works...

    He says around the 4 minute mark pretty much what I said above. B mode uses engine braking to save your brake pads.

    When you downshift in a traditional car the engine is forced to turn faster. This means more energy is converted to heat and hence you get more engine braking. Energy from the wheels goes back through the gearbox and clutch and into the engine.

    You can't downshift in a vehicle that doesn't have gears to select though. Nor does energy normally flow from the wheels back into the engine in a Pruis, as it's instead directed into a generator to charge the battery. B mode does 2 things - it allows energy to flow from the wheels into the engine (i.e. instead of the generator), and it changes the CVT position (this is basically downshifting) to make the engine run faster relative to the road speed.
     
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  8. bisco

    bisco cookie crumbler

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  9. The Professor

    The Professor Senior Member

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    I feel N and R are a little unloved at this point...
     
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  10. Starship16

    Starship16 Senior Member

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    I'd like to see a test & video like that, but at freeway speeds. I was on a long steady downhill of several miles, at freeway speed of 65. Very early morning, no traffic. I was enjoying the scenery, and didn't even notice and soon I was doing close to 90. I switched into B Mode, but it didn't slow me down. I had to ride the brakes several times down that hill.
     
    #10 Starship16, Jul 16, 2018
    Last edited: Jul 16, 2018
  11. sam spade 2

    sam spade 2 Senior Member

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    Who knows. Probably not but why take the chance ?

    It accomplishes pretty much exactly nothing useful to screw around with it like that.

    Put it in B and leave it there. Use gas and brakes as needed.
    OR
    Set your cruise for the desired speed and let the car do it all for you.
     
  12. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    [The Plug in Prius have different rules and limits, ignore all I say if you have a PHEV]

    He seems surprised that the engine is running, but that is the point of B mode, engine braking. You can't be in EV mode and use engine braking.

    If your downhill has 600 feet of vertical drop, you can fill the battery, a ski hill should drop over 600 feet.

    Even in B, you will charge the HV Battery, adding engine braking does not disable regen. It just reduces it.

    (I will say, the engine makes a fair amount of noise in B mode, even though no gas is being used.)
     
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  13. Mendel Leisk

    Mendel Leisk Senior Member

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    Yeah I would reserve B for long downhill declines, say coming down a ski mountain, or a mountain pass, that sort of thing. Sustained and long downhills, where the the hybrid battery is going to for sure be full to the gills and you've still got a long ways down to go.

    For around town, regular hills, just use the brakes.
     
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  14. mudworm

    mudworm Member

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    Got it! I think I fully understand it now. Now I know my down hill does not call for B mode.
     
    #14 mudworm, Jul 17, 2018
    Last edited: Jul 17, 2018
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  15. alanclarkeau

    alanclarkeau Senior Member

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    They have their places ... but not going down hills.

    Though - I did know a chap who did worse than "N" on a work '70s Holden (GM Australia). He'd had one before and we gave him a new model "Ute"** and he turned the engine off like he had any times before going down the range - said it saved fuel. Trouble was, the new "Ute"** had a new-fangled ignition switch which locked the steering too - before they put an interlock to prevent that happening. The tree he hit didn't do too much damage as he almost stopped before he hit it, but he never did that again.

    ** This is what an Australian UTE looks like - 2nd photo is a PANEL VAN - we had a fleet with both.
    upload_2018-7-17_19-0-33.png upload_2018-7-17_19-1-25.png
     
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  16. alanclarkeau

    alanclarkeau Senior Member

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    I suspect that Gen 4 moves into something like B Mode in normal travelling on occasions. I've had it on undulating roads - up and down hills where it regenerates going down, then the ICE comes on almost immediately going up again - and that might be recharging the battery too. Eventually, on this particular drive, I get to some gentle driving at 50 km/hr, the battery is showing full, but it won't go into EV Mode wen I get close to stopped, and going down hills, I can hear the engine sounding like it's spinning in B Mode. It's happened a few times for me like that.
     
  17. The Professor

    The Professor Senior Member

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    I don't really understand why there's a manual B mode at all. The Prius knows the battery SOC, the angle of the vehicle, the power demand, speed, accelerator position, etc. It should be able to apply B mode automatically, and only when needed.

    Pixel 2 ?
     
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  18. WilDavis

    WilDavis Senior Member

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    Both hands on the wheel, and cut the cack musak, fer Christ's sake!
     
  19. The Electric Me

    The Electric Me Go Speed Go!

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    What makes you believe the Prius is designed on any level to "know" what angle it is at?
    It's an amazing machine, and an amazing driving system, but I don't think it is designed to evaluate the gravitational forces of it possibly being on a incline.

    It doesn't even implement "Hill Keep Assist" automatically. It counts on "users" to know they are on a hill and implement if needed.

    Unless future incarnations of Prius are engineered to automatically execute a type of "B" mode, I think Prius drivers are stuck with the "B" on the selector switch

    And from time to time Prius Chat is destined to get the inevitable "What is B mode?, Am I using it right? " questions and debate.

    Since I agree that the vehicle does know Battery SOC, Power Demand, Accelerator Position, etc, I think the fact that "B" IS a user selectable mode, demonstrates that Toyota has designed the Prius to allow the user to evaluate the hill or incline before us, and let the driver make the decision.

    Also, I speculate IF Toyota somehow made "B" mode engine braking automatic? We'd almost immediately get a percentage of owners asking how to disable this feature, because they would rather fully coast down the hill or mountain. Whether it's good for them or not.
     
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  20. The Professor

    The Professor Senior Member

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    It has a tilt sensor for the alarm, it has 3-dimensional accelerometers in all of the air bags, it also knows because one of the criteria of the ICE turning off under certain circumstances is that the vehicle must not be stationary on a "slope".

    I hear what you're saying, but bear in mind when Dynamic Cruise Control is active it will use engine braking when needed, so it actually already does this to some extent.

    Pixel 2 ?
     
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