B Mode: A Cautionary Note

Discussion in 'Gen 4 Prius Main Forum' started by HighSpeedTofuDelivery, Apr 10, 2021.

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  1. HighSpeedTofuDelivery

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    B Mode, as every Prius driver should know, is a drive mode for providing braking on long hill descents to avoid excessive use of friction brakes that can lead to brake fade (temporary reduction/failure of the brakes to absorb more heat/perform work to slow the vehicle). This is obviously a scenario some may find themselves in which is why the Prius was engineered with a dynamic braking system very similar in function to a diesel-electric locomotive. Due to the design of the hybrid system, direct, mechanical engine braking is not possible. Regenerated power from MG2 is fed into MG1 which spins the engine to perform work and slow the vehicle so that friction brakes can be used sparingly or not at all, meaning they will remain completely serviceable and reliable to use. Ok, so what?

    Someone I knew who owned a Gen 3 Prius and now has a Gen 4 was under the impression that B Mode, because it provided more braking, provided more regen, and thus, would be the most economical drive mode. He drove his Gen 3 in B mode for its entire life. But there was a problem. The car began burning oil later in life, and not a small amount. His mechanic told him after a thorough inspection of the engine that his cylinders were out-of-round. This is obviously something we don't want to happen. The question is, why does excessive use of B Mode cause this? Well, I believe I have a good theory now.

    It has to do with Toyota's use of the offset cylinder bore design of the engine which increases power efficiency by lowering the friction of the pistons on the power stroke. The trade off is that if force (engine braking) is applied in the opposite direction, the force acting on the piston moving against the cylinder is angled in such a way that it pushes the piston against the cylinder wall. This increases friction, displaces oil film and increase wear leading to oval, out-of-round cylinders and a lot of burnt engine oil. There are all kinds of problems that happen when engines start burning excessive amount of oil. Definitely not something anyone wants to happen both for environmental and economic reasons.

    In an engine without an offset cylinder, this does not happen and engine braking can be done safely all the time without consequence. But for a Prius or other modern engines with offset cylinder bores, frequent, prolonged engine braking braking in B mode will destroy your engine and shorten its service life drastically.

    The bottom line? Only use B Mode when absolutely necessary and switch back to D as soon as it makes sense.
     
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  2. FuelMiser

    FuelMiser Senior Member

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    We are in Colorado and use B Mode quite a bit when descending hills. It would help to know how many miles on the Gen 3 that developed the oil burning. Also, don't Gen 3's have a propensity to burn oil "later in life" anyway, regardless of B mode use?
     
  3. ChapmanF

    ChapmanF Senior Member

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    Well, the first question is, does excessive use of B Mode cause this?

    At this point, we are looking at one report of a "later in life" Gen 3 Prius (specific year and mileage not reported), for which constant use of B mode was one of the conditions it was driven under (others not reported), with later-in-life high oil consumption, which is kind of widely reported for Gen 3 anyway, and out-of-round cylinder wear. That seems a bit thin for pinning the wear and oil consumption on the excessive use of B mode specifically, though probably that didn't help.

    Beyond that, even if this were granted to be persuasive evidence that driving exclusively in B mode for a car's entire life might have a damaging effect, there is a big enough difference between that and anybody's normal use of B mode for descending long hills that nobody really needs to start worrying "did I shift out of B fast enough at the bottom of that hill?" And any claim like "frequent, prolonged engine braking braking in B mode will destroy your engine and shorten its service life drastically" seems way out over the skis at this point.

    Interesting idea about the offset bores being a contributing factor. Yes, that sounds like something that could sensibly be studied (assuming it hasn't been already, which could be dubious). No, having an interesting idea and feeling that it makes sense to you is not yet the same as establishing it.

    It might be worthwhile for you to post a diagram with worked-out force vectors as a check on what you're saying happens to the angles. I'm not saying you're necessarily wrong about that, just that 35 words of informal English aren't going to be quite enough for anybody to check your work.
     
  4. HighSpeedTofuDelivery

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    I did claim that it was a theory only. I figured that would be enough of a disclaimer about my "informal english". ;)

    That said, there could certainly be other contributing factors but the Prius I am referring to had such high oil consumption, it needed to be scrapped and replaced because the cost to fix it was impractical. I am not an engineer so perhaps I am not the best person to expand on this theory further but I do believe I am onto something. It would be interesting to hear from any actual engineers on here to see what they think.
     
  5. HighSpeedTofuDelivery

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    There's burning oil, and then there's burning so much oil that you need a new car. That's what happened to the individual in question. You are using B mode correctly. This is more of an extreme use case where B mode is the default drive mode and highlighting a potential serious consequence for doing so.
     
  6. ChapmanF

    ChapmanF Senior Member

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    Disclaimers are what you need if you're afraid someone is going to accuse you of something, or sue you.

    Nobody's doing that, only pointing out that when you don't show enough of your work for anybody to check it, then nobody can.
     
  7. HighSpeedTofuDelivery

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    I was under the impression this was Prius "Chat", not Prius University. Lol If that's the case, I'm sorry I even bothered to share my thoughts.
     
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  8. ChapmanF

    ChapmanF Senior Member

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    Well, it's a little bit both, if you look around at some of the threads.

    Sometimes a thread gets kicked off with an interesting idea and enough info for the community to dig in and carry the ball further and a lot of stuff ends up getting learned.

    Sometimes it doesn't, so it gets a little bit of "hmm, interesting idea" and then kind of fizzles. Or at the other extreme it turns into one of those viral ideas that everyone "knows" even though the how-would-we-check-that part never happened.
     
  9. meeder

    meeder Active Member

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    Using the word theory suggests that there is ample evidence for the claims made.
     
  10. HighSpeedTofuDelivery

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    Alright, it's a thought, not a theory. Happy?
     
  11. sam spade 2

    sam spade 2 Senior Member

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    And I think your "theory" is a fairy story.

    But I do agree with the last part, only because B mode can slow the car rapidly without any brake lights.
     
  12. Tideland Prius

    Tideland Prius Moderator of the North
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    Why are we being pedantic on the word theory? He’s just sharing his thoughts on what happened to his friend. Would you rather he kept this to himself?
     
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  13. rjparker

    rjparker Senior Member

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    All early gen3s are prone to sticking piston rings that cause uneven cylinder wear and excessive oil consumption. This was acknowledged by Toyota who offered piston and ring replacements under the powertrain warranty. However most don't exhibit the required excessive oil consumption (greater than a quart every 1200 miles) until well past 120,000 miles. These engines are not repairable without a good short block which usually means a used engine will be needed. They also clog up egr coolers and blow head gaskets. Low tension rings for mpg combined with an excessive 10,000 mile oil change interval are usually defined as the root causes.
     
    #13 rjparker, Apr 10, 2021
    Last edited: Apr 10, 2021
  14. Mendel Leisk

    Mendel Leisk Senior Member

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    I beginning to think your super power is hyperbole lol. The other day it was disconnection of the 12 volt “breaking” the cars electronics, resetting the odometer.

    How you get out-of-round cylinders from driving in B mode is a stretch. Yes it’s not the brightest thing to do, but don’t connect dots that ain’t there.
     
  15. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    Superstitious vibe to it from the get-go. I haven't got time to jump into every.... hang on be right back, gotta point the sunvisors down to keep moonlight from drying out the insulation on the HV cables...
     
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  16. ChapmanF

    ChapmanF Senior Member

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    I wasn't trying to start a pile-on, honest ... I was about to try to reassure the OP that the person who asks to see your work, supporting your idea, is not your enemy ....

    Cloudy/rainy here tonight, so I'm not sweating the moonlight.
     
  17. Mendel Leisk

    Mendel Leisk Senior Member

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    Point taken. :oops:
     
  18. alanclarkeau

    alanclarkeau Senior Member

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    It is an interesting topic. I can see that it could well happen.

    But it does point out one thing - the importance of reading the (lengthy) Owner's Manual which TOYOTA provided - good bedtime reading, will put you to sleep quite quickly.

    AND - of a salesperson ensuring that his/her customer KNOWS how to drive it. Maybe neither of those happened - not sure.

    Can someone elaborate. In "D" - the ICE cuts in and out - with the end result that it's "holidaying" for maybe 30-40% of the driving time.

    In "B" Mode - which I've only used 2 or 3 times, and only on a downhill mountain pass - does the ICE spin 100% of the time. Just trying to get my mind around it - can someone help.
     
  19. fuzzy1

    fuzzy1 Senior Member

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    My understanding is that in B-mode, once the ICE starts spinning, it then stays on 100% of the time.

    But at low speed, it is possible to shift into B-mode while the ICE is not spinning, and have it remain off, using regenerative braking only while the battery isn't full. But once the battery fills enough to need the ICE for engine braking, then it turns on and stays on.

    I haven't played enough with this non-plug-in low speed regenerative-only B mode to know its bounds and speed / braking force thresholds. Hopefully others can add more details, or even (gulp) corrections.

    While I do have lots of B-mode experience, almost all of it is at higher speeds where the ICE must spin.
     
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  20. Tideland Prius

    Tideland Prius Moderator of the North
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    I haven’t tried it in the Gen 4 Prius but yeah in the Gen 2 and 3, B mode will force idle the car if the engine is already running when you come to a stop. It feels like the engine is almost stalling.
     
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