Difference in MPG between B and D in normal driving conditions

Discussion in 'Gen 3 Prius Fuel Economy' started by PriQ, Jun 28, 2016.

  1. PriQ

    PriQ CT+iQ

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    A large auto magazine (Motor by FDM in Denmark) has reviewed the Auris Hybrid (Gen III Prius drive train) while remarking that driving in 'B' gives better regen. This auto magazine also does a fuel economy evaluation and noted engine noise as a negative point for the car.

    Now. With the vast knowledge in here. Does anyone know how much fuel economy is affected during normal city/highway driving when you drive in 'B' rather than 'D'? And as a bonus. How much more will the engine run due to the missing regen?

    Unfortunately the writers are immune to facts, so pointing out that they were wasting fuel and ran the engine more than necessary, was met with a reply of "'B' does indeed increase regen". Now I want to try again by pointing out how much fuel they wasted. Unfortunately my own commute is too dependent on traffic for me to be able to test for myself (my CT200h has the same drive train). Can the community help with this information?
     
  2. booke02

    booke02 Active Member

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    Bollocks

    "B" actually gives less regen as it uses engine for braking. Perhaps they mistook the sound of engine braking for regen???

    And, yes, it would be noisy driving in "B" all the time.
     
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  3. tpenny67

    tpenny67 Active Member

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    "B" does not give more regen, however in "D" you need to press the brake pedal to get a significant amount of regen, while in "B" mode it happens when you take your foot off the gas, but some energy is wasted spinning the engine.

    In normal driving I don't notice a huge difference in MPG between the two. I'm most likely to use "B" in slightly hilly terrain when following traffic in the 40-50 mph range, as it prevents the engine from toggling on/off each time the car goes above/below 46 mph, and you don't need to hit the brakes constantly when the traffic slows a tad downhill. Coincidentally, this is the same type of traffic where I usually lock out overdrive on an automatic.

    Note that the engine will run more in "B". Driving in "B" at slower speeds will have an increasing impact on MPG, as the energy required to keep the engine spinning will become larger relative to the energy required to move the car. If you come to a complete stop in "B", the engine will still be turning and burning gas if I recall correctly from my scangauge, which is a complete waste of fuel.

    In practice, the MPG difference is probably only significant in slow stop and go traffic or if you're trying to hypermile. If you're driving like an automotive journalist, "B" mode approximates driving a manual where you'd leave it in a lower gear during aggressive driving.
     
  4. bwilson4web

    bwilson4web BMW i3 and Model 3

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    One of the Prius tricks is to turn off the engine at every opportunity. But in "B" mode, this is defeated so the engine constantly burns gas.

    The "B" mode has use when descending tall hills as this reduces heating of the batteries. It also allows descents with 'feet on the floor' on steep grades over 8%. Otherwise, avoid it.

    GOOD LUCK!
    Bob Wilson
     
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  5. Redpoint5

    Redpoint5 Senior Member

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    I haven't heard of anyone who has tested the difference in fuel economy with driving in B mode vs D. Even if they did, the data would be applicable only to their test route and conditions, along with their driving style.

    You can only present the facts to the authors, which were explained in above posts about the engine always running, and the fact that B mode does not allow any greater regen ability when compared to feathering the brakes in D mode.

    I have only used B mode on 2 occasions, and that was after nearly completely charging my PiP 4.4 kWh battery from zero range due to sustained descents from Yosemite and Crater Lake. The car kept kicking out of regen and would start the engine, so I used B mode to reduce my need to use friction brakes.

    Anybody that is getting better MPG from driving in B mode is driving like an ape, or simply mistaken about it being more efficient.
     
  6. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    Any number of automotive 'journalists' confused Braking for Battery as the meaning of B.

    That said there is a tiny window where B can be useful, between about 24 MPH when it can absorb energy without starting the engine as an air pump, and about 7 MPH when it is rotating too slowly to get any regenerative Braking at all. I do not drive in that range as I avoid school zones for safety reasons.
     
  7. Mendel Leisk

    Mendel Leisk Sunday driver DIY’r

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    The Owner's Manual's explanation of B is very spotty, really doesn't do it justice.

    Unless you're encountering a LONG downhill run, long enough that there's a danger of the battery becoming as fully charged as the car will allow, and the car will fall back to using friction brakes only, and you still have a long downhill stretch: there's no reason to use B, it's use is detrimental to fuel economy, and battery charging.

    In day-to-day driving, anything that doesn't involve descending a mountain, B is not needed.
     
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  8. CR94

    CR94 Senior Member

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    I generally agree, but would add this: Using "B" might be wise if you must deccelerate from cruising speed down a medium-sized hill to a stop sign or red light when the battery is already warm. In that situation, the value of energy that could recovered by staying in "D" might be less than the cost of the added wear on the battery.
     
  9. qdllc

    qdllc Senior Member

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    Are you sure on this? The engine uses compression braking, and that shouldn't require any fuel flow (which can be totally shut off by the ECU).

    I know with cruise control, B engages as needed on downhill runs when the battery reaches max charge. If you didn't have cruise control on, I would expect that if the battery maxes out, the regen braking would stop and you'd just keep coasting faster and faster unless you hit the brakes or go into B mode.
     
  10. Lucifer

    Lucifer Senior Member

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    The gen2 owners manual warns not to use the "B" for extended periods as it will heat up and cause damage, does't say what will heat up but does say damage,
     
  11. Redpoint5

    Redpoint5 Senior Member

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    Quite right about fuel consumption while using B mode. The injectors should be shut off, and the pistons move air around, bleeding off energy. B-mode does have a practical function, but it's a very rare occasion where it's advantageous, especially in a PiP with a larger battery.

    Speaking of the PiP, I have yet to see the cruise put the car in B mode, presumably because there has always been some room in the battery to regen. I do see the car kick in and out of regen mode on long descents, a phenomenon that I don't have an explanation for yet, but never has it gone into B mode.
     
  12. CR94

    CR94 Senior Member

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    Even if fuel is completely shut off, "engine braking" is braking, and needless braking is not the secret to decent fuel consumption.
     
  13. qdllc

    qdllc Senior Member

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    B exists as an option to using physical brakes to maintain or reduce speed...especially on downhill grades. To bring up fuel consumption and braking as if the two have a harmonious relationship is fallacious. You don't use B to save gas. You use B as a means of slowing down. It's only worse than regen braking when regen braking is still available, but once your battery is maxed, there's no place to send the power you recover.
     
  14. macman408

    macman408 Electron Guidance Counselor

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    The fuel is minimal, but not zero, when using B for a long time; I've noticed fuel consumption on the MFD around 90 mpg sometimes in B mode when I really wouldn't expect to be using any fuel. I'd assume that fuel flow is usually cut off, but if you ever need to coast a bit without the drag, I believe it will start using fuel. Personally, I have gotten better fuel economy on long downhills by switching between B and D than by leaving it in B and pressing the gas to reduce the drag. I'd *hope* that the car is smart enough to leave the fuel cut off if you're truly coasting, but I wouldn't be certain of it.

    Maybe the PiP is different, but I've seen this said about the 3G hatchback too, and I think there are some subtle distinctions to be made. The car never actually switches mode on you (i.e. the display shows B); however, if the battery fills, and you coast with your foot off the gas, it can rev up the engine to provide the missing drag that would normally be provided by siphoning power off into the battery. This is *distinct* from B mode, because if you shift to B while this is happening, the engine will rev even higher, because B has a higher rate of drag.

    Worth noting is that this is separate from cruise control; notably, because cruise doesn't work in B mode (I assume it will kick you out if you try to switch to B while cruising). However, when in D (whether in cruise or not), the engine will rev to provide the same amount of "simulated" engine drag that you get when the battery is not full.

    So I guess it's a simulated simulated engine drag - normally, the car simulates engine drag by putting power into the battery. When that's not possible, it simulates the simulated engine drag with... engine drag! A bit higher revs than normal though, to make up for the fact that the Prius is a bit more efficient than your average sedan, and coasts really nicely.
     
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