B Mode Questions

Discussion in 'Gen 3 Prius Main Forum' started by mtngal, Dec 20, 2011.

  1. mtngal

    mtngal New Member

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    Is there a maximum speed for using B Mode? I do know that it's used only for engine braking.

    I've read here the couple of threads that talk about how it works and based on my very limited experience, I wondered if there's a speed where you shouldn't use it. Also, isn't there some sort of regulator for the battery that would shut off regenerative charging when it was full?

    The reason I ask is that I live in the mountains and spend my life either going up or down hill, some pretty steep. I once put the car in B mode to go down the main road in my subdivision, to see if I could cut down on using the brakes (it did that very nicely). So then I decided to see if it keep the car from reaching full battery charge before I reached the freeway (it did). The only thing is that I forgot it was in B by the time I reached the freeway, and proceeded to drive southbound down the I-5 Grapevine. For the most part I didn't notice anything other than I wasn't getting as good gas mileage, until I got to the steepest, long section from Templin Hwy to Castaic (for those who know Southern California). Its very easy to coast this whole section at 70 in regular mode, using the brakes a little when the regeneration isn't enough drag to keep the car from speeding up. However, about half-way down the hill the engine suddenly started racing as fast as it could, sounded scary to me - especially since I had long since forgotten about it being in B mode - until I pulled off and realized what I had done and then put it in D mode.

    The reason why I'm asking about how safe it is to use B mode at high speeds is that when I drive northbound on the Grapevine, going to Bakersfield, the battery is completely charged before I hit the valley floor. While I've read here that the car is supposed to automatically use the B mode, it seems to me that as soon as the battery is full, I lose the drag I normally feel with the regeneration, and have to start using the brakes more. I admit that it could be because the road at that point is steeper than the section that fills up the battery, but the timing just seems more connected to the state of the battery than it does to road conditions. I'm thinking that this might be a place where I'd like to use some engine braking rather than keep using the actual brakes, but after my one experience with B mode at 70 mph, I've been afraid to try.

    Does anyone know if it would be a bad idea to try B mode at high speeds? If so, am I safe in assuming that there's a regulator on the battery and the car won't keep trying to charge it when it's full, so there's no harm to the battery by continuing downhill, using my brakes to keep the speed down? The cost of the extra wear on the brakes isn't much compared to replacing the battery or the engine, so I don't consider this a big matter if all it does is wear down the brakes sooner. This car has such good aerodynamics that it picks up more speed going downhill than any other car I've ever owned, and it's taking me some time to get used to that.

    By the way, the other time I thought having B mode was useful was going to work when the roads were snowy/potentially icy. I'm still using the original tires and haven't bought chains yet (it wasn't bad enough for chains anyway), so having the ability to use engine braking instead of depending on brakes to slow down was nice. Though I suppose with all of the safety/traction control devices it's not as much an issue, is it?
     
  2. David Beale

    David Beale Senior Member

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    There is no maximum speed for "B" mode.

    Mechanically (electronically) what "B" mode does is use the engine with no fuel injected to help slow the vehicle. It becomes an air compressor, exhausting the compressed air out the tailpipe. It also moves the regeneration up a notch or two - makes it more powerful.

    When the battery gets "full" (it isn't actually full, just as full as the HSD wants it to be), the car stops regenerating and will do several things.
    First, if you are still in "B" mode and going downhill the engine braking will still be used.
    Second, the HSD system will try to use up the battery power. If you level out and apply the accelerator pedal the HSD will use electric power whenever possible, trying to drop the charge level back down to about the 60% range ("full" or indication of full is actually about 80% charge). It is limited by battery current and battery temperature, but you get the idea.

    There are a few things you should keep in mind when using "B" mode for braking, or even just regenerative braking.
    First, for a "what the heck?" moment, when going downhill in "B" if you press the accelerator pedal the engine will -drop- RPM. Very different than what you would expect.
    Second, and very important in slippery conditions, is "B" and regenerative braking is only on the -front- wheels. It's -possible- this could destabilize the car (it might want to swap ends). So be careful when it's slippery. I haven't heard any reports of this and I suspect VSC will activate to prevent this (apply rear brakes for you), but something to keep in mind.
     
  3. dogfriend

    dogfriend Human - Animal Hybrid

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    That is exactly what B mode is for. You do lose regeneration when the battery is full (actually not full, but over 80% SOC). B mode helps by using engine braking to supplement friction braking. Engine braking reduces the need to use friction braking and lessens the danger of overheating the friction brakes.

    By the way, there is no reason you can't use B mode before the battery gets "full"; this will give you more control over your speed on the downhill sections. The only trade off is that you will be throwing some energy away and so your mileage will theoretically be lower. Safety vs. mpg ; safety should come first IMO.

    There is no maximum speed limit for B mode.
     
  4. fuzzy1

    fuzzy1 Senior Member

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    ... only if the grade is short enough to not fill the battery in B mode too. If it also fills in B mode, then MPG should be a wash.
     
  5. mtngal

    mtngal New Member

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    Interesting about the B mode only being the front wheels, guess I'll just trust to VSC and get chains for really bad days (we're supposed to have them, but I just haven't gotten around to buying them yet).

    Glad to know that there's no maximum speed for B mode, I might try it again next time I'm headed to Bakersfield. Also glad to know that I was right in my assumption that the car will protect the batteries from being over-charged.

    Given my accidental experience with B mode at 70 mph, I'm not surprised that pressing the accelerator petal would drop the rpm - that tracks with my experience. As long as I had my foot lightly on the accelerator (gliding) or actively using the accelerator, I didn't notice a difference. However, when I got too much speed up (going above 70 mph) and took my foot off the accelerator to glide a bit, then the engine started really to rev and that's what worried me.
     
  6. David Beale

    David Beale Senior Member

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    No need to worry. The car will protect both the battery and the engine. It will not over-rev. The GII will not go over 5000 RPM and the GIII is higher.

    The HSD will not allow you to discharge the battery too much as well as not charge it too much. The reason is a bit complex, but suffice to say it's to ensure long life.
     
  7. tumbleweed

    tumbleweed Senior Member

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    Actually you don't need to select B mode to use it. If your battery gets full (aprox 80%) the car will go into B mode instead of continuing to regenerate when you push the brake pedal.

    If you forget and leave it in B mode you will usually notice because the cruise control won't work.
     
  8. fuzzy1

    fuzzy1 Senior Member

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    No it doesn't. While D mode's synthetic drag will also spin up the ICE faster when the battery gets full, the resulting ICE RPM and braking drag force are well below what B mode produces in the same situation.
     
  9. mtngal

    mtngal New Member

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    Based on my one experience with leaving the B on, the RPM is definitely much higher with B than D with the battery full (which I've experienced several times). In fact, I don't notice much additional RPM when the battery is full, just less drag than when it's recharging.

    I really haven't used the cruise very much - it seemed to me that I can get better gas mileage on my own. That's different than my last two cars, which got better gas mileage with the cruise control. I should probably try cruise going to work (downhill) and see if it can do better, along with not allowing the car to gain too much speed on the steeper parts.
     
  10. qbee42

    qbee42 My other car is a boat

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    This is a matter of degrees. Pressing on the brake will produce more braking in either mode, although B mode is more aggressive with regeneration.

    The main purpose of B mode is to forestall friction braking. If you know that regeneration will max out on a descent, switch to B mode early to dissipate excess energy. Early dissipation will save on friction braking.

    Tom
     
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  11. tumbleweed

    tumbleweed Senior Member

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    I agree Tom, I have also noticed that using the brake pedal for regeneration in D can cause more current to flow to the battery than B mode without brakes applied. Using the brake pedal in D from a fairly high speed, with the battery not at full SOC, can generate over 100 Amps. My ScanGauge observations indicate the B mode will not generate that much, more in the neighborhood of 80 Amps as I recall, of course it could be higher with a very low SOC.

    RPM in the B mode without applying the brakes seems dependent on speed. I have seen over 5000 RPM at higher speeds on a long steep hill using the B mode without the brakes. When using the brakes in D, with a full SOC, RPM seems dependent on both speed and pedal position.
     
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  12. Old Bald Guy

    Old Bald Guy Old Bald Guy

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    What the "B" mode is, is a Jake brake ... like what semi trucks use. When a semi uses its engine brake, it shuts off the fuel and turns the diesel into a big air compressor, thus slowing the truck.

    The Prius does the same thing. B (engine brake) cuts off the fuel and ignition ... turning the engine into an air compressor. IF you do not touch the GAS pedal, the computer will let the gas engine spin up against compression, making for a very effective way to slow the car.

    The engine WILL wind up to what sounds like high RPM ... and hold it there ... to slow the car. If you slow down too much and need to go faster and push the gas pedal, the computer will bring the gas engine back to running speed, turn on the fuel and spark, and you will be driving ALMOST like being in Drive.

    If you go down a loooong or steep grade in Drive with your foot OFF of the gas pedal, the computer will let the regenerative brake effect work until the battery is full ... and then you will HAVE TO use the brake pedal, possibly frying your brakes.

    However, If you go down a loooong or steep grade in B (engine brake) with your foot OFF of the gas pedal, the computer will use the regenerative brake effect until the battery is full, BUT the computer will also use the engine as a brake until the hill ends or you have to speed up.

    On one trip, I went across Interstate 8, over a pass west of Yuma, Arizona. On the downhill side was a sign saying 6% grade for 13 miles. I shifted into B (engine brake) and never had to touch the brakes for those 13 miles downhill. I DID have to use the throttle several times to keep from slowing down TOO much. Engine braking on a Prius is very strong.

    By the way, the Prius DOES NOT use B automatically ... you have to shift into B. Just like if you want to back up, you have to shift into reverse.

    There isn't a speed limit to when you use B, but the faster you are going when you shift, the more noticeable will be the sudden slowing down of your car.
     
  13. qbee42

    qbee42 My other car is a boat

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    A couple of clarifications for the above post:

    1) While B mode is similar to a jake brake, B mode relies on the closed throttle plate to induce pumping losses. Since diesel engines lack throttle plates, jake brakes use a compression release valve system to dump pressure at the top of each compression stroke. This is an effective method for inducing pumping losses, but it is very noisy. Because of this, jake brakes are outlawed in some locations.

    2) While the Prius will never automatically switch to B mode, it will switch to engine braking when the HV battery reaches a high SoC limit. This is effectively the same as B mode, although the details differ a bit.

    Tom
     
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  14. mtngal

    mtngal New Member

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    Thanks for the wonderful information in this thread, it's given me lots of confidence about using B mode when I might not have (long, steep, high speed down grades), thinking it was only for slower speeds. It's nice to know that I can use it instead of the brakes at freeway speeds. That's very nice to know.
     
  15. tumbleweed

    tumbleweed Senior Member

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    Yes you can certainly use it at any speed you wish, I have used B at over 80MPH coming down long steep grades, works great.

    But remember you are wasting energy if you use B mode instead of normal regeneration with the brake pedal to slow down unless you have a full SOC. To put maximum energy into the battery for future use press the brake pedal in D and keep an eye on the HSI, keep it in the regeneration band. There is no point in using the engine as an air compressor and throwing that energy away if the battery has room for it. As you probably already know the small area on the left side or the HSI indicates the amount of regeneration, if you completely cover that small area with the bar you have started using the friction brakes, also wasting energy.
     
  16. mtngal

    mtngal New Member

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    It actually surprised me that I could get the battery up to it's maximum level while driving on the I-5 freeway. I also do it on a regular basis before I get on the freeway, but it doesn't reach full until I'm on a fairly level place so don't have to use the brakes much. By the time I then reach freeway driving speeds the battery level is down a bit and my gas mileage is down even more. Works out well, because for the next 10-30 miles I get to work on building the mpg back up again. This car is fairly addicting.
     
  17. David Beale

    David Beale Senior Member

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    Not sure on the specifics, but it's normal for mileage to be bad for the first 10 min or so after you start a cold car. It then improves dramatically. This is even more noticeable with a Prius.

    Highway mileage is usually worse than city mileage, and the best mileage will be when you are on a road with a limit around 40 to 50 MPH with few stops (provided you obey the limit ;) ) . Surprisingly this is true even when the road is a wild one with huge grades, such as the "Duffey Lake" road from Lilloet to Pemberton, BC.
     
  18. Dolce_Vita

    Dolce_Vita Member

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    Question; does B mode turn on the tail lights? I never really use B mode, but the times i have, it decelerates the car enough to make the tailights needed (IMO)
     
  19. ItsNotAboutTheMoney

    ItsNotAboutTheMoney EditProfOptInfoCustomUser Title

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    No.

    The brake lights come on when you use the brakes.
     
  20. fuzzy1

    fuzzy1 Senior Member

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    Mine does not decelerate as fast as a two-gear downshift on any previous car, none of which activated the brake lights.
     
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