Controversial B mode thread :-)

Discussion in 'Gen 2 Prius Main Forum' started by mikewithaprius, Jul 2, 2011.

  1. mikewithaprius

    mikewithaprius New Member

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    I feel somewhat awkward posting this. World is no longer flat sort of thing.

    F8L, from his own experience and intuition, had suggested B mode recharges the battery better on downhills, and I said I'd look into it. Of course, we all post all the time in newbie question areas that this is incorrect, it's actually less efficient, energy thrown away as heat, yada, yada. I've learned everything I know about the car on these forums, and didn't question it, since everything else seemed to be dead on.

    So off I go today. I'm just driving once a week for my 12v battery in the summer, so I took off some on some unknown route here in Vermont. Luckily I picked a nice mountainous one. Mileage was in the high 30s (that's right, I was getting in the 30s like a common peasant) fifteen minutes into the drive, it was was all uphill basically.

    On the way back down, I remembered to test out F8L's idea. I'm going downhill, not too steep yet, and put it into B mode with BTA showing about -20 amps at that point. After a quick flash of of -40 or so (or was it +40? Uh oh, someone else go out and check, or else it'll be another week :-D) it settled right in at -25!! So B mode was 25% more efficient in packing in the amps.

    My first thought was if this had something to do with more lost momentum, so you're getting more energy because you're slowing down more, but still less than if you slowed down that much, in terms of mph, while in drive. Someone will have to explain that to me if it's the case. Nonetheless, there are times when braking is a function of time and not speed, so putting it in B mode over the same period of time as drive gives more regen. :eek:

    I tried it a bunch of times to make sure. Same result over and over down this 10 minute mountain. It was nice to see the 100 mpg bars after a couple down near or below 25 mpg from the uphill. Got to the bottom, switched SG back to SOC, and discovered I was already at 83% even though I had more to go. Managed 62 mpg roundtrip even with those huge hills, thanks to the regen. It helped me go a long way, including an 80 mpg bar with acceleration from 0 to 50 mph.

    I also verified that in B mode, even though the engine comes on quite hard, at like 2400 RPM for what I was doing, gallons per hour still showed 0.00.

    I wish I had pictures of all this, but with SKS turned off for the summer, my key was stuck in the ignition, couldn't mechanically lock the car to run and get my camera.

    So - no gas use, but more energy regenerated in B. F8L was right. I feel like I'm either missing something somewhat big, or else the conventional wisdom just sort of got passed down like folklore without testing.

    Thoughts? Comments? Insults? :D
     
  2. fuzzy1

    fuzzy1 Senior Member

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    You tried this multiple times. Do you mean multiple engagements of B during a single descent, or multiple runs down the same hill, some in B and some with just regen braking? Only the multiple descent version is a valid comparison.

    Amps are not energy, nor a proxy for energy. To settle this question, we need to see the change in SOC for descending the entire hill in B mode, versus the same descent with just regen braking, at the same speed.

    It doesn't surprise me that BTA increases when you switch from D to B, but that doesn't make B more efficient. It is just part of the increased overall drag.
     
  3. mikewithaprius

    mikewithaprius New Member

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    You're right, just did it many times in downhill portions, not over same one. I'll be happy to try it out next week like you say, starting with same SOC at same point a couple times and descending. I would then look at SOC at a specified distance after I start, correct (as opposed to specified time)?

    There are just some times when you want to be losing momentum the whole time (stop sign, lower speed limit coming, etc.), and if you can pass that entire distance with more amps into the battery, I'm confused how that theoretically charges the battery less.
     
  4. dtuite

    dtuite Silverback

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    Does the initial SoC make a difference?
     
  5. mikewithaprius

    mikewithaprius New Member

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    dtuite,

    I'm guessing so because just from observation, SOC seems to fill very quickly at higher percentages, like 70 to 80, than from 60 to 70. Even if not, if I'm going to go use the gas to check it out I figure it should be controlled enough so that it will mean something either way.
     
  6. Corwyn

    Corwyn Energy Curmudgeon

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    Energy-in [goes as] amps * time, not amps * distance.

    Imagine you are stopped at the top of a hill with a stop sign at the bottom. You nudge forward, and coast to the bottom. The energy available to charge the batteries is solely the potential energy of the car at that height. And all of it must be accounted for. B mode has some energy is going into the batteries, some into spinning the engine, and some into aerodynamic drag. D mode has no energy going into spinning the engine, but more into aerodynamic drag (since the car will be traveling faster on average), and possibly into friction braking. B mode is therefore better only when engine spinning (someone said this was 1 kW) is less than the difference in drag, and heat in the brakes (if any).
     
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  7. F8L

    F8L Protecting Habitat & AG Lands

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    Thanks for taking the time to test this out and post the thread, Mike.

    I 'venot done any extensive testing of this idea. Like you, I ready the posts stating B mode does not regen faster than just braking but it seemed like I always regen faster when in B mode. On my commute, I have a short steep section of road that ends with a stop sign. I watch my SOC on the scangauge and when I switch to B mode the SOC rises faster than in Drive.

    I think performing a series of runs on the same downhill stretch of road in B mode then again in Drive would help clear this up.
     
  8. xs650

    xs650 Senior Member

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    Energy is power*time, it is only proportional to amps*time as long as the voltage is constant and we ignore power factor.
     
  9. xs650

    xs650 Senior Member

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    I think that by picking the gradient and driving conditions, you could have either mode regen faster.

    In B, the car is controlling amount of regen (+ engine braking) In D, the amount of regen is highly dependent on how the driver uses the brakes. It depends on how good a job the car does at maximizing regen.

    I could see situations where the car isn't generating the maximum power system allowable amount of regen on a short down grade because the car has no way of knowing it's a short downgrade, while a driver does.
     
  10. uart

    uart Senior Member

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    Remember that you can use the normal brake pedal to increase the amount of regeneration too. So you could maintain the same speed profile in "D' mode (gently using the brake) as you would get in "B" mode. Under these conditions I'm pretty sure "D" mode would be overall more efficient, but particularly if the decent speed was mostly between 20 and 41 MPH, where the ICE is not spinning in "D" mode but is spinning in "B" mode.

    Finally it's worth mentioning that the ICE is not always stationary in "D" mode nor is it always spinning in "B" mode. Above 41MPH the engine also spins in "D" mode. And below some minimum speed (not sure but I think it's about 20 MPH) the engine stops spinning in "B" mode. So certainly in some situations there's probably not a lot of difference in overall regeneration efficiency. Where there is a difference however I think it's always going to favor "D" over "B".

    BTW. Can any experts chime in with info about the valve timing when the ICE is spinning in "D" mode (above 41MPH) as compared to what it is in "B" mode. Is there also a difference here?
     
  11. qbee42

    qbee42 My other car is a boat

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    You are confusing "recharging better" with better efficiency. They are not the same. B mode will indeed force more electricity back into the battery given the same brake pedal position, but it doesn't do it more efficiently. B mode also invokes engine braking, which deliberately discards energy.

    For an analogy, let's look at payday loans, where you borrow against your paycheck at very high interest rates. Taking out a payday loan will fill your bank account faster, but in the long run you end up with less money. The same is true with B mode, except instead of cash we use charge.

    Tom
     
  12. mikewithaprius

    mikewithaprius New Member

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    Lucky for you guys I left my windows open overnight. And it rained. :( After sopping up water from the seats with probably hundreds of Kleenex, I drove around to air out the car some more.

    I happened to find a perfect hill tucked away, apparently created for regen braking testing, since it was newly paved, had a speed limit of 35 mph, and had literally no traffic on it...perfect. Quite steep, I might add. From the bottom heading up I was getting between 11 and 18 mpg instant. I sacrifice for PriusChatters :p

    Conditions: overcast, 68°F, no AC or any other accessory use in car, all four windows wide open for my poor, wet seats. Length of hill 0.4 miles. Started from same location at top of hill (trucks use low gear sign) and stopped at same location at bottom each time (stop sign). Started each trial when SOC got down to 58.0% exactly. Accelerated gently to 7 mph each time (next telephone pole basically) and let gravity do the rest. I would brake naturally when I felt I needed to, so I did not brake in the same manner, at the same place, everytime, which represents what I was looking to find out - how D regen differs from B regen in "normal" driving. Numbers below are SOC at stop sign at bottom of hill once fully stopped.

    D mode - from 58.0% SOC
    1: 65.0%
    2: 65.0%
    3: 66.0%

    B mode - from 58.0% SOC
    1: 65.5%
    2: 65.5%
    3. 65.5%

    So B mode works better or just as good as D most of the time. But there's one more critical thing. In Drive I would easily hurtle down the hill, and if I hadn't braked, I imagine I would've been going 60 mph by the bottom. I never went over 40 mph to make sure engine stayed off. In B mode, I was going much slower the whole time, since it was better at slowing the car (by the way, like uart says, engine does not start spinning in B mode until 20 mph, so that extra drag I felt slowing the car down comes from more regen). So B mode produced as much energy, and usually more, than D, but at much slower speeds. If I had been going the same speed, B mode would have produced more.

    One question I have, which may reveal another advantage of B mode, is whether traction control is easily tripped in it as well. The road was so smooth it never happened, but would B limit or eliminate the switch to friction brakes over a pothole unlike D mode, thus keeping you in regen longer? If you're not aware, if you go over a pothole while braking, even letting up on the brake for a second or two and reapplying still will have you stop in "friction mode". Thus one bump while braking while down a steep eliminates almost all of regen, excepting about 10 amps or so. If B mode avoids this, it would be a huge plus.

    I guess what I don't understand is where our ideas about B mode NEVER equalling D come from, since it does. B basically beat D on this one little test, and would thus seem to do it even moreso if travelling the same speed in both "gears".

    Anyway, I would invite others to do more testing, but I'm pretty goshdarn convinced of using B on any hill of decent length. I won't be using it in daily driving to stop signs, but the idea that is completely worthless and will harm your fuel economy seems to be unfounded, and it may actually help.
     
  13. mikewithaprius

    mikewithaprius New Member

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    Hey Tom, thanks. I know I have some limitations in terms of technically describing things in which I have no expertise. I guess regardless of how I'm trying to say it, if braking in B can give equal or better increase in SOC compared to D, overall the use of that stored energy during the trip will help give better mpg.
     
  14. uart

    uart Senior Member

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    So there's the difference right there!

    Under the same conditions "D" mode should be as efficient or more efficient than "B" mode. Same conditions is important when making this comparison because slower speed is well known to make regeneration more efficient (less windage losses).

    There is no good reason to not use the same speed profile for both tests (other than to deliberately distort the results :confused:). As I said in my previous post just use the brake pedal in "D" mode so as to maintain a similar speed profile to the "B" mode test.
     
  15. mikewithaprius

    mikewithaprius New Member

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    uart, that means that B mode got me to the same SOC without having to go as fast as in D.

    Let's say you're coming over the crest of a bridge. You're going 35 mph. You let your foot off the pedal, or brake somewhat, and come to the stop at the bottom to pay a toll.

    Now let's say same thing, but you use B mode once over the crest, but you're starting downhill at the same 35 mph. You would expect it to do better in that situation in terms of SOC boost given what I've seen, no?
     
  16. uart

    uart Senior Member

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    Yes but not going as fast is an advantage (as long as the starting speed is the same). The faster you go the more windage losses. You need to keep the same speed profile to make it a fair comparison.
     
  17. mikewithaprius

    mikewithaprius New Member

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    I see what you mean now. That experiment is beyond an individual, we'd need a team to try to pass the same point at the exact speed and exact SOC everytime, as well as maintain the same speed profile over 0.4 miles (or get a self-driven Google Prius to do it!).

    I think the point about bumps triggering traction control is important though. One bump and your entire hill in D is wasted. There is also this graphic:

    [​IMG]

    It shows us how easy it is to mix into friction braking by pushing enough on the brake. If you're going down a steep hill, B mode is supposed to avoid friction brakes. So I'm starting to understand the theoretical the more everyone explains, but in the practical, it could very well help avoid riding your friction brakes down a hill without realizing it.

    Also in actual driving, I wouldn't be able to control my speed profile in a precise manner. I get that for a real study of this, we would need that. I'm just not as convinced of the dogma that B doesn't work as well *practically* in terms of regen, or even better since it would eliminate some of the unnecessary brake pad wear/heat with more brake pedal depression, and potentially avoid traction control circumstances.
     
  18. efusco

    efusco Moderator Emeritus
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    There's a whole lot more to this guys, I think a few people have alluded to it, but I'm going to try to go all "B-mode for dummies" to simplify.

    Consider that when quantifying "efficiency" you can't just take into account the easily quantifiable passive energy (ie. Battery SOC), but also the kinetic energy (suggested by speed/momentum).

    Thus let's assume the following situation(s). You hit the top of a hill at 40mph and will have the following options:

    1)You can go into an ICE-off glide and put the car into neutral. In this situation all the energy will be kinetic energy (discounting that lost due to friction from air resistance and in the mechanics of the wheel bearings/etc.) Your battery SOC will likely not change at all, but you'll have obtained the maximum energy that the hill is capable of "giving" you in the form of speed.

    2)Leave the car in "D" and take your foot off the accelerator, but don't apply any regen braking. You'll gain a little bit of regen, you'll probably gain a little speed or maybe a lot depending upon the grade of the hill. Thus some balance of kinetic and passive energy, but in no case can it be more than situation #1 as some will be lost in the conversion to heat and passive form of energy as well as some heat loss from ICE spin. You have a net energy loss over situation #1.

    3)Leave the car in "D" and apply regen braking. Depending upon the force of regen, the grade of the hill, the battery's starting SOC you will get, potentially, quite a lot of regen. If you have no net delta speed you'll have just a small energy loss from the ICE, and a small energy loss in conversion to battery storage. Net energy is probably close to situation #2, but, again, it can't be as good as situation #1.

    4)Drop into "B" mode. This is equivalent to situation #3 with very mild regen braking. You'll regen more than in "D", but at the cost of forced ICE spin which requires energy (from the battery, no fuel burn). Thus this is the worst situation of the 4 as MORE heat is lost from ICE spin than any of the above, less is regened than in the #3 scenario, and you have less kinetic energy left than in either of the above 3 situations.

    Now, #4 is just fine if you don't care about maximizing energy--it's a pretty small difference among all 4 after all. And it's great if you just don't like having your foot on the brake pedal and regulating that yourself on long hills. But there's no situation where B-mode can/will be MORE energy efficient than the previous 3 situations.
     
  19. fuzzy1

    fuzzy1 Senior Member

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    I must object here. A fair comparison must include continuous light braking in D mode, to achieve the same retarding force and speed profile as the B mode run. When braking 'naturally' by pulsing them when you feel it is needed, it is too easy to exceed the regen limit and throw away a bunch of energy in friction, as I observe with certain drivers not conditioned to hybrids.

    Release and reapply the brakes. I already had a similar reflex for snow and ice driving from the non-ABS days, to regain traction after a slip. This reflex quickly changed to a regain-regen reflex in the Prius.
     
  20. F8L

    F8L Protecting Habitat & AG Lands

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    I'm driving back from LA so I can't go into depth with this post but please keep in mind that my observation was based on a hill with a, stop sign at the bottom and a low speed limit so gliding cannot be done legally or safely. :)
     
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