Driving in "B" mode or "gear" - GreenHybrid - Hybrid Cars


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Old 04-13-2008, 10:39 PM
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Default Driving in "B" mode or "gear"

I just bought an 08 prius.
I read the manual section on driving and I don't fully understand
what the purpose of the "B" mode is.

if I am decelerating, what is the difference between D and B modes?
when is the B mode beneficial and why?
In both cases, the battery is being recharged.
The manual has a warning about not driving in B mode for a long time.
what are the drawbacks of the B mode?

I appreciate any clues

thanks
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Old 04-13-2008, 10:46 PM
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Default Re: Driving in "B" mode or "gear"

"B" mode is inefficient and will give you worse mileage. Also, the cruise control, an excellent tool for good mileage, won't work. "B" should best be used when descending large hills to minimize the amount of work the brakes have to handle.

In contrast, "D" provides excellent handling in all conditions and allows the hybrid modes to excel.

GOOD LUCK!
Bob Wilson
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Old 04-14-2008, 08:50 AM
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Default Re: Driving in "B" mode or "gear"

In our old Ford station wagon coming down the mountain from the ski area near Wenatchee, FULLY loaded, the brakes would overheat, even with downshifting teh automatic (within reason) and I would have to pull over to let them cool.

Your Prius brakes aren't likely to EVER approach that point (Pike's Peak maybe..??) by that is an example of what "B" mode might be used for. But for the Prius that is yet another matter of importance. If the hybrid battery, by happenstance, is alreay fully charged and you have a long downhill drive ahead of you then "B" mode is appropreate for use along with the frictional braking you will undoubtedly be using.

But yes, when frictional braking is used, ANYTIME frictional braking is being used, you might be "wasting" energy that could otherwise be recovered for charging the hybrid battery. So the same is true of "B" mode, engine compression braking.

"B" mode could be made even more efficient, a HIGHER level of engine compression braking, if you could somehow open the throttle plate but without the fuel injectors powered.

Your ICE becomes one HUGE air compressor.
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Old 04-15-2008, 08:48 AM
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Default Re: Driving in "B" mode or "gear"

Quick version, the difference between "B" and "D" is in "B" the engine is used in compression braking when you release the throttle. In "D" it is not.
I think in "B" the "base rate of regen" is a little higher as well, but I could be wrong on this.

You use "B" for descending long hills to keep the car under control. Generally the Prius will coast better than most other vehicles, so when you descend a long hill you will release a lot of energy into the brakes or regeneration system. If you just use "D" and the brake to control the vehicle speed, you will quickly fully charge (to 80%) the "traction battery" and will then be using the friction brakes only to keep the speed under control. In "B" the car will use the engine to dump the excess energy (and it does so aggressively - I've seen the ICE RPM go up to 5000 on really long steep hills such as a mountain pass!). So you reduce the load on the brakes in "B". On short hills you will be "wasting" the energy by dumping it into the engine compression rather than using it for recharging the traction battery (regeneration).
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Old 04-15-2008, 10:14 AM
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Default Re: Driving in "B" mode or "gear"

Quote:
Originally Posted by David Beale View Post
Quick version, the difference between "B" and "D" is in "B" the engine is used in compression braking when you release the throttle. In "D" it is not.
I think in "B" the "base rate of regen" is a little higher as well, but I could be wrong on this.

You use "B" for descending long hills to keep the car under control.

Actually NOT...!!

Your statement "under control" implies, implicitly, adverse roadbed conditions, snow, ice, etc. One should never, NEVER, bias braking toward the front of the vehicle in any way when traveling downhill on a slippery surface.

Absent any other method (say frictional braking w/ABS) of slowing the vehicle a light or slight application of the E-brake (almost always rear implemented) would be appropreate.

The Prius is FWD, ALWAYS.

Engine compression braking and/or regenerative braking, in those conditions can turn DEADLY instantly. To that end Ford now has a US patent that applies to techniques they developed for use in the hybrid Escape and Mariner.

If the OAT is near freezing or below the level of regenerative braking that can be used is significantly reduced. The second technique involves the INSTANT disabling of regenerative braking should ABS activate when you have pressure on the brake pedal.


Generally the Prius will coast better than most other vehicles, so when you descend a long hill you will release a lot of energy into the brakes or regeneration system. If you just use "D" and the brake to control the vehicle speed, you will quickly fully charge (to 80%) the "traction battery" and will then be using the friction brakes only to keep the speed under control. In "B" the car will use the engine to dump the excess energy (and it does so aggressively - I've seen the ICE RPM go up to 5000 on really long steep hills such as a mountain pass!). So you reduce the load on the brakes in "B". On short hills you will be "wasting" the energy by dumping it into the engine compression rather than using it for recharging the traction battery (regeneration).
Now, if the industry could just find a way to quickly, INSTANTLY, eliminate all levels of engine compression braking on a FWD or F/AWD when ABS activates. Or maybe INSTANTLY upshift, or shift an automatic transaxle into neutral, when ABS activates.
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altima, b, car, difference, driving, gear, highlander, hybrid, ice, mode, mountain, nissan, prius, shift, steep, toyota

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