To "B" or not to "B"

I don't understand this idea. If I go down a hill in D, the ICE does not run.
If I put it in B, doesn't the ICE run?

 

Yes, in "B" mode, the ICE does run. It stands for engine "braking" and even states this in the owners manual. Its purpose is to provide additional capacity to hold the car back while travelling long steep downhill grades. Toyota also states the fuel economy will be lower running the car in "B" mode.

 

Using the "B" Setting

I've experimented several times with the "B" setting on my '04 and find it to be effective - even on short hills. There is one particular hill in the Pittsburgh area - the approximately 50 or so foot long stretch at the bottom of Potomac Ave as it intersects with Banksville - where drivers of automatics fight gravity so hard that using neutral is more effective than the lowest available transmission selection. I imagine the Prius will be no different. My feeling is that in general, the B" setting is not really practical unless one is travelling down long, steep hills - something like route 40 into Uniontown, PA from the summit or perhaps the interstate (route#?) travelling west out of Denver, or maybe Going To The Sun highway in Glacier N.P. Actually, the "B" function did not seem as effective as one of the low gear functions in a regular automatic tranny.

Bruce Webster

 

Evan, I drove my Prius over Raton pass on my Denver trip. I never used the "B" setting, and the car never wavered from 55 (on the way up due to ice), and 65 on the way home on the downside of those 6% grades. The regen system did a far better job of holding the car back on its own than the Saturn did over the same hills with its CVT, or even the Dodge truck on the same hills with its manual tranny.

 

pls correct me. but on steep hill descents w/ b mode, the ice is turning but not by combustion, it is precisely the b-mode effect that turns the ice, so you are not using gas??????

at least, that is how i understood it

 

My understanding is yes you are using a little bit of gas. Toyota even points out in the owners manual that fuel economy will be lower in "B" mode. Basically an engine is a large air compressor. Without a bit of fuel, all of that cold air that is being brought in, would quickly cool off the coolant in the cooling system, as well as cool off the catalytic convertor enough so that it would no longer be functioning efficiently. My Dodge truck has a diesel engine in it. Diesels will totally shut down fuel delivery while coasting. I've been down long steep hills in that truck in winter. The engine occasionally cools off sufficiently to miss and smoke heavily from raw fuel untill the cylinders build up a little bit of heat again, once a load is placed on the engine. While the Prius would not missfire like a diesel, the emissions would still be high untill the catalytic convertor heated up enough to work properly again.

 

good posts, all!

one comment to the original question, though....

even if the Prius has a mechanical fuel pump (and i'd guess it's electric, not mechanical), it's not like the old days where some fuel always went through the primary jets into the manifold.... nearly everything today is fuel-injected, and the engine's computer CAN tell the injectors to not inject at ALL, so there really would not be any fuel buildup, etc.

my taurus would "surge", or slightly accelerate, while coasting down some of the long downhills in the Sierras to and from the Tahoe/Truckee area in CA. the computer had to turn on the injectors to keep the catalytic converter hot, so the engine-braking effect would occasionally "go away", and it would speed up a bit downhill. i had to explain that to one of the editors at Car & Driver, but they never replied...

same for the diesel, too, if the injectors are electrically triggered. i owned an Isuzu diesel, and it had mechanical-pump driven injectors, so even at "closed throttle", which is a misnomer, because there's no throttle on a diesel's intake, it never could go to zero fuel injection. on the other hand, it used so little fuel at idle, i once calculated it would take something like one quart of fuel to let the thing idle 8 hours overnight on a REALLY cold night, rather than risk it not starting the next morning. never had to test that, though... it always started.

 

On page 137 of the manual it's stated that to use engine braking one only has to shift from D to B. Does that mean you do not have to stop the car before doing so? And that you can shift from B to D while driving as well.
I think that's the way it's intended, but I'd like to know for sure...

 

You can shift from D to B and back without stopping the car. Actually, you can also shift from any gear to any other gear (including D to Reverse or to Park) without stopping the car, but the computer will not engage those gears.

I use B frequently when going down steep hills and the ICE never starts under these circumstances. Htmlspinnr's answer is accurate.

 

Thanks! Just as I thought, just checking