driving two months now and just discovered the engine brake

You can't. The engine brake will never help you gain fuel efficiency, it will only help you save the friction brakes. It's used on long downhills where the battery will charge to its max state. Once the battery gets to max, the HSD can no longer use regenerative braking and must switch to friction brakes. B mode uses the engine to throw away excess energy, thereby delaying the transition to friction braking. Think of it as down-shifting.

Tom

 

This paper from PC member Hobbit goes into some more technical detail on the "why" of what Tom has said.

 

Best way to use the B mode for fuel economy is to unfind it.

 

couldn't agree more, I wondered the longest time why my mileage went down after I discovered "B" mode. There is one exception though, below 20mph, it is saving, but not any more than the regular brake pedal. (learned all this from fandom-tech)

frosh

 

Jimbok... Thanks for forwarding that excellent paper by Hobbit. Very informative and answered many questions I've had. I would add, however, that even as a new 3,000 mile owner, I've found the B mode most helpful here in rural coastal Maine these last two months....! We've had an extremely heavy winter with more snow than normal and the roads have been, for the most part, deplorable. Snow and ice-covered most of the time. Being used to much heavier vehicles with studded snow tires, etc. all my life (now at age 67), I was very apprehensive when first driving the Prius some weeks ago in a heavy snow storm. I've never used much braking on snow and ice in the past but have found that in a heavy vehicle with the then-new anti-lock braking I could experimentally lock the brakes on snow/ice and the vehicle would, essentially, slide "straight" (given no slope, paving pitch, curve, etc.). When I tried it initially in the Prius I was disappointed to find how quickly this lite little car would start to slide (again, I'd never put studded tires on my Prius). However, with experimentation, I'm now tending to use the B mode on any inclines and going into any curves when on ice and snow—as well as on ice-covered flat stretches with cross winds. I know that there's a cost but I am very impressed with how the car handles and, given its lightness, slows so well and rapidly in B mode with no brakes to perhaps precipitate a "slide or skid".... (In B mode on ice I simply have grown to feel 'more in control...' over the last couple of months.) I'm going out just now into a snow/freezing rain storm with temp at 32.6º for a necessary 15± miles of driving and I will run, at between 15 and 35 mph constantly in B mode. Again, I know it's costing me in mpg but I tend to feel "safer" and "in more and better control"...... Any old timers who disagree? Thanks, Wes at the 'end of the world' in Downeast coastal Maine. ( : >)

PS—My mpg aves. in sig. below will improve when we're finally out of this damned winter...! Now, in retrospect, one of my biggest regrets in having to wait until November to get my Prius is that I wasn't able to go into winter with a much more respectable 52 to 56 pmg ave. than my embarrassingly low present ones in Greenhybrid, et al.... It is a frustration to make some 100± mile runs and pull into the yard at 62.7 mpg to only find the effort 'wasted' by running in snow, ice, and slush (doesn't slush create enormous frictional drag!!!) the next day. http://priuschat.com/forums/images/icons/icon8.gif

 

wesshaw1, from the perspective of braking, using B mode going into a corner or simply pressing on the brake pedal accomplish exactly the same thing: both apply drag to the front wheels. I suspect B mode feels better to you because you have to do it more deliberately, reaching over and flipping the lever. This forces you to anticipate the upcoming curve and react in a more timely fashion. B mode brakes in exactly the same fashion as regen braking, but throws away some of the energy. Both work through the front wheels only, and both revert to ABS friction brakes all around if the front wheels slip.

As for sliding, what type of tires are you using? Also, does your Prius have VSC. Ours has good tires and VSC, and it is very difficult to make it slide.

Tom

 

I suppose the source of the confusion lies in the fact that the Prius fakes some engine braking using regen braking whenever you lift off the accellerator.

 

Exactly. It's smarter than the driver (well, sort of).

Tom

 

the best thing I've discovered about "B" mode is undiscovering it and never using it, even here in the mountains. I really don't ever see a need for it, and even on long extended downhills, the engine will take care of itself very well and protect the hybrid battery without me trying to take over and shift to B..... just my 2 cents..

 

I found it useful when heading into *long* downhills that I knew
would top out the battery a third of the way down. I try to
use "B" *before* hitting all-greens in the battery, because the
engine isn't forced to scream quite as loudly and I basically
try to get my energy wastage over and done with earlier in the
game rather than later. By judicious moving between "B" and "D"
I can regulate descent speed somewhat, often without having
to go for the brakes at all, depending. If I've anticipated
it all the right way, I arrive in the town in the valley just
as the battery goes all green and then I can tool through it
in total silence and get ready for the big burn on the next
climb-out, which will hopefully be helped by any excess charge
remaining in the pack. It was a very fun game to strive for
good MPG with minimal energy waste all through those VA/WV/NC
hills last summer.
.
_H*