B or Jake brake Question

[fuzzy1]

Originally Posted by Matt Herring

... but would not take that route again at 4am over Mt. Greylock...there is no help for miles and cell service is not available!

Out here that is normal. The real cell service coverage maps still have more white space than colored space. While many folks do stick to the Interstates and the few other major roads with full cell coverage, or never leave the urban and suburban cores, I couldn't live with such tight travel restrictions.

The National Parks in this region are nearly cell-free.

[Mike Dimmick]

B mode is low gear for descents. If you see a sign with a high percentage downward gradient, or low numbered ratio (1 in 3, 1 in 4, 1 in 5) or a 'low gear now' sign, select B. Otherwise, leave it in D.

I'm going to stop saying anything about how B mode works because the car has a number of tricks to achieve this 'extra retardation when lifted off the accelerator' and it picks different things to do based on road speed, inclination, and battery state-of-charge. All you need to know is that it stops the car running away on a down-grade. That's what it's for and all you should use it for.

[fuzzy1]

B dumps energy as heat via the ICE.

During my recent vacation, I did find a situation were B mode simply increased the regeneration drag without spinning up the ICE for compression braking. This worked consistently one evening, at speeds of typically 25 mph and sometimes as high as 35. While it did repeat on some other days, it was not at all consistent, so there are other factors coming into play. I was paying more attention to the many rutting elk herds in Rocky Mountain National Park, and likely missed some clues.

When this B-mode pure regeneration was happening, it worked only with 6 or fewer bars on the battery. The instant Bar 7 turned on, ScanGauge showed the ICE spin up.

Also on this trip, I noticed that the D-mode fake drag regeneration can spin up the ICE for compression drag when the traction battery is full. It was usually quiet and smooth enough that I wouldn't have noticed without the ScanGauge RPM display.

[Old Bald Guy]

Originally Posted by Matt Herring

The OP lives in MA as I do...relative info for a fellow MA'er since neither of us lives in "mountains" as they are described in relation to the rest of the country. While the "air pump with fuel" using B Brake is a good analogy there are many negatives to using B Brake.

If you're going to use B Brake you should understand what it's implications are in terms of mpg...unless you don't care because the Prius gives you high mpg just by it's design...but why own a Prius if you don't care about mpg. There are many threads on this site and others that detail the do's and don'ts of B Brake use...most of which are don'ts. The only positive I can see from using B Brake is to save a little brake pads but ultimately you sacrifice mpg's for brake pads and in the end you should be in favor of saving mpg's because on most hills you can ride the coast out without drastically exceeding the speed limit rather than riding the brakes or using B Brake down a hill.

1. When applying B Brake you are obviously impeding momentum which results in lower mpg because you are robbing the vehicle of speed that can not be regained without burning fuel.

From real world experience driving through the Rockies ... twice ... in August, "B" does NOT result in lower MPG. My mileage was about the same in the mountains as on flatland. If you are going down a 7% or 8% for several miles, you have two choices ... ride the brakes or use "B".

2. When applying B Brake while the battery is at or near full charge you are adding energy to the battery that will ultimately force the ICE to burn that energy off which causes the car to use more fuel.

This just ain't so. When NOT using "B" ... and when your battery gets to the limit the computer will allow ... the computer SHUTS OFF the regenerative braking and you are forced into using the mechanical brakes. (ever go down a long hill and lose your brakes because you cooked them?)

When using "B" ... which is basically a Jake Brake ... the computer TURNS OFF THE FUEL TO THE ICE, shuts off the spark to the ICE, and alters the valve timing ... which turns the ICE into a big air compressor.

I have gone down some looooong and steep grades while in "B" ... and have had to use the gas pedal some to keep from slowing down TOO much.

Using "B" on a downgrade does NOT make your car use more gas and it does NOT make the battery overcharge.

The Prius battery governing system is constantly monitoring the SOC to make sure it's in the ideal 60-70% charge range. When you use B brake and significantly rise above that level (on a bigger hill it could put you at full bars) the ICE engages and wastes fuel to burn off excess battery energy...this is highly counterproductive.

NO MATTER WHAT ... you cannot make your car overcharge the battery. "B" does NOT charge the battery more than the computer will allow. It just doesn't happen. When the ICE is spinning, it is being turned by the electric motors ... NOT by fuel. The fuel and spark are shut off.

3. When using B Brake it causes the ICE to spin at a higher rate (when at a higher speed) and what do you think propels the ICE to spin higher...fuel. At lower speeds the ICE spins much lower but it still spins at a higher rate than when not using B Brake.

Sure "B" makes the ICE to spin faster ... it creates a lot more drag and slows the car better. But it is NOT being spun up with fuel.

I have found no data that supports using B Brake vs. driving the car normally. If you really want to maximize the vehicle (when operating under 60 mph) when going down a hill get it up to 40 mph and ease off the go pedal into EV mode, put it in Neutral, and coast down the hill and as far as it will take you in the flow of traffic. While this will not produce battery regen it will produce the highest mpg...and again, operating the vehicle efficiently will never put your battery level at a low rate causing ICE to regen the battery. Coast down the hill in Drive if you need a little regen...but I would never coast it down in B Brake unless it was a mile or more of heavy grade descent and I was in danger of drastically exceeding the speed limit.

Putting your car in neutral and coasting downhill is not only stupid and dangerous, it is also illegal in every state I have been in ... and I have driven in most of them.

Your very last line about going down a long and steep grade is what "B" is about.

[Matt Herring]

Originally Posted by Old Bald Guy

From real world experience driving through the Rockies ... ". Stop there...I was giving advice on the B Brake to someone driving in MA...not in the Rockies.

(ever go down a long hill and lose your brakes because you cooked them?) No, I haven't because I don't live in the mountains and, again, my advice was for a MA driver in our terrain.


I have gone down some looooong and steep grades while in "B" ... and have had to use the gas pedal some to keep from slowing down TOO much. Good for you...not sure how that improves your argument that B Brake is useless in MA.

Using "B" on a downgrade does NOT make your car use more gas and it does NOT make the battery overcharge.

I never said it would make the battery overcharge...I said it wasn't necessary to use B Brake on MA terrain. And, if you are using more brake via B Brake and finish the bottom of the hill even 1 mph slower than when using regular brakes...at some point you have to use fuel to go faster to make up that 1 mph...all things being equal. And that lowers mpg.

NO MATTER WHAT ... you cannot make your car overcharge the battery. "B" does NOT charge the battery more than the computer will allow. It just doesn't happen. When the ICE is spinning, it is being turned by the electric motors ... NOT by fuel. The fuel and spark are shut off. I never said you can overcharge your battery.

Putting your car in neutral and coasting downhill is not only stupid and dangerous, it is also illegal in every state I have been in ... and I have driven in most of them. Why is it stupid and dangerous? Have you ever read a news article that claimed the driver caused an accident by driving in neutral? Please provide facts and not something you heard from someone. Do you read all the state driving laws before you drive in all the states you have driven in? And if you read the MA driving laws you will find that only on the interstate highway do the laws state you cannot disengage the gears while driving (and it only specifically states it cannot be done on the Mass Pike and in tunnels)...do your homework before you speak.

See my replies in your quotes

[codybigdog123]

A lot of information here. Unfortunately, not all of it is accurate, and some is misleading. But here's a very informative article on the subject. It's for GenII, but I will assume the function and operation of the B setting has not changed from II to III?

http://www.techno-fandom.org/~hobbit/cars/b-mode.html


Regarding the legality and safety issues of putting the car in neutral while driving/coasting:

(1) It is illegal in most states.
(2) It is unsafe for a number of reasons.

About a year ago, Click & Clack answered this very question - They said it's both unsafe and illegal in most states. By the way, most driving laws are enacted because of safety issues. That is, one usually follows from the other.

Is it bad to put an automatic car into neutral while driving to save gas?

Car Talk - Driving in Neutral on a manual transmission Nissan Sentra

[patsparks]

Prius transmission is never in neutral anyway.

[richard schumacher]

Picky, picky... Pat means that the mechanical components are always engaged with each other, but of course the effect of "N" is exactly as in a conventional car: the drive system gives no resistance to motion. In a Prius coasting at high speed "N" has the added excitement of creating potentially harmfully high RPM and voltage in one motor/generator.

[hill]

Originally Posted by fuzzy1

You apparently do not have many mountain descents of 2000 to 5000 vertical feet in your area.

The search tool can be your friend. Use 'B' for downhill / high speed descents and you'll get worse mpg than if you'd simply taken your foot off the pedal.

.

[codybigdog123]

Originally Posted by hill

The search tool can be your friend. Use 'B' for downhill / high speed descents and you'll get worse mpg than if you'd simply taken your foot off the pedal.

.

Why? When you take your foot off the gas, no fuel is consumed...whether in B or D.