In pos B engine is a air pump on decel. I use it frequently on grades etc. as needed, sometimes for a moment I've accelerated and driven in this position [B mode ] for a bit then realize and shift to D You can feel difference but whats going on in the motor how can it provide power but still feel and sound like air pump with fuel??? Its a great feature, jake brake that saves your brake pads and charges bats too
IMO, the B Brake is a waste unless you have somehow drained your battery down to 1 bar and need quick regen for an upcoming large hill for lower rpm's to continue high mpg driving rather than wasting energy recharging the battery with rpm's. If you are driving the vehicle properly you will never need to force regen to the battery. Same goes for the brake pads...I don't think I use my brakes more than 5-6 times over an 11 mile commute to work. If you learn to anticipate traffic and coast rather than brake you will do more to extend the life of your brake pads vs. using B Brake. The highest mpg is gained by keeping the car in motion...not by slowing it's progress. Momentum is mpg's friend.
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. 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. 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. 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. 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. Why, do you use B Brake fuzzy1 and for what purpose?
I'd swear that the OP's location read Medford OR, not Medford MA when I first looked at it. I use B on those descents that are too steep to coast at reasonable speed, too twisty to use cruise control regen, and too much descent for the available regen capacity of the traction battery. When lucky, I can start descents with only 3 or 4 bars on the battery, but most often have 6 bars, so pure regen will fill the battery within only a few hundred feet of descent. B is necessary to prevent brake overheating. Coming down Pike's Peak, there is a mandatory brake check stop, where the ranger tests with a non-contact IR thermometer. I registered 173F, quite acceptable compared to the 300F limit, though it helped that with the top three miles of road was closed so we began the descent 1500 feet low. While we were taking a voluntary rest stop regardless, a vehicle with a serious smell of burning brakes pulled over into the mandatory 30-minute cooling-off zone. It was a full size pickup with Florida plates. During the past month's roadtrip, I tallied 8500 vertical feet of screaming compression B-mode descent, with the traction battery topped out, on a single tank of fuel. Too bad I didn't keep track of a total for the whole trip, but it was certainly over 20,000 feet.
Medford MA and Medford OR are probably two very different terrains I would suspect! I was just relating my local experience to the OP's as I would guess they are similar terrain with little elevation drop during travel. Under these circumstances, B Brake has little value other than in very low battery charge operation. Pike's Peak or similar descents would require quite different operation of the vehicle for sure and I understand your use of B Brake. Personally, I've never been at those altitudes (only one encounter with the Berkshire Mountains in Western MA at 4am with no traffic when I had a little fun with 10 miles of steep downgrade after crawling up the other steep ascent prior to that)...scary on the downhill which I did in Drive at speeds my car has never seen before. The descent was only about 2,500 to 3,000 feet from Mt. Greylock in MA. On a side note, ironically I happened upon Mt. Greylock when I decided to try a new driving route for better economy on 45-55 mph roads rather than take highways. In the end, I gained 4 mpg taking my economy route but would not take that route again at 4am over Mt. Greylock...there is no help for miles and cell service is not available!
B dumps energy as heat via the ICE. It actually regens less than light braking. For more information about B mode and how to use it search B mode in the search at the top of the page. http://priuschat.com/forums/search/
Keep in mind that speed is also a factor, not just feet of descent. Traveling out west, I've gone down many long mountain descents at highway speed and never needed B mode. This is because at highway speed a considerable amount of energy is needed to overcome aerodynamic drag. On some of these occasions I've used B mode as a convenience, since it allows good speed control without using the brake, but it wasn't necessary or even good for mileage. Contrast that with a couple thousand foot descent of two track, where B mode comes into play in the first few hundred feet. At fifteen or twenty miles per hour there is almost zero drag, so speed control is all brakes or B mode. Tom
Hi Matt - Since you and the OP and I are in the same general area and our day-to-day driving likely does not take us into mountainous areas, I agree - B is not particularly useful. But I have used it in the Berkshires and in the White Mtns. coming down the Kancamagus Highway where, as Fuzzy describes the decent, it is very steep with a lot of hairpins and without B, you'd be burning brake pads all the way down.
Please note that as long as there is headroom in the battery (i.e. the battery is not yet full) stepping moderately on the brake pedal uses regen to slow the car, so your brake pads never get wear. Only when the battery is as full as the computer will allow it to get, or if you are driving slower than 8 mph, or you step very hard on the brakes, will the pads be engaged. The B position uses engine compression to slow the car. This means, as Pat says above, that energy is dissipated as heat. You want this to happen if and only if the battery is full, or your downhill is so great that you know the battery will be full before reaching the bottom. Conclusion: Use the B position for very long, steep downhills, and do not use it under any other circumstances. Note that in ordinary mountain driving, where the downhills are broken by occasional uphills, the battery will get nearly full on the downhills, but the car will use a lot of that energy to get back up the shorter uphill segments, and B mode may not be needed. And when B mode is not needed, it is wasteful. So only use B for very long, extended, steep downhills where the battery would otherwise become completely full and still have some downhill remaining. And note also that when you see 8 bars on the battery icon, it is not yet full. It will accept some more charge before it is really as full as the computer will let it get.
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.
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.
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.
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". 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. 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. 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. 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.
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
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.
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. .
