In my first couple days with my new 2004 Prius I have been using the shift to B frequently to use the Generator / Engine braking and recharge and then shifting back to D. Is there a downside to this? Can you shift too much? Also, what is the downside to leaving the car in B? Higher revs and poorer gas milage?
Very poor gas mileage if you use it at any time besides steep downhills. There is no indication to use it at all most of the time, but if you're tired of using the brake pedal on steep long downhills you can drop into "B", but this will actually result in lower overall mileage due to the engine braking involved. You can also use it at speeds below 20mph if you really want to, when slowing, without the ICE kicking in and it should, theoretically, maximize regenerative braking. It's hard to imaging it makes much difference though, and no regen braking occurs below 9mph anyway.
Well, it does defeat the purpose of the system. As Evan stated "B" is used for heavy mountain driving, and even then, only when it's needed to safely keep the car at speed downhill. Personally, I've never used the "B" selection, even on 6 and 7 percent grades. My car typically stays within 5 mph of the posted limit, and has only required a light touching of the brakes on occasion to bring that down. For normal everyday use, just leave it in "D" and forget about it.
Ok sorry n00b here. What is "B" anyway? I saw that and thought it might be a parking brake. But since you guys drive with it, I'm guessing its not. Alpha
"B" is for engine braking. Basicly it acts sort of as a "Jake Brake" where the back pressure of the ICE helps slow the vehicle. However, in the Prius it also maximally engages regenerative braking (MG1) to reclaim some of that kinetic energy.
I find that B is only useful when going downhill for a long time, as on a long and twisty country road several miles long. In this case, B could keep the brakes from overheating. The rest of the time there's no need to use B (may be useful, though, if you're bored and want to play with the shifter).
I use B every work day coming down a 6 story spiral parking garage ramp. Keeps me from having to ride the brake. Of course, for a quick thrill, I don't use B or the brake!
Excuse the ingnorance of an old retired engineer here, but please explain to me what it is about 'B' mode that would cause gas mileage to suffer when using it. If you watch the monitor when in 'B' mode as you use it slow your speed going down hill, the engine isn't fired up and the battery is being recharged through regeneration. Jake brake reference has been used, but that entails using engine compression with no fuel applied (air compressing on each stroke and released at the top of that stroke). If no fuel is applied to the engine in 'B' and regeneration is occuring in 'B', how on earth is this going to affect gas mileage? I'm sure I'm missing something here. Please enlighten me. Ken
being in B mode would only hurt your gas mileage if you were trying to increase your vehicle's speed by accelerating. accelerating in B mode would feel similiar to accelerating in 1st gear in a normal automatic transmission ICE. you should also know that B mode creates unnecessary wear on the transmission and should not be used for long periods of time. although many people here seem to think that the Prius slows down fairly rapidly when coasting in Drive, i feel that it doesnt. many times when i am coming to a stoplight i do not want to start coasting too early especially if traffic is heavy so i will use the B mode as it basically slows the vehicle at a much greater rate. and as an experiment, i decided to use it as often as was possible in town to see if there was a noticable change in my gas mileage. since i just started this experiment this weekend and havent tested it enough to say for certain, it does appear that my mileage has increased due to more regen power being captured. for the first time in recent memory, i achieved 150 Wh of recharge in a segment, then got a 100 Wh charge in the next segment. this was achieved on level road at speeds from 35-45 mph. as 50% of the stops were stop signs, couldnt avoid the complete stop. as it looks so far, im nearly 2 mph over my typical in town average of 49 mpg. like i said its still early and will have a better picture of it after a few weeks of trying the B mode.
How does 'B' mode create unnecessary wear on your transmission (power split device that is always working in one way or the other anyway).
It seems to me that using B mode for stop signs would be a lot more bother than stepping on the brake pedal. Plus, the pedal gives you control over how much braking you apply. And you still get regen. Presumably, all your braking is regen until you reach the cut-off speed, unless you stomp so hard on it that the car gives you brake-pad braking for an emergency stop.
Couple answers. First, the original author strongly suggested that he IS using B mode as a second Drive mode which will greatly impact gas mileage. Second, I'm not convinced that the ICE is off during B-mode. You can frequently hear the ICE running when you drop into B mode (i.e. not a true Jake brake--but it's a good tool for comparison). Please remember that the Energy screen does not and is not intended to precisely display what is really happening at every moment. Just like hyperstealth driving above 41mph there may be no arrows from the ICE, but rest assured that it is running. Likewise with B mode above 20mph. I do NOT have proof that fuel is being consumed in B mode other than my personal experience of listening to engine noise. I do not think it is a large amount of fuel, in any case, but it is a slight loss none-the-less. Indirectly, any unnecessary slowing ultimately is loss of energy/decrease in fuel efficiency. Certainly braking/slowing is, often, necessary, but I prefer to be in direct control of the process and to titrate the amount as necessary.
Using B may reduce the amount of regeneration, by using the engine as load as well as MG2 to slow the vehicle. This would be useful on long descents, when the battery could become fully-changed and leave nowhere to dissipate excess energy. But I expect that the Prius would use the battery SOC to determine how much regeneration and how much engine braking to do, and B mode would only be a hint to the system to expect a lot of braking. From a driving point of view, B mode is most like having constant gentle brake pressure without having to touch the pedal. And like gentle braking, it's good and regenerative when possible. So no advantage over ordinary braking for stopping on the flat. This is all guesswork, of course. If anyone has concrete facts, would they please stand up now? Thank you.
Um...I'm missing something. Isn't that what the brake pedal is for? :-D I mean...especially in a car where the brake-pedal will, initially, do the same thing as shifting into B -- use regenerative engine-braking magic Which isn't to say that I haven't done this exact same thing in a conventional car with automatic transmission -- downshifting rather than braking. But it doesn't seem to make as much sense in a car that does engine-braking on its own when signalled by the brake pedal to slow down...
i think the confusion here is stemming from the manual. it doesnt specify what B mode really does other than to slow the car down by using engine braking. now is that the same as regeneration? i dont know. i do know that B mode slows the car down at a steadier rate and the braking doesnt really feel the same as when regen-ing the car. i tried using regen extensively the other day when i was doing a lot of surface street driving that involved a lot of slow downs but not necessarily full stops to see if i would notice an increase or decrease in mpg. the mpg did go up and i hardly ever used the brake since the B mode was sufficient nearly everytime to stop or slow the car in time. without side by side comparisons done at the same time, it would be hard to tell. but i will say that it seemed that my regen power levels were much higher than normal.
No. "B" adds engine braking, that is, it uses the engine without fuel or spark as a big air compressor. To the extent that this happens as you slow down you have lost energy that might otherwise have gone into the traction battery, and thus your economy will be reduced. This is why Toyota advises against using it in normal driving. Sure, people who like fiddling with knobs as they drive can use it all they like, but is poorer fuel economy the reason that they bought a Prius in the first place?
Richard, All due respect, but I don't think your description is completely accurate. Also, if you know that there is no fuel/spark used as part of Engine compression please point me to the source of that information. I believe that there is fuel and spark used (minimal amount), but I have no proof and haven't seen proof either way. I think that the miniscanner will help answer that question eventually. Also, before engine braking begins the electric motor regenerates maximally to slow. Thus, it's a 2 part process, motor braking with regen AND engine braking for additional slowing.
Uhh, oops, I withdraw the bit about "no fuel or spark" since I can't document it. And I did note that "B" adds engine braking; regeneration still occurs.
Since brakes are much cheaper to replace when they wear than transmissions, I don't think it is advisable in any case.
