transmission B mode WHY?

If a Prius requires continuous braking descending a 6% grade in B, then we need to stay off steeper mountain roads. I wouldn't have bought the car if I knew it were so unsuited for mountain roads.

 

My Experience with Gen II prius is it automatically switches to engine brakes (equivalent to 'B') when going downhill once the traction battery is full even if I do not touch the shifter. The display keeps showing 'D'. But I feel and see on scanguage engine rev up. And battery charging stop.

Doesn't Gen III do that?

Only purpose of 'B' in my mind is going downhill in icy and snowy conditions where you do not want to use brakes. Something like using lower gear -- Although lower gears in conventional cars, both automatic and manual transmissions are far more effective than 'B' in my opinion.

 

I don't believe the Gen III does that. I have sometimes used B mode when descending I-70 after the battery is full. My experience is that the engine will provide braking in B but not in D. Realistically, even driving in the Colorado mountains, I have not had to use B mode very often.

 

XS650 I am unsure how providing better mountain brakes makes the car more 'unsuitable' than not providing them. It is true that the design of the Prius 'transmission' makes lower gears impossible, so Toyota did their best to achieve similar functionality.

 

I didn't say or imply that B made it less suitable for downgrades, i said that your words indicated that it wasn't very suitable for steep downgrades.

I was going by what you said about how ineffective the B mode was on a lousy 6% grade.

You are the one that made it sound like B is better than nothing but not very effective.

 

Only you consider this 'a lousy 6% grade', the state of Oregon considers it the most dangerous stretch of highway in the state, beating Siskiyou summit.

http://www.oregon.gov/ODOT/MCT/docs/SiskiyouPass.pdf?ga=t

Note the lack of mention of hairpin turns in the Siskiyou pamphlet, and that by comparison the road map is straight.

I have played on the Siskiyous, Donner Pass, Grapevine, Cajon, Parley's Summit, Snoqualimie, White and Chinook Passes and Immigrant Hill beats them all. (I confess I have not traveled through Colorado, I tend to use Wyoming) Forester Pass is steeper, but I walked over Forester.

 

XS650. I don't think the Prius is unsuitable for steep roads. I've used it many times in the mountains, and it's quite capable of keeping up with traffic and staying in control on down grades. In fact, I just drove it like a "normal" car and didn't use the "B" position at all.

I do see your point that 6% grades aren't that steep when compared to some two lane mountain roads, but don't discount them either when they come in 12 miles down grades.

I just drive it, and it seems very capable on any road I've been on. Where you run into trouble with the Prius is when the heavy Colorado ski traffic comes to a standstill traffic jam on the uphill 6% grade on icy snow packed roads. This is where an all wheel drive option would be most helpful.

 

I often use the B mode just to slow down before stop signs if my batter is fully charged. Only problem is to remember to shift back to drive. Fortunately, you can't put cruise on while in B--and the ECO light is off, so there are reminders to do so--as well as the engine winding up as you approach highway speed! Reminds me of using the exhaust brake on our diesel pickups....

 

Nop, GenIII does engine braking in "D" when battery full to provide the deceleration that should be given by electric means (which at the time is not avaliable anymore).

 

I agree, Prius is capable on any road. Downhill every other non-hybrid car starts heating brakes, and Prius goes further without doing it, as long as battery remains not top full. After the descent, 1.5mile full EV is available, or a fuel consumption reduced next few miles if ICE needed. Uphill the throttle pedal is always very easy to deal with, controlling thrust, while other manual gbox cars have to be squeezed rpm every gear, and sometimes a gap is felt (with auto box also), and reducing steering wheel control by missing one hand.

 

This is so true!!!For some reason if the speed of the car remains hi for some time without using the gas pedal, you can hear the engine rev up but with no fuel consumption.B mode is more aggressive...its like using second gear in a manual transmission car ,which actually freaks me out because makes me feel than the engine will explode

 

I use B mode sometime to slow down going around corners. It just makes the car feel like the manual transmission downshift.

 

True, you could use 'B' Mode that way. But why do you want to use it unless battery is full? Regenerative braking, by braking lightly, would capture some of the energy back atleast.

With 'B' you are loosing all that energy in heat at the engine in terms of compressing and thus heating air, and on frictional losses in engine.

'B' Is still better than frictional braking. In frictional braking, you are converting kinetic energy into heat anyways as in B, But you have added break wear, plus tracking problems in poor road conditions.

Telmo744 has confirmed that Gen III uses engine braking when coasting downhill with traction battery full. Thus 'B' in my opinion is only useful in icy snowy conditions.

(Technically another use would be to slow down , as in light braking, again when traction battery is full . Since prius would use frictional brakes instead of regen in such situation. This situation though would is not a very common situation and can well be ignored.)

 

Just for the fun of it !!! I understand all of the regen and all that but still like to do it and makes it feel like a sports car with manual tranny.

Sometimes I just like to drive the car for fun and not worry about regen and mpg and all that other stuff.

 

"B" releases more energy using the engine, compared when in "D" revving (past battery full) that is not so hard. So we must assume that in steep descents, "B" can effectively save more hydraulic brakes wear.

 

i wouldn't use B mode, i would just lightly tap the brakes and use regenerative braking. Even when the battery meter shows full, we know that it isn't because that meter on the dash only shows 30-70% or so of the total battery capacity. The ECU keeps it in the middle to prolong battery life, so don't worry about overcharging.

 

Many times I have filled the battery to the point that an ECU halts further regen. When one is near the bottom of the grade, no problem, just let the brake heat up a little. But when the downslope lasts several more miles, B mode is necessary.

When I know the battery is going to fill, B mode is applied at the top of the hill, before regen even starts.

Prius is adequate, but not as capable at this as any manual transmission car I've owned.

Try coming down Pikes Peak road, behind a manual transmission in probably second gear, an excellent choice for that car. But once my battery topped out and regen ceased, the speed was too low for B mode to spin up the engine fast enough for sufficient drag, so I had to ride the brakes. And pull out for several cooling stops.

Too bad that pickup with Florida plates neither downshifted nor stopped for cooling, until the mandatory brake checkpoint. His brake smell was very pungent.

 

It just means that the computer stops using any regenerative braking function and solely rides the friction brakes... Just like my dad told me to never do coming down a pass. So the B function just uses the engine to act like a brake, just as if you had a stick and put it in 2nd, or an auto and put it in L or 2... The only difference is that the Prius CVT is calling it "B"

With 4 wheel disc brakes on a car this size and weight, you have more than enough braking power to out-brake just about any car on the road even without regen. I can tell you this from experience, as a deer popped out on me the other day a few car lengths away, and i was going about 55. I stopped COMPLETELY and SUDDENLY. Everyone in the car with me was amazed how fast the thing came to a halt with nary a chirp from the tires. This is the best braking car I have ever driven other than my friend's C6 Corvette.

But like every other car since the beginning of car history, if you make the brakes red-hot by riding them down a 5000' elevation drop, its not gonna brake for crap. Thus, after your battery is charged, all you have to do is put it in B and reset your trip odometer so you can see your 99.9mpg from your engine being off the entire way down (unlike every other car since the beginning of car history)

 

I was going to mention Pikes Peak.

Considering it is in my backyard, it is a pretty awesome road. Not steep, but long which I think is worse than steep and short.

Any road with mandatory brake checks is an awesome road.

And as to Colorado having interstate exceptions, I think it is because it was the first section of interstate to be constructed through mountains and they had to do a lot of creative design.

Those sections on I-70 are definately fun to drive as long as you dont get stuck in ski traffic. We generally have dinner after we hit the slopes to let most of the traffic clear out. You may spend an hour eating, but it only makes a 10minute difference in time since you would have spent most of that hour going 2 miles an hour in bumper to bumper traffic. They really need to put in a high speed rail system between Denver and Silverthorn/Vail.