What's up with the B position??!!

When you use B, the computers are more aggressive about regen, simulating being in a lower gear, and use the engine as a air pump, also simulating a lower gear, but without regen. In addition, the cruise control will not work in B.

Since the air pump wastes energy, B is best suited to long downhills that would fill the HV battery anyway.

 

Hi Bernie. Even when you're in "D" the Prius is simulating the engine braking effect that you get in a normal car. If they didn't apply this little bit of regenerative braking then the Prius would tend to just glide (like in neutral) any time the foot was off the accelerator. Presumably they choose not to make it just glide like that so that it feels as close as possible to a normal car.

If "D" mode provides just a little bit of regeneration to make it feel like a regular automatic transmission, then "B" mode provides a lot more regeneration (braking effect) so that it feels more like a manual in third gear. Also at speeds over about 25 MPH it keeps the engine spinning (without fuel injection) when you back off the gas to provide even more (real) engine braking resistance.

It's predominantly meant for people who like the feeling of extra engine braking (like driving in third gear) in really hilly or mountainous terrain. Technically it's not as efficient as driving in "D", but in very mountainous terrain you often end up generating more energy than the battery can hold anyway (8 green bars), so it may actually be beneficial under those conditions.

 

The important use of "B" is to keep the brake fluid from boiling. If you were driving a semi and needed the jake brake, that's when you'd use "B" in a Prius.

 

We drive in the Rocky Mountains some. Quite often the battery will get deplted when going up long passes. When the battery gets that low cruise control quits, the engine screams just to go 50 or so and you can forget about passing slower traffic.

If I put the Prius in B mode at the start of a long mountain pass, would it keep the battery charged up better? I'd like to be able to pass a semi going 45 mph without holding up traffic behind me. As it is (with a depleted battery), if I get behind a truck going 45 and have to slow down and wait for a break in left lane traffic to pass, it could take miles to get around the semi and by then I have a whole line of cars behind me.

 

No it wouldn't really make much difference to going up hills. Like I said before, it's actually less efficient at storing energy in B mode, but that can be a good thing in mountains because once the battery is fully charged it has to use the friction brakes. So using B mode can delay the point where your battery reaches 8 green bars and so save your brakes to some extent, that's all.

As for improving the going uphill situation in mountains, about the only thing that would help there is a higher capacity traction battery. The problem is that at full assist the traction battery really only has about one minute capacity (eg 40% of 1.3KW-hr lasts only about one and a half minutes at 20kW assist). Since climbing a mountain is going to take more than one minute then the traction battery just isn't going to be able to provide assistance the whole way.

 

I think that it is helpful to ascend the mountain passes at a fairly high rate of speed. For example, if the speed limit is 65 mph then proceed at 65-70 mph.

It seems easier for the Prius powertrain to maintain that constant speed on an incline, vs. traveling at a slower speed, then trying to accelerate to pass another vehicle.

 

No. Using the brake pedal without B mode will better charge the battery. B mode throws away energy. Using the brake puts it into the battery. However, once the battery is full it is full. You can't charge it past the high limit, regardless of mode.

Tom

 

Yes definitely, if you can keep it in a power band where there are no arrows in/out of the battery then that's ideal for hill climbing.

 

Would if I could. But sometimes you get stuck behind a semi or motor home and have to slow down. Then, with a depleted battery, you can't get your speed up again.

 

I had that happen four-fifths the way up a 12,000 foot pass with a fully loaded 2006. Once the left lane opened up, I pulled out and floored the accelerator pedal. Pretty much nothing happened. I kept it floored and the Prius sloooowly got back up to speed. Under those conditions, you can time zero to sixty with a calendar.

Tom