B positon on drive selector

That is the "engine braking" option. It keeps the engine in gear to create friction, slowing the car down while traveling downhill. Its fairly popular ICE technique. On some steep roads you will se signs saying trucks must use engine braking traveling down hill.

 

On any automatic transmission car you drive, you'll notice that the car will coast much faster in Neutral than in drive. The reason is that, when you release the gas on an auto tranny (or a manual, for that matter), the transmission spins the engine. This slows the car without using the friction brakes.

B mode in a Prius works like that. When the battery is fully charged, the transmission cannot generate any more electricity and so the tranny must spin the engine instead.

Many Prius owners think it accelerates car charging when going down big hills, but it's the opposite. I don't know to what extent the Prius computer automatically switches between engine braking an battery charging... but I'm pretty sure it will. I've heard people say their battery still charges in B mode, so that tells me that the computer is smarter than the driver.

 

1) It is the law, in the US, you have to offer a low speed gear with increased drag.

http://www.gpo.gov/fdsys/pkg/CFR-2002-title49-vol5/pdf/CFR-2002-title49-vol5-sec571-102.pdf There is no upper limit to when you can use B.

2) Sustained steep downhills can fill the HV Battery, so that you lose regenerative braking. Using B from the summit can delay this. Overheating your friction brakes is bad, perhaps fatal.

3) B attempts to rotate the engine without fuel, making it an air pump to add drag, slowing the car. This will never be as economical as regenerative braking, so if the descent is short or gentle, do not use B. (Use B where your would use L in a 'normal' automatic, almost never)

 

So how would that apply to the Prius? There is no notion of discrete gear ratios. Power from the ICE can be arbitrarily divided between wheels and the generator.

 

If you want to feel what it does go find a long hill and coast down it in "D" then go back up coast down in "B". What you should feel is that you aren't riding the brakes quite so much.

 

It is continuously variable, ( the law did not say they had to be discrete, although in my Gen 2 I think it was every 6 RPMs) so there are multiple forward gear ratios. (675 of them I think)

 

I have a question about this. Not sure if you know... Does the Prius electronically govern this operation? That is, if you engage B mode, will it prioritize battery charging until the battery fills, and then use engine braking? I would assume the same is true with ordinary braking: The system will prioritize battery charging, and then use engine braking when you gently press the brake pedal, and then use friction braking. But I'm only guessing at all of this.

 

No, if you don't press B that is the behavior, if you KNOW you will fill the HV Battery, then press B at the top.

 

I use the B setting when going down a long hill. It works pretty good on holding your current speed.

DBCassidy

 

Isn't B just engaging engine compression, which is essentially wasting energy? I my experience, just setting the cruise control works fine to maintain speed on downhills. Unlike standard ICE cars, whose cruise controls don't attempt to slow you down beyond what the engine braking in Drive provides in idle mode, the Prius can engage regeneration to usefully recover the energy, and since the PiP has a bigger battery than the standard Prius, it can handle longer downhills before having to resort to engine braking. If you actually manage to have enough downhill to fill the battery, then the PiP will use engine baking on its own when the battery is full, without the driver having to set B mode.

 

Being able to use cruise control to go downhill is actually one of my favorite features. You can see a big difference in regeneration when cruising down at 30mph or when coasting down at 30mph. As said before, you shouldn't use B unless your EV battery is topped off.

 

I think that it would be a very rare situation that would advise use of B. On a long downhill, you just use regenerative braking (press lightly on the brakes, or engage cruise control). It takes a LONG, serious downhill to fill the EV battery, which would presumably be empty at the top of a long incline. After the battery is full, then the system will automatically use engine compression braking (B). And only if that is not enough, would you need the friction brakes. Maybe coming down from the summit of Pike's Peak would be sufficient to push you to friction brakes, and indicate that you might want to preemptively use B mode.

I really think that B is there due to lawyers' demands (that U.S. code mentioned), not engineers' demands.

 

Here are my personal observations with B in my PiP..

When in EV mode, as long as you're NOT near "full", B will just increase the amount of regenerative braking. Yes, you could just put your foot on the brake pedal, but I do not know.. it always bugs me when I see people riding their brakes down a long hill.. so I personally will put it in B just so I dont look like I'm riding my brakes.

If while in EV you are near "full", the engine will start. If the engine is cold, it will go through its warm-up routine.. and yes, it will consume gasoline while doing this.

If you are in HV and you engage B, then I believe it works just like a regular Prius (dont know since I never drove a regular Prius in B). The engine sounds like its running at a higher RPM than normal (engine braking). I've never tried this while logging data so I dont know if it consumes fuel. However, it appears from various posts here on PC that the regular Prius does not consume fuel while in B.

The one condition I havent tried is if I am in EV with a warmed-up engine. However, I would think that being in EV mode would make B just increase the regenerative braking until it approaches the battery's maximum SoC.