Using B gear in winter?

6 of one, ½ dozen of the other.

In B mode, the car will do slightly more aggressive powertrain braking as soon as you lift your foot from the go pedal. Sometimes maybe that might be what you want; a lot of the time it isn't. (A good rule of thumb for driving in treacherous conditions is "avoid doing aggressive things").

At the same time, the car is a Prius, which means powertrain braking is going to be what it starts with anyway when you put your foot on the brake. So to B or not to B is really a question of whether you prefer that to happen right away when you lift from the go pedal, or only when you push on the brake.

Whether in B or not, using more powertrain braking to "reduce the necessity of [friction] braking" isn't a definite win. There are friction brakes on all four wheels, but the powertrain can only brake the two in front (there's no AWD Prime, is there?). Trying to do the same braking with only half as many wheels puts you twice as much at the mercy of your limited road grip. Also, the two in front are the ones you steer with.

At the same time, the car is a Prius, so it is watching what happens with your road grip, and if there's any problem it's going to switch directly to your four-wheel friction braking anyway.

 

B would be mostly the opposite of what you need for winter traction, and have other negatives. ECO mode “might” help. Snow tires are a big plus.

 

In winter driving with manual transmissions, I've also been known to upshift, to reduce available torque at the drive wheels.

Long long ago I had an automatic car that could lock out first gear and force second gear. This was needed several times to get up through dad's farm entrance gate, in extremely icy conditions, after having to stop to open it. (The other side was level ground, so there was no risk of the car sliding into it during or after closing it.)

 

Shifting to B IS braking. It is a different form of braking only the front wheels. The advantage is (a) puts more regenerative charge into the battery, (b) limits heat buildup in the friction brakes, not often a concern in winter. The disadvantage is, as noted above, if front tire traction isn't great, the fronts may skid (not good), or if tire traction is great the fronts may grip and the rears lose grip and the car pinwheels (really bad). So---use B on slippery roads with discretion. Never use winter tires on just the fronts, always on all four corners.

 

I remember using lower gear on my stick-shift cars during slippery winter road conditions. In fact, I did similar low gear shifting on winterly roads with my previous automatic cars as well. But for PP, B mode is nothing like the low gear on other regular automatic cars. One thing you will find out is that during winter in a colder climate, using B mode will tend to start the engine even when the traction battery is half full. If it is full, then B mode will definitely start the engine even in a warmer temp.

 

When possible, B mode just does more aggressive regenerative braking when you let off the throttle. If the battery is full, then it will do engine braking (spinning the gasoline engine with no fuel). In the former case, energy is recovered into the battery. In the latter case, it is dissipated as heat. In the Prime, there are really not many situations where B mode make sense. I think it was put there to comply with some prehistoric government regulation that cars with "automatic" transmissions had to be able to downshift to keep from overheating the friction brakes on long downhills. On a long downhill, using B mode and pressing the brake pedal lightly does exactly the same thing: more regeneration.

 

Unless you are in a “select low gear now” situation when B mode is very useful

 

This may be true with the plug-in; I’ve heard something like that before. FWIW, with a reg Prius B reduces regen; it’s prime (sorry for the pun) use is to avoid full-to-the-gills battery on protracted downhills.

 

I agree they do "the same" thing, but not quite "exactly the same" thing.

Riding the brake lightly will make some powertrain braking happen. Shifting to B mode also makes some powertrain braking happen.

But B mode is a little more conservative in how much regenerative battery charging it will do; it will be quicker to shunt energy off to engine braking, so it charges the battery a little more slowly. When you simply apply the brake in D, it balances the recovered energy more in favor of battery charging, and usually spins less of it off in the engine.

The idea is if you know you're at the top of a long descent, you can select B, and that way the battery will be charged a little more gently, but probably still maxed out by the bottom. If you stay in D and ride the brake, you'll max out the battery charge earlier in the descent, harder on the battery, and then be stuck with just engine braking and friction the rest of the way down.

 

Since this is a Prime thread, we probably should take some care in distinguishing between HV-B and EV-B.

 

Is there a difference? Basically they are the same car just the Prime has a different battery.

 

In regards to B, that bigger battery makes a difference.

In a standard Prius, going back to the first, putting the car into B "gear" has the system use more engine braking. Meaning the engine is allowed to spin after a higher rpm when foot off throttle and brake. This is engine braking like in any other car.

In the Prime/PHV, the bigger battery can take in more energy, so the car uses higher regen braking when in B, than when normally taking the foot off the throttle. That is, if the battery isn't near full charge. When the charge is near full, it acts like a standard Prius. Which is an issue for those trying to stay in EV mode. If the engine spins up, even for a non-fuel burning purpose like braking, the system switches to hybrid mode, and the engine stays on until the warm up cycle is complete.

In terms of what is happening between the tires and pavement, it probably doesn't matter.

For the OP, I've used B in winter weather in a 2005 Prius without issue, but only when already going slower. I didn't use it to slow down from higher speeds.

 

For degree of actual regeneration into the battery, there is a very real difference. For other behavior, I don't really know or remember well enough, as I don't (yet) have a plug-in.

When B-mode questions come up, I often don't initially don't notice that some are in Prime forums, and point towards HV-B answers for non-plug-ins that don't properly address the plug-in's EV-B mode.

 

It’s the same transaxel so therefore engine braking is the same in both models

 

The transaxle isn't exactly the same, there are a few differences (e.g. MPx powers and RPM limits, sprag clutch). I just don't know if they are relevant here, and need to defer to experienced Prime drivers on that.

It isn't the same battery, so it is practical for the Prime to have regen-only HV-B-mode braking down some very long hills, without spinning up true engine braking. While a non-Prime could also be programmed to do so (and does in at least one corner case), it fills up the battery so quickly as to be less useful, so usually spins up true engine braking immediately.

 

You say “could be” and “ I’m not sure but” a lot in these posts. In the UK the battery’s are the same

 

Agreeing: the difference with the Prime is that there is somewhere to store the energy on the downhill. It takes a pretty serious downhill to fill the Prime battery from empty. The tiny battery on the standard Prius would fill very quickly, to the point of not being worth doing.

FWIW, and not really relevant to this discussion (sorry), engine braking with gasoline engines is not compression braking, like it is on diesels. It is instead pulling against the manifold vacuum with the fuel injectors closed on the input stroke that generates the drag.

 

The Prius Prime/PHV battery has a 6.x kWh usable capacity.

The non-plug in Prius has less than 1 kWh of capacity, regardless of chemistry.

When the battery isn't near fully charged, the plug in model just increases regen braking when in B. That is done with M/G2, which is between the transaxle and wheels. When the battery is full, it goes to true engine braking. The battery is big enough that may not happen though.

The NP Prius battery has no where near the capacity to take in the charge of sustained downhill coasting, so the system goes right to engine braking. It actually reduces the amount of coasting regen that happens in D.