Prius B mode during engine break-in?

Unless you panic stop, the pads don't touch the rotors at speed. Regen! Braking!

 

That's something they should stress more after a brake pad replacement, say a card on the dash or something: you always want to take it easy on brand new pads, for 100~200 kms or miles.

 

With a protracted downgrade, enough to fill the battery, you could start using the pads, a lot.

 

That is a serious overstatement. A lot of people, not just my spouse, have normal braking habits that easily exceed the regen limit and break over into friction pad mode.

For perspective, the Gen3 regen braking is limited by the battery to 27 kW, anything more requires pads and rotors. Panic braking at highway speed can be 500 kW, so even moderate braking goes well above 27.

 

Yeah that is true, but going down for 1-2 miles, the battery gets fully charged and no more regenerative braking then the ICE is used to brake the car, even while in drive with the foot off the pedals.

Most of the time there's no need to use B, can just regen brake the whole way and occasionally engage the friction brakes...but when you regen to a full charge and still coasting down a steep hill probably fine to use B.

 

B is fine in the hills. If it was bad during break-in, Toyota would have added yet another caution to the manual.

 

This weekend I traveled to the hills to better understand the car's behavior.

On the way down I decided to use position B to regenerate the battery and after it was charged the engine started to make a very loud snore.

A snore similar to riding at 120km / h on the second gear of a manual car.

When I shifted the gearshift back to D, the engine returned to normal.

I repeated the process a few miles ahead when the battery dropped, and again when it was in position B and the battery full the engine roar became loud again.

I hope this is normal behavior.

 

Completely normal. Some people get distressed by the engine sound but that’s just the way it is. It can sustain that all day every day if you need it to. Fortunately most hills aren’t that long.

 

Yes it is. It is a bit alarming. I would only use B on long, sustained downhills, say a km long downhill or more. Try to keep your speed down too. If the downhill sometimes becomes flatter, to the point that you would need to give gas to maintain speed: shift back to D, for just that flatter portion.

Note that B charges the battery less, and that is the primary reason for using it: to increase the time it takes to fully charge the battery, so that you delay the point at which the car ceases regen braking and relies completely on friction brakes, possibly overheating them, if the hill is long enough.

 

"B" won't regenerate the battery much at all, if at all. I rarely use it - in "D" it will do much the same on downhill once the battery is full. From what I gather, it spins the engine to provided resistance to save the Brakes. The Manual unfortunately doesn't say much about it - apart from:

 

The other side of the myth used to be the opposite of what American-car engine break-in procedures were - the myth, or theory, was:

Japanese cars' engines last so long because they are hot-rodded onto and out-of the ships that bring them over from Japan. The thinking was that the hard initial use by car movers / boat unloaders was seating in the pistons earlier/better.

Now this isn't a myth, and you may be approaching it the wrong way.

Every time I put on new brake pads to one of my own vehicles, I wait to drive it until the middle of the night when nobody is out on the streets. Brake-pad bed-in procedures are hot and heavy, with cooling periods in between absolutely melting brake pad material to the new, or old, rotors.

The earlier one beds in their new brakes (assuming a clean braking surface), the faster they attain the shortest stopping distance, and most brake friction, that that system will produce.

But, again, we're talking panic-slows from 55 or 60 MPH to 5 MPH (not coming to a complete stop), then letting off and allowing everything to cool for minutes before repeating the procedure. You may not have been able to do that on the route home.

 

Most if not all manufacturers say to go EASY on brand-new brakes.

 

In a vehicle with a computer moderating everything between your right toe and the engine throttle plate, I didn’t feel the need to do much for a break-in procedure. Anything Toyota felt to be critical could/would have been done via automation before I ever laid eyes on the car. And if the prodedure needed to continue longer than Toyota’s ownership of the inventory? Again, the computer would be doing that for them and me. They can continue to print the manual with statements like “drive easy” and ”use varying speeds” and it will work out fine for everyone.

Racing brakes need bedding in, and a racetrack is the perfect place to do it. On the other hand I don’t think this is essential for street cars.

Quite a lot of engineering has gone towards making safe brakes without any need for bedding-in whatsoever, and I have faith that a lot of that work has been successful. I have had a few cars that benefitted from bedding-in after a problem was discovered, and I have no problem doing it when it becomes necessary… but I think it’s wrong to assume that all cars always need it.

 

I completely filled my battery with B-mode regeneration numerous times this week, on long steep downhills while supporting a bicycling event. Though it would have filled up faster with regular light foot-brake use in D-mode, i.e. no engine braking mixed in.

Once the battery is filled, D and B may be conceptually similar on engine braking, but have a HUGE difference in degree. My past testing on a particular grade found B giving almost the full braking needed to keep speed steady, with the engine screaming at 4600 RPM. D provided much lighter braking, nowhere near enough to keep speed steady or safe, while the engine spun at a measly 2000-ish RPM.

 

I tried B mode on a down-hill off-ramp from the main highway. I didn't have anyone behind me, and was going to have to either stop or slow down for the next road anyway. It helped charge the battery up a little more (one week of owning it, and the battery has never been near full - is that normal?), so I may play with it again on higher-speed off-ramps that are down hill when nobody is behind me.

Begs the question - is there any value in having the battery fully charged? Is it OK being in the mid-range for most of its life?

 

Normally, just let it do it automatically. I understand that "B" Mode charges at a much SLOWER rate, not boost it. I think you'll find that the vast majority of Hybrid drivers have never or rarely used "B" Mode except in the very limited situations as described in the manual.

The battery will vary between nearly empty and nearly full over the course of a trip.

 

The Owner's Manual has near zero explanation of B mode (in it's 700 plus pages). B mode efficacy and purpose has been mostly pieced together by discussion here.

 

Oh, OK. I threw it in B mode, and thought it seemed that the dash graph was showing an increased charge rate on that down ramp, but I must have been mistaken.

 

B mode is adding engine braking to the mix, reducing regen braking. Consensus seems to be save it for long downhills, where there's a danger of fully charging the battery, having regen braking stop, and all the braking is taken over by friction brakes, and a long downhill still to come. Anything under a mile of steep downhill you'll never run into trouble just using the brakes.

I've used it on a local steep downhill, that's about 3 kms, but even that is borderline, not sure if it's needed. I use it coming down Mount Seymour, a local ski mountain, and there I'm pretty sure it is needed.

FWIW, I've never seen the regen indicator shut off. Even with Seymour, where the battery will be showing completely full (long before you reach the bottom), it'll continue to indicate charging, all the way to the base of the hill. In that scenario I'll switch to B when it's steep, back to D when it periodically levels out, enough that I need to give gas.

 

You will never need B mode in FL, you have no 600 foot vertical descents. (Vacation in the Great Smokies to try it out. Expect it to sound busy!)

In the Gen2, if you were traveling under 24 MPH, you could B mode on regen alone, above that it turned the engine as an air pump. I am not sure the limit on a Gen4, you usually can hear it spin.

Ideally, the battery is never 'full', as that implies you have no regen braking ability: Where would the electricity go?

In winter running the engine for heat, long downhills, and rapidly stopping from very high speed can fill it; the computers will then work on using it up.