Pulsing brakes at the bottom of a long steep hill

I think what fuzzy1 meant is that the computer will never apply friction for you if you're not touching the brake pedal. In other words, if you are descending a hill with your foot off the brake but either in B mode or cruise, the computer will use a combination of regen and engine braking to control speed, but it won't do any friction braking on its own initiative.

Once you have put your foot on the brake, you've given the computer permission to use all means at its disposal, which it then does, in a constantly recalculated balance taking into account how hard you are pressing, how much battery headroom is available, what the road surface is doing, etc.

Now, the rule that the computer won't use the friction brakes with your foot off the pedal gets relaxed, I believe, in some models with adaptive cruise or collision avoidance.

The v and c were introduced in 2012 and show the most genealogical connections to Gen 3.

 

Chapman covered it.

Those graphs are for what the computers do when you push the brake pedal. I was referring to foot-off-brake operation at my trim level, which has only regular cruise control, the car never engages the friction brakes without the driver's foot on the brake pedal. Though as Chapman notes, higher level features such as adaptive cruise and collision avoidance can apply friction brakes to avoid or reduce the speed of a crash. And these features are moving down to lower trim levels on newer models.

Maybe I should also mention traction control and stability control as also using friction brakes, but only momentarily at single wheels for other purposes, not sustained all-wheel braking to slow the car.

 

No, I have never experienced anything I would describe as 'pulsing' brakes as a consequence of regen capacity, etc.

The pretty much canonical response to a 'pulsing brake' question would be 'have you checked your rotor runout?'.

But maybe if you said something more about what you're describing as 'pulsing'. For example, what would you estimate as the period of the pulse?

 

Sometimes the best recourse is to pull over at a vantage point; enjoy the view for a while.

 

You can get that idea from the following posts in that thread where you found the diagram, where I was asking Elektroingenieur about the puzzling things in Gen 4 (the extra pumpy bits in the actuator, and the gap hold valve) that Gen 3 hasn't got. Otherwise it's pretty similar.

 

The battery gets too full for further regeneration, but it never gets "too full" for simply being a battery in a hybrid car. The computers don't allow that to happen.

If you want a car that regenerates all the way down that hill, then you don't want a hybrid, you want a true plug-in car with its much larger battery.

This case has several hallmarks of overheated brakes, so I'd chalk it up to pilot error. A Hawaiian member here, with a similar descent elevation in a much warmer climate, needed to replace his overheated brakes twice before learning to use B mode.

This "Trucks Gear Down" sign at the top of Strathcona Parkway should indicate to most drivers of traditional vehicles that a downshift is either necessary or at least advisable. For a Prius, a downshift means shifting to B-mode:



I've experienced pulsing brakes from warped rotors, in an old car model susceptible from lack of proper matched lug nut torque, not from overheating. Old family mechanical 'wisdom' had never mentioned that torque mattered. That incident got me started in properly torquing them in all my cars.

Pulsing only in the lower section of the hill, not at the top nor after cooling down beyond the hill, sure sounds like possible temporary heat warping of the rotors. I've heard of others getting warped rotors from overheating, but those were not temporary.

PS. 2nd warning sign, same hill:



Continuing Street View down the hill, I see another 8% grade / gear down warning, another truck brake check pullout, and another 12% grade / gear down warning.

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Another note: on the steepest sections, B mode engine compression will produce a high RPM with loud noise, reminiscent of a huge vacuum, or jet engine, or screaming banshee. This is normal. The computers won't let it reach redline, and with no fuel-burning power strokes, it is actually under less stress than when you climbed the same hill. So don't disengage B mode for fear of engine damage.

And you may still have to use the brake pedal some, but it will be far less than on your D- and N-mode descents. On the shallow and flat segments, you'll have to give it some gas pedal too, to maintain speed, but there is no reason to disengage B mode until the hill is done.

 

You’ll sometime see runaway lanes too. They’ll end in a stretch of gravel, intended to bog down a vehicle to a stop in a controlled manner.

Runaway truck ramp - Wikipedia

 

We aren't talking about slick roads.
I did NOT say that they are identical.

Whether it is a "good idea" on a twisty decline depends mostly upon what speed you set.
For the person who is put off by the early engagement of B mode, it might be a good compromise.
Once it engages to slow the vehicle down, there is little difference.

 

We are talking about a road down from a ski resort, during ski season. Such roads are very commonly not bare and dry.

These typically have a minimum set speed. For long straight-ish sections, OK. For others ...

 

The annoying physicist in me (but really, there is a point beyond being annoying!) would point out that letting the speed go up really won't do anything to "burn off" that potential energy. What letting the speed go up does is convert that potential energy, roughly without loss, into kinetic. You still have that energy needing to be burned off when you reach the next switchback or slow spot, and the car still has only the same three tools for doing so.

But letting the speed go up means that, when you do reach the next point where you need to burn it off, you'll have to burn it at a higher rate, and that rate may be harder on the car, or beyond what the car can do.

For example, it's not uncommon to find that you can set the cruise at a certain speed but find that it can't hold your speed down to that setting on downhills, but if you set it to a lower speed, it can. The difficulty in the higher-speed case is that it is needing to burn off energy at a rate higher than (regen + engine braking) can supply, but at the lower speed, the rate is manageable.

 

Not sure the engineers would let that go: a car where pressing the brakes too hard could damage the rotors.

Protracted, heavy brake use will take a toll over time yeah, but the cars are designed to handle heavy braking, emergency braking. There’s several things that can “let go”, well before rotor damage. First of tire traction (the car will skid), and in a lot of modern cars anti-lock braking.

Look at it this way: if there’s a driver behaviour that can easily and irrevocably damage the car, it’ll never leave the factory. Or it shouldn’t.

 

I wonder if something more than I intended is being made of my comment in #32 about burning power at a higher rate being possibly "beyond what the car can do".

I was not in any way suggesting that damage would be done or magic smoke would be let out. Simply that whatever is "beyond what the car can do" isn't going to happen.

If you are descending a certain grade at a certain speed and trying to use cruise or B to maintain it, and the rate of potential energy change is beyond what the car can absorb in regen / dissipate through the engine, the speed holding isn't going to happen, and you will pick up speed. If you choose a lower speed to maintain, the rate of energy change will be within what the car can manage, and the speed holding will happen.

If you pick up a bunch of extra speed and then have to jab the brake pedal harder before the next switchback, you will be both pumping amps harder into the battery and heat harder into the rotors. That's not going to make either go kablooey right there on the roadway, but it just in the grand scheme of things might not be the way you'd want to routinely employ the car.

 

I am going to dispute this.

To a first order (neglecting second order effects such as differing I^2*R losses of different battery charging strategies, and air drag losses from different speed profiles), absent the engine compression drag of CC/B-mode, then friction braking & charging strategy is irrelevant to the energy and heat outcome. Regardless of your profile, the end result is the same.

This is a matter of the First Law of Thermodynamics, a.k.a. energy conservation. The gravitational energy to be processed is fixed by the vehicle-cargo mass and elevation change. Braking/charging profile isn't part of the equation.

The energy that can be accepted by the battery is a function of only its size and state of charge at the beginning of the trip leg. Charging rate profile isn't part of the equation. Whether charged rapidly or slowly, it will absorb the same amount on a tall hill like this.

Conservation of energy requires that all the gravitational energy not put into battery charge, be turned into friction brake heat. Since the other elements of the equation are independent of braking and charging profile, that forces the total brake heat element to also be independent of braking profile. It doesn't matter what your brake pedal strategy is, the total friction brake heat will be the same!

And since there will be little opportunity for the brakes to naturally cool between pulses, the resulting brake temperatures will also be about the same. (Second order effects may create some differences, but they will remain a small part of the overall picture.)

The only easily practical ways to alter this hot brake issue are:

(1) Increase the trip time to allow more natural cooling, i.e. take some brake cooling stops, the road has two turnouts for exactly this purpose;

(2) Divert much of that gravitational energy to another outlet, specifically engine compression braking. Cruise control set low will work on the straight-ish legs when the road is bare and dry. But for the curves and switchbacks, or when the road is slick, stick with B-mode.

After writing that response, I went skiing today, 230 miles from his hill, and at only a bit higher altitude.

The last 8 miles of the climb were almost entirely on hardpacked snow and ice, hardly any asphalt was ever visible on that segment. It was fairly slick. And a common condition during winter in the vicinity of ski areas.

For the descent, the day's traffic had worn and melted a bit over 6 miles of that down to bare asphalt. Still very wet. The rest remained as packed ice and slush.

 

I suppose that first-order analysis is about right, but I remember that the U. Mich Transportation Research Institute found that the higher-order effects made a bigger difference than I expected, and came out in favor of repeated snubbing rather than a constant light drag. The light dragging caused heat-discoloration to a seven times greater area of the drum surface than the repeat-snubbing strategy, in their test.

All that said, their main recommendation was simply to

That is, of course, possible even without turnoffs, if you're willing to go slowly enough (and don't have impatient drivers behind you). The rotors don't only gain heat from your braking; they do also shed it, and if you are converting gravity potential at a slow enough rate, the shedding can keep up. Might be really slow with a heavy load on a steep grade though. And at the lower rate your engine braking is able to handle a bigger fraction of the job.

 

Pulling over occasionally will have a couple of benefits: allow the hot-heads to get by, and give the brakes a rest.

 

For popular ski hills, the impatient drivers behind can easily create a very serious conflict with the slow-enough-to-shed-the-heat approach.

Most places have laws requiring slow vehicles holding up X or more vehicles to take turnouts to let the trailing vehicles pass. The states where I've lived set X to 3 and 5.

Of course, turning out to let them pass, is also a good time and place for cooling stops. Most of the faster vehicles are avoiding brake overheating by being adequately downshifted. Though we can smell a few that are not.

 

I’d slow to a crawl and let them by with even a good straight stretch with nothing oncoming.