2010 Prius engine braking down very, very steep slope!

So did the prius make it down the hill?

 

If you engage B mode, and your braking does not fill the CHG bar, then you have until the battery is at 8 bars before the friction brakes will engage.

 

Dave - I'm sure that others will chime in, but I think that the way the engine braking works is that it causes the gas engine to spin while letting air into the cylinder chambers to create drag - I do not believe that it involves changing the gearing ratio at all.

Remember that the Prius actually has 3 braking systems - the engine, regen and also friction. Also remember that to maximize the regen breaking on a long downslope that you want to keep the hybrid battery from filling up - once full, regen will stop working so turn on lights, ac, radio, wipers and anything else. On a slope like this I might actually start the car at the top and then let it sit running on battery power until you are at the point where the engine kicks on to recharge the battery (2 bars I believe). At the beginning of the decent you will be using the engine and regen breaking almost exclusively so you might want to slow down to almost a crawl early on - scrubbing off speed and letting the friction breaks do the work and heat up - then you can then let off the breaks a bit and go back to engine and regen - this will allow you to use the friction breaks for more of the decent, but for shorter bursts and give them time to cool.

It sounds like this road is quite extreme - the fact that you say a few cars run off it each year would lead me that conclusion at least.

Is there some reason that you need to drive it in your Prius?

 

I am tracking now. And you have a good point though I suspect your brakes held up just fine.. right (when you got to the bottom)? Will you replace them earlier than the rest of us... probably.

 

Haven't been on this particular road in Wyoming (my trips have been up in the Big Horns, and not with the Prius), but I'd imagine there's a few points where you can pull off and let the brakes cool down for awhile. Might take awhile, but you should get down safely.

If it's a 4WD road, I don't think you should expect the Prius to drive it without some difficulty.

 

Agreed with the use of regen as much as possible, and with dumping power to the A/C or something like that - make sure the temperature is set to the minimum, and Eco Mode is off, for best power consumption. And do stop to let the brakes cool if you're worried about them - it's possible to overheat the battery when there's a large current either to or from it (especially when the battery is near full), causing you to switch to friction and engine braking even when the battery isn't full. You might consider getting a laser thermometer to measure them to know whether it's safe to continue or not. (Though I have no idea what a safe temperature would be.)

Unfortunately, it's not possible for the engine to spin at high RPMs when the car is moving very slowly; doing so would exceed the maximum speed of MG1. But the car will do what it can, even if it's not very much. Alternately, you can try taking the road at 50 mph, since engine braking will be more effective at that speed.

 

My Prius crossed this range twice, on highways 14 and 16, but those paved roads were easier than what appears to be NF-26. An online topo shows a descent of 5500 feet in 30-ish miles, 2500 of that in 5 miles.

The slope appears similar to the Pikes Peak road, so search here for those threads. Yes, a non-hybrid with a bigger engine and low fixed gears has an advantage there. My short Prius answer is that regen braking fills the battery very quickly, and speeds are too low to get much B-mode compression, so brake cooling turnouts are the only remaining answer. I took enough of them that the attendant at the mandatory brake cooling checkpoint was quite happy with my disk temperature.

I have also opened the windows, turned on AC full blast, and turned on the rear window defroster, but didn't figure out whether this added up to any significant braking load.

 

Open the hatchback, too, that will add a lot of drag.

 

If you set the cruise control to a low speed, it will use everything available to slow you down, short of applying the friction brakes. When the battery fills up, it will try engine braking, even if you haven't selected it.

Hey, even the Space Shuttle gets hot on the way down. There's no faking it.

 

At the slow speeds OP is talking about, cruise control won't engage, and engine braking is too weak even with B mode selected.

Additionally, at that steep slope, regen braking is not strong enough, and vanishes when the battery is filled very early in the descent.

 

at lower speeds, it's kinda hard to overheat the prius brakes...

the only time i've had the brakes overheat, is on purposes... it's a several mile downhill road that i used to race/test cars on... keeping at an average of 60 to 80 mph on this downhill 35mph road (quasi controlled road).. extremely hard braking in the corners and ends of straightaways.. it took me twice as long to stop at the end (still pretty fast)

when i was in colorado next to pikes peak, i used to cruise those roads all the time... i never ever had problems there... even racing down mountain passes... unless you're extremely speeding and applying way too much force to the friction brakes alone, you'll never overheat...

take in account.... my overheating was probably mainly the rear drums... the genIII has all disk brakes... (plus i was going from gas to brake.... no coasting or rest..)

btw... all colorado roads i used to cruise were 4x4 roads... when you see a 4x4 that can't proceed without abusing his 4x4 system... it's a good spot for you to stop and turn around. one winter i was stuck on an ice patch on a 40% (probably less?) or so incline... i kept scooting left and right.. made a little bit of a climb but that was my end point for the day... i stopped to rest and play in the snow with friends... a 4x4 jeep came along.. asked if we wanted a tow past that point.. i said no because there isn't enough grip... he chuckled because he has 4 wheel drive... as he proceeded he hit the hillside by sliding into it... grip is grip...

 

engine braking always works... just as a note... i used it every day going over mountain passes at 15 to 25mph... it always helps.. if you don't use it, the car will automatically do it for you when the battery fills up...

 

this should be more in speed limits.:

 

What is the clearance on your Odyssey? Most roads that are described as 4WD roads, I would be leery of taking my Prius because of that issue.

Off topic, many years ago, I really like driving Wyo. Alt 14 in your neck of the woods in my wife's Dodge Aspen (I think it was). But it was paved.

Further off topic, a few days ago, a fawn ran out in front of me and I slammed on the Prius brakes. They worked as advertised--I think the anti-lock feature engaged. Slowed down quickly enough that the fawn got to safety (even though I hadn't quite stopped by the time the car crossed the spot where I first saw it).

 

I haven't confirmed this, but I believe that if you drive in "B" mode for a while (5 - 10 minutes) prior to reaching that hill then the traction battery may move towards the bottom of its range -- providing additional capacity for brake regeneration.

 

Sorry, I'm not familiar with the other nearby roads. How did yours perform on Pikes Peak road itself?

On my trip down Pikes Peak, speed was constrained by heavy traffic. The leaders were deeply downshifted and moving at speeds appropriate for their cars, but very wrong for my Prius. Passing, to move up to a better B-mode speed, was not a safe option.

Yes, engine braking helped. But no, it was not sufficient to keep my car from ramming the car ahead. So I had to ride the brakes, and they were smelling hot.

Not being a racer, I don't know the point where they are too hot. While we were stopped at Glen Cove for other reasons, only one vehicle was ordered into the brake cooling timeout zone, a 3/4 ton pickup with Florida plates. Its brake stench arrived well ahead of the truck.

On mine, there is still a difference in degree. Engine braking is considerably weaker in D-mode than B-mode. I first detected the D-mode version not by feel or sound, but by seeing ScanGauge's RPM spinning up from zero to slightly above idle speed.