Why does ICE come on going downhill at <50 °F?

If the battery is near full this is normal. Don’t fully charge the battery before departing on your trip.

 

Selecting B on the shift lever engages the engine to slow the car. That's the purpose of that position. It's designed to save the brake pads when descending long grades. If you don't want the engine to come on, leave it in D and use the brake pedal to maintain safe speed. By the way, never select N because that will disengage the entire hybrid drive and you'll be relying entirely on your brake pads to control the car.

 

This sounds more like a regular Prius answer. But OP's Prime is a bit more complex than this, and can use B in some situations where the gasoline engine is not intended to spin up.

I'd want to know the level of charge on his traction battery, if too full to take the regeneration then it will spin the engine. But I don't yet know the other issues that may come into play, such as ambient temperature.

 

B mode + EV mode will turn on the engine sometimes.

• If the battery is full
• If the hill you're going down is very long and at relatively high speeds. There seems to be a limit to how much charge the battery can accept, since regen is the equivalent of super fast charging.
• If it's cold you will hit the limit of the battery sooner.

B mode + HV mode will almost always turn on the engine, so you're better off using the brakes for more regen to hopefully get enough charge to get into EV mode. In EV mode + B mode, using the brakes might not turn on the engine if it's a short hill, not too step, and not too fast. But for a longer hill the engine will likely come on because you'll be putting too much charge into the battery too fast with regen braking.

Cruise control in cold weather will also turn on the engine sometimes, but it's not clear when or why. Once the engine comes on, it will stay on until it's warmed up.

Some things you could do that might help:

• Use the charge timer so the battery is warm and freshly charged when you leave in the morning
• Set the charge timer to finish about 1 hour after you normally leave, so the battery isn't completely full

A couple anecdotes:
The road down from Mt. Evans is a winding narrow road with a speed limit of something like 30mph. It descends from 14,000 feet and is relatively gradual. In the summer I was able to get to almost 80% charge (from nearly empty) by the time I got to Idaho Springs, using a combination of B mode and the brakes.

The road down from Freemont Pass descends steeply to Copper Mountain, with a speed limit of 55 or 65. I was barely able to get any regenerative braking whether I used B mode or the brakes. Temperature was around 40, but the battery was probably toasty from the drive up to the top of the pass.

 

I have seen the ICE turn on with a full battery on a downhill, but this winter I am seeing it come on when going down a steep hill. Normally, I get about 10% charge increase going down the hill, but lately the ICE has been turning on. I suspect that there is a rate of charge constraint in place during cold weather.

 

Someone above mentioned:

Maybe you could try driving down the hill in neutral if it's safe. Since the car won't be able to charge the battery, it would seem like there's no reason for it to turn on the engine. Of course it will waste the energy you could recover going downhill, and it will put more wear on the brakes. Also be aware that it might not be legal to drive in neutral.

 

what is 'breaking mode'?

is there any way to go a different direction and avoid the steep downhill?

 

up?

 

I would rather burn a few ounces of gasoline than wear down my brake pads. Gasoline is much easier to replace and cheaper than the pads. Using neutral mode is emotionally satisfying but economically unwise.

 

That is part of the reason. With lower temperature, charging is slower. So when I drive downhill, even if the battery is not full, ICE could still be turned on.

For the same 0.7 mile downhill, I used to get charged by 1.2-1.3 miles, but now with 40F outside, it can only get charged by 0.7 mile. And ICE just kicks in almost every morning.

 

Thanks to all for this interesting thread. My experience is similar to OP, except (1) I do not use B mode going down the hill, (2) when the ICE kicks in the traction battery is only around 75%, and (3) at bottom of hill (with ICE now running), it's somewhere around 80-85%. This is on my morning commute, and it was plugged in to household AC to charge the previous evening with no timer, so fully charged several hours before I start in the AM.

A few comments from above I don't understand-- a couple of folks suggested starting with battery at less than full charge could prevent the ICE coming on, and there was some related talk about "too much regen" going down the hill. I don't see why any of this is relevant-- in my '08 Prius, if I went down a long hill and the battery came up to 100%, it did not start the ICE (I've always wondered what became of the excess regen in those cases). Why would the Prime be different?

 

How much vertical drop were you experiencing on those hills?

Both my 2010 and 2012 Prii spin up the ICE to screaming banshee levels for engine braking (not gas burning) on our 7%, 2000 vertical foot mountain descents. I don't know why your 2008 wouldn't do the same.

Note that these are at highway speed, above the auto-off threshold speed, so the ICE must already be spinning ~1000 RPM when first starting downhill.

 

Based on measurements of elevation and distance in Google Earth, the hill I had in mind when referencing the '08 Prius is 138 m (453 ft) drop over 2600 m (1.6 miles), a grade of 5.3%. That's a state highway, so typically going around 60 mph, either coasting or light touch on the brake pedal, no engine braking. The hill I had in mind re my 2018 Prime ICE kicking on is shorter (1026 m), steeper (7.6% grade) and lower-speed (40-45 mph), and again no B mode.

I have experienced, as you describe, the ICE "wailing like a banshee" in B mode in the '08, which is one reason I rarely use it. It just sounds like that can't really be good for the car (but I acknowledge that impression is uninformed by any deep technical understanding of what's going on). Is it really OK to have the engine sounding like it's protesting so much?

 

Toyotas are really reliable, especially the engines. You have to trust that their cars are designed to be essentially idiot proof. There's not much you can do in normal use that will damage them.

If the engine is cold, B mode is probably not great for it. But it probably still won't damage anything. The oil is synthetic so it flows at cold temperatures. And the car will warm up the engine if it needs to.

The engine on the Prime gets so little use for most people that it will probably last forever. For me about 2/3 of my miles are in EV mode, and then in HV mode the engine is probably only on half of the time.

 

On a 25 mph hill with a similar drop (400 feet), my Gen3 will sometimes barely fill the battery, sometimes not, in D mode with pure regen braking. On the days it does fill, it barely starts winding up the ICE before the road flattens out at the bottom.

At 60 mph, you are getting significantly more air drag than I get at 25, so the battery would be filling less. And at least on my Gen3, the battery gauge displays 8 bars before the battery is really filled. I.e. it seems that an internal reading of 7.5 gets rounded up to 8 displayed bars, and regen continues for a short time longer before quitting and winding up the engine.

So I'm guessing that your battery wasn't quite fillled.

I'm figuring that as only 256 vertical feet, so less regenerated energy is pushed into the battery.

For those of us facing hills with a greater risk of crashing from overheated brakes, this loud engine noise is definitely a better alternative. But even for most of our hills, this risk is mostly limited to commercial trucks, not passenger cars. I don't recall anyone here burning their Prius brakes with less 3000 vertical feet.

On your hills, the brakes will not overheat to dangerous levels, so B is not necessary. And since it would generally prevent the battery from filling, it is even counterproductive compared to leaving the car in D and harvesting more energy.

 

Can someone explain why having a full battery would force the car to switch from EV to ICE?

 

The ICE does not actually start but the MG1 motor spins the ICE to use excess electricity from braking to avoid overcharging the battery.

 

Where's it going to put the electrons from the regenerative braking? The battery is full. It has to spin the ICE to soak up the energy..