Cruise control and fuel economy

Autoist · Aug 5, 2016

The car doesn't seem to apply brakes when CC is engaged going down hill, only "engine braking" to keep at the speed set, no fuel usage.

CR94 · Aug 8, 2016

MrMischief said: ↑

When in the mountains I've taken to cancelling cruise when I know the next uphill is getting relatively close and there's enough room in traffic to do it. I then let the car build up speed as it coasts up to whatever I feel comfortable with so the momentum carries me farther up the next grade before the ICE has to fire up to make it the rest of the way. I have nothing to say this improves gas mileage, but it kind of has to, right? ...
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Right, if you don't needlessly brake (whether friction, regenerative, or "B" type) on the downhill, you'll get a little farther beyond the downhill before needing to burn fuel.

econo-one · Sep 12, 2016

MrMischief said: ↑

When in the mountains I've taken to cancelling cruise when I know the next uphill is getting relatively close and there's enough room in traffic to do it. I then let the car build up speed as it coasts up to whatever I feel comfortable with so the momentum carries me farther up the next grade before the ICE has to fire up to make it the rest of the way. I have nothing to say this improves gas mileage, but it kind of has to, right? I don't really do this in say Wyoming on I-25 because the car doesn't really pick up any speed freewheeling on the hills there, but I do it a lot in the mountains of Colorado.

It seems to me that B-mode just free spins the ICE as fast as it can but does not feed it any fuel. The oil pump is providing pressure at that point, I'm assuming that the coolant is circulating, just no gasoline being fed into the cylinders. This might cause extra wear, but I expect it to be minimal, if even measurable.
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MrMischief said: ↑

When in the mountains I've taken to cancelling cruise when I know the next uphill is getting relatively close and there's enough room in traffic to do it. I then let the car build up speed as it coasts up to whatever I feel comfortable with so the momentum carries me farther up the next grade before the ICE has to fire up to make it the rest of the way. I have nothing to say this improves gas mileage, but it kind of has to, right? I don't really do this in say Wyoming on I-25 because the car doesn't really pick up any speed freewheeling on the hills there, but I do it a lot in the mountains of Colorado.

It seems to me that B-mode just free spins the ICE as fast as it can but does not feed it any fuel. The oil pump is providing pressure at that point, I'm assuming that the coolant is circulating, just no gasoline being fed into the cylinders. This might cause extra wear, but I expect it to be minimal, if even measurable.
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I have a scan gauge on my 2016 Prius and it definitely shows fuel consumption when going downhill with the engine spinning. The fuel gauge on the dash may be pegged at 150 mpg, but the scan gauge show gas mileage at somewhere between 50 and 100 mpg. So it appears that fuel is being sent to the engine. This is a shame, because engine braking is so minimal going down long grades after the traction battery becomes fully charged. I find that even putting the transmission in "B", the engine spins faster, but any additional compression braking is minimal--barely noticeable. Thus on long downgrades I end up using the brakes far more than I'd like.

tzx4 · Sep 13, 2016

dangrass said: ↑

Yesterday I drove the Prius from Oakland, CA to Donner Summit and back. For those who don't know, Donner Summit is the high point of interstate 80 at the crest of the Sierra at 7,000. So, sea level to 7,000 ft.

I found that the car dealt with the climb reasonably well, regenerating/EVing whenever it could and doing a fine job of blending gas and electric power. Fuel economy to the summit was a tad under 53. Heading back down the car got about 73 mpg, for an average of about 63.

What I noticed coming down was that the car behaved very differently with the cruise control engaged than not engaged. When not engaged the car would quickly switch to EV mode and would gradually speed up, requiring braking to keep it within reasonable limits. With the CC on the car would tend to spin the ICE rather than freewheel, presumably to help maintain the desired downhill speed.

While this level of automation is kinda cool, I found myself disengaging the cruise control to encourage EV operation and benefit from additional momentum when there was a small hill coming up.

Has anyone done any analysis on this and come to any conclusions as to fuel economy implications of using CC on long downhill runs?
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That is my experience too. My former Prius c behaved differently. It would fill the battery, and then spin up the motor on long descents. The '16 Prius, not. Considering that my 550 pound heavier '16 is getting overall average 63 MPG is 6PMG better than the c, it is obvious that Toyota engineers know what they are doing.

That_Prius_Car · Sep 26, 2016

I did the same with my 2016. Driving to Pittsburgh the other day, I engaged cruise control and noticed it would not go into EV. Even on slight downgrades where it's not braking, but just giving a little bit of throttle, it spun the engine, so I turned off the cruise in favor of the engine not spinning and using any oil life.

Cruise control and fuel economy

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Cruise control and fuel economy

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