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Cruise control and fuel economy

Discussion in 'Gen 4 Prius Technical Discussion' started by dangrass, Aug 4, 2016.

  1. dangrass

    dangrass Junior Member

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    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?
     
  2. Mendel Leisk

    Mendel Leisk Senior Member

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    Sounds like cruise is employing B mode some. That could be good, especially on extremely long down hill runs.
     
  3. dangrass

    dangrass Junior Member

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    Yes, that's what it seems to be doing. Certainly helps maintain the desired speed, but I suspect isn't helping fuel economy.
     
  4. krmcg

    krmcg Lowered Blizzard Pearl Beauty

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    battery management is more important than gas mileage
     
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  5. CR94

    CR94 Senior Member

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    That's evil, when the grade is not steep enough to require braking. That behavior wastes time, wastes fuel, and wastes wear 'n tear. I don't think cruise control has any business taking over braking (including aggressive regenerative braking, as well as "B" braking). If a downhill is so steep, long, and dangerous that braking is actually needed, the driver should wake up and take control.
     
  6. Mendel Leisk

    Mendel Leisk Senior Member

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    I've used cruise heading downhill, but I did feel two little bumps growing on my forehead. :whistle:
     
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  7. DonDNH

    DonDNH Senior Member

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    I love the DRCC and use it when on the open road.
     
  8. Ente

    Ente Junior Member

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    I only use cruise control when the road is flat, because I like to reduce the amount of power going uphill and then increase going downhill while cruise control would try to maintain a relatively steady speed.
     
  9. dangrass

    dangrass Junior Member

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    ...but only when you are using cruise control?
     
  10. dangrass

    dangrass Junior Member

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    This is the first automatic transmission car I've ever bought and I have to say coming from a 6 speed Jetta TDi there's a definite feeling of loss of control with the Prius. Having said this I've noticed in other peoples' automatic transmission cars that I've recently driven, a Subaru Outback and a Honda Accord, both which have real CVTs, that they automatically downshift when going down grades under cruise control. Actually, they seem smart enough to downshift even without cc. So, I guess this is just the way life is these days.
     
  11. krmcg

    krmcg Lowered Blizzard Pearl Beauty

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    Always
     
  12. dangrass

    dangrass Junior Member

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    Right...so I'm still unclear as to the fuel economy implications of cc-enacted B mode versus freewheeling mode.
     
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  13. William Redoubt

    William Redoubt Senior Member

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    The Prius does not have an "automatic tranmission" in the sense any other car does. It is a single speed car and does not shift. So no need to be automatic. Power is redirected to differing tasks via the power split unit. While the car may feel different there is no loss of control. If anything, there is maximum control, as the car takes care of itself to deliver the right amount of power and efficiency.
     
  14. dangrass

    dangrass Junior Member

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    Yes, not a traditional automatic, that's for sure, but certainly not a manual transmission. The point is that when compared to a manual transmission vehicle there's a loss of driver control. As you say "the car takes care of itself..."

    While there's no way that the Prius could be a manual transmission car what with all the drivetrain trickery going on, for those of us accustomed to manual transmissions there's a distinct feeling of loss of control.
     
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  15. Mendel Leisk

    Mendel Leisk Senior Member

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    Yeah, regardless of vagaries of what's in the transaxle, it behaves like a Continuously Variable Transmission, ie: an automatic without distinct shift points.
     
  16. aforkosh

    aforkosh Active Member

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    Actually, one of the only times I used cruise control on my Gen 2 was to keep the car under 75 mph on the long straight downgrade north of Mojave, CA on State Route 14. The road is straight, boring, and uncrowded with a mild downgrade (maybe 3% or so). I got tired hitting the brake to control the speed, so I just let it sit in Cruise Control for several miles.
     
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  17. Autoist

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    Here is the observation I have seen via the instantaneous MPG gauge. (didn't have external scan gauge tool attached)

    Going down hill when in CC the engine will spin to act in braking mode, and not fuel is consumed (instantaneous MPGs are pegged at the top, 100). This appears to be done to keep the car at the set speed. This is very similar to the B mode on the shifter. It is also done when the battery indicator reaches full or nearly full.

    So I say it is save and economical to leave it in CC when going down hill. (You may encounter grades too steep and despite the engine spinning VERY fast you will still gain speed, then you will have to take over and brake.)
     
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  18. MrMischief

    MrMischief Active Member

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    I don't want cruise engaging friction brakes while going downhill, but a couple of my cars won't downshift to hold speed going downhill and it irritates the crap out of me. All cars should downshift or use regenerative braking or the b mode when required to maintain speed. If it can prevent you from having to get on your friction brakes it is much safer.
     
  19. CR94

    CR94 Senior Member

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    The same as needlessly braking down hills to the same extent with conventional friction brakes (aside from non-fuel-economy implications, which clearly are not the same).
     
  20. MrMischief

    MrMischief Active Member

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    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.