Fuel economy, hills, and cruise control

Discussion in 'Gen 3 Prius Fuel Economy' started by CivicQc, Jun 6, 2010.

  1. CivicQc

    CivicQc The world needs more prius

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    We often hear that cruise control is good for flat road, but not for hilly areas, as the CC system makes the engine work too hard to keep a constant speed when going uphill, which kills fuel economy.

    Over the past 2 months, I have done a little experiment. I had to cross a mountain area 4 times (2 return trips), the road is 200 km long, mostly uphill on the first half, mostly downhill on the second half. Maximum altitude about 800 m.

    On the first return trip, I have not use CC. I let the car slow down to sometimes 70 km/h when going uphill, and used gliding when going downhill as much as I could, as long as there was enough slope to preserve speed. Result: 4.1 L/100km on the way out, 4.0 L/100km on the way back. Those are displayed results.

    On the second return trip, on the way out I have used cruise control. So speed was always about 90 km/h, uphill and downhill. Result was: 4.1 L/100 km.

    Although this test was not scientific (it would need to be done again, in more "controlled" conditions), I believe that in the Prius 3, the fact that CC regens when going downhill compensates a bit, and makes CC appropriate even for hilly regions.

    Opinions? Anyone had a similar experience?

    (by the way, I was quite impressed with the fuel economy I got for those trips...)
     
  2. mainerinexile

    mainerinexile No longer in exile!

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    I agree that CC works better than might be expected in hilly terrain. Personally, I like to think of the Prius as an HGEV (hybrid gravity electric vehicle) in hilly terrain, and can usually gain back my mileage on the downhill--sometimes even improve my mileage rating by coasting. The problem with CC is that it can often really rev the engine just as you come to the top of the hill, so I take it out of CC frequently on uphills, or tap it for 1-2 mph slower, to prevent this.
     
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  3. mad-dog-one

    mad-dog-one Prius Enthusiast

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    CivicQc:

    If your downhill glide using both methods was about the same at 90+ KPH, while the uphill segment was faster using cruise control, then you got the best mileage and also covered the route in the least time using cruise control.
     
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  4. ItsNotAboutTheMoney

    ItsNotAboutTheMoney EditProfOptInfoCustomUser Title

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    The human has an advantage:
    - being able to anticipate terrain ahead.

    The CC has two advantages:
    - precise control of the throttle: a person driving has to select the correct throttle position and can easily misjudge it. A person can also fail to keep the throttle absolutely steady.
    - sensing change of speed: the CC will know quickly whether there's a change of speed and can respond quickly.

    CC can also use some tricks to help:
    - tolerance - by allowing some variation in speed (and I notice the Prius varying by +/- 2mph, more in one significantly undulating section) it can "smooth" the road and keep consistent throttle.
    - guessing - if it assumes that a hill is "normal", it can respond pretty quickly to acceleration following deceleration and reduce the throttle without too much delay.

    I would expect two things would particularly hit CC economy:
    - Large variation in incline: would cause frequent throttle changes.
    - Speed: the higher the speed, the more likely the CC is forced to use the power zone and the less efficient it will be.
     
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  5. nparker13

    nparker13 Member

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    Interesting, I did a recent trip and decided to use cruise control the entire trip. I beat any previous record at 59.1 MPG indicated over about 150 miles. I used DRCC but there werent very many other cars that triggered the radar. I also tried something when going down a hill, i applied slight pressure to remain at a 0 energy state (allowing it to collect speed past the cc limit temporary. I held it there until the speed burned down and cc began to kick in). I drove at 55mph the whole time, and it was VERY hilly (CT15 & I91 in CT). Part of the reason I think it was so high is because of the mental speed creep. I find myself wanting to go faster if im driving manually even up to 65, but cc keeps me under control.
     
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  6. tumbleweed

    tumbleweed Senior Member

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    I seem to be able to beat the CC most of the time and the steeper the hills the better I can do. But after driving a 100 miles or so I sometimes get tired of it and just turn the CC on and take a mileage hit.

    I use the CC most of the time on busy freeways because it maintains a steady speed and that's what other drivers expect, which probably makes it safer.

    But on back roads through the mountains I can do better because I tend to let the car coast in "warp stealth" up to whatever speed I can coming down the hills and then use that velocity to help get up the next hill. I think that's a more efficient use of energy than using the down slope to cram the battery full then pulling the Amps back out again.
     
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  7. Tideland Prius

    Tideland Prius Moderator of the North
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    There's a highway near me that has a 80km/h limit and is undulating. With CC, I have seen it do 2.5L/100km on the slight downslope bits on the instant FE bar. However, I tried finding the SHM and I'm able to hold 1.5L/100km (so 3 bars on the instant FE) a lot longer (pulse to 90km/h, coast to 80km/h) and use the slight downhill sections to accelerate and coast on the flat and slight uphill parts.
     
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  8. cit1991

    cit1991 New Member

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    Speed is a killer of consumption too. When you do these trial runs of CC vs. manual, could you also look at average speed?

    I always wonder about pulse and glide vs. CC at the same average speed...or are the pulse and glide numbers better because you are going slower?
     
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  9. CivicQc

    CivicQc The world needs more prius

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    Good point. Although in this case, when gliding downhill, I go much faster than 90 km/h, while when using CC the speed remains constant at 90. Overall the average speed might be similar for both situations since CC keeps speed constant when going uphill, but I will make the check - thanks for pointing that out.
     
  10. Tideland Prius

    Tideland Prius Moderator of the North
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    Depends... if it's a slight downhill, P&G wins cause you're holding speed with the engine off rather than cruising with the engine on at low fuel consumption (2.5L/100km like I said)
     
  11. hobbit

    hobbit Senior Member

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    I go on 15 through CT routinely myself, and I would *never* use
    CC on that road. Too many surprises, as it's one of those older
    highways with really short ramps. The hills are fun, particularly
    toward the southern end, as it's a sort of manic-depressive game
    of 3000 RPM, warp stealth, 3000 RPM, warp stealth, ... but all
    closely controlled and timed.
    .
    _H*
     
  12. Micnbrb

    Micnbrb New Member

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    Hi
    I use the CC also. I watch the MPG graph and when it hits 75 MPG on the down hills I click up the CC lever 1 MPH. I keep doing this until I reach the maximum speed I want to go at the bottom of the hill. Now I've built up some momentum for the up hill where I click down the speed 1 MPH each time the bar hits 25 MPG which has increased my mileage from 43 to 50 MPG in very hilly Westchester NY in my 2005 Prius.
     
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  13. phoenixgreg

    phoenixgreg Senior member

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    I'd sure like to do that but we have state patrols hidden at the bottom of hills and mountains here. They'd love to pick off someone coasting at high speed just for the sake of the ticket. But I did learn something about my CC that wasn't apparent in the instructions: If you set it for the speed limit and you are going down a hill/mountain faster than the set speed, the CC will engage engine braking automatically to slow you down. That's cool - all my American cars would just coast out of control.
     
  14. kgall

    kgall Active Member

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    I've wondered about this:
    In my old automatic transmission cars, going down a steep hill always caused me to speed up whether or not cruise control is on.

    But in the ECVT Prius, being in cruise control keeps speeds steady even downhill, whereas the car speeds up when out of CC if the downhill slope is steep.

    Why does this happen? Do ECVT or hybrid cruise controls work differently?
    Or is this an illusion, and they don't work any differently and it's all in my head?
     
  15. tumbleweed

    tumbleweed Senior Member

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    The previous generations of Prius behaved pretty much like your old auto trannie cars as far as the CC was concerned. The Gen3 Prius uses regen braking to slow you down when descending. I think it may use friction brakes too if needed to keep the speed at the set point. Sometimes a good feature because it can keep you within the speed limit; but it may be more energy efficient to let the speed build up and use the extra velocity. You can do that by using the throttle to keep the HSI out of regen.
     
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  16. marvmark

    marvmark Junior Member

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    I just made a trip diagonally across the state of Ohio. I used AC and CC in both directions. I set the speed to 61 mph on the route going SW to NE (about 300 miles). I was able to achieve 65 MPG indicated (yes I was passed by everyone including tractor trailers). On the return trip the next day, I used AC CC and set the speed to 65 mph. I only got 53.8 MPG indicated. I was surprised that is was that much different. The only other factor that I can think of that may have been in play here is wind.
     
  17. bwilson4web

    bwilson4web i3 and Prime

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    You might consider running a test on a 10 mile flat section of a local interstate doing:

    • pair of runs in opposite directions without AC, record temperatures
    • pair of runs with AC, record temperatures
    This would quantify the effect while taking out elevation and wind effects.

    Bob Wilson
     
  18. CivicQc

    CivicQc The world needs more prius

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    I have driven over the same mountain road recently. Both ways using CC, set at 92 km/h. My Prius is now equipped with snow tires (blizzaks). I have made interesting observations:

    • Way out: Outdoor Temp.: -9 C, Fuel economy: 4.7 L/100 km
    • Way back: Outdoor Temp: -12 C, Fuel economy: 4.7 L/100 km
    For both trips, the road was clear and dry.

    I realize there is a clear increase in fuel use compared with the summer, that is most certainly caused by the low temperature and the fact that I use different tires.

    But I also noted something interesting. As mentioned by many, normally when using CC downhill, the system will use regen to slow the car down, up to a point when the battery is fully charged, and from that moment engine brake is used. On the way out, CC behaved exactly that way. However, on the way back, it was different. Whenever I was driving downhill, engine brake was used instead of regen to slow the car down, in spite of the fact that the battery was only 6 bars. It never regarged the battery more than 6 bars.

    Did any of you make similar observations? Might it be caused by the lower temperature (the hybrid system protecting the battery at those temperatures by not recharging it much?). Note that during the trip, I once switched off cruise control once, and actually used the brake pedal downhill to recharge the battery. And I did raise it to 7 bars. So it seems to be a logic specific to cruise control.

    Comments?
     
  19. rcshaw777

    rcshaw777 Junior Member

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    I read this thread with great interest.

    I have tried using CC initially and find it doesn't help in fuel economy because the roads here are very undulating.

    Now I am experimenting combination of P&G & CC.

    I maintained at CC speed at uphill climb, then as it reaches the peak toward the downward slope, I gently apply the accelerator and maintain at lower quadrant at the HSI.

    The vehicle speed will increase of course; this will enable the Prius to build momentum in anticipating the next hill climb. The speed will gradually decline to the CC set speed, then I will let go the accelerator to let CC take control when it climbs the hill.

    The advantages of doing this is the following
    1) The increase downward slope speed; will not inconvenient other motorists trailing behind who tend to increase speed to take advantage of downward slope

    2) Take advantage of the downward slope to build up momentum for next hill climb; this will reduce engine high rev.

    3) Easier to control the accelerator pedal. Making use of CC to activate the Pulse when doing hill climbing, and then Glide when going down hill by gently pressing the accelerator and maintaining at lower quadrant.
     
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