TMC is an independent, primarily volunteer organization that relies on ad revenue to cover its operating costs. Please consider whitelisting TMC on your ad blocker or making a Paypal contribution here: paypal.me/SupportTMC

Hypermiling Technique in an EV

Discussion in 'Technical' started by smorgasbord, Feb 11, 2013.

  1. smorgasbord

    smorgasbord Active Member

    Joined:
    Jun 3, 2011
    Messages:
    2,379
    Location:
    SF Bay Area
    #1 smorgasbord, Feb 11, 2013
    Last edited: Feb 11, 2013
    I think regen affects hypermiling technique.

    For instance, imagine you're at the top of a big hill, straight road. You have been driving at 50MPH on flat roads. If you coast down in neutral, you'll hit 75MH pretty quickly and stay there for miles. You have two choices:
    1) Standard Hypermiling Technique: Coast down in neutral at 75MPH. When you reach the bottom of the hill and hit flat road, you continue to coast until you hit 50MPH, then put cruise control on.
    2) My Proposal: Use cruise control down the hill at 50MPH, which means regen will be engaged. When you reach the bottom of the hit and hit flat road, continue at 50MPH.

    In scenario #1, your "excess" potential energy goes into more speed. At the bottom of that hill, that extra speed will let you coast a little bit longer on flat land until you reach 50MPH.

    In scenario #2, your "excess" potential energy goes into the battery. Some of it is lost since regen isn't close to perfect, what you do capture will be greater than momentum you have at the bottom of the hill. Therefore, it's the more energy frugal way to drive. The longer the hill, the more regen will help.

    Now, I think it's possible on a short and/or not steep hill where the additional speed gain above 50MPH is modest and you reach max speed the foot before the road levels out, that since you haven't gained much speed there isn't much additional aero losses and so the extra momentum you have at the bottom will carry you further than the juice you got into the battery. But, I don't know where the transition point is.
     
  2. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

    Joined:
    Jun 21, 2012
    Messages:
    7,019
    I thought about this as well - my lane road heads downhill for about 1/10 a mile, then back uphill. I spent some time trying to figure out the best way to approach it -- use neutral, starting at the beginning of the road (which can get me almost to the top of my hill), or just rely on regen. I could model this, considering the points that we have -- regen can recover about 75% of the energy put into making the car go, which will intersect with the increase in air drag as speeds increase.

    I came to one conclusion in my consideration:

    I want simple.

    So on long stretches of road, I don't want to have to consider this. I expect the cruise control to get smarter over time so I don't have to worry. :)
     
  3. MikeC

    MikeC Active Member

    Joined:
    Jul 9, 2012
    Messages:
    2,405
    Location:
    Los Angeles
    Not sure about this - but isn't regen only beneficial if you actually want to slow down anyway?

    I'm thinking scenario 1 would be more energy efficient because all of your kinetic energy goes into forward motion which is what you ultimately want. In scenario 2, once you reach the bottom, you'll have to expend energy to keep your speed at 50MPH that you wouldn't if you were coming down at 75MPH. Some of this will come from the regen you got, but since regen is ~75% efficient, this discrepancy would be the deficiency compared to scenario 1.

    On the other hand, you'd also have to account for the difference in drag between 50 and 75MPH. Is this right?
     
  4. Rodolfo Paiz

    Rodolfo Paiz P85 "Plug and Play"

    Joined:
    Nov 19, 2012
    Messages:
    681
    Location:
    Miami, FL
    #1 is more efficient if there is no speed limit. But if you've been doing 50 because the limit is 45, then you can't really let the car accelerate to 75 anyway, so regen is preferable to braking and #2 is more realistic.
     
  5. smorgasbord

    smorgasbord Active Member

    Joined:
    Jun 3, 2011
    Messages:
    2,379
    Location:
    SF Bay Area
    The way I wrote the scenario, in #1 the downhill is just steep enough that the car reaches 70MPH and stays at that speed in neutral. So, whatever energy you're getting from the downhill is all spent making the car go 70 MPH. When you reach the bottom of the hill, your battery is at the same SOC as it was at the top. All you've done is travel the length of the hill, and then have some momentum.

    In scenario #2, the car only goes 50MPH. That means you're putting juice into the battery from keeping the car going 20MPH slower. When you reach the bottom of the hill, you have more juice in your battery. If the hill were 10 miles long, you can go a lot further on the regened juice than the extra 20MPH momentum at the bottom would take you.

    How much more juice? Well, 50MPH takes 205Wh/mile, going 70MPH takes 300Wh/mile. That's 95Wh/mile savings from going slower and regen captures about 2/3 of that, or about 60Wh/mile of downhill.
     
  6. rolosrevenge

    rolosrevenge Dr. EVS

    Joined:
    Feb 7, 2009
    Messages:
    1,851
    Location:
    Right behind you...
    The big question with this is: is the areodynamic drag losses at 70 mph compared to 50 mph greater than the battery charge/discharge efficiency of the drive train. Since drag goes up by velocity cubed, I'm going to put my money on that the charging/discharging has a higher efficiency.
     
    • Like x 1
  7. smorgasbord

    smorgasbord Active Member

    Joined:
    Jun 3, 2011
    Messages:
    2,379
    Location:
    SF Bay Area
    The only question is whether you're driving long enough at the higher speed that capturing the extra energy is larger than the momentum change. That's a no-brainer.

    Whether 70MPH takes 37% more energy (as Tesla's charts show), or only 5% more energy, capturing 2/3 of that positive number for the length of the downhill is better than capturing nothing and ending up at 70MPH at the bottom versus 50MPH.
     
  8. stevezzzz

    stevezzzz R;SigS;P85D;SigX

    Joined:
    Nov 13, 2009
    Messages:
    6,062
    Location:
    Colorado
    Aero drag increases as a function of velocity squared, not cubed. You may be confusing drag with the power required to maintain a particular steady-state speed.

    Drag equation - Wikipedia, the free encyclopedia

    Squared is bad enough, and I'm with the OP that if the hill is long enough (and steep enough), using the cruise control and regen to maintain a constant speed results in a better overall Wh/mi value than allowing aero drag to limit the downhill speed. What is 'long enough and steep enough'? At the shallow end, where aero drag limits your speed in neutral to the desired speed of travel (i.e., the car doesn't speed up downhill in neutral), it doesn't matter how long the hill is: you arrive at the bottom with the same SOC (ignoring vehicle overhead and climate control losses) and no extra speed: your Wh/mi for the downhill stretch is zero. If the hill is really steep, only a very short downhill run in neutral can be tolerated, to keep speed within reasonable limits: if the hill is long enough and steep enough that in neutral you're tempted to reach for the friction brakes anyway, might as well let regen recapture some of the potential energy you're losing on the way down.

    In normal driving, I think that choosing your speed of travel wisely and setting the cruise control is likely to be your 'easy button' best bet. To use real hyper-miling techniques, you'd have to allow the car to slow nearly to a stop at the top of every hill, and glide down the other side in neutral, accelerating by gravity alone, until you reach some pre-determined speed that is likely to be not a whole lot faster than the speed at which the car achieves max range on the flat. Once you reach that speed you'd engage cruise control and let regen kick in. Basically, you'd do anything you can to reduce the amount of juice it takes to get to the top of the next hill, and to avoid throwing away potential energy due to excessive aero drag on the way down the other side. You'd also drive with the windows and pano roof closed, with minimal heating or cooling for the cabin, while trying to avoid pavement imperfections and driving with two of your four over-inflated tires on the solid painted lane margin lines to reduce rolling resistance.

    Sounds like a lot of work (and you'd be a hazard to others on the road), unless you're in the middle of nowhere with barely enough SOC to reach the only charging station within range, and you have to choose between bordering the car or making it.
     
  9. GSP

    GSP Member

    Joined:
    Dec 28, 2007
    Messages:
    1,995
  10. yobigd20

    yobigd20 Well-Known Member

    Joined:
    Oct 28, 2012
    Messages:
    5,793
    Location:
    Skaneateles, NY
    It's called enable cruise control @ 20mph.

    graph1.jpg
     
  11. stevezzzz

    stevezzzz R;SigS;P85D;SigX

    Joined:
    Nov 13, 2009
    Messages:
    6,062
    Location:
    Colorado
    Good stuff. And it's a lot easier to hypermile in an S than any ICE or hybrid: just ignore all the techniques that work around the inefficiencies of an ICE. Aside from picking a slower cruising speed, the two easiest and most fruitful techniques are what he calls 'Driving with Load' and 'Driving without Brakes/Driving with Buffer' ('brakes' includes both regen and friction brakes, with the caveats this thread has brought up concerning long descents).
     
  12. AC1K

    AC1K Member

    Joined:
    Mar 18, 2013
    Messages:
    468
    Location:
    Calgary Alberta Canada
    i think its proven in ANY vehicle regardless of power train is you should maintain momentum, try not to speed up or slow down, stopping is the worst thing you can do.
    speeding up takes energy, slowing down takes energy (in regards to regen, you are not gaining 100% back, maybe 40-50%, so regen is not efficient at all but its better the than negative results ICE gives)

    take corners as fast as possible without risking safety,
    slow down in advance of a red light in hopes that it will change back to green before you get there,
    try and take routes that have less obstacles (red lights, stop signs, etc),
    obstacles also include other cars, if you can leave earlier before rush hour and leave later, after rush hour to avoid stop and go.
    also tires are a big factor, the less rolling resistance you have, the better, usually tires that are really quiet have lower rolling resistance at the cost of traction, tires with a hard solid center line gives higher millage (see picture)
    http://images.productwiki.com/upload/images/michelin_energy_saver_1.jpg

    using all these techniques, im able to get 25MPG in my genesis coupe V6 during regular day to day driving instead of the rated 17MPG
     

Share This Page