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Best way to drive efficiently off-highway?

Discussion in 'Model S: Battery & Charging' started by AnOutsider, Jan 4, 2013.

  1. AnOutsider

    AnOutsider S532 # XS27

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    We have this thread: Best energy usage at high speeds and it's confirmed what I've thought: it's easier to maximize range on the highway. Now there's a lot of reasons for this (constant speed, higher speeds so more distance covered, drafting etc), but what about at low speeds?

    For example, heading out to work or the restaurants (20 mins away, but only partially highway) I see really high usage. The roads are all 30-45MPH areas, and I've yet to master a way of being really efficient in this. So what are some tips? Do you creep off from lights? Heavy regen to lights? How about hilly areas at these lower speeds? Accelerate before and at the bottom of the hill to use less energy to crest it? Slowly putt putt up the hill (wouldn't that use more energy)?
     
  2. GSP

    GSP Member

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    Check out hypermiling techniques at CleanMPG.com.

    1) look ahead 2-3 traffic lights and set your speed so that you don't stop and minimize slowing down.
    2) if you have to slow down, try to coast at zero power or in neutral, instead of using regen
    3) if you have to slow down faster than coasting in neutral, then use regen
    4) only in an extreme case regen will not be enough, so use the brakes
    5) best way to not slow down is often to keep speed low in the first place

    CleanMPG calls this "DWB" or Driving Without Brakes. They have many other hints, such as cracking driver and right rear windows 1/4" instead of using HVAC or fan.

    See section II and especially section III at this link:

    Beating the EPA - The Whys and How to Hypermile - CleanMPG Forums

    GSP
     
  3. Todd Burch

    Todd Burch Electron Pilot

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    Marcus,

    I think some of the issue is that it's winter, and on our short trips a higher percentage of total trip energy is used to get the pack up to temperature. i bet as Spring rolls around and it warms up, energy usage will go down quite a bit.

    My rules of thumb are:
    1) Coasting is better than regen. Find the sweet spot (0 kW--no orange or green bar). As you're probably seen, it's fairly easy to hold very close to 0 kW with your foot.
    2) Regen is better than braking.
    3) Accelerate gently and evenly.
    4) Unless I have someone riding my tail, if I see the light ahead of me turn yellow, I immediately start coasting. If you're lucky, the light will turn green before you get to it. Of course, go into regen at the right moment when needed.
    5) i suspect creep uses very little energy. Probably insignificant. I don't think having creep on or off makes a difference, but if you follow (4), ideally your car doesn't come to a stop before the light turns green and traffic starts moving again.
    6) In theory--in a frictionless vacuum, the speed at which you go over the hill makes no difference. In reality, air resistance is obviously less at lower speeds. I haven't seen definitive tests to prove it, but I suspect that coasting up the hill, then letting gravity speed the car back up on the far side, is the most efficient method. Slower though, obviously. If you speed up beforehand, you're wasting more energy due to increased drag.
     
  4. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    If you don't press your foot down on the brake hard enough on the Prius, creep will use 500W (assuming flat road). More if you are moving under creep or are on a hill.

    That is correct. It works as you say. Speed up downhill, use as little power as possible going up hill. I have several of those on my commute and it works great. I speed up to about 45 on the down hill side, then glide up until the car slows to 30 (speed limit 40), then I use as little power as possible to get up the remaining part of of the hill. Obviously, the effectiveness varies with the hill and the road surface. The worst situation is to have a stop signal at the bottom of a hill and then having to power up after the stop.
     
  5. AnOutsider

    AnOutsider S532 # XS27

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    Can you explain #2? Why avoid regen?

    Also, overall, these are good for hyper-miling and such, but I mean what about "normal" driving? Are you just doomed to average 500wh/mi in these conditions?


    Good point re: winter, I didn't think too much about that -- though at home I'm garaged, so the pack shouldn't be TOO cold. On #1, as above, why avoid regen? To avoid losing speed?

    I actually have creep off, but I do creep slightly when needed (luckily traffic around here is rare).

    As for going up the hill, let's imagine a hill is a mile long. If I creep up that hill at 30mph I might be in the say, 20 range on the gauge but let's say it takes me 2 minutes to do that. Now if I accelerated and went up the hill at 60mph, I'd spend only 1 minute at that usage level (whatever it might be). Would it be break even? Worse? Better? Inquiring minds and all.
     
  6. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    Regen turns kinetic energy into electricity and heat. Gliding turns kinetic energy into forward motion. Basically, regen throws some energy away as heat. Friction brakes throw it all away as heat. So regen is better than friction brakes, but that's about it.

    - - - Updated - - -

    If you let it, a car with a lot of instrumentation will teach you how to drive economically. After you've driven economically for awhile, you'll find that you arrive at your destination at about the same time.
     
  7. GSP

    GSP Member

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    Coasting is better than regen because 100% of your kinetic energy is used to travel more distance. Regen can only capture and restore about 75% of the car's kinetic energy. This is due to energy conversion losses, which generate waste heat converting energy from kinetic to electrical to chemical, then back to electrical and to kinetic. Better than the brakes, which is 100% conversion to waste heat.

    GSP
     
  8. PeterW

    PeterW Member

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    To put it another way; regen is a form of braking and it puts energy back into the battery, but it does not put all of it back, you lose some. OTOH if you were not going fast enough to require regen then you do not lose any.
     
  9. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    For starters there will be a big difference in aerodynamic resistance between the two speeds. Other than that it depends on how efficient the motor is at one speed compared to another. It's been way too many years since I did calculus regularly, but this is an area-under-the-curve problem. The smaller the area under the curve, the less power is used.

    But the original idea was to speed up going down hill and then glide up hill until some minimum speed before you apply power--and then only apply enough power to reach the top of the hill, and repeat. If you go up the hill at 60 (or worse, use cruise to keep a steady 60 downhill and uphill) you'll be using a lot more power. The total time won't be much different for both because what you lose in time going uphill, you gain going down hill.

    Here is an exaggerated case of traveling from DFW to Phoenix and back. Lots of long climbs (and long descents). Sorry it's not in a Model S but none were available then:

    DATE__________ODO____INC_____AVG
    --- Trip to AZ starts here
    12/04/08____85560____501____62.3 (3.8)
    -- 27 F here
    12/06/08____85929____368____53.4 (4.4)
    12/07/08____86322____392____56.7 (4.1)
    12/08/08____86800____478____60.8 (3.9)
    12/13/08____87232____431____57.4 (4.1)
    12/14/08____87673____440____63.3 (3.9)
    -- 25 F here and 50 mph winds
    12/17/08____88133____459____56.3 (4.2)
    --- Trip to AZ ends here
     
  10. GSP

    GSP Member

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    #10 GSP, Jan 4, 2013
    Last edited: Jan 5, 2013
    Some thoughts on climbing and descending hills, from the cleanMPG link that I posted earlier:

    DWL: Driving w/ Load: Instead of relying on CC (Cruise Control) to maintain speed, you rely on your iFCD (Instantaneous Fuel Consumption Display) and accelerator pedal for those automobiles that have them to stay locked in at a given fuel economy. One example would be when climbing an overpass. Instead of holding a steady speed up, over, and down the other side, you allow speed to droop as you climb while maintaining load or FE on the ICE and climb back to initial target after the decline on the backside. Begin the overpass climb at 65 mph, drop off speed as you climb, reach 62 mph at the crest, increase speed on the decline back to 65 mph. The technique depends on elevation deltas and traffic conditions. This can be simulated in a non iFCD equipped Accord or other automobile by locking in the accelerator pedal when approaching the overpass. Just hold the accelerator steady into, up, over, and down the back side at the same exact angle while arriving at the same initial target speed after the overpass has been cleared. There are slight accelerator pedal changes that can maximize the technique for those with iFCD’s but the locked down accelerator will work well for those just starting out and with a lack of an iFCD. An even easier way to understand the technique is to drive like a roller coaster coasts over the peaks and through the troughs. Pros are increased FE over any small terrain delta with a minimum of work. Cons are that there is thought and user input involved as well as slightly lowering your overall average speed to a given Point B.
     
  11. Todd Burch

    Todd Burch Electron Pilot

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    I agree with this approach (let speed bleed as you go uphill, then regain on the downhill), although it should be noted that several tips on cleanMPG assume ICE propulsion, and many of those assumptions don't apply with an electric drivetrain. For instance, with an electric drivetrain, driving slower is generally more efficient with an electric, but only applies up to a point in an ICE (driving 20 mph in an ICE, for instance, is generally less efficient than driving at 25 mph).
     
  12. AnOutsider

    AnOutsider S532 # XS27

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    Honestly, I tried this today on my trip home, and it was too tedious for me. I drove up the hills at speed and then coasted back down (trying not to regen). Worked fine, though it likely would have worked better that way as well -- but so would driving 25 mph on flat roads sooooo...
     
  13. Todd Burch

    Todd Burch Electron Pilot

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    Yep--nobody ever said it was fast! I honestly would only do it if there was no traffic behind me, I wasn't in a hurry, and I was worried about range.
     
  14. brianman

    brianman Burrito Founder

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    And put your hazards on so people don't stand on the horn because you're 20 below the speed limit.
     
  15. dmetcalf

    dmetcalf S258

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    Good points. I used this technique, along with waiting to engage cruise control until I was at speed to avoid heavy acceleration during my 400+ mile attempt. Not many hills, but I kept the numbers pretty even using the glide method downhill.
     
  16. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    Normally going 10 below the speed limit up hills is enough to make a significant difference in energy use. I seldom go slower than that and still get pretty good results (which have been posted elsewhere).
     
  17. bbmertz

    bbmertz Model S: P4909

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    Sorry if this is an ignorant question, but when you mention "coasting" (not in Neutral) on the MS do you mean just lightly pressing the accelerator pedal to maintain a constant speed, rather than completely releasing the pedal which causes regen to kick in? Why is this considered coasting -- doesn't using the pedal drain a bit of energy from the battery or is your point that it drains less energy than slowing via regen then accelerating again? (As an EV newbie, I'm used to "coasting" meaning completely releasing the gas pedal.)
     
  18. GeekGirls

    GeekGirls Kid in Candy Store

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    I don't hypermile too aggressively, generally driving the speed limit except when it's 65, and then I tend to drive 60. There are no shortage of hills around here. I think my lifetime average is under 340 Wh/mi and it's getting better. Coming to a complete stop is definitely the main thing I try to avoid, using regen to slow and anticipate light changes when it's practical to do so.

    My main additional tip? If you need to regen, or brake, do so early rather than late. Bleeding speed off early means you don't need to lose as much as if you wait until the last minute.

    Bonus tip: watch out for red light cameras coasting around corners. They really need to change the law to encourage energy conservation. As it stands right now you are required to come to a complete stop even when it's completely safe to do otherwise.
     
  19. brianman

    brianman Burrito Founder

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    I thought Todd described it pretty well, but I'll try to help clarify nonetheless.

    In every ICE car I've driven, I refer to coasting as various degrees of "decreasing the throttle/gas pedal but not pressing the break". "Full coasting" in such terminology involves releasing the gas pedal entirely and letting the momentum of the vehicle (as well as interaction with location with the gear, automatic downshifting, etc.) gradually slow down the vehicle (uphill/friction), maintain speed (slight downhill that counters friction), or speed up (strong downhill).

    In the Model S, releasing the accelerator invokes regen mode -- there's currently no "Off" setting. Consequently, one has to decide on which definition of coasting to use: (a) releasing of accelerator or (b) momentum-based motion without accelerator or braking intervention. I tend to go with (b) rather than (a). As does Todd. To obtain (b) in a Model S, your foot needs to find the sweet spot (of partial depress) of the acceletator where you are not applying energy (green) or recovering energy (orange). These colors refer to the arc on the right side of the speedometer in the Model S.

    Hope this helps.
     
  20. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    In the Prius groups this is referred to as gliding. Coasting is foot off the accelerator pedal and regen happens (just not as strongly as in the Model S).
     

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