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Is 200Wh/mi achievable?

Discussion in 'Cars and Transportation' started by Topher, Jan 6, 2017.

  1. Topher

    Topher Energy Curmudgeon

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    Nah, that's only 200 Wh / mile (Model 3's 55kWh/215miles range = 255Wh/mile). We could probably do that with weight drops through better batteries, and reduction of safety equipment (no longer needed due to moving cargo to trains, and autonomous driving). Aerodynamics can also be improved through similar methods. Even if all needed to come from drag coefficient, that is only a Cd of 0.16 (from 0.21), which the VW XL1 has now.

    Thank you kindly.
     
  2. Doug_G

    Doug_G Lead Moderator

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    Right, a tiny, ugly car with limited capacity under ideal driving conditions.

    I have a Roadster, which can do 240 under good conditions. But that's not my average. And that car is tiny!
     
  3. Topher

    Topher Energy Curmudgeon

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    Tiny cars have WORSE Cd than larger cars. Particularly in length. (and the rest is taken into consideration by the frontal area part of the equation).

    Thank you kindly.
     
  4. Doug_G

    Doug_G Lead Moderator

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    Cd is only part of the equation. A larger object generates more drag than a small one, even with the same Cd.
     
  5. Topher

    Topher Energy Curmudgeon

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    Yes, which is precisely why I said "Even if [it] all needed to come from drag coefficient".

    If we take a Model 3, improve its Cd (without making any size changes) to 0.16 and we will have a 200 Wh/mile (highway) car.

    We know this is possible, since Cd is a scale independent value, so we can scale the shape of a VW XL1 up to the size (frontal) of a Model 3, and it remain the same Cd. Now stretch that (intelligently) into the longer length of the Model 3, and the Cd will IMPROVE. The frontal area of the Model 3 hasn't changed (much, and only for the better), so that part of the equation remains the same. However, it is multiplied by the reduced Cd, so the aerodynamic portion has improved by 22%, giving us a 200 Wh/mile Model 3 (for situations dominated by aerodynamics).

    In other words, the XL1 has low consumption due to its size AND Cd, it gets the equivalent of 140 Wh/mile. A larger Model 3 with the XL1's Cd, could get 200 Wh/mile.

    Thank you kindly.

    p.s. not at all sure how 'ugly' affects efficiency. :rolleyes:
     
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  6. ohmman

    ohmman Maximum Plaid Member

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    Sorry that I'm not up to date on the XL1 specs, but at what speed does it achieve 140Wh/mi equivalent?

    The references I can find also state that the XL1 has a Cd of 0.186, not 0.16. So using your math, it seems that the aerodynamic portion improves by 11.4%, not 22%. 0.186 is impressive, just as 0.21 would be for the 3 if that actually happens. But hundredths are huge in drag coefficient, so we need to be sure we're speaking accurately.
     
  7. Garlan Garner

    Garlan Garner Well-Known Member

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    The XL1 is not attractive at all. It really doesn't matter what the CD is of an ugly car.
     
  8. Topher

    Topher Energy Curmudgeon

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    I got my numbers from Wikipedia. Take up your argument with them. I figured it was good enough for a feasibility question for 100 years from now.

    Thank you kindly.
     
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  9. ohmman

    ohmman Maximum Plaid Member

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    Wikipedia doesn't agree with your numbers.

    Cd is 1.86, not 1.6.

    A 31 mile electric range on a 5.5kWh battery tells me that it consumes 177.4Wh/mi, not 140Wh/mi.

    I don't mind if you're approximating to bolster an argument, but being clear about your assumptions makes us all know how much weight to give the quoted data.
     
  10. Topher

    Topher Energy Curmudgeon

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  11. ohmman

    ohmman Maximum Plaid Member

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    That 0.159 number appears to be the prototype, not the production version. As they approached reality, it increased.

    The broader point was that 200Wh/mi was wishful thinking. You said it wasn't, using the XL1 as an example of why not. What I guess you meant was that it wasn't wishful thinking in 100 years. Now I'm on the same page.

    100 years from now we will operate vehicles in near vacuums so very little fluid dynamics exercises are necessary. Why even talk about it? :rolleyes:
     
  12. Topher

    Topher Energy Curmudgeon

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    Do you need MORE examples, and are you going to be as nit-picky about them as well?

    So you are going to put the 'wishful thinking' on the mere difference between prototype and production? Could that prototype have been driven?

    Thank you kindly.
     
  13. ohmman

    ohmman Maximum Plaid Member

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    I don't know. Could it? What of the consumption? How does it affect your estimates?

    FWIW, you are the one putting the wishful thinking on it. Not me. I'm saying it's a poor example. You're digging in heels and saying it's valid.
     
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  14. Doug_G

    Doug_G Lead Moderator

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    Even if it is possible, is it practical?

    We have to optimize motor vehicles for many different things - safety (mirrors, pedestrian impacts, collision protection), practicality (fitting stuff in them, being able to get in/out, etc.), size (this is a biggie), aesthetics (so people actually buy them), etc. That often works against ideal aerodynamics.

    Weight has a modest impact on range. At highway speeds the dominant effect is aero.

    Here's some data for the Roadster (from Tesla):

    roadster.jpg

    Weight really only affects the "Tires" number - rolling friction. (It does affect hill climbing. Also accel/decel losses, but that's more about drive train efficiency.)

    As you can see, the majority of the impact is from aero. You might be able to improve the drive train efficiency a little, but it's already pretty darn good. The main thing you can do is improve aerodynamics. And that's not simple to do.
     
  15. bxr140

    bxr140 Member

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    This is a fun topic. :)

    5 kwh/mi for a car is practically a certainty, probably in 30-50 years. The only thing that stands in the way is the fundamental definition of a “car” at that time. I’d contest it won’t be too much different than today, with the major difference being that fewer people will outright own cars.

    Enablers of 200wh/mi are:
    1. Mass. As battery technology improves, mass is going to come down significantly. Also, as CFRP technology improves, mass will come down even more. Less mass has a huge knock on effect, where the rest of the car can be downsized as well, like tires/wheels, brakes, safety structures, etc. Mass is the major resistance to acceleration. Decreasing mass will have a major effect on transient efficiencies.
    2. Aero. As computing power and production technology improves, more aerodynamically optimized surfaces can be manufactured more easily. As battery technology improves volume density will decrease, decreasing overall size and improving packaging--we could end up with a small center tunnel back in our cars filled with 200kwh of capacity. There's also pecific things like no mirrors, smaller tires (because of less mass), and tighter panel gaps (from better manufacturing) that will play more specific parts.
    3. Automation and networked transportation. This is the nail in the coffin of 200wh/mi. Networked transportation will lead to much more efficient travel and much less wasted energy on acceleration/deceleration/unnecessary speed (why speed up to 45mph then stop for the next light when a steady 30mph will get your there at the same exact time?) and most importantly, automation will lead to peloton drafting. That right there could get us down to 200kwh/mi on the freeway, maybe even with today's Model S.
     
  16. omgwtfbyobbq

    omgwtfbyobbq Member

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    #16 omgwtfbyobbq, Jan 9, 2017
    Last edited: Jan 9, 2017
    For what it's worth, the 2014 i3 uses *~246Wh/mile at 60mph. I think the 3 could get to ~200Wh/mile at the same speed.

    Plugging the i3's weight (~3000lbs with driver), drag coefficient (.29), and frontal area (~34.6ft^2) into **ecomodder's calculator results in energy requirements of ~230Wh/mile. Assuming the inverter/motor is ***~90% efficiency, that puts energy consumption at ~255Wh/mile, which is close enough for jazz IMO.

    Using the same ecomodder calculator, increasing the weight to 4000lb, reducing the frontal area by 10%, and dropping the drag coefficient to .21 results in ~180Wh/mile in energy requirements, which would be ~200+Wh/mile at the battery. The i3's a little lower in practice, so a 3 with these specs might be too, but I think ~200Wh/mile @ 60mph is doable. Hard, but still doable.

    If Tesla could get the Cd down to .18, [email protected] is possible with the above specs, and at .16 [email protected] is possible. Getting down to the Cd of the XL1 car/prototype would be really tough, but Tesla doesn't have to deal with cooling an ICE which could be a slight advantage.

    * https://avt.inl.gov/sites/default/files/pdf/fsev/fact2014bmwi3ev.pdf

    ** Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com

    *** https://energy.gov/sites/prod/files/2016/06/f32/edt006_burress_2016_o_web.pdf
     
  17. deonb

    deonb Supporting Member

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    5kwh/mi? Do people in the future drive at 500 mph?
     
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  18. jaguar36

    jaguar36 Active Member

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    This thread is confusing, was it split off some other topic or something? What sort of car are you talking about? A Tesla S? At what speed? It seems highly unlikely without things like drafting to get to 200wh/mi at 70mph

    Thats a typo, units got reversed, according to the first post its 5mi/kwh = 200wh/mi.
     
  19. ohmman

    ohmman Maximum Plaid Member

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    It was split from a Model 3 thread. The discussion is about the feasibility of a future vehicle that could achieve 200Wh/mi.
     
  20. jaguar36

    jaguar36 Active Member

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    With some ground rules being:

    4 Seater mass market car.
    Production car (aka, its going to need some cooling/heating, reasonable traction and braking and so on)
    Driving solo on an existing interstate (no drafting, or crazy infrastructure enhancements)
    70 Mph, 70 degrees out, no wind or rain.
    2030 Production year (somewhat arbitrary, but beyond that the ability to predict what might happen is very poor)


    Currently a Model S uses ~340Wh/mi under these conditions.
    Therefore based on the Tesla provided graph in posted by Doug in #19:
    Aero = 150Wh/mi (based on C&D's estimate of 14hp)
    Drivetrain = 115 Wh/mi
    Tires = 70 Wh/mi
    Ancillary = 5 Wh/mi

    To get to 200, you're going to need to improve all these, so reasonable improvements would be:
    Aero, increase Cp from 0.24 to 0.19. I personally think even this is a stretch, as you'll still need things like heat exchangers (for AC and drivetrain cooling) and a usable trunk which are going to make even that 0.19 number a stretch. Also assume its a smaller car, so the frontal area from 25.2 to 20. Thats rather unrealistic as well, as its the same as a 2001 Honda insight, which was a tiny 2-seater. Both of these result in 94.2 wh/mi

    Drivetrain, you're not going to get much more efficient than the Model S's drivetrain. There aren't a ton of gears in it, being a single speed transmission. the Drivetrain number is based on a RWD Model S 85, so switching to the smaller front motor in the D gets you 4% (based on the difference between the RWD and D models) So maybe you could go smaller still, get another 4%? Still, not a lot of room for improvement there. Maybe put one motor per side to do away with a differential? (course then you lose efficiency by needing two motors) Still, lets be generous and give it 25% resulting in 86.2 Wh/mi

    Tires, Bridgestone committed between 2005 and 2020 to gain a 25% reduction in rolling resistance. Thats 1.7% per year, lets say they actually make that (Seems highly unlikely as they are already 10 years in and have only gotten 12%), and that they could continue that pace (even less likely as gains become harder and harder to make over time). That results in a 25% reduction to 52.5 Wh/mi

    Lastly Ancillary's. Not going to be a ton of room for improvement here, but its small enough to not really effect the anwser, so lets just say a 50% drop. that means just 2.5Wh/mi

    Grand Total: 235Wh/mi. So to review, if you make a bunch of extremely aggressive assumptions, and then assume they are all able to happen, you still end up way short.
     

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