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MPGe and Range Calculation

Discussion in 'Tech Talk' started by JimB, Sep 15, 2017.

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  1. JimB

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    This has been bugging me for some time and I hope someone has the answer.

    Given:
    Model 3 Long Range
    Range = 310 mi, per Electrek and others
    126 MPGe, per Electrek and others
    Equivalent gallon of gasoline = 33.7 kWh, per Wikipedia
    Battery capacity = 75 kWh, per Electrek and Elon

    Then:
    126/33.7 = 3.739 mi/kWh X 75 kWh = 280.4 mi, not 310

    What am I doing wrong?

    Using this approach, it would require an 83 kWh battery to get a 310 mile range. It can’t be that big.

    I would think that MPGe and Range would be mathematically related.

    Help
     
  2. garsh

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    MPGe also takes into account inefficiencies in charging the battery (wall-to-wheels).
     
  3. JimB

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    Interesting, so without taking into account the charging inefficiencies, it would be:

    310/75 = 4.133 mi/kWh X 33.7 kWh/ge = 139 MPGe.

    Thanks
    I'm now unbugged

    PS: Maybe they should take into account the inefficiencies of taking an ICE to the gas station.
     
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  4. Kbm3

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    There is a great thread on TMC called "Tesla created a monster".

    There are calculations to show that range should really be 334 miles and that Tesla voluntarily lowered it.
     
  5. garsh

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    Yeah, they way they come up with the EPA range value is kind of funny.
    • Using a dynamometer with various coefficients set to simulate actual vehicle drag at various speeds, run through a carefully-defined program of changing speeds until the vehicle runs completely out of energy. Record the total distance travelled (495 miles city & 455 miles highway for LR Model 3)
    • Apply a fudge factor of 0.7 to this value (347/318 miles for LR Model 3)
    • Then, throw all of that out and report something even lower so that people don't start complaining about not being able to reach these unrealistic distances in the real world (and that's how we got to 310 miles).
     
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  6. JimB

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    All very interesting, thanks.
    They may also not want to compete with the 100D which is 335 miles
     
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  7. MelindaV

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    or, alienate an existing ~75kWh Model S owner by having the 'not the more advanced Model 3 than the Model S' Model 3 with that much better range.
     
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  8. JimB

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    What really counts is what we get in our ow
    When all is said and done, what really counts is the range each of us gets in our own real world of driving. It appears that we may be pleasantly surprised.
     
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  9. Troy

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    #9 Troy, Sep 19, 2017
    Last edited: Sep 19, 2017
    Hi, @JimB. @garsh is correct. The MPGe numbers are calculated from the 89.41 kWh wall consumption in the EPA CSI file (the file that shows the dyno scores). Here is the correct calculation:

    126/33.7 = 3.739 mi/kWh X 89.41 kWh = 334.3 mi, not 310.

    You might say, why do we get 334 instead of 310 miles? The reason is that Tesla voluntarily lowered the range from 334 to 310 miles. However, MPGe numbers remain unaffected by voluntary reductions.

    There is a topic called "
    EPA Certification Data". In that thread, I've posted a calculation exercise for the Nissan Leaf. Feel free to check it out. I don't want to give links because my messages get stuck in moderator approval queue.

    By the way, in real-world driving, I think the Model 3 80 will have the same range as the Model S 100D because they both have almost the same highway dyno score. The S100D scored only 0.7 miles higher. The Model 3 80D should have better range than the Model S 100D but this might change when the Model S switches to the PMAC motors.
     
  10. garsh

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  11. mkg3

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    In one of these threads, cannot recall which one nor could easily find it again, someone stated that having AC on does not reduce range much but the heater does. Why is that?

    It makes no sense to me. Both uses electric fan and heat exchanger of some type.

    It seems to me that one can easily capture the heat from the batteries and motor running to reduce the heating requirement for the vehicle. By doing so, the air can be preheated and reduce delta T required to desired temperature when cold.

    The AC system, one would think, would require more energy to reduce the air temperature to low 60F while its over 90~100F+ outside.

    Since I don't know how the heat system works in Tesla (versus regular car, where the radiator water heat is used), it may make perfect sense to someone who knows the system....
     
  12. Bokonon

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    Running the AC definitely does reduce the range, especially (in my experience) above 90 degrees and/or with high humidity. It just doesn't reduce range as much as running the heater. While Tesla's HVAC system can draw on some amount of waste heat from the drivetrain and battery pack to warm the cabin, it does not have a heat pump to extract heat from the outside air, so it relies on resistive heat instead. As I understand it, that's the main reason for the difference.

    There are also a variety of air-temperature-related factors that coincide with the need to turn on the heat or the A/C, such as air density and tire-rubber stiffness, which can also impact range one way or the other.

    FWIW, I currently drive an EV with a heat pump, and the range hit I experience when I heat the cabin to 70 degrees when it's 50 outside is roughly equivalent to what I see when I cool the cabin to 70 when it's 90 outside, as you might expect. However, when it's closer to freezing, resistive heating takes over, and the range hit becomes much more pronounced.
     
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  13. mig

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    Perhaps this is obvious, but with A/C you are just moving heat from inside the car to the outside. Thus, you can remove more heat than the energy you put in, say for a typical case a vehicle AC can remove 2x the heat energy than what is put into the compressor (just a wild guess).

    With a resistive heater, you have a heating efficiency of near 100%, however that is obviously only half as efficient as the AC. An AC is just a heat pump in reverse which is why it seems strange to some that Tesla doesn't just use a heat pump. But I trust there is some good reason.
     
  14. oneshortguy

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    Tesla has a 'Range Per Charge' section for the Model S and X. You can turn on or off the AC and simulate different scenarios.

    https://www.tesla.com/models

    Screen Shot 2017-09-19 at 2.33.50 PM.png
     
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  15. tonymil

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    This article confirms the Model 3's EPA ratings, and although the numbers have been available before, it now occurs to me how little the range and MPGe varies between highway and city driving.

    https://electrek.co/2017/11/29/tesla-model-3-official-epa-rating/

    The range drops by only 26 miles (8%) and the MPGe by 11. That's outstanding! Here a website that shows the city/highway/combined MPG-e for new ev's and you can see that the Model 3 has the least deviation.

    https://www.fueleconomy.gov/feg/Pow...2017&year2=2018&vtype=Electric&srchtyp=newAfv
     
  16. Scuffers

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    quite right, other point is AC you're only trying to shift a few KW of heat, heating car needs somewhat more than that, and realistically, to pull that heat from outside (that's going to be somewhat cold in the first place) would be pretty hard going energywise.

    one thing I would hope Tesla have done, is insulate the car somewhat better than conventional cars, not losing heat in the first place is the best way to economise on power to keep it warm.

    for an ICE car, this was never a problem, heat was a waste product in the first place.
     
  17. MelindaV

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    interestingly, the EPA site, under the 'Specs' tab, list the Model 3 as having 17CF of luggage room, with an A4 and 330i both listed at 13CF. (maybe the MotorTrend article flipped those two numbers when comparing the Model 3 trunk to a 3 Series)
     
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  18. Sandy

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    As well comparing the MS and M3 the 3 has 3 more cubic feet of passenger space than the S. I would have thought the S was bigger inside.
     
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  19. Sandy

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    I’m guessing here but the narrow gap between city and highway on the 3 is most likely due to the low CD compared to the others in the chart.
     
  20. tonymil

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    That's my guess too. Kudos to the engineers who designed the initial version that we saw at the reveal and who then re-engineered the car to address all the complaints about the "duckbill" front end and the small trunk opening, yet still kept the CD low. That's quite impressive.
     
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