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MPG equivalent

Discussion in 'Roadster' started by malcolm, Jun 22, 2008.

  1. malcolm

    malcolm Active Member

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    The value on the TM homepage has jumped from 135 to 256. Anyone got any details on this?

    Presumably this is for the smaller American gallon, so us Brits can expect to see around 307 miles to the gallon (equivalent)! :biggrin:
     
  2. TEG

    TEG TMC Moderator

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    That is odd. I wonder what has changed. So many places I see a rule of thumb of 33kWh = equivalent of 1 gallon of gasoline. So the simple calculation of 220 * 33 / 54 = 134 MPGe

    Check the end of this document:

    Amount Equivalent to
    One Gallon of Gasoline
    @ 114,500 Btu per gallon = 33.558 KWH
     
  3. MDR

    MDR Member

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    Here's a couple of quotes from the owners site:


     
  4. doug

    doug Administrator / Head Moderator

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    Thanks for the info, MDR.

    Most references I can find for the energy content in 1 gallon of gasoline convert out to 33 to 38 kWh. 82kWh is quite a jump (over 215%).

    I don't really understand this statement by Darryl:
    How is that supposed to help big companies with CAFE calculations? All of a sudden everyone looks a lot less efficient.
     
  5. Kardax

    Kardax Member

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    In my opinion, MPG is meaningless for EVs.

    Gasoline cars get a City/Highway rating.
    EVs should get a City/Highway range rating.
    Plug-in hybrids of any kind should get both ratings, showing EV range and gasoline fuel economy.

    Simple :)

    -Ryan
     
  6. Chris H.

    Chris H. Member

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    This will turn into a "Crisis of Credibility" for Tesla Motors, mark my words...
     
  7. stopcrazypp

    stopcrazypp Well-Known Member

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    From what I am reading from the link given by MDR, it still uses the 33,705Wh/gal conversion factor but it factors in average efficiency of US fossil fueled powerplants & transmission and efficiency of petroleum refining and distribution. So far it's fine, except I like the consistent straight 33,705Wh/gal which doesn't change over time or depend on your local powerplant. It's adequate for plug-to-wheel efficiency. You can find the efficiency for specific powerplants and factor that in later for fuel-to-wheel efficiency.

    Then they have this "fuel content" factor that they don't explain very well that boosts the number 6.67 times. Sounds like it's just a boosting factor for alternatively fueled vehicles from the comments section of the document. So that's just artificially inflating the efficiency numbers of alternatively fuel vehicles based on it's "benefits". It's an incentive factor to help boost a manufacturer's CAFE.

    I'm sticking to the 33,705 Wh/gal number, which seems to the most scientifically sound number for plug-to-wheel efficiency conversions. MPG for electric vehicles don't make much sense (Wh/mi is much better), but it's still useful for comparing, esp for explaining to the layman.

    Since we are talking about efficiency anyways I used the 0.321kWh/mi provided by Tesla and then the 12,307 Wh/gal number given in the EPA document which factors in the avg efficiency of US fossil fuel plants, transmission, petroleum refining & distribution and came up with 38.34mpg. Which means on average, powered by US fossil fuel plants, the efficiency from fuel-to-wheel in the Tesla Roadster is about 38.34mpg, which is not bad. The Lotus Elise SC (probably the closest gasoline powered car to the Tesla Roadster) gets about 20/26 mpg.
     
  8. TEG

    TEG TMC Moderator

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    Some people will look at the MPGe numbers as a way to estimate costs to operate. For instance, how much do you pay for the electricity to drive 100 miles, compared to drive 100 miles in a gasoline car. A Prius could go maybe 100 miles on 2 gallons. Using $5/gallon gas thats $10 for 100 miles. So ~$0.10 per mile. If we said an electric car costs $0.02 per mile, then the EV is 5x cheaper per mile than the gas car. Given 50MPG for a Prius, then the EV would be in line to say that it gets 250MPGe.

    Now if you go back a few years, gas was closer to $2/gallon, not $5/gallon, so it would line up better to say 135MPGe back then.

    For this reason alone, it makes sense to permit higher MPGe if the cost of gas has gone up so much more than the cost of electricity.

    Just another way to look at it. And yes my numbers are horribly rounded.
     
  9. malcolm

    malcolm Active Member

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    Tesla's hands would seem to be tied by the EPA so I think it's unfair to lay gloom at their door over this one Chris.

    Although I agree with Kardax that an mpg value is meaningless for BEVs, it could all become unnecessarily complex in a couple of years with the sales literature for REEVs.

    Miles per charge, electricity cost per mile need to be promoted and explained clearly rather than publishing these mpg equivalent values.

    In the meantime, I'm not sure what Tesla can do. 300 mp(imperial)g sounds like science fiction. Yet that is the consequence of the EPAs calculations. Does publishing this encourage the EV doubters and damage Tesla's engineering credibility?
     
  10. mt2

    mt2 Member

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    I have to chirp in with my "me too" post; MPGe is irrelevant. It depends on what you mean by "equivalent".

    As an average consumer, my bottom line is my pocketbook. If I switch from an ICE to an EV but keep all my driving habits, I'd measure the difference in my energy costs. That, of course, is totally irrelevant for anyone else who would be switching from a different vehicle with a different MPG. And it would always be in flux because gasoline and electricity costs change. Never mind that some folk would get evening rates, some don't, and others may have solar panels on their roofs.

    My brother, on the other hand, is a mechanical engineer and a stickler for this kind of thing. He'd probably want to put the EV on the EPA driving cycle until it uses up 33.558kW and then note the mileage. Simple.

    Simple, but not good enough for environmentalists, economists, and policy makers. There is an energy cost to harnessing energy, so we have to look at the well to wheel calculation that Tesla did early on. That's probably the most accurate - but the formulas vary according to who writes them. Personally, I'd like to know what the X-Prize spreadsheet reveals about the Roadster. But the government sets the standards when it publishes papers written by committees and special interest groups, so that's what we get stuck with.

    Despite the whining about getting stuck with the EPA numbers, I bet Tesla was overjoyed with the 256 MPGe figure.
     
  11. pgwoosley

    pgwoosley Member

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    Tesla might want to footnote the MPG figure with an explanation that the number is computed as required by EPA. This would help Tesla's credibility.
     
  12. DaveD

    DaveD EVs Kick Gas!

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    This is the text that I'm seeing at the bottom of the TeslaMotors.com site:

    "* Range is based on our most recent testing with a Validation Prototype car at an EPA-certified location in February 2008. EPA range is subject to change pending future testing. Actual range will vary with driving style and conditions. Conversion from electric consumption to gallons of gasoline equivalent is calculated using the Department of Energy equivalence factor documented in the Code of Federal Regulations, Section 10, Part 474. "

    In your opinion, is this adequate? If not, how would you like to see it worded?
     
  13. pgwoosley

    pgwoosley Member

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    Suggested rewording:

    "Manufacturers are required to convert from electric consumption to gallons of gasoline equivalent using the Department of Energy equivalence factor specified in the Code of Federal Regulations, Section 10, Part 474, and Tesla has done so. "
     
  14. vfx

    vfx Well-Known Member

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    How 'bout

    The EPA requires manufacturers to display electric consumption converted to an equivalent in burned gallons of gasoline using the Department of Energy's equivalence factor specified in the Code of Federal Regulations, Section 10, Part 474.
     
  15. doug

    doug Administrator / Head Moderator

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    Extended Interview: Tesla Motors Chairman Elon Musk | Online NewsHour | June 25, 2008 | PBS
     
  16. pgwoosley

    pgwoosley Member

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    Sounds good.
     
  17. stopcrazypp

    stopcrazypp Well-Known Member

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    #17 stopcrazypp, Jun 26, 2008
    Last edited: Jun 26, 2008
    I'm not sure about the CO2 per mile, but isn't the 135 miles per gallon equivalent the same PLUG-to-wheel figure originally quoted on the website. That was never a WELL-to-wheel calculation in my knowledge.

    Edit:
    Scratch what I said below about Ohmexcited's page, I've mistaken "gas-fired plant" to be gasoline-fired plant. So it's still unsure how the CO2 compares if gasoline was used in a powerplant to make electricity to power the Roadster vs. the Prius.

    But thanks to Ohmexcited's page (a member here)
    An Investigation of the Electric Vehicle's Impact on CO2 Emissions
    at least the claim that a Tesla Roadster running on a gasoline fired plant emits about half the CO2 as a prius is right (though it might be older efficiency data on the Tesla).
     
  18. doug

    doug Administrator / Head Moderator

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    Sorry, I posted that quote without any comment. Yeah I wanted to point out that I thought 135 MPGe was calculated from the amount of energy contained in a gallon of gas, and not the energy required to refine and transport (etc.) it. Where as the discussion Elon goes through is a Well-to-wheel comparison. My question is does that comparison alone justify the new 256 "MPGe" (i.e. the oil was used to make electricity), or is there even more fudging to it.
     
  19. stopcrazypp

    stopcrazypp Well-Known Member

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    #19 stopcrazypp, Jun 26, 2008
    Last edited: Jun 26, 2008
    You are right, 135 MPGe was calculated by directly converting the energy usage from PLUG-to-wheel using the amount of energy contained in a gallon of gas.

    The 256 "MPGe" number includes a 6.67x incentive factor designed to boost alternatively fueled vehicles (it's not unique to EVs).

    If it were not for the incentive factor, the Tesla would get 38.4 MPGe (fuel-to-wheel, where the fuel has already arrived at the powerplant and is ready for use; note here the "fuel" just refers to fossil fuels, since the numbers involve the average efficiency of US fossil fuel plants, not gasoline/petroleum specifically) as I mentioned before. Without the dividing by "Petroleum refining and distribution efficiency", it would be 31.9MPGe (this should be well-to-wheel).

    So you can see it is a little of misleading to describe the 135MPGe number as an "apples-to-apples" comparison to Prius efficiency, since if gasoline was used to make electricity, by the time it gets to the wheels of the Tesla Roadster it would likely be around 38.4MPGe, b/c of losses from the powerplant and transmission.

    Reading the public comments section of the DOE document, people argue for the incentive factor to be higher for EVs, or argue for nuclear, hydro, & renewables to be counted as 100% efficient to boost the numbers, so the it isn't as high as it could have been. Either way, the incentive factor makes the 256MPGe pretty much useless for the consumer, it turns into just a number for the calculating CAFE numbers (the quote from Darryl that MDR gave actually mentioned this so at least he was honest here). Again, it sounds like the number is madated by the EPA, so I'm not sure if Tesla has the freedom to not use that number.
     
  20. vfx

    vfx Well-Known Member

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    I just wanted to used the word "burned" :)
     

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