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Suggestion for an EV Mileage Designation

Discussion in 'Energy, Environment, and Policy' started by tdelta1000, Jan 29, 2010.

  1. tdelta1000

    tdelta1000 Active Member

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    If gasoline vehicles are cover by "MPG" designation then my suggestion for covering EV mileage should be "MPC" which is Miles Per Charge.

    Would this designation make it easier the determine the traveling distance of any EV or is it important for manufactures, government agencies and others to have a marker for savings?
     
  2. bolosky

    bolosky Member

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    Those measure different things. Probably the best EV equivalent to MPG is Wh/mile. (Or I suppose miles/kWh, but it's really more sensible the other way and the switch to EVs is a great chance to fix it.)

    What you're asking for is usually called range, and as anyone who's spent much time with a Roadster will tell you, your mileage will vary (a whole lot).
     
  3. tdelta1000

    tdelta1000 Active Member

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    Bolosky,

    If the distance an EV travels is called "range" then the same would hold true for gasoline engines because their "range" vary. The standard for gasoline range has been known as MPG for some time now. As for EVs the designation has not been set in stone. I like idea of setting a single designation like MPC (Miles Per Charge) or your suggestion MPK (Miles per Kilowatt). What do you think?
     
  4. ChargeIt!

    ChargeIt! Member

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    Miles-per-Charge is still really a range "kind-of-thing". In the Tesla Roadster there are several modes you can charge and drive with. Quoting 244 MPC is really not a real-world practical equivalent of the ICE's MPG rating. Just like the size of an ICE's gas-tank will vary, so will the "range". Therefore quoting 244 MPC would also necessitate mentioning that that is achievable only (in addition to a lot of other efficient driving stuff) if you charged in Range Mode (on the Roadster). In the ICE world, either the range is so large that you don't care (or so quickly "re-fillable") or the advertised total range on a tank of gas is so less well advertised that most of us can't tell you what it is. In an EV its "range" is a much more critical factor ... but still it would be a disservice to equate it with MPG.

    The ICE's MPG really is meant to tell you something about "efficiency" of the vehicle compared to other ICE vehicles. That's where the Wh/mile makes the most sense for the EV ( I am purposely for now avoiding discussion of hybrids ... added complexity ), or its inverse, MPK. (However, tdelta1000, you should say MPK=Miles-per-kWh, not Miles-per-kW.) From a real-world practical perspective, the MPG helps the ICE driver learn about the car's fuel efficiency, which (at least in my mind) is immediately translated to economic efficiency, assuming a certain price-per-gallon for the fuel the car uses (diesel, premium gas, regular gas, etc at -- let's say -- $3/gallon). So ... MPG tells me how many miles I can go with $3 in my pocket (at today's gas prices).

    For the EV its MPK would tell me how many miles I can go with <insert you home electricity price here> ... 16 cents. The $0.16 will likely not fluctuate as much as gasoline prices ... and unfortunately $0.16 is such a "small" amount it is not very relatable to $3 spent on gas for the ICE. But I could simply multiply the MPK by 10 or 20 ($3.20) to get some reasonable comparison "RANGE" :wink: to that $3 spent on the ICE.

    ( I am ignoring the difference here (but it should not be ignored for an in-depth MPK ratings discussion) of the actual electricity (think $s from your pocket) fed to the EV to charge it (including miscellaneous inefficiencies, cooling, heating, etc) versus its Wh/mile used while driving. )
     
  5. johnr

    johnr Member

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    The measurement of efficiency in a gasoline-fueled vehicle is MPG - this of course tells you how much fuel it will burn to travel a given distance.
    The equivalent measure for electric vehicles would appear to be watt-hours per mile (Wh/Mi), or better yet, miles per kilowatt-hour (Mi/KWh) - this tells you how much energy you'll use in KWh to travel a given distance or how far you'll travel on each KWh you buy from the electric company.
    MPG (miles per gallon) => Wh/Mi (watt-hours per mile) or Mi/KWh (miles per killowatt-hour)

    For a gasoline-fueled vehicle, range is measured by the size of the fuel tank in gallons.
    For an electric vehicle, range is measured by the size of the battery pack in KWh.
    If a gas car is rated at 30 MPG and has a 14 gallon tank it will go 420 miles before it needs another fill-up. An EV at 5.65 Mi/KWh and a 53 KWh battery pack will go 240 miles on a full charge (factoring in 80% battery utilization).
    Range => Gallons or KWh

    Since electricity costs so little compared to gasoline, I don't think the efficiency of an electric car has any real significance at this time - the range is far more important to me.
     
  6. JRP3

    JRP3 Hyperactive Member

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    We use wh/mi to calculate pack size for a needed range, and like mpg this can change with driving conditions. On average a vehicle that uses 200 wh/mi needs a 20,000 watt hour, or 20 kwh, pack. That same vehicle driven hard might use 300 wh/mi so range would be reduced. Total average range and average wh/mi. are good metrics of EV efficiency. If you know pack size and range you can calculate efficiency, wh/mi., if you know range and wh/mi you can calculate pack size, and if you know wh/mi and pack size you can figure out range.
     
  7. vfx

    vfx Well-Known Member

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    Didn't we cover this is another thread? Probably the GM 235 debacle.

    This is related:

    Instead of the intelligent "gallons per mile"

    From MPG to EPM: Plan for “Electricity Per Mile” Ratings Takes Hold
     

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