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Decoding Electric Car MPG

Edmunds.comEdmunds.com Member, Administrator, Moderator Posts: 10,315
edited September 2014 in General

imageDecoding Electric Car MPG

How does the fuel economy rating of an electric car differ from gas vehicles? Forget MPG. Meet the kilowatt-hour, and learn why smaller is better.

Read the full story here


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    rbdiamond1rbdiamond1 Member Posts: 1
    The article misses a major point, which is the total cost of ownership. An electric motor has no oil changes, etc. I would like to see an analysis that calculates typical maintenance of an internal combustion engine vs. an electric motor.
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    iisi50mhziisi50mhz Member Posts: 2
    If the SI abbreviation for "meters" is "m" and the standard (Imperial unit) abbreviation for "miles" is "mi.", then shouldn't "kWh/100m" be interpreted as "kWh per 100 meters" and "kWh/100 mi." be used for "kWh per 100 miles"?
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    cecil9cecil9 Member Posts: 1
    So where does the efficiency of the power station and the transmission losses in the distribution cable fit in to these equivalent MPG numbers-plus transformer losse and battery losses
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    doctorsteamdoctorsteam Member Posts: 1
    The calculation of MPGe is bogus. It is based on a faulty assumption and grossly overstates the MPGe. The EPA should know that heat content in gasoline is not the same as mechanical work at the drive shaft. Using the right conversion, the Leaf and Volt (using just battery) give about 26-28 MPGe for a gasoline -fueled vehicle.
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    kkirby1kkirby1 Member Posts: 1
    In order to objectively compare the efficiency of electric vehicles to gas burners to gage environmental benefits, such as generation of greenhouse gasses, etc., it is necessary to consider other factors, such as the energy of conversion, and how many times it has to be converted. For example, a power plant that generates electricity may only achieve about 30% to 40% efficiency in converting heat to electricity. There is also energy lost in transmission, and in conversion from electrical to chemical energy to charge a battery, and again in converting it back to electrical to mechanical energy to move the car. I would like to see and accounting of the total amount of fuel per gallon of an electrical car, considering all of the energy of conversion throughout the entire process, to a similar acccounting for an internal combustion engine. It seems there are more conversions necessary to power an EV than to power a gas burning vehicle, and if so, when all efficiencies in the process are considered, EVs may not be nearly as efficient as is being reported simply as kWh per mile or miles per kWh.
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    WhoaItsAFactorialWhoaItsAFactorial Member Posts: 2
    kkirby1 said:

    It seems there are more conversions necessary to power an EV than to power a gas burning vehicle, and if so, when all efficiencies in the process are considered, EVs may not be nearly as efficient as is being reported simply as kWh per mile or miles per kWh.

    If we are going to factor in these, then you have to factor in the energy cost to get oil out of the ground, the transportation cost of getting the crude oil to a refinery, the energy cost of refining the oil into gasoline, and the transportation cost of getting the gasoline to the gas stations.
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    LensilvaLensilva Member Posts: 1
    EVs do not yet pay a road use tax, which in Florida amounts to about 22% of the fuel cost. I'm sure that will change one day, but it must be part of the equation.
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    KN4TXKN4TX Member Posts: 2
    In Alabama, I pay $200 extra road tax above and beyond my annual license for my EV. If I drove an ICE that got 25 mpg, and drove 12,000/year, that 22% equates to $105. So... I pay double for driving my EV compared to my wife's Lexus... I'm sure FL will find a way to gouge EV owners soon enough.
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    KN4TXKN4TX Member Posts: 2
    The efficiency math has been done: An example is the comparison between an electric vehicle (EV) and a fossil fuel powered car. The EV has an overall efficiency of about 60% while the fuel powered car is about 20% (or less) efficient. When one considers the total path from primary energy, the efficiency of the electric car is 60% x 40% (grid efficiency) for a total of 24% while the fuel powered car has an efficiency of about 20%. When the bigger picture is considered, the two forms of transportation are not all that different.
    https://www.energycentral.com/c/ec/grid-efficiency-opportunity-reduce-emissions
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    Jonathan_AllenJonathan_Allen Member Posts: 1
    KN4TX raises an interesting point. The MPGe figures consider only the First Law of thermodynamics but ignores the Second. That is, electricity is high grade energy (work) which can be used directly to propel the vehicle. Heat, however, is lower grade since under the Second Law (and the Carnot Equation) only a fraction of that energy can be harvested as work. If this happens in the steam (Rankine) cycle cycle of the power plant, then that efficiency has included in the calculation. It still comes out better than a gas vehicle, but not so much as implied by the published MPGe numbers.

    An electric vehicle charged from a conventional power station is actually a coal-fired steam car. To realize the full environmental value of electric vehicles and their reduced "carbon footprint" the charging source must be renewable, or at least not based on fossil fuels.
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    Captain_QuirkCaptain_Quirk Member Posts: 1
    "Start wrapping your head around a new fuel economy measure where smaller is better."

    This is why the kWh/100 miles formulation is bassackwards. It's literally upside down. Consumers are used to fuel economy being measured in distance per amount of fuel used, not the other way around. The economy of EVs should be measured in miles per kWh, not kWh per 100 miles.

    And as others have noted, the EPA's inflated "MPGe" figures, which are based solely on the energy content of gasoline, fail to take into account the very large inherent inefficiency of thermal engines (in this case, internal combustion engines) -- that is, those engines' ability to convert the chemical energy in gasoline into actual work (i.e., miles of travel). As far as I can tell, the EPA is treating internal combustion engines as if they were 100% efficient, which is ridiculous. By my calculations, the equivalent fuel efficiency of the Ford Focus EV mentioned in the article, rated at 105 MPGe, is actually about 64.4 MPGe. (Based on its stated efficiency of 32 kWh per 100 miles.)
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    kyfdxkyfdx Moderator Posts: 237,153
    Captain_Quirk said:

    "Start wrapping your head around a new fuel economy measure where smaller is better."

    This is why the kWh/100 miles formulation is bassackwards. It's literally upside down. Consumers are used to fuel economy being measured in distance per amount of fuel used, not the other way around. The economy of EVs should be measured in miles per kWh, not kWh per 100 miles.

    And as others have noted, the EPA's inflated "MPGe" figures, which are based solely on the energy content of gasoline, fail to take into account the very large inherent inefficiency of thermal engines (in this case, internal combustion engines) -- that is, those engines' ability to convert the chemical energy in gasoline into actual work (i.e., miles of travel). As far as I can tell, the EPA is treating internal combustion engines as if they were 100% efficient, which is ridiculous. By my calculations, the equivalent fuel efficiency of the Ford Focus EV mentioned in the article, rated at 105 MPGe, is actually about 64.4 MPGe. (Based on its stated efficiency of 32 kWh per 100 miles.)

    Actually, it's only the US that measures it that way.
    In all metric countries they measure fuel mileage as Litres/100 miles.
    The kWh/100 miles just follows along with that.

    No comment on the rest of it, or how it accurate it might be.

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